Herbivores affect structural diversity, plant diversity, nutrient cycling. To understand these effects we need a common framework. She shows a cross-site study on grasslands in North America and the Netherlands. They have large herbivores (cows, bison) and also smaller herbivores (deer, pronghorns). They do exclosure studies on these grassland sites and look at effects.
1. Impact on plant diversity (Bakker et al 2006 Ecology Letters). Change in diversity is proportional to primary productiction. In more productive systems the herbivores increase diversity. In less productive systems they decrease species richness. Why is this?
In a low productivity system (e.g. Short grass step) the above ground biomass is low (below ground biomass is high) so herbivores have little influence on the vegetation. The small herbivores can get at plants and remove some species
In a high productivity system (e.g. Flood plain grassland), the herbivores prevent domination by a few plant species. The large herbivores facilitate small herbivores such as rabbits. Rabbit burrowing (1% bare soil coverage) provides a regeneration niche.
2. Impact on nitrogen availability
In N poor systems, herbivores have a positive effect on nitrogen availability by transforming biomass into dung. In N rich systems however, herbivores can have a slight negative effect on nitrogen availability She shows a conceptual pathways by Schrama et al (2013). This may be an effect of soil compaction and trampling having an effect of slowing down the nitrogen cycle.
She shows that clay fraction is a good proxy for soil compactability. Under intermediate soil texture and moisture conditions herbivores increase N availability, but under high clay or high soil moisture conditions thet can inhibit N cycling.
Should we introduce livestock into nature reserves? Are they the best proxy we have for extinct fauna?
In productive grasslands, there is good quality food for herbivores, and well-defended shrub species. In Netherlands, thorny shrub is much moore common on rich soils. This is a type of habitat where grazers have been influential, and would be suitable for livestock grazing.
She did an exclusion experiment in a shrub-grassland ecotone ecotone. There was woody encroachment, but actually the most important influence was from rabbit grazing, not from livestock.
In aquatic systems, similar principles may apply. She did exclosures in ten different wetland areas. There was expansion of vegetation inside exclosures, driven by muskrats as “aquatic rabbits”
Aquatic megafauna role? These include hippos, moose and capybaras. Very little known about their roles. Meso fauna such a beaver very important too.
Conclusions
There are some general rules of herbivore impact. They increase plant species in productive ecosystems, facilitate smaller herbivores.
1. Impact on plant diversity (Bakker et al 2006 Ecology Letters). Change in diversity is proportional to primary productiction. In more productive systems the herbivores increase diversity. In less productive systems they decrease species richness. Why is this?
In a low productivity system (e.g. Short grass step) the above ground biomass is low (below ground biomass is high) so herbivores have little influence on the vegetation. The small herbivores can get at plants and remove some species
In a high productivity system (e.g. Flood plain grassland), the herbivores prevent domination by a few plant species. The large herbivores facilitate small herbivores such as rabbits. Rabbit burrowing (1% bare soil coverage) provides a regeneration niche.
2. Impact on nitrogen availability
In N poor systems, herbivores have a positive effect on nitrogen availability by transforming biomass into dung. In N rich systems however, herbivores can have a slight negative effect on nitrogen availability She shows a conceptual pathways by Schrama et al (2013). This may be an effect of soil compaction and trampling having an effect of slowing down the nitrogen cycle.
She shows that clay fraction is a good proxy for soil compactability. Under intermediate soil texture and moisture conditions herbivores increase N availability, but under high clay or high soil moisture conditions thet can inhibit N cycling.
Should we introduce livestock into nature reserves? Are they the best proxy we have for extinct fauna?
In productive grasslands, there is good quality food for herbivores, and well-defended shrub species. In Netherlands, thorny shrub is much moore common on rich soils. This is a type of habitat where grazers have been influential, and would be suitable for livestock grazing.
She did an exclusion experiment in a shrub-grassland ecotone ecotone. There was woody encroachment, but actually the most important influence was from rabbit grazing, not from livestock.
In aquatic systems, similar principles may apply. She did exclosures in ten different wetland areas. There was expansion of vegetation inside exclosures, driven by muskrats as “aquatic rabbits”
Aquatic megafauna role? These include hippos, moose and capybaras. Very little known about their roles. Meso fauna such a beaver very important too.
Conclusions
There are some general rules of herbivore impact. They increase plant species in productive ecosystems, facilitate smaller herbivores.