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 1999Global Patterns of Plant Invasions and the

Concept of Invasibility

- 4 Themes of Invasion Ecology as described by Lonsdale1. Ecosystem properties (EP) such as disturbance2. 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) Temperate ecosystems are more invasible ANDd) 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?