Guns, birds and squirrels

Tropical forests are getting ever more fragmented, human population in the tropics is increasing and guns are now widely available.

All of this has led to an explosion in the number of people hunting for food in the tropics.

This hunting can cause local extinction of bird and mammal species, with large bodied species being particularly at risk. This can lead to loss of species that eat fruit and therefore act as dispersers of plant seeds.

This dispersal is important since it means that species are dispersed widely around forest, rather that just being concentrated in small areas around their parent plant. However, those species that eat seeds, causing them to be damaged and therefore unable to germinate, may also be lost as a result of hunting. The balance between the losses of these two types of species will determine their effects on plant reproduction.

In general, it appears that losses of animal species, particularly larger species, as a result hunting tends to lead to an increase in the abundance of plant species which don’t require animals for dispersion. However, the results of these studies can sometimes be unclear due to lack of replication and because they have tended to be over a relatively short period of time.

A new study in Ecology Letters aims to tidy up our view of how hunting affects plant species. This work studied the dynamics of Lambir forest in Malaysian Borneo, which looks like this:

Lambir forest
photo credit: berniedup on flickr

Though it looks nice, this forest has been hunted for over 15 years and this has caused the local extinction of seed dispersing species like the white crested hornbill, which looks like this:

White crested hornbill
photo credit: berniedup on flickr

as well as the red giant flying squirrel, which looks like this:

Red giant flying squirrel
photo credit: vil.sandi on flickr

However, seed predators such as the sambar deer have also become locally extinct

photo credit: Smithosian Wild on flickr

This situations mirrors that of other study sites and makes it hard to determine how hunting will affect plant biodiversity.

For their study Rhett Harrison and colleagues investigated the changes in diversity and distribution of plant species in Lambir by monitoring nearly 500,000 (!) individual trees. They found that the density of seedlings tended to increase – suggesting a reduction in the amount of seed predation going on as well as a reduction in dispersal by animals. They also found that tree richness was reduced, though this reduction was relatively modest.

Figure 1 - richness and seedlings
Number of seedlings (a) and tree species richness (b) change during the study period. Error bars are 95% confidence intervals.

Most interestingly the study also suggests that plant species that need animals to disperse their seed tended to become relatively more clustered than species which didn’t rely on animals.

Figure 2- seed mode
Degree of clustering by dispersal mode during study. Lines around dots represent 95% confidence intervals.

All of these results suggest that hunting can have marked effects on tropical forest plant biodiversity – in the long run leading to a potential decline in some animal dispersed species.

Reading this study reminded me of attempts to link traits of species which determine the probability of extinction and those which affect ecosystem functions and services. In this case large body size is associated with a dietary preference for fruit or seeds – with obvious consequences for seed dispersal. What really sets this study apart is the length and size of it which means it is the most precise study of its kind. Linking these traits will allow us to generalise about the ecology of hunting in tropical forests but this is only part of the solution.

Large areas of South East Asia, West Africa and the Atlantic forest in Brazil are facing similar pressures from hunting, so this phenomenon may be quite widespread. Though it is obviously less of a threat to biodiversity when compared to deforestation and other more dramatic degradation the subtle effects of hunting may occur both inside and outside protected areas going relatively unnoticed. To tackle this problem effectively we need to know the motivation for this hunting. Only then can we start to deal with what to do to stop it.


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