Hans Van Dyck

Costs of dispersal

Dispersal costs can be classified into energetic, time, risk and opportunity costs and may be levied directly or deferred during departure, transfer and settlement. They may equally be incurred during life stages before the actual dispersal event through investments in special morphologies. Because costs will eventually determine the performance of dispersing individuals and the evolution of dispersal, we here provide an extensive review on the different cost types that occur during dispersal in a wide array of organisms, ranging from micro‐organisms to plants, invertebrates and vertebrates. In general, costs of transfer have been more widely documented in actively dispersing organisms, in contrast to a greater focus on costs during departure and settlement in plants and animals with a passive transfer phase. Costs related to the development of specific dispersal attributes appear to be much more prominent than previously accepted. Because costs induce trade‐offs, they give rise to covariation between dispersal and other life‐history traits at different scales of organismal organisation. The consequences of (i) the presence and magnitude of different costs during different phases of the dispersal process, and (ii) their internal organisation through covariation with other life‐history traits, are synthesised with respect to potential consequences for species conservation and the need for development of a new generation of spatial simulation models.

Landscape connectivity and animal behavior: functional grain as a key determinant for dispersal

Landscape connectivity can be viewed from two perspectives that could be considered as extremes of a gradient: functional connectivity (refers to how the behavior of a dispersing organism is affected by landscape structure and elements) and structural connectivity (depends on the spatial configuration of habitat patches in the landscape like vicinity or presence of barriers). Here we argue that dispersal behavior changes with landscape configuration stressing the evolutionary dimension that has often been ignored in landscape ecology. Our working hypothesis is that the functional grain of resource patches in the landscape is a crucial factor shaping individual movements, and therefore influencing landscape connectivity. Such changes are likely to occur on the short-term (some generations). We review empirical studies comparing dispersal behavior in landscapes differing in their fragmentation level, i.e., with variable resource grain. We show that behavioral variation affecting each of the three stages of the dispersal process (emigration, displacement or transfer in the matrix, and immigration) is indeed likely to occur according to selective pressures resulting from changes in the grain of the landscape (mortality or deferred costs). Accordingly, landscape connectivity results from the interaction between the dispersal behavior of individuals and the grain of each particular landscape. The existence of this interaction requires that connectivity estimates (being based on individual-based models, least cost distance algorithms, and structural connectivity metrics or even Euclidian distance) should be carefully evaluated for their applicability with respect to the required level of precision in species-specific and landscape information.

Butterfly diversity loss in Flanders (north Belgium): Europe's worst case scenario?

We illustrate the strong decrease in the number of butterfly species in Flanders (north Belgium) in the 20th century using data from a national butterfly mapping scheme. Nineteen of the 64 indigenous species went extinct and half of the remaining species are threatened at present. Flanders is shown to be the region with the highest number of extinct butterflies in Europe. More intensive agriculture practices and expansion of house and road building increased the extinction rate more than eightfold in the second half of the 20th century. The number of hot spots decreased considerably and the present-day hot spots are almost exclusively in the northeast of Flanders. Species with low dispersal capacities and species from oligotrophic habitats decreased significantly more than mobile species or species from eutrophic habitats. We discuss these results in a northwest European context and focus on concrete measures to preserve threatened butterfly populations in Flanders.

Habitats and Resources: The Need for a Resource-based Definition to Conserve Butterflies

Current definitions of habitat are closely allied to the concept of patch and matrix. This concept is, for instance, central to the prevailing metapopulation models of population dynamics. But, butterfly population dynamics, mobility and spatial structure can only properly be understood in the context of a resource-based definition of habitats. In criticising current definitions of habitat, we illustrate how habitat is best understood in terms of resource distributions. These transcend vegetation-based definitions of habitat and lie at the root of life history strategies, the vulnerability of butterflies to environmental changes and extinction, and govern conservation status. We emphasise the need for a resource-use database and demonstrate the shortcomings of current data for conserving butterflies; patch based definitions of habitats are inappropriate for some species and for others do not provide a universal panacea, inadequately explaining spatial occurrence when scaled over space and time. A resource-based habitat definition challenges the bipolar, patch vs. matrix view of landscape; the alternative is to view landscape as a continuum of overlapping resource distributions. We urge greater attention to the details of butterfly behaviour and resource use as the keys to understanding how landscape is exploited and therefore to successful conservation at the landscape scale.

Quantitative analysis of changes in movement behaviour within and outside habitat in a specialist butterfly

BackgroundDispersal between habitat patches is a key process in the functioning of (meta)populations. As distance between suitable habitats increases, the ongoing process of habitat fragmentation is expected to generate strong selection pressures on movement behaviour. This leads to an increase or decrease of dispersal according to its cost relative to landscape structure. To limit the cost of dispersal in an increasingly hostile matrix, we predict that organisms would adopt special dispersal behaviour between habitats, which are different from movements associated with resource searching in suitable habitats.ResultsHere we quantified the movement behaviour of the bog fritillary butterfly (Proclossiana eunomia) by (1) assessing perceptual range, the distance to which the habitat can be perceived, and (2) tracking and parameterizing movement behaviour within and outside habitat (parameters were move length and turning angles distributions). Results are three-fold. (1) Perceptual range was < 30 m. (2) Movements were significantly straighter in the matrix than within the habitat. (3) Correlated random walk adequately described movement behaviour for 70% of the observed movement paths within habitat and in the matrix.ConclusionThe perceptual range being lower than the distance between habitat patches in the study area, P. eunomia likely perceives these habitat networks as fragmented, and must locate suitable habitats while dispersing across the landscape matrix. Such a constraint means that dispersal entails costs, and that selection pressure should favour behaviours that limit these costs. Indeed, our finding that dispersal movements in the matrix are straighter than resource searching movements within habitat supports the prediction of simulation studies that adopting straight movements for dispersal reduces its costs in fragmented landscapes. Our results support the mounting evidence that dispersal in fragmented landscapes evolved towards the use of specific movement behaviour, different from explorative searching movements within habitat.

How is dispersal integrated in life histories: a quantitative analysis using butterflies

As dispersal plays a key role in gene flow among populations, its evolutionary dynamics under environmental changes is particularly important. The inter-dependency of dispersal with other life history traits may constrain dispersal evolution, and lead to the indirect selection of other traits as a by-product of this inter-dependency. Identifying the dispersals relationships to other life-history traits will help to better understand the evolutionary dynamics of dispersal, and the consequences for species persistence and ecosystem functioning under global changes. Dispersal may be linked to other life-history traits as their respective evolutionary dynamics may be inter-dependent, or, because they are mechanistically related to each other. We identify traits that are predicted to co-vary with dispersal, and investigated the correlations that may constrain dispersal using published information on butterflies. Our quantitative analysis revealed that (1) dispersal directly correlated with demographic traits, mostly fecundity, whereas phylogenetic relationships among species had a negligible influence on this pattern, (2) gene flow and individual movements are correlated with ecological specialisation and body size, respectively and (3) routine movements only affected short-distance dispersal. Together, these results provide important insights into evolutionary dynamics under global environmental changes, and are directly applicable to biodiversity conservation.

The evolution of movements and behaviour at boundaries in different landscapes: a common arena experiment with butterflies

As landscapes change, mobility patterns of species may alter. Different mechanistic scenarios may, however, lead to particular patterns. Here, we tested conflicting predictions from two hypotheses on butterfly movements in relation to habitat fragmentation. According to the resource distribution hypothesis, butterflies in more fragmented landscapes would have higher levels of mobility as resources are more scattered. However, these butterflies could have lower levels of mobility as they experience ‘hard’ habitat boundaries more frequently (i.e. higher crossing costs) compared with butterflies in landscapes with continuous habitat; i.e. the behaviour–at–boundaries hypothesis. We studied movements, habitat boundary crossing and habitat preference of laboratory–reared individuals of Pararge aegeria that originated from woodland and agricultural landscapes, by using an experimental landscape as a common environment (outdoor cages) to test the predictions, taking into account sexual differences and weather. Woodland butterflies covered longer distances, were more prone to cross open–shade boundaries, travelled more frequently between woodland parts of the cages and were more at flight than agricultural butterflies. Our results support the behaviour–at–boundaries hypothesis, with ‘softer’ boundaries for woodland landscapes. Because the butterflies were reared in a common environment, the observed behavioural differences rely on heritable variation between populations from woodland and agricultural landscapes.

The effect of wing colour on male behavioural strategies in the speckled wood butterfly

The behaviour of male speckled wood butterflies, Pararge aegeria L. (Satyrinae), was studied in relation to the following phenotypic traits: dorsal wing colour, submarginal wing spots, wing length and generation. Wing colour was of overriding importance. Pale males spent most of their time resting within a sunlit patch and engaged in short flights and intra- and interspecific interactions. Darker males flew more frequently between sunlit patches through the shaded forest. Thus males differing in phenotype did not have the same probability of showing perching, patrolling or an intermediate type of mate-locating behaviour. Darker and larger males were also more likely to leave the forest. There was very little effect of spot type, but a clear influence of generation on the behavioural variation. It is hypothesized that the observed variation is related to differences in thermoregulation between differently coloured individuals. Darker individuals are expected to have an increased warming rate which means that they can spend more time flying through the shady wood from one sunlit patch to another, while pale males will not suffer from overheating as soon as dark ones when settled on a sunlit patch. This in turn causes variation in their adaptedness to different behavioural strategies

Take-off performance under optimal and suboptimal thermal conditions in the butterfly Pararge aegeria.

Realized fitness in a fluctuating environment depends on the capacity of an ectothermic organism to function at different temperatures. Flying heliotherms like butterflies use flight for almost all activities like mate location, foraging and host plant searching and oviposition. Several studies tested the importance of ambient temperature, thermoregulation and butterfly activity. Here, we test the influence of variation in flight morphology in interaction with differences in body temperature on locomotor performance, which has not been thoroughly examined so far. Take-off free flight performance was tested at two different body temperatures in males and females of the speckled wood butterfly Pararge aegeria. We found that both males and females accelerated faster at the optimal body temperature compared to the suboptimal one. The multivariate analyses showed significant sex-specific contributions of flight morphology, body temperature treatment and feeding load to explain variation in acceleration performance. Female and male butterflies with a large relative thorax (i.e. flight muscle investment) mass and large, slender wings (i.e. aspect ratio) accelerated fast at optimal temperature. However, high aspect ratio individuals accelerated slowly at suboptimal temperature. Females of low body mass accelerated fast at optimal, but slowly at suboptimal body temperature. In males, there was an interaction effect between body and relative thorax mass: light males with high relative thorax mass had higher performance than males with a low relative thorax mass. In addition, relative distance to the centre of forewing area was positively related to acceleration at both temperatures in males. Males and females with higher feeding loads had lower levels of acceleration. Finally, males that were able to accelerate fast under both temperatures, had a highly significantly heavier relative thorax, lower body and abdomen mass. More generally, this study shows that the significance of butterfly flight morphology in terms of flight performance is at least partially dependent on body temperature.

Does habitat fragmentation affect temperature-related life-history traits? A laboratory test with a woodland butterfly

Habitat fragmentation may change local climatic conditions leading to altered selection regimes for life-history traits in small ectotherms, including several insects. We investigated temperature-related performance in terms of fitness among populations of the woodland butterfly Pararge aegeria (L.) originating from populations of a closed, continuous woodland landscape versus populations of an open, highly fragmented agricultural landscape in central Belgium. Female fecundity and longevity were evaluated in a temperature-gradient experiment. As predicted, females of woodland landscape origin reached higher maximum daily fecundity and lifetime number of eggs than did agricultural landscape females at low ambient temperatures, but this reversed at high ambient temperature. Egg weight decreased with temperature, and eggs of woodland butterflies were smaller. Contrary to what is generally assumed, remaining thorax mass was a better predictor of lifetime reproductive output than was abdomen mass. Since we used the F2 generation from wild-caught females reared under common garden conditions, the observed effects are likely to rely on intrinsic, heritable variation. Our results suggest that differential selection regimes associated with different landscapes intervene by intraspecific variation in the response of a butterfly to variation in ambient temperature, and may thus be helpful when making predictions of future impacts on how wild populations respond to environmental conditions under a global change scenario, with increasing temperatures and fragmented landscapes.