Philippe Rivalan

Rarity Value and Species Extinction: The Anthropogenic Allee Effect

Standard economic theory predicts that exploitation alone is unlikely to result in species extinction because of the escalating costs of finding the last individuals of a declining species. We argue that the human predisposition to place exaggerated value on rarity fuels disproportionate exploitation of rare species, rendering them even rarer and thus more desirable, ultimately leading them into an extinction vortex. Here we present a simple mathematical model and various empirical examples to show how the value attributed to rarity in some human activities could precipitate the extinction of rare species—a concept that we term the anthropogenic Allee effect. The alarming finding that human perception of rarity can precipitate species extinction has serious implications for the conservation of species that are rare or that may become so, be they charismatic and emblematic or simply likely to become fashionable for certain activities.

Contrasted demographic responses facing future climate change in Southern Ocean seabirds

1. Recent climate change has affected a wide range of species, but predicting population responses to projected climate change using population dynamics theory and models remains challenging, and very few attempts have been made. The Southern Ocean sea surface temperature and sea ice extent are projected to warm and shrink as concentrations of atmospheric greenhouse gases increase, and several top predator species are affected by fluctuations in these oceanographic variables. 2. We compared and projected the population responses of three seabird species living in sub-tropical, sub-Antarctic and Antarctic biomes to predicted climate change over the next 50 years. Using stochastic population models we combined long-term demographic datasets and projections of sea surface temperature and sea ice extent for three different IPCC emission scenarios (from most to least severe: A1B, A2, B1) from general circulation models of Earths climate. 3. We found that climate mostly affected the probability to breed successfully, and in one case adult survival. Interestingly, frequent nonlinear relationships in demographic responses to climate were detected. Models forced by future predicted climatic change provided contrasted population responses depending on the species considered. The northernmost distributed species was predicted to be little affected by a future warming of the Southern Ocean, whereas steep declines were projected for the more southerly distributed species due to sea surface temperature warming and decrease in sea ice extent. For the most southerly distributed species, the A1B and B1 emission scenarios were respectively the most and less damaging. For the two other species, population responses were similar for all emission scenarios. 4. This is among the first attempts to study the demographic responses for several populations with contrasted environmental conditions, which illustrates that investigating the effects of climate change on core population dynamics is feasible for different populations using a common methodological framework. Our approach was limited to single populations and have neglected population settlement in new favourable habitats or changes in inter-specific relations as a potential response to future climate change. Predictions may be enhanced by merging demographic population models and climatic envelope models.

Density-dependent nest destruction and population fluctuations of Guianan leatherback turtles

Approximately 40% of the world’s leatherback marine turtles (Dermochelys coriacea) nest in Suriname and French Guiana. Trends in nest numbers reconstructed for the last 34 years indicate several up and down events. We undertook computer simulations to determine whether a density-dependent phenomenon might be involved because the period of high-density nesting coincides with a high level of nest destruction by nesting females. The parameters of density-dependent nest destruction were calculated for the Ya:lima:po-Awa:la beach. We show that: (1) density-dependent nest destruction occurs, but (2) it promotes a density-dependent feminization of hatchling sex ratio, and consequently (3) the global production of juveniles continues to increase in relation with the increasing number of deposited nests even for the highest densities observed at that beach. Mean annual production of female hatchlings per adult female, although density dependent, is less than two juveniles even at the lowest densities of nesting females.

Modeling Approaches to Quantify Leatherback Nesting Trends in French Guiana and Suriname

ABSTRACT Nesting of leatherbacks in French Guiana and Suriname has been monitored for more than 30 years. Given the documented exchange of tagged females across the nesting beaches, leatherbacks found on the 6 principal nesting areas in French Guiana and Suriname are considered to be a single large nesting population. Despite more than 3 decades of work on this population, this populations status remains unclear. Here we describe the most recently available estimates of various life history parameters and describe the trend of the number of nests laid in the region over the past 36 years. Our analyses suggest that the trend of the whole population in French Guiana and Suriname is stable or slightly increasing over this time period. We strongly urge the continued monitoring of the population by the different research groups in the region so that future data sets will enable an accurate description of the status of this leatherback population.

MAXIMUM LIKELIHOOD ESTIMATES OF TAG LOSS IN LEATHERBACK SEA TURTLES

Abstract Leatherback sea turtles (Dermochelys coriacea) nesting in French Guiana have been the focus of a long-term monitoring and metal flipper-tagging program and were subjected to a double-tagging experiment starting in 1994. We developed a new method to estimate the rate of tag loss based on the number of days at liberty after tagging, using maximum likelihood function fitting. Metal tag loss in leatherback sea turtles is characterized by a high rate of loss just after tagging (>30% of turtles lose at least 1 tag within a year) and is followed by higher retention over a longer time period. Using additional data from permanent internal tags that were subsequently applied to turtles in French Guiana, we have shown that the rate of loss of the second metal tag was higher than that of the first metal tag. More accurate estimates of tag loss rates are essential for correctly generating demographic parameters from Capture–Mark–Recapture (CMR) analyses. We have developed a software package to facilitate the use of these models by others researchers in estimating tag loss.

Beach rock as a keystone habitat for amphibious sea snakes

SUMMARY The shorelines of coral islets are subject to strong anthropogenic pressure, being highly coveted for tourism. These landforms contain unique biotic assemblages but unfortunately are limited in size making them extremely vulnerable to perturbation. Robust information linking habitat structure and species requirements is urgently needed to promote and guide the conservation of these fragile areas. New Caledonia contains critical shore habitats for two species of amphibious sea snakes. One species (Laticauda laticaudata) shelters almost exclusively under mobile beach rocks, which are both easily accessible from the sea and regularly submerged at high tide. The scarcity of such specific and spatially limited habitat restricts the distribution of this species to highly localized areas. The other species (L. saintgironsi)usesagreatervarietyofterrestrialrefuges, but has a preference for shores with abundant beach rocks. These findings offer a robust basis to promote the conservation of these crucial habitats and to justify their inclusion in marine protected areas(MPA), which in turn should benefit a wide array of other organisms also dependent on beach rocks.

Phenology of marine turtle nesting revealed by statistical model of the nesting season

BackgroundMarine turtles deposit their eggs on tropical or subtropical beaches during discrete nesting seasons that span several months. The number and distribution of nests laid during a nesting season provide vital information on various aspects of marine turtle ecology and conservation.ResultsIn the case of leatherback sea turtles nesting in French Guiana, we developed a mathematical model to explore the phenology of their nesting season, derived from an incomplete nest count dataset. We detected 3 primary components in the nest distribution of leatherbacks: an overall shape that corresponds to the arrival and departure of leatherback females in the Guianas region, a sinusoidal pattern with a period of approximately 10 days that is related to physiological constraints of nesting female leatherbacks, and a sinusoidal pattern with a period of approximately 15 days that likely reflects the influence of spring high tides on nesting female turtles.ConclusionThe model proposed here offers a variety of uses for both marine turtles and also other taxa when individuals are observed in a particular location for only part of the year.

The Inverse Problem Applied to the Observed Clutch Frequency of Leatherback Turtles from Yalimapo Beach, French Guiana

ABSTRACT Observed clutch frequency is the observed number of clutches deposited by a female turtle in a single nesting season. This value is generally biased compared to the true nest number because some clutches could have been missed by beach patrols. We constructed an analytical model to search for the distribution of observed clutch frequency according to 1) the distribution of the clutch number per female, 2) the probability of female capture on the beach, and 3) the probability that a female was an intraseasonal one-time nester. We used data from 1987 to 2003 collected on leatherbacks nesting on Yalimapo beach in French Guiana to test the power of the model. Whereas for 13 of the 16 years, the model produced estimates concurrent with the field data; for the other 3 years, there was no concordance, indicating a lack of power of the model. We describe a required level of capture effort on the beach that will be sufficient for the model to adequately describe clutch frequency across the nesting season. This level is the product of the proportion of the season monitored times the instantaneous probability of the presence and the detection for a particular female during 1 nesting event.