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From: Andow et al 1990
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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?
Exotics travel along edges
Distance to Edge
Per
cent
Exo
tics
0 50 100 150 200 250
010
2030
100 m2
10 m2
1 m2
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