Diffusion, which approximates a correlated random walk, has been used by ecologists to describe movement, and forms the basis for many theoretical models. However, it is often criticized as too simple a model to describe animal movement in real populations. We test a key prediction of diffusion models, namely, that animals should be more abundant in land cover classes through which they move more slowly. This relationship between density and diffusion has rarely been tested across multiple species within a given landscape. We estimated diffusion rates and corresponding densities of 25 Israeli butterfly species from flight path data and visual surveys. The data were collected across 19 sites in heterogeneous landscapes with four land cover classes: semi-natural habitat, olive groves, wheat fields and field margins. As expected from theory, species tended to have higher densities in land cover classes through which they moved more slowly and lower densities in land cover classes through which they moved more quickly. Two components of movement (move length and turning angle) were not associated with density, nor was expected net squared displacement. Move time, however, was associated with density, and animals spent more time per move step in areas with higher density. The broad association we document between movement behaviour and density suggests that diffusion is a good first approximation of movement in butterflies. Moreover, our analyses demonstrate that dispersal is not a species-invariant trait, but rather one that depends on landscape context. Thus, land cover classes with high diffusion rates are likely to have low densities and be effective conduits for movement.
In systems where indirect effects are mediated by traits (e.g., behavior) rather than density, direct and indirect effects may be measured in different biological currencies, making it difficult to quantify their relative contributions to a net interaction. For instance, the hydroid Hydractinia symbiolongicarpus, which colonizes shells occupied by the hermit crab Pagurus longicarpus, affects its host directly by reducing its fecundity and indirectly by modifying the behavior of its predators. In this study, I construct population projection matrices for P. longicarpus, which were housed with different assemblages of hydroids and predators. By defining direct and indirect effects as combinations of log‐transformed λ's from these matrices, I translate direct effects on fecundity and indirect effects on survival into a common currency (population growth rate), allowing me to assess the relative strength of these effects. In the presence of a fish predator, strong positive indirect effects countered negative direct effects, producing a net mutualism. In the presence of a crab predator, indirect effects were equivalent in size and magnitude to direct effects, resulting in a net parasitism. These results provide insight into the mechanisms underlying context‐dependency and help to explain the geographical and temporal variation observed in outcomes of symbiotic interactions.
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