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kirtland’s warblers and cowbirds, revisited

KiwabpfeifferLate last year, I did a series of posts on the endangered Kirtland’s Warbler, with an emphasis on the pros and cons of using cowbird control as an open-ended management technique (posts in the series listed after the jump).  One point I made is that trapping cowbirds as a long-term management tool is not a great idea, and one reason is that it prevents the warblers from developing defenses against the brood parasitism of the cowbirds.

In the July-September 2006 issue of Conservation in Practice, a magazine published by the Society for Conservation Biology, there is an article [1] by Scott Norris entitled “Evolutionary Tinkering.” Norris began by envisioning what species would have survived the current wave of extinctions by the year 2303: it would be a world of biological survivors. He summarized a growing concept in the conservation world — that we need to use active intervention in evolutionary trajectories as a conservation strategy.

This idea is built on these points:

  • Major drivers of extinction today are also drivers of evolutionary change,
  • Natural selection acting on wild populations can produce big changes in short periods of time, known as “contemporary evolution,”
  • Endangerment is a function of both environmental change and adaptive response — or lack thereof,
  • For threats that are not going away any time soon, endangered species management approaches that are based solely on impact reduction or mitigation are break-even propositions at best.

Some ecologists are increasingly concluding that in some cases, management efforts may be getting in the way of evolutionary progress.  Norris notes:

“One of the most jarring lessons of contemporary evolution is how often conservation itself is a significant selective force, altering evolutionary trajectories even as it tries to maintain the status quo.”

One researcher that did careful work on this theme is Marm Kilpatrick, who looked at a major threat to some native Hawaiian birds: avian malaria. These birds have been forced into cooler high elevation zones where malaria-carrying mosquitoes do not occur.  But these zones will disappear due to global climate change, and the only way for these species to persist will be if they are allowed to develop resistance against malaria [2]. Norris discussed this approach with Kilpatrick regarding the Kirtland’s Warbler:

“In a sense, the approach argues for letting go of the species a little bit so it can respond to the stressor and selective pressure,” Kilpatrick says. “For Kirtland’s Warbler, it would mean letting some cowbirds parasitize warbler nests — which would result in some nest failures.”  This may be too great a risk, Kilpatrick concedes, if the population is already very small. But there is another kind of risk — management dependency that has no stopping point. “In my mind the advantage of facilitated evolution is that it offers a way to let the species take care of itself in the long run.”

These were exactly the points I made in my posts as well as in private conversations I’ve had with people involved in the Kirtland’s Warbler recovery. Kilpatrick notes in his paper that for species impacted by multiple stressors — as is Kirtland’s Warbler — reducing the impact of one factor and encouraging evolutionary change that allows a species to deal with other stresses may allow long-term recovery.  Increasing the specialized habitat of Kirtland’s Warblers has been very successful — once again, the 2006 census has broken records, and the warbler population is the highest ever officially recorded. It may be time to set the wheels in motion to model various management strategies and explore allowing some subpopulations of the warbler to be exposed to cowbirds.

There is another curve ball in the ultimate survival of Kirtland’s Warbler, and that is global warming.  The warblers rely exclusively on jack pines for nesting.  While jack pines have a range that extends into Canada, the warblers typically only nest in trees that grow in a particular type of soil, which is only found on the southern edge of the trees’ range. This is likely because the birds nest low or on the ground, and require fast-draining substrate. Some models have predicted the elimination of jack pines on these soils within 60 years due to climate change (as they are very sensitive to temperature and soil moisture).

This means that we must also be considering developing heat- and drought resistant jack pines.  This statement probably raises no hackles at all, since humans have used selective breeding in plants to develop food and cash crops throughout much of our history. Of course, we have done the same with animals, and many people probably never think twice about the origins of the majority of our companion animals, dairy cows, or broiler chickens. This is not new and revolutionary methodology, and using these skills to save endangered species is a logical next step, offering hope for the future of our dramatically changing world.

Here are links to the original posts:

[1] Norris, S.  2006.  Evolutionary tinkering.  Conservation in Practice 7(3):28-34.

[2] Kilpatrick, A.M. 2006.  Facilitating the evolution of resistance to avian malaria in Hawaiian birds.  Biological Conservation 128:475-485.

I once again thank Bryan Pfeiffer at Wings Photography for use of his stunning Kirtland’s Warbler photo…so good, I’m rerunning it here.  Cowbird photo from istockphoto.

Filed in Birds, Science

Comments on this entry are closed.

  • John August 15, 2006, 10:00 am

    I wonder if drought-resistant jack pines would still be appealing for Kirtland's Warblers. This species's habitat requirements are so demanding that there may be a risk that changing the trees' chemistry could result in driving them away. It is still worth a try, if the alternative is extinction anyway.

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