Christopher X J. Jensen
Associate Professor, Pratt Institute

Carrying capacity — but not growth rate — varies with habitat quality (at least for moose)

Posted 17 Aug 2015 / 0

EcosphereCharacterizing demographic parameters across environmental gradients: a case study with Ontario moose (Alces alces)

What I find interesting about this study — besides its unprecedented investigation of something that would seem pretty critical to basic conservation efforts — is what was and was not intuitive in its findings. Moose have higher carrying capacities where they encounter the vegetation in general and specifically the mixedwood forests they prefer. Counter-intuitively, they do less well in harvested areas, even though timber harvest ought to create successional stages that increase moose forage supply. The authors of this paper interpret this as suppression of population size due to increased hunting pressure, as forest clearing makes for better access to hunters and easier hunting. I get this idea and think that it probably makes sense, but also shows how the term “carrying capacity” can be really confusing, even as used in professional publications. What the authors are actually saying is that the carrying capacity — as a measure of the maximum population that can be sustained by existing food supply — is not being realized by populations where there is increased timber harvesting. Instead, the effective growth rate of those local populations goes to zero due to increased hunting mortality. This is really more a change in the effective rate of growth than the carrying capacity. Of course there is rampant confusion in ecology on this issue (for example, see this article, which basically considers carrying capacity to be the same as “equilibrium population size” regardless of what factors lead to that equilibrium — and then contrast it with the Wikipedia article on carrying capacity, which gets it right in the second sentence!). Call me a purist, but I like the meaning of K in the original logistic equation: the population size that can be sustained by existing resources in the absence of other factors that may lead effective growth rate to be zero. In other words, carrying capacity is not just any equilibrium but the actual equilibrium caused by resource limitation (in other words, by bottom-up factors).

Equally interesting is the finding that the intrinsic growth rate of moose is not affected by habitat quality. As the article nicely points out, moose biology pretty much determines the best case scenario for moose-baby-making. A radically better habitat does not speed up moose offspring production, but it would likely reduce the mortality rate of these offspring. Obviously this is not going to be the case for all organisms, but I imagine this is true for a lot of large vertebrates. In essence, the evolutionary process — responding to the average quality of habitat over long periods of time — has already fine-tuned moose biology to a particular rate of offspring production… although it is really interesting to think about what evolutionary factors are responsible (for example, see Fryxell et al. 2007).

A Minor Post, Articles, Carrying Capacity, Community Ecology, Conservation Biology, Ecological Modeling, Habitat Destruction, Intrinsic Growth Rate, Parasitism, Population Growth, Sustainable Harvesting

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