When making conservation decisions, we never have all the information we need. One knowledge gap is that we don’t know where all species are (or even what they are – so many species remain undiscovered and undescribed). Nor do we know how they will react to changes to their environment including management. So inevitably, we have to use a subset of species as proxies for how biodiversity as a whole will be affected by management. The question is, how do we go about selecting which species to use as proxies, and how does the method for selecting them, and therefore the species selected, affect the final conservation decision.
In the 1990s, Robert Lambeck proposed the idea of using a set of ‘focal species’ that are the ones that are most affected by key threats. His and subsequent studies focused on selecting species to guide restoration or revegetation of native woodland. For example, to decide how connected or close together patches of bush targeted for restoration should be, you would be guided by the needs of the species that is most dispersal limited; to define the how big patches of habitat should be, you would be guided by the species that has the largest area requirements, such as the biggest home range.
The focal species approach is quite intuitive, but has faced a fair amount of criticism. First, there is a lack of evidence to support the underlying principle that focal species confer protection to co-occurring species facing similar threats; results from previous studies are ambiguous. Second, it is not clear what the objectives of management are: what sort of landscape are we aiming for? The best for each of the focal species, or for certain (undefined) aspects of biodiversity, or for all biodiversity? The landscape that is best for all species is impossible: species have different needs – the ideal landscape for one species will not be the best for another or for all other species. These two things are of course interrelated: it is difficult to evaluate a theory without explicit goals and measures of how well the theory performs.
Along with colleagues Hugh Possingham, Karin Frank and David Lindenmayer, I sought to understand the conditions under which the focal species concept has merit for making sound conservation decisions (just published in Diversity and Distributions). Because the focal species concept is based on population processes and the persistence of species (that is, wanting to keep viable populations of the species), both the goal and the responses of the focal species and the other species they are supposed to represent should be measured in terms of population viability, such as probability of persistence over a given time frame, which gives a common currency across the species.
We used a case study that David Lindenmayer has been working on for decades near Tumut in NSW, a fragmented landscape of patches of native forest embedded in pine plantation. In a previous study, we modelled ten species of vertebrates (four bird species, five marsupials and one native rodent) representing a range of body sizes and life history strategies. We used a method for choosing a reserve system that maximizes the persistence of multiple species, where species persistence is estimated using a metapopulation model, and is a function of the amount, quality and configuration of habitat patches and the ecology of the species.
We related the criteria for selecting the focal species to key model parameters for dispersal capability, home range size and fecundity – linking species characteristics to ecological theory and to our decision criteria. Using these criteria, we identified a subset of three focal species from the set of ten species. The three focal species were the bush rat, which has very limited dispersal, the red browed tree creeper, which has the largest home range size of our suite of ten species, and the mountain brushtail possum or bobuck, which had the lowest reproductive rate of our ten species, which means that it has a low population growth rate and doesn’t make many little possums to disperse out into the world.
Our goal was to find the reserve solution that maximised the combined viability across the species (summing the probabilities of persistence over 100 years). We asked if the reserve system that maximised persistence over the three focal species was the same as the system that maximised viability across all ten species. That is, our measure of the performance of the focal species approach was whether the resultant decision was the same.
And what did we find? Well, the best reserve system using the three focal species was the same as the best reserve system for all ten species. The focal species approach seemed to work. Not only that, we tested all 120 combinations of three of the ten species, and the one containing the three focal species was the only one that was the same as the decision for all ten species. The reserve system that maximised the viability across the ten species and the three focal species was different to the best landscape for any one of the species – that is, it was a compromise, and there were trade-offs between the needs of different species. This is key – it is impossible to find the best landscape for any one species without compromising the viability of other species. Nor is it the best landscape for a ‘super-bad species’ that is bad a dispersing, has a huge home range size and has few babies – that is not realistic. Instead, we have a clearly defined aim: maximise the viability of the suite of species we have enough info on and care about. And we have a way of testing the focal species approach: does it give us the same answer that we would get if we used a larger but defined subset.
We don’t claim that the best reserve system for the focal species or the set of ten species represents the needs of all other species or other components of biodiversity (such as ecosystem processes). We bounded the aspects of biodiversity we wanted our focal species to represent, and were explicit about its limits. Therefore the best ten-species decision may not be the best for biodiversity overall, but it’s the best we can make with the information we have.
We also didn’t account for landscape change and dynamics – our model is based on a snapshot. In fact, since we did the modelling analyses, the whole system has been radically altered by bushfire and drought. How the species will respond is a matter of time, though David and his team will be monitoring it, giving us valuable information about how many aspects of biodiversity respond to change, and how we can improve our management of such dynamic and unpredictable systems.
Despite these caveats, our study not only tested the focal species approach and found that it can work, but we provided a framework for testing this and other methods for selecting the subsets of biodiversity used for decision-making. The next step is to apply similar tests to these approaches over many more case studies to find generalisations, and answer key questions such as, how many focal species are needed to make robust decisions, and how sensitive are our results to uncertainty in our information and the rules for their selection? More to follow, I hope.
Nicholson E., Lindenmayer D.B., Frank K. & Possingham H.P. (2013). Testing the focal species approach to conservation planning for species persistence. Diversity and Distributions, 19 (5-6): 530–540 [link]
Lambeck, R.J. (1997) Focal species: a multi-species umbrella for nature conservation. Conservation Biology, 11, 849-856 [link].
Note: I’m a co-author in another paper in the same issue: Ponce-Reyes, R., Nicholson, E., Baxter, P., Fuller, R.A. & Possingham, H.P. (2013) Extinction risk in cloud forest fragments under climate change and habitat loss. Diversity & Distributions, 19 (5-6): 518–529 [link].