Tuesday, June 23, 2015

When the mountain fears its deer: predators, trophic cascades, and ecological integrity - Part 1

"I have watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every edible bush and seedling browsed, first to anemic desuetude, and then to death. I have seen every edible tree defoliated to the height of a saddlehorn. Such a mountain looks as if someone had given God a new pruning shears, and forbidden Him all other exercise. … I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer."

-Aldo Leopold

Our story begins in 1963, in a small bay that opens into the eastern Pacific Ocean. It's a rainy, gray day, of the kind that only an anxious and somewhat downtrodden coastal Washington can provide. The foamy seawater ripples with the eager anticipation of a coming storm. And, standing in a small rectangular enclosure, a man is fervently tossing starfish.

Pisaster ochraceus - purple sea star

Photo by Stephen Bensten.

In the Mukkaw Bay, the starfish Pisaster feeds on several species of barnacle. Where these starfish are present, the barnacles are restricted to a small band in the intertidal zone, outside of which one can find a litany of ocean dwellers, including anemones, chitons, limpets, algae, and sponges. To understand how Pisaster influences the structure of the tidal ecosystem, Robert T. Paine devised an experiment where seastars were removed from the enclosure, and the species living in the enclosure were monitored. Upon the predator's removal, barnacles surged in number, and crowded much of the available space. With the high barnacle population, the other intertidal species began to disappear. By controlling barnacle density, the sea star was the foundation of an entire intertidal ecosystem.

From this research, the keystone species concept was born.

Much as a keystone holds together an arch, a keystone species is one that disproportionately affects the other species in an ecosystem. When the keystone species is a predator, a 'trophic cascade' may occur, where species that the predator does not feed on change in abundance or behavior. While the concept of a trophic cascade can be simple, finding evidence for it is a surprisingly tricky endeavor. Ecosystems are complicated webs of interactions, and different parts are influenced by multiple factors. A barnacle is affected by predation, but also by weather, temperature, land use, and ocean acidity. These influences are further complicated by interactions: one effect may be stronger in the presence of another. For example, predation may affect a population more strongly when the weather is extreme than when times are calm.

In this upcoming series, I will be discussing a number of famous studies documenting trophic cascades across diverse ecosystems. I will describe the origins of the studies discovering the cascades, and the potential pitfalls of the available evidence. In a rapidly changing world, predators are an important component of the world's ecosystems, and I hope that this series of posts will inspire you and teach you about a key conservation concept.

Next week: sea otters, orcas, and shallow water ocean diversity 

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