It was right there in the shadows under the stairs, glaring lidlessly at me with its round yellow eye above the sharp, bloody beak. A cooper’s hawk had caught itself what looked like a mourning dove (thanks for the ID’s, Max!), possibly at one of the campus feeders. An urbanized bird, it was vigilant as it ate, but unbothered by my photographing it from barely ten feed away or the laughing students passing by with cafeteria lunches or in gym clothes.
As I watched, a little mouse ran through the open dirt of the planter where the hawk was feeding, and up the concrete wall before disappearing into the bushes. A brave little guy, I thought at first. Or was it cagey? Did it understand the hawk posed no danger while occupied with its current meal? The time and effort it takes for the hawk to pluck the feathers and gorge itself on the insides of the dove is what biologists call “handling time” (thanks to Hollings 1959) and this hawk spent at least an hour at it. Handling time is important to models of predator-prey dynamics, because without handling time, predators can eat everything they encounter. That results in different system behavior than when there is some level of satiation, when the predators cannot or choose not to eat one more bite (like us before Thanksgiving dinner, eating everything in sight versus after stuffing and cranberry sauce – satiated).
All these different models can make very different predictions about how biologically diverse a place “should” be. All the models are, of course, wrong (they are all simplified versions of the real world), but some are useful. As my advisor, Peter Chesson, frequently reminds me, good modeling is driven by the biology of the system. The math and the biology should agree.
Yet biologists have argued for a long time about whether predation or competition are more important forces regulating ecosystems. This was officially kicked off by a paper in 1960 (by Hairston and others) that hypothesized the world is green because predators of plant-eaters keep them from eating all the plants. But isn’t competition important too? I think so – I saw some last weekend.
Over the weekend, I went to Mexico in my role as the Graduate Teaching Assistant for Conservation Biology. We hiked and camped through the borderlands and lava flows all the way to the beach where we scavanged the tide pools for octopus and the 23-legged sunstars.
While there, I took this picture of brown carpet anemones and a competing green anemone elbowing one another for space among the algaes on this rock. My guess there is that they directly compete for space. This kind of competition is kind of a lottery (Sale 1977). Each time one dies, a prize spot opens up. Its (now former) neighbors have all bought all the tickets they could by producing as many little gametes as possible, which turn into larvae.
Further north, in the desolate lava flows and Maar craters and drifting dunes of the Pinacate Biosphere Preserve, I was startled to see so many plants growing there. The annual 3 inches of rain supports dozens of reptiles and mammals, too! Even big pronghorn antelope! How might you model the competition of the plants for the water? Bill Ricker has a model that makes growth of each individual decrease exponentially as other individuals compete for its resources.
All that is a lot to learn from to explain biodiversity, agriculture, cities, global warming’s effects, etc. Here’s a question: what do you think ecologists should use these to answer? If you got to ask for ecology’s next top model, what would it model? What would it tell us?
More details on papers I referenced above:
Hairston N G, Smith F E, Slobodkin L B. Am Nat. 1960;44:421–425
Holling, C. S. 1959. “Some characteristics of simple types of predation and parasitism.” Canadian Entomologist 91: 385-98
Ricker, WE (1954). Stock and recruitment. Journal of the Fisheries Research Board of Canada.
Sale, P. 1977. Maintenance of high diversity in coral reef fishes. American Naturalist111:337–359