Universal theories are few and far between in ecology, but that is what makes it fascinating.
To some scientists in other fields, ecology must seem relatively straightforward. Many of the organisms live at a very human scale and are easy to access, especially in community ecology. Ecologists do not need special equipment to see and count elk. There are no electron microscopes, space telescopes or drilling rigs that can go wrong. Easy.
And yet, ecologists know that their subject can prove as troublesome as any other. Ecology would be easy, were it not for all the ecosystems — vastly complex and variable as they are. Even the most austere desert or apparently featureless moor is a dense, intricate network of thousands of species of photosynthesizers, predators, prey animals, parasites, detritovores and decomposers. As naturalist E. O. Wilson put it: “A lifetime can be spent in a Magellanic voyage around the trunk of a single tree.” And not all of what one might learn from such a voyage would be transferable to the next tree. History, chance, climate, geology and — increasingly — human fiddling mean that no two ecosystems work in the same way.
Scientists like to impose structure and order on chaos, and ecologists are no different. Ecology has its grand theories, but they are riddled with conditional clauses, caveats and exceptions. There are clear patterns at the global and single-species scales, but the middle ground is, as biologist John Lawton affectionately put it in 1999, “a mess”. It is doubtful that the generalities that underlie the complex patterns of nature will ever be phrased succinctly enough to fit on a T-shirt.
This complexity is demonstrated by work that questions a famous and elegant ‘trophic cascade’ in Yellowstone National Park, Wyoming, discussed on page 158. The theory goes that wolves, restored to the park in the 1990s after decades of absence, scare elk away from certain areas. That has a knock-on effect for the rest of the food chain, allowing aspen and willows to flourish after decades of being browsed nearly to death. But studies in recent years suggest that wolves alone do not control the ecosystem. Other factors — the presence of beaver dams and grizzly bears, weather, hunting by humans and even climate change — also affect the elk population and the growth of trees and shrubs.
It would be useful to have broad patterns and commonalities in ecology. To know how ecosystems will respond to climate change, or to be able to predict the consequences of introducing or reintroducing a species, would make conservation more effective and efficient. But a unified theory of everything is not the only way to gain insight.
More ecologists should embrace the non-predictive side of their science. Teasing out what is going on in complex systems by looking at how ecosystems evolved, and by manipulating the environment in experiments, is just as much a science as creating formulae for how ecosystems work.
“If ecosystems all worked in the same way, they would lose much of their mystery, their surprise and their beauty.”
Paradigm shifts, after all, are rare in ecology. Debates are often resolved when competing concepts combine, rather than when one pushes the other completely off the table. Take the contrasting ideas of top-down regulation of ecosystems by carnivores and bottom-up regulation effected by the nutrition available from plants. The field is slowly working towards an integrated theory to predict when the top will rule and when the bottom will be in charge — and that theory will take the time to consider the middle players, the herbivores.
Other ecological debates have followed a similar path. Disagreement over whether complex ecosystems are more or less stable than simpler ones, for example, is also settling to a consensus: it depends.
Useful practical predictions need not stem from universal laws. They may come instead from a deep knowledge of the unique workings of each ecosystem — knowledge gained from observation and analysis. Proposing sweeping theories is exciting, but if ecologists want to produce work useful to conservation, they might do better to spend their days sitting quietly in ecosystems with waterproof notebooks and hand lenses, writing everything down.
Ecological complexity, which may seem like an impenetrable thicket of nuance, is also the source of much of our pleasure in nature. If ecosystems were simple puzzles that all worked in the same way, they would lose much of their mystery, their surprise and their beauty. A lot of conservation work aims to protect the complexity and variability that makes ecosystems so hard to understand, and indeed to conserve.
Ecological rules are not the only reasons to promote conservation and fight extinctions. Sometimes we can argue for the conservation of particular species because ecology provides a scientific basis for it. At other times, we make the argument because there is a good chance that ecology will soon catch up and explain why the species are important.
But even if some predators do little but sit at the top of their food pyramids, creaming off a few herbivores, would we really want to live in a world without them? Answering that question really is easy.
Nature 507, 139–140 (13 March 2014) doi:10.1038/507139b