Landscape structure, habitat fragmentation, and the ecology of insects

My office doesn't have a window. If it did, and I could look across campus, my view would be a mosaic of buildings, grass, trees, parking areas, a couple of small streams, and even a lake. There are oak trees with diverse Cynipid assemblages, a myriad of arthropods in the litter beneath the ornamental plantings, and abundant pollinators on the Hypericum that lines the path from my office to the library. There are also diesel-belching buses and some insane planner's nightmare of one-way roads and dead-end footpaths. As a microcosm of the world at large, our campus is a study in fragmentation. Areas of natural, semi-natural, and grossly artificial vegetation are divided and subdivided into patches of various sizes in various degrees of isolation. Boundaries abound. A demented four-year-old with Lego® could not create a three-dimensional architecture this complex. Yet within this fragmented landscape, insects persist in some abundance and diversity. As insect ecologists, we're obligated to understand the processes that influence the abundance, richness and diversity of insects in fragmented landscapes. As pest managers, we need to know how the architecture of landscapes influences pest population dynamics and their interactions with natural enemies and agents of control. As conservation biologists, we must develop strategies to maintain focal insect species, faunal diversity and the trophic interactions that drive key ecosystem processes. Whatever our entomological interests, landscape structure and habitat fragmentation have an influence on the ecology of the insects that we study. If space is the final frontier of ecological theory (Kareiva, 1994), then fragmentation of space is the warp engine that drives research in spatial ecology. Critical issues such as local extinction of bird populations (Renjifo, 2001; Robinson et al., 2001; Sekercioglu et al., 2002), declining densities of small mammals and herpetofauna (Maisonneuve Cane, 2001) and the management of landscapes for insect biodiversity (Ehrlich Samways, 1992; Thomas, 1995; Di Giulio et al., 2001) catch the imaginations of scientists and the public alike. There is a growing sense of urgency and awareness that understanding the role of habitat fragmentation in ecological processes is a prerequisite for sound science, policy and management. Whether the goal is to predict the presence and abundance of species in fragmented landscapes (Cowley et al., 2000), or to conserve species interactions and trophic structure (Fisher, 1998), the behavioural and dynamical responses of insects to landscape structure are key. The features of landscapes that influence the population and community ecology of species, including insects, are well known. The ratio of habitat edge to interior (Chen et al., 1995; Radeloff et al., 2000), the isolation of habitat fragments (Collinge, 2000), patch area (Kruess Kuussaari et al., 2000; Hanski Varchola Onstad et al., 2001), genetic change in insect populations (Singer Ronce With et al., 2002) into their understanding of the spatial ecology of insects. Even with a growing awareness of the features of landscapes that contribute to variation in insect populations and communities, there remain clear gaps in our understanding of the links between landscape change and insect dynamics. Below, I describe some of the recent work that has explored these links and suggest some approaches for future work. Much can be learned about the impacts of fragmentation on insect populations by studying the movement of predators and prey among patch types in complex landscapes (Holland Martin et al., 2001; With et al., 2002). Simple models assume that rates of movement are independent of landscape structure, typically assume constant movement rates whatever the landscape mosaic in question (Goodwin Zabel Braman et al., 2000; Collinge, 2000; Di Giulio et al., 2001). Studying the movement of particular species among isolated patches is important, but a critical question remains; can fine-scale movement behaviours be used to predict broad scale patterns of distribution on heterogeneous landscapes? According to Jonsen Steffan-Dewenter Tscharntke et al., 1998). In for management that focus upon diversity appear to have a than those related to the arrangement of patches on the landscape, and the of insect pollinators may less to fragmentation than some other insect 2001). remain that fragmentation may particular species of In forest, for example, fragmentation by bees but by bees 1999). The of population is generally related to such as population size and However, patch quality may also be an important of patch colonization and et al., 2001). of movement and increasing in local habitat patches has been as a cause of in some et al., 2001). There is even that fragmentation can for variation in dispersal within For example, and of the forest damselfly vary between populations streams and those streams through 2000), and the of links among insect physiology, rates of movement and landscape structure. There are it would be to develop some and of both the processes of landscape change and the that The of landscape is actually more than the simple processes of fragmentation or change in et al., 1996; 1998; et al., 2000; & 2001). We need to know the effects that losses in insect have on higher trophic levels such as insectivorous et al., 2001). We need to the effects of changes such as et al., or species et al., 2001) with habitat fragmentation. by the weed into fragmented forests in has a in and beetle diversity in the forest et al., 2001), because of of the natural forest This is a clear of fragmentation and the of trophic dynamics within within complex and with other and the The true effects of landscape change and fragmentation upon the ecology of insects will be by the ecological complexity that the world within which insects I the for our work on habitat and the population dynamics of insect herbivores.