Cross-continental analysis shows that disturbance effects on reptile body condition do not predict abundance responses

Ecological disturbances are discrete events that alter or transform the physical, chemical, or biological characteristics of ecosystems. Animal populations are vulnerable to disturbance, and the risk-disturbance hypothesis and population collapse framework propose that population declines can be predicted by declines in animal body condition. However, no research has empirically examined the general relationship between body condition and abundance, nor their relationship in response to disturbance.

We used a combined dataset representing 33 studies and >42,000 observations of 75 species from Australia, New Zealand, Spain, and the United States to test predictions relating to the relationship between reptile body condition and abundance. We first investigated the relationship at the site level and then used meta-analytical models to test whether populations showed linked changes in abundance and body condition in response to disturbance. We further tested whether key environmental and species traits influenced this relationship and whether there was a time-lagged effect of body condition responses on abundance.

Our results provided no strong support for the risk-disturbance hypothesis or population collapse framework. We found a positive relationship between mean reptile body condition and abundance at the site level. However, the relationship was largely lost when investigating population responses to disturbance. We provide a new conceptual framework that shows how disturbances can modify or uncouple the relationship between abundance and body condition by influencing underlying drivers, such as predation, competition, and resource availability. As such, the impacts of disturbance on reptile body condition cannot be assumed to reflect or predict abundance responses. Monitoring programs that infer population impacts based on changes in body condition should confirm the relationship between these two variables in the relevant study system.

Methods
Between October and December 2023, datasets suitable for analyses were identified through three main methods: (1) reviewing the data sources of a recent global meta-analysis investigating reptile body condition responses to disturbance (Macdonald et al. 2023), (2) using ‘Scopus’ to identify prominent authors publishing in the field of reptile abundance responses to disturbance, and (3) asking existing networks and research collaborators for recommendations. Adopting a formal systematic review approach was not suitable for this study because we needed to obtain raw, unpublished data from data custodians (the author group) and consult with them regarding data cleaning and interpretation. We compiled 33 author datasets that investigated reptile responses to disturbance. Each underlying dataset contained (1) field data collected at sites with and without disturbance, (2) reptile capture data at each site, and (3) individual reptile snout-vent length and weight measurements at each site.