Nathan, R., U. N. Safriel and H. Shirihai. 1996. Extinction and vulnerability to extinction at distribution peripheries: an analysis of the Israeli breeding avifauna. Israel Journal of Zoology 42: 353-376.
Abstract. We hypothesized that peripheral populations, being mostly small and remote, are more prone to extinction than non peripheral populations within the same region. Between 1863 and 1993, 204 bird species were recorded as breeders in Israel. Of the 185 species that bred regularly, 14 species have become extinct and 58 species are threatened. Many breeding species of Israel are at the periphery of their distribution, and most of these are at their southern periphery. The extinct species were significantly more peripheral than others, and the 113 non threatened species are significantly less peripheral than others. Most extinct species are raptors and waterfowl, and their extinction is related to human pressure. However, even within these groups extremely peripheral species tend to become extinct, whereas less peripheral species tend to persist. We conclude that because peripheral populations are inherently vulnerable, they are also more sensitive to human pressure. [Full Text - PDF]
Abstract. We review and analyze the altitudinal distribution of reptiles (from museum collections) and breeding birds (from literature records and a recent field sampling project) on Mt. Hermon (latitude approx. 33o30'N), over the range 300-2814 m. Species richness was greater in breeding birds (n=86-90) than in reptiles (n=34), declining with increasing altitude in both groups, but more steeply so in birds. We suggest that the simplification of vegetation structure with increasing altitude may explain this difference, because birds probably better use the three-dimensionality of densely-vegetated habitats that dominate low elevations. But in both taxa the overall decline was nonlinear: species richness actually increased from low to mid-elevations, forming a hump-shaped pattern. Standardization of species richness for surface area, across altitudinal belts of 100 m up to 1,900 m, revealed the area effect was both significant and unpredictable. It did not affect the overall decline, but contrary to a general expectation of a linear trend turning hump-shaped, the hump-shaped curve became concave, i.e., from low to mid-elevations the number of species per unit area declined sharply, and at higher elevations it remained fairly stable. In a comparison with other mountains in the Mediterranean Basin (between latitudes 35o00' and 42o30'N), the very same reptile species reached higher maximum elevations on Mt. Hermon, while the same bird species reached similar elevations in both areas. This difference may reflect lesser ability of reptiles, compared to birds, to inhabit high elevations at higher latitudes, but their altitudinal distribution on Mt. Hermon showed no evidence for such a physiological difference. In general, in contrast with other reports, we found no positive correlation between the extent of altitudinal range of the species and their position on the altitudinal gradient, as would have been implied by Rapoport's altitudinal rule. Future mapping of local distributions of the Mt. Hermon biota, required for efficient conservation, should derive from planned, extensive field sampling, augmented with museum records for clarification of taxonomic uncertainties. [Full Text - PDF]
Abstract. 1 Although serotiny is frequently considered to have evolved under the selective pressure of fires, the drying conditions that induce cone opening are not necessarily associated with fire. We hypothesized that in Pinus halepensis, a Mediterranean serotinous wind-dispersed tree, xeriscence (seed release induced primarily by drying conditions not generated by fire) bears intrinsic adaptive values, independent of those of pyriscence (fire-induced seed release). 2 We used seed-traps to quantify seed release in two scrubland pine stands in Israel. Contemporary meteorological data were used to seek correlations with climatic factors affecting seed release. 3 Substantial seed release, estimated to be about 60% of the annual crop, was observed in the absence of fire. Seed release was distinctly seasonal, with high rates in spring and autumn, and was strongly correlated with short, infrequent but temporally predictable Sharav events (dry and hot weather). In the most extensive Sharav-induced seed release, seed density reached 117 seeds m(-2) and 15% of the annual crop was released within 6 days. Stepwise multiple regression revealed that mean relative humidity (in both stands) and maximum temperature (in one stand) were significant predictors of seed release. 4 Vertical (upwards) wind velocity was significantly positively correlated with dry and hot weather. Seed counts in distant traps (> 20 m from the nearest tree) were significantly greater in periods in which Sharav events occurred than in other periods. Xeriscence may therefore have an adaptive value in promoting dispersal distance by wind. 5 Both xeriscence and pyriscence appear to be involved in determining serotiny in P. halepensis and provide means of exploiting establishment opportunities generated either by fire or by other factors. [Full Text - PDF]
Abstract. Growing interest in spatial ecology is promoting new approaches to the study of seed dispersal, one of the key processes determining the spatial structure of plant populations. Seed-dispersion patterns vary among plant species, populations and individuals, at different distances from parents, different microsites and different times. Recent field studies have made progress in elucidating the mechanisms behind these patterns and the implications of these patterns for recruitment success. Together with the development and refinement of mathematical models, this promises a deeper, more mechanistic understanding of dispersal processes and their consequences. [Full Text - PDF]
Abstract. Spatiotemporal variation in the wind-generated dispersion pattern of Aleppo pine (Pinus halepensis) seeds was examined by placing seed traps up to 110 m away from a small. isolated stand in Israel during six successive dispersal seasons. Subsequent recruitment was surveyed two years later. Of the 5487 seeds, 97% were trapped 120 m from the nearest adult tree. Seasonal dispersal curves were consistently right-skewed and leptokurtic. The inverse power law and the negative exponential model accounted for a similar fraction of the variation in the number of dispersed seeds at different distances (79-86% and 76-88%, respectively). Seed dispersal rates varied significantly among seasons. Eighty saplings (3-5 yr old) became established during the three years of the study, most within 15 m of the nearest canopy, some farther away, and none directly under the tree canopies. The estimated probability of seed survival to sapling stage increased significantly with increasing distance from adults, as predicted by the escape hypothesis. Winds varied significantly between seasons. generating significant interseasonal variation in seed deposition patterns far from, but not near to, adult trees. Coefficients of variation of seed dispersal rates were lower near adult trees than farther away. Both Moran's I correlograms and partial Mantel tests revealed interseasonal consistency in seed deposition patterns for the total study area and for the area near adult trees but showed considerable variation farther away. We suggest that the low spatiotemporal variation near adults and the high variation far away act to intensify the effects of predation (by increasing the efficiency of predators near adults and reducing it far away) and competition (by increasing the intensity of seedling competition near adults and reducing it far away) in structuring the observed survivorship curve predicted by the escape hypothesis. [Full Text - PDF]
Abstract. We present a temporally and spatially explicit mechanistic model of tree seed dispersal by wind, incorporating full stochasticity based on natural variation. The model simulates the dispersal of each individual seed by integrating the temporal effects of climatic conditions on the rate of seed release, and the spatial effects of wind direction and horizontal and vertical velocities, the terminal velocity of seeds (i.e., the constant descent velocity in calm air), and the height of seed release, partitioned into tree height and the distribution of seeds with tree height. The model was tested for two Pinus halepensis stands within the Mediterranean region of Israel, in which seed dispersal has been extensively monitored by seed traps. The predicted dispersal curve verified expectations of a positively skewed leptokurtic distribution and of peak location at some distance from a point source and at zero distance from an area source. Long-distance dispersal events occurred with very low frequency, but given the large seed crop in P. halepensis, even a small fraction should result in a considerable number of seeds dispersed far from their source. The model reliably simulates the observed dispersal pattern in a spatial resolution of 1 m(2) (R-2 between 60% and 90%), as revealed from comparisons of the predicted and observed proportions of seed dispersed to seed traps. A sensitivity analysis using Latin hypercube sampling along with stepwise multiple rank regression showed that the effects of the horizontal and vertical wind velocities on the dispersal distance override those of the biotic factors. This suggests that the synchronization of seed release with favorable winds is the most effective plant-controlled mechanism to increase the distance of dispersal in wind-dispersed species such as P. halepensis. [Full Text - PDF]
Abstract. Some mechanisms that promote tong-distance dispersal of tree seeds by wind are explored. Winged seeds must be lifted above the canopy by updrafts to have a chance of further dispersal in high velocity horizontal winds aloft or in landscape-scale convection cells. Shear-induced turbulent eddies of a scale up to one-third of canopy height provide a lifting mechanism. Preliminary data suggest that all seeds of a given species may be viable candidates for uplift and long-distance dispersal, despite the evidence that slow-falling seeds are dispersed farther under any given wind conditions. Turbulence is argued more often and more extensively to advance long-distance dispersal than to retard it. Seeds may take advantage of 'Bernoulli sailing' to move with faster than average winds. Elasticity of branches and trees may play a rote in regulating the release of seeds into unusually favorable winds. Dispersal is at least biphasic, and the study of long-distance dispersal calls for mixed models and mixed methods of gathering data. [Full Text - PDF]
Abstract. Long-distance dispersal (LDD) is central to species expansion following climate change, re-colonization of disturbed areas and control of pests(1-8). The current paradigm is that the frequency and spatial extent of LDD events are extremely difficult to predict(9-12). Here we show that mechanistic models coupling seed release and aerodynamics with turbulent transport processes provide accurate probabilistic descriptions of LDD of seeds by wind. The proposed model reliably predicts the vertical distribution of dispersed seeds of five tree species observed along a 45-m high tower in an eastern US deciduous forest. Simulations show that uplifting above the forest canopy is necessary and sufficient for LDD, hence, they provide the means to define LDD quantitatively rather than arbitrarily. Seed uplifting probability thus sets an upper bound on the probability of long-distance colonization. Uplifted yellow poplar seeds are on average lighter than seeds at the forest floor, but also include the heaviest seeds. Because uplifting probabilities are appreciable (as much as 1-5%), and tree seed crops are commonly massive, some LDD events will establish individuals that can critically affect plant dynamics on large scales. [Full Text - PDF]
Abstract. It has been argued that nonstandard mechanisms of dispersal are often responsible for long-distance dispersal in plants. For example, plant seeds that appear to be adapted for wind dispersal may occasionally be dispersed long distances by birds, or vice versa. In this paper, we explore whether existing data on dispersal distances, colonization rates, and migration rates support the idea that dispersal processes suggested by the morphology of the dispersal unit are responsible for long distance dispersal. We conclude that the relationship between morphologically defined dispersal syndrome and long-distance dispersal is poor. This relationship is poor because the relationship between the morphology of dispersal units and the multiple processes that move seeds are often complex. We argue that understanding gleaned from the often anecdotal literature on nonstandard and standard means of long distance dispersal is the foundation for both statistical and mechanistic models of long-distance dispersal. Such models hold exciting promise for the development of a quantitative ecology of long-distance dispersal. [Full Text - PDF]
Abstract. Long-distance dispersal (LDD) includes events in which propagules arrive, but do not necessarily establish, at a site far removed from their origin. Although important in a variety of ecological contexts, the system-specific nature of LDD makes "far removed" difficult to quantify, partly, but not exclusively, because of inherent uncertainty typically involved with the highly stochastic LDD processes. We critically review the main methods employed in studies of dispersal, in order to facilitate the evaluation of their pertinence to specific aspects of LDD research. Using a novel classification framework, we identify six main methodological groups: biogeographical; Eulerian and Lagrangian movement/redistributional; short-term and long-term genetic analyses; and modeling. We briefly discuss the strengths and weaknesses of the most promising methods available for estimation of LDD, illustrating them with examples from current studies.The rarity of LDD events will continue to make collecting, analyzing, and interpreting the necessary data difficult, and a simple and comprehensive definition of LDD will remain elusive. However, considerable advances have been made in some methodological areas, such as miniaturization of tracking devices, elaboration of stable isotope and genetic analyses, and refinement of mechanistic models. Combinations of methods are increasingly used to provide improved insight on LDD from multiple angles. However, human activities substantially increase the variety of long-distance transport avenues, making the estimation of LDD even more challenging. [Full Text - PDF]
Higgins, S.I., J.S. Clark, R. Nathan, T. Hovestadt, F. Schurr , J.M.V. Fragoso, M.R. Aguiar, E. Ribbens and S. Lavorel. 2003. Forecasting plant migration rates: managing uncertainties for risk assessment. Journal of Ecology 87:659-669.
Abstract. 1 Anthropogenic changes in the global climate are shifting the potential ranges of many plant species. 2 Changing climates will allow some species the opportunity to expand their range, others may experience a contraction in their potential range, while the current and future ranges of some species may not overlap. Our capacity to generalize about the threat these range shifts pose to plant diversity is limited by many sources of uncertainty. 3 In this paper we summarize sources of uncertainty for migration forecasts and suggest a research protocol for making forecasts in the context of uncertainty. [Full Text - PDF]
Abstract. Models of seed dispersal—a key process in plant spatial dynamics— have played a fundamental role in representing dispersal patterns, investigating dispersal processes, elucidating the consequences of dispersal for populations and communities, and explaining dispersal evolution. Mechanistic models of seed dispersal have explained seed dispersion patterns expected under different conditions, and illuminated the circumstances that lead to long-distance dispersal in particular. Phenomenological models have allowed us to describe dispersal pattern and can be incorporated into models of the implications of dispersal. Perhaps most notably, population and community models have shown that not only mean dispersal distances but also the entire distribution of dispersal distances are critical to range expansion rates, recruitment patterns, genetic structure, metapopulation dynamics, and ultimately community diversity at different scales. Here, we review these developments, and provide suggestions for further research. [Full Text - PDF]
Abstract. Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species. [Full Text - PDF]
Abstract. Human effects on plant colonization capacity have not been studied mechanistically because a crucial component of colonization capacity, long-distance seed dispersal, could not be quantified. Now, development of mechanistic models has progressed sufficiently to estimate long-distance seed dispersal by wind. We used a recently developed mechanistic dispersal model to quantify seed dispersal by wind in grasslands for three important human effects on natural systems: habitat fragmentation, eutrophication, and an increase in extreme wind velocity events due to climate change. We combined the dispersal data with data on seed production and germination ability to estimate effects on colonization capacity. Habitat fragmentation decreases the number and size of populations. This does not affect the populations’ seed dispersal ability, but it reduces the number and germinability of produced seeds, and hence the colonization capacity. Site eutrophication strongly reduces seed dispersal distances, but in some species it also increases seed production and germinability. Thus, long-distance colonization decreases, but short-distance colonization may increase. Wind velocity is the key determinant of dispersal distances. An increase in extreme winds increases long-distance dispersal and long-distance colonization capacity. However, increases in extreme winds predicted for climate change scenarios are not sufficient to compensate the reductions in long-distance colonization due to habitat fragmentation or eutrophication. [Full Text - PDF]
Abstract. 1 Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump-shaped (Janzen-Connell (J-C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns. 2 We introduce a simple mechanistic model showing that distance and density-dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J-C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms. 3 The model shows that the J-C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host-specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds. 4 The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators. 5 The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance-dependent predation promotes either short or long dispersal distances, or both (dimorphism). [Full Text - PDF]
Abstract. To assess the canopy seed bank structure of Pinus halepensis, we measured the level of serotiny and the seed bank size and density of trees in unburned stands and post-fire regenerated stands in Israel. We analysed the effects of tree size, tree density and fire history on the level of serotiny. The level of serotiny decreased with an increase in tree height. The high level of serotiny in short trees could be explained by selection to increase regeneration chances after burning at pre-mature age. Also, limitation of long-distance seed dispersal opportunities in short trees may favour high serotiny levels. The level of serotiny was higher in post-fire stands than in unburned stands, suggesting a fast selection for serotiny by fire. Unburned stands had a higher total stand seed density than post-fire regenerated stands, but the proportion of seeds in serotinous cones of the total stand seed density was higher in post-fire regenerated stands. The fact that P. halepensis bears simultaneously serotinous and non-serotinous cones reflects its dual strategy as both a post-fire obligate seeder, mainly from serotinous cones and an early coloniser during fire-free periods, mainly from non-serotinous cones. The relative investment in these strategies is dependent on fire history and varies with tree height. Furthermore, mature brown cones can contribute to post-fire regeneration in case of spring fires, and serotinous cones are known to open partially also in dry spell events. Thus, post-fire regeneration and invasion are strategies, which seem to complement each other. [Full Text - PDF]
Abstract. Fire is known to be a major factor in shaping plants and vegetation worldwide. Many plant traits have been described as adaptations for surviving fire, or regenerating after it. However, many of the traits are also advantageous for overcoming other disturbances. The fact that fire in the Mediterranean Basin has been almost exclusively of anthropogenic origin, and thus is of short duration in an evolutionary time scale, cast doubt on the possibility that fire can act as a selective force in the Mediterranean Basin. Our aim here is to review the ecological advantages of Pinus halepensis traits and their possibility to be selected by fire. The non-self pruning of cones and branches, and the high resin content increase the probability of canopy fires and consequent death of P. halepensis trees. Postfire regeneration of P. halepensis depends totally upon its canopy-stored seed bank. The seedlings grow quickly and they first reproduce at an early age. Young reproductive trees function first as females with a high percentage of serotinous cones. Thus, young P. halepensis trees allocate many resources to seed production, reducing their‘immaturity risk’ in a case of an early successive fire. The proportion of serotinous cones is higher in post-fire naturally regenerating stands than in unburned stands, and seeds from serotinous cones germinate better under simulated post-fire conditions. The extremely high pH of the ash-bed under the burned canopies creates the post-fire regeneration niche of P. halepensis exactly under their parent trees. All these traits are advantageous for post-fire regeneration, but could they also be selected during the time scale of anthropogenic fires in the Mediterranean Basin? Pinus halepensis is a relatively short living tree with almost no recruitment under forest canopy. The longest estimated fire-return interval and generation length are about 125 years. The earliest solid evidence for the first hominid-controlled fire in the Mediterranean basin is 780,000 years ago, and thus the estimated number of post-fire generations is 6240. We suggest that such a number of generations is sufficient for the selection and radiation of fire adaptive traits in P. halepensis. [Full Text - PDF]
Abstract. This special issue of Diversity and Distributions presents six papers that contribute to the assembly of a general research agenda for studying long-distance dispersal (LDD) across a variety of taxonomic groups (e.g. birds, fish, aquatic invertebrates and plants), ecosystems (e.g. terrestrial and marine ecosystems, wetlands and grasslands) and thematic fields (e.g. biological transport, marine biology, biogeochemistry and biodiversity conservation). This editorial emphasizes the need to develop a network integrating different research approaches (‘yellow brick roads’) to address the great challenge (‘finding the end of the rainbow’) of quantifying, understanding and predicting LDD and its implications. I review the key avenues for future research suggested in the special issue contributions, and stress the critical importance of properly considering the spatial and temporal scales relevant to the process and system of interests. I propose combining absolute and proportional definitions of LDD as a default practice in any investigation of LDD processes. When LDD is defined primarily by an absolute critical distance that characterizes key feature(s) of the system of interest, a quantitative assessment of the proportion of dispersal events expected to move beyond this critical threshold distance should also be provided. When LDD is defined primarily by a certain small fraction of dispersal events that travel longer than all others, an estimate of the absolute distance associated with this high percentile at the tail of the dispersal curve should also be added. [Full Text - PDF]
Nathan, R., N. Sapir, A. Trakhtenbrot, G. G. Katul, G. Bohrer, M. Otte, R. Avissar, M. B. Soons, H. S. Horn, M. Wikelski, and S. A. Levin. 2005. Long-distance biological transport processes through the air: can nature's complexity be unfolded in-silico? Diversity and Distributions 11: 131-137.
Abstract. Understanding and predicting complex biological systems are best accomplished through the synthesis and integration of information across relevant spatial, temporal and thematic scales. We propose that mechanistic transport models, which integrate atmospheric turbulence with information on relevant biological attributes, can effectively incorporate key elements of aerial transport processes at scales ranging from a few centimetres and fractions of seconds, to hundreds of kilometres and decades. This capability of mechanistic models is critically important for modelling the flow of organisms through the atmosphere because diverse aerial transport processes — such as pathogen spread, seed dispersal, spider ballooning and bird migration — are sensitive to the details of small-scale short-term turbulent deviations from the mean airflow. At the same time, all these processes are strongly influenced by the typical larger-scale variation in landscape structure, through its effects on wind flow patterns. We therefore highlight the useful coupling of detailed atmospheric models such as large eddy simulations (LES), which can provide a high-resolution description of turbulent airflow, with regional atmospheric models, which can capture the effects of landscape heterogeneity at various scales. Further progress in computational fluid dynamics (CFD) will enable rigorous exploration of transport processes in heterogeneous landscapes. [Full Text - PDF]
Abstract. Dispersal is universally considered important for biodiversity conservation. However, the significance of long- as opposed to short-distance dispersal is insufficiently recognized in the conservation context. Long-distance dispersal (LDD) events, although typically rare, are crucial to population spread and to maintenance of genetic connectivity. The main threats to global biodiversity involve excessive LDD of elements alien to ecosystems and insufficient dispersal of native species, for example, because of habitat fragmentation. In this paper, we attempt to bridge the gap in the treatment of LDD by reviewing the conservation issues for which LDD is most important. We then demonstrate how taking LDD into consideration can improve conservation management decisions. [Full Text - PDF]
Katul, G. G., A. Porporato, R. Nathan, M. Siqueira, M. B. Soons, D. Poggi, H. S. Horn, and S. A. Levin. Mechanistic analytical models for long-distance seed dispersal by wind. The American Naturalist (In Press).
Abstract. We introduce an analytical model, the Wald Analytical Long-distance Dispersal (WALD) model, for estimating dispersal kernels of wind-dispersed seeds and their escape probability from the canopy. The model is based on simplifications to well-established three-dimensional Lagrangian stochastic approaches for turbulent scalar transport resulting in a 2-parameter Wald (or Inverse Gaussian) distribution. Unlike commonly-used phenomenological models, WALD's parameters can be estimated from the key factors affecting wind dispersal – wind statistics, seed release height, and seed terminal velocity – determined independently of dispersal data. WALD’s asymptotic power-law tail has an exponent of -3/2, a limiting value verified by a meta-analysis for a wide variety of measured dispersal kernels and fatter than the exponent of the bivariate student t-test (2Dt). We tested WALD using three dispersal data sets on forest trees, heathland shrubs and grassland forbs and compared WALD’s performance with other analytical mechanistic models (revised versions of the tilted Gaussian Plume model and the advection-diffusion equation), revealing fairest agreement between WALD predictions and measurements. Analytical mechanistic models such as WALD combine the advantages of simplicity and mechanistic understanding and are valuable tools for modeling large-scale long-term plant population dynamics. [Full Text - PDF]
Abstract. Seed terminal velocity and release height are recognized as key biotic determinants of long-distance dispersal (LDD) of seeds by wind. Yet, potential determinants at the ecosystem level, such as seasonal dynamics in foliage density characterizing many deciduous forests, have received much less attention. We integrated detailed field observations and experiments with a mechanistic wind dispersal model to assess how seasonal variation in foliage density, estimated by leaf-area index (LAI), affects LDD in deciduous forests. We found that the model, previously shown to accurately predict seed dispersal by wind, also reliably describes the effects of LAI variation on wind statistics for a wide range of canopy types. Sparser canopies are characterized by more organized vertical eddy motion that promotes LDD by uplifting seeds to higher elevations where winds are stronger. Yet, sparser canopies are also characterized by reduced mean windspeed aloft. We showed that former effect more than compensates for the latter, i.e., conditions of low LAI are favorable for LDD. This may account for the tendency of many temperate tree species to restrict seed release to either early spring or late fall, when LAI is relatively low. Sensitivity analysis reveals that the typical seasonal variation in LAI can be more important to LDD of seeds by wind than the natural variation in seed terminal velocity. Because our model accurately describes the effects of LAI variation for distinctly different sites, species, and life forms, we suggest that its results reflect a general association between LDD and foliage density dynamics. [Full Text - PDF]
Bohrer, G., R. Nathan, and S. Volis. 2005. Effects of long-distance dispersal for metapopulation survival and genetic structure at ecological time and spatial scales. Journal of Ecology 93: 1029–1040.
Abstract. 1 Long-distance dispersal (LDD) of seeds by wind plays an important role in population survival and structure, especially in naturally patchy or human-fragmented metapopulations. However, no study has tested its effects using a realistic dispersal kernel in a metapopulation context with explicit spatial structure and local extinctions. 2 We incorporated such kernels into a newly proposed simulation model, which combines within-patch (population) demographic processes and a simplified maternally inherited single-locus, two-allele genetic make-up of the populations. As a test case, we modelled a typical conservation scenario of Aleppo pine ( Pinus halepensis ) populations. 3 The effects of LDD were rather diverse and depended on initial population conditions and local extinction rates. LDD increased metapopulation survival at intermediate local-extinction probabilities. LDD helped maintain higher total genetic variability in populations that were initially drifted, but facilitated random genetic loss through drift in initially ‘well mixed’ populations. LDD prevented population differentiation in low extinction rates but increased it at intermediate to high extinction rates. 4 Our results suggest that LDD has broader evolutionary implications and would be selected for in populations facing intermediate local-extinction pressures. Our modelling approach provides a strong tool to test the effects of LDD on metapopulation survival and genetic variability and to identify the parameters to which such effects are most sensitive, in ecological and conservational scenarios. [Full Text - PDF]
Neilson, R. P., L. F. Pitelka, A. M. Solomon, R. Nathan, G. F. Midgley, J. M. V. Fragoso, H. Lischke and K. Thompson. Forecasting regional to global plant migration in response to climate change: challenges and directions. BioScience 55: 749-795.
Abstract. The rate of future climate change is likely to exceed the migration rates of most plant species. The replacement of dominant species by locally rare species may require decades, and extinctions may occur when plant species cannot migrate fast enough to escape the consequences of climate change. Such lags may impair ecosystem services, such as carbon sequestration and clean water production. Thus, to assess global change, simulation of plant migration and local vegetation change by dynamic global vegetation models (DGVMs) is critical, yet fraught with challenges. Global vegetation models cannot simulate all species, necessitating their aggregation into plant functional types (PFTs). Yet most PFTs encompass the full spectrum of migration rates.Migration processes span scales of time and space far beyond what can be confidently simulated in DGVMs. Theories about climate change and migration are limited by inadequate data for key processes at short and long time scales and at small and large spatial scales. These theories must be enhanced to incorporate species-level migration and succession processes into a more comprehensive definition of PFTs. [Full Text - PDF]
|P31||Buckley, Y. M., S. Anderson,
C. P. Cattarall, R. T. Corlett, T. Engel, C. R., Gosper, R. Nathan, D. M.
Richardson, M. Setter, O. Spiegel, G. Vivan-Smith, F. A. Voigt, J. E. S.
Weir, and D. A. Westcott. Management of plant invasions mediated by frugivore
interactions. Journal of Applied Ecology (In Press).
Abstract. 1. Some of the most damaging invasive plants are dispersed by frugivores and this is an area of emerging importance in weed management. It highlights the need for practical information on how frugivores affect weed population dynamics and spread, how frugivore populations are affected by weeds and what management recommendations are available. 2. Fruit traits influence frugivore choice. Fruit size, the presence of an inedible peel, defensive chemistry, crop size and phenology may all be useful traits for screening and eradication programmes to consider. By considering the effect of these traits on the probability, quality and quantity of seed dispersal it may be possible to rank invasive species by their desirability to frugivores. Fruit traits can also be manipulated with biocontrol agents. 3. Functional groups of frugivores can be assembled according to broad species groupings and further refined according to size, gape size, pre- and post-ingestion processing techniques and movement patterns to predict dispersal and establishment patterns for plant introductions. 4. Landscape fragmentation can increase frugivore dispersal of invasives as many invasive plants and dispersers readily use disturbed matrix environments and fragment edges. Dispersal to particular landscape features such as perches or edges can be manipulated to function as seed sinks if control measures are concentrated in these areas. 5. Where invasive plants comprise part of the diet of native frugivores there may be a conservation conflict between control of the invasive and maintaining populations of the native frugivore, especially where other threats such as habitat destruction have reduced populations of native fruit species. 6. Synthesis and applications. Development of functional groups of frugivore dispersed invasive plants and dispersers will enable us to develop predictions for novel dispersal interactions at both population and community scales. Increasingly sophisticated mechanistic seed dispersal models combined with spatially explicit simulations show much promise for providing weed managers with the information they need to develop strategies for surveying, eradicating and managing plant invasions. Possible conservation conflicts mean that understanding the nature of the invasive plant-frugivore interaction is essential for determining appropriate management.
Troupin, D., R. Nathan, and G. G. Vendramin. Analysis of spatial genetic structure in an expanding Pinus halepensis population reveals development of fine-scale genetic clustering over time. Molecular Ecology (In Press).
Abstract.We analyzed the change of spatial genetic structure (SGS) of reproductive individuals over time in an expanding Pinus halepensis population. To our knowledge, this is the first empirical study to analyze the temporal component of SGS by following the dynamics of successive cohorts of the same population over time, rather than analyzing different age cohorts at a single time. SGS is influenced by various factors including restricted gene dispersal, microenvironmental selection, mating patterns and the spatial pattern of reproductive individuals. Several factors that affect SGS are expected to vary over time and as adult density increases. Using air photo analysis, tree-ring dating and molecular marker analysis we reconstructed the spread of reproductive individuals over 30 years beginning from five initial individuals. In the early stages, genotypes were distributed randomly in space. Over time and with increasing density, fine-scale (<20m) SGS developed and the magnitude of genetic clustering increased. The SGS was strongly affected by the initial spatial distribution and genetic variation of the founding individuals. The development of SGS may be explained by fine-scale environmental heterogeneity and possibly microenvironmental selection. Inbreeding and variation in reproductive success may have enhanced SGS magnitude over time.
C., J. Burczyk, R. Nathan, N. Nanos, L. Gil, and R. Alia.
2006. Effective gene dispersal and female reproductive success in Mediterranean
maritime pine (Pinus pinaster Aiton). Molecular Ecology 15:4577-4588.
Abstract. Understanding population-scale processes that affect allele frequency changes across generations is a long-standing interest in genetic, ecological and evolutionary research. In particular, individual differences in female reproductive success and the spatial scale of gene flow considerably affect evolutionary change and patterns of local selection. In this study, a recently developed maximum-likelihood (ML) method based on established offspring, the Seedling Neighbourhood Model, was applied and exponentially shaped dispersal kernels were fitted to both genetic and ecological data in a widespread Mediterranean pine, Pinus pinaster Aiton. The distribution of female reproductive success in P. pinaster was very skewed (about 10% of trees mothered 50% of offspring) and significant positive female selection gradients for diameter (? = 0.7293) and cone crop (? = 0.4524) were found. The selective advantage of offspring mothered by bigger trees could be due to better-quality seeds. These seeds may show more resilience to severe summer droughts and microsite variation related to water and nutrient availability. Both approaches, ecological and of parentage, consistently showed a long-distance dispersal component in saplings that was not found in dispersal kernels based on seed shadows, highlighting the importance of Janzen-Connell effects and microenvironmental variation for survival at early stages of establishment in this Mediterranean key forest tree.[Full Text - PDF]
|P35||Van der Veken, S., J.
Rogister, K. Verheyen, M. Hermy, and R. Nathan. 2007. Over
the (range) edge: a 45-year transplant experiment with the perennial forest
herb Hyacinthoides non-scripta. Journal of Ecology 95:343-351
Abstract. 1. Most experimental populations of perennial
forest herbs have been monitored for only a few years and thus only apply
to germination and early seedling survival. Moreover, there is a lack
of long-term, quantitative data on the rates of spread of populations.
|P36||Spiegel, O. and R. Nathan.
2007. Incorporating dispersal distance into the disperser effectiveness
framework: frugivorous birds provide complementary dispersal to plants in
a patchy environment. Ecology Letters 10: 718–728.
Abstract. Fleshy-fruited plants are usually dispersed by an array of frugivores, differing in the effectiveness of the dispersal service they provide to the plant. Body size differences among frugivores are hypothesized to affect seed dispersal distances and consequently their effectiveness as dispersers. We tested this hypothesis by comparing the effectiveness of two passerine birds, grackles (Onychognathus tristramii) and bulbuls (Pycnonotus xanthopygos), dispersing the desert shrub Ochradenus baccatus. Laboratory experiments, quantifying gut retention time and the effect on germination, were combined with field observations quantifying bird movements and fruit consumption rates. An empirically parameterized mechanistic model showed that the two dispersers switch roles as a function of spatial-scale: while most seeds within the local habitat were dispersed by bulbuls, the larger grackles were exclusively responsible for between-patches, long-distance dispersal. We suggest that distance-related differences are common and important to plant fitness, and thus should explicitly be considered in studies of disperser effectiveness.[Full Text - PDF]
Avgar, T., I. Giladi, and R. Nathan. 2007. Linking traits of foraging animals to spatial patterns of plants: social and solitary ants generate opposing patterns of surviving seeds. Ecology Letters 11:224-234.
Foraging traits of seed predators are expected to impact the spatial structure of plant populations, community dynamics and diversity. Yet, many of the key mechanisms governing distance- or density dependent seed predation are poorly understood. We designed an extensive set of field experiments to test how seed predation by two harvester ant species interact with seed dispersal in shaping the spatial patterns of surviving seeds. We show that the Janzen–Connell establishment pattern can be generated by central-place foragers even if their focal point is located away from the seed source. Furthermore, we found that differences in the social behaviour of seed predators influence their sensitivity to seed density gradients and yield opposing spatial patterns of surviving seeds. Our results support the predictions of a recent theoretical framework that unifies apparently opposing plant establishment patterns, and suggest that differences in foraging traits among seed predators can drive divergent pathways of plant community dynamics.
Bohrer, G., G. G. Katul, R. Nathan, R. L. Walko, and R. Avissar. 2008. Effects of canopy heterogeneity, seed abscission, and inertia on wind-driven dispersal kernels of tree seeds. Journal of Ecology 96: 569-580.
1. Understanding seed dispersal by wind and, in particular, long-distance dispersal (LDD) is needed for management of plant populations and communities, especially in response to changes in climate, land use and natural habitats. Numerical models designed to explore complex, nonlinear atmospheric processes are essential tools for understanding the fundamental mechanisms involved in seed dispersal. Yet, thus far, nearly all such models have not explicitly accounted for the spatial heterogeneity that is a typical feature of all ecosystems. 2. The recently developed Regional Atmospheric Modelling System (RAMS)-based Forest Large Eddy Simulation (RAFLES) is used here to explore how within-stand canopy heterogeneity impacts LDD. RAFLES resolves microscale canopy heterogeneity such as small gaps and variable tree heights, and it simulates their impacts on turbulence inside and above the canopy in the atmospheric boundary layer (ABL). For that purpose, an Eulerian–Lagrangian module of seed dispersal is added to RAFLES to simulate seed trajectories. 3. Particular attention is paid to the sensitivity of statistical attributes of the dispersal kernels (i.e. mean, mode, variance, tail) to key simplifications common to all seed dispersal models, such as horizontal homogeneity in the canopy and flow field, and the tight coupling between air parcel trajectories and seed trajectories (i.e. neglecting seed inertia). These attributes appear to be sensitive to various factors operating at scales ranging from the seed scale to the ABL scale. 4. Simulations with RAFLES show that LDD is characterized by a dispersal kernel with a ‘tail’, asymptotically approaching a power law decay of –3/2 (mainly occurring for lighter seeds at high wind speeds). This is consistent with asymptotic predictions from analytical models. The wind speed threshold at which seed abscission occurs, set-up to be twice the standard deviation of the vertical wind speed, is shown to affect short-distance dispersal, but has no significant impact on LDD. Ignoring the effects of seed inertia on the seed trajectory calculations has a minor effect on short-distance dispersal and no effect on the probability of seed uplift. Thus, it has no significant impact on LDD. 5. Synthesis. Tree-scale canopy heterogeneity affects the turbulence characteristics inside and above the canopy and, consequently, this affects dispersal kernel statistics. A key finding from this study is that ejection is enhanced above the shorter trees of the canopy. Seeds dispersed above shorter trees have a higher probability of experiencing LDD while their short-distance dispersal remains practically the same. At inter-annual time scales, such interactions could affect species composition.
1. Plant fecundity and seed dispersal often depend on environmental variables
that vary in space. Hence, plant ecologists need to quantify spatial environmental
effects on fecundity and dispersal. 2. We present an approach to estimate
and model two types of spatial environmental effects: source effects cause
fecundity and dispersal to vary as a function of a source’s local
environment, whereas path effects depend on all environments a seed encounters
during dispersal. Path effects are described by first transforming physical
space so that areas of low seed permeability are enlarged relative to
others, and then evaluating dispersal kernels in this transformed ‘movement
space’. 3. Models for source and path effects are embedded into
the established inverse modelling (IM) framework. This enables the statistical
estimation of environmental effects from easily available data on the
spatial distribution of seeds, seed sources and environmental covariates.
4. The presented method is applied to data from a well-studied population
of the wind-dispersed Aleppo pine ( Pinus halepensis ). We use local tree
density as an environmental covariate, model fecundity as a function of
a tree’s basal area, and consider four dispersal kernels: WALD (a
closed-form mechanistic model for seed dispersal by wind), log-normal,
exponential power and 2Dt. 5. The inclusion of source and path effects
of tree density markedly improves IM performance. IM analyses and independent
data agree in the parameter range of the mechanistic WALD kernel and in
suggesting weak negative density-dependence of fecundity. Of 64 IMs considered,
the best four involve the WALD kernel and negative source effects on its
shape parameter. The best IM predicts that increasing tree density at
the source shortens median dispersal distance while enhancing long-distance
dispersal (LDD). Additionally, path effects lead to lower seed permeability
of high density areas. These results shed light on the mechanisms by which
environmental variation affects fecundity and dispersal of P. halepensis
. Moreover, the predicted density-dependent dispersal causes a pronounced
lag-phase in simulations of population spread. 6. Synthesis . The presented
method can quantify environmental effects on fecundity and dispersal in
a wide range of study systems. The movement space concept may furthermore
promote a unified
Nathan, R., W. M. Getz, E. Revilla, M. Holyoak, R. Kadmon, D. Saltz, and P. E. Smouse. 2008. A movement ecology paradigm for unifying organismal movement research. Proceedings of the National Academy of Sciences USA 105: 19052-19059.
Movement of individual organisms is fundamental to life, quilting our planet in a rich tapestry of phenomena with diverse implications for ecosystems and humans. Movement research is both plentiful and insightful, and recent methodological advances facilitate obtaining a detailed view of individual movement. Yet, we lack a general unifying paradigm, derived from first principles, which can place movement studies within a common context and advance the development of a mature scientific discipline. This introductory article to the Movement Ecology Special Feature proposes a paradigm that integrates conceptual, theoretical, methodological, and empirical frameworks for studying movement of all organisms, from microbes to trees to elephants. We introduce a conceptual framework depicting the interplay among four basic mechanistic components of organismal movement: the internal state (why move?), motion (how to move?), and navigation (when and where to move?) capacities of the individual and the external factors affecting movement. We demonstrate how the proposed framework aids the study of various taxa and movement types; promotes the formulation of hypotheses about movement; and complements existing biomechanical, cognitive, random, and optimality paradigms of movement. The proposed framework integrates eclectic research on movement into a structured paradigm and aims at providing a basis for hypothesis generation and a vehicle facilitating the understanding of the causes, mechanisms, and spatiotemporal patterns of movement and their role in various ecological and evolutionary processes.
Holyoak, M., R. Casagrandi, R. Nathan, E. Revilla, and O. Spiegel. 2008. Trends in the study of movement ecology: What are we missing? Proceedings of the National Academy of Sciences USA 105: 19060-19065.
Movement is important to all organisms, and accordingly it is addressed in a huge literature. Of nearly 26,000 papers referring to movement, an estimated 34% focused on movement by measuring it or testing hypotheses about it. This enormous amount of information is difficult to review, and highlights the need to assess the collective completeness of movement studies and identify gaps. We surveyed 1000 randomly selected papers from 496 journals, and compared the facets of movement studied with a suggested framework for movement ecology, consisting of internal state (motivation, physiology), motion and navigation capacities, and external factors (both the physical environment and living organisms), as well as links among these components. Most studies simply measured and described the movement of organisms without reference to ecological or internal factors, and the most frequently studied part of the framework was the link between external factors and motion capacity. Few studies looked at the effects on movement of navigation, or internal state, and these were mainly from vertebrates. For invertebrates and plants most studies were at the population level, whereas more vertebrate studies were conducted at the individual level. Consideration of only population-level averages promulgates neglect of between-individual variation in movement, potentially hindering the study of factors controlling movement. Terminology was found to be inconsistent among taxa and subdisciplines. The gaps identified in coverage of movement studies highlight research areas that should be addressed to fully understand the ecology of movement. Comparison with the suggested framework will facilitate identification of underlying mechanisms behind observed patterns.
Wright, S. J., A. Trakhtenbrot, G. Bohrer, M. Detto, G. G. Katul, N. Horvitz, H. C. Muller-Landau, F. A. Jones, and R. Nathan. 2008. Understanding strategies for seed dispersal by wind under contrasting atmospheric conditions. Proceedings of the National Academy of Sciences USA 105: 19084-19089.
Traits associated with seed dispersal vary tremendously among sympatric wind-dispersed plants. We used two contrasting tropical tree species, seed traps, micrometeorology, and a mechanistic model to evaluate how variation in four key traits affects seed dispersal by wind. The conceptual framework of movement ecology, wherein external factors (wind) interact with internal factors (plant traits) that enable movement and determine when and where movement occurs, fully captures the variable inputs and outputs of wind dispersal models and informs their interpretation. We used model calculations to evaluate the spatial pattern of dispersed seeds for the 16 factorial combinations of four traits. The study species differed dramatically in traits related to the timing of seed release, and a strong species by season interaction affected most aspects of the spatial pattern of dispersed seeds. A rich interplay among plant traits and seasonal differences in atmospheric conditions caused this interaction. Several of the same plant traits are crucial for both seed dispersal and other aspects of life history variation. Observed traits that limited dispersal are likely to be constrained by their life history consequences.
Growing recognition of the importance of long-distance dispersal (LDD) of plant seeds for various ecological and evolutionary processes has led to an upsurge of research into the mechanisms underlying LDD. We summarize these findings by formulating six generalizations stating that LDD is generally more common in open terrestrial landscapes, and is typically driven by large and migratory animals, extreme meteorological phenomena, ocean currents and human transportation, each transporting a variety of seed morphologies. LDD is often associated with unusual behavior of the standard vector inferred from plant dispersal morphology, or mediated by nonstandard vectors. To advance our understanding of LDD, we advocate a vector-based research approach that identifies the significant LDD vectors and quantifies how environmental conditions modify their actions.
Plant recruitment in nature exhibits several distinctive patterns ranging from hump shaped to monotonically decreasing with distance from the seed source. We investigate the role of postdispersal seed predation in shaping these patterns, introducing a new mechanistic model that explicitly accounts for the movement strategy used by seed eaters. The model consists of two partial differential equations describing the spatiotemporal dynamics of both seed and predator densities. The movement strategy is defined by how predators move in response to the different cues they can use to search for seeds. These cues may be seed density, seed intake, distance from the plant, density of conspecific foragers, or a mixture of these four. The model is able to reproduce all the basic plant recruitment patterns found in the field. We compare the results to those of the ideal free distribution (IFD) theory and show that hump-shaped plant recruitment patterns cannot be generated by IFD predators and, in general, by foragers that respond exclusively to seed density. These foragers can produce only nonincreasing patterns, the shapes of which are determined by the foragers’ navigation capacities. In contrast, hump-shaped patterns can be produced by distance-responsive predators or by foragers that use conspecifics as a cue for seed abundance.