Researchers at Wageningen University find that in South America, Africa and Australia, under climate change, forests, savannas and treeless systems are not gradually succeeding, but rather tilting.

[Picture: Amazon rainforest, close to 100 percent tree cover optimum, under condition of daily oscillating rainy cycle. Wageningen University handout.]

This is because intermediate states, with about 5% and about 60% tree cover in nature, do not appear to exist, because somehow in the tropics and subtropics the ecology interaction -climate is operating towards one of the three optimal states.

This means that species have much less time to migrate – and a much smaller chance of surviving. It also means that there may be some truth in the dreaded Amazon transition, where drought results in damage to vegetation, which in turn can further disrupt the rainy cycle. Where the tropical rainforest is currently (maximum biomass density), in the future, the savannah may remain – with an average tree cover of only around 20 percent. [Speaking of tipping points: apart from biodiversity loss, this transition would also result in the release of many gigatonnes worth of carbon to the atmosphere.]

Mechanisms of climate-biodiversity thresholds at all ecosystem levels

Earlier today, we looked at the impact of climate change on biodiversity at the smallest level of the ecosystem: a dry aquatic desert ecosystem. This is a classic case study to show that ecological damage most likely occurs after a disturbance (such as drought) exceeds a threshold. A normal intermediate drought may have no effect, as the stream will only temporarily decrease in size, or there may at least remain puddles for aquatic species to find shelter – until the rains return. With the introduction of more extreme drought, the entire ecosystem can dry out – and sudden local extinctions occur, as recently happened in the Arizona desert.

Now the new research, which has been published in Science, shows that such thresholds also exist in macrotropical ecosystems, perhaps in all biomes of the rainforest, savannah and ecosystems of the “tree-less state” (scrubland without trees, grasslands without trees and desert).

Satellite observations show current tipping points

The researchers hope their findings will also serve practical use. The same satellite data that revealed ecosystem transitions to them also shows where changes in precipitation can lead to the current biome boundary. There, local conservation measures could be put in place to, for example, try to improve the resilience of vegetation.

Considering a recent Nature publication that quantified what is really at stake in the context of the [conservative] range of IPCC scenarios, we are slowly starting to think that perhaps the wisest thing would be to consider a top-down approach. Remember, at the end of the day, it’s all about emissions.

© Rolf Schuttenhelm | www.bitsofscience.org