ECOLOGY BIOL 2311 FINAL EXAM STUDY QUESTIONS 1 a What is meant by indirect effects in food webs b Describe two types of indirect effects do not use an example for these c Now describe one real world example of an indirect effect Indirect effects describe when one species has an effect on another species that is mediated by a third species In other words Species A has an effect on Species B mediated by Species C One type of indirect when Species C modifies the interaction between Species A and B interaction modification A second type of indirect effect occurs when Species A and B are competing for a resource and depletion of that resource by Species C affects both Species A and B apparent competition A real world example of indirect effects is a trophic cascade in Yellowstone National Park Introduction of wolves into the system positively impacted aspen populations by reducing elk grazing 2 Your new job at the National Park Service has placed you in the limpet conservation assessment unit Staff ecologists tell you last year had really low densities of birds on rocky intertidal shores of the NW Pacific Knowing what you know about rocky shore food webs in this area predict the likely result of this lack of bird predators with regards to the abundance of three different limpet species Be sure to explain the rationale for your answer With a decrease in the bird population in this area the three limpet species will be released from heavy predation pressure L digitalis abundance will increase L strigatella abundance will decrease and L pelta abundance will remain the same L digitalis abundance will increase due to a release from predation and increased habitat availability L pelta abundance will remain the same because although they experience a release from predation they are subjected to habitat loss as a result of competition between barnacles and mussels that does not occur when there is heavy bird predation on barnacles as well The absence of birds also decreases algal cover again through competition for space with barnacles and L strigatella abundances will decrease due to resource limitation 3 Consider the kelp food web including the sea otter off the coast of California Using this example illustrate and explain three general ecological principles about the structure of food webs When comparing the food chain length California to that of Alaska we see that the primary producers are affected differently In the three trophic level food web in California primary producers are negatively affected and in a four trophic level food web in Alaska primary producers are positively affected So the effect on the primary producers depends on whether there are an even or odd number of links trophic levels on the food chain We can also see that the greater complexity connectance of the food web in California buffers the effects of trophic cascades In addition a greater species diversity and functional redundancy on a given trophic level can decrease the effect of a species decline 4 a Define the term trophic cascade in your own words and B give an example C Provide at least one characteristic that is shared by systems in which trophic cascades are prevalent What about this these characteristic s affect the likelihood of trophic cascades occurring A trophic cascade is a type of top down control in which changes in the top trophic level either positively or negatively affect the primary producers at the bottom of the food chain An example of a trophic cascade occurred when otter populations declined off the coast of California and kelp abundances declined as a result of increased urchin herbivory Trophic cascades are more prevalent in aquatic systems as opposed to terrestrial systems This is due to the fact that aquatic herbivores and primary producers are much more similar in size aquatic herbivores have better assimilation efficiencies and thus consume more productivity and there is much more edible plant material in aquatic systems For each of the reasons listed aquatic herbivores have a larger effect on primary producers than in terrestrial systems 5 You have been hired as the manager for a small estuary that has experienced local deforestation with increasing intensity of agriculture increasing runoff and nutrient addition and intense harvesting of oysters which once covered the floor of the estuary but now are relatively rare Describe how each of these human impacts may have resulted in the extreme eutrophication high levels of phytoplankton biomass experienced by the estuary in the last 50 years Deforestation and increased intensity of agriculture would increase nutrient runoff upstream of the estuary because there are less plants to uptake the nutrients before they run into the stream and there is an additional artifical input of nutrients into the system in the form of fertilizers These nutrients would travel into the estuary and resulting in eutrophication In addition intense harvesting of oysters would increase the abundance of bethic diatoms and therefore contribute to a high biomass of phytoplankton 6 You are an environmental consultant hired by a town adjacent to a small lake in the Sierras The figure below describes what you know about the current food web of the lake The town is economically depressed and residents have proposed two ideas for stimulating the local economy 1 introduce northern Pike a piscivore into the system to attract sportsfishermen and their money to the area and 2 harvest filter feeding bivalves from the lake for food they are quite a delicacy and fetch a hefty price The town wants you to predict for them the consequences of each proposal for the health of their lake long renowned for its crystal clear waters If both a piscivore as well as sportfishermen into the food web the lake has the potential to become eutrophic Although the introduction of a piscivore could result in a trophic cascade in which phytoplankton would be negatively affected introduction of another top predator humans could potentially reverse the effects of the trophic cascade and allow phytoplankton to proliferate even more As for harvesting filter feeding bivalves a decrease in bivalves would increase the number of plankton in the benthos thus clouding the lake 7 A What is the difference between a keystone and a foundation species B Give an example of each from the kelp forest A keystone species is one that has a effect on the community that is disproportionately large compared to its