How one bird combated the attack of an invading army of rats

The invasion of outside predators into a new habitat often spells trouble for the native species. In fact, attack by newly introduced predators is a leading cause of extinction for many species and is becoming a focus for conservationists.

This susceptibility to foreign predation holds particularly true for island species that have less exposure and genetic diversity that would prepare them to adapt to attack from a new species. Because of this generally poor response of native island species to foreign predators, there is limited documented research on island species adaption; however, a study from Pacific Rim Conservation in Honolulu, Hawaii, examines how one type of bird adapted to attack from rats. In learning how a species addresses these sorts of new predatory dangers, conservationists can more confidently create a conservation plan for other types of species that encounter similar problems.

In this particular study, Eric Vaderwerff spent 16 years detailing how the nesting of the Ohau Elapio bird species changed in adaptation of attack by the black rat. By measuring the average nest height of almost 300 different nests from 1996 to 2011, Vanderwerff found that there was a 50% increase in nesting height over that span of time. He also ascertained that the nesting height increase was not learned but evolved through natural selection. Natural selection refers to the “survival of the fittest” theory in which the “weaker” die out leaving only the “stronger.” In this context, nesting closer to the ground made the bird more susceptible to attack by the black rat. Over time, low nesters died out leaving behind only the high nesters. As a result, over 16 years, the average height of nesting dramatically increased.

The results of this study, that the adaptation to an invasive predator occurred through natural selection, have the potential to shape how the conservation world treats similar situations. Traditionally, predator control is used to limit the amount of damage done by those predators; however, it can be difficult to sustain application of predator control for an indefinite period of time. Depending on the form of control, there are environmental factors, economic considerations and political practicality that may hinder the passage of this sort of endless program. However, the results of this study indicate that a species will adapt via natural selection, so it is only necessary to install the predator control for enough time to allow the native species to evolve. If no predator control is implemented, then the native species may die out before it has time to evolve. However by implementing the predator control, the species is being put on a sort of “life support,” giving it just enough time to recover on its own. The fact that the control is only needed for enough time to allow natural selection to do its work makes it much more feasible to implement.

Nest success rate over the span of the study. Nest success rate is defined as nests in which the parents survive and give birth to at least one offspring. The upward trend of nesting success coincides with a general upward trend in nest height over the study. As nesting occurs higher, the likelihood of predation decreases and chances of survival increase.

This study was certainly an intricate one. To maintain this sort of data over a 16 year span is no small task, and the labor-intensive qualities of this study are a large reason that there are not many similar ones. However, moving into the future, it is essential that more research is done into the average time required for natural selection to salvage a species. This will give conservationists and legislators the data they need to determine the length of predator control that will need to be budgeted for in regions with invasive species.



Vanderwerf E. 2012. Evolution of Nesting Height in an Endangered Hawaiian Forest Bird in Response to a Non-Native Predator. Conservation Biology. 26(5): 905-911.

This entry was posted in Conservation Biology Posts, Conservation Blogs 2012-2013. Bookmark the permalink.

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