L17 - Soils, plants and herbivores Flashcards
How does plant biomass consumption by herbivores vary across ecosystems?
It ranges from 1-99% of net primary productivity (NPP), depending on plant allocation to woody or non-woody structures.
How does herbivore density correlate with rainfall?
High annual rainfall supports higher herbivore densities as plants grow abundantly, providing food for herbivores
What are the negative and positive impacts of herbivores on soil biota and ecosystems?
- NEGATIVE: Selective grazing and reduced quality of litter inputs to soil
- POSITIVE: Retardation of succession and faecal returns that speed up litter decomposition
How do herbivores affect low-fertility sites (deceleration effect)?
Selective grazing favors plants with low tissue nitrogen, leading to reduced soil nutrient availability and promoting slow-growing species like conifers.
What was the feedback loop observed in moose browsing on boreal forests (Pastor et al., 1993)?
- Moose selectively graze hardwoods (e.g., aspen and birch)
- Conifer dominance increases, producing low-quality litter.
- Soil microbial activity and nitrogen mineralization decrease
- Ecosystem productivity declines, leading to moose population collapse
What was the impact of red deer browsing in the Scottish Highlands (Harrison & Bardgett, 2003)?
- Reduced regeneration of birch trees (Betula pubescens).
- Four-fold reduction in nitrogen mineralization.
- Grazing reduced soil biological activity and ecosystem productivity.
How do herbivores affect high-fertility sites (acceleration effect)?
Herbivores favor tolerant, high-nitrogen plants, leading to increased soil fertility, nitrogen cycling, and net primary productivity (NPP).
How did sheep grazing affect temperate grasslands (Bardgett et al., 1997)?
- Increased microbial biomass (50-100%).
- Increased nitrogen mineralization (50-60%).
- Increased plant nitrogen content (10-25%).
- Enhanced NPP through the faecal short-cut decomposition pathway
How do African grazers in the Serengeti modify nutrient cycling (McNaughton et al., 1997)?
- Grazing increases nitrogen mineralization and soil sodium (important for reproduction).
- Grazers enhance soil fertility, alleviating nutrient deficiencies and boosting ecosystem carrying capacity.
How does grazing impact stomatal conductance in grasses in the Serengeti?
Grazing increases stomatal conductance and transpiration, which leads to sodium accumulation in the surface soil.
How does grizzly bear digging affect alpine meadows (Tardiff & Stanford, 1998)?
- Increased inorganic nitrogen in the soil.
- Glacier lily bulbs have higher nitrogen and carbohydrate content, doubling seed production.
- Improved nutrition benefits seedling establishment and grizzly bears’ food supply.
Why is global grassland degradation a major concern?
Grasslands cover ~40% of Earth’s land, and degradation impacts 550 million poor livestock herders, with on-farm costs estimated at $6.8 billion annually.
What are the key causes of grassland degradation on the Qinghai-Tibetan Plateau?
Overgrazing, reduced soil fertility, climate change, extreme weather events (e.g., drought), and pika infestations
What percentage of grasslands on the Qinghai-Tibetan Plateau are degraded, and what are the consequences?
- 30-50% of grasslands are degraded.
- Reduced productivity, loss of soil fertility, and decreased ecosystem service capacity.
What are the goals of global restoration efforts for grasslands?
To understand the extent, causes, and consequences of grassland degradation and implement strategies to restore soil function and ecosystem resilience.
