What is a food web? What are the major types of food webs?
- Food webs are representations of trophic relationships and energy flow in a community.
- 3 major types of food webs:
- Connectance web: depict feeding linkages (primary producer to herbivore to carnivore)
- Flow web: represent fluxes of energy and nutrients (the thicker the arrow, the larger the flux)
- Interaction web: identify strong interactions influencing community / ecosystem structure. The thicker the arrow, the stronger the interaction.
What is a strong interactor? What are the different kinds of strong interactors?
- Species that play a major role in maintaining community structure and function. The loss or introduction of a strong interactor results in significant community / ecosystem changes.
- Dominant species: a strong interacting species that compromises a large amount of biomass in the community. Also known as foundation species.
- Keystone species: A strongly interaction species whose impact on the community is disproportionately large relative to its biomass.
What are ecosystem engineers?
- Species can also affect one another through the creation, modification and maintenance of critical habitats.
- Species that alter the habitat and thus affect the availability of resources to other species are called ecosystem engineers. Beavers, corals etc.
What are indirect effects?
Effects that are transmitted via the food web to species beyond those which the exploiter directly interacts with.
What is the effect of snail grazing on algal diversity?
- Increases when the preferred food is completely dominant, then decreases due to high snail densities
- Leading to grazing on both dominant and subordinate species.
How does increased nutrient availability affect diversity of higher trophic levels?
First increases, then decreases as best competitors for nutrients begin to dominate as resources become scarce due to increased plant diversity.
What is extirpation?
This is when a species is locally extinct, as opposed to globally extinct.
If the world is green, what must be true?
According to Hairston, Smith and Slobodkin (1960), this means that predators are limiting herbivores and herbivores aren’t limiting plants.
What is top down control?
This is when the top trophic level limits the size of the trophic level they consume. When wolves remove elk, aspen populations grow exponentially until the limiting factors of sunlight, water and space begin affecting the population.
What is a trophic cascade?
This is a specific type of top-down effect in which the addition or removal of a top carnivore cascade down to affect the primary producers of the food chain. A classic trophic cascade is when the food chain is an odd number of trophic levels in the food chain, primary producers will be abundant; and when there is an even number in the food chain, primary producers will be less rich. Trophic cascades don’t have to be a result of a decrease in the population size of the herbivores, but could also be a result of the change of behavior. (behaviorally mediated trophic cascade)
What is a behavioral cascade?
Specific type of top-down effect like the trophic cascade where the impacts of adding or removing top predators influences primary producers. However, in this case, neither the population size nor the biomass of the herbivore is affected. It alters the behavior of the herbivore to such an extent that they can no longer exert grazing pressure on the primary producer population.
What does autogenic mean?
It means internally produced or generated independently of external influence.
What does allogenic mean?
It means externally sourced.
What are autogenic engineers?
Autogenic engineers change the physical environment through their own structures. (such as corals altering ocean currents and situation rates) Trees alter the physical environment of the forest floor, and are thus called autogenic engineers.
What are allogenic engineers?
They alter the environment through the direct redistribution or transformation of living or nonliving material. Examples include burrowing mammals redistributing soils, altering the flow of water, availability of nutrients and structure of soil. In turn, they affect plant productivity and species composition.
Why is ecological theory important?
Ecologists rely heavily on careful observations and experiments conducted in the field to analyze and comprehend community dynamics. Field research is only useful, however, when it is designed to address questions and test hypotheses derived from theoretical analyses.
What are the various perspectives regarding the question of what determines the structure of ecological communities?
- Top-down control by consumers: Simply put, since the world is green, consumers must be controlling community structure. “Green World Hypothesis”, HSS, now serves as the foundation for much of current food chain theory.
- Bottom-down control by resource availability: support comes from farms where the addition of nitrogen and phosphorus have increased vegetation. Ecologists taking this approach hypothesize that organisms on each trophic level are limited by resource availability. As the amount of energy running through the food chain increases, it should also alter the size of all trophic levels. As the amount of energy depends on primary productivity, this theory suggests that community structure ultimately depends on the supply of plant resources such as nutrients, water and sunlight. Many communities aren’t green at all! Also, the world is prickly and tastes bad (plants have defenses against herbivory).
- The result is a mixture of two. Debate still continues regarding the relative importance. An hypothesis is the exploitation hypothesis, which states that top-down forces become increasingly important as more trophic levels are added.
- Disturbance regimes also play an important role. Bottom-up is more important in frequently distrubed communities.
What are the various hypotheses regarding the question of what determine food chain length?
- Productivity hypothesis: More productive ecosystems will have longer food chains.
- Ecosystem-size hypothesis: As ecosystem size increases, species diversity, habitat diversity and habitat availability also increase. Thus, larger ecosystems will have longer food chains.
- Productive-space hypothesis: Both productivity and ecosystem size are important.