Estimating the consequences of multiple threats and management strategies for semi‐aquatic turtles

Abstract

Summary1. Roads are pervasive fixtures on most landscapes but are typically among many factors contributing to wildlife population declines. Addressing road mortality as part of larger conservation efforts is challenging because it can be difficult to measure per capita mortality from roads and other concurrent threats. 2. We used 4 years of mark-recapture-recovery data for diamondback terrapins Malaclemys terrapin on a causeway in Georgia, USA, to directly estimate threats of adult road mortality and nest predation, contrast the consequences to population growth using stage-based matrix models and make management recommendations to stabilize the population. 3. Mean estimated annual adult road mortality was 11Á1% (range = 4Á4-16Á4%). Estimated annual nest predation was 61Á9%. We estimated that the population was declining (k < 0Á98) in all scenarios where both threats were included. Variation in adult survival was the most influential (highest elasticity) contributor to population growth relative to other demographic rates; however, k would remain below 1 with any nest predation rate exceeding our estimate even if actions to mitigate road mortality were 100% effective. 4. Synthesis and applications. Our study provides some of the first direct estimates of vehicle mortality rates and shows that mortality can remain sufficiently high among years to cause population declines. We also demonstrate that management actions focused on singular threats are inadequate for recovering populations. We conclude that integrated road and predator management is necessary to conserve turtle populations, and we suggest alternative strategies to compensate for some vehicle mortality and nest depredation.

When Drivers and Terrapins Collide: Assessing Stakeholder Attitudes Toward Wildlife Management on the Jekyll Island Causeway

“…For example, Crawford et al (2014a) found that current rates of terrapin road mortality and raccoon predation of nests were sufficiently high that both must be addressed to stem the decline in the Jekyll Island terrapin population. In other words, reduction of raccoon predation on nests is needed to conserve terrapin populations; however, lethal raccoon removal was among the most contentious management options across patron groups.…”

“…In addition to mortality from vehicles, predators such as raccoons (Procyon lotor) can cause high rates (50%-90%) of mortality for eggs laid on road shoulders (Feinberg & Burke, 2003;Szerlag-Egger & McRobert, 2007). Likely representative of conditions on many heavily-traveled coastal roads throughout the terrapin's range, road mortality and nest predation on the Jekyll Island Causeway (JIC) leading to Jekyll Island, GA, USA were both contributing to terrapin population declines (Crawford, Maerz, Nibbelink, Buhlmann, & Norton, 2014a). Because terrapin populations are expected to decline as pressure from development and tourism increases in coastal areas, management solutions are needed that recover and stabilize terrapins, as well as other wildlife affected by roads.…”

A question of scale: Replication and the effective evaluation of conservation interventions

“…Elasticity analyses of population models show that adult female survivorship has the highest impact on population growth rates, and several authors have interpreted these results to mean that efforts to increase juvenile recruitment are better spent on protecting adults (Congdon et al 1993;Heppell et al 1996;Heppell 1998;Mitrus 2005;Enneson and Litzgus 2008). These criticisms tend to focus on headstarting in isolation, without consideration for concurrent mitigation of the initial causes of population declines (Dodd and Seigel 1991;Frazer 1992;Heppell 1998;Meylan and Ehrenfeld 2000;Vander Haegen et al 2009;Smeenk 2010;Crawford et al 2014). Models incorporating multiple conservation actions may be more realistic and may lead to more successful management plans for threatened populations.…”

“…Models incorporating multiple conservation actions may be more realistic and may lead to more successful management plans for threatened populations. For example, models of conservation interventions for turtle populations simultaneously impacted by road mortality and nest predation showed that reducing adult mortality is insufficient to increase population growth rate without simultaneous nest protection (Crawford et al 2014). …”

“…Recovery from population-level mortality events is extremely slow, requiring several decades (Brooks et al 1991;Congdon et al 1993Congdon et al , 1994Doak et al 1994;Heppell et al 1996;Heppell 1998). This life history complicates conservation efforts, particularly where multiple, emergent threats exist (e.g., Crawford et al 2014). …”