On Subsidies to Ecological Systems

After reading the important paper by Killengreen et al. (2011) it dawned on me that I had not thought enough about subsidies to ecosystems in the modern era. If we put the idea of subsidies together with the idea that at least many terrestrial systems are strongly influenced top down by predation, some pieces of a few puzzles seem to come together for me. For anyone working in northern Canada, one puzzle has always been how small predators like weasels survive over the winter when lemmings or voles are in very low abundance. Certainly these are highly efficient predators, but there is a limit to being a good predator when you are small and your prey is under the snow at densities that might be only a few individuals per square kilometre. But if you are a weasel and happen to find a caribou or muskox carcass, you might well be in heaven for the winter.

There are many examples of predator subsidies provided by humans. Dingos in the outback of Australia utilize garbage dumps from mining companies (Newsome et al. 2014). Feral cats in the outback of Australia travel long distances to human habitations to get food when drought has reduced prey abundance (Molsher et al. 1999). Snow geese have increased greatly in abundance from winter food provided by agricultural crops in southern USA (Alisauskas et al. 2011). There must be many more examples in the literature, even without looking at the data on rats in city dumps.

But what does all this mean, since in some sense we knew these facts long ago? First and foremost I think it means we are studying a world that did not exist in the past, so that the ‘balance of nature’ is changed in a variety of ways we do not comprehend or understand. If predators are subsidized for example over the winter period, prey populations may on average be more heavily exploited by additional predators that have not died by starvation. Or to take another view of the matter, additional predators surviving might increase intraguild predation providing a variety of indirect effects we can only guess at now.

There is an extensive literature on the effects of nutrient subsidies to aquatic ecosystems, going back to phosphorus in soap (Schindler 1977) and acid rain (Likens et al. 1996). The difference in perspective now is that while most of the effects of nutrient subsidies to lakes and forests are bottom-up, many of the more recent subsidies being recognized are top-down in affecting predator survival. Subsidies can in fact be negative in the case of the reduction of top predators by human persecution (Ripple et al. 2014).

Perhaps what concerns me most about this is that we will never be able to stop doing ecology and testing theories about how populations and communities work if the world keeps changing under our feet. The laws of physics and chemistry may not change over time, but the generalizations of ecology may change faster than we can imagine because of human perturbations.

Alisauskas, R.T., Rockwell, R.F., Dufour, K.W., Cooch, E.G., Zimmerman, G., Drake, K.L., Leafloor, J.O., Moser, T.J. & Reed, E.T. (2011) Harvest, survival, and abundance of midcontinent Lesser Snow Geese relative to population reduction efforts. Wildlife Monographs, 179, 1-42.

Killengreen, S.T., Lecomte, N., Ehrich, D., Schott, T., Yoccoz, N.G. & Ims, R.A. (2011) The importance of marine vs. human-induced subsidies in the maintenance of an expanding mesocarnivore in the arctic tundra. Journal of Animal Ecology, 80, 1049-1060.

Likens, G.E., Driscoll, C.T. & Buso, D.C. (1996) Long-term effects of acid rain: response and recovery of a forest ecosystem. Science, 272, 244-245.

Molsher, R., Newsome, A. & Dickman, C. (1999) Feeding ecology and population dynamics ofo the feral cat (Felis catus) in relation to the availability of prey in central-eastern New South Wales. Wildlife Research, 26, 593-607.

Newsome, T.M., Ballard, G.-A., Fleming, P.J.S., van de Ven, R., Story, G.L. & Dickman, C.R. (2014) Human-resource subsidies alter the dietary preferences of a mammalian top predator. Oecologia, 175, 139-150.

Ripple, W.J., Estes, J.A., Beschta, R.L., Wilmers, C.C., Ritchie, E.G., Hebblewhite, M., Berger, J., Elmhagen, B., Letnic, M., Nelson, M.P., Schmitz, O.J., Smith, D.W., Wallach, A.D. & Wirsing, A.J. (2014) Status and ecological effects of the world’s largest carnivores. Science, 343, 1241484.

Schindler, D.W. (1977) Evolution of phosphorus limitation in lakes. Science, 195, 260-262.

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