Foraging behavior and patch size distribution jointly determine population dynamics in fragmented landscapes

Increased fragmentation caused by habitat loss presents a major threat to the persistence of animal populations. Whereas the negative effects of habitat loss on biodiversity are well-known, the effects of fragmentation per se on population dynamics and ecosystem stability remain less understood. How fragmentation affects populations is strongly determined by the rate at which individuals can move between separated habitat patches within the fragmented landscape. Here, we use a computational, spatially explicit predator-prey model to investigate how the interplay between fragmentation per se and optimal foraging behavior influences predator-prey interactions and, ultimately, ecosystem stability. We study cases where prey occupies isolated habitat patches and let predators disperse between patches following a Lévy random walk. Our results show that both the Lévy exponent and the degree of fragmentation strongly determine coexistence probabilities. Brownian and ballistic predators go extinct in highly fragmented landscapes and only scale-free predators can coexist with prey. Furthermore, our results reveal that predation causes irreversible loss of prey habitat in highly fragmented landscapes due to the overexploitation of smaller patches. Moreover, our results show that predator movement can reduce, but not prevent not minimize, the amount of irreversibly lost habitat. Our results suggest that incorporating optimal foraging theory into population- and landscape ecology models is crucial to assess the impact of fragmentation on biodiversity and ecosystem stability.

Performance of Multicriteria Evaluation and Heuristic Methods in the Delineation of Green Infrastructure in Areas with Fragmented Landscapes

The EU Commission has established Green infrastructure as one of the tools to preserve biodiversity and grant the provision of ecosystem services that reduce impacts on natural values like those produced by climate change. Therefore, a European green infrastructure strategy has been created that commit member states to incorporate green infrastructure to their territorial planning. Yet, methodologies to delimit green infrastructure so as to facilitate its inclusion in territorial plans are still scarce. The available methods are mainly based in multicriteria evaluation and focus on zoning general green infrastructure areas taking into account the provision potential of just a few ecosystem services. Considering the provision of a wide range of ecosystem services to delimit green infrastructure elements is key to grant their multifunctionality and increase their efficiency mitigating climate change impacts in natural values and human population. However, the lack of data or the high cost to accurately map ecosystem services provision potential, leads most of the time to infer it from land cover data. This creates problems when using these maps to delimit green infrastructure in areas with fragmented landscapes; since identified green infrastructure areas may be irregular and scattered. There are heuristic methods like simulated annealing that have been used to identify ecosystem services hot spots which consider the regularity and size of the identified patches. These methods can be used to delimit green infrastructure in fragmented landscapes finding a balance between the regularity of the areas and their potential to provide multiple ecosystem services. In the current work, a comparison has been made between the performance of simulated annealing and current multicriteria evaluation methods to delimit green infrastructure multifunctional buffer zones in an area of north-western Spain with a very fragmented landscape. Results have shown that simulated annealing delimits more regular multifunctional buffer areas but with a less average potential for providing multiple ecosystem services. The conclusions of the paper indicate that simulated annealing is good produces more regular multifunctional areas but with a lower ESs provision potential. It was observed that in the case of ESs that were mapped considering factors at landscape scale, their provision potential did not vary too much between the multifunctional buffer areas delimited with each of the methods. This indicates that delineation methods may produce more regular GI elements if ESs provision potential is mapped considering the influence of biophysical factors at a wider landscape scale.

Host Plant Limitation of Butterflies in Highly Fragmented Landscapes

Insect herbivores can be limited by host plants in two ways: density‐dependent competition for food resources, or density‐independent search time limitation. Our understanding of density‐dependent host plant limitation is relatively well developed and well integrated into conservation plans for at‐risk insects. Search time limitation is much less well developed. Here, we explore both mechanisms using empirically‐based models of monarch butterfly population dynamics. These mechanisms differ fundamentally in their predictions: Resource competition leads to matching of herbivore densities to host plant densities, and visible competition via consumption of host plants. Search time limitation leads to changes in population growth rate that can cause herbivore numbers to decline when host plant densities are constant. Search time limitation also implies that host plants can limit herbivores, even when many individual plants are uneaten. For monarch butterflies, our calculations suggest that many parts of North America have host plant densities below the threshold for search time limitation, which contrasts with the typical assumption of resource competition. More generally, incorporating search time limitation into conservation plans is important for reframing our understanding of how host plants limit insect herbivores in highly fragmented landscapes.