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.

Changes in ecosystem function with realistic extinction patterns

The huge amount of work on the effects of species richness on ecosystem function have generally shown that with greater plant species richness, you tend to have increased primary productivity, nutrient uptake and greater stability to disturbances. This has been interpreted as meaning that maintaining ecosystem functions at certain thresholds is dependant upon species richness, and decreases in species richness reduce productivity as well as ecosystem stability and multifunctionality.

However, the shape of this relationship depends on how functions react to losing species.

Most evidence for the effects of species richness on ecosystem function has come from controlled experiments which manipulate plant species richness randomly. Random manipulation of the plant communities is equivalent to supposing that extinction risk is the same for all species.

This is obviously not true in nature, where particular characteristics of a species can determine the likelihood of extinction (e.g. body size in mammals; dispersal mechanism, seed size and reproductive strategies in plants). Indeed, the studies that have examined the effects of realistic extinction patterns on ecosystem function have tended to show quite different results to those which assume random extinction.

Researchers would argue that up until now they have tried to look at the general patterns of ecosystem function with changes in richness. This is fine, and more of this kind of work needs to be done, particularly outside of grasslands. However, we also need to understand what characteristics determine extinction probabilities and how these link to traits which might influence ecosystem function. Conceptually this has been characterised as the linkages between response traits, those traits which determine the establishment or persistance of a species, and effect traits, those traits which influence ecosystem properties and functions.

Hypothetical linkages between response and effect traits

To explore this idea I will use the example of primary productivity. If traits which promote primary productivity are positively correlated with those that encourage persistence following disturbance, then primary productivity may be relatively robust to species loss. However, if there is an negative relationship, then there would be a disproportionate effect on productivity. Finally, if there is no obvious link between traits then the assumptions of biodiversity ecosystem function experiments and random extinction would hold true.

The situation is obviously more complex than I have described here with abundance playing a massive role in determining ecosystem functions. However, I think investigating the linkages between traits is likely to be best hope we have of forming generalisations about how species extinctions can alter ecosystem functioning in the real world.

Possible relationships between species richness and productivity given different relationships between traits promoting resistance to disturbance and productivity. Colours refer to the corresponding relationships in previous graph.

This is a massive task and up until now I have seen little work which attempts to deal with this directly, rather than as a purely conceptual issue (though there has been some very good work on non-random extinctions). As such these ideas remain largely untested. However, with the surge in interest in ecosystem services it is vital we try to work out how the functions that support services that we depend upon work.  We are living in a time of unprecedented alteration of biodiversity by humans and these changes could have dramatic effects on our life support system. Extinctions form part of this problem, but we have little idea about the consequences of real extinctions on ecosystem functions and services.

Should we try to save every species?

I woke up yesterday morning to find my Twitter feed full of news about the new list of 100 most endangered species that ZSL and IUCN have produced for a document called ‘Priceless or worthless.’ In it Jonathon Baillie and others argue that all biodiversity on earth has an intrinsic value. In this and in a radio interview on the Today programme yesterday Baillie said he thinks we should try to preserve all species in existence.

Say what you like, but I don’t agree.

I think ZSL and IUCN are being slightly disingenuous when they claim that they aim to preserve all species on earth. This is an impossible task. And they know it.

Any type of human development, let alone the unprecedented destruction of the biosphere that is currently going on, almost definitely endangers some species somewhere. There are countless examples from the 1500s on of settlers causing the extinction of species all over the place, as a result of relatively low-impact activities – at least by today’s standards. Even if we all embraced clean energy and reduced consumption to a minimum overnight we would still drive species to extinction by altering ecosystems to meet our own needs.

Given that we can’t, and won’t, save all species what should we do?

The answer really depends on what you think we should preserve and why you think we should try to preserve it.

I am of the opinion that we should:

  1. Be pragmatic and target our efforts on particular species that we think we can save and factor in the costs of doing this. We should target the most cost-effective options as a priority. Ignoring costs of conservation is idiocy. Even if it seems a little hard-hearted factoring in costs is vital, just ask Hugh Possingham.
  2. View ecosystems as the unit of conservation and push the development of the ecosystem red-list as a means of international prioritisation. This doesn’t preclude the use of flagship species or even species specific conservation in some cases but in general we should be trying to maintain unique ecosystems and the interactions within them.
  3. Be explicit about what our values are. People need ecosystem services to live and pretending otherwise is fantasy. On the other hand completely ignoring the beauty of nature dehumanises us. We need to find a middle way where cultural services are valued as well as those that are vital to life. The two approaches need not be mutually exclusive.

These are not the only answers, or even the best ones, but they are important and should be addressed. I fear that by aiming to save everything we will waste valuable resources and will ultimately fail to protect much. Even if we can change people’s current attitudes and raise more money for biodiversity conservation we will have to make hard choices. We should at least admit it.