Lecture 12

Question 1

What is a community level?

Answer 1

Multiple interactions between different species at different trophic levels

Question 2

What is a direct interaction between trophic levels?

Answer 2

Where one animal consumes another (interaction between two trophic levels)

Question 3

Describe an indirect interaction between trophic levels?

Answer 3

. Community level- multiple interactions between different species at different trophic levels
. Additional complexity and unpredictability (compared to direct interactions between trophic levels)

Question 4

Give an example of indirect interactions between trophic levels

Answer 4

(Darwin in Origin of Species notes that:)
. Bee pollination in parts of S England is affected by cats (indirect)
. Cats predating field mice
. Mice attacking combs

Question 5

What was originally thought to be the main process affecting community structure?

Answer 5

Interspecific competition

Question 6

How was interspecific competition thought to affect community structure? What experiments were done to show this and what did the experiments show?

Answer 6

. Competitive exclusion principal , niche packing
. Role of manipulative field experiments: exclude or add a species to a community
. Literature reviews indicated approx 90% of studies found competition
(. Schoener (1983): 164 experiments
. Connell (1983): 214 species in 527 experiments)

Question 7

Describe trophic cascades and give examples

Answer 7

. Where consumption at one trophic level causes change in abundance/ composition at lower trophic levels
. E.g. where carnivores eat herbivores, resulting in increase in primary producer
. Latter may have impacts on whole ecosystem
. E.g. Sea Otter predate sea urchins in N America

Question 8

Explain the Sea Otter example of trophic cascade

Answer 8

. California 1990
. Sea Otters eat the sea urchins and the sea urchins eat the kelp
. Sea Otter’s eating the sea Urchins has a positive effect on the primary producer abundance because it is getting rid of the consumer of kelp
. After 1990 Orcas/ killer whales have started eating otters in vast numbers so sea urchins grow in abundance and eat the kelp, depleting the kelp forests

Question 9

What is trophic facilitation? Give an example

Answer 9

. Where consumer is indirectly facilitated by positive interaction between its prey and another species
. E.g. commensal interaction between marsh rush and shrub has indirect positive impact on aphids feeding on shrub in New England marshes

Question 10

Give the mechanism for the commensal interaction (trophic facilitation) between marsh rush and shrub as an indirect positive impact on aphids

Answer 10

. Removal of Juncus led to decline in Iva growth
. Removal of Iva had no impact on Juncus growth
. Juncus led to decrease in soil salinity, increase in soil O2
. Shade from Juncus reduces evaporation (impact on H2O salinity)
. Juncus root aerenchyma increases soil O2 (helps Iva)
. Aphid growth rates higher when Juncus present

Question 11

What are competitive networks in communities?

Answer 11

Where every species has a negative impact on other species in the community (can lead to stability —> all have a negative effect on each other so cancel each other out so, none tend to dominant

Question 12

What is a competitive hierarchy?

Answer 12

Where the community is linear, one species dominants etc.

Question 13

Give an example of a competitive network community

Answer 13

Buss & Jackson (1979)- coral reefs in the Caribbean; encrusting invertebrates and algae on rocks- no one species consistently ‘won’ competition

Question 14

What happens in the absence of keystone predator? Give an example

Answer 14

Leads to extinction or huge changes in abundance of other species.
Example: starfish predate molluscs, limpet’s, snails, barnacles on a rocky shore. The starfish is a keystone species- with it present there are 15 species on the shore as it controls Mytilus numbers. But, if you take it away then there are only 8 as Mytilus out-compete many other species. If you take the starfish away it will massively alter the whole store
(‘Key’ role of Pisaster starfish is structuring the whole community)

Question 15

Who developed the keystone predator concept?

Answer 15

Paine in classic 1966 study of rocky stores

Question 16

What trophic level are keystone species on? Give examples

Answer 16

Any; ’ecosystem engineers’ (have large structural impact on the environment) such as beavers creating a dam; mutualists such as pollinators; nitrogen-fixing plants; trees in woodlands

Question 17

How do you measure the importance of a keystone species?

Answer 17

Quantify by measuring the percentage of other species lost when a keystone is removed (difficulty of manipulative experiments)

Question 18

Give the example of a keystone species Beavers

Answer 18

Beavers (dramatic effect on the community around them) were hunted to extinction (nearly) in 1940 and there were few wetlands.
After reintroduction the beavers slowly increased the amount of wetland. They are also ecosystem engineers because they have a big effect on the physical environment (wetland has increased by 13-fold)

Question 19

Is a tree a keystone species?

Answer 19

No, it is a dominant species. It can provide a number of habitats, fertilises soil, they also effect temperature and moisture

Question 20

Give an impact of disturbance

Answer 20

May reduce populations to the extent that resources are no longer limiting (very important)

Question 21

Disturbance is one of the numerous factors that affect what?

Answer 21

Community species richness

Question 22

Why are most communities non-equilibrium systems?

Answer 22

Because they are subject to disturbance (not in a static state- they are always changing)

Question 23

Describe disturbance

Answer 23

. Normal event in most communities
. Can vary in intensity and in frequency
. Can be abiotic or biotic

Question 24

Give some abiotic disturbances

Answer 24

. Forest fires, fires on grasslands

. Severe storms on rocky shores, forests, rainfall

Question 25

Give some biotic disturbances

Answer 25

. Grazing e.g. insect herbivore outbreak
. Predators
. Trampling
. Disease

Question 26

What does disturbance do? Why is this very important?

Answer 26

Disturbance opens up gaps in the community.

Very important for sedentary or immobile species (less so for mobile animal species)

Question 27

What do real communities comprise?

Answer 27

A mosaic of patches

Question 28

Describe patches

Answer 28

Species interact within them.

Species migrate between patches (key point)

Question 29

Explain patch connectivity

Answer 29

. Patch-to-patch migration is crucial to species survival
. A species might go extinct in one patch
. But does not mean extinction from whole community if reinvasion from another patch
. The number of interconnections is huge, even with small number of patches
(Different regions in a community may have barriers between them)

Question 30

How do time lags affect patches?

Answer 30

. After disturbance to one patch, may take several generations before it returns to its original state
. With numerous patches, the entire community may never reach equilibrium (will never reach a steady state) if there is periodic disturbance of individual patches

Question 31

Give an abiotic disturbance example

Answer 31

. Succession in forests and woods
. Disturbance (e.g. storms) where trees fell there is a small patch/ a ‘mini-succession’ occurs
. Species richness highest at mid-successional stage
. Species richness falls in climax community due to competitive species
. E.g. succession after glacial retreat (can study different species as the glacier moves back as it exposes plain earth underneath it), Alaska

Question 32

What happens to species richness if there is extremely high disturbance? Why is this?

Answer 32

Species richness is low because the species are kept at poor early successional stage (e.g. human intervention)

Question 33

What happens to species richness if disturbance events are rare? Why is this?

Answer 33

Species richness is low because you get dominant species dominating the community: competitive exclusion effects the climax community

Question 34

At what disturbance levels is species richness highest?

Answer 34

At intermediate levels

Question 35

Explain the ‘intermediate disturbance hypothesis’

Answer 35

. At low disturbance levels, competitive exclusion reduces diversity
. At high disturbance levels, diversity declines as mortality rises
. At intermediate disturbance levels, a balance between disruption of convoy and mortality leads to high density