Lecture 19 - Community Organisms and Dynamics I

Question 1

Communities are complex.

Give 3 ways ecologists reduce complexity.

Answer 1

1. Species richness - each species is equal to all others
2. Define feeding roles - who eats whom?
3. Define species of importance

Question 2

Define the food chain.

Answer 2

An arrangement of the organisms of an ecological community according to the order of predation in which each uses the next usually lower member as a food resource.

Question 3

Define the food web.

Answer 3

The totality of interacting food chains in an ecological community.

Question 4

Define trophic levels. What are the 4 trophics levels? Which organisms compose each?

Answer 4

1. First trophic level = Producers = Green plants
2. Second trophic level = Primary consumers = Herbivores
3. Third trophic level = Secondary consumers (mesopredators) = Carnivores, insect, parasitoids
4. Fourth trophic level = Tertiary consumers = Higher carnivores, insect hyperparasites.

Question 5

Define Cannibalism and Omnivory in Food web terminology

Answer 5

Cannibalism: preys on the juveniles of the same
Omnivory: preys on multiple trophic levels
species.

Question 6

Define trophic position. How is it calculated

Answer 6

Trophic position (TP) is the weighted mean length of all pathways of energy flow leading from primary producers to the consumer.
	TP = TPdiet +1

Question 7

What is the trophic position of a bird that eats 25% seeds, 25% fruits, 25% herbivorous insects & 25% spiders?

Answer 7

TP = TPdiet + 1
TPdiet = (0.25 × 1) + (0.25 × 1) + (0.25 × 2) + (0.25 × 3) = 1.75 
TP = TPdiet + 1 = 1.75 + 1 = 2.75

Question 8

How do we know about feeding relationships and trophic positions? Explain 3 methods.

Answer 8

1. Direct analysis. (Watching predators eat prey)
2. Gut or stomach content analysis:
3. Stable isotope analysis (of tissues): The excretion rate of the lighter isotope (14N) is greater than that of the heavier isotope (15N) during metabolism resulting in about 3% of accumulation per trophic level. → As you go up the trophic levels, more 15N.
   a) Isotopic fractionation: Cumulative effect across the food chain.
   b) A time-integrated measure: its showing what that organism has made into its flesh over the duration of its life.
   c) Allows for non-integer trophic position
   d) It covers omnivory and is a food web approach.
   e) Advantages????? She talks about it in lecture 21/11 at the end 1:20 h

Question 9

Define isotopic fractionation

Answer 9

Cumulative effect across the food chain.

“You are what you eat + ~3‰ δ^15N”

Question 10

What is the average prey-to-predator in many communities?

Answer 10

Average of 2.5:1 (constant ratio but not a linear relation)

Question 11

Define Connectedness and its relation with diversity.

Answer 11

Connectedness: each species is connected to more species as diversity increases.

Question 12

Define food chain length

Answer 12

The number of links running from the top predator to the basal species. (Average connectedness always end up in a length of 4.)

Question 13

Explain the idea that the food chain length is limited.

Answer 13

There is inefficiency as you go up every trophic level, then you can only take up so much of the energy from the trophic level before you can make into new biomass. You get to a point in the end where there would have been way more primary productivity to support more trophic levels.

Question 14

Name 3 hypothesis of the food chain length limit.

Answer 14

1. Energetic hypothesis: Length is limited by inefficient transfer of energy.
   Evidence: Fake tree holes experiment using yogourt containers so that they could put different levels of primary productivity in each to see if the number of trophic links would increase.
   Hard to show in large ecosystems.
2. Dynamic stability hypothesis: Long food chain are not stable. Lower positions in a food web will be able to bounce back from disturbance more quickly and higher positions, because they have to assimilate all the energy from the lower positions are more likely not to exist.
3. Ecosystem size hypothesis:
   Support from lake studies
   Longer food chains in larger ecosystems.
   Unclear which attributes of ecosystem size are responsible for effects on food chain length.

Question 15

What are the 4 generalizations we make in communities?

Answer 15

1. As diversity increases, each species is connected to more and more species.
2. Food chains are short
3. Constant proportions of top predator, intermediate species and basal species.
4. Omnivory is common.

Question 16

Name 3 indirect effects in food webs

Answer 16

1. Mutualisms: some interactions are positive in both directions.
   a) Cleaner shrimper
   b) Mycorrhizae fungi provide plants with greater access to inorganic nutrients and are important to plant performance.
2. Competition. (indirect, often shown as dashed lines with signs (-)).
3. Preferential or density-dependent predation, known as “Predator-mediated coexistence”
   Predator allows inferior competitor to remain

Question 17

Name 2 benefits of Preferential or density-dependent predation

Answer 17

• > mediates coexistence of competitors, prevents competitive exclusion
• > enhances biodiversity

Question 18

Define Keystone species

Answer 18

Keystone species: a species whose removal has drastic effects on many other species in the community.
First found = starfish.

Question 19

Define 3 categories of keystone species.

Answer 19

1. Major predator.
   e. g. Pisaster ochraceuswhich preys on the rocky intertidal zone of the North American Pacific coast.
2. A unique food source.
   e. g. In Central America, frugivores have to rely on a very limited number of tree species for food (Tergorgh 1986).

Ecosystem engineer: a species that maintains critical ecosystem processes.

Question 20

Define dominant species. How are they different from keystone species?

Answer 20

Dominant species: high biomass.

Keystone species is one whose EFFECT is large, and disproportionately large relative to its abundance.

Question 21

What’s the disadvantage of Gut or stomach content analysis.

Answer 21

disadvantage → decomposition → not accurate