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Favoring recruitment as a conservation strategy to improve the resilience of long-lived reptile populations: insights from a population viability analysis

Citation

Warret Rodrigues, Chloé; Angin, Baptiste; Besnard, Aurélien (2022), Favoring recruitment as a conservation strategy to improve the resilience of long-lived reptile populations: insights from a population viability analysis, Dryad, Dataset, https://doi.org/10.5061/dryad.9p8cz8wgv

Abstract

In long-lived species, although adult survival typically has the highest elasticity, temporal variations in less canalized demographic parameters are the main drivers of population dynamics. Targeting recruitment rates may thus be the most effective strategy to manage these species. We analyzed 1136 capture–recapture histories collected over 9 years in an isolated population of the critically endangered Lesser Antillean iguana, using a robust-design Pradel model to estimate adult survival and recruitment rates. From an adult population size estimated at 928 in 2013, we found a yearly decline of 4% over the 8-year period. As expected under the canalization hypothesis for a long-lived species, adult survival was high and constant, with little possibility for improvement, whereas the recruitment rate varied over time and likely drove the observed population decline. We then used a prospective perturbation analysis to explore whether managing the species’ immature cohorts would at least slow the population decline. The prospective perturbation analysis suggested that a significant and sustained conservation effort would be needed to achieve a recruitment rate high enough to slow the population decline. We posit that the high recruitment rate achieved in 2014 – likely due to the maintenance in 2012 of the main nesting sites used by this population – would be sufficient to slow this population’s decline if it was sustained each year. Based on the results of diverse pilot studies we conducted, we identified the most likely threats targeting the eggs and immature cohorts, stressing the need to improve reproductive success and survival of immature iguanas. The threats we identified are also involved in the decline of several reptile species, and species from other taxa such as ground-nesting birds. These findings on a little-studied taxon provide further evidence that focusing on the immature life stages of long-lived species can be key to their conservation.

Methods

Between 2012 and 2020, we conducted a yearly mark–recapture survey at the end of the males’ reproductive peak of activity (i.e. 9 primary sessions). Each primary session consisted of five secondary sessions (i.e. 5 consecutive days of sampling for 7.5 hours per day). We thus sampled the population over 45 secondary sessions. The capture teams consisted of 10 to 12 people, at least half of whom were proficient at spotting and capturing iguanas. Field assistants were divided into two-person teams, with each inexperienced person partnered with someone experienced. To ensure comprehensive and uniform coverage of the islet, we divided it into five zones that could be covered in a similar duration. Consequently, zones with dense cover or high iguana density were smaller than open zones or with few iguanas. We minimized observer bias by randomly assigning a two-person team to a zone at the start of the five-day sample period. Each two-person team systematically searched for iguanas in their zone before moving to the next zone on the following day. Therefore, all teams had the opportunity to search each zone for a total of 7.5 hours. We captured iguanas by hand or with a noose and scanned them for an existing PIT tag (11.5 x 2.12 mm; 0.1g, Trovan®, Ltd., EID, U.S.); if they lacked a tag, we inserted a new one on the ventral side of the right thigh.

We analyzed the capture–recapture data using a robust design Pradel model, which is based on two nested temporal scales of sampling: the year and the repeated intra-year sessions (Pradel, 1996). Robust design schemes allow the population to be open between the primary sessions, but assume closure for the secondary sessions (Kendall, Nichols, & Hines, 1997). Recruitment is defined as the number of adults that recruit into the population in a given year for any adult present in the population the year before (Pradel, 1996). We analyzed only adult data because juvenile Lesser Antillean iguanas (SVL < 18 cm) usually have substantially lower capture and recapture probabilities than adults. They represented only 0 to 0.7% of total captures per year and thus were too few to model age-specific parameters. Their inclusion could have generated strong recapture heterogeneity, leading to biased estimates of demographic parameters (Arsovski et al., 2018). We compared models using Akaike Information Criteria with correction for small sample size (AICc) (Hurvich & Tsai, 1989). All analyses were conducted using the MARK program (White & Burnham, 1999). (See Material and Methods - Statistical Analysis for more information).

Usage Notes

This dataset contains the raw capture-recapture history of the Adult and Subadult iguanas (1156 individuals) captured between 2012 and 2020 on Chancel Islet, Martinique. We provide this dataset as a csv (comma separated) file. Additional information is available in the ReadMe file.

Funding

European Regional Development Fund

French Government

Parc Naturel Regional de la Martinique

Agence française pour la biodiversité

Office National des Forêts

Le Carouge, NGO

French Government

Parc Naturel Regional de la Martinique

Office National des Forêts

Le Carouge, NGO