7. Soil Food Webs

Question 1

Describe early food webs. 4

Answer 1

1. Richard Bradley, 1718, and Jonathon Swift, 1733, are some of the earliest accounts of food webs
2. A famous example is form Darwin’s on the origin if species - cat eats mouse eats bee pollinates clover
3. it was later argued that, as cows ate clover, and imperial sailors ate beef, the maids owning the cats were protecting the british empire
4. top of food chain controls everything else

Question 2

What are the quantitative relationships in food webs? 3

Answer 2

1. mass and energy are lost at every trophic level
2. Semper, 1881, assumed 1:10 ratio of food to flesh
3. 1000 plant units sustain 10 carnivores

Question 3

How were food webs developed? 3

Answer 3

1. in 1946, George Clarke realised we can use food webs to quantify human impact on ecosystems
2. webs became more quantitative and complex
3. large variety of symbols employed

Question 4

Prove some examples of when food webs have been used to quantify human impacts on ecology, and general ecology. 7

Answer 4

1. DDT - grazed by cows, in milk, in human fat
2. insects travelling up food chain in increasing concentrations
3. removal of top predators from ecosystems, leading to overgrazing
4. removal of prey/plant biomass from ecosystems
5. green world hypothesis
6. modelling behaviour and dynamics of complex communities
7. Used to calculate flow of energy and matter through ecosystems

Question 5

What is the green world hypothesis? 7

Answer 5

1. most of life sustained by plants - how are there any left?
2. plants are not passive
3. nutrients limit herbivores, not energy
4. abiotic factors limit herbivores
5. spatial variability reduces availability of plants and protects them from herbivores
6. herbivores limit their own numbers
7. enemies limit herbivore numbers

Question 6

What increases the stability of ecological networks? 8

Answer 6

1. Lord May studied randomly generated ecosystems and found the more complex, the less stable - real ecosystems aren’t random
2. trophic coherence - organized trophic levels increase stability
3. compartmentalisation increases stability
4. if something from one compartment goes extinct, it only affects that compartment
5. occurs in nature
6. Many weak links are stronger than a few strong links
7. if a strong link is removed, there is a greater impact
8. if a weak link is removed, prey controlled by other predators

Question 7

What is the difference between primary producer and detritus based food chains? 6

Answer 7

1. primary producers are photosynthetic plants, fueled by sunlight
2. detritus based feed on dead organic matter - energy transferred but returned on death/excretion
3. both release co2 from respiration into global system
4. green organisms contribute to brown on death
5. brown may eat green
6. systems interlinked

Question 8

Describe primary consumers in the soil. 6

Answer 8

1. huge diversity and collective biomass
2. bacteria, algae, fungi and actinomycetes
3. responsible for breakdown of organic matter and nutrient recycling
4. some form symbiotic relationships
5. some are autotrophic
6. millions bacterial species in 10g soil, 1000s of metres of fungal hyphae

Question 9

Describe secondary and tertiary soil consumers. 3

Answer 9

1. Protists, nematodes, collembola, mites (top predators)
2. huge species diversity, according to morphological and dna studies
3. can survive in extreme places eg. alaskan tundra

Question 10

What is collembola? 1

Answer 10

1. Order of insects, incl springtails

Question 11

Describe fungal based soil food webs. 6

Answer 11

1. Prevail in undisturbed, late successional systems
2. are sensitive to agricultural intensification
3. consist of relatively slow growing organisms
4. Linked to efficient carbon and nitrogen cycling
5. mites eat springtails, eat predatory nematodes and fungi, predatory nematodes eat fungi, fungi eats detritus
6. ratio of this compared to bacterial characterised by top trophic levels present

Question 12

Descrobe bacterial based soil food webs. 5

Answer 12

1. Prevail in more nutrient rich, disturbed habitats
2. Flourish under agricultural intensification
3. consist of fast growing organisms
4. more leaky of carbon and nitrogen
5. Mites eat predatory nematodes, which eat protozoa and nematodes. They both eat bacteria, which eats detritus

Question 13

Describe Clarholm’s wheat growing experiments in 1985. 4

Answer 13

1. Wheat grew with and without bacteria
2. more grew with bacteria as more nitrogen mineralised
3. adding protozoa increases available n further for plant growth
4. Microbial loop- grazing on bacteria liberates nutrients for plant growth

Question 14

How do you calculate energy flow in soil food webs? 6

Answer 14

1. Calculate how much energy is released via food webs - quantified biomasses
2. annual feeding rates of functional groups are calculated, assuming that the biomass production rate of a group balances the rate at which material is lost through natural death and predation
3. if a predator feeds on more than one prey, both the preference for a prey and the abundances of these prey types determine feeding rates
4. work out how much n and carbon is mineralised in each ecosystem, based on above assumptions
5. when you know C:N, can calculate biomass created/excreted by organisms
6. can compare results to soil samples - results line up well

Question 15

Describe the points of argument for the top down vs bottom up regulation of ecosystems. 9

Answer 15

1. Which one depends on productivity of soil
2. If less primary consumption, they are in control
3. system eventually becomes unstable and too many upper levels, which further limits lower levels
4. Other interactions influence this eg. prey hiding places
5. Trophic cascade - when this pattern is unstable and oscillates
6. Plants fuel food webs with nutrients and grow on nitrogen that food webs release
7. can sustain plant growth from bottom up
8. different plant inputs select for different soil food webs, which in turn affect plant nutrient quality
9. This selects for different types of food webs eg. fungal produces less nitrogen but better at breaking down calcified materials

Question 16

Describe the Jena experiment. 5

Answer 16

1. Land split into plots with 2,4,8 etc. plant species, coupled above and below ground communities
2. Top trophic levels connect lower trophic levels
3. Top trophic levels dampen fluctuations of lower trophic levels
4. slow fungal energy channel contains more weak links and higher trophic levels than bacterial energy channel
5. Slow growing organisms more likely to have stress tolerant traits

Question 17

Describe the De Vries et al. study in 2012 of the stability of soil food webs and their response to drought. 7

Answer 17

1. Resistance- immediate response to disturbance
2. resilience- magnitude and speed of return to initial state
3. areas of wheat and grassland tested in control and drought
   4,. drought favours fungal pathway - bacterial lacked top predators in drought
4. grass - fungal - drought increased abundance and more higher trophic levels
5. when a second drought was introduced, grass had a lower impact, higher resistance
6. But more sensitive to drought - less reslient

Question 18

Belowground biodiversity and ecosystem functioning. Bardgett and Van der Putten. 2014. Nature.

Describe the spatial patterns of soil biodiversity. 4

Answer 18

1. Biodiversity hotspots have low soil diversity
2. Above and below ground diversity governed by different mechanisms
3. spatial patterns may be explained by soil architecture, rooting patterns, soil water and nutrients, climate and continental isolation
4. 3 levels - smallest, local and ecosystem/regional/contintental

Question 19

Belowground biodiversity and ecosystem functioning. Bardgett and Van der Putten. 2014. Nature.

Describe the temporal patterns of soil biodiversity. 4

Answer 19

1. populations in soil vary on timescales from seconds to millennia
2. rain after drought causes microbial changes within minutes
3. In alpine soils, summer-winter sees a complete change in soil biota
4. Become more compex and stable, with greater reliance on mycorrhizal fungi for plants, as succession progresses

Question 20

Belowground biodiversity and ecosystem functioning. Bardgett and Van der Putten. 2014. Nature.

What is special about mycorrhizal fungi? 1

Answer 20

1. They can pass chemical signals between plants, indicating a herbivore attack

Question 21

Belowground biodiversity and ecosystem functioning. Bardgett and Van der Putten. 2014. Nature.

How do soils respond to environmental change? 3

Answer 21

1. Soil fungi can adapt rapidly to climate change.
2. Change fruiting dates and reduce growth and respiration
3. May impact decomposition patterns

Question 22

Top down is bottom up. J, Moore et al. 2003. Ecology.

What is the significance of the rhizosphere? 4

Answer 22

1. Root energy channel - root feeding insects, nematodes and microbes in symbioses with plant roots
2. Bacterial energy channel - bacteria, protozoa, rotifers, nematodes and some arthropods
3. Fungal energy channel - saprophytic fungi, arthropods and nematodes
4. If both disrupted, fungal bacterial channels have different recovery times, changing the dominance and relationships between them

Question 23

Top down is bottom up. J, Moore et al. 2003. Ecology.

How is runaway consumption attenuated? 3

Answer 23

1. Predators control prey densities top down
2. Microbial consumers respond by remineralising nutrients that limit plants
3. Above and below ground herbivores increase as there are more plants

Question 24

Integrating food web diversity, structure and stability. Rooney and McCann. 2012. Trends in ecology and evolution.

What is the importance of structure in the food web?

Answer 24

1. Lord May’s experiments showed that, in the absence of structure, diverse food webs are less stable.
2. With structure, diverse food webs have lower connectance, making them stable.
3. Slow energy channels are less stable
4. It takes longer for these to return if less stable.