April 26: Fieldnotes from the Edge of the Wild
In my last blog post, I discussed trophic cascades and humility in the face of the complexity of nature. We’re in the midst of our field season conducting research on golden eagles in the Bighorn Basin now, and trophic cascades and humility are on my mind again! Of about 60 Golden Eagle nesting areas we’ve mapped in our study area, we can only confirm 20 that are active this year. Eagle pairs may refurbish one or more alternate nests in a given breeding season, but ultimately choose not to carry on with breeding activities. Presumably, this choice has much to do with food availability, though disturbance near the nest may also play a factor.
During the past two years, we’ve documented a population crash in cottontail rabbits and white-tailed jackrabbits; key prey for eagles in our area. If there is enough alternate prey available, we’d expect to see eagles maintain similar breeding rates while feeding on different prey. Although we’ve seen eagles take more ground squirrels, songbirds, snakes, and even pronghorn fawns, rabbits remain the chief prey, and the breeding rate and productivity of eagles have declined. Apparently, alternate prey is not available in sufficient abundance in our study area to completely replace rabbits in the diet. Scientists have been studying rabbit population fluctuations in different areas of the world, but the fundamental cause of these cycles remains unclear.
One possibility is that high rabbit density tends to create stress for rabbits that reduces reproduction in some way. Another hypothesis is that high rabbit densities reduce food availability or affect plants in a way that makes them less nutritious or palatable to rabbits. Others point to the fact that high rabbit densities lead to an increase in predators preying on rabbits. And, of course, weather conditions may exert pressures on rabbit populations. As with most ecological puzzles, rabbit population fluctuations are probably caused by a combination of factors acting together. Because rabbits are such a core prey species where they occur, fluctuations in rabbit populations create widespread effects felt by predators, other prey species, and the plants that rabbits use as food. As we continue our long-term study of golden eagles in the Bighorn Basin, we’re learning much more about the interrelationships among all the inhabitants in the sagebrush-steppe ecosystem and how human activities influence these interrelationships.
We’ve just completed our early season aerial surveys to determine eagle nesting activity, and during the next few weeks we’ll be conducting ground surveys to gather additional information. This year is shaping up to be unusual—warm weather has come early, and hot, dry conditions are forecast for the summer. It will be interesting to see how these conditions impact our eagles and the ecosystem. Stay tuned to this blog for updates and enjoy Spring!