Sunday, September 5, 2010
In keeping with the beach theme from the last post, as loose as it may be, here's an article published in the journal Ecological Applications all about sea turtle nesting.
The loggerhead sea turtle (Caretta caretta) migrates internationally, forages in subtropical and temprate oceans, and comes up onto sandy beaches in order to build nests and lay eggs. The populations of these turtles are have been depleted, especially in the Pacific, and the species has been listed as Endangered on the IUCN Red List and as Threatened on the U.S. Endangered Species Act. Nesting beaches are typically located between 19 and 36 degrees latitude in each hemisphere with large assemblages (up to 80-90% of the world population) in southern Florida and Masirah, Oman. Based on mitochondrial DNA, it is known that the turtles nesting in different geographic regions are genetically distinct from each other. Even looking on a finer scale you can see genetic differences within even a local region; for example, Florida has four distinct subpopulations. Why the divide? Most likely it is due to females returning to their natal beaches to nest and males providing the gene flow between beaches.
When it comes to counting nesting females, the job is relatively easy. After all, a giant turtle pulls herself up the beach, digs a big hole for her eggs, covers the nest, and then pulls herself back to the water. So if you don't see the turtle herself then you will most likely see the marks of her passage in the sand. You can even tell which species has passed just by looking at the tracks. As for clutch (nest) frequency, a female can lay approximately four clutches per season. By using these nests/marks organizations such as the Fish and Wildlife Research Institute of the Florida Fish and Wildlife Conservation Commission (FWC) are able to estimate the populations sizes of loggerheads, green turtles (Chelonia mydas), and leatherbacks (Dermochelys coriacea). Coordinated nest counting programs began in Florida in 1979, and as of 1989 a consortium of conservation groups has established a subset of Florida beaches to survey to take into accound the inherent variablility in nest counting over a broad geographic scale. The FWC leads what is called the Florida Index Nesting Beach Survey program which generates nesting indices. This program is especially useful in resolving spatial and temporal nest counts and generating trend assessments. This particular paper uses these data in this way with loggerhead turtle nesting on Florida beaches, particularly in characterizing a decline in nest counts and assessing the efficacy of recovery efforts.
When talking about nest-count surveys, the paper defines two complementary programs: "Statewide" beaches (n = 190) that aim to be complete in their seasonal and geographic coverage but are not highly consistent, and "Index" beaches (n = 32) that aim to be consistent and have a higher resolution but are not as complete in their coverage. With these data, the researchers conducted a trend analysis on both annual survey-region (SR) nest count totals (n = 18) and annual zone-level (ZL) nest counts (n = 18 x 368 = 6624). For more info on these zones and the stats used then take a look at the paper - there's also a ton of figures.
Whew. Ok, so what did they find? Over an 18 year period, loggerhead nest counts on Index beaches increased and then decreased, with a net decrease; Statewide beaches also showed a declining trend. Lots and lots and lots of stats and models simplified....everything declined and more steeply more recently, although there are some "peaks" and "valleys" going on, possibly due to loggerhead nesting distribution resulting from beach attributes. Locally, declining beach conditions, due in large part to chronic erosion, coastal armoring, artifical lighting, human beach activity, and cold water coastal upwelling are likely causing some of the effects within the spatiotemporal scale. These adverse conditions are probably not influencing the subregional differences in nesting trends as you would see similar effects on other sea turtle species in the same area and you don't (green turtle nesting is actually increasing), although subregionally loggerheads are still declining.
Are you confused yet? The authors conclude that the observed decline in loggerhead nests is best explained by a decline in the number of adult female loggerheads in the population. So why the decline in females? Well, that's a little harder to get at, especially considering that they travel all over the oceans and have a wide array of threats throughout their various life stages. After all, they don't even reach sexual maturation until they are 25-30 years old. Increased moratality at any life stage (hatchling, juvenile, and adult) could affect the overall population and nesting frequency. Let's add another complication: nest temperature determines hatchling sex ratios. It has been observed that female hatchling production on nesting beaches has also declined. These authors postulate that it is due to low incubation temperatures a generation ago that brought about a male-biased primary sex ratio. However, global warming might actually have positive effect in skewing the ratio towards more females.
How do we fix it? More attention from resource managers is one way. Turtles are charismatic megafauna and when people find out that the projected decline for loggerhead nest counts is 80% by 2017 I think they might care. That means that the individual person can do quite a bit too. Contact your local resource managers and state representatives and/or join your local conservation group. Have a voice.
Here's the paper:
Witherington, Blair, et al. (2009) Decreasing annual nest counts in a globally important loggerhead sea turtle population. Ecological Applications: 19(1), 30-54.
Since this is paired with my Ft. Lauderdale visit, I'll give you two great links for turtle nesting in that area:
Greater Ft. Lauderdale's article on Nesting Sea Turtles
Nova Southeastern's page on Broward County's Sea Turtle Conservation
(image from seatutlenet.com)