Notes on Pimentel et al

Invasion Biology
JDB Sample Discussion Notes

Pimentel et al. 2001

Economic and Environmental threats of alien plant, animal, and microbe invasions

Hypothesis Tested / General Goal / Themes of Paper:

Summarize the number of introduced species in six countries (US, UK, Brazil, South Africa, India, and Brazil), and estimate the economic impacts of these species on the native biota, the overall ecological health, and that of the humans in different areas.
Primarily was an extension of Pimentel et al. 2000 that summarized these factors for the US.
Main contribution of the rather tediously written paper are tables 1, 2, and 3 which summarize the relative number of native vs. introduced species (1), economic losses by each major general taxonomic category of invasives (2), the environmental losses to each government, measured in dollars (3).

Structure of Paper:

Most of the control costs are associated with agricultural production
Usually these efforts are not very successful and only succeed in reducing pest populations (sole exception was the complete eradication of the Med fly (Ceratitis capitata)in Florida
There has been a 10-fold increase in the number of invading species (annually? Not clear in paper)

120,000 non-indigenous spp have invaded these 6 countries (relatively few are pests (tens rule)
Reasons why introduced spp succeed is competitive and predation release, development of new associations (hosts), newly effective predators, abundance of disturbed habitats (for edge species), introduction of highly adaptable and successful alien species.
Impossible to assess the costs of species that are driven to extinction
Per capita losses in the 6 countries is estimated at US$240
98% of the world’s food supply comes from introduced species
Only control method suggested is that we increase governmental restrictions on movement of species – hedged and didn’t call for restrictions on movement of people.

Comments:

Estimates of rats and cats were wildly high and seemingly without too many bases – perhaps they were discussed elsewhere in other publications? These estimates accounted for almost 80% of the total costs due to introduced species.
Point out that most of the cells in the matrix in table 1 were empty because of lacking data – could lead to a huge underestimation of the costs

Questions:

How accurate are the estimates? What problems would you find with the assumptions behind them? Are most of them too liberal or too conservative?
Would native weeds (which are at least 25% of the weeds, if not more) have moved in to fill the gap left if the introduced weeds were removed? Perhaps much of the problem, with weeds anyway, is that of habitat modification and fragmentation? This would have mitigated the supposed losses from the introduced species.
Perhaps many of these influences were acting simultaneously on endangered species – this would reduce the total cost losses from introduced species
Costs related to the depletion of birds (by cats) and a decrease in the money spent by hunters or dollars per hunter decreasing linearly may be erroneous. Hunters would still go out and hunt, irrespective of the number of ducks or hunting birds and birders would still go out even if a huge percentage of the birds were gone – down to a threshold, of course, below which the costs would really go down. As a consequence the amount that a hunter or birder or specialists would spend per bird would escalate with a decrease in the total number of birds remaining. This is not a linear relationship.
Are government restrictions and regulations really enough to reduce the flow of introduced species?
How would the authors feel about huge expenses related to control of introduced species?