Transcript Trends in numbers of Invaders: Part II

Trends in numbers of
Invaders:
Part II
By Issy Cless and Amber Orr
Cohen et al 1998
Accelerating Invasion Rate in a Highly Invaded
Estuary
• Assembled data on exotics in the San Francisco Bay
• San Francisco Bay is possibly “the most invaded estuary in the
world” due to
– Variety and number of transport vectors
– (maybe) Low native species richness
– High disturbance
• Identified a total of 234 established exotics and found that the
proportion of exotics has been increasing
• Rate of exotic species introduction has increased from one
species every 55 weeks to one species every 14 weeks
Cohen modified the data by excluding samples
whose introduction date could not be verified.
Do you agree with this method and how do
you think it affected the results?
“Although greater numbers or greater dominance of exotic species
in disturbed areas relative to undisturbed areas has often been
observed, it is unclear whether this occurs because these areas
are more easily invaded, because they are more heavily
inoculated with exotic organisms, or because a greater number
of the inoculated organisms are adapted to the disturbed
environments from which they were imported.”
What do you think is the mechanism by
which disturbance increases the rate of
exotic establishment and invasion?
Which do you think is a more likely cause
for the level of invasion in the bay:
diversity/degree of transport vectors or
the characteristics of the bay itself?
Lonsdale 1999
Global Patterns of Plant Invasions and the
Concept of Invasibility
- 4 Themes of Invasion Ecology as described by Lonsdale
1. Ecosystem properties (EP) such as disturbance
2. Propagule pressure (PP) - rate of exotic introduction
3. Properties of Exotic Species (SPE)
4. Properties of Native Species (SPN)
- This Paper focused on EP- Invasibility- an emergent
property of ecosystems that describes a given site’s
susceptibility to establishment of non-natives
- Lonsdale compiled data from previous studies on
invasibility and attempted to eliminate the confounding
variable of site area.
Statistical techniques
• The concept of the residual is based on
separating the effects of variable A on
variable B before comparing B to C.
• Lonsdale uses this method many times,
in particular when determining the
extent effect of site size on exotic
richness as determined by native
richness.
Lonsdale examined 6 hypotheses regarding
Invasibility of various sites:
a)
The Old World is less susceptible to invasion than the New World: Found
some support for this, but only when effects of site differences in native
richness was factored out.
b)
Richer communities are less invasible: Actually found the opposite- that
exotic species introduction increases with native richness
c)
d)
Temperate ecosystems are more invasible AND
Islands in Europe/North America are less invaded than tropical islands:
Found little support for either of these.
e)
Islands are more invasible than mainlands because are species poor.
Found that yes, islands are more invasible than mainlands (3 times larger
proportion of exotics compared to average proportion), but not because of
differences in species richness
f)
An increase in number of human visitors to an area leads to an increase in
exotics:
Confirmed. Nature reserves have 1/2 the proportion of exotics.
Lonsdale finds that the number of exotics
for mainland sites increases with
latitude. The textbook finds the exact
opposite trend (recall the scatter plots).
Thoughts?
Lonsdale confirmed that New World sites have a
higher number of exotics than Old World sites,
but only when variation due to differing native
diversities is factored out.
Do you think factoring out this variable could
lead to greater understanding of the
differences in the two areas? Or is it too
important to ignore?
What do you think of his finding that
native species richness is correlated
with non-native richness? Is it possible
that native biodiversity could increase
non-native richness while
silmultaneously deterring actual
invasion?
Lonsdale found that “visitors” to sites
significantly increase the number of
exotics at a site. With this in mind, do
you think the potential benefits of
ecotourism outweigh its potential
dangers?
When considering the number of exotic
species present in different areas, which
do you think is more important: number
of introduced species (propagule
pressure, PP) or characteristics of the
area itself (ecosystem properties, EP)?
How do you think Lonsdale’s results
would be different if he had analyzed
data on all invasive species rather than
just plants? Do you think the
mechanism by which invasive plants
establish themselves differs from that of
other species?