Species richness is the iconic measure of biodiversity. It is simple to interpret* and it is one of the most commonly measured metrics in ecology. From the early beginnings of ecology Darwin, Wallace and von Humbolt noted the striking differences in the number of species found in different places and ecologists are still fascinated by it . However, over the last few months I have begun to question how useful it is for applied research.
These thoughts were probably triggered by two recent papers that suggested that there have not been systematic losses of species richness in plant and animal communities in the last century. This result is startling. Up until the publication of these papers it had been widely assumed that the global extinction of species led to a reduction in species richness at a local scale (although some papers had previously hinted there may be little change in local richness). However, this doesn’t appear to be the case, at least where change in ecosystems is not a result of dramatic changes in land-use. One of these studies found that the species communities investigated had changed in composition over time even if there hadn’t been a reduction in the total number of species. This change in communities could well have resulted in the loss of species considered important for maintaining ecosystem services, such as so-called ecosystem engineers, or species of perceived conservation importance. It is also possible that widespread species may be moving in to colonise areas forcing out rarer species. However, just by looking at changes in species richness we don’t gain any insight into what species are in these communities.
This begs the question, just what is species richness good for, in the context of conservation and applied ecology? A question I have asked in various different forms on Twitter recently…
You can probably tell that I am somewhat sceptical about the value of species richness as a metric, even though I have used it in plenty of my own work. However, there are some good reasons for measuring it and like so many things in conservation whether or not you think it is a valuable metric largely depends on what you think conservation is for.
Firstly, there is the utilitarian argument for species richness being important. Countless experiments have suggested that increased species richness leads to increased provision of some ecosystem services, an increase in the number of services that can be provided in any one location and an increase in the stability of provision of these ecosystem services. In addition there have been some studies that show that people value diverse systems with lots of species more than those with less and get more aesthetic appreciation from them. Of course every single one of these statements is contentious, and some have more support than others. However, they represent the broad arguments I have heard in favour of species richness and I largely agree with these points.
While there are sometimes good reasons for using species richness there are also situations in which the logic for the use of species richness as a metric is flawed. One clear problem with it is that it doesn’t tell you anything about the characteristics of the species counted such as their functional traits or how common/widespread they are. When the aim is to conserve species that are most at threat of local/global extinction, as is often the case, measures of rarity are important. An example that I have encountered a lot recently is that related to the debates surrounding land-sharing/land-sparing strategies to protect biodiversity (see my previous posts on the topic here and here). A number of studies suggest that as the proportion of land that is used for low intensity agriculture increases the species richness of an area remains stable until a threshold is passed at which species richness declines. This would suggest that low intensity farming systems retain a lot of their conservation value. However, when you look at the same issue taking into account the identity of the species you see that the populations of many specialist species decline rapidly and the populations of generalists increase, thereby limiting the change in species richness**. If we consider species that are specialists to be of greater conservation concern than generalist species then species richness is clearly not very useful as a metric in this context. The same is true of any study looking at land-use change.
In addition if you want an idea of how a change in land-use might influence ecosystem functions and services using a measure of functional diversity related to their traits is probably more useful than a measure of species richness. This approach is more mechanistic than simply using species richness as a large body of work has already tried to identify which traits link to particular processes and functions. In an ideal world doing this allows you to infer how land use may influence ecosystem processes. Doing this will give us a more general idea of the consequences of changes in biodiversity for human well-being.
I’m sure there are countless other arguments for or against the use of species richness in applied research, so feel free to comment below. At the moment I feel that people measure it because it is relatively easy to measure and because everyone else does. We should think why it might be useful to know about species richness changes rather than just blindly measuring it. So next time you think it would be a useful thing to measure, ask yourself why.
*I know richness is not that simple to interpret because it depends on exactly how it has been measured, whether it is rarefied by area or number of individuals etc but I couldn’t be bothered to go into that. Not here, not now.
**I realise we’re getting dangerously close to talking about the intermediate disturbance hypothesis here, but again I’m going to ignore the problems of that for the time-being.