20 November 2009

This morning we went to the Refuge earlier than usual so that we could make certain that all the tech stuff worked for our Skype session with Jonathan's class at 1:00 pm EDT (2:00 pm AST). It is convenient to have wireless access in numerous locations at Cottages By The Sea, however the connection is much slower than the DSL line at Refuge headquarters. Skype worked fine from the balcony of Refuge headquarters. After we made certain that everything worked we went to check traps at about 12 noon. Unfortunately I got tied up in a conversation with a wildlife specialist who works for the USDA. He will be attempting to eradicate mongoose from Sandy Point National Wildlife Refuge because of their perceived negative influence on the green, hawksbill, and leatherback sea turtle eggs and hatchlings. Their is very little hard data to support the contention by many that mongoose are serious threat to the survival of these threatened and endangered species of sea turtles. Most of the "evidence" is the appearance of mongoose tracks at turtle nests. The argument goes something like this. People have actually observed (very rarely) mongoose digging up turtle eggs from green and hawksbill sea turtle nests. Their nests are generally less than 18 inches below the surface. Therefore, observation of tracks at a nest site of all turtles (including leatherbacks with nests typically 36 inches or more below the surface) means that mongoose have destroyed the eggs and/or hatchlings. As a scientist I have a difficult time accepting such an argument based upon such scant data. There are certainly other possible explanations of the existence of mongoose tracks at nest sites. For example, hatchlings left the nest and mongoose investigated an empty nest, or mongoose walk by nests in search of ghost crabs.

To me the larger question of mongoose predation impacts upon recruitment into the next generation of turtles is much more interesting. I attempt to estimate their impacts in the following argument based upon published data. One study on St. John (Small, V. 1982. Sea turtle nesting at Virgin Islands National Park and Buck Island Reef National Monument, 1980 and 1981. U.S. Dept. of the Interior Research/Resources Management Report SER-61, 54 pp. V. Small, Virgin Islands National Park, Box 110, Cruz Bay, St. John, U.S. Virgin Islands, 00830.) determined that intensive trapping and removal of mongoose during turtle hatchling season would be effective in reducing local mongoose population size. The author "estimated" that mongoose destroyed 23% of the eggs and hatchlings of hawksbill turtles. Although this estimate sounds alarming, I have not seen a mathematical model that determines how this translates into the overall recruitment into the population. For example, USFWS estimates 15,000 female hawksbill nest around the world each year (see http://www.fws.gov/northflorida/SeaTurtles/Turtle%20Factsheets/hawksbill-sea-turtle.htm), lay an average of 140 eggs per nest, and an average of 4.5 nests per year. Therefore, we can estimate 9.45 million hawksbill turtle eggs produced per year. Using USFWS estimates of approximately 300 nests on US territories and states where mongoose live, we get approximately 42,000 eggs laid. Using Small's estimate of a 23% loss, we get approximately 10,000 hawksbill eggs/hatchlings being removed by mongoose each year from a total potential of 9.45 million - a number that is approximately one-tenth of one percent. Most people would argue that 0.1% or 0.001 is a very small number indeed! So, I don't believe the existing data demonstrate that mongoose have a significant impact on recruitment rates for hawksbill turtle populations and we have no data for impacts upon mongoose on other endangered turtle populations.

So, the USDA has decided to remove all mongoose from Sandy Point National Wildlife Refuge over the next 16 months to determine if that reduces mongoose predation on sea turtle nests. This seems a little odd to me. The premise is that mongoose prey on turtle nests. The hypothesis is that removal of mongoose from nesting areas will prevent mongoose predation on turtle nests. Isn't that similar to making the prediction that if I don't put my boat in the water it won't sink? If our desire is to increase the size of the sea turtle population, which it should be, shouldn't the question be what impact does mongoose removal from turtle nesting areas have on turtle recruitment? Maybe that question is not as easy as the boat question. I don't understand how these government agencies work.