“Grazing away the resilience of patterned ecosystems”

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Eric Siero, Koen Siteur, Arjen Doelman, Johan van de Koppel, Max Rietkerk, and Maarten B. Eppinga (Mar 2019)

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Extending classic reaction-diffusion models shows how herbivory reduces the resilience of spatially patterned ecosystems

The eye of the tiger bush: herbivory accelerates desertification of patterned ecosystems

Giraffe in an area covered with tiger bush south of Niamey, Niger, in 1999. Local people suggested that the giraffe were driven into the area, and hence herbivore grazing pressure is possibly determined by human impact.
(Credit: Johan van de Koppel)

Vegetation in semi-arid grazing systems self-organizes into strikingly regular spatial patterns, often referred to as tiger bush. Apart from being appealing to the human eye, theory has suggested that formation of these patterns actually helps to retain vegetation density when rainfall decreases. Moreover, these patterns may also serve as early warning signals that a transition to a bare desert ecosystem is imminent.

Most theory, however, has not considered that the herbivores of semi-arid ecosystems move through the landscape, thereby creating variations in grazing pressure. In a new study appearing in The American Naturalist, a research team led by Eric Siero (University of Oldenburg) reports how predictions from previous pattern formation models change when more realistic movement behavior of herbivores is included. Importantly, the team found that herbivores may accelerate transitions in ecosystem states, and can possibly obscure early warning signals that such a transition is imminent. However, the results also suggest that if there is still vegetation present, temporary, small-scale grazing exclosures are surprisingly successful in catalyzing a restoration process of ecosystems' productivity at the larger landscape scale.


Abstract

Ecosystems’ responses to changing environmental conditions can be modulated by spatial self-organization. A prominent example of this can be found in drylands, where formation of vegetation patterns attenuates the magnitude of degradation events in response to decreasing rainfall. In model studies, the pattern wavelength responds to changing conditions, which is reflected by a rather gradual decline in biomass in response to decreasing rainfall. Although these models are spatially explicit, they have adopted a mean-field approach to grazing. By taking into account spatial variability when modelling grazing, we find that (over)grazing can lead to a dramatic shift in biomass, so that degradation occurs at rainfall rates that would otherwise still maintain a relatively productive ecosystem. Moreover, grazing increases the resilience of degraded ecosystem states. Consequently, restoration of degraded ecosystems could benefit from the introduction of temporary small-scale exclosures, to escape from the basin of attraction of degraded states.