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The Landscape Ecology of Invasive Spread • Question: How is spatial pattern expected to affect invasive spread? • Premise: Habitat loss and fragmentation leads to spread of invasives • Definition: Landscape ecology: not regional level but the study of spatial pattern of resources, habitat, etc. on ecological processes History • Reaction Diffusion Models – Theory by Skellam (1951) – Tested by Andow et al (1990)(animal) and Higgins and Richardson (1996 and others) (plants) – Parameters: population density, population growth rate and diffusion coefficient across a heterogeneous landscape From: Andow et al 1990 Integrodifference equation models • Parameters: population growth at each spatial point and the ease of movement between points (dispersal kernel). • Stage structured dispersal important. Long distance dispersal, though rare, determines spread • Problem: As in Reaction-diffusion, assumes the landscape is homogeneous Neubert and Caswell (2000): Traveling Invasion Wave Neutral Landscape Models • Theory: Invasive Spread occurs above a threshold limit of disturbance, which depends on the spatial pattern of the disturbance • Definition: Connectivity: The ability of organisms to move among patches • Implication: To reduce invasive spread, reduce the extent and connectivity of disturbances. A: RANDOM: Spread in a random landscape B: FRAGMENTED: Spread across a fragmented fractal landscape C: CLUMPED: Spread across a clumped fractal landscape with spatially autocorrelated disturbance A: An invasive species with poor dispersal ability able to move only to adjacent cells B: Invasive spread for a species with better dispersal. Low levels fo disurbance, fragmentation limits spread, but at intermediate levels, species able to spread farther by using fragments as stepping stones. C: Invasive species in fragmented fractal landscapes that vary with dispersal ability. Good dispersers less influenced by fragmentation Landscape transformation is the final stage of a terminal invasion Dispersal success declines as lacunarity thresholds, especially on clumped landscapes Bergelson et al 1993: Senecio disperse well when a landscape is fragmented but population growth rates higher when a landscape is not fragmented! What about landscape structure leads to invasive spread? 20 1 m2 10 10 m2 100 m2 0 Percent Exotics 30 Exotics travel along edges 0 50 100 150 Distance to Edge 200 250 Facilitation? Invasibility of Systems • • • • • Parasitism (e.g. Cowbirds) Enhanced Competition Population Sinks are more vulnerable Edge Effects Lower genetic diversity of native species Questions • Habitat fragmentation in order to stop invasives: Is this practical? • How can we manage for both spread of invasives as well as keeping the native populations healthy? • Which parameters are needed in a model predicting spread of invasives? • Dispersal vs. Demography: which is more important in invasive spread? Higgins and Richardson 1998