Spider Populations Flourish When Birds Leave

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Scope Correspondent

When the birds are away, the spiders will play, a recent study in the journal PloS One has found.

A press release describes the forests of the island of Guam, situated in the western Pacific ocean, as “eerily quiet” without the calls of songbirds, but spiders are now silently proliferating in the jungle landscape.

The brown treesnake, Boiga irregularis, an invasive species that was introduced onto the island during World War II, preyed easily upon Guam’s birds and their eggs, which were previously unaccustomed to predation, resulting in the extinction of ten out of the twelve native bird species.

Haldre Rogers of Rice University has discovered that the over thirty-year absence of insectivorous birds on Guam has dramatically increased spider populations in limestone karst forests, a jungle ecosystem where trees grow directly on limestone bedrock.

One of the benefits of studying bird exclusion on Guam is that the entire island has been without birds for a long period of time and so is a study site without the problems that small-scale bird exclusion experiments wrestle with, such as systems not having enough time to reach equilibrium or spiders and their prey migrating in and out of study cages used to keep birds out. Full Article »

Spiders Adopt Their Enemy’s Weapons

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Spiders can fend off ants by coating their web with the predators’ own alarm chemical, reports a new study of Singapore spiders and ants.

Only spiders whose webs are thick enough for ants to crawl on seem to use the chemical, suggesting that different spider species may have evolved targeted chemical weaponry.

The insect world is full of mysteries, and spiders and ants were the center of this one. Spider webs are made of silk, a biomaterial whose strength and elasticity make an efficient trap for catching prey. But staying on or near the web makes the spider a sitting duck for potential predators, including wasps as well as ants. Yet ants steer clear of webs.

Spiders are known for the defenses they have evolved for escaping wasps, their main flying predator: “leaf-refuges” for hiding; camouflage silk decorations to make the spider look threatening; “drag-lines” to drop from the web, and the ability to color-shift, as octopi sometimes do in corral reefs. But little was known about spider defenses against ants—or how a particular spider species’ weapons might adapt to its specific predators.

Orb-weaver spiders, a common spider family worldwide, are an attractive meal to ants, since they are large, meaty, sedentary, and easily immobilized by an ant army. Yet ants are rarely found on webs. Why not? Full Article »