Competitive interactions limiting the number of species in rock pools: experiments with Sigara nigrolineata

Rock-pool corixids (Hemiptera) Arctocorisa carinata and Callicorixa producta were excluded from small rock pools. First-stage larvae of Sigara nigrolineata, separately or together with larvae of either of the rock-pool species, were then introduced daily at a rate simulating reproduction by typical populations. In all three treatments, the survival of S. nigrolineata larvae was high at the beginning of experiment, 0.39–0.83 between the 1st and 5th stages, but later high mortality was observed. C. producta adults tended to invade experimental rock pools and towards the end of the experiment larvae of this species were dominant. Oviposition rates and developmental rates of the three species are comparable. In laboratory tests, young larvae of S. nigrolineata were more vulnerable to interspecific predation than those of the rock-pool species, and older larvae proved to be less efficient predators. In rock pools, there are no environmental factors preventing successful reproduction by S. nigrolineata. However, competition and predation by the two other rock-pool species seem to prevent successful reproduction. Dispersal in S. nigrolineata is too slow for efficient colonization of rapidly changing rock-pool environments.     

Spatial and environmental effects on a rock‐pool metacommunity depend on landscape setting and dispersal mode

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The ecology and mechanisms of overflow‐mediated dispersal in a rock‐pool metacommunity

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Evolution of dispersal in metapopulations with local density dependence and demographic stochasticity

In this paper, we predict the outcome of dispersal evolution in metapopulations based on the following assumptions: (i) population dynamics within patches are density-regulated by realistic growth functions; (ii) demographic stochasticity resulting from finite population sizes within patches is accounted for; and (iii) the transition of individuals between patches is explicitly modelled by a disperser pool. We show, first, that evolutionarily stable dispersal rates do not necessarily increase with rates for the local extinction of populations due to external disturbances in habitable patches. Second, we describe how demographic stochasticity affects the evolution of dispersal rates: evolutionarily stable dispersal rates remain high even when disturbance-related rates of local extinction are low, and a variety of qualitatively different responses of adapted dispersal rates to varied levels of disturbance become possible. This paper shows, for the first time, that evolution of dispersal rates may give rise to monotonically increasing or decreasing responses, as well as to intermediate maxima or minima.     

Effects of inbreeding on traits that influence dispersal and progeny density in Cakile edentula var. lacustris (Brassicaceae)

Inbreeding may influence the intensity of sibling competition by altering the number of offspring produced or by changing plant morphology in ways that influence seed dispersion patterns. To test this possibility, effects of inbreeding on seed production and on traits that influence progeny density were measured using experimental pollinations of flowers of Cakile edentula var. lacustris. Different flowers on a plant were either hand pollinated with self pollen (with and without emasculation) or foreign pollen, or they were allowed to be pollinated naturally. Selfed flowers matured significantly fewer viable seeds than outcrossed flowers (10.3% less seed maturation with inbreeding depression of 19.2%), due in large part to a greater percentage of proximal seed abortions and lower germination success. Plants grown from selfed seeds tended to have lower seed production (37 fewer seeds on average, with inbreeding depression of 16.2%), caused in part by an increase in the percentage of fruits with proximal seed abortions, although this effect was not significant. Inbreeding depression in total fitness was 29.0%, which corresponds to a difference of 46 seeds per pollinated ovule. Selfing rate estimates were usually intermediate to high, indicating that inbreeding effects observed in this study would be present in naturally pollinated progeny. Although the influence of inbreeding directly on dispersal was negligible, the predicted reduction in sibling competition caused by reduced seed production resulted in an estimate of inbreeding depression of 17.5%, which is 11.5% lower than that measured under uniform conditions. Consequently, inbreeding depression estimated under natural dispersion patterns may be lower than that estimated under uniform conditions since seeds from self- and cross-pollination may not experience the same competitive environment in the field. Inbreeding in the maternal generation, therefore, could influence progeny fitness not only by determining the genetic composition of progeny, but also by influencing the competitive environment in which progeny grow.     

Dispersal evolution diminishes the negative density dependence in dispersal

In many organisms, dispersal varies with the local population density. Such patterns of density-dependent dispersal (DDD) are expected to shape the dynamics, spatial spread, and invasiveness of populations. Despite their ecological importance, empirical evidence for the evolution of DDD patterns remains extremely scarce. This is especially relevant because rapid evolution of dispersal traits has now been empirically confirmed in several taxa. Changes in DDD of dispersing populations could help clarify not only the role of DDD in dispersal evolution, but also the possible pattern of subsequent range expansion. Here, we investigate the relationship between dispersal evolution and DDD using a long-term experimental evolution study on Drosophila melanogaster. We compared the DDD patterns of four dispersal-selected populations and their non-selected controls. The control populations showed negative DDD, which was stronger in females than in males. In contrast, the dispersal-selected populations showed DDD, where neither males nor females exhibited DDD. We compare our results with previous evolutionary predictions that focused largely on positive DDD, and highlight how the direction of evolutionary change depends on the initial DDD pattern of a population. Finally, we discuss the implications of DDD evolution for spatial ecology and evolution.     

Heterogeneity in local density allows a positive evolutionary relationship between self‐fertilisation and dispersal

Despite empirical evidence for a positive relationship between dispersal and self‐fertilization (selfing), theoretical work predicts that these traits should always be negatively correlated, and the Good Coloniser Syndrome of high dispersal and selfing (Cf. Baker's Law) should not evolve. Critically, previous work assumes that adult density is spatiotemporally homogeneous, so selfing results in identical offspring production for all patches, eliminating the benefit of dispersal for escaping from local resource competition. We investigate the joint evolution of dispersal and selfing in a demographically structured metapopulation model where local density is spatiotemporally heterogeneous due to extinction‐recolonization dynamics. Selfing alleviates outcrossing failure due to low local density (an Allee effect) while dispersal alleviates competition through dispersal of propagules from high‐ to low‐density patches. Because local density is spatiotemporally heterogeneous in our model, selfing does not eliminate heterogeneity in competition, so dispersal remains beneficial even under full selfing. Hence the Good Coloniser Syndrome is evolutionarily stable under a broad range of conditions, and both negative and positive relationships between dispersal and selfing are possible, depending on the environment. Our model thus accommodates positive empirical relationships between dispersal and selfing not predicted by previous theoretical work and provides additional explanations for negative relationships.     

Effect of predator density dependent dispersal of prey on stability of a predator-prey system

This work presents a predator-prey Lotka-Volterra model in a two patch environment. The model is a set of four ordinary differential equations that govern the prey and predator population densities on each patch. Predators disperse with constant migration rates, while prey dispersal is predator density-dependent. When the predator density is large, the dispersal of prey is more likely to occur. We assume that prey and predator dispersal is faster than the local predator-prey interaction on each patch. Thus, we take advantage of two time scales in order to reduce the complete model to a system of two equations governing the total prey and predator densities. The stability analysis of the aggregated model shows that a unique strictly positive equilibrium exists. This equilibrium may be stable or unstable. A Hopf bifurcation may occur, leading the equilibrium to be a centre. If the two patches are similar, the predator density dependent dispersal of prey has a stabilizing effect on the predator-prey system.     

Natal dispersal in great bustards: the effect of sex, local population size and spatial isolation

1. We investigated the causes of natal dispersal in four Spanish areas where 35 breeding groups of the polygynous great bustard Otis tarda were monitored intensively. A total of 392 juveniles were radio-tracked between 1991 and 2006 by ground and via aeroplane to avoid potential biases derived from the non-detection of long-distance dispersers. 2. We explored 10 explanatory variables that were related to individual phenotypic features, habitat and conspecific traits in terms of group size and breeding performance, and spatial distribution of available breeding groups. Probability of group change and natal dispersal distances were investigated separately through multifactorial analyses. 3. Natal dispersal occurred in 47.8% of the birds and median natal dispersal distance of dispersers was 18.1 km (range 4.97-178.42 km). Sex largely determined the dispersal probability, with 75.6% of males being dispersers and 80.0% of females being philopatric, in contrast to the general pattern of female-biased dispersal found in most avian species. 4. Both the frequency of natal dispersal and dispersal distances were affected by the spatial distribution of breeding groups. More isolated groups showed a higher proportion of philopatric individuals, the effect being more evident in males than in females. This implies a reduction in gene flow in fragmented populations, as most genetic exchange is achieved through male dispersal. Additionally, dispersers hatched in more isolated groups tended to exhibit longer dispersal distances, which increases the associated energetic costs and mortality risks. 5. The dispersal decision was influenced by the number of conspecifics in the natal group. The individual probability of natal dispersal was related inversely to the size of the natal group, which supports the balanced dispersal model and the conspecific attraction hypothesis. 6. Overall, our results provide a good example of phenotypic plasticity and reinforce the current view that dispersal is an evolutionary complex trait conditioned by the interaction of individual, social and environmental causes that vary between individuals and populations.     

Effects of dispersal on local population increase

Work to date on biological invasions and the spread of biological control agents has been focused on the explicitly spatial aspects, such as rate of spread and shape of the wave front. There has been relatively little attention paid to the influence of dispersal on the rate of increase of local populations. We use a simple general model for logistic local growth and one-dimensional diffusive dispersal to show that dispersal can act as a substantial drain on local populations. Local increase at a site of introduction is always slower than would be expected in the absence of dispersal, while the rate of increase of other populations is initially enhanced, then reduced by dispersal. This may have an important effect when estimating population parameters for invading organisms or biological control agents during the initial stages of their spread, and helps explain the "latent period" typically observed in biological invasions.     

扩散对破碎化景观上宿主-寄生种群动态的影响

景观破碎化和扩散是空间种群模型的重要因素,对生物入侵存在着深远的影响。本章将基于偶对近似模型,探讨由局部和全局宿主-寄生相互作用共同决定的扩散模式对破坏性景观上疾病入侵与传播的影响。其中,生境破坏由生境丧失量与生境破碎化程度来描述。模拟结果显示,宿主和病毒的全局扩散对疾病的入侵与种群密度产生不对称效应：病毒的全局扩散对系统产生的影响较宿主的全局扩散更为显著。不同扩散模式下,生境丧失越高或破碎化程度越低,均将越有害于寄生病毒的入侵;同时,生境的破坏程度也显著地影响了入侵阈值对扩散模式的响应机制。本文研究结果暗示,景观破碎化的空间分布格局以及病毒扩散的限制均可作为物种保护与管理中有效的疾病控制策略。该研究结果在一定意义上丰富和发展了寄生感染理论,为物种保护提供了生态学理论依据。     

Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder

It remains poorly understood how effects of anthropogenic activity, such as large-scale habitat fragmentation, impact sociality in animals. In cooperatively breeding species, groups are mostly formed through delayed offspring dispersal, and habitat fragmentation can affect this process in two opposite directions. Increased habitat isolation may increase dispersal costs, promoting delayed dispersal. Alternatively, reduced patch size and quality may decrease benefits of philopatry, promoting dispersal. Here, we test both predictions in a cooperatively breeding bird (placid greenbul, Phyllastrephus placidus) from an Afrotropical cloud forest archipelago. Males born in fragmented forest dispersed about 1 year earlier than those born in continuous forest. Contrary to females, males also started to reproduce earlier and mostly settled within their natal patch. Females only rarely delayed their dispersal for more than 1 year, both in fragmented and continuous forests. Our results suggest that early male dispersal and reproduction is jointly driven by a decrease in the value of the natal territory and an increase in local breeding opportunities in fragmented forest. While plasticity in dispersal strategies of cooperative breeders in response to anthropogenic change is believed to optimize reproduction-survival trade-offs, to what extent it shapes the ability of species to respond to rapid environmental change remains to be studied.     

Antagonism between local dispersal and self-incompatibility systems in a continuous plant population

Many self-incompatible plant species exist in continuous populations in which individuals disperse locally. Local dispersal of pollen and seeds facilitates inbreeding because pollen pools are likely to contain relatives. Self-incompatibility promotes outbreeding because relatives are likely to carry incompatible alleles. Therefore, populations can experience an antagonism between these forces. In this study, a novel computational model is used to explore the effects of this antagonism on gene flow, allelic diversity, neighbourhood sizes, and identity by descent. I confirm that this antagonism is sensitive to dispersal levels and linkage. However, the results suggest that there is little to no difference between the effects of gametophytic and sporophytic self-incompatibility systems (GSI and SSI) on unlinked loci. More importantly, both GSI and SSI affect unlinked loci in a manner similar to obligate outcrossing without mating types. This suggests that the primary evolutionary impact of self-incompatibility systems may be to prevent selfing, and prevention of biparental inbreeding might be a beneficial side-effect.     

Post-breeding dispersal by female red squirrels (Tamiasciurus hudsonicus): the effect of local vacancies

Red squirrels (Tamiasciurus hudsonicus) defend food-based territoriesyear round, and juveniles must acquire a territory before winterto survive. We experimentally removed territory owners duringthe time that juveniles were becoming independent to examinethe effect of local vacancies on dispersal patterns. Juvenilesattempted to take over removal territories most frequently.However, females with offspring still on the natal territoryactually took over twice as many territories as juveniles. Thesefemales did not appear to move because of low reproductive potentialor to increase territory quality. Instead, moving to a removalterritory allowed more of their offspring to remain on the natalterritory, which appeared to increase juvenile survival.     

Egg masses on the move: corixid oviposition on terrapin shells

In a very dry year, when aquatic habitats were scarce, we observed exceptionally massive corixid oviposition on terrapin shells in Doñana National Park, southwestern Spain. Most terrapins inhabiting a large pond devoid of aquatic vegetation exhibited a dense cover of corixid eggs, whilst terrapins from vegetated ponds had no egg cover. Corixid oviposition had also been observed on terrapin shells in previous periods of severe drought. Considering that terrapins frequently alternate between aquatic and terrestrial environments, we experimentally assessed the viability of corixid eggs in varying degrees of dry-exposure. A significant proportion of corixid eggs did not hatch when they were occasionally dried under sun exposure, but egg mortality was lower when they were dried in the shade. The massive oviposition on terrapin shells was probably a consequence of the limited availability of adequate oviposition sites, and it might increase the odds of egg survival when terrapins migrate to other ponds.     

A truce with neutral theory: local deterministic factors, species traits and dispersal limitation together determine patterns of diversity in stream invertebrates

1. Studies seeking to explain local patterns of diversity have typically relied on niche explanations, reflected in correlations with local environmental conditions, or neutral theory, invoking dispersal processes and speciation. 2. We used macroinvertebrate community data from 10 streams that varied independently in local ecological conditions and spatial proximity. Neutral theory predicts that similarity in communities will be negatively associated with distance between sites, while niche theory suggests that community similarity will be positively associated with similarity in local ecological conditions. 3. Similarity in total invertebrate, grazer and predator assemblages showed negative relationships with distance and, for grazers and predators, positive relationships with local ecological conditions. However, the best model predicting community similarity in all three cases included aspects of both local ecological conditions and distance between sites. 4. When assemblages were analysed according to dispersal ability, high-dispersal species were shown to be freely accessing all sites and community similarity was not well predicted by either local ecology or spatial separation. Assemblages of species with low and moderate dispersal ability were best predicted by combined models, including distance between sites and local ecological factors. 5. The results suggest that the perceived dichotomy between neutral and local environmental processes in determining local patterns of diversity may not be useful. Neutral and niche processes structured these communities differentially depending on trophic level and species traits. 6. We emphasize the potential for both dispersal processes and local environmental conditions to explain local patterns of diversity.