I'm wondering if there is a way to determine if a beach that is being used by sea turtles reflects a true nesting colony or if it is just incidental nesting (i.e. the beach is not a nesting colony, but just an available beach that the turtles would happen to nest on). I understand that sea turtles exhibit natal homing, but the range of their natal homing is quite wide and they may nest on any beach within that range, and it may be an ideal beach or not. Is there a way to determine if a beach reflects a nesting colony or just incidental nesting? Would it be the number of times that a particular female (or females) returns to the beach over the course of several years, the nest-density of that beach, or something else that I have not thought of? Is there a "magic number" to demarcate between a beach used by a colony or just incidental?
Based on the data we have in several places, the common strategy for marine turtles is as followed. During a nesting season, a female nest several time. Most of the nests from a female are deposited in the same beach but a small fraction (#10%) are deposited in other beaches of the region (bet hedging strategy). To separate between a "colony" beach and an occasional beach, a strategy could be to estimate the proportion of females seen only once for each beaches (corrected of course for the captured probability). If you calculate the ratio (female seen once)/(total number of females) for each beach, the ratio will be around 0.5 for colony beach and around 0.1 for occasional beach (the values could be different for the species/region). You can find a way to estimate the proportion of single nest females in this paper:
Briane, J.-P., Rivalan, P., Girondot, M., 2007. The inverse problem applied to the Observed Clutch Frequency of Leatherbacks from Yalimapo beach, French Guiana. Chelonian Conservation and Biology 6, 63-69.
Do not hesitate to contact me if you need some advice.
Sincerely
Marc Girondot
It is just a coincidence but remember that majority of vertebrates are sociable unless otherwise
We are talking about two very different events: in the first case, "the beach problem" affects a beach in the vicinity of a tested colony. This can be described inside the phenomena of natal homing. Thus as a variation in a normal pattern of a nesting site. But I guess you are talking about an incidental beach, a location where have never been reported -at least in the last 100-50 years- a nesting event, a colony, etc. This summer there has been two events of this kind in the spanish mediterranean coast, one in Delta del Ebro región, and another attempt in Santa Eulalia, Ibiza, Balearic Islands. Both sites have never been reported in bibliography, oral information, etc as nesting sites, (historical biogeographyand as a consequence this unsuccesful nesting attemps can be considered incidents, most probably without demographic and/or biogeographc consequences. So a comprehensive bibliographic revision, and oral interviews to local stake holders is compulsory in my eyes, before deciding conservation measures.
In short, I do not think there is a "magic number". What species are we talking about? It would be interesting to determine if the beach is ongoing a colonization event and/or if other nesting places are being lost. Maybe there are just more turtles and found new beaches. Please let us know
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