Do all inter-patch movements represent dispersal? A mixed kernel study of butterfly mobility in fragmented landscapes.

1. In times of ongoing habitat fragmentation, the persistence of many species is determined by their dispersal abilities. Consequently, understanding the rules underlying movement between habitat patches is a key issue in conservation ecology. 2. We have analysed mark-release-recapture (MRR) data on inter-patches movements of the Dusky Large Blue butterfly Maculinea nausithous in a fragmented landscape in northern Bavaria, Germany. The aim of the analysis was to quantify distance dependence of dispersal as well as to evaluate the effect of target patch area on immigration probability. For statistical evaluation, we apply a 'reduced version' of the virtual migration model (VM), only fitting parameters for dispersal distance and immigration. In contrast to other analyses, we fit a mixed dispersal kernel to the MRR data. 3. A large fraction of recaptures happened in other habitat patches than those where individuals were initially caught. Further, we found significant evidence for the presence of a mixed dispersal kernel. The results indicate that individuals follow different strategies in their movements. Most movements are performed over small distances, nonetheless involving travelling between nearby habitat patches (median distance c. 480 m). A small fraction (c. 0·025) of the population has a tendency to move over larger distances (median distance c. 3800 m). Further, immigration was positively affected by patch area (I∼A(ζ) ), with the scaling parameter ζ = 0·5. 4. Our findings should help to resolve the long-lasting dispute over the suitability of the negative exponential function vs. inverse-power one for modelling dispersal. Previous studies on various organisms found that the former typically gives better overall fit to empirical distance distributions, but that the latter better represents long-distance movement probabilities. As long-distance movements are more important for landscape-level effects and thus, e.g. for conservation-oriented analyses like PVAs, fitting inverse-power kernels has often been preferred. 5. We conclude that the above discrepancy may simply stem from the fact that recorded inter-patch movements are an outcome of two different processes: daily routine movements and genuine dispersal. Consequently, applying mixed dispersal kernels to disentangle the two processes is recommended.

Hovestadt T ，Binzenhöfer B ，Nowicki P ，Settele J ... -  《-》

Modelling population redistribution in a leaf beetle: an evaluation of alternative dispersal functions.

1. Dispersal is a fundamental ecological process, so spatial models require realistic dispersal kernels. We compare five different forms for the dispersal kernel of the tansy beetle Chrysolina graminis moving between patches of its host-plant (tansy Tanacetum vulgare) in a riparian landscape. 2. Multi-patch mark-recapture data were collected every 2 weeks over 2 years within a large network of patches and from 2226 beetles. Dispersal was common (28.4% of 880 recaptures after a fortnight) and was more likely over longer intervals, out of small patches, for females and during flooding. Interpatch movement rates did not differ between years and exhibited no density dependence. Dispersal distances were similar for males and females, in both years and over all intervals, with a median dispersal distance of just 9.8 m, although a maximum of 856 m was recorded. 3. A model of dispersal, where patches competed for dispersers based on their size and distance from the beetle's source patch (scaled by the dispersal kernel) was fitted to the field data with a maximum likelihood procedure and each of five alternative kernels. The best fitting had relatively extended tails of long-distance dispersal, while Gaussian and negative exponential kernels performed worst. 4. The model suggests that females disperse more commonly than males and that both are strongly attracted to large patches but do not differ between years, which are consistent with the empirical results. Model-predicted emigration and immigration rates and dispersal phenologies match those observed, suggesting that the model captured the major drivers of tansy beetle dispersal. 5. Although negative exponential and Gaussian kernels are widely used for their simplicity, we suggest that these should not be the models of automatic choice, and that fat-tailed kernels with relatively higher proportions of long-distance dispersal may be more realistic.

Chapman DS ，Dytham C ，Oxford GS 《JOURNAL OF ANIMAL ECOLOGY》

Anthropogenic landscape change promotes asymmetric dispersal and limits regional patch occupancy in a spatially structured bird population.

1. Local extinctions in habitat patches and asymmetric dispersal between patches are key processes structuring animal populations in heterogeneous environments. Effective landscape conservation requires an understanding of how habitat loss and fragmentation influence demographic processes within populations and movement between populations. 2. We used patch occupancy surveys and molecular data for a rainforest bird, the logrunner (Orthonyx temminckii), to determine (i) the effects of landscape change and patch structure on local extinction; (ii) the asymmetry of emigration and immigration rates; (iii) the relative influence of local and between-population landscapes on asymmetric emigration and immigration; and (iv) the relative contributions of habitat loss and habitat fragmentation to asymmetric emigration and immigration. 3. Whether or not a patch was occupied by logrunners was primarily determined by the isolation of that patch. After controlling for patch isolation, patch occupancy declined in landscapes experiencing high levels of rainforest loss over the last 100 years. Habitat loss and fragmentation over the last century was more important than the current pattern of patch isolation alone, which suggested that immigration from neighbouring patches was unable to prevent local extinction in highly modified landscapes. 4. We discovered that dispersal between logrunner populations is highly asymmetric. Emigration rates were 39% lower when local landscapes were fragmented, but emigration was not limited by the structure of the between-population landscapes. In contrast, immigration was 37% greater when local landscapes were fragmented and was lower when the between-population landscapes were fragmented. Rainforest fragmentation influenced asymmetric dispersal to a greater extent than did rainforest loss, and a 60% reduction in mean patch area was capable of switching a population from being a net exporter to a net importer of dispersing logrunners. 5. The synergistic effects of landscape change on species occurrence and asymmetric dispersal have important implications for conservation. Conservation measures that maintain large patch sizes in the landscape may promote asymmetric dispersal from intact to fragmented landscapes and allow rainforest bird populations to persist in fragmented and degraded landscapes. These sink populations could form the kernel of source populations given sufficient habitat restoration. However, the success of this rescue effect will depend on the quality of the between-population landscapes.

Pavlacky DC Jr ，Possingham HP ，Lowe AJ ，Prentis PJ ，Green DJ ，Goldizen AW ... -  《-》

Combining measures of dispersal to identify conservation strategies in fragmented landscapes.

Understanding the way in which habitat fragmentation disrupts animal dispersal is key to identifying effective and efficient conservation strategies. To differentiate the potential effectiveness of 2 frequently used strategies for increasing the connectivity of populations in fragmented landscapes-corridors and stepping stones-we combined 3 complimentary methods: behavioral studies at habitat edges, mark-recapture, and genetic analyses. Each of these methods addresses different steps in the dispersal process that a single intensive study could not address. We applied the 3 methods to the case study of Atrytonopsis new species 1, a rare butterfly endemic to a partially urbanized stretch of barrier islands in North Carolina (U.S.A.). Results of behavioral analyses showed the butterfly flew into urban and forested areas, but not over open beach; mark-recapture showed that the butterfly dispersed successfully through short stretches of urban areas (<500 m); and genetic studies showed that longer stretches of forest (>5 km) were a dispersal barrier, but shorter stretches of urban areas (≤5 km) were not. Although results from all 3 methods indicated natural features in the landscape, not urbanization, were barriers to dispersal, when we combined the results we could determine where barriers might arise: forests restricted dispersal for the butterfly only when there were long stretches with no habitat. Therefore, urban areas have the potential to become a dispersal barrier if their extent increases, a finding that may have gone unnoticed if we had used a single approach. Protection of stepping stones should be sufficient to maintain connectivity for Atrytonopsis new species 1 at current levels of urbanization. Our research highlights how the use of complementary approaches for studying animal dispersal in fragmented landscapes can help identify conservation strategies.

Leidner AK ，Haddad NM 《-》

Sex-biased topography effects on butterfly dispersal.

Metapopulation persistence in fragmented landscapes is assured by dispersal of individuals between local populations. In this scenario the landscape topography, although usually neglected, may have an important role in shaping dispersal throughout the matrix separating habitat patches. Due to inter-sexual differences in optimal reproductive strategies, i.e., males maximizing the number of mating opportunities and females maximizing the offspring survival chances, topography-related constraints are expected to exert a different effect on male and female dispersal behaviour. We tested sex-biased topography effects on butterfly dispersal, with the following hypotheses: (1) females are constrained by topography in their movements and avoid hill crossing; (2) male dispersal is primarily driven by two-dimensional spatial structure of the habitat patches (i.e. their geometric locations and sizes) and little influenced by topography. Following intensive mark-recapture surveys of Maculinea (= Phengaris) nausithous and M. teleius within a landscape characterised by an alternation of hills and valleys, we investigated sex-specific patterns in their inter-patch movement probabilities derived with a multi-state recapture model. In particular, we (1) analysed the fit of dispersal kernels based on Euclidean (= straight line) vs. topography-based (= through valley) distances; (2) compared movement probabilities for the pairs of patches separated or not by topographic barriers; and (3) tested the differences in the downward and upward movement probabilities within the pairs of patches. Euclidean distances between patches proved to be a substantially stronger predictor of inter-patch movement probabilities in males, while inter-patch distances measured along valleys performed much better for females, indicating that the latter tend to predominantly follow valleys when dispersing. In addition, there were significantly lower probabilities of movements across hills in females, but not in males. Both above results provide support for the hypothesis that topography restricts dispersal in females, but not in males. Since the two sexes contribute differently to metapopulation functioning, i.e., only female dispersal can result in successful (re)colonisations of vacant patches, the topography effects exerted on females should be considered with particular attention when landscape management and conservation actions are designed in order to maintain the functional connectivity of metapopulation systems.

Plazio E ，Bubová T ，Vrabec V ，Nowicki P ... -  《-》