To  observe the consequences of removing a top predator, ecologists have traditionally relied on small scale experiments. Recently humans have caused a number of accidental mass extinctions, which have produced “natural experiments” across entire ecosystems. One such natural experiment in the forests of Guam has found results that far exceed the expectations of small-scale experiments; in the decades since the island’s birds were wiped out by invading snakes, spider populations have reached levels no one could have predicted: Guam has 40 times the number of spiders as neighboring islands!

Dr. Haldre Rogers, a Huxley Faculty Fellow at Rice University, has long been fascinated with the eerily silent forests of Guam. Almost three decades ago, the brown tree snake was first introduced to Guam as a stow-away aboard American military planes. This arboreal reptile was the first snake species on the island and birds were naive to the threat. Now almost all native species of birds are extinct on the island. Strict regulations have so far allowed the neighboring islands of Rota, Tinian, and Saipan to remain snake-free, but otherwise essentially similar to the ecosystem of Guam. This region of the Philippine Sea provided a perfect natural experiment for Rogers to study the absence of insectivorous birds. Her findings were published on PLOS ONE.

As Rogers explains, past small scale experiments using bird nets to create a bird-free branch or tree have shown consistent increases in spider populations. Her team decided to examine this trend on a larger scale by counting the spiders in several similar tracts of forest on Guam and nearby islands in both the dry and rainy seasons. The results showed an average forty-fold increase in total spider populations on Guam compared to nearby islands. Rogers describes having to carry a stick  with her to knock down spider webs when trekking through the forest of Guam — a precaution not needed on neighboring islands.

Previous small scale experiments predicted at most a 25-fold increase in spider populations, a far cry from the results of Roger’s natural experiment. However she stresses the importance of both study types. Natural experiments have a timeframe of decades compared to weeks or months and can demonstrate larger impacts, but the results are only correlative. Smaller scale experiments, on the other hand, can control more variables and isolate specific mechanisms. Together, “manipulative and comparative experiments give the strongest predictive power.”

“Past  studies have primarily focused on the loss of species from direct predation,” says Dr. Betsy Von Holle, Assistant Professor at the University of Central Florida. Few studies have “focused on trophic level effects and ecosystem responses.” Von Holle sees a great untapped potential for future studies of invasive species to shift their attention toward secondary effects down the food chain. She says the increased number of spiders on Guam will likely alter the populations of herbivorous insects on which the spiders prey, which in turn could mean long term changes in the communities of plant species which comprise the silent forests.

 

Sources:

Phone interview with Dr. Haldre Rogers (9-14-12)

Phone interview with Dr. Betsy Von Halle (9-17-12)

Rogers H, Hille Ris Lambers J, Miller R, Tewksbury JJ (2012) “‘Natural experiment’ Demonstrates Top-Down Control of Spiders by Birds on a Landscape Level.” PLoS ONE 7(9): e43446. doi:10.1371/journal.pone.0043446

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