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? Exotics travel along edges

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