Sunday, April 14, 2013

Ripples in the ecosystem

Humans have been moving animals around the planet for thousands of years. Hunter and gathers took dogs with them as they migrated, and with the evolution of agriculture soon other domesticated animals were traded between populations. Like all animals, human alter the environments they live-in and so do their commensal species. But humans are special in that they can move quite large animals around unintentionally, bring together species that have not co-evolved and setting the stage for long term changes in ecosystems.

In 2012 Rogers et al. described the first landscape level natural experiment showing the impact of bird loss on the control of their prey, spiders, and the magnitude of effects generated from long-term, landscape-scale bird loss to the effects generated from bird exclusion experiments elsewhere.

They took advantage of the only place in the world where all avian insectivores have been extirpated from the landscape, the Western Pacific island of Guam. The brown tree snake, Boiga irregularis, was introduced to Guam in the mid-1940's. The snakes ate their way through the bird fauna, leading to the extirpation of all native insectivorous bird species from the majority of the island in the mid-1980's. There are only two insectivorous bird species remaining today, in extremely localized populations; the Micronesian Starling has a small population on Andersen Air Force Base at the northern tip of Guam and the Mariana Swiftlet inhabits three caves on the Naval Base in southern Guam. No non-native insectivorous bird species have colonized the forests of Guam, therefore, aside from these two locations on the military bases, the forests are devoid of insectivorous birds.

Rogers and colleagues focused on spiders because experimental studies showed a consistent top-down effect of birds on spiders. They conducted spider web surveys in native forest on Guam and three nearby islands with healthy bird populations. They found spider web densities on Guam were 40 times greater than densities on islands with birds during the wet season, and 2.3 times greater during the dry season.

The results confirm the general trend from manipulative experiments conducted in other systems however, the effect size was much greater in this natural experiment than in most manipulative experiments. In addition, bird loss appears to have removed the seasonality of spider webs and led to larger webs in at least one spider species in the forests of Guam than on nearby islands with birds.The results suggest that effect sizes from smaller-scale experimental studies may significantly underestimate the impact of bird loss on spider density as demonstrated by this large-scale natural experiment.

However, changes in spider populations are not the only impact of bird loss on Guam. Spider-eating birds were decimated by the brown tree snake, but so were fruit eating birds that disperse seeds. McConkey and colleagues (2012) note that seed dispersal interacts decisively with the major drivers of biodiversity change: habitat fragmentation, over-harvesting, biological invasions, and climate change.

So, Guam’s forests are not only exceptionally quiet due to the loss of birds, but they are also thinning out. A four year study planned for this summer will examine the island for 16 tree species, looking at how the loss of seed dispersing birds, is affecting tree distribution.

While the brown tree snakes have been in Guam for almost 75 years, the presence of invasive constrictors in the Florida Everglades is more recent, probably less than 30 years. So, the Everglades, like Guam, will likely become a laboratory for studying the impact of invasive snakes on the ecosystem.

Martin, C. 2013. Where have the trees of Guam gone?

McConkey, K.R., S. Prasad, R. T. Corlett, A. Campos-Arceiz, J. F. Brodie, H. Rogers, & L. Santamaria, 2012. Seed dispersal in changing landscapes, Biological Conservation, 146, 1-13

Rogers H, J. Hille Ris Lambers, R. Miller, & J.J. Tewksbury (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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