National Wildlife Research Center (NWRC)

PROJECT GOALS:

Develop information and methods applicable to avian populations of Wildlife Services' management concern and control techniques to reduce or eliminate invasive species.

Project Accomplishments 2008

Invasive Black Spiny-Tailed Iguanas Threaten Resources in Florida—The native range of the black spiny-tailed iguana extends from southern Mexico to Panama.  The species was introduced into Florida around 1980 when three animals were brought from Mexico and released on the southern end of Gasparilla Island near Port Charlotte in southwest Florida.  Since then, the species has spread to occupy the entire island, and with some human assistance, there are also iguana populations on the adjacent mainland, small islands nearby, and in Key Biscayne near Miami.   Over the years, as the iguana population has grown, these lizards—once an interesting novelty—have become a serious problem.  Residents of Gasparilla Island in both Lee and Charlotte counties have funded initiatives to control this invasive pest species, which eats valuable landscaping plants and occasionally enters buildings.

Black spiny-tailed iguanas also impact natural resources.  Scientists suspect that the iguanas disperse seeds of nonnative invasive plant species.  They consume many native plants as well, including some of the same food plants used by gopher tortoises, which is listed by the Florida Department of Environmental Protection as a threatened species.  In addition, the iguanas occupy burrows made by gopher tortoises.  The gopher tortoise is slow to reach reproductive maturity, produces small egg clutches, and grows slowly.  Juvenile tortoises are vulnerable to predation for months after emergence.

In 2008, through an agreement with Charlotte County, APHIS biologists in Florida began to trap and remove iguanas on Gasparilla Island.  Concurrently, using iguanas captured during the trap and removal operation, NWRC scientists in Gainesville, Florida, initiated research to identify a toxicant to augment the ctenosaur trapping effort.  Furthermore, as WS trappers removed iguanas, NWRC scientists examined the carcasses to quantify body size, reproductive condition, and stomach contents.  Analyses of stomach contents confirmed that iguanas are primarily herbivorous.  However, they also appear to be opportunistically predatory, as indicated by the remains of a juvenile gopher tortoise found in the gut of a large male iguana.  This is the first documented instance of such predation by iguanas on this State-listed threatened species.  Scientists do not yet know the extent to which predation by the invasive black spiny-tailed iguana affects the gopher tortoise population on Gasparilla Island. 

NWRC is using multiple monitoring methods that address the differing habitats and circumstances on the island to collect data for assessing iguana populations.  One of these methods will be strip transects for iguana tracks to compare areas having received control to similar areas where control has not been initiated.  NWRC scientists will also use tracking plots to establish baselines in dune areas prior to control and for comparison after control begins.  They will also apply visual transects for the same purposes in residential areas and along rip-rap areas by the shore.  Initial observations indicate the species could pose a threat to some snakes, which is of concern because indigo snakes are a listed as a threatened species on the island.  NWRC scientists are continuing to work with APHIS biologists and Charlotte County cooperators to eliminate threats posed by this invasive species.

Artificial Effigies are Effective in Managing Urban Crow Roosts—In the United States, congregations of crows numbering in the tens of thousands have been documented for many years.  A recent trend is for crow roosts to form in urban/suburban areas as opposed to rural sites.  Roosting aggregations comprising thousands of crows have become familiar in towns and cities across the country.  For the past several years, wintering crows have roosted in and around the city of Lancaster, Pennsylvania, to the consternation of Government officials, homeowners, and businesses.  The Lancaster area is a matrix of residential communities, farmland, shopping centers, and industrial and light commercial development.  In winter, crows use all of these areas for staging and/or roosting.  Scientists from the NWRC field station in Florida evaluated the effectiveness of artificial effigies as a new technique for managing crow roosts.

While the use of effigies (i.e., dead crows) to affect crow behavior is not a new idea, there is divergent information as to whether or not crow effigies or carcasses actually are effective in disrupting crow behavior.  Furthermore, no study had previously investigated the use of effigies specifically for dispersing crow roosts.

In 2006 and 2007, NWRC researchers installed effigies in trees and at ground level in sites where crows roosted.  When necessary, they augmented the effigies using lasers and recorded crow alarm calls.  Roosts were monitored before and after effigy installation, and crow numbers substantially decreased at the treated sites.  The scientists concluded that effigies are potentially a valuable component of integrated roost dispersal programs.

In 2008, based on the findings of the first two years, researchers advised the Lancaster community-based crow roost dispersal effort on how best to implement harassment using effigies.  This integrated dispersal effort resulted in approximately 35,000 crows being moved into an area where they were considered inoffensive to the community.  The crows did not abandon the Lancaster area, but they ended up roosting at a location where their impact was negligible.  The artificial crow effigy is a simple, inexpensive tool that has proven to be effective when applied in an integrated roost management approach.

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Gainesville, FL, Field Station