Size of Island's Food Chain Linked to its Size

New research suggests that the size of an island's food chain is correlated with its size.

That a smaller island will typically sustain fewer species than a larger one is a widespread phenomenon in nature, but the new research on food chains suggests that animal communities on small islands may function differently than those on larger ones.

By studying a set of 20 small islands off the Finnish coast, researchers found that most of the small islands lacked the highest levels of the food chain.

"Ecologists have known for decades that less area means fewer species", said research leader Tomas Roslin. "What we show is that the decrease in species richness with decreasing area gets steeper when you climb up the food chain. That means that when you move towards smaller island size, you run out of top predators before you run out of intermediate predators, and that you lose the last plant-eaters before you lose the last plant."

While the research was literally conducted on islands, Roslin said that figurative islands around the world could be facing the same pressures as the Finish islands in the study.

"While we worked on a set of real islands, you can probably think of habitat fragments as 'islands' in a broader sense," Roslin said. "What our results then mean is that if we keep splitting natural habitats into smaller and smaller pieces, we may not only lose a lot of species from the resultant fragments, but also change the structure and functioning of local food webs."

To carry out the research, Roslin and his colleagues took samples of food chains on four levels: plants, predators feeding on the herbivores, and top predators feeding on the predators themselves.

The researchers focused on parasitic wasps as the predator of study.

"To test ideas about food chain length, you really cannot deal with raw counts of species - instead, you need to know which species form actual feeding chains" said Gergely Várkonyi, a wasp expert involved in the research.

"Among the wasps encountered on these islands, we were able to pick out the species truly dependent on the lepidopteran herbivores," Várkonyi said, referring to the large order of insects that included moths and butterflies. "As we see it, knowing not just what the species are but what they do in their lives is the key to sensible ecology", Gergely said.

Marko Nieminen, one of the researchers involved in the fieldwork conducted over three summers, said the study is unique because it was able to look at food chain patterns across a large pool of species.

"Where other people have looked at effects of island size on restricted numbers of species or restricted levels in food chains, we did the full thing across four levels," Nieminen said. "Overall, we dealt with 200 species of plants, 415 species of lepidopteran herbivores, 42 species of parasitic wasps attacking herbivores and seven species of wasps attacking parasitic wasps."

"In choosing the islands, we deliberately went for a simple system," Nieminen said. "Our islands were essentially smallish pieces of rocks with some forest and heathland on them. Historically, they all rose from the sea just some millennia ago, after being submerged and scraped clean of life by the last ice age. This similarity in structure and history allowed us to look at effects of island size, without having to worry about other differences among islands."

The researchers concluded that if conservationists want to succeed in saving ecological interactions, efforts to conserve much larger areas should be made.

"If we keep splitting habitats into ever-smaller pieces, then we will be losing upper links from food chains, and important control functions along with them," the researchers said in a statement.

The work is published in the journal Ecography.