Trading off target-persistence levels and numbers of species conserved
Setting high-level persistence targets means few species can benefit from a limited budget
No matter how low your budget is, it is always better to set a high persistence target (above at least 75% probability of persistence)
The persistence level that delivers the highest conservation outcome is influenced by the available budget
Targets such as a species’ minimum viable population size or the optimum proportion of land that should be protected (Decision point #83) are important for translating the complexities of biodiversity conservation into clear, generalizable rules. However, setting the same high-aspirational target across different species and landscapes may not be very efficient. To begin with, it is unlikely that different species will respond in exactly the same way to the same conservation target. This could result in unequal levels of protection, and eventually lead to an overestimation in the amount of conservation actually achieved.
On top of this, setting high-level targets (ie, aiming for high probabilities for a species to persist into the future) will mean that fewer species can benefit from conservation funding when the budget available for these activities is limited. That is because it costs a lot more to achieve those high probabilities.
We recently investigated this 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 of species surviving at lower persistence levels; Di Fonzo et al, 2016).
Our analysis involved modifying the species persistence target of a well-known framework for prioritising management of threatened species, the Project Prioritisation Protocol (PPP, see Decision Point #29). PPP ranks species according to their cost-efficiency, and selects the set of species for conservation in order, until the budget is expended (see the box Dial triple P).
As a case-study for our analysis, we used a dataset of 700 threatened species from New Zealand with relevant information on the cost, likelihood of success, and the potential benefit of working on each species project. 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 on the basis of 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 two main findings (summarised in Figure 1) are:
- it 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; and
- 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.
These findings demonstrate 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 gains in conservation. We hope that our findings can be used to encourage conservation planners to maintain high targets (above 75% probability of persistence), but also to get them to question the value of setting even higher goals of 95% probability of persistence, which we believe might be ‘over precautionary’. Indeed, working with a severely constrained budget makes the aspiration of a 95% probability of persistence sub optimal, because many other species will miss out.
More info: Martina Di Fonzo email@example.com
Di Fonzo MMI, HP Possingham, WJM Probert, JR Bennett, S O’Connor, J Densem, LN Joseph, AIT Tulloch & RF Maloney (2016). Evaluating trade-offs between target persistence levels and numbers of species conserved. Conservation Letters 9(1): 51-57. http://onlinelibrary.wiley.com/doi/10.1111/conl.12179/abstract
Dial triple P for better conservation outcomes
In 2009, Liana Joseph worked with conservation managers from New Zealand to develop a framework to guide the allocation of limited resources to save the greatest number of threatened species (Joseph et al, 2009). The framework was called the Project Prioritization Protocol and it comprises nine steps:
1. Define objectives (eg, maximise the number of species with 95% probability of persistence in 50 years),
2. List biodiversity assets (eg, threatened species),
3. Weight assets (eg, according to cultural significance, economic importance, evolutionary significance and so forth),
4. List management actions required for each asset to meet the objective,
5. Estimate the cost of each action,
6. Estimate the benefit to the species generated by carrying out the complete set of actions required to meet the objective,
7. Estimate the likelihood of success of each action,
8. State the constraints (eg, the budget limitation), and
9. Combine information on costs, values, benefits and likelihood of success to rank projects according to their benefits per unit dollar (E), using the following equation:
E=(W . B . S)/C
Where W is the weight, B is the difference between the probabilities that a species will persist in 50 years with and without this project; S is the product of the probabilities of ‘input’ (ie, implementation) and ‘output’ (ie, technical) success of all actions, based on expert opinion; and C is the cost of all the actions required to ensure a specific level of persistence. The process of prioritization starts by funding all species projects, then successively excluding projects with the lowest cost-efficiencies until the target budget is met. The weightings of the remaining species are iteratively updated as species are excluded.
Joseph LN, R Maloney & HP Possingham (2009). Optimal allocation of resources among threatened species: a project prioritization protocol. Conservation Biology, 23:328-338. (And see the story on PPP in Decision Point #29).