Florian Hartig

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The time horizon and its role in multiple species conservation planning

Florian Hartig & Martin Drechsler

Biological Conservation, 141(10), 2625–2631 (2008)
Cite this
@article{hartig2008time,
  author = {Florian Hartig and Martin Drechsler},
  title = {The time horizon and its role in multiple species conservation planning},
  journal = {Biological Conservation},
  volume = {141},
  number = {10},
  pages = {2625--2631},
  year = {2008},
  doi = {10.1016/j.biocon.2008.07.028},
}

DOI: 10.1016/j.biocon.2008.07.028
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Abstract

Survival probability within a certain time horizon T is a common measure of population viability. The choice of T implicitly involves a time preference, similar to economic discounting: conservation success is evaluated at the time horizon T, while all effects that occur later than T are not considered. Despite the obvious relevance of the time horizon, ecological studies seldom analyze its impact on the evaluation of conservation options. In this paper, we show that, while the choice of T does not change the ranking of conservation options for single species under stationary conditions, it may substantially change conservation decisions for multiple species. We conclude that it is of crucial importance to investigate the sensitivity of model results to the choice of the time horizon or other measures of time preference when prioritizing biodiversity conservation efforts.

What the paper shows and why it matters

(AI-generated)

Conservation planning compares options with objective functions, and species’ survival probabilities are a measure tied directly to the goal of persistence. A survival probability always refers to a time horizon T: success is judged at T and everything that happens later is ignored. Ignoring everything after T is a time preference, much like discounting in economics, where a short horizon resembles a high discount rate. Environmental economists debate discounting at length, yet conservation planning appears to have largely neglected the choice of T. One possible reason is that, under constant environmental conditions, T does not change which option is best for a single species, and the same was assumed to hold for several species. The paper tests that assumption.

The paper shows that the time horizon T of a survival-probability analysis can change which multi-species conservation option is best: survival probability decays exponentially with T, so small differences between options grow with longer horizons. In a stylised model that splits a fixed budget between two species, objectives that add up survival probabilities (the expected number of surviving species), with or without extra weight on an even split, favour an even split at short horizons and, beyond a critical horizon, concentrating the money on one species. This happens when each extra unit of spending buys less and less survival, or when an even split is weighted strongly and the gain from spending does not increase steeply. The objective “probability that all species survive” is provably unaffected by T, but the paper does not see this as an argument for it: if the budget is too small to save every species, it can minimise the expected number of survivors. A numerical analysis finds that a switch within typical horizons occurs over a fairly large range of parameters.

The results mean that T cannot be treated as a technical default. The paper calls it a fundamental, normative part of the objective function, to be chosen with care, with its influence tested and communicated alongside any conservation recommendation, all the more as conservation turns to dynamic problems such as climate change.