RT Journal Article
SR Electronic
T1 Critical transitions and the fragmenting of global forests
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 091751
DO 10.1101/091751
A1 Leonardo A. Saravia
A1 Santiago R. Doyle
A1 Benjamin Bond-Lamberty
YR 2016
UL http://biorxiv.org/content/early/2016/12/06/091751.abstract
AB Global forests provide critical habitat for many species, essential ecosystem services, and are coupled to atmospheric dynamics through exchanges of energy, water and gases. One of the most important changes produced in the biosphere is the replacement of forest areas with human dominated landscapes. These areas contain many fewer species than the natural ecosystem they replaced, and this change is one of the main drivers of extinctions in the Anthropocene. Another effect is that habitat replacement leads to fragmentation, altering the sizes of patches, the structure and function of the forest. In different kinds of vegetation, including forests, the patch size distributions follow power laws. These have been used as a signal of critical phase transitions, where the system state changes suddenly at a critical point; in the case of forest patches, this implies an abrupt change in connectivity that causes a increased fragmentation level, posing an additional risk to forest function as an habitat and ecosystem. All these patterns have been observed only at relatively small scales, however. Here we studied the distribution of forest patch sizes at a global level—wide regions of connected forest across continents and big islands—examining their changes over the last fourteen years and different signals of a critical transition. The conditions that indicate that a region is near a critical fragmentation threshold are: a patch size distribution following a power law; temporal fluctuations of the largest patch following a power law; variance of largest patch fluctuations increasing in time; and a negative skewness of the largest patch fluctuations. We found that most regions, except the Eurasian mainland, followed a power-law distribution. Only the tropical forest of Africa and South America met the five criteria and thus seemed to be near a critical fragmentation threshold. This implies that the combined influence of human pressures and climate forcings might trigger undesired effects of fragmentation, such as species loss and degradation of ecosystems services, in these regions. The simple criteria proposed here could be used as early warning to estimate the distance to a fragmentation threshold in forest around the globe, and provide a guide to direct conservation efforts at a continental level.