Low propensity for aerial dispersal in specialist spiders from fragmented landscapes

Aerial dispersal by ballooning is a passive flight, by which wind drag generates an upward lift on a silk thread. It is likely to reflect an aerial lottery, in which the absence of flight direction control is a serious cost for long-distance dispersal in a fragmented landscape. For species occurring in one patchily distributed habitat type, dispersal should evolve in a different way from morphological traits, directly linked to active dispersal. Therefore, we expect that if the risk of landing in an unsuitable habitat is lower than the probability of reaching a suitable habitat, selection should benefit a well-developed ballooning behaviour. We investigated interspecific variation in the ballooning-initiating tiptoe behaviour as it is linked to spider dispersal performance. Our results indeed indicate that ballooning performance is negatively related to habitat specialization in spiders from patchy grey dunes, so habitat specialists are characterized by poorly developed dispersal behaviour. These findings are concordant with recent insights that dispersal is selected as risk spreading in generalists, while it is selected against in specialist species.     

Genetic variation for aerial dispersal behavior in the Banks grass mite

Banks grass mites, Oligonychus pratensis (Banks), exhibit a behavior that facilitates their dispersal as aerial plankton over long distances. The propensity to display this behavior and its latency varies among individual mites. Some of this variation can be attributed to genetic differences among individuals. Mother-daughter resemblance and bi-directional selection were used to estimate the genetic component of latency of the aerial dispersal behavior in a wild population collected from corn plants. Mother-daughter regression analysis suggested that up to 10% of variation in initiation of aerial dispersal behavior was attributable to an additive genetic component. Subsequent selection for shorter and longer latency resulted in significant divergence between selected lines after 9 generation, but the response was not symmetrical. Realized heritability for shorter latency was 0.09, while that for longer latency was not different from 0. This response to selection was consistent with results of the mother-daughter analysis, and the response asymmetry may explain the high variability in the mother-daughter regression. In both their natural habitat and in a corn agroecosystem, mites survive by using a succession of temporarily available hosts. Aerial dispersal behavior by Banks grass mites facilitates colonization of new hosts. The genetic basis and selection response of behavior initiating aerial dispersal is consistent with its role in mites population dynamics in this setting.     

Sex-biased dispersal and adaptation to marginal habitats

If gene flow occurs through both sexes but only females contribute to population growth, adaptation to marginal (sink) habitats should be differentially affected by male versus female dispersal. Here I address this problem with two models. First, I consider the fate of a rare allele that improves fitness in the marginal habitat but reduces fitness in the core (source) habitat. Then I study the evolution of a polygenic character mediating a trade-off in fitness between the habitats. Both approaches led to qualitatively similar predictions. The effect of a difference in the dispersal rate between the sexes depends on the degree to which immigration from the core habitat boosts the reproductive output from the marginal habitat. This boost is slight if the marginal habitat is able to sustain well a population without immigration. In that case, both female- and male-biased dispersal is more favorable for adaptation to marginal habitats than equal dispersal of both sexes (assuming that the dispersal rate averaged over the sexes is kept constant). In contrast, if the marginal habitat is an absolute sink unable to sustain a population without immigration, the conditions for adaptation to that habitat are least favorable under highly male-biased dispersal and most favorable under highly female-biased dispersal. Under some circumstances, high average (male+female) dispersal is more favorable than low dispersal. Thus, gene flow should not be seen solely as thwarting adaptation to marginal habitats. The results are interpreted in terms of how male and female dispersal affects the relative rate of gene flow from the source to the sink habitat and in the opposite direction. This study predicts that ecological niches of taxa with female-biased dispersal should tend to be broader and more evolutionarily flexible.     

Community trait response to environment: disentangling species turnover vs intraspecific trait variability effects

Ecological communities and their response to environmental gradients are increasingly being described by various measures of trait composition. Aggregated trait averages (i.e. averages of trait values of constituent species, weighted by species proportions) are popular indices reflecting the functional characteristics of locally dominant species. Because the variation of these indices along environmental gradients can be caused by both species turnover and intraspecific trait variability, it is necessary to disentangle the role of both components to community variability. For quantitative traits, trait averages can be calculated from ‘fixed’ trait values (i.e. a single mean trait value for individual species used for all habitats where the species is found) or trait values for individual species specific to each plot, or habitat, where the species is found. Changes in fixed averages across environments reflect species turnover, while changes in specific traits reflect both species turnover and within‐species variability in traits. Here we suggest a practical method (accompanied by a set of R functions) that, by combining ‘fixed’ and ‘specific averages’, disentangles the effect of species turnover, intraspecific trait variability, and their covariation. These effects can be further decomposed into parts ascribed to individual explanatory variables (i.e. treatments or environmental gradients considered). The method is illustrated with a case study from a factorial mowing and fertilization experiment in a meadow in South Bohemia. Results show that the variability decomposition differs markedly among traits studied (height, Specific Leaf Area, Leaf N, P, C concentrations, leaf and stem dry matter content), both according to the relative importance of species turnover and intraspecific variability, and also according to their response to experimental factors. Both the effect of intraspecific trait variability and species turnover must be taken into account when assessing the functional role of community trait structure. Neglecting intraspecific trait variability across habitats often results in underestimating the response of communities to environmental changes.     

On a population pathogen model incorporating species dispersal with temporal variation in dispersal rate

In the present paper, we consider a mathematical model of ecosystem population interaction where the population suffers from a susceptible–infectious–susceptible disease. Dispersal of both the susceptible and the infective is incorporated using reaction–diffusion equations. We first study the stability criteria of the basic (non-spatial) model around the disease-free and the infected steady states. We find that the loss rate of the infective species controls disease prevalence. Also without predation pressure, the disease will continue to exist among the population. Then we analyze the spatial model with species dispersal in constant as well as in time-varying form. It is observed that though constant dispersal is unable to generate diffusion-driven instability, dispersal with sinusoidal variation in dispersion rate can generate diffusive instability when the wave number of the perturbation lies within a given range. Numerical simulations are performed to illustrate analytical studies.     

Inter- and intraspecific genetic and morphological variation in a sibling pair of carabid species

Background

Pogonus littoralis and Pogonus chalceus are very close related species with quite different ecological preferences within salt marshes. We study the evolutionary processes in and between these presumably young species. Therefore, we compare the variation in ecologically relevant characters and the genetic variation within one of the species (intraspecific differentiation) with the variation of the two types of characters between the two species (interspecific variation). Data are compared between two independent sets of populations, one set at a small geographical scale (the ecologically diverse Guérande area in France) and the other set at a Atlantic-Mediterranean scale.

Results

Body and relative wing size and IDH1 allozyme data show that the intraspecific variation in P. chalceus is high and in the same range as the interspecific variation (P. chalceus versus P. littoralis). Based on neutral markers (other allozymes and mitochondrial DNA) on the other hand, the intraspecific variation in P. chalceus is much lower in comparison to the interspecific variation.

Conclusion

The different ecotypes in the highly polytypic species P. chalceus are as highly differentiated in ecological characters as true species, but are not recognised as such by screening neutral DNA polymorphisms. This can be interpreted as a case of ongoing speciation driven by natural selection adapting each ecotype to its respective ecological niche. The same ecological process can be recognised in the differentiation between the two sister species, where en plus reproductive isolation between the two gene pools occurred, allowing independent drift and mutation accumulation in neutral genetic characters.     

Ecological stoichiometry of Eurasian perch – intraspecific variation due to size,habitat and diet

The turnover and distribution of energy and nutrients in food webs is influenced by consumer stoichiometry. Although the stoichiometry of heterotrophs is generally considered to vary only little, there may be intraspecific variation due to factors such as habitat, resources, ontogeny and size. We examined intraspecific variation in Eurasian perch Perca fluviatilis stoichiometry, a common species that exhibits habitat and resource specialization, ontogenetic niche shifts and a large size range. This study investigated the elemental stoichiometry of a wide size range of perch from littoral and pelagic habitats. The mean C:N:P stoichiometry of whole perch was 37:9:1 (molar ratios). However, %C, %P, C:N, C:P and N:P varied with size, morphology, habitat and diet category. These factors together explained 24–40% of the variation in C:N:P stoichiometry. In contrast, perch stoichiometry was not related to diet stoichiometry, suggesting that the former is homeostatically regulated. The results suggest that the high P content of perch may result in stoichiometric constraints on the growth of non‐piscivorous perch, and that piscivory is an efficient strategy for acquiring P. Resource polymorphism, individual diet specialization and intraspecific size variation are widespread among animals. Thus changes in stoichiometry with size, habitat, morphology and resource use, and therefore also stoichiometric demands, are probably common.     

Drought impacts and recovery are driven by local variation in species turnover

In forest systems, drought acts as a disturbance that can generate impacts on community structure and composition at multiple scales. This study focused on a 2-month drought event within an early successional forest system to determine the mechanism(s) of community response to, and recovery from, drought. Drought induced a 28% decline in neighborhood species richness and cover as a result of decreased colonization and increased extinction rates. Following drought, neighborhood richness quickly recovered via increased colonization rates while extinction rates were unaltered. Drought had little long-term effect on neighborhood structure (species richness and cover) and generated only subtle changes in neighborhood composition. Ruderal (annual and biennial) species were more likely to change (increase or decrease) in cover and frequency than the more stress tolerant perennial and woody species. However, population dynamics appeared to be generally driven by stochastic species turnover among fields and not by uniform shifts in species performance across the site. Although drought impacts and recovery appeared predictable at the neighborhood scale, population responses to drought within the site were rather unpredictable. Our findings suggest that stochastic fine-scale processes can generate predictable course-scale dynamics within a disturbed system. The scale-specific mechanisms of community change presented here should be explored in other systems to determine the extent of their generality in driving disturbance impacts on communities.     

Y-Chromosome variation in hominids: intraspecific variation is limited to the polygamous chimpanzee

Background

We have previously demonstrated that the Y-specific ampliconic fertility genes DAZ (deleted in azoospermia) and CDY (chromodomain protein Y) varied with respect to copy number and position among chimpanzees (Pan troglodytes). In comparison, seven Y-chromosomal lineages of the bonobo (Pan paniscus), the chimpanzee''s closest living relative, showed no variation. We extend our earlier comparative investigation to include an analysis of the intraspecific variation of these genes in gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus), and examine the resulting patterns in the light of the species'' markedly different social and mating behaviors.

Methodology/Principal Findings

Fluorescence in situ hybridization analysis (FISH) of DAZ and CDY in 12 Y-chromosomal lineages of western lowland gorilla (G. gorilla gorilla) and a single lineage of the eastern lowland gorilla (G. beringei graueri) showed no variation among lineages. Similar findings were noted for the 10 Y-chromosomal lineages examined in the Bornean orangutan (Pongo pygmaeus), and 11 Y-chromosomal lineages of the Sumatran orangutan (P. abelii). We validated the contrasting DAZ and CDY patterns using quantitative real-time polymerase chain reaction (qPCR) in chimpanzee and bonobo.

Conclusion/Significance

High intraspecific variation in copy number and position of the DAZ and CDY genes is seen only in the chimpanzee. We hypothesize that this is best explained by sperm competition that results in the variant DAZ and CDY haplotypes detected in this species. In contrast, bonobos, gorillas and orangutans—species that are not subject to sperm competition—showed no intraspecific variation in DAZ and CDY suggesting that monoandry in gorillas, and preferential female mate choice in bonobos and orangutans, probably permitted the fixation of a single Y variant in each taxon. These data support the notion that the evolutionary history of a primate Y chromosome is not simply encrypted in its DNA sequences, but is also shaped by the social and behavioral circumstances under which the specific species has evolved.     

Joint evolution of sex ratio and dispersal: conditions for higher dispersal rates from good habitats

We analyze models of evolution of sex ratio conditional on habitat quality and with sex specific dispersal. Previous analysis concluded that the main constraint on sex ratio is habitat choice and leads to overproduction of the most dispersing sex in low quality habitat. Here, we analyze three models with finite local populations and show that constraints on sex ratio can balance constraints on habitat choice. In the first model, dispersal rates are fixed. In the second, sex specific dispersal can evolve independently of the habitat quality. These models suggests that sex ratio evolution can lead to higher global dispersal rates (mean of male and female dispersal rates) from high quality habitats. In the last model dispersal evolves conditionally with both sex and habitat. Our models suggests that conditions for overproduction of the most dispersing sex in high quality habitat are frequent. The predictions of the models with evolving dispersal contrast with patterns generally described in nature. We discuss possible reasons of this difference.     

Increased temperature variation poses a greater risk to species than climate warming

Increases in the frequency, severity and duration of temperature extremes are anticipated in the near future. Although recent work suggests that changes in temperature variation will have disproportionately greater effects on species than changes to the mean, much of climate change research in ecology has focused on the impacts of mean temperature change. Here, we couple fine-grained climate projections (2050–2059) to thermal performance data from 38 ectothermic invertebrate species and contrast projections with those of a simple model. We show that projections based on mean temperature change alone differ substantially from those incorporating changes to the variation, and to the mean and variation in concert. Although most species show increases in performance at greater mean temperatures, the effect of mean and variance change together yields a range of responses, with temperate species at greatest risk of performance declines. Our work highlights the importance of using fine-grained temporal data to incorporate the full extent of temperature variation when assessing and projecting performance.     

Avian seed dispersal and seedling distribution of the endangered tree species,Taxus chinensis,in patchy habitats

Background: There is limited understanding about bird dispersal behaviour and seedling distribution of endangered tree species in patchy environments, although these processes are important for plant species persistence.

Aims: We tested how patch features affected bird behaviour and seed dispersal, and thus seedling distribution of the endangered Chinese yew tree (Taxus chinensis).

Methods: In the present study, we combined field data of bird dispersal behaviour and GIS-based information to elucidate the influence of spatial features of habitat patches on bird dispersal behaviour, and the resulting effects on the seedling distribution of the endangered Chinese yew in two patchy habitats.

Results: Our results showed that the only seed source patch could attract eight bird species for dispersal at the two sites. Post-foraging movements of bird dispersers was strongly related to both topography and the relative locations of habitat patches. Yew seedlings aggregated only at the seed source and bamboo recruitment patches, which was affect by both the spatial distribution of recruitment patches and patch use by dispersers.

Conclusions: Our results emphasise that bamboo patches in both patchy environments provide the necessary conditions for germination of yew seeds, and the post-foraging behaviour of dispersers determines seed deposited in these patches. Our study highlights the importance of the dispersal behaviour of frugivorous birds in the successful regeneration and colonisation of yew populations in patchy habitats.     

Seed dispersal in Erythronium dens-canis L. (Liliaceae): variation among habitats in a myrmecochorous plant

Erythronium dens-canis is a geophyte which produces a single flower each season. The fruits produce small seeds with relatively large elaiosomes. We performed experiments to investigate primary and secondary seed dispersal mechanisms of this species in different habitats in the western part of the Cantabrian Range in northwest Spain. Sticky traps were used to measure primary dispersal of seeds up to 0.5 m from mother plants. Seed cafeteria experiments were performed in different habitats to examine the role of ants and rodents in secondary seed transport and seed predation. Our results indicate that: (a) primary seed dispersal is positively skewed (99% of seeds fall within 20 cm of the mother plant) and seed dispersal distances vary significantly among plants; (b) secondary dispersal is exclusively by myrmecochory, although the proportion of seeds removed by ants differs significantly among habitats; (c) ant species composition and abundances vary among habitats; and (d) freshly dropped seeds are more likely to be removed than seeds that have begun to dry out. We conclude that secondary dispersal of seeds is greatly influenced by habitat but not by small-scale microhabitat. This revised version was published online in June 2006 with corrections to the Cover Date.     

The link between behavioural type and natal dispersal propensity reveals a dispersal syndrome in a large herbivore

When individuals disperse, they modify the physical and social composition of their reproductive environment, potentially impacting their fitness. The choice an individual makes between dispersal and philopatry is thus critical, hence a better understanding of the mechanisms involved in the decision to leave the natal area is crucial. We explored how combinations of behavioural (exploration, mobility, activity and stress response) and morphological (body mass) traits measured prior to dispersal were linked to the subsequent dispersal decision in 77 roe deer Capreolus capreolus fawns. Using an unusually detailed multi-trait approach, we identified two independent behavioural continuums related to dispersal. First, a continuum of energetic expenditure contrasted individuals of low mobility, low variability in head activity and low body temperature with those that displayed opposite traits. Second, a continuum of neophobia contrasted individuals that explored more prior to dispersal and were more tolerant of capture with those that displayed opposite traits. While accounting for possible confounding effects of condition-dependence (body mass), we showed that future dispersers were less neophobic and had higher energetic budgets than future philopatric individuals, providing strong support for a dispersal syndrome in this species.