Is farther seed dispersal better? Spatial patterns of offspring mortality in three rainforest tree species with different dispersal abilities

The paradigm that tropical trees with farther seed dispersal experience lower offspring mortality is currently based on within-species studies documenting higher survival of offspring located farther from conspecific adults and/or closer to light gaps. We determined whether the paradigm also holds among species by comparing spatial patterns of offspring mortality among three sympatric Neotropical rainforest tree species with the same seed dispersers but with different dispersal abilities. First, we assessed spatially non-random mortality for each species by measuring spatial shifts of the population recruitment curve (PRC) with respect to conspecific adults and light gaps across three early life stages: dispersed seeds, young seedlings and old seedlings. Then, we determined whether PRC shifts were greater for the species with short dispersal distances than for the species with greater dispersal distances. We found that the PRC shifted away from conspecific adults consistently across life stages, but we found no consistent PRC shifts towards gaps, suggesting that mortality was related more to the proximity of conspecifics than to absence of light gaps. PRC shifts away from adults were greatest in the species with the lowest dispersal ability, supporting the paradigm. Differential PRC shifts caused the spatial distribution of offspring to become almost independent with respect to adult trees and gaps in all three species, despite large differences in seed dispersal distance among these species. Our results provide direct empirical support for the paradigm that among tropical trees, species with farther seed dispersal are less impacted by spatially non-random mortality than are similar species with shorter dispersal distances. Thus, greater dispersal effectiveness merits extra investments of trees in seed dispersal ability, even at the cost of fecundity.     

Scale-dependence and mechanisms of dispersal in freshwater zooplankton

Communities of organisms form as a result of both interspecific and abiotic interactions within local habitat patches and dispersal among patches in a region. Local processes are expected to play a dominant role when dispersal occurs much more often than extinction. We performed two field experiments to examine rates and mechanisms of dispersal in freshwater pond zooplankton communities. First, we tested the effect of distance from a source on the rate of colonization of artificial habitat by placing wading pools at 5, 10, 30 and 60 m from two natural fishless ponds and observing the succession of zooplankton. Seventy-eight percent of the species in the source ponds that were capable of living in the pools colonized at least once during the experiment. A new species was found in the pools on average once every four days, suggesting that colonization events occur on the order of days to weeks for many species. Colonization rates declined further from the source at one pond but not the other, and the effect of distance was relatively weak at both ponds. This suggests that many species disperse broadly over short distances. The second experiment tested the role of animal vectors for zooplankton dispersal by restricting access to the pools. Eight treatments were imposed that excluded potential animal vectors along a body size gradient from large mammals to small insects. While the treatments affected zooplankton colonization, many species invaded even when all animals larger than 1 mm were excluded. Animal vectors may therefore be less important for dispersal than wind. Our results suggest that zooplankton are highly effective dispersers over short distances, and can disperse via several mechanisms. Local interactions should therefore play a dominant role in structuring these communities at small regional scales.     

Primary assembly of soil communities: disentangling the effect of dispersal and local environment

It has long been recognised that dispersal abilities and environmental factors are important in shaping invertebrate communities, but their relative importance for primary soil community assembly has not yet been disentangled. By studying soil communities along chronosequences on four recently emerged nunataks (ice-free land in glacial areas) in Iceland, we replicated environmental conditions spatially at various geographical distances. This allowed us to determine the underlying factors of primary community assembly with the help of metacommunity theories that predict different levels of dispersal constraints and effects of the local environment. Comparing community assembly of the nunataks with that of non-isolated deglaciated areas indicated that isolation of a few kilometres did not affect the colonisation of the soil invertebrates. When accounting for effects of geographical distances, soil age and plant richness explained a significant part of the variance observed in the distribution of the oribatid mites and collembola communities, respectively. Furthermore, null model analyses revealed less co-occurrence than expected by chance and also convergence in the body size ratio of co-occurring oribatids, which is consistent with species sorting. Geographical distances influenced species composition, indicating that the community is also assembled by dispersal, e.g. mass effect. When all the results are linked together, they demonstrate that local environmental factors are important in structuring the soil community assembly, but are accompanied with effects of dispersal that may ??override?? the visible effect of the local environment.     

Dispersal distance is influenced by parental and grand-parental density

Non-genetic transmission of information across generations, so-called parental effects, can have significant impacts on offspring morphology, physiology, behaviour and life-history traits. In previous experimental work using the two-spotted spider mite Tetranychus urticae Koch, we demonstrated that dispersal distances increase with local density and levels of genetic relatedness. We here show that manipulation of parental and grand-parental density has a significant effect on offspring dispersal distance, of the same order of magnitude as manipulation of offspring density. We demonstrate that offspring exposed to the same density disperse further if they were born to parents exposed to higher density compared with parents exposed to low density. Offspring dispersal distance also increases when grand-parents were exposed to higher density, except for offspring exposed to low densities, which disperse at shorter distances whatever the grand-parental density. We also show that offspring from mothers exposed to higher densities were overall larger, which suggests that parents in high densities invest more in individual offspring, enabling them to disperse further. We propose that our findings should be included in models investigating the spread rate of invasive species or when predicting the success of conservation measures of species attempting to track changing climates.     

Sex biased natal dispersal in a closed, saturated population of Seychelles warblers Acrocephalus sechellensis

The distances that individuals disperse, from their natal site to the site of first breeding and between breeding sites, have important consequences for the dynamics and genetic structure of a population. Nearly all previous studies on dispersal have the problem that, because the study area encompassed only a part of the population, emigration may have been confounded with mortality. As a result long-distance dispersers may have been overlooked and dispersal data biased towards short distances. By studying a virtually closed population of Seychelles warblers Acrocephalus sechellensis we obtained almost unbiased results on several aspects of dispersal. As in the majority of other avian species, natal dispersal distance was female biased in the Seychelles warbler. Female offspring also forayed further from the natal territory in search of breeding vacancies than male offspring. The sex bias in natal dispersal distance did, however, depend on local breeding density. In males, dispersal distance decreased as the number of territories bordering the natal territory increased, while in females, dispersal distance did not vary with local density. Dispersal by breeders was rare and, unlike in most species, distances did not differ between the sexes. We argue that our results favour the idea that the sex bias in natal dispersal distance in the Seychelles warbler is due to inbreeding avoidance and not resource competition or intrasexual competition for mates.     

The role of local and regional processes in structuring larval dragonfly distributions across habitat gradients

Despite the importance of community-structuring processes operating at both local and regional scales, there is relatively little work examining both forces within a single system. I used a combination of observational and experimental approaches to examine the processes structuring larval dragonfly distributions in lentic habitats that encompass a gradient of both permanence and top predator type. I compared the relative vulnerability of species to predators from different portions of this gradient to assess the role of predation as a local force structuring communities. I also assessed the role of regional processes on species' distributions by examining species' propensity to disperse to and colonize artificial ponds distributed across a landscape. In both studies I contrasted habitat specialist species, which had larvae restricted to permanent lakes, with habitat generalist species, which had larvae that occur broadly across the habitat permanence and top predator transition. Results from this work suggest that dispersal and colonization behavior were critical mechanisms restricting the distributions of habitat specialist species, but that predation may act to reinforce this pattern. The habitat specialists dispersed less frequently, colonized artificial ponds less often when they did reach them, and most moved shorter distances than the habitat generalist species. Habitat specialists were also more vulnerable than habitat generalists to an invertebrate top predator with which they do not co-exist. Results from these studies suggest that species distributions can be shaped by processes operating at both regional and local spatial scales. The role of dispersal and recruitment limitation may be generally underestimated as a force shaping species distributions and community structure across habitat gradients in which there is a transition in both the biotic interactions and the disturbance interval across that gradient.     

The adaptive significance of larval dispersal in coral reef fishes

Synopsis Coral reef fishes almost universally disperse over relatively great distances during a pelagic larval phase. Barlow (1981) suggested that this dispersal is adaptive because adult fishes inhabit a patchy, uncertain environment. This reiterated an older idea that the random extinction of local populations necessarily favours dispersal, since ultimately all populations of non-dispersers will disappear. Whereas this view is based on adult survival, we emphasize a less frequent view that substantial larval dispersal may be adaptive when offspring experience patchy and unpredictable survival in the pelagic habitat. We do not address the question of why these animals ‘broadcast’ rather than ‘brood’, but suggest that species committed to pelagic offspring will be under selection to disperse siblings to spread the risk of failure among members of a cohort. Our arguments are supported by a heuristic computer simulation.     

The lengths birds will go to avoid incest

Cooperatively breeding species, in which some individuals help others to rear their offspring, face a high risk of inbreeding because of close relatedness within social groups. Many species circumvent this problem via sex-biased dispersal, in which one sex is more likely to disperse (and to disperse further), while the other stays and helps. In the absence of sex-biased dispersal, more complex dispersal patterns can arise, based on kin recognition. However, this can also present challenges when dispersal distances are short, leading to clusters of relatives on neighbouring territories. In this issue, Nelson-Flower et al. (2012) break important new ground by unravelling adaptive incest-avoidance mechanisms in a cooperatively breeding bird without sex-biased dispersal. They provide an elegant demonstration of finely tuned dispersal distances together with cognitively based methods for knowing whom to avoid as mates.     

Dispersal of Amazonian birds in continuous and fragmented forest

Many ecologists believe birds disappear from tropical forest fragments because they are poor dispersers. We test this idea using a spatially explicit capture data base from the Biological Dynamics of Forest Fragments Project near Manaus, Brazil. We measure bird movements directly, over relatively large scales of space and time, both before and after landscape fragmentation. We found that species which disappear from fragments move extensively between plots before isolation, but not after, and often disperse to longer distances in continuous forest than in fragmented forest. Such species also preferentially emigrate from smaller to larger fragments, showing no preference in continuous forest. In contrast, species that persist in fragments are generally less mobile, do not cross gaps as often, yet disperse further after fragmentation than before. 'Heavy tailed' probability models usually explain dispersal kernels better than exponential or Gaussian models, suggesting tropical forest birds may be better dispersers than assumed with some individuals moving very long distances.