Lecture 16: Potential Exam Questions

Question 1: What are the three types of food web classifications?

Connectedness webs: emphasize feeding relationships, based on observations of who ingests what. Who eats who?
Energy flow webs: weight interaction strengths by flux of energy, require measures of food and energy consumed, and generally show dominance of a small number of species. Who gets energy from what?
Functional webs: emphasize influence of individual species on the growth rate of other species. Revealed by removal experiments and may not correspond to energy flux. This tries to explain how the rest of the web is influenced by a single species by estimating effects of removal or insertion of a single species.

Question 2: What is the difference between top down and bottom up theories of the trophic cascades?

Bottom Up: nutrients or plants control relative abundance of upper trophic levels. Plants or nutrients are the limiting factor in trophic level interactions.
Top Down: Predators control the population of next trophic level down.

Question 3: Considering a top down trophic cascade, how might sea otters indirectly affect the abundance of kelp forests?

Sea otters feed on sea urchins who in turn feed on kelp. When there are healthy populations of sea otters, kelp forests are more prominent because sea otters control the population of sea urchins. The top down trophic cascade described here, the sea otter depresses the trophic level it feeds on, indirectly increasing the biomass of the next lower trophic level below it (kelp).

Question 4: What is Disturbance?

An unpredictable event that varies in magnitude and frequency. Can create a trigger for some species to be decimated and other species to succeed. Areas with frequent disturbances are characterized by rapid species turnover which tend to be small in size and mature rapidly.
Example of disturbance = fire, flood, earthquake

Question 5: Define and provide one example of a keystone species

A keystone species is any species within an ecosystem that when removed results in dramatic shifts in the other organisms within the ecosystem. Similar to a keystone in an arch, the keystone species may not appear to have such a momentous role in the ecosystem when measured by biomass or overall numbers but the small role that the species plays supports the equilibrium structure of the system.

One example from class is the starfish on the Pacific coast. The starfish, though small in numbers, are essential for controlling the overall mussel population since they both compete for space in the intertidal zone. Removing that species results in an explosion of mussels driving out most other species. Another example: sea otters.

Question 6: What is it called when a species feeds on more than one trophic level?

Answer: Omnivory

Question 7: Describe the differences in opinion of a "community" versus an "assemblage" of species.

The key differences between these two terms has to do with species interaction within an ecosystem. Clements argued a holistic concept of community that there exists a tight interaction between species in an ecosystem, thus they could be characterized as a "community" with distinct, clearly defined boundaries. Gleason, however, advocated that species had very individualistic responses that resulted in loose associations, or "assemblages" and therefore, there is no distinct boundary where one community type meets another. The debate has not been resolved, but there is evidence that both hold true. Modern ecology links the individualistic premise that most species lack distinct boundaries and the holistic view that community structure and function arise from interactions among species; however, the extent of these interactions should be determined through experimentation, such as the starfish removal experiment.

Question 8: Describe the different techniques in constructing a connectedness food web, energy flow web and a functional food web.

A connectedness food web is constructed simply by observing an ecosystem and documenting who ingests what. An energy flow web shows the weight interaction strengths by flux of energy and therefore requires measuring the amount of food/energy consumed by a species. Generally this results in demonstrating how vital a small number of species can be in an ecosystem. Lastly, a functional food web is structured to show the influence of individual species on other species. For example, removing all starfish (keystone species) from an ecosystem will result in the mussel population exploding. Therefore there is a strong functional relationship between the two species. Functional webs are valuable in analyzing these relationships but they generally cannot show how energy flows through trophic levels very reliably. Rather, they demonstrate how species interact as they compete for resources.

Question 9: Define the term succession and distinguish between its two types.
Succession is the tendency of a community's species composition to be replaced over time in a sequence of changes, known as seres, to progress towards a stable state. Secondary succession occurs in a habitat that has been disturbed, but in which some aspects of community remain. Secondary succession can be seen after a forest fire where the seed bank has been left intact. Primary succession is different in that it occurs in a newly formed or exposed habitat devoid of life, such as a new stream or soil left behind as a glacier retreats. Primary succession is the establishment of a community whereas secondary succession is the regeneration of a community. (It may help to remember that r-selected species are typically primary successors, and k-selected species are secondary successors. R-selected species do very well in disturbed and unstable environments and reproduce quickly. K-selected species reproduce much less quickly, are more competitive, and require more stable environments in which to grow.)

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