Conservation Maven

Brown tree snake (Boiga irregularis) introduction to Guam has caused the extinction of most native vertebrates on the island including 9 of the 11 native bird species.A new study in the jornal Conservation Biology provides an interesting counterpart to our article on the conflicts between conservationists and animal rights groups over the use of lethal control of invasive wildlife. Rebecca Smith and fellow researchers conducted a meta-analysis of 83 predator removal studies and found strong evidence that lethal control measures can help conserve threatened bird species.

Human disturbance can lead to an increase of both native and non-native predators that can subsequently lead to a decline in bird populations. For example, introduced predators such as the brown tree snake on Guam have been responsible for 34% of the 110 bird extinctions that have occurred on islands since the 1600s.

Therefore managers have undertaken predator eradication efforts to try and protect at-risk bird species. However given the cost of these programs and the controversy they can generate, it is important to critically evaluate whether or not they support the recovery of threatened species.

In their meta-analysis, the researchers found that predator removal areas saw on average a 77% increase in hatching success and 79% increase in fledging success compared to control sites. The post-breeding populations in removal areas were 60% greater, however, this number was not statistically significant largely due to an insufficient sample size. The longer-term measure of breeding population numbers was 71% greater which was statistically significant.

The analysis found interesting heterogeneity in the study results. Mainland predator removal areas saw a significant increase in post-breeding populations while island areas did not. Given the heterogeneity of the results the authors warn,

"Generalizing results across different ecological situations can be misleading, so we stress how important it is to assess the evidence from similar situations before deciding on interventions to be carried out and to monitor the effects of interventions once they are implemented."

Ultimately the effects of predator control will depend on whether or not managers can realistically kill all the predators. The authors write,

"This type of management can have long-lasting benefits to prey populations, particularly following the eradication of predators from islands that cannot easily be reinvaded by predators. In contrast, on mainlands, if predator removal is not continued, any positive effects on prey populations soon disappear as predators move back into the area. This means predator removal needs to be a long-term management strategy. It also needs to be effective."

--Reviewed by Rob Goldstein

SMITH, R., PULLIN, A., STEWART, G., & SUTHERLAND, W. (2010). Effectiveness of Predator Removal for Enhancing Bird Populations Conservation Biology DOI: 10.1111/j.1523-1739.2009.01421.x