Transcript PowerPoints

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