Volume 11, Number 38: 17 September 2008
In their recent essay on the subject of threats to the ability of animals to successfully complete their historic annual migrations, Wilcove and Wikelski (2008) write that "in general, the threats to migrants fall into four nonexclusive categories: habitat destruction, the creation of obstacles and barriers such as dams and fences, overexploitation, and climate change." However, there is something about the several examples they report that leads us to believe that climate change should not have been included in their list of offending phenomena.
Consider the salmon of Pacific Northwest USA fame. As young fish they leave their natal rivers and head to sea, where they dramatically increase in size, returning a year or two later to their points of origin to spawn and die. Wilcove and Wikelski report that "prior to European settlement, 160-220 million kilograms of salmon migrated each year up the rivers of Washington, Idaho, Oregon, and California," but that "today, after decades of dam construction, overfishing, water withdrawals for irrigation, logging, and streamside grazing by livestock, salmon populations have plummeted," to where they say that only 12-14 million kilograms of salmon make the return trek each year, all courtesy, we would add, of something other than climate change.
Then there are the bison of the U.S. Great Plains, hundreds of thousands of which could be seen "trekking across the prairies, as was possible less than two centuries ago," according to the two researchers, but which are only seen today in national parks and private ranches. And, of course, there are -- or rather were -- the famed passenger pigeons of eastern North America, which once "temporarily obscured the sun as they migrated to and from their breeding grounds," but which today no longer cast a shadow there -- or anywhere -- all courtesy of something other than climate change.
Also of note are the more recent declines in the breeding populations of migratory songbirds in eastern North America and Europe, of which Wilcove and Wikelski report that "no one can say with confidence the degree to which the observed declines are a function of the loss of breeding habitat, the loss of winter habitat, heightened mortality during migration (due to habitat destruction, pesticides, communications towers, and other factors), or some combination of the three," in addition to which challenges they mention wind farms in the case of migratory bats. But, of course, we can say "with confidence" that the declines have been due to something other than climate change.
Some people, however, would like to challenge this statement, expressing concern, as Wilcove and Wikelski phrase it, "that the phenology of migration could be disrupted by climate change." As an example, they write that "the spring migration of many songbirds in both Europe and North America coincides with the leaf-out of deciduous trees and the emergence of caterpillars, which the birds eat." If these caterpillars "are emerging earlier in the season due to warming temperatures, but the birds are not migrating earlier because they are relying on different cues (e.g., minor changes in day length in the tropics), then," as they continue, "the songbirds could face serious food shortages during the migration or breeding season."
This concept, however, has not been proven. It is only a scenario, which the two scientists say in prefacing their prior remarks "some have theorized," and which they say must be thoroughly tested in order to determine "whether such a scenario is truly plausible." That it is probably not plausible -- or at least not of much consequence -- is suggested by the fact that the ancestors of today's songbirds had to have successfully dealt with the cooling following the Roman Warm Period, the warming following the subsequent Dark Ages Cold Period, the cooling following the subsequent Medieval Warm Period, and the warming following the subsequent Little Ice Age that ushered in the Current Warm Period.
Yes, the very real impediments to historical animal migrations that plague their current populations are not the result of global warming, CO2-induced or otherwise. They are the result of a host of much more direct human impacts on the natural environment, such as those that fall within the other three categories of threats listed by Wilcove and Wikelski. Consequently, if people are concerned about the abilities of today's migrating animals to continue their ages-old seasonal travels, they need to focus on the much more mundane things we do to make their trips miserable ... or even impossible.
Sherwood, Keith and Craig Idso
Wilcove, D.S. and Wikelski, M. 2008. Going, going, gone: Is animal migration disappearing? PLoS Biology 6: 1361-1364.