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