Reading group: Index this

Sumatran tiger on camera trap. Photo via wikimedia commons (which states that “this animal didn’t like camera traps and proceeded to destroy three over a weekend”)

In our latest QAECO reading group, Kate Giljohann selected an editorial published in the Journal of Applied Ecology titled “Management by proxy? The use of indices in applied ecology”. The article, led by Philip Stephens and editors of the journal, discussed the role of proxies in applied ecology and ways to improve their use in environmental management.

Most of us use proxies or indices in our research. But what are they? Well, scientists and managers often want to measure and keep track of key attributes of ecological systems to inform decision-making. For example, estimating population size and the rate at which it is changing is fundamental for determining the status of species, whether they should be managed, or how they have responded to management intervention. But, often these quantities can be difficult or time consuming to measure directly, so instead we measure something else that’s closely related. The assumption is that if we keep track of what our proxy is doing, we can also keep track of what our real quantity of interest is doing as well.

In this paper, Stephens et al. (2015) discuss what they believe are the three most commonly used indices/proxies in applied ecology. Firstly, they discuss proxy-based measures of population abundance. This is perhaps the most common form of proxy in applied ecology and is based on the fact that wildlife surveys – such as track and scat counts or camera trapping rates – often yield indices of abundance, rather than estimates of abundance. Secondly, the authors discuss proxy-based measures of habitat quality. Habitat suitability maps are routinely used to indicate habitat quality and therefore the ability of a species to occupy a region. Finally, the paper discusses proxy-based measures of ecosystem function; these are broad measures of how an ecosystem is performing as a whole.

This paper was a great choice because it’s relevant to almost everyone in QAECO. Whether it be population indices or habitat suitability maps, most of our research uses proxies to some degree. Although this topic has been widely discussed in the ecological literature, we thought the authors did a good job of highlighting the need to calibrate and validate proxies, and question their underlying assumptions (e.g. that fitness or per-capita population growth increases in higher quality habitats). A good example they discussed was the study by Nimmo et al. (2015) who demonstrated the linear relationship between track counts and true abundances of Australian dingos.

The article also highlighted current methods to improve the use of indices in applied ecology, such as accounting for detectability (Guillera-Arroita et al. 2010) or varying fecal decay rates (Jenkins and Manly 2008). We would have liked more examples of cutting edge methods given the authors were highlighting problematic implementation issues and the applied/management focus of the journal.

The authors acknowledged upfront that their list of proxies was not exhaustive and was centred on three they commonly encountered in their role as editors. Other proxies we thought may be even more problematic are: indicator/umbrella/flagship species, taxonomic or functional surrogates, many of the commonly employed fitness proxies (i.e. body size or male attractiveness) and indices that have no direct or explicit relationship with the quantity of interest (e.g. SAFE index: McCarthy et al. 2011).

In summary, we agreed that if using proxies we should fully understand the relationship between the proxy and the attribute of interest, and incorporate emerging methods to improve the accuracy of the proxy.

We were left with some unresolved questions, such as: Do we quantify the relationship between the proxy/index and the quantity of interest enough? And what would be the impact of holding off making conservation decisions until we have calibrated the proxy?

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