Lecture 10: Trophic relationships in communities
Trophic eco = who eats what?
Flow of energy through food
Plants don’t eat anything, but sunlight
Organisms came up with defensive mechanisms to not be eaten – to boost up their fitness
Trophic levels – based on the function in the trophic flow in the whole community
- Primary Producers = plants
- Primary consumers = herbivores(insects)
- Secondary consumers = carnivores who eat herbivores
- Tertiary consumers = carnivores who eat secondary consumers
- Detritivores = eat dead organic matter
Food chain and food webs
- Trophic connections among species can be analyzed as interaction webs with network
properties like ‘connectance’. Getting idea from internet web.
- If we have a lot of organisms who eat each other we get a food web
- Trophic relations can determine community structure in ways that can be investigated by
Interaction strengths are assessed by removal experiments. Think of tomato experiment from the lab:
control group and insecticide treatment
Example from the lecture:
Control treatment Insecticide treatment
Beetles abundant Goldenrod beetles suppressed
High plant diversity Low plant diversity
Hairston, Smith & Slobodkin, 1960: trophic cascades (HSS)
Proposition: by just looking at the world we can understand how tropic levels are connected
- The world is green: plants are not being limited by herbivores, because of the indirect effect of
carnivores eat the herbivores, thus, not controlled by food supply, but by predation.
- Indirect effect: one trophic level exerts influence on a second by affecting a third
- Cascades involve effects that alternate across trophic levels Spiller& Schoener, 1992: research on Caribbean; plants -> beetles -> spiders and Anolis,
lizard: carnivore, eats spiders and beetles. Remove the lizard, if lizards are mostly killing
the beetles then they are helping the plants, if spiders then negative effect on plants.
Result of the experiment: Lizards do not benefit plants because of unequal interaction
strengths between lizards and beetles and lizards and spiders.
Knight et al., 2005: far-reaching effects of fish in ponds;
Did an experiment to know whether fish is actually has a
positive effect on bees and plant, and negative effect on
Removed fish – if a pond has fish, there are less dragonflies,
therefore, more bees, hence, plants around the pond get a
- Direct and indirect effects can be opposed
- Indirect effects can be strong as direct
- Keystone consumers can shift communities between alternative states
- Outcomes not fundamentally predictable, depend on interaction strengths
- Experiments needed, perhaps long-term (to get the strength)
Special difficulties of herbivores
- Easy to be carnivore: animal tissues easy to convert into animal tissues, all is protein, easy for
- Plant tissues hard to convert into animal tissues
o Cellulose and lignin tough – complex carbohydrates difficult to digest, indigestible
without microbial symbionts. Our guts do not have those enzymes to break down
o Plant tissues heavily defended against herbivores, mechanically and chemically.
Herbivore also comes up with mutatio