Disentangling the effects of supplementary feeding on population abundance and the aggregation of wild ungulates as drivers of the prevalence of tuberculosis in south-central Spain

Abstract

AbstractThe supplementary feeding of wild ungulates is a common and pervasive practice throughout Europe, but the understanding on its unintended ecological effects is still limited. This management action has different degrees of intensity from artificial grasslands to supplementation in feeders, since the type and criteria of each supplementary feeding programme depend on managers’ objectives (e.g. maintaining harvesting yields, increasing animal body condition, mitigate agriculture or forest damage). The decision on how, when and where feed may determine the magnitude of the effects. Here, based on a long-term and well‐replicated dataset and using mixed structural equation model (mixed-SEM), we investigated the effects of supplementary feeding on red deer and wild boar abundance and aggregation and how directly and indirectly it affects the prevalence of tuberculosis-like lesions (TBL). We, therefore, hypothesized that supplying food avoiding the use of feeders would have less effect as regards increasing transmission rates, despite the fact that its efficiency as regards modulating population dynamics may be similar to the provision at feeders. Supplementary feeding was characterised in field surveys carried out on 60 hunting estates (south-central Spain) over four seasons (2002/2003, 2006/2007, 2012/2013 and 2018/2019). The abundance, aggregation and prevalence of TBL in red deer and wild boar were also determined. Collectively, our results showed that most of the variability in population aggregation could be directly explained by the population abundance and, to a lesser extent, by the degree of supplementation. Regarding red deer, there was a positive interaction between population abundance and aggregation in relation to the prevalence of TBL, suggesting that the positive effect of abundance on the prevalence of TBL is intensified by population aggregation. The model also showed a significant and positive correlation between the prevalence of TBL in both species. Finally, we reported a negative relationship between the degree of supplementary feeding and the prevalence of TBL in wild boar. Our findings shed light on the drivers of TBL prevalence in wild ungulate populations. Our results are relevant for the design of disease-control actions aimed at reducing the prevalence of tuberculosis and other shared diseases favoured by intensive wildlife management.