Research warns of ecosystem conversion

Human-driven deforestation has been touted as one of the greatest threats facing wildlife and ecosystems worldwide. But a Princeton research team led by Carla Staver, a graduate student in the Department of Ecology and Evolutionary Biology, has suggested that the loss of savannas to forests as a result of human alteration of climate and landscapes is an equally serious issue.

In tropical and subtropical areas, savanna wildfires generally maintain boundaries between savanna and forest. Fires eliminate tree cover and prevent forests from encroaching on grassland; in turn, areas with high tree cover — such as forests — prevent fires from spreading. This natural balance is crucial for organisms that have adapted to living in either biome.

However, the researchers with whom Staver worked have proposed that human-induced factors including climate change, construction and fire prevention interfere with this natural balance. Low rainfall, an effect of climate change, encourages more wildfires and thus turns more forest biomes into savannas. On the other hand, fire prevention would allow forests to develop where savannas might once have existed by curtailing wildfires. Similarly, construction would break up the pattern of natural fires and encourage more forests to sprout.

Such changeovers, the team found, could wreak havoc on forest and savanna ecosystems. Plants and animals would find it difficult to transition from living in a forest to a savanna, and vice versa. More importantly, as Staver noted in an email, “the effects ... on the distribution of savanna and forests are likely to kick in suddenly and will probably be hard to reverse once they occur.”

Staver, collaborating with ecology and environmental biology professor Simon Levin and Sally Archibald, a senior research scientist at the Council for Scientific and Industrial Research in South Africa, used satellite data on fire distributions combined with climate, soil and tree cover data to survey regions of Africa, Australia and South America.

In particular, the South African parks where the team conducted research were full of “ecological and evolutionary puzzles,” Levin said in an email.

The team’ s research is also some of the first to affirm the fire feedback theory, which proposes that fires primarily maintain the savanna-forest boundary and determine the flourishing of these habitats. The team found that the frequency of fires was more of an indicator of whether savanna or forest would dominate in these areas than factors like soil texture, rainfall or seasons, especially in areas with moderate rainfall.

According to Staver, these findings have major implications in understanding how climate change and other types of global human-driven change are likely to affect the biosphere. Because changes in savanna and forest distributions have been projected to occur quickly and irreversibly, the team hopes to focus next on “looking at actual trajectories of change and how quick ‘sudden’ is, which could affect how we interpret these results,” she said.

“There is great interest in the ecological community on what makes ecological systems robust or stable,” Levin said, elaborating on the team’s interest in studying savanna-forest dynamics. “This is an ideal system of study.”

The report was published on Oct. 13 in the journal Science and was supported by funds from the Andrew W. Mellon Foundation.