Invasive Species Impact on Island Ecology

By Stephen Holle

Planet earth is intimately linked through various modes of transportation such as ships and planes. However, unlikely passengers (flora and fauna) sometimes stroll past security and embark on free rides across the world. The unintended passengers can then become introduced into environments with favorable conditions and limited competition, and the introduced species is able to out compete native flora and fauna. Invasive species are one of the largest threats to biodiversity on a global scale and as a result very few environments are characterized by completely native species. Biogeographic conditions also further compound the issue of nonnative species and islands are especially sensitive to their spread partly due to the limited amount of land surface area, low fecundity, and an absence of coevolution with the invasive species. Therefore, the ecological balance of isolated regions such as Guam is greatly impacted by invasive species, which degrade localized biota (Wiles 2003).

Guam has a number of invasive species which greatly disrupt the ecological balance of the island. However, the brown tree snake (Boiga irregularis) has been one of the most impactful and has decimated native bird populations across the island. B. irregularis was introduced in Guam following World War II and their expansion and growth across the island coincided with a steep decline in native bird populations such as the Marianas Crow and the Island Collared-Dove.

A table depicting the decline of native bird populations on Guam due to the introduction of B. irregularis. Table: Guam Division of Aquatics and Wildlife Resources (Wiles 2003).

However, their destruction was not limited to bird populations and B. irregularis also preyed on lizards, non-native mammals, and flying fox populations. Native bird populations are critical components to the ecology of Guam because they perform valuable functions such as seed dispersal and pollination and when they are absent it greatly disrupts the critically sensitive balance of nature and propagation of native plant species. In order to quantify how detrimental brown tree snakes are to local bird populations the Department of Aquatics and Wildlife Resources (DAWR) on Guam conducted roadside surveys in 11 specific locations on the northern end of the island. In the past Guam has supported approximately 23 native bird species, but since the introduction of the B. irregularis ten of these species were entirely lost from the survey areas and other species experienced a decline around 90%, while others experienced complete extirpation within a short time frame. The eradication of native bird populations occurred rapidly, and within 6.9 years a majority of native bird species experienced significant declines (Wiles 2003).

A photo showcasing a juvenile brown tree snake. Photo credit: Stephen Holle.

In order to conserve natural resources and local conditions, Guam is looking for various ways to control and eradicate B. irregularis populations. On Guam the DAWR has developed a number of mitigation measures such as captive breeding programs and B. irregularis control programs to minimize impacts on the islands ecology. Field workers within DAWR distribute modified minnow traps, which are baited to capture snakes for termination. However, as Diane Vice, director of the brown tree snake control program points out, “the long term goal of the brown tree snake program is eradication, but current technology and staffing needs only allow for population control measures.” Currently, DAWR is focused on controlling outbreaks of B. irregularis on other islands across Micronesia.

A photo depicting modified minnow traps, which are used to capture B. irregularis. Photo credit: Stephen Holle.

DAWR uses trained dogs to inspect cargo entering passenger and commercial planes and ships to curve the export of invasive species like B. irregularis. Other mitigation measures also exist at the federal level and are being developed by the National Wildlife Research Center (a division of the USDA) to control populations of B. irregularis and potentially lead to more effective long term solutions. One application is a biological control in which “toxic mouse bombs”, which are mice laced with the chemical acetaminophen (an active ingredient in aspirin) are dropped out of planes. The brown tree snakes consume the mice and perish by this unsuspecting trap. The chemical is lethal to snake populations, but will not affect other species and humans, which make it a viable control measure. Only time will tell if the mitigation measures are enough to stop this unwelcomed predator and restore baseline conditions (Wiles 2003 & Vice 2012).

Works Cited

Vice, Diane. Personal Interview. 22 May. 2012.

Wiles, Gary et al. “Impacts of the Brown Tree Snake: Patterns of Decline and Species Persistance in Guam’s Avifauna.” Conservation Biology 17 (2003): 1350-1360.

About the Author: Stephen Holle is a senior working toward a bachelor’s degree in environmental studies at USC Dana and David Dornsife College of Letters, Arts and Sciences. With his ENST scientific diving experience he hopes to move on to a career focused on policy and natural resource management.

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