Defaunation in Tropical Forests May Exacerbate Climate Change

When one considers the relationship between rainforest biota and climate change, thoughts tend to be that climate change has the potential to cause species loss and may present a serious threat to some of these animals. However, a paper published earlier this month turned this on its head by presenting new evidence suggesting that the loss of large mammals and birds in tropical environments through deforestation, hunting and other anthropogenic factors may actually exacerbate climate climate.

The logic behind this is placed in the role of these animals as biotic seed dispersal vectors. Large frugivorous mammals, such as the woolly spider monkey, are able to consume and consequentially disperse large seeds and fruits. Some seeds actually germinate preferentially after being passed through the digestive tract of a certain species, others such as the calvaria tree actually require to be digested to germinate at all. Large hardwood rainforest trees tend to have bigger seeds and fruits, meaning that they rely on these animals to successfully reproduce and disperse. Crucially, these trees are also those which store the most carbon from the atmosphere and are a major component of rainforest carbon sinks. Whilst they only made up 21% of trees included in the study, they are estimated to hold the majority of stored carbon in rainforests. Smaller softwood trees have their seeds dispersed by smaller mammals and birds but these experience less impact from hunting and the trees are less important carbon sinks.

How mammals and birds aid dispersal in a healthy vs unhealthy forest system; Source.

Large animals provide almost all of the seed dispersal services for hardwood rainforest trees, so play a crucial role in maintaining the rainforest carbon sink which stores 40% of the world's terrestrial carbon. Several of these animals are threatened by anthropogenic activity including deforestation, hunting and habitat loss, a brief look at the IUCN Red List will make this abundantly clear. Many of these threats are growing, for example, unsustainable hunting has increased in tropical forests in recent decades and threatens 19% of all tropical vertebrates, with larger vertebrates affected at disproportionately higher rates. Smaller vertebrates such as rodents are also impacted by defaunation of larger mammals and can become locally extinct in overhunted areas. Extirpations of large mammals in tropical forests, whilst a tragedy in itself, has clear potential to limit the ability of rainforests to act as carbon sinks. Whilst policy has mostly focused on limiting deforestation, more focus is needed on the conservation of large mammals through prevention of forest degradation and restricting hunting.  Between 7-17% of global carbon emissions could be at stake unless defaunation is prevented, which is a huge amount of CO2 to be at risk.