Distribution dynamics of a great bustard metapopulation throughout a decade: influence of conspecific attraction and recruitment

Dispersing individuals can use conspecifics as indicators of habitat quality and aggregate at traditionally occupied sites, leaving other favourable patches unoccupied. Here we test the predictions of the conspecific-based habitat selection hypothesis on a Spanish great bustard (Otis tarda) metapopulation, currently fragmented due to recent human-induced habitat changes. The number of birds had increased by 23% between 1988 and 1998, but not consistently among leks. Leks that were large in 1988 increased, while those that were small decreased, which suggests that dispersing individuals used the numbers of conspecifics as cues for breeding-site selection. Moreover, leks with high productivity increased, while those with low productivity decreased. Finally, lek distribution was markedly stable throughout the decade, with no establishment of new leks, and suitable habitat patches remained unoccupied, as predicted by the conspecific attraction hypothesis. These results were corroborated by a simulation model which incorporated natal dispersal rates between leks as obtained through radio-tracking of 15 birds that survived throughout their 4-year dispersal period. In conclusion, in spite of the apparent increase in total numbers throughout the decade, both conspecific attraction and local differences in reproductive success contributed to a more aggregated distribution, increasing the species' vulnerability to local catastrophes, and the risks of reduced genetic diversity and extinction of small leks.     

Frequency-dependent conspecific attraction to food patches

In many ecological situations, resources are difficult to find but become more apparent to nearby searchers after one of their numbers discovers and begins to exploit them. If the discoverer cannot monopolize the resources, then others may benefit from joining the discoverer and sharing their discovery. Existing theories for this type of conspecific attraction have often used very simple rules for how the decision to join a discovered resource patch should be influenced by the number of individuals already exploiting that patch. We use a mechanistic, spatially explicit model to demonstrate that individuals should not necessarily simply join patches more often as the number of individuals exploiting the patch increases, because those patches are likely to be exhausted soon or joining them will intensify future local competition. Furthermore, we show that this decision should be sensitive to the nature of the resource patches, with individuals being more responsive to discoveries in general and more tolerant of larger numbers of existing exploiters on a patch when patches are resource-rich and challenging to locate alone. As such, we argue that this greater focus on underlying joining mechanisms suggests that conspecific attraction is a more sophisticated and flexible tactic than currently appreciated.     

Alternative models of conspecific attraction in flies and crabs

Animals are often attracted to one another when selecting habitats,but little is known about the rules governing conspecific attraction.We use Akaike Information Criterion to evaluate alternativemodels of the effects of conspecifics on individual choice inthe context of habitat selection. One set of models was testedusing data collected on virgin female flies, Drosophila melanogaster,selecting habitats in the laboratory; a second set of modelswas tested using data collected on crabs, Ocypode rotundata,selecting foraging patches in the field. Patterns of space usein the flies were most consistent with models indicating thatindividuals were attracted to other females that selected trapsduring the same hour, as well as to cues left by females thathad entered traps during the previous hour. Results for thecrabs were most consistent with a model which assumes that individualspreferred to join the patch with the most crabs but that theirability to assess the number of crabs in alternative patcheswas constrained by basic psychophysical principles (Weber'slaw). These results provide support for hypotheses about thefunctional significance of conspecific attraction in the fliesand the crabs and illustrate the richness of information aboutconspecific attraction that can be obtained when the same dataset is confronted with alternative models of the ways that animalsrespond to one another when selecting habitats.     

Assessing human impact despite uncertainty:viability of the northern spotted owl metapopulation in the northwestern USA

We demonstrate the effect of uncertainty (resulting from lack of information or measurement error) on the assessment of human impact, with an analysis of the viability of the northern spotted owl throughout its range in the United States. We developed a spatially-explicit, stage-structured, stochastic metapopulation model of the northern spotted owl throughout its range in the United States. We evaluated the viability of the metapopulation using measures such as risk of decline and time to extinction. We incorporated uncertainty in the form of parameter ranges, and used them to estimate upper and lower bounds on the estimated viability of the species. We analysed the effect of this type of uncertainty on the assessment of human impact by comparing the species' viability under current conditions and under an assumed loss of spotted owl habitat in the next 100 years. The ranges of parameters were quite large and resulted in a wide range of risks of extinction. Despite this uncertainty, the results were sensitive to parameters related to habitat loss: under all assumptions and combinations of parameters, the model predicted that habitat loss results in substantially higher risks of metapopulation decline. This result demonstrated that even with relatively large uncertainties, risk-based model results can be used to assess human impact reliably.     

Structured models of metapopulation dynamics

I develop models of metapopulation dynamics that describe changes in the numbers of individuals within patches. These models are analogous to structured population models, with patches playing the role of individuals. Single species models which do not include the effect of immigration on local population dynamics of occupied patches typically lead to a unique equilibrium. The models can be used to study the distributions of numbers of individuals among patches, showing that both metapopulations with local outbreaks and metapopulations without outbreaks can occur in systems with no underlying environmental variability. Distributions of local population sizes (in occupied patches) can vary independently of the total population size, so both patterns of distributions of local population sizes are compatible with either rare or common species. Models which include the effect of immigration on local population dynamics can lead to two positive equilibria, one stable and one unstable, the latter representing a threshold between regional extinction and persistence.     

The influence of habitat characteristics and conspecifics on attraction and survival of coral reef fish juveniles

In marine species with a pelagic larval stage, search behavior and selection of a suitable reef habitat can maximize the settlement success of recently settled juveniles and their subsequent performance (growth and survival of juveniles). Our objective was to test this hypothesis for a single target coral reef fish species (Chromis viridis) at Moorea Island. C. viridis settle on living coral colonies of Porites rus already populated with conspecifics. In the present study (conducted in experimental cages), we found that: 1) mortality rate of recently settled juveniles of C. viridis was lower in the settlement habitat (living coral colonies of P. rus) than in other habitats having physical structure different from those of P. rus colonies; 2) C. viridis juveniles preferentially colonized coral heads of P. rus with conspecifics present rather than uninhabited coral heads and they also preferentially colonized uninhabited coral heads rather than coral heads with heterospecifics; 3) mortality rate of C. viridis juveniles did not vary with the presence or absence of conspecifics or heterospecifics on P. rus colonies. Overall, the study allows us to highlight that site selection by juveniles for habitat containing conspecifics does not benefit their short term mortality rates, suggesting that in the short term at least, site selection has little importance.     

Population dynamics of epiphytic orchids in a metapopulation context

Background and Aims

Populations of many epiphytes show a patchy distribution where clusters of plants growing on individual trees are spatially separated and may thus function as metapopulations. Seed dispersal is necessary to (re)colonize unoccupied habitats, and to transfer seeds from high- to low-competition patches. Increasing dispersal distances, however, reduces local fecundity and the probability that seeds will find a safe site outside the original patch. Thus, there is a conflict between seed survival and colonization.

Methods

Populations of three epiphytic orchids were monitored over three years in a Mexican humid montane forest and analysed with spatially averaged and with spatially explicit matrix metapopulation models. In the latter, population dynamics at the scale of the subpopulations (epiphytes on individual host trees) are based on detailed stage-structured observations of transition probabilities and trees are connected by a dispersal function.

Key Results

Population growth rates differed among trees and years. While ignoring these differences, and averaging the population matrices over trees, yields negative population growth, metapopulation models predict stable or growing populations because the trees that support growing subpopulations determine the growth of the metapopulation. Stochastic models which account for the differences among years differed only marginally from deterministic models. Population growth rates were significantly lower, and extinctions of local patches more frequent in models where higher dispersal results in reduced local fecundity compared with hypothetical models where this is not the case. The difference between the two models increased with increasing mean dispersal distance. Though recolonization events increased with dispersal distance, this could not compensate the losses due to reduced local fecundity.

Conclusions

For epiphytes, metapopulation models are useful to capture processes beyond the level of the single host tree, but local processes are equally important to understand epiphyte population dynamics.     

How interactions between animal movement and landscape processes modify local range dynamics and extinction risk

Forecasts of range dynamics now incorporate many of the mechanisms and interactions that drive species distributions. However, connectivity continues to be simulated using overly simple distance-based dispersal models with little consideration of how the individual behaviour of dispersing organisms interacts with landscape structure (functional connectivity). Here, we link an individual-based model to a niche-population model to test the implications of this omission. We apply this novel approach to a turtle species inhabiting wetlands which are patchily distributed across a tropical savannah, and whose persistence is threatened by two important synergistic drivers of global change: predation by invasive species and overexploitation. We show that projections of local range dynamics in this study system change substantially when functional connectivity is modelled explicitly. Accounting for functional connectivity in model simulations causes the estimate of extinction risk to increase, and predictions of range contraction to slow. We conclude that models of range dynamics that simulate functional connectivity can reduce an important source of bias in predictions of shifts in species distributions and abundances, especially for organisms whose dispersal behaviours are strongly affected by landscape structure.     

Linking extinction-colonization dynamics to genetic structure in a salamander metapopulation

Theory predicts that founder effects have a primary role in determining metapopulation genetic structure. However, ecological factors that affect extinction-colonization dynamics may also create spatial variation in the strength of genetic drift and migration. We tested the hypothesis that ecological factors underlying extinction-colonization dynamics influenced the genetic structure of a tiger salamander (Ambystoma tigrinum) metapopulation. We used empirical data on metapopulation dynamics to make a priori predictions about the effects of population age and ecological factors on genetic diversity and divergence among 41 populations. Metapopulation dynamics of A. tigrinum depended on wetland area, connectivity and presence of predatory fish. We found that newly colonized populations were more genetically differentiated than established populations, suggesting that founder effects influenced genetic structure. However, ecological drivers of metapopulation dynamics were more important than age in predicting genetic structure. Consistent with demographic predictions from metapopulation theory, genetic diversity and divergence depended on wetland area and connectivity. Divergence was greatest in small, isolated wetlands where genetic diversity was low. Our results show that ecological factors underlying metapopulation dynamics can be key determinants of spatial genetic structure, and that habitat area and isolation may mediate the contributions of drift and migration to divergence and evolution in local populations.     

Metapopulations of moths on islands: a test of two contrasting models

1. We describe a generalized mainland-island metapopulation model which includes migration among the island populations. We test model predictions with quantitative data on more than 200 species of moths in two contrasting networks of small islands. The data include a direct measure of migration rate, based on trapping of moths on rocky skerries with no local populations of the vast majority of species.
2. We predicted that moths which are strong fliers but uncommon on the islands have a higher incidence on scattered islands than on islands in a group, because the latter 'compete' for immigrants from the mainland. In contrast, we predicted that weakly flying species with potentially large local populations on the islands occur more frequently on islands in a group due to enhanced colonization rate.
3. Both predicted patterns were observed. Island occupancy increased significantly with the number of individuals caught on the rocky skerries, which is our measure of migration rate from the mainland, supporting the basic assumption that the species occur on the islands in a balance between colonizations and extinctions.
4. These results demonstrate that the moth metapopulations on islands represent a mixture of Levins's and mainland-island metapopulations, and that the mixture is different for different species in the same landscape.     

The evolution of conspecific gamete precedence and its effect on reinforcement

Conspecific gamete precedence, the usage of conspecific sperm by a female that mates with both a conspecific and a heterospecific male, has been found in many taxa. We construct a population genetic model to examine the evolution of conspecific gamete precedence and its coevolution with premating isolation in the process of reinforcement. Our findings suggest that conspecific gamete precedence can evolve via a process very similar to reinforcement. We explore the nature of the selection against hybridization necessary to drive this evolution. Moreover, our results confirm the prediction of Marshall et al. (Trends Ecol. Evol. 2002;17:558-563) that conspecific gamete precedence will inhibit the evolution of reinforcement between two species. We further find that reinforcement will inhibit the evolution of conspecific gamete precedence. Both reinforcement and conspecific gamete precedence increase reproductive isolation and contribute to the process of speciation. We discuss factors that may affect which of these phenomena are likely to become predominant between incipient species.     

The use of conspecific reproductive success for breeding patch selection in terrestrial migratory species

Classical models of breeding habitat selection rarely deal with the question of information gathering for patch quality assessment. In this paper, we present two models comparing the fitness outcomes of behavioural strategies based on conspecific reproductive success as a cue to assess local environmental quality before selecting a new breeding habitat. The models deal with two phases of the life-cycle of a territorial migratory species: recruitment to a breeding population (model 1) and breeding site fidelity of subsequent breeding attempts (model 2). The first model shows that prospecting breeding patches before recruiting is the best strategy if the environment is predictable and contains a low proportion of good patches, even if it implies losing a breeding opportunity. The second model shows that dispersing after a breeding attempt according to the patch's breeding success rather than the individual's own success is the bests own success is the best strategy if the environment is patchy. These results underline the importance of studying the spatio-temporal variations of factors affecting reproductive success when considering the importance of habitat selection strategies based on conspecifics. Moreover, they allow the understanding of individual behaviour patterns observed in natural populations and their potential consequences at the metapopulation level.     

A stochastic metapopulation model accounting for habitat dynamics

A stochastic metapopulation model accounting for habitat dynamics is presented. This is the stochastic SIS logistic model with the novel aspect that it incorporates varying carrying capacity. We present results of Kurtz and Barbour, that provide deterministic and diffusion approximations for a wide class of stochastic models, in a form that most easily allows their direct application to population models. These results are used to show that a suitably scaled version of the metapopulation model converges, uniformly in probability over finite time intervals, to a deterministic model previously studied in the ecological literature. Additionally, they allow us to establish a bivariate normal approximation to the quasi-stationary distribution of the process. This allows us to consider the effects of habitat dynamics on metapopulation modelling through a comparison with the stochastic SIS logistic model and provides an effective means for modelling metapopulations inhabiting dynamic landscapes.     

Spatially-correlated extinction in a metapopulation model of Leadbeater's Possum

The importance of considering spatially-correlated extinction in metapopulation viability analyses was investigated using a model of the population dynamics of Gymnobelideus leadbeateri McCoy (Leadbeater's Possum). Fire caused local extinction of G. leadbeateri and induced changes in the suitability of the habitat over a period of decades and centuries. Spatially-correlated fires, in which the correlation between the incidence of fire declines with distance, and uniformly-correlated fires were simulated. The predicted risk of metapopulation extinction increased: (i) as the variance in the number of fires each year increased, (ii) as the mean fire interval decreased, and (iii) as the mean dispersal distance decreased. Incorporating spatial correlation in the incidence of fires between patches had little effect on the results, provided the variance in the number of fires per year remained the same and fires modified habitat quality. The predicted risk of metapopulation extinction was greater for spatially-correlated fires than for uniformly-correlated fires when fires only caused local extinction but did not change habitat suitability. Incorporating spatial correlation in the incidence of fire within patches, which allowed partial burning of patches, reduced the predicted risk of extinction. This effect was only slight when patches were smaller than about 50 ha. The results of our simulations demonstrate the importance of considering correlations in disturbance regimes in metapopulation models, especially if these models are used to assist the design of nature reserves.     

Density dependence in resource exploitation: empirical test of Levins' metapopulation model

Levins' model of metapopulation dynamics is modified to incorporate variable degrees of density dependence in the per capita exploitation of resource patches. We demonstrate a simple means of testing for this density dependence in a sample of metapopulations, each at its equilibrium balance of local colonization to extinction. The fraction of habitable unoccupied patches equilibrates to a constant number under the null model of density independent colonization, and to a constant proportion under strong density dependence. We compare the null model to two density dependent alternatives, using data on exploitation of nest boxes by collared flycatchers Ficedula albicollis . The analysis shows how predicted trends in the equilibrium unoccupied fraction are similar for both spatial interference and net immigration. This needs to be recognized, since the null hypothesis of a constant unused resource applies also to the dynamics of consumable resources, where it is expressed in a constant stock of uneaten prey at the dynamic equilibrium of predators to prey.     

Islands on the plains: metapopulation dynamics and female biased dispersal in hyraxes (Hyracoidea) in the Serengeti National Park

Two species of hyrax, Heterohyrax brucei and Procavia johnstoni, inhabit rock outcrops, or kopjes, in the Serengeti National Park, Tanzania. Such distinct 'island' habitats provide an excellent model to investigate natural metapopulation dynamics with distinct small populations with extinction and colonization events, as well as migration between populations. Allele frequencies, genetic variability and genetic distances between populations were calculated based on DNA microsatellite markers. The genetic diversity in both species of hyrax, especially P. johnstoni, was surprisingly low: allelic diversity ranged from 2 to 7 alleles per locus. This may have been induced by colonization by a small number of individuals from single source populations. F-statistics, assignment tests and calculations of pairwise relatedness all indicated female-biased dispersal in H. brucei but not P. johnstoni. Values of FIS in P. johnstoni showed an excess of homozygotes indicative of high rates of inbreeding; evidence for inbreeding could not be detected in H. brucei. Although female dispersal patterns in H. brucei seem to prevent inbreeding and consequently reduce risk of local extinction, this seems not to be the case in P. johnstoni.