Elephants and wildfires as important as climate change

Scientists dealing with the impact of climate change on African ecosystems should not ignore the proverbial elephant in the room.

This is the advice of prof. Guy Midgley, a world-leading climate change scientist in the Department of Botany and Zoology at Stellenbosch University.

In an article published in Nature Climate Change this month, Prof Midgley and co-author, Prof. William Bond, call for an urgent reassessment of our current understanding of the impacts of climate change on African ecosystems. Prof. Bond is chief scientist at the South African Environmental Observatory (SAEON) and emeritus professor at the University of Cape Town.

"African ecosystems are unique, with big herbivores like elephants and frequent wildfires sweeping through the landscape. We are now finding that disturbances by herbivores and fire can be as important as climate in shaping the structure, functioning and diversity of African ecosystems," Prof. Midgley says.

However, most projections published so far (the so-called niche-based models) are based on the assumption that vegetation and biodiversity patterns are determined by climate alone.

"In Africa, grasslands can be maintained by frequent fires even where rainfall is high enough to support a forest. But when fire is suppressed, trees can take over. This means that some ecosystems can flip quite quickly from one kind to another independent of any climate change," explains Prof. Midgley.

Scientists are already observing these changes taking place. Many southern African sub-tropical grasslands and savannas are being transformed into woodlands: "More carbon dioxide in the atmosphere is causing the tree saplings to grow faster, allowing trees to establish. More trees suppress wildfire and result in the spread and thickening of woodlands. A niche-based model would not be able to take these effects into account."

This means that many current predictions of climate change impacts on African ecosystems need to be reassessed. But that is not the end of the problem.

Lack of empirical data from African ecosystems

Prof. Midgley says even though a new class of models (the so-called Dynamic Global Vegetation Models) are able to account for disturbances and the effects of carbon dioxide, the data is lacking: "There is a dire lack of empirical evidence and information on interactions between climate, disturbances and the impact of atmospheric CO₂ on ecosystems in tropical and sub-tropical climates.

"In the Nature article we argue that these issues require urgent resolution, because there are significant and immediate implications for biodiversity risk assessments, and adaptation and mitigation responses relevant for policy makers and land managers."

In other words, the widely accepted approach is to increase tree cover in order to sequester atmospheric CO_2. But in Africa the opposite might be the case. With better models and more data it may be better to suppress trees in order to maintain savanna and grassland diversity.

"Managing the responses of African ecosystems to climate change requires home-grown knowledge, as well as an understanding of the trade-offs between biodiversity and carbon sequestration objectives in Africa," he concludes.

In September 2015 Prof. Midgley and Prof. Bond will co-lead an international workshop, partly funded by the National Research Foundation (NRF), to address some of these issues.

Photos courtesy of Prof. Brian van Wilgen