Fueling a predator death-trap: Trophic subsidies and the risk of management-induced collapse in a predator-prey system

Anthropogenic subsidies create complex ecological dynamics, yet their interaction with invasive species management is poorly understood. Managing subsidies or predators in isolation risks perverse outcomes, including population collapses, demanding a more holistic understanding.

We employed a Pattern-Oriented Inference framework to synthesize multiple lines of evidence for the endemic Noronha skink ( Trachylepis atlantica ) across an archipelago. We analyzed three key patterns using complementary methods: (i) population density, estimated via capture-mark-recapture and negative binomial GLMs of count data; (ii) individual body size, using gamma GLMs; and (iii) injury rates, via tail-autotomy analysis.

We documented a dramatic density gradient driven by predation pressure. Skink populations were over three times denser on predator-light secondary islands (0.411 ind/m ² , 95% CI: 0.297–0.568) than in predator-rich PARNAMAR (0.125 ind/m ² , 95% CI: 0.090–0.174). Although anthropogenic subsidies boosted density by 82% in the inhabited APA (0.227 ind/m ² , 95% CI: 0.174–0.297), this was insufficient to overcome the severe impact of invasive predators.

This created a paradox where density, a traditional population success metric, was inverse to size, an indicator of individual fitness. On the main island, although the subsidized APA was denser than PARNAMAR, individual condition was no better. Adult males in the APA were as small as those in the lowest-density sites and significantly smaller than males on the secondary islands. Predation pressure explains this gradient, with tail-loss injury rates peaking in the APA (29.3%), remaining high in PARNA (25.0%), and dropping about 50% on the secondary islands (15.0%).

Synthesis and applications: The convergence of these patterns supports a “predator death-trap” hypothesis, where trophic subsidies in the inhabited APA fuel high skink recruitment, masking extreme mortality from a subsidized predator community and other anthropogenic threats. This dynamic produces high population turnover and truncates size structure by selectively removing larger, older adults and may explain patterns seen elsewhere. Our findings have critical management implications: removing food subsidies without concurrent, effective control of key invasive predators could trigger a population collapse. We advocate integrated, multi-species management and provide robust evidence for the threatened status of this endemic reptile.