15-17: Herbivores and grasses Flashcards
What do herbivores influence?
Nutrient recycling eg. N, P
Where do you find herbivores?
In all ecosystems
What % of plant production is consumed by herbivores in oceans?
60-99%
Where is plant consumption lowest (by biomass)?
Forests
As there is a large allocation of biomass to woody material, which can’t be eaten
Where is plant consumption greatest (by biomass)?
Grassland
Most biomass is non-woody
Negative effects that herbivores have on soil biota and ecosystems
Selective grazing
Reduced quality of litter inputs to soil
Positive effects that herbivores have on soil biota and ecosystems
Retardation of succession Faecal return (short-cut litter decomposition)
How are water and herbivores connected?
More water = more food = higher densities of herbivore
How do herbivores effect soil?
Reduction in soil quality
How do we study the effect of herbivores on resource quality and quantity?
Field studies
Fence off areas, compare where herbivores graze to where they can’t
What is the deceleration effect?
Plants that thrive are less palatable
Animals eat deciduous plants
Litter decomposes slowly as it is rich in tanins
This reduces the N supply
So low soil nutrient availability for the growth of slow-growing conifer species
RE WATCH LECTURE
Case study of moose browsing boreal forests
Pastor et al. 1993
Lake Superior
Moose were introduced 100 years ago
Selective grazing on hardwood lead to dominance of conifers
Produce litter of poor quality (high polyphenols, low N)
Reduce microbial activity and N mineralisation
What is the degenerative feedback loop (moose example)?
Increased consumption from moose- selective grazing on hardwoods
Reduced tree production due to conifer dominance
Decreased leaf litter quality
Reduced soil microbial activity and nitrogen mineralisaton- not compensated by faeces
Reduced productivity of ecosystem- more moose consumption than supply
So collapse of moose population
Case study of regenerating birch forest by red deer
Lots of reindeer, natural birch forest not growing as high as should be
Culling regime in 1986
Couldn’t use fences as would stop movement of birds
The size of trees increased
Tree growth promotes soil biological activity due to root activity and litter inputs
Grazers reduce ecosystem productivity
What is the positive feedback loop in fertile sites?
Acceleration effect
Herbivores favour tolerant plants with high tissue N
Grazing benefits grasses
High soil fertility
Case study of sheep grazing on fertile temperate grasslands
Grazing closures to compare with or without grazing
Increased microbes, N mineralisation and plant N in grazing areas
Grazing leads to more productive ecosystem and plants of a higher nutrient content
More dung than litter (x3)
Case study of African grazers on the Serengeti
Serengeti grazers prefer to forage plants enriched in minerals important for late stages pregnancy, lactation, and growth of young
Forage vegetation on soil with greater nutrient availability
Animal activities increase nutrient availabilty
Supply of N to plants (annual net N mineralisation) doubled when animals were present
So modify system
Case study of disturbance effects of grizzly bears digging alpine meadows
Forage on bulbs- higher nutrition
In digs (compared to undisturbed meadows), found:
More inorganic N in soil
Glacier Lily bulbs of greater N and carb content
Produce twice as many seeds due to improved nutrietion
Seedlings establish better, so better food for bears
Case study of grassland degradation on Qinghai-Tibet plateau
30-50% of grassland is degraded
Around 8 million people live there, 48% live in poverty
