Double-crested Cormorant Population not at Biological Carrying Capacity
Labeling Double-crested Cormorant Population as Overabundant is “Inappropriate”
Scientists Linda Wire and Francesca Cuthburt, of the University of Minnesota’s department of Fisheries, Wildlife and Conservation biology, find that describing the population of Double-crested Cormorants as overabundant is “inappropriate” given historical evidence that this fish-eating bird has previously been very abundant.
Biological Carrying Capacity vs. Wildlife Acceptance Cpacity
Ecologists often describe the biological carrying capacity as the population limit that a particular ecosystem can support. Wire and Cuthburt instead discuss the “wildlife acceptance capacity” as the population number desired or tolerated by human society. They stress that society should aim to maintain bird populations at historic levels, and not set population goals based on how they are perceived to help us reach our fisheries goals. Perhaps we need to take a step back and evaluate the contribution of the double-crested cormorant’s activities versus our own activities that impact the fisheries. The cormorants were abundant long before the demise of fish populations. This is evident upon inspection of the historical records that Wires and Cuthburt have included in their study. At the very least, we should not claim that this bird is overabundant if there is no evidence that it is at its true carrying capacity.
Human Activities and the Double-crested Cormorant Population
Cormorants have had a reputation as a competitor of humans for fish as far back as 1634. Wire and Cuthburt cite that there are only a few locations where a decline of natural resources have been directly linked to double-crested cormorant populations and that the perceived overabundance is due to the double-crested cormorant populations rebounding closer to their historic levels after recent declines. The double-crested cormorant population dropped due to the use of DDT from the 1940s to 1970s, combined with population control by humans (legal and illegal methods) and habitat change. While it may be perceived that the cormorant population has increased, the perceived increase is really a population rebound after a decline due to human activities. The population along the Atlantic Coast and in the interior of the United States and Canada has rebounded from 32,000 breeding pairs (early 1970s) to more than 226,000 breeding pairs (late 1990s).
Future Population Goals
The core issue here is that we need to decide if we should keep our bird populations at historic levels (keep common birds common) or try to reduce population sizes if they compete with us for the same food source. At the very least, we should not misrepresent their population size to the public as “overabundant” if the historic population size was equal or greater than the current population size. We need to be honest about the difference between the biological carrying capacity and the “wildlife acceptance capacity”, which is the number that certain human populations think should exist. That number may have little to do with scientific evidence about how a particular population size affects the rest of the ecosystem.
Find the full study on historic Double-crested Cormorant population sizes here.