Metapopulation Landscape Research Articles

Reliability of migration between habitat patches with heterogeneous ecological corridors

Natural and designed ecological corridors are key elements for the survival of a species, as they allow the species to avoid local extinction by migrating to more suitable habitat patches. This paper studies various reliability metrics for the process of migration in a metapopulation landscape network from a critical habitat patch to destination habitat patches via perfect stepping stones and imperfect (deletable) corridors. The work presented herein generalizes earlier work on the application of reliability theory in ecology by allowing corridors to be heterogeneous (of non-identical unreliabilities). The paper is a tutorial exposition of modern reliability techniques, which formulate a problem in the Boolean domain, manipulate formulas to achieve disjointness of logically added subexpressions and retain statistical independence of logically multiplied ones, and finally reach a probability-ready expression that is directly transformed back to the probability domain. Several metrics are covered including system unreliability, life expectancy (MTTF), and component importance measures. An interesting finding is that the life expectancy of a classical landscape network is more than double that of a single corridor. Extensions to quantification of uncertainty in the above metrics and to evaluation of more sophisticated metrics of landscape connectivity are also pointed out.

A reliability-theory approach to corridor design

Natural habitats are small islands in the sea of human environment. Area loss and fragmentation affect seriously the survival of species, for small, isolated populations are exposed to several risks. As the area of suitable habitats decreases, one way to escape local extinction is to migrate between habitats through ecological corridors, and utilize the whole metapopulation landscape. Thus, natural and designed corridors can be key elements for survival. Here, we present a method to study corridor pattern from a reliability-theory viewpoint. We analyze the probability of successful migration in a metapopulation landscape network of habitats, stepping stones and corridors. We examine the situation when individuals of a local population must migrate from a disturbed, critical habitat to others. In the landscape graph, points represent habitats and stepping stones, while edges represent corridors. If corridors can be destroyed, migration probability depends on the pattern of permeable corridors. Engineered corridors can enhance the reliability of migration, depending on their position in the network. We present some general rules for designing reliable landscape patterns (e.g. ‘necklace’ arrangement is less reliable for migration than ‘loop’). Then, we illustrate our viewpoint by presenting two hypothetical landscape networks and comparing the possibilities for designing reliable corridor topologies by creating one engineered corridor. Further, we determine the preferred topology for the engineered corridor. Our hope is that this reliability-theory analysis will stimulate further development of the method and in the fieldwork.

Hops as a metapopulation landscape for tetranychid-phytoseiid interactions : perspectives of intra- and interplant dispersal

Population densities, distributions and dispersal of Neoseiulus fallacis (Garman) and Tetranychus urticae (Koch) on individual hop plants, Humulus lupulus L. were studied for attributes of metapopulations such as empty patches, asynchrony of subpopulations, extinction of subpopulations, and dispersal of predators and prey among patches. Occupancy of hop leaves by predators or prey was stable over a season with 69–75% of leaves having neither predators nor prey, 4–15% with prey mites only, 9–17% with both predators and prey mites and 6–10% with predaceous mites only. Stability of occupancy classes through time indicated that inherently unstable predator and prey subpopulations developed asynchronously. Flagged hop leaves showed the existence of many empty individual leaves, colonization of some by prey, then by predators, then extinction of both, and then recolonization by spider mites. This illustrated the existence of empty patches, extinction of subpopulations, and dispersal of predators and prey to empty patches. This differed from spider mites and phytoseiid predators on apple foliage where there was a progression of occupancy status, indicating synchronous development of subpopulations on individual plants. Studies of predator and prey dispersal between hop plants showed that removal of basal leaves to 1.5 m high, a common agronomic practice, greatly limited dispersal of the predaceous mites but not the spider mites. Retaining basal leaves facilitated interplant movement of predators and improved the extent and timing of biological control. Through management, N. fallacis dispersal may be adjusted so that the entire hop planting becomes a metapopulation landscape, leading to greater stability and persistence of predator–prey within a season.