# Interpreting a Sudden Population Decline in a Long‐Lived Species ( Malaclemys terrapin rhizophorarum )

## TL;DR

This study investigates a sudden population decline in mangrove diamond-backed terrapins using long-term capture data and survival models.

## Contribution

The study uses an 18-year dataset and population projection models to explore potential causes of a sudden population decline in a long-lived species.

## Key findings

- Survival estimates for terrapins dropped sharply starting in 2015, with female survival falling below 36% by 2018.
- A scenario of low adult survival combined with no reproduction after 2010 best matches observed population trends.
- The decline may be due to adult emigration, human removal, or reduced recruitment.

## Abstract

Long‐term ecological studies are critical for providing insight into population dynamics and detecting population declines, particularly for species of conservation concern. However, spatiotemporal variation and logistical challenges make the identification of sudden population declines difficult. We conducted an in‐water capture‐mark‐recapture study of mangrove diamond‐backed terrapins (
Malaclemys terrapin rhizophorarum
) within Big Sable Creek, in Everglades National Park, Florida. We used an 18‐year dataset (2001 to 2019) incorporating year, sex, hurricane occurrence, and sampling effort to estimate survival using Cormack–Jolly–Seber (CJS) models in Program Mark. Annual survivorship estimates were high from 2001 to 2003 for both sexes (91%–96%) and variable from 2006 to 2014 (77%–92%). Beginning in 2015, survival estimates exhibited a steeper decline (females: 65%, males 75%), and dropped to below 36% by 2018. Because the driver of this apparent population decline is unknown, we created a population projection matrix and used model‐estimated annual survival to simulate annual terrapin population size. We then generated competing scenarios of low survival at various age classes to attempt to reproduce a simulated decline mirroring what we observed from our capture data. A scenario of low adult survival (75%–85%) from 2012 to 2018, possibly in conjunction with no reproduction after 2010, provides estimates of abundance that appear to match simulated annual population size and may indicate that adult emigration/human removal or a drastic drop in recruitment could be responsible for the apparent decline in survival. We explore reasons for this apparent decline and highlight difficulties common to long‐term studies that may influence how declines are interpreted.

Annual sex‐specific survival estimates for mangrove diamond‐backed terrapins (
Malaclemys terrapin rhizophorarum
) in Everglades National Park from 2001 to 2019.