Inverse link between density and dispersal distance in butterflies: field evidence from six co-occurring species

A pertinent question in animal population ecology is the relationship between population abundance, density, and mobility. Two extreme ways to reach sufficient abundance for long-term persistence are to inhabit restricted locations at high densities, or large areas in low densities. The former case predicts low individual mobility, whereas the later predicts high one. This assumption is rarely tested using across-species comparisons, due to scarcity of data on both mobility and population sizes for multiple species. We used data on dispersal and local population densities of six butterfly species gained by mark-recapture, and data on their (relative) regional abundance obtained by walking transects in a landscape surrounding the mark-recapture sites. We correlated both local density and regional abundance against slopes of the inverse power function, appropriate for describing the shape of dispersal kernel. Local densities correlated negatively with the dispersal kernel slopes both when sexes were treated as independent data points and if treated together. For regional abundance, the correlation was also negative but only marginally significant. Our results corroborate the notion that a trade-off exists between living in dense populations and having poor dispersal, and vice versa. We link this observation to resource use by individual species, and distribution of such resources as host plants in the study landscape.     

Integrodifference models for persistence in fragmented habitats

Integrodifference models of growth and dispersal are analyzed on finite domains to investigate the effects of emigration, local growth dynamics and habitat heterogeneity on population persistence. We derive the bifurcation structure for a range of population dynamics and present an approximation that allows straighforward calculation of the equilibrium populations in terms of local growth dynamics and dispersal success rates. We show how population persistence in a heterogeneous environment depends on the scale of the heterogeneity relative to the organism's characteristic dispersal distance. When organisms tend to disperse only a short distance, population persistence is dominated by local conditions in high quality patches, but when dispersal distance is relatively large, poor quality habitat exerts a greater influence.     

Which traits promote persistence of feral GM crops? Part 2: implications of metapopulation structure

Transgenes may spread from crops into the environment via the establishment of feral populations, often initiated by seed spill from transport lorries or farm machinery. Locally, such populations are often subject to large environmental variability and usually do not persist longer than a few years. Because secondary feral populations may arise from seed dispersal to adjacent sites, the dynamics of such populations should be studied in a metapopulation context. We study a structured metapopulation model with local dispersal, mimicking a string of roadside subpopulations of a feral crop. Population growth is assumed to be subject to local disturbances, introducing spatially random environmental stochasticity. Our aim is to understand the role of dispersal and environmental variability in the dynamics of such ephemeral populations. We determine the effect of dispersal on the extinction boundary and on the distribution of persistence times, and investigate the influence of spatially correlated disturbances as opposed to spatially random disturbances. We find that, given spatially random disturbances, dispersal slows down the decline of the metapopulation and results in the occurrence of long-lasting local populations which remain more or less static in space. We identify which life history traits, if changed by genetic modification, have the largest impact on the population growth rate and persistence times. For oilseed rape, these are seed bank survival and dormancy. Combining our findings with literature data on transgene-induced life history changes, we predict that persistence is promoted by transgenes for oil-modifications (high stearate or high laurate) and, possibly, for insect resistence (Bt). Transgenic tolerance to glufosinate herbicide is predicted to reduce persistence.     

The role of density-dependent dispersal in source-sink dynamics

I investigate two aspects of source-sink theory that have hitherto received little attention: density-dependent dispersal and the cost of dispersal to sources. The cost arises because emigration reduces the per capita growth rate of sources, thus predisposing them to extinction. I show that source-sink persistence depends critically on the interplay between these two factors. When the emigration rate increases with abundance at an accelerating rate, dispersal costs to sources is the lowest and risk of source-sink extinction the least. When the emigration rate increases with abundance at a decelerating rate, dispersal costs to sources is the highest and the risk of source-sink extinction the greatest. Density-independent emigration has an intermediate effect. Thus, density-dependent dispersal per se does not increase or decrease source-sink persistence relative to density-independent dispersal. The exact mode of dispersal is crucial. A key point to appreciate is that these effects of dispersal on source-sink extinction arise from the temporal density-dependence that dispersal induces in the per capita growth rates of source and sink populations. Temporal density-dependence due to dispersal is beneficial at low abundances because it rescues sinks from extinction, and detrimental at high abundances because it drives otherwise viable sources to extinction. These results are robust to the nature of population dynamics in the sink, whether exponential or logistic. They provide a means of assessing the relative costs and benefits of preserving sink habitats given three biological parameters.     

Population synchrony and stability in environmentally forced metacommunities

A general prediction from simple metapopulation models is that spatially synchronized forcing can spatially synchronize population dynamics and destabilize metapopulations. In contrast, spatially asynchronous forcing is predicted to decrease population synchrony and promote temporal stability and population persistence, especially in the presence of dispersal. Only recently have studies begun to experimentally address these predictions. Moreover, few studies have experimentally examined how such processes operate in the context of competition communities. Stabilizing processes may continue to operate when placed within a metacommunity context with multiple competing consumers but only at low to intermediate levels of dispersal. High dispersal rates can reverse these predictions and lead to destabilization. We tested this under controlled conditions using an experimental aquatic system composed of three competing species of zooplankton. Metacommunities experienced different levels of dispersal and environmental forcing in the form of spatially synchronous or asynchronous pH perturbations. We found support that dispersal can have contrasting effects on population stability depending on the degree to which population dynamics were synchronized in space. Dispersal under synchronous forcing or no forcing had either neutral of positive effects on spatial population synchrony of all three zooplankton species. In these treatments, dispersal reduced population stability at the local and metapopulation levels for two of three species. In contrast, asynchronously varying environments reduced population synchrony relative to unforced systems, regardless of dispersal level. In these treatments, dispersal enhanced temporal stability and persistence of populations not by reducing population synchrony but by enhancing population minima and spatial averaging of abundances. High dispersal rates under asynchronous forcing reduced the abundance of one species, consistent with increasing regional competition and general metacommunity theory. However, no effects on its stability or persistence were observed. Our work highlights the context‐dependent effects of dispersal on population dynamics in varying environments.     

Canonical functions for dispersal-induced synchrony

Two processes are universally recognized for inducing spatial synchrony in abundance: dispersal and correlated environmental stochasticity. In the present study we seek the expected relationship between synchrony and distance in populations that are synchronized by density-independent dispersal. In the absence of dispersal, synchrony among populations with simple dynamics has been shown to echo the correlation in the environment. We ask what functional form we may expect between synchrony and distance when dispersal is the synchronizing agent. We formulate a continuous-space, continuous-time model that explicitly represents the time evolution of the spatial covariance as a function of spatial distance. Solving this model gives us two simple canonical functions for dispersal-induced covariance in spatially extended populations. If dispersal is rare relative to birth and death, then covariances between nearby points will follow the dispersal distance distribution. At long distances, however, the covariance tails off according to exponential or Bessel functions (depending on whether the population moves in one or two dimensions). If dispersal is common, then the covariances will follow the mixture distribution that is approximately Gaussian around the origin and with an exponential or Bessel tail. The latter mixture results regardless of the original dispersal distance distribution. There are hence two canonical functions for dispersal-induced synchrony     

Predation across spatial scales in heterogeneous environments

A hierarchy of scales is introduced to the spatially heterogeneous Lotka-Volterra predator-prey diffusion model, and its effects on the model's spatial and temporal behavior are studied. When predators move on a large scale relative to prey, local coupling of the predator-prey interaction is replaced by global coupling. Prey with low dispersal ability become narrowly confined to the most productive habitats, strongly amplifying the underlying spatial pattern of the environment. As prey diffusion rate increases, the prey distribution spreads out and predator abundance declines. The model retains neutrally stable Lotka-Volterra temporal dynamics: different scales of predator and prey dispersal do not stabilize the interaction. The model predicts that, for prey populations that are limited by widely ranging predators, species with low dispersal ability should be restricted to discrete high density patches, and those with greater mobility should be more uniformly distributed at lower density.     

Metapopulation persistence in dynamic landscapes: the role of dispersal distance

Species associated with transient habitats need efficient dispersal strategies to ensure their regional survival. Using a spatially explicit metapopulation model, we studied the effect of the dispersal range on the persistence of a metapopulation as a function of the local population and landscape dynamics (including habitat patch destruction and subsequent regeneration). Our results show that the impact of the dispersal range depends on both the local population and patch growth. This is due to interactions between dispersal and the dynamics of patches and populations via the number of potential dispersers. In general, long-range dispersal had a positive effect on persistence in a dynamic landscape compared to short-range dispersal. Long-range dispersal increases the number of couplings between the patches and thus the colonisation of regenerated patches. However, long-range dispersal lost its advantage for long-term persistence when the number of potential dispersers was low due to small population growth rates and/or small patch growth rates. Its advantage also disappeared with complex local population dynamics and in a landscape with clumped patch distribution.     

Linking demography and host dispersal to Trichuris arvicolae distribution in a cyclic vole species

Spatial structure in the distribution of pathogen infection can influence both epidemiology and host-parasite coevolutionary processes. It may result from the spatial heterogeneity of intrinsic and extrinsic factors, or from the local population dynamics of hosts and parasites. In this study, we investigated the effects of landscape, host dispersal and demography (population abundance and phase of the fluctuation) on the distribution of a gastro-intestinal nematode Trichuris arvicolae in the fossorial water vole Arvicola terrestris sherman. This rodent exhibits outbreaks occurring regularly in Franche-Comté (France). Thirteen out-of-phase populations were studied in autumn 2003. They exhibited highly different T. arvicolae prevalences. The heterogeneity in prevalences was not explained by population structure, landscape or vole abundance, but by the phase of the vole population fluctuations. Populations at the end of the high density phase showed null prevalence whereas populations in increase or outbreak phases exhibited higher prevalences. Population genetic analyses based on microsatellites revealed significant differentiation between vole populations, and higher dispersal rates of young voles compared with old ones. These younger individuals were also infected more frequently than older voles. This suggested a role of host dispersal in the distribution of T. arvicolae. However, there was a strong discrepancy between the spatial patterns of prevalence and of host genetics or demographic phase. Genetic differentiation and differences in demographic phase exhibited significant spatial autocorrelations whereas prevalence did not. We concluded that the distribution of T. arvicolae is influenced by vole dispersal, although this effect might be overwhelmed by local adaptation processes or environmental conditions.     

物种多度格局研究进展

物种多度格局研究始于20世纪30年代,是种群生态学和群落生态学研究的起点。物种多度格局研究主要在两个水平上进行：1）初期研究主要集中于群落水平,希望在不同群落之间发现一个共同的整体格局来描述群落的组织结构。常用模型包括几何级数、对数级数、对数正态和断棍模型,不同模型代表了不同的生态学过程。2）目前转向重视物种水平,并以物种多度的区域分布规律及其生态学机制研究为主。物种分布区多度关系有正相关、无相关和负相关3种形式。局部多度高的物种一般趋于广布,而局部多度低的物种趋于受限分布。物种多度区域分布的生态位模型预测为单峰型,还经常会出现“热点地区”;而异质种群模型预测为双峰型。物种多度的区域分布主要由环境资源特性、物种生态位和扩散过程等因素决定。3）物种多度格局的时间变化与空间变异类似,代表了这些生态学过程的时间异质性。4）物种多度格局的尺度变化经常表现出自相似性,但该规律并非一直存在,因为生物多样性由不同尺度上的不同生态学过程决定。5）多度（稀有度）是物种保护的基本依据,而群落多度模型能够指示生态学和干扰过程变化对群落结构的影响。物种多度格局的模型手段仍需改进,机制研究尚不系统,应用研究亟待扩展,对于物种多度格局的深入理解将为揭示生物多样性分布机制和有效保护提供帮助。     

Orchid biogeography and factors associated with rarity in a biodiversity hotspot,the Southwest Australian Floristic Region

Aim The causes of orchid diversification and intrinsic rarity are poorly resolved. The Orchidaceae of the Southwest Australian Floristic Region use a diversity of pollination strategies and sites of mycorrhizal infection, and occupy a diversity of habitats. We combined a biogeographic analysis with analysis of factors associated with rarity to establish: (1) the landscape features correlated with taxon turnover and speciation, and (2) the possible role in taxon rarity of geographic region, pollination strategy, edaphic habitat and site of mycorrhizal infection. Location Southwest Australian Floristic Region. Methods The distributions of 407 orchid taxa (species and subspecies) were mapped at the quarter‐degree scale using 13,267 collections in the Western Australian Herbarium. This database was used to map taxon richness, for a biogeographic analysis and to quantify rarity of taxa. Using herbarium records, rarity was expressed as mean abundance, mean distribution and incidence of rarity based on abundance and distribution for each genus. We tested for differences in rarity of species between pollination strategies, edaphic habitats and sites of mycorrhizal infection. Results Taxon richness was highest in the High Rainfall Province. Biogeographic provincial boundaries for orchids were aligned with rainfall, while district boundaries tended to follow geological formations. When rarity was defined as either low abundance or small distribution, the greatest number of rare taxa occurred in areas of high taxon richness and naturally fragmented edaphic environments. For both abundance and distributional extent, sexual deception had a significantly higher incidence of rarity than food‐rewarding taxa. There was no significant difference in rarity with site of mycorrhizal infection. Main conclusions While large‐scale edaphic and climatic variation are correlated with orchid taxon turnover and speciation in a similar fashion to the flora in general, the processes responsible for patterns of diversity may differ. Fragmented edaphic environments appear to be associated with a higher incidence of rare species due to limited dispersal/colonization opportunities or radiations of taxa in allopatry. The high incidence of rarity in sexually deceptive taxa could be due to either low fruit set or the risk of specializing on a single pollinator species.     

Density-dependent dispersal and relative dispersal affect the stability of predator-prey metacommunities

Although density-dependent dispersal and relative dispersal (the difference in dispersal rates between species) have been documented in natural systems, their effects on the stability of metacommunities are poorly understood. Here we investigate the effects of intra- and interspecific density-dependent dispersal on the regional stability in a predator-prey metacommunity model. We show that, when the dynamics of the populations reach equilibrium, the stability of the metacommunity is not affected by density-dependent dispersal. However, the regional stability, measured as the regional variability or the persistence, can be modified by density-dependent dispersal when local populations fluctuate over time. Moreover these effects depend on the relative dispersal of the predator and the prey. Regional stability is modified through changes in spatial synchrony. Interspecific density-dependent dispersal always desynchronizses local dynamics, whereas intraspecific density-dependent dispersal may either synchronize or desynchronize it depending on dispersal rates. Moreover, intra- and interspecific density-dependent dispersal strengthen the top-down control of the prey by the predator at intermediate dispersal rates. As a consequence the regional stability of the metacommunity is increased at intermediate dispersal rates. Our results show that density-dependent dispersal and relative dispersal of species are keys to understanding the response of ecosystems to fragmentation.     

Competition and predation as possible causes of bacterial rarity

We assembled communities of bacteria and exposed them to different nutrient concentrations with or without predation by protists. Taxa that were rare in the field were less abundant at low nutrient concentrations than common taxa, independent of predation. However, some taxa that were rare in the field became highly abundant in the assembled communities, especially under ample nutrient availability. This high abundance points at a possible competitive advantage of some rare bacterial taxa under nutrient-rich conditions. In contrast, the abundance of most rare bacterial taxa decreased at low resource availability. Since low resource availability will be the prevailing situation in most soils, our data suggests that under those conditions poor competitiveness for limiting resources may contribute to bacterial rarity. Interestingly, taxa that were rare in the field and most successful under predator-free conditions in the lab also tended to be more reduced by predation than common taxa. This suggests that predation contributes to rarity of bacterial taxa in the field. We further discuss whether there may be a trade-off between competitiveness and predation resistance. The substantial variability among taxa in their responses to competition and predation suggests that other factors, for example abiotic conditions and dispersal ability, also influence the local abundance of soil bacteria.     

Spatial heterogeneity and rates of spread in experimental streams

Theoretical models predict that environmental heterogeneity can decrease or potentially increase rates of spread in biological populations depending on the relationship between the scale of dispersal and the scale of heterogeneity. These effects arise from the interaction between habitat quality and the processes of dispersal, colonization and growth. Flowing water environments provide a unique opportunity to test these predictions. If advection influences dispersal, flow can alter the relative scale of dispersal to environmental heterogeneity in the upstream versus downstream direction. We explored the influence of heterogeneity on the spatial spread of a species of diatom in experimental streams. Environmental heterogeneity was created by maintaining agar diffusing substrata at different nutrient levels. Diatoms were placed at the midpoint of each stream, and spatial spread rates were determined by monitoring algal abundance non‐destructively. Our results reveal that, relative to homogeneous streams, resource heterogeneity decreases spread rate in the upstream direction but increases spread rate in the downstream direction. Empirical estimates of growth rates and colonization times reveal that heterogeneity predominantly influenced colonization rates. Colonization rates estimate successful dispersal events, and thus relate to both colonization and dispersal. These results are one of the first empirical tests of general theories regarding the impact of heterogeneity on rates of spread and highlight the importance of understanding the impact of heterogeneity on colonization and dispersal in continuous habitats.     

Eryngium maritimum, biology of a plant at its northernmost localities

The prospects for persistence of Eryngium maritimum in its northern distribution area was evaluated by studying historical data, matrix modelling, dispersal, seed germination and molecular variation. Historical data revealed a fragmented distribution of small populations with decrease in both population numbers and size during the last 150 years. Fruits of E. maritimum were found to have a relative low floating ability, making the prospects of long distance water dispersal more limited than would be expected from the world distribution of the species. Germination was found to be low (< 25 %). Elasticity matrices showed that survival of reproductive plants were more important than reproduction. High juvenile mortality and low germination activity emphasised the importance of rapid growth and survival. The population insurance against natural catastrophes and environmental stochasticity is suggested to be in a "root bank" of established individuals that are able to tolerate disturbances due to clonal reproduction and regrowth from root fragments. Isozyme electrophoresis revealed low molecular variation on a large geographical scale. The chromosome number is 2n = 16. A high degree of fixed heterozygosity suggests that the species is tetraploid with basic chromosome, number x = 4. The persistence prospects of E. maritimum in its northernmost distribution area are considered low considering the small population sizes, the low dispersal ability and low germination.     

Does lack of available suitable habitat explain the patchy distributions of rare calcicole fern species?

In Canada, as in many countries, a relatively large number of fern species occur in specialized habitats and have low numbers of known populations containing few individuals. It has been suggested that the distribution of ferns is mostly determined by factors of climate and habitat due to relatively low limitations in ferns for dispersal and establishment. Here, we attempt to explain whether the local patchy distribution of three calcicole fern species rare in Canada ( Asplenium ruta-muraria , Pellaea atropurpurea and Woodsia obtusa ) is due to a lack of available habitat. Analyses based on micro-scale differences between sites occupied by the ferns and nearby, unoccupied sites did not reveal any significant differences, thereby indicating that the rarity of these species is not entirely driven by the rarity of their microhabitat at a local scale. Our results suggest that the widely accepted premise that ferns are not limited by dispersal or establishment should be reconsidered, as such limitations are the only likely explanation for empty available and suitable habitat.     

The impact of neutrality, niche differentiation and species input on diversity and abundance distributions

We present a spatially-explicit generalization of Hubbell's model of community dynamics in which the assumption of neutrality is relaxed by incorporating dispersal limitation and habitat preference. In simulations, diversity and species abundances were governed by the rate at which new species were introduced (usually called 'speciation') and nearly unaffected by dispersal limitation and habitat preference. Of course, in the absence of species input, diversity is maintained solely by niche differences. We conclude that the success of the neutral model in predicting the abundance distribution has nothing to do with neutrality, but rather with the species-introduction process: when new species enter a community regularly as singletons, the typical J-shaped abundance distribution, with a long tail of rare species, is always observed, whether species differ in habitat preferences or not. We suggest that many communities are indeed driven by the introduction process, accounting for high diversity and rarity, and that species differences may be largely irrelevant for either.     

How does intraspecific density regulation influence metapopulation synchrony and persistence?

Intraspecific density regulation influences the synchronization of local population dynamics through dispersal. Spatial synchrony in turn may jeopardize metapopulation persistence. Joining results from previous studies suggests that spatial synchrony is highest at moderate over-compensation and is low at compensating and at very strong over-compensating density regulation. We scrutinize this supposition of a unimodal relationship using a process-based metapopulation model with explicit local population dynamics. We extend the usually studied range of density regulation to under-compensation and analyse resulting metapopulation persistence. We find peaks of spatial synchrony not only at over-compensatory but also under-compensatory density regulation and show that effects of local density compensation on synchrony follow a bimodal rather than unimodal relationship. Persistence of metapopulations however, shows a unimodal relationship with a broad plateau of high persistence from compensatory to over-compensatory density regulation. This range of high persistence comprises both levels of low and high spatial synchrony. Thus, not synchrony alone jeopardizes metapopulation persistence, but only in interplay with high local extinction risk. The functional forms of the relations of density compensation with spatial synchrony and persistence are robust to increases in dispersal mortality, landscape dynamics, or density dependence of dispersal. However, with each of these increases the maxima of spatial synchrony and persistence shift to higher over-compensation and levels of synchrony are reduced. Overall, for over-compensation high landscape connectivity has negative effects while for under-compensation connectivity affects persistence positively. This emphasizes the importance of species life-history traits for management decisions with regard to landscape connectivity: while dispersal corridors are essential for species with under-compensatory density regulation, they may have detrimental effects for endangered species with over-compensation.     

Dispersal altering local states has a limited effect on persistence of a metapopulation

Metapopulations are collections of local populations connected by dispersal. Metapopulation models often assume would-be colonists affect the states of local populations they disperse from and those they disperse to. Here, we build a new framework to include that effect and to assess the impact of dispersal. Our model predicts that a metapopulation will, in general, be found either in the state of global extinction or in the state of persistence. Our key finding is that dispersal, and the state changes associated with dispersal, have significant qualitative and quantitative effects on long-term dynamics only in a narrow range of parameter space. We conclude that life history features other than dispersal (e.g. mortality rate) have a greater influence over metapopulation persistence. We discuss the implications of our results for conservation biology, the future application of our model to the study of cooperative breeding, as well as our model's limitations.     

Temporal autocorrelation can enhance the persistence and abundance of metapopulations comprised of coupled sinks

In spatially heterogeneous landscapes, some habitats may be persistent sources, providing immigrants to sustain populations in unfavorable sink habitats (where extinction is inevitable without immigration). Recent theoretical and empirical studies of source-sink systems demonstrate that temporally variable local growth rates in sinks can substantially increase average abundance of a persisting population, provided that the variation is positively autocorrelated--in effect, temporal variation inflates average abundance. Here we extend these results to a metapopulation in which all habitat patches are sinks. Using numerical studies of a population with discrete generations (buttressed by analytic results), we show that temporal variation and moderate dispersal can jointly permit indefinite persistence of the metapopulation and that positive autocorrelation both lowers the magnitude of variation required for persistence and increases the average abundance of persisting metapopulations. These effects are weakened--but not destroyed--if variation in local growth rates is spatially synchronized and dispersal is localized. We show that the inflationary effect is robust to a number of extensions of the basic model, including demographic stochasticity and density dependence. Because ecological and environmental processes contributing to temporally variable growth rates in natural populations are typically autocorrelated, these observations may have important implications for species persistence.