New paper: Trading off Persistence Targets with Numbers of Species

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Targets such as a species’ minimum viable population size or the optimum proportion of land that should be protected are important for translating the complexities of conserving natural resources into clear “rules of thumb”; however setting the same high-aspirational target across different species and landscapes may not be very efficient.  Firstly, it is unlikely that distinct species will respond in exactly the same way to the same conservation target, which could result in unequal levels of protection and eventually lead to an overestimation in the amount of conservation achieved.  Secondly, setting high-level targets will mean that fewer species can benefit from conservation funding when the budget available for these activities is limited.  Our paper investigates this second point by evaluating the trade-off between carrying out intensive levels of conservation effort to provide a high level of persistence for a few species against applying lower amounts of effort across more species, resulting in greater numbers surviving at lower persistence levels.

We carry out this analysis by modifying the species persistence target of a well-known framework for prioritizing management of threatened species, the “Project Prioritization Protocol” (PPP), which ranks species according to their cost-efficiency, and selects the set of species for conservation in order, until the budget is expended.  We used a dataset of 700 threatened species from New Zealand (see three examples of these above!) with relevant information on the cost, likelihood of success, and the potential benefit of working on each species project as a case-study for our analysis.  Specifically, we compared the conservation outcomes for our 700 species under different budgets when we reduced the PPP’s target from 95% down to 5% probability of persistence.  Conservation outcomes were evaluated based on the ‘expected number of species saved’  in each scenario, which is a metric that takes into account the number of species prioritized for conservation management, their respective probabilities of persistence, as well as the total probability of persistence of all unmanaged species.

Our study has two main findings (summarized in Figure 1): First of all, we show that is always better to set a high persistence target (above at least 75% probability of persistence) in order to maximise the expected number of species saved, no matter how low your budget is.  Secondly, we find that the persistence level that delivers the highest conservation outcome is influenced by the available budget, such that lower budgets have slightly lower optimal targets.  It is important to note that we identify a threshold target of 75% probability of persistence, below which it is never optimal to aim for.  This finding demonstrates how the practice of undertaking low levels of management on more species (to give the impression of working on a wider range of species) can become inefficient when resources are spread too thinly.

The key message of our study is that it is important to carefully consider what target to aim for in order to achieve the greatest conservation gains.  We hope that our findings can be used to encourage conservation planners to maintain high targets (above 75% probability of persistence), and also lead them to question whether setting an overprecautionary goal of ensuring 95% probability of persistence is indeed optimal, considering their budget.

Reference:  Di Fonzo, M.M.I., Possingham, H. P., Probert, W.J.M., Bennett, J.R., O’Connor, S., Densem, J., Joseph, L.N., Tulloch, A.I.T., and Maloney, R.F. Evaluating trade-offs between target persistence levels and numbers of species conserved. In press in Conservation Letters.

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