by Whit Gibbons

April 24, 2005

'"Will you walk into my parlor?' said the Spider to the Fly." That 19th-century rhyme's underlying premise, predator-prey relationships, was the focus of an ecological study with far-reaching implications. In this case, however, island-dwelling spiders were the prey. Lizards were the predatory hosts.

A fairly reliable ecological principle is that the number of species inhabiting islands varies with island size. Small islands generally have fewer species of a particular group of animals than larger islands in the vicinity. Relative to this phenomenon, Tom Schoener and David Spiller of the University of California at Davis conducted a difficult but informative experiment.

They wanted to determine how factors other than habitat size influence success or failure of species colonizing a new area. In particular, they sought to answer whether island size or the presence of predators had more influence on invasion success. Such questions are of increasing interest because of the high level of worldwide travel by humans and the likelihood that small organisms might be transported to new areas. Understanding the ecological principles involved can only serve to our advantage.

Several common species of orb spiders that build silk webs on the ground occur on a chain of islands in the Bahamas. The chain is about a dozen miles long and includes more than a hundred islands that range from the size of a backyard to the size of a football field. The larger the island, the more likely spiders are to occur, but some of the smaller islands have spider populations. Lizards were present on some of the larger islands and absent on others; none were present on the smallest islands.

To test the effect of lizard predation on spiders, the scientists selected fifteen islands. Five were large islands with lizards; five were large ones without lizards; five were small islands, which had no lizards. One of the most common species of orb spiders in the Bahamas was chosen for artificial colonization of islands. A selected number of male and female spiders was released on each of the fifteen islands. The following year, three times as many of each sex were released on each island.

After the simulated colonization, the researchers checked how many spiders remained after various intervals of time. Even the larger islands were small enough to allow an arachnologist (someone who studies spiders) to find individuals because of their characteristic webs.

If lizards were present, the bleakest time for spiders was four days after they were introduced to the island, when fewer than 40% of the spiders survived. In the absence of lizards, more than 65% of the spiders survived.

To determine the long-term persistence of colonizing spiders, the investigators sampled all fifteen islands each year. Most of the small islands had more than a dozen spiders remaining. Most of the large, lizard-free islands had more than 200 spiders. But only one of the islands where lizards lived had any spiders at all: two. By the end of the five-year experiment, the introduced orb spiders were extinct on all the islands on which lizards were present. One small island still had spiders, and three of the large, lizardless islands had enormous orb spider populations.

One conclusion drawn from the study is that the presence of predators had a stronger influence on survival success and persistence of spiders than did the size of the island. The investigators suggest that from a conservation ecology perspective more emphasis should be given to studying predation effects on islands.

Another point to be gained from the study is that subtle ecological phenomena, such as control of a prey species by a small, natural predator like a lizard, can have dramatic and long-lasting effects on the ecology of a region. Lizards and spiders may seem like insignificant creatures. But the interaction of predators and prey--and the ecological effects of that interaction--is far from insignificant. Though mostly unrecognized by us, such interactions are in constant progress in natural habitats among thousands of species. Studies such as this one add to our understanding of the complexities of the natural world. And the more we understand about the environmental mosaic around us, the more we can appreciate the significance of the diversity of life.

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