4) Soil Biota, Plants and Aboveground Herbivores

How do herbivores modify soil processes, and what are the potential consequences?

Can modify soil processes in ways that have positive or negative consequences for their species, affecting soil fertility, nutrient cycling, and plant regrowth.

How does herbivore consumption vary across ecosystems?

• Lowest in forests (<1-5%) due to woody plant material.

• Greatest in grasslands (10-60%) where most biomass is non-woody.

At what level of rainfall is herbivore biomass highest?

Herbivore biomass is highest at approximately 600 mm of annual rainfall.

What happens when herbivores selectively graze on low fertility sites?

• Herbivores favor defended, low tissue N plants (e.g., evergreen plants) because deciduous plants are more palatable.

• Defended plants decompose slowly, reducing nutrient cycling rates.

What is the deceleration effect in herbivory?

Grazing on low fertility sites leads to reduced nutrient cycling and slowed ecosystem processes due to poor litter quality and slower decomposition.

How can grazing effects be measured?

By creating enclosures to exclude animals; compare grazed vs ungrazed areas.

What were the ecological effects of moose browsing in Isle Royale, Lake Superior?

• Increased consumption of aspen and birch led to conifer dominance.

• Poor-quality litter reduced soil microbial activity and nitrogen mineralisation, leading to ecosystem productivity decline.

• The degenerative feedback loop caused moose population collapse

How did reducing red deer populations in Creag Meagaidh, Scottish Highlands, affect regenerating birch forests?

• A culling regime reduced red deer density, allowing tree numbers and diameters to increase.

• Enhanced tree growth promoted soil biological activity via root activity and litter inputs.

What is the acceleration effect in herbivory?

Grazing stimulates plant regrowth, litter decomposition, and nutrient cycling, often improving soil fertility and productivity.

What role do faeces and urine play in fertile sites?

Enhance soil fertility by returning high-quality nutrients to the soil, stimulating plant growth.

What were the effects of sheep grazing on fertile temperate grasslands?

• 50–100% increase in microbial biomass in grazed areas.

• 50–60% increase in nitrogen mineralisation.

• 10–25% increase in plant nitrogen, improving net primary productivity (NPP).

• The main mechanism was nutrient-rich dung from sheep.

How do Serengeti grazers affect soil fertility?

• Grazers forage in areas enriched with minerals critical for reproduction and growth.

• Grazing activities augment nutrient availability in the soil, benefiting plant growth

How do grizzly bears affect alpine meadow ecosystems?

• Grizzlies dig for glacier lily bulbs, enriching soil nitrogen content.

• Digs lead to bulbs with greater nitrogen and carbohydrates, producing twice as many seeds.

• Enhanced seedling establishment provides better food for bears.