Heterophil-to-lymphocyte ratio varies with habitat fragmentation and canopy cover in a tropical understory insectivore

Abstract

Habitat fragmentation and degradation are major problems in tropical forests, leading to a decline in biodiversity. However, it is often unclear which aspects of habitat change are responsible for the decline of specific species. Physiological studies can help identify potential stressors and elucidate causal relationships between habitat change and species decline. In this study, we measured the heterophil-to-lymphocyte (H:L) ratio as an indicator of long-term physiological stress and ecological pressures, such as pathogen exposure or risk of injury, in the tropical understorey insectivore Cabanis’s Greenbul (Placid) ( Phyllastrephus cabanisi placidus ). We then related this ratio to fragment size and forest degradation (canopy cover and vertical vegetation structure). Greenbuls breeding in forest areas with low canopy cover tend to have higher H:L ratios than those breeding in areas with high canopy cover, indicating that low canopy cover creates challenging conditions for greenbuls during breeding. Unexpectedly, however, greenbuls had higher H:L ratios in the largest, most intact forest fragment. These higher H:L ratios in larger fragments may reflect physiological and immunological adjustments to higher ecological pressures, such as predation, parasite prevalence, and competition for territories, due to intact trophic networks. This study therefore advocates concerted efforts to preserve large forest fragments, while smaller fragments should be rehabilitated to strengthen the ecological corridors and networks of these key biodiversity areas, which are of high conservation priority within the isolated mountain peaks of the Taita Hills in Kenya.

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