1 What did we do?

The work combines global layers of forest cover (Hansen et al. 2013) with current and future mean annual temperature (Hijmans et al. 2005), to assess how connected tropical forests are to cooler habitat. The thinking is that tropical forest species sensitive to climate change (which is thought to be many of them) will need to move towards the poles or – more likely – uphill, to stay within their current thermal niche as global temperatures rise. Unfortunately, movement across landscapes is made increasingly difficult for forest specialists by habitat loss and fragmentation.

Fig 1. Forest fragmentation in the tropics. Photo credit: David Edwards.

2 How did we do it?

To calculate connectivity we directly adapted the methods from a great paper by McGuire et al. (2016), which focused on the USA. Everything is based on movement between forest patches, where a ‘patch’ is a continuous area of forest with the same temperature. For each patch we used some simple rules to determine the coldest other patch that could be reached by following a gradient of hotter to cooler adjacent patches. This is a bit like a game of draughts (aka checkers), where you can only move in one direction, one step at a time, and each move has particular rules that determine which of the various options is the best. Once you know the coldest patch that can be reached from your starting patch, you add in the projected future temperature under climate change. If the future temperature of the end patch is cooler than (or the same as) the current temperature of the starting patch, then resident species could potentially avoid rising temperatures by moving.

Fig 2| Schematic diagram of the different steps involved in calculating climate connectivity. Step 1 = identify forest patches and their current temperature. Step 2 = trace each patch to its coldest destination patch, and also assign projected future temperature of each patch. Step 3 = calculate climate connectivity as the current temperature of the starting patch minus the future temperature of the destintion patch.

3 What did we find?

When we do this for all forest in the tropics the conclusions are, well, not good. In the year 2012, some 62% of tropical forest area failed to connect to anywhere cool enough to allow resident species to avoid climate warming by moving. Alarmingly, if species moved as far as possible to the coldest reachable forest, they would still experience 2.6°C of warming by 2070. That’s a lot for a region that has a characteristically stable climate over days, seasons and, indeed, millennia.

How does the current situation compare to the past? We can also use the Hansen et al. (2013) data to look at the year 2000. What we see is that from 2000 to 2012, 27% of tropical forest lost climate connectivity (compared to just 10% that gained connectivity). In terms of the analyses, the only thing that changed in those 12 years was forest cover. Just as with forest fragmentation (Taubert et al. 2018), the loss of connectivity accelerates with increasing forest loss, which leaves the door wide open for further, more drastic loss of climate connectivity in the future.

Fig 3| Maps of current climate connectivity and change in climate connectivity over time. (a) Climate connectivity in 2012, with positive values in blue indicating successful connectivity and negative values in red indicating unsuccessful climate connectivity. (b) The change in climate connectivity from 2000 to 2012, where positive values in blue indicate a gain of connectivity and negative values in red indicate a loss of connectivity. To aid visualization we have shifted and magnified land masses in Oceania.

4 What does this mean?

Is there any hope? I tend to believe that there’s always hope. In this case, a little bit of reforestation in the right places could reinstate a crucial climate corridor, for example, and this disproportionately improves climate connectivity in that region. Where are the right places? This is going to be regionally specific, but a general rule is to connect forests from warmer lowlands right up to cooler highlands. Elsen, Monahan, and Merenlender (2018) found that protection along elevational gradients is generally pretty poor, so there’s huge scope for improvement here.

In addition, we need to be minimising further forest loss in the tropics and, globally, reducing greenhouse gas emissions to mitigate the worst effects of climate change. In the first case, a certain amount of deforestation seems inevitable, to feed a growing and developing human population. However, when we compared observed patterns of connectivity loss to the results if forest were lost at random in a piecemeal fashion (knocking out forested cells at random across the landscape), it seems that a land sparing approach (Green et al. 2005) to forest conversion may be the better way to reconcile agriculture with the need to maintain climate connectivity. Essentially, this means that production is intensified on already converted land, and forest is ‘spared’ and protected. And on top of that, if we get our act together and move towards the less severe warming scenario (RCP 2.6), the prognosis is much improved – species moving to the coldest reachable forest would only experience an average of 0.77°C of warming.

5 Final remarks

As with many of the big environmental problems we face, there is cause for alarm but not for terror-induced paralysis. There are solutions. In future work we are hoping to investigate the options more fully, among lots of other ideas and extensions that I’ve tried to keep track of as we’ve been going along!

Full article here: https://www.nature.com/articles/s41558-019-0529-2 _______________________________________

This is work I did as part of my PhD, with Prof David Edwards (University of Sheffield) and Prof Jane Hill (University of York). I was funded by ACCE (Adapting to the Challenges of a Changing Environment), which is a NERC (Natural Environment Research Council) Doctoral Training Partnership. I am very grateful to the many people who made this work possible, particularly Dr Jenny McGuire, whose 2016 study (see here) was the inspiration for this project. Also to Prof Chris Thomas and the reviewers for their invaluable comments and suggestions.