Measuring connectivity in plantations
Forest plantations are everywhere. You’ll find them in almost every vegetated country in the world. They cover a surprisingly large portion of our planet – some 260 million hectares, corresponding to 7% of global forest cover. And their size is increasing at an impressive rate: according to the Food and Agriculture Organization every year an area roughly equivalent to the size of Switzerland (around 5 million hectares) is converted to planted forest. It is estimated that by 2020 planted forest will cover 300 million hectares, which is equivalent to half the size of the Amazonian rainforest.
Why are we converting such a large area of the planet to this ‘artificial biome’? There are two main reasons: an increasing demand for wood used for timber and paper, and the need to sequester carbon to mitigate global warming.
Despite the considerable amount of research that has been conducted on the impact of plantations on biodiversity, there is little agreement on whether plantations are a ‘biological desert’, or a ‘lesser evil’ compared to agricultural land. As a consequence, landscape managers have a confusing body of empirical evidence to support their decision-making on whether or not to convert agricultural land to plantations. Considering the extremely large areas of the planet that are converted to planted forest each year and the parlous state of our biodiversity, this is far from an academic question. Our study sought to bring some clarity on the topic.
Our analyses were based on data from the ‘Nanangroe experiment’, one of the largest landscape transformation experiments ever established (managed by David Lindenmayer’s research group since 1998). It involves monitoring changes to fauna inhabiting woodland remnants set in a grazing landscape as that landscape is converted to a massive pine plantation.
The setup of the Nanangroe experiment is very simple: bird populations inhabiting 50 treatment Eucalyptus patches (ie, patches that are surrounded by pine plantations) are compared to 50 control Eucalyptus forest patches (patches surrounded by grazed fields). A key feature of this experiment is that the habitat within the Eucalyptus patches has remained unchanged in control and treatment sites, while the ‘matrix’ surrounding the treatment patches was transformed. It is thus possible to focus on the impact of pine plantation with limited background noise from other confounding factors.
In our study we focused on the connectivity between populations (ie, on the flow of dispersers and immigrants between populations in the patches of woodland remnants). Population connectivity is a key ecological parameter; however, estimating it directly is an extremely difficult task which becomes prohibitive when working on many species. In this study we adopted a novel approach to estimate connectivity. We quantified it by measuring the synchrony in abundance between populations (ie, the coincidental changes of population density over time) over 15 years.
Our results are surprising; especially if, like many people, you believe that trees in the landscape make it easier for birds to get around. What we found is that pine plantations did not increase connectivity for any of the 52 bird species examined. The pine plantations acted as a barrier for four species, whereas for the remaining 48 species the effect of pine plantations was neutral (ie, it did not significantly affect the connectivity between populations).
The implications of our findings for landscape management are clear. The conversion of agricultural areas to plantation forestry is unlikely to promote substantial movement of individuals. Therefore, we strongly suggest that expansion of pine plantations should not be promoted in the belief that it may provide increased connectivity for birds compared with an agricultural matrix.
More info: Alessio Mortelliti email@example.com
Mortelliti A, MJ Westgate & DB Lindenmayer (2014). Experimental evaluation shows limited influence of pine plantations on the connectivity of highly fragmented bird populations. Journal of Applied Ecology. doi: 10.1111/1365-2664.12313
About the animation:
The science in Mortelliti et al, 2014, was the inspiration for an award-winning stop animation video produced by Alessio and Christina Thwaites. You can see their 3 minute, 52 second extravaganza at the Environmental Decisions Group YouTube Channel.