EAT MORE SPIDERS THAN YOU THINK
by Whit Gibbons
April 24, 2005
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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