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New global study positioned to dominate thinking about herbivores and plant biodiversity in grasslands and savannas

By Dr Dave Thompson, Biodiversity Scientist, SAEON Ndlovu Node
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Herbivores impact on plant biodiversity in many of the world’s ecosystems, but the magnitude and direction of these herbivore effects (positive, negative or no effect) vary widely within and among ecosystems.

Understanding such impacts is vital for conserving plant diversity and ecosystem functioning in a human-altered world, especially in disturbance-dependent grasslands and savannas where herbivore communities are typically species-poor, and often numerically reduced versions, of their former selves.

Prevailing theory predicts that the activity of herbivores – primarily being feeding by grazers, should increase plant biodiversity in environments where rainfall is high (high productivity), and have the opposite effect in dry environments (low productivity). However, isolated studies reveal that not all grassy systems conform to this theory, with deviations from the pattern being seen.

This calls into question the generality of the role of system productivity in governing herbivore effects on diversity and suggests that alternative mechanisms may be driving how animals impact on their environments.

New global study

Now, a study recently published in Nature Ecology & Evolution is providing just that alternative mechanism, and is offering rather compelling evidence which explains how and why herbivores impact plant biodiversity.

In grappling to explain different patterns in how herbivore-exclusion plots at the Konza Prairie LTER (Long Term Ecological Research) station (USA) and in the Kruger National Park (South Africa) affected plant biodiversity immediately after and in the years following herbivore removal, a group of researchers hit on an intriguing idea. Could the ability of herbivores to change the abundance of the dominant plant species – which is linked to those species being palatable or not to the herbivores, affect resource availability and so either encourage or prohibit additional species?

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Figure 1. Herbivore exclusion plots erected at Konza Prairie LTER station (left) and in the Kruger National Park in 2006 showed differing effects of grazing (or rather, the lack of grazing) on plant diversity. In attempting to explain this, researchers were forced to challenge conventional thinking.

To test this hypothesis, the researchers – led by Dr Sally Koerner of the University of North Carolina Greensborough and including Dr Dave Thompson from SAEON’s Ndlovu Node, established the ‘Grazing Exclosure Consortium’ to conduct a meta-level analysis comparing findings from large herbivore exclosure experiments from around the world. Ultimately plant species composition data from 252 sites spanning six continents and a large rainfall gradient (mean annual precipitation 45-1511 mm) were ‘donated’ for inclusion in the analyses.

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Figure 2. Map of the 252 site localities included in the meta-analysis, with mean annual rainfall shown. Many sites overlap, so not all are visible.

To be included in the Grazing Exclosure Database, sites had to meet five criteria: (1) exclosures had to be located in herbaceous dominated communities – sites ranged from tallgrass prairie to alpine meadows to desert; (2) herbivores with adult body mass > 45 kg were excluded from plots using fencing, with adjacent plots exposed to herbivores; (3) data had to be collected after at least three years of herbivore exclusion; 60% of sites provided data reflecting 10 or more years of grazing manipulation and 18% of sites provided herbaceous data following 50 years of exclusion; (4) plots inside and outside the exclosure had to be sampled at the same time and sampling intensity; and (5) data had to be available at the species level.

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Figure 3. Herbivore type and number varied among exclusion sites, and included domesticated cattle, sheep, goats, burros and horses, as well as native wildlife such as bison, caribou, kangaroo and the full complement of large African herbivores.

And the result?

A positive, but weak relationship between grazing-induced change in species richness and annual rainfall (as a proxy for productivity) was found. But the pattern emerging from the data showed a much stronger relationship between grazing-induced changes in dominance and changes in species richness.

Where herbivores decreased the relative abundance (biomass, cover) of dominant species, plant species richness increased, while increased dominance caused a decline in species richness. This relationship holds true across all continents and their assortment of wild and domestic herbivores, and across the rainfall gradient.

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Figure 4. The change in species richness (as a measure of diversity) between grazed and ungrazed treatments at each of the 252 sites correlated more strongly with change in the biomass or cover of dominant species than with precipitation (as a proxy for system productivity). Each dot represents a single site.

Change in dominance explains positive and negative herbivore impacts on plant biodiversity across vastly different grassy systems globally by considering the traits of the dominant plants – those characteristics of a species that impart competitive advantage, that result in resource limitation for others, and which allow grazing tolerance or avoidance. Strong dominance by just a few species is a nearly universal feature of herbaceous ecosystems.

As a consequence, this new thinking points to ‘dominance management’ – essentially altering competition for resources using herbivores as an effective conservation strategy.

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