Decapitated ecosystems come back to haunt us

What happens when you cut off something’s head? In the case of an ecosystem, it doesn’t die, but transforms into something very different — and sometimes scary. A zombie, if you like.

Decapitation is essentially what humans are doing to food webs throughout the world’s islands, continents, and oceans. Meaning that we’re cutting off the top level of the food chain, which consists of large predators, everywhere you look. John’s recent report of the horrific massacre of sharks in the Galapagos is a particularly grisly manifestation of a trend that has been underway now for thousands of years.

Large animals either (1) provide food for us, as tuna and other large fish do, so that we hunt them intensively, or (2) they compete with us for food and are otherwise real or perceived nuisances to our activities, and so are actively persecuted, as wolves and lions have been, or (3) they simply need huge expanses of wild territory to survive, as polar bears and tigers do, and our activities are eating up wild land.

For all these reasons big beasts are almost inherently incompatible with human civilization. And so depletion or extinction of large animals has followed human arrival on every continent and island we’ve colonized since leaving Africa in the Pleistocene.

But while this initially controversial proposition has become accepted in recent years, the profound consequences of those losses are less well known. The gradual decline of large animals on planet earth is an alarming trend in its own right. But might it also have more far-reaching effects?

Changes to marine ecosystem wrought by loss of top predators

We know, for example, that most of the largest animals are near the top of the food web, and that disturbing the top of the food chain often ripples down and out in unexpected but important ways, a phenomenon known as a “trophic cascade”.

Examples include the decimation of sea otters by Russian fur traders in the Aleutians during the 18th century, which resulted in epidemic outbreaks of their prey, seaweed-eating sea urchins, which in turn wiped out the lush kelp beds of the area and turned the bottom into “urchin barrens” with virtually no algae, nor fish, nor much of anything else other than urchins. The role of otters in this scenario has since been supported by the return of kelp beds where otter populations were allowed to come back and thrive.

And it’s not just big vertebrates. The lowly ochre starfish plays a somewhat similar role in the rocky shore zone of Washington State. Bob Paine’s classic experiments in the 1960s showed that removal of starfish allowed the blue mussels that they feed on to take over the shore, converting a rich and diverse shore community with dozens of species of invertebrates and seaweeds to a monotonous expanse containing little other than blue mussels. Top predators in both cases were the key to maintaining rich biodiversity.

Evidence is growing that this phenomenon is very general. Big coral reef fishes–both predators like sharks and large herbivores–appear  to similarly maintain healthy reefs, through complex interactions.

Now a new paper in Science summarizes the evidence for just how far those ripples travel and what happens when the waves hit the shore. Amid all the (justified) clamor over climate warming, this paper reminds us of humanity’s first and far older global change — the decapitation of food webs on land and sea worldwide.

The new paper focuses on the underappreciated and astonishingly far-reaching indirect impacts that predator loss has on all kinds of ecosystems. Predator impacts don’t just allow unmolested herbivores to damage a few leaves —  they can transform entire landscapes by releasing disease epidemics, facilitating wildfires, exotic invasions, and even changing atmospheric composition. From Mark Hay’s review at Faculty of 1000:

“Trophic cascades are ubiquitous and that top-down forcing must be included in conceptual overviews if there is to be any real hope of understanding and managing the workings of nature. Examples are many, but some far-reaching ones include the following ones: 1) exclusion of native birds, leading to an 80-fold increase in non-native spiders; 2) collapse of large demersal fish populations, leading to a 20% reduction in silica available to pelagic diatoms in the Baltic Sea; and 3) the realization that Pleistocene megaherbivores could have contributed to or even largely accounted for the reduced atmospheric methane concentration and the resulting abrupt 9°C temperature decline that defines the Younger-Dryas period. Overall, these authors convincingly argue that top-down control of ecosystems is omnipresent and that the loss of apex consumers is arguably humankind’s most pervasive influence on the natural world.”

We need big animals. Not just because they’re magnificent, or because livelihoods depend on them, or even because it’s the right thing to do. Which it is. In addition to all these reasons, we need big animals because the world unravels without them.

Source: Estes JA et al. 2011. Trophic downgrading of planet Earth. Science 333:301-6.

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