Caught in the snare : the indirect effects of hunting-induced mammal defaunation on bird communities

The Anthropocene has been marked by substantial declines in global biodiversity occurring at a rate and magnitude that is reminiscent of previous mass extinction events. This collective degradation of faunal assemblages through species loss and local population declines is known as defaunation and has become a major driver of global change. In general, large-bodied vertebrates, particularly large mammals, have been most adversely impacted by this trend, leading to a downsizing effect as smaller mammal species persist or proliferate. Previous research has demonstrated that large mammals often play key roles in the provisioning of many ecological functions, implying that the loss of these influential species may directly disrupt ecosystems. However, large mammal declines are also likely to initiate secondary, cascading impacts on communities of other taxa, such as birds, which play similarly critical roles in maintaining ecological functions. These indirect effects of defaunation are much less studied and their influence on ecosystems remains comparatively unknown. To elucidate the secondary consequences of mammal loss on birds, it is necessary to compare avian communities across contrasting conditions of defaunation in which other confounding factors such as habitat disturbance or turnover are kept constant. In this dissertation, I do this by describing assemblages of birds found within areas of continuous primary rainforest in the Congo Basin experiencing contrasting levels of human hunting pressure on mammals. Although contemporary large mammal diversity is centered in Africa, the effects of size-biased defaunation on wildlife assemblages across the continent are clear. One of the greatest threats to mammals in many regions is the hunting of wild game, also known as bushmeat. Increasing human population, along with a lack of viable financial or nutritional alternatives, mean that current bushmeat harvest rates across much of Africa are highly unsustainable, despite being practiced continuously for thousands of years. Yet for the purposes of this research, bushmeat offtake may also be ideal for isolating the indirect effects of defaunation as hunting effort often disproportionately targets larger species, does not necessarily modify habitat, and can vary dramatically in intensity across short distances. This may create areas of adjacent, identical habitat, parts of which have been hunted (defaunated) and parts of which have not. Here I focus on the Dja Faunal Reserve in Cameroon and adjacent community-owned forests, areas of approximately consistent habitat which likely experience differing levels of hunting pressure. I conduct a focused case study in this region which I then scale up to reveal broadly applicable global patterns of mammal defaunation and its indirect effects on bird communities. More specifically, I begin in Chapter 1 with an introduction of bushmeat hunting and global defaunation patterns. In Chapter 2, I compare the estimated biomass of humans and large mammalian wildlife captured on camera traps in and around the Dja Reserve. I show that hunting facilitates the nearly complete replacement of wildlife by humans across the study site while total cumulative biomass remains approximately constant. In Chapter 3, I expand on these results by examining bird communities within the same study site but at points experiencing different conditions of mammal defaunation as described by the previous chapter. I show that avian functional composition is disrupted by the removal of mammals and that this occurs independently of any change in habitat. In Chapter 4, I assess the generalizability of this pattern of avian community disruption by conducting a global meta-analysis quantifying bird population responses to changing mammal abundance. The results from this large-scale evaluation were strikingly similar to those from the focused field study presented in Chapter 3. This suggests that mammal defaunation may consistently disrupt bird communities in ecosystems around the world, with similar impacts on ecological functioning. This work therefore represents some of the first empirical evidence describing a generalizable indirect effect of mammal defaunation. Finally, in Chapter 5, I identify key knowledge gaps requiring future research and contextualize this work within a broader social environment, especially pertaining to the occurrence of hunting in Central Africa

Keywords: Biology.
Advisor: Dirzo Rodolfo
De Leo Giulio A.
Hadly Elizabeth Anne
Smith Thomas B. (Thomas Bates)