As I mentioned, I've been researching and working on this concept of "keystone species" as part of my Wild Earth column for Aontacht, and I also began this morning's work by looking at the Pagan Blog Project letters I've missed over the past couple weeks to see what topics I might cover in order to catch up. Two topics I definitely want to write about are "Death and Decomposition" and "Fear: Living at Peace with Predators." I've written a lot recently about loving nature, and it's too easy to slip into an idealized, idyllic view of the natural world as pastoral and largely domesticated, so turning back to more disturbing and/or morbid topics will hopefully provide a good counterbalance to that tendency. In the course of doing that, though, I finally had a chance to read "Living in a Landscape of Fear: How Predators Impact an Ecosystem," which is Chapter 2 of Cristina Eisenberg's new book, The Wolf's Tooth, and was reprinted on Scientific American here.
In reading this, though, I was reintroduced to the fact that even the idea of "keystone species" is somewhat controversial in the study of ecology. Keystones are generally part of the broader theory known as the "green world hypothesis," which suggests that the planet is "green" (i.e. covered so thickly with plant life) because apex/keystone predators help to control the populations of herbivorous prey that would otherwise explode and consume much of the vegetation. Eisenberg points out that this is just one way of conceiving of the relationships between predators and prey, and that it involves top-down forces being exerted on populations which not all ecologists agree are the most important:
When [Nelson Hairston, Frederick Smith, and Lawrence Slobodkin (HSS)] formulated their hypothesis, the scientific community commonly accepted that bottom-up processes, mostly related to competition between species, were the primary forces shaping populations. The green world hypothesis provided an alternative view of population regulation driven by top predators, via carnivory. It enabled predation and grazing to play roles equally important to resources or habitat, with predation having the key role for some populations and resources having the dominant role in others.
[...] Not everyone bought the green world hypothesis. William Murdoch's counterargument, termed the plant self-defense hypothesis by conservation biologist John Terborgh, suggests that food (bottom-up control) has the strongest influence, that the world may be green because not all plants are palatable to herbivores, and that predators are unnecessary for ecosystem regulation. Or, as evolutionary ecologist Stevan Arnold puts it, the world is green, but that doesn't mean it's edible.
Murdoch asserted that food shortage and plant defense strategies may be regulating herbivore numbers. He hypothesized that while plants are essential for the survival of the trophic levels above, the reverse is not true. He and other critics of the green world hypothesis, such as ecologists Donald Strong and Gary Polis, suggested that HSS' failure to address the full complexity of systems, which includes omnivory, weakened their hypothesis. Some have identified HSS' tri-level trophic model as a hypothetical construct because in the real world food webs are not so tidy and can have fewer or more than three levels. Still others, such as conservation biologist Michael Soulé, believe that top-down versus bottom-up, like all dualisms, is false, because the natural world is complex and bottom-up forces (nutrient flow) interact with top-down forces (the effects of predation). Other scientists, such as Rolf Peterson, concur; in the wolf–moose–balsam fir system he studies in Isle Royale National Park, it's never one or the other but a synergy of the two.
I feel distinctly uncomfortable with my own less-than-firm grasp on the complexity of this issue, particularly considering the rant I left on Brendan's blog post a couple weeks back, in which I roundly reject the idea that any serious ecologist would recommend mass murder or forced sterilization as a way of controlling human population problems because, as I wrote at the time, "the study of population dynamics in the science of ecology clearly links population growth to resource availability." This would be the bottom-up dynamic Murdoch is talking about. And obviously, I agree with it to a certain extent even though I have spent the last few months studying keystones and trophic cascades. Arguably, self-imposed human genocide is not really the same thing as a predator-prey relationship in a naturally functioning non-human ecosystem.... but still, that I stumbled so blithely into this kind of false dualism (because I agree with Peterson and Soulé that it is a false dualism) in order to defend a particular ethical stance feels very embarrassing. I do still think that the whole environmentalists-hate-people-and-want-us-to-kill-ourselves is a straw man argument, but I should have done a better job of explaining why (besides the fact that I've hardly ever seen someone actually make that claim).
One aspect of why predator-prey relationships aren't all about mortality rates is exactly what Eisenberg gets into in her book excerpt: the influence of fear and vigilance on prey populations in the potential presence of predators. Basically, the presence of predators in an ecosystem will affect the behavior of prey populations in significant ways, so that prey will spend more time being vigilant and looking for predators (rather than with their heads down grazing), they spend more time on the move from one grazing spot to another rather than staying in a single area, and they are more likely to spend time in areas where escape from predators is easier (so, depending on the prey species, this might be open areas without obstacles to vision or flight, or densely forested areas where hiding is much easier). In the absence of predators, prey species (like deer and other ungulates) are more likely to behave like domesticated livestock, showing little fear response, spending much more time grazing, especially in one spot. As a result, they tend to severely deplete vegetation growth above a certain height (the height they can comfortably reach), preventing the natural succession of a field or meadow into a forested area, and generally reducing the biodiversity of the area. Predators like gray wolves tend to target larger herds, because they are easier to find -- but larger herds are often those which have family groups containing both young and potentially sick or injured animals. So wolf predation acts as a natural population control not only by picking off the sick and weak, but by disturbing what would otherwise be the most passive and stationary herds with the most young in them. Eisenberg also gets into how apex predators influence the behavior of mesopredators (predators such as, in some ecosystems, coyotes, which also change their behavior in the presence of more dominant predators).
She also talks about trophic levels as conceptual tools for thinking about more complex relationships, in a paragraph that I admittedly found a little difficult to follow:
Ecologists Lauri Oksanen and Stephen Fretwell proposed that if we treat trophic levels as units, systems having four or more trophic levels may have more than one level representing predation. In this scenario a predator at the fourth level will dominate a predator at the third level, and this will release the herbivore population at the second level from predation, causing its numbers to increase. This in turn will cause overgrazing of plant communities at the first level. Trophic cascades in which a top predator controls its herbivore prey via top-down forces will therefore always have an odd number of trophic levels. Removal of the top trophic level in such systems will have a radical effect on lower levels, causing herbivore irruption and over-consumption of vegetation. In any food chain, energy flow alternates; in odd-linked systems, plants will be limited by resources available to them (top-down control); in even-linked systems, plants will be limited by grazers (bottom-up control). Thus systems with an odd number of levels will be green, while systems with an even number of levels will be brown or barren.
One example she gives is the effect that orcas had on kelp forest biodiversity when they began eating more sea otters, a keystone predator that helped to keep sea urchin populations in check (which would otherwise overgraze on the roots of kelp and greatly reduce the kelp forests and the diverse other creatures that depend on them). In this case, the sea otter-sea urchin-kelp relationship went from an odd numbered trophic cascade, to an even numbered one (orca-sea otter-sea urchin-kelp). I think her way of phrasing this is confusing, since she refers to "top-down" systems as one where plants are limited by resources (which sounds more like bottom-up, where resource availability (i.e. food) exerts the most influence over plant populations), while "bottom-up" systems are ones where plants are limited by grazing (which sounds like top-down, because plant populations are being influenced most by the beings above them in the food pyramid). It only makes sense once you remember that we tend to be animal-centric and somewhat biased in how we conceive of things. What's actually going on is a complex dynamic between bottom-up forces that support populations and top-down forces that suppress populations.
For instance, if you take into consideration inorganic resources that kelp depend on (like sunlight, water, nutrients and suitable habitat), you get something that looks like this:
With the green arrows representing a relationship in which the lower trophic level supports the population of the higher level through resource availability, while the red arrows represent a relationship in which the higher level suppresses the population of the lower level through predation. Thicker, darker arrows represent greater influence or impact, compared to lighter, thinner arrows.
I have more thoughts on this, but it's now after lunch and we need to get ready and head out to the ponds for some homeschooling time. (Honestly, I'm feeling pretty damned exhausted and not up for teaching, and the weather is basically going to be chilly and rainy for the next several days. I would much rather just stay home, but the kids are going stir crazy and are already obsessed with their visit "not being fair" compared to the other two, since they had nicer weather last week and an extra weekend day here. Not to mention, last night's frog walk was a flop because it was just too chilly for the frogs to be out -- we heard them briefly as they approached the pond, but they quieted down almost immediately as we got close. Sigh. This whole arrangement is so disruptive and difficult, and I know the kids don't mean to guilt trip us... but they do. Never mind that in the autumn, it was J and F who had the gorgeous week or sunny weather and the following week when S and J came, it rained the entire time.)
Okay. Ponds time.