Natal Patch Research Articles

The evolution of an ‘intelligent’ dispersal strategy: biased, correlated random walks in patchy landscapes

Theoretical work exploring dispersal evolution focuses on the emigration rate of individuals and typically assumes that movement occurs either at random to any other patch or to one of the nearest‐neighbour patches. There is a lack of work exploring the process by which individuals move between patches, and how this process evolves. This is of concern because any organism that can exert control over dispersal direction can potentially evolve efficiencies in locating patches, and the process by which individuals find new patches will potentially have major effects on metapopulation dynamics and gene flow. Here, we take an initial step towards filling this knowledge gap. To do this we constructed a continuous space population model, in which individuals each carry heritable trait values that specify the characteristics of the biased correlated random walk they use to disperse from their natal patch. We explore how the evolution of the random walk depends upon the cost of dispersal, the density of patches in the landscape, and the emigration rate. The clearest result is that highly correlated walks always evolved (individuals tended to disperse in relatively straight lines from their natal patch), reflecting the efficiency of straight‐line movement. In our models, more costly dispersal resulted in walks with higher correlation between successive steps. However, the exact walk that evolved also depended upon the density of suitable habitat patches, with low density habitat evolving more biased walks (individuals which orient towards suitable habitat at quite large distances from that habitat). Thus, low density habitat will tend to develop individuals which disperse efficiently between adjacent habitat patches but which only rarely disperse to more distant patches; a result that has clear implications for metapopulation theory. Hence, an understanding of the movement behaviour of dispersing individuals is critical for robust long‐term predictions of population dynamics in fragmented landscapes.

The evolution of dispersal – the importance of information about population density and habitat characteristics

The evolution of mobility patterns and dispersal strategies depend on different population, habitat and life history characteristics. The ability to perceive and make use of information about the surrounding environment for dispersal decisions will also differ between organisms. To investigate the evolutionary consequences of such differences, we have used a simulation model with nearest‐neighbour dispersal in a metapopulation to study how variation in the ability to obtain and make use of information about habitat quality and conspecific density affects the evolution of dispersal strategies. We found a rather strong influence of variation in information on the overall rate of dispersal in a metapopulation. The highest emigration rate evolved in organisms with no information about either density or habitat quality and the lowest rate was found in organisms with information about both the natal and the neighbouring patches. For organisms that can make use of information about conspecific density, positively density‐dependent dispersal evolved in the majority of cases, with the strongest density dependence occurring when an individual only has information about density in the natal patch. However, we also identified situations, involving strong local population fluctuations and frequent local extinctions, where negatively density‐dependent dispersal evolved.

Evolution of condition-dependent dispersal under kin competition

Dispersers often differ in body condition from non-dispersers. The social dominance hypothesis explains dispersal of weak individuals, but it is not yet well understood why strong individuals, which could easily retain their natal site, are sometimes exposed to risky dispersal. Based on the model for dispersal under kin competition by Hamilton and May, we construct a model where dispersal propensity depends on body condition. We consider an annual species that inhabits a patchy environment with varying patch qualities. Offspring body condition corresponds to the quality of the natal patch and competitive ability increases with body condition. Our main general result balances the fitness benefit from not dispersing and retaining the natal patch and the benefit from dispersing and establishing somewhere else. We present four different examples for competition, which all hint that dispersal of strong individuals may be a common outcome under the assumptions of the present model. In three of the examples, the evolutionarily stable dispersal probability is an increasing function of body condition. However, we found an example where, counterintuitively, the evolutionarily stable dispersal probability is a non-monotone function of body condition such that both very weak and very strong individuals disperse with high probability but individuals of intermediate body condition do not disperse at all.

Lethal male–male combat in the parasitoid Melittobia acasta: are size and competitive environment important?

Interactions between organisms can lie anywhere along the spectrum from fatal conflict to total cooperation. To understand this variation, knowledge of the strategies that individuals use to acquire their resources is required. We tested theoretical predictions relating to conflict behaviour in a species of parasitoid wasp, Melittobia acasta, in which males compete for mates in their closed, natal patches. They engage in lethal combat to gain access to the females developing in their host. Environmental factors and individual traits both influenced conflict behaviour and contest outcome: fighting behaviour increased with the density of competitors and larger contestants were more successful. Our results also indicate that opponent assessment did not influence conflict behaviour and contests were always resolved by fighting.

Settlement decisions in blue tits: difference in the use of social information according to age and individual success

Dispersers are expected to assess breeding habitat quality before settlement. Although cues reflecting habitat quality are well studied, social cues have not been as well evaluated. In this paper, we studied breeding habitat selection during 3 years in a natural population of blue tits, Cyanistes caeruleus, breeding in nest-boxes. Our aim was to investigate if this species used conspecific density and/or reproductive success of con- and heterospecifics (i.e., social cues) in settlement decisions. The patterns observed were consistent with the idea that juveniles, when dispersing from their natal patches, did not react to any of the cues that we tested. In contrast, breeders that dispersed seemed to respond to both conspecific mean patch reproductive success (PRS) and breeding density of the settlement patch in the year of dispersal, their response differing according to their own reproductive success. Indeed, failed breeders moved to areas with high PRS and low density relative to source patches, while successful breeders behaved the opposite. The comparison between juveniles and adults might be modulated by the limited time available to juveniles to gather information on PRS and density at the end of the dispersing year. Adults lacking these time constraints, however, seemed to rely on these conspecific cues although limited by their own quality. Additionally, breeders were more likely to be immigrants in patches with relatively low breeding success and density the previous year, suggesting that settlement is influenced by multiple cues, which may reveal information on different aspects of habitat and be available at different moments. Collectively, our results support the importance of social cues for blue tits' settlement.

How do birds search for breeding areas at the landscape level? Interpatch movements of male ortolan buntings

Animal movements at large spatial scales are of great importance in population ecology, yet little is known due to practical problems following individuals across landscapes. We studied the whole Norwegian population of a small songbird (ortolan bunting, Emberiza hortulana) occupying habitat patches dispersed over nearly 500 km2. Movements of colour‐ringed males were monitored during ten years, and extensive long‐distance dispersal was recorded. More than half of all cases of breeding dispersal took place within one breeding season, and males moved up to 43 km between singing territories, using 1–22 d. Natal dispersal was usually to a habitat patch close to the natal patch, or within the natal patch if it was large. Breeding dispersal movements were often long‐distance, beyond neighbouring patches, and up to 11–19 patches were overflown. Movements of at least 6–9 km across areas of unsuitable habitat occurred regularly. The number of patches visited was low (1–4) even though search costs in terms of time spent moving from one site to another were relatively low (often only a few days even for distances >10 km). Most males seemed to use a threshold tactic when choosing a patch, but returns to previously visited patches were recorded, including some cases of commuting. In conclusion, male ortolan buntings have a surprising ability to move quickly at the landscape level, and this resulted in a high connectivity of patches. We discuss our results in relation to optimal searching strategies, in particular the use of within‐breeding season versus post‐breeding season search, conspecific attraction and adaptive late arrival of young birds.

SEX RATIOS WHEN HELPERS STAY AT THE NEST

This study investigates the evolution of the sex ratio (parental investment in sons) when breeding adults are supported by help provided by nonbreeding individuals of one sex. The study also assumes that the helping sex remains on its natal site to compete for the opportunity to breed, whereas the nonhelping sex disperses. Two kin-selection models are presented, both of which incorporate the age structure found in many natural populations where such helping occurs. The first model assumes that helpers increase the survival of their parents. The second model assumes that helpers are indiscriminant: a helper chooses to increase the survival of a random pair of adults breeding on its natal patch. In both models, sex ratios are not always biased toward the sex that provides the most help. When helpers do not discriminate (second model), the direction of sex-ratio bias is determined solely by the size of the benefit of helping behavior. When this benefit is small, sex-ratio evolution is primarily influenced by local resource competition and sex ratios are biased toward the nonhelping (dispersive) sex. If the benefit of help is large enough, the effect of local resource competition is reduced and sex-ratio bias favors the helpful sex. When helpers help only their parents, the same qualitative relationship exists between the direction of sex-ratio bias and the benefit of helping. In this case, however, the direction of sex-ratio bias is also influenced by the size of the social group, mortality, and which individual (mother or father) controls the sex ratio. This study also investigates a sex-ratio conflict that exists between mates. Helping behavior of nonbreeders can act to alleviate the disparities between the optimal sex ratio from the perspective of a mother and that from the perspective of a father. This consequence of helping has not been previously recognized.

Single locus complementary sex determination in Hymenoptera: an "unintelligent" design?

The haplodiploid sex determining mechanism in Hymenoptera (males are haploid, females are diploid) has played an important role in the evolution of this insect order. In Hymenoptera sex is usually determined by a single locus, heterozygotes are female and hemizygotes are male. Under inbreeding, homozygous diploid and sterile males occur which form a genetic burden for a population. We review life history and genetical traits that may overcome the disadvantages of single locus complementary sex determination (sl-CSD). Behavioural adaptations to avoid matings between relatives include active dispersal from natal patches and mating preferences for non-relatives. In non-social species, temporal and spatial segregation of male and female offspring reduces the burden of sl-CSD. In social species, diploid males are produced at the expense of workers and female reproductives. In some social species, diploid males and diploid male producing queens are killed by workers. Diploid male production may have played a role in the evolution or maintenance of polygyny (multiple queens) and polyandry (multiple mating). Some forms of thelytoky (parthenogenetic female production) increase homozygosity and are therefore incompatible with sl-CSD. We discuss a number of hypothetical adaptations to sl-CSD which should be considered in future studies of this insect order.

Immigration and emigration in the Sinai Baton Blue butterfly: estimates from a single patch

The movement of individuals among patches of suitable habitat is a key process in metapopulation biology, but is very difficult to observe under natural conditions. Thus, many estimates of rates of movement are indirect and incomplete, and there is little empirical knowledge of the factors affecting immigration and emigration. I studied intensively a local population of Sinai Baton Blue butterflies in a discrete habitat patch. The study lasted the entire adult flight period, and involved almost every individual in the local population. Using these data I attempted to estimate the number of immigrants and emigrants, and identified an important factor affecting inter-patch movement. Early in the season, individuals were resident in the study patch for only a very short time; I assumed this was because they were emigrating, and thus estimated that at least 14% of the population emerging in the study patch emigrated. I assumed that butterflies not caught until they were relatively old, had immigrated to the patch (rather than being missed at a young age), and thus estimated that 13% of the population caught in the patch was composed of immigrants, most of which were females. Individuals assumed to be immigrants arrived in the patch throughout the adult flight period, but older individuals generally arrived later in the season. Timing of migration was almost certainly linked to phenology of the butterfly’s only hostplant, Sinai Thyme. Most butterflies remained in their natal patch. The degree of inter-patch movement estimated using this method was consistent with other metapopulation studies, and would suffice for this species to exist as a metapopulation.

Dispersal‐related life‐history trade‐offs in a butterfly metapopulation

1. Recent studies on butterflies have documented apparent evolutionary changes in dispersal rate in response to climate change and habitat change. These studies often assume a trade-off between dispersal rate (or flight capacity) and reproduction, which is the rule in wing-dimorphic species but might not occur equally in wing-monomorphic species such as butterflies. 2. To investigate the relationship between dispersal rate and fecundity in the Glanville fritillary butterfly Melitaea cinxia we recorded lifetime individual movements, matings, ovipositions, and maximal life span in a large (32 x 26 m) population cage in the field. Experimental material was obtained from 20 newly established and 20 old local populations within a large metapopulation in the Aland Islands in Finland. 3. Females of the Glanville fritillary from newly established populations are known to be more dispersive in the field, and in the cage they showed significantly greater mobility, mated earlier, and laid more egg clutches than females from old populations. The dispersive females from new populations exhibited no reduced lifetime fecundity in the cage, but they had a shorter maximal life span than old-population females. 4. These results challenge the dispersal-fecundity trade-off for nonmigratory butterflies but instead suggest a physiological trade-off between high metabolic performance and reduced maximal life span. High metabolic performance may explain high rates of dispersal and oviposition in early life. 5. In fragmented landscapes, an ecological trade-off exists between being more dispersive and hence spending more time in the landscape matrix vs. having more time for reproduction in the habitat. We estimate with a dispersal model parameterized for the Glanville fritillary that the lifetime egg production is 4% smaller on average in the more dispersive butterflies in a representative landscape, with much variation depending on landscape structure in the neighbourhood of the natal patch, from--26 to 45% in the landscape analysed in this paper.

Natural Enemies and the Evolution of Resistance to Transgenic Insecticidal Crops by Pest Insects: The Role of Egg Mortality

We explore the influence of egg mortality dynamics on the rate at which target pests evolve resistance to high-dose transgenic insecticidal crops. We develop a two-patch deterministic population genetic model in which pests can develop in either toxic or nontoxic (refuge) fields, and their eggs are subject to varying levels and forms of egg mortality. The three standard forms of egg mortality are studied: density independence (DI), positive density dependence (PDD), and inverse density dependence (IDD). Resistance is modeled as a single locus with a fully recessive allele that confers complete resistance with no fitness cost. Insect movement and oviposition is modeled as follows: males move panmictically before mating and females may either stay in their natal patch to oviposit or move after mating before oviposition. While our simulations show that both the magnitude and form of egg mortality can influence the rate of resistance evolution, important caveats apply. Higher levels of DI egg mortality can lead to substantial delays in resistance evolution, but this effect is dependent on the presence of intraspecific competition among larvae. The rate of resistance evolution is affected by the form of density dependence (DI versus PDD versus IDD), but these effects are dependent on at least some females ovipositing in their natal field. If this condition is met, the rate of resistance evolution is fastest when eggs are subject to PDD mortality and slowest when eggs are subject to IDD egg mortality. DI egg mortality produces intermediate rates of resistance evolution.

Interactions between maternal effects and dispersal

Maternal effects and spatial processes are rightly recognised as being of great importance in the life histories of a number of different organisms. Previously, little effort has been made for investigating the potential for changes to arise from interactions between these two crucial processes. Here I tested the outcome of introducing a maternal effect on a population of dispersing organisms in different ways, under both spatially homogeneous and heterogeneous conditions, through novel modification of a well‐known population model. First, I proposed that parents contribute a proportion of offspring based on their own developmental experience. This is applied to dispersing organisms as well as those that remain in their natal patch to breed. I go on to ask how the density of dispersing offspring might be influenced by a maternal effect. What happens if parents that developed under relatively good (or bad) conditions compared to current conditions invest in dispersing offspring? This relaxes the assumption that populations have perfect knowledge of the local carrying capacity. The results highlight an interesting range of outcomes. By introducing a maternal effect into the potential future reproductive output otherwise highly unstable population dynamics were simplified (but not stabilised). If maternal effects affect dispersal decisions, it is possible for regular cycles with a period consistent with many examples from ecological time‐series’ to arise. These results are in agreement with previous studies, however, by investigating the effects in different aspects of life history, it is possible to predict the direction of the impact on population dynamics. This work also highlights the potential for regular cycles in population dynamics to arise due to maternal effects, in agreement with recent field studies.

ESTIMATING POSTNATAL DISPERSAL: TRACKING THE UNSEEN DISPERSERS

Postnatal dispersal, defined as dispersal from the natal patch soon after reaching maturity, is common in a variety of insect species. Postnatal dispersal may increase connections among subpopulations and, thus, significantly affect local population dynamics in metapopulation and source–sink contexts. Yet most mark–recapture studies of insects miss the postnatal dispersal phase because it comes soon after eclosion. Cephaloleia fenestrata (Chrysomelidae) is a specialist herbivore of the rolled leaves of Pleiostachya pruinosa (Marantaceae, common name “prayer plant”). Some P. pruinosa patches were flooded prior to the onset of this study, extirpating the majority of C. fenestrata in the flood zone and providing a natural experiment and an opportunity to observe recolonization dynamics. We built a spatially explicit population projection matrix model to simulate spatial population dynamics of C. fenestrata. The model was a poor fit to the empirical data. Adding postnatal dispersal to the model resulted in a much better model fit. This study suggests that 81% of C. fenestrata disperse from the natal site soon after eclosion. Model simulations indicated that postnatal dispersal increases the mean population size in the flood zone more than six-fold and the standard deviation more than three-fold. Herein we present an implicit method of estimating postnatal dispersal when marking animals before the postnatal dispersal phase is problematic.

THE TRANSITION TO SOCIAL INBRED MATING SYSTEMS IN SPIDERS: ROLE OF INBREEDING TOLERANCE IN A SUBSOCIAL PREDECESSOR

The social spiders are unusual among cooperatively breeding animals in being highly inbred. In contrast, most other social organisms are outbred owing to inbreeding avoidance mechanisms. The social spiders appear to originate from solitary subsocial ancestors, implying a transition from outbreeding to inbreeding mating systems. Such a transition may be constrained by inbreeding avoidance tactics or fitness loss due to inbreeding depression. We examined whether the mating system of a subsocial spider, in a genus with three social congeners, is likely to facilitate or hinder the transition to inbreeding social systems. Populations of subsocial Stegodyphus lineatus are substructured and spiders occur in patches, which may consist of kin groups. We investigated whether male mating dispersal prevents matings within kin groups in natural populations. Approximately half of the marked males that were recovered made short moves (< 5m) and mated within their natal patch. This potential for inbreeding was counterbalanced by a relatively high proportion of immigrant males. In mating experiments, we tested whether inbreeding actually results in lower offspring fitness. Two levels of inbreeding were tested: full sibling versus non-sib matings and matings of individuals within and between naturally occurring patches of spiders. Neither full siblings nor patch mates were discriminated against as mates. Sibling matings had no effect on direct fitness traits such as fecundity, hatching success, time to hatching and survival of the offspring, but negatively affected offspring growth rates and adult body size of both males and females. Neither direct nor indirect fitness measures differed significantly between within patch and between-patch pairs. We tested the relatedness between patch mates and nonpatch mates using DNA fingerprinting (TE-AFLP). Kinship explained 30% of the genetic variation among patches, confirming that patches are often composed of kin. Overall, we found limited male dispersal, lack of kin discrimination, and tolerance to low levels of inbreeding. These results suggest a history of inbreeding which may reduce the frequency of deleterious recessive alleles in the population and promote the evolution of inbreeding tolerance. It is likely that the lack of inbreeding avoidance in subsocial predecessors has facilitated the transition to regular inbreeding social systems.

THE TRANSITION TO SOCIAL INBRED MATING SYSTEMS IN SPIDERS: ROLE OF INBREEDING TOLERANCE IN A SUBSOCIAL PREDECESSOR

The social spiders are unusual among cooperatively breeding animals in being highly inbred. In contrast, most other social organisms are outbred owing to inbreeding avoidance mechanisms. The social spiders appear to originate from solitary subsocial ancestors, implying a transition from outbreeding to inbreeding mating systems. Such a transition may be constrained by inbreeding avoidance tactics or fitness loss due to inbreeding depression. We examined whether the mating system of a subsocial spider, in a genus with three social congeners, is likely to facilitate or hinder the transition to inbreeding social systems. Populations of subsocial Stegodyphus lineatus are substructured and spiders occur in patches, which may consist of kin groups. We investigated whether male mating dispersal prevents matings within kin groups in natural populations. Approximately half of the marked males that were recovered made short moves (< 5m) and mated within their natal patch. This potential for inbreeding was counterbalanced by a relatively high proportion of immigrant males. In mating experiments, we tested whether inbreeding actually results in lower offspring fitness. Two levels of inbreeding were tested: full sibling versus non-sib matings and matings of individuals within and between naturally occurring patches of spiders. Neither full siblings nor patch mates were discriminated against as mates. Sibling matings had no effect on direct fitness traits such as fecundity, hatching success, time to hatching and survival of the offspring, but negatively affected offspring growth rates and adult body size of both males and females. Neither direct nor indirect fitness measures differed significantly between within patch and between-patch pairs. We tested the relatedness between patch mates and nonpatch mates using DNA fingerprinting (TE-AFLP). Kinship explained 30% of the genetic variation among patches, confirming that patches are often composed of kin. Overall, we found limited male dispersal, lack of kin discrimination, and tolerance to low levels of inbreeding. These results suggest a history of inbreeding which may reduce the frequency of deleterious recessive alleles in the population and promote the evolution of inbreeding tolerance. It is likely that the lack of inbreeding avoidance in subsocial predecessors has facilitated the transition to regular inbreeding social systems.

Mating system and sex allocation in the gregarious parasitoid Cotesia glomerata

The gregarious parasitoid Cotesia glomerata (L.) is often presumed to possess the characteristic attributes of a species that manifests local mate competition (LMC), as it commonly produces female-biased broods. However, our field surveys of sex ratio and laboratory observations of adult behaviour showed that this species is subject to partial local mate competition caused by natal dispersal. On average, 30% of males left their natal patch before mating, with the proportion of dispersing males increasing with an increase in the patch's sex ratio (i.e. proportion of males). Over 50% of females left their natal patch before mating, and only 27.5% of females mated with males emerging from the same natal patch. Although females showed no preference between males that were and were not their siblings, broods from females that mated with siblings had a significantly higher mean brood sex ratio (0.56) than broods from females that mated with nonsiblings (0.39). Furthermore, brood sex ratios increased as inbreeding was intensified over four generations. A field population of this wasp had a mean brood sex ratio of 0.35 over 3 years, which conformed well to the evolutionarily stable strategy sex ratio (r=0.34) predicted by Taylor's partial sibmating model for haplodiploid species. These results suggest that the sex allocation strategy of C.glomerata is based on both partial local mate competition in males and inbreeding avoidance in females. In turn, this mating system plays a role in the evolution of natal dispersal behaviour in this species.Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Extremely female-biased sex ratio and lethal male-male combat in a parasitoid wasp, Melittobia australica (Eulophidae)

Melittobiaaustralica (Hymenoptera: Eulophidae) is a gregarious ectoparasitoid of the prepupae and pupae of solitary wasps and bees. The males never disperse from their natal patch, and mating takes place only on the host from which they emerged. We measured the offspring sex ratio of M. australica with differing foundress numbers and examined combat between emerged males. The offspring sex ratios were extremely female biased and almost independent of foundress number in all cases. The population of M. australica used in the experiment was infected with the cytoplasmically inherited symbiotic bacterium Wolbachia. However, although Wolbachia is a potential sex-ratio distorter, noninfected individuals showed the same sex ratio patterns as the Wolbachia-infected individuals. An arena experiment showed that younger males were almost always killed by older males that had eclosed earlier. These results suggested that lethal male–male combat is an additional factor distorting the sex ratio toward a more female-biased sex ratio. This provides a new perspective on current local mate competition models.

Brood-size and sex-ratio variation in field populations of three species of solitary aphid parasitoids (Hymenoptera: Braconidae, Aphidiinae).

Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.

Individual and population level determinants of immigration success on local habitat patches: an experimental approach

The effect of population density on immigration success of young root voles was studied in a factorial experiment where immigrants of three different types were translocated from donor populations to recipient habitat patches with experimentally manipulated population sizes. The different types of immigrants were: (1) residents, animals that had remained in their natal patch; (2) colonists, animals that already had successfully dispersed and settled in a new patch; and (3) transients, animals roaming around in a non‐habitable matrix area of the donor population. Generally, we found evidence for a negative density‐dependent immigration success in terms of survival, rate of sexual maturation and body growth rate. With respect to the survival rate, the degree of density dependence differed among the three immigrant types. Our findings are discussed with respect to the degree of rescue effect immigrants may have on a recipient population depending on population density, and also its relevance concerning translocation of individuals for conserving endangered populations.

Boundary effects on dispersal between habitat patches by forest birds (Parus major, P. caeruleus)

The behaviour of individuals in response to patch boundaries is acrucial element in many dispersal models. Diffusion-based models of dispersalpredict that both the likelihood and direction of dispersal from a habitatpatchare influenced by the starting position of a disperser in relation to the patchboundary. An alternative view is that the decision to disperse between patchesis uncoupled from movements within the patch of departure. The latter situationis most likely in the case of relatively mobile animals living in small patcheswith strongly reflecting boundaries. I tested the relationship betweenproximityto a boundary and natal dispersal in Great and Blue Tits born in relativelysmall (7-11 ha) forest patches with high population density innorthern Belgium. Birdsthat were born closer to the forest edge were not more likely to be recruitedoutside than inside the natal patch. However, Great Tits showed a significanttendency to emigrate in the direction of the nearest patch border. No sucheffect was found in Blue Tits. A possible explanation is that in Great Tits thedirection of dispersal, but not the decision to emigrate, is influenced by aprocess of familiarization with the area around the natal territory, includingareas across the patch border.