Ask your doctor whether you should consider a surrogate species

Conservation practitioners often use surrogates to make decisions for connectivity planning or determining priorities in general. For example, in my research, instead of directly measuring the effect of habitat restoration on an endangered butterfly, we tracked populations of a more common species with similar habitat requirements. Another example would be using the species richness of butterflies, which is easy and fun to measure, to represent the response of all insects – the Earth’s most diverse group – to environmental changes, which may be more challenging or tedious to measure.

A new paper published in Oikos argues that the use of surrogates in conservation ecology could be improved by appreciating how surrogates have been used in the medical field with humans. Doctors also use easily-measured surrogates, such as blood pressure, to estimate disease risk they cannot measure directly. Barton and colleagues walk through the steps used in medicine to choose valid surrogates, and evaluate their ability to predict the outcomes of treatments.

Here are key takeaway messages from the comparison between medical and ecological surrogates:

1. Choose a good surrogate! This might seem obvious, but the surrogate needs to relate to the outcome of interest. In their example, they measure an animal taxon that mechanistically links to the woodland condition they are restoring. Acknowledge that there will be variables that affect the outcome that are not captured by the surrogate, indicated by the top arrow on their chart (above). The treatment, in this case land management, will also affect both the surrogate and outcome indirectly, like through vegetation structure in their example.
2. Validate the surrogate’s performance. Make sure that the surrogate and outcome both respond to the treatment in similar ways. Barton and co-authors recommend that statistical techniques, such as path analysis or structured equation modeling, be used to link the measured variables to each other. In my opinion, this recommendation will be the biggest obstacle to their framework being adopted broadly, since these types of analyses require a high level of skill. As an alternative, I wonder if comparing different plausible surrogates would be enough for land managers to choose one with the best performance.
3. Evaluate the surrogate in different places and times. The same surrogate may not work well in different habitat types or may change in its ability to predict the outcome over time. A willingness to adapt when confronted with new information is essential, especially when ecologists do not have the luxury of laboratory controls used in medical trials.  This is especially true when planning for connectivity, where individuals may use corridors differently over space or time.