Ecology

Geology/Geography/Oceanography/Atmospheric Sciences

Quiz 17 :

Species Interactions and Community Structure

Quiz 17 :

Species Interactions and Community Structure

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Explain how the experiments of Lubchenko (1978) showed that feeding preferences, population density, and competitive relations among food species all potentially contribute to the influences of "keystone" consumers on the structure of communities. What refinements did the work of Lubchenko add to the keystone species hypothesis?
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The study on the influence of intertidal snails, and on the species diversity of intertidal algal species reveals, the following factors. It contributes to the influence of keystone species, Littorina littorea on the other species of the community:
• Feeding preferences :
The snails preferred small, ephemeral, and tender algae in their food like the green algae ( Enteromorpha species). They completely avoided the tough perennial species like the red algae ( Chondrus species). This preference of food influenced the population, competitive interactions and species diversity. It is clear by the fact that the species diversity of the emergent substrata decreased, as the snails excluded the preferable species through consumption.
• Population density :
The population density showed an important role in the influence of key stone species. The areas where, the population density of Enteromorpha was high showed a lower density of Littorina and very low density of Chondrus. On the other hand, the areas that showed a high density of Chondrus , showed a high density of Littorina and low density of Enteromorpha.
In addition, unusual density of species contradictory to the food preference was linked with the competitive relationships. The presence of Littorina species restricts the competitive exclusion of Chondrus by Enteromorpha ; however this competitive exclusion of Chondrus was clearly seen in areas where Littorina was absent or in low density.
• Competitive relation among food species :
The two food species Enteromorpha and Chondrus were strong competitors. The species Enteromorpha competitively exclude Chondrus from any area, if the predator is not present. Therefore, the presence of the keystone species maintained the species diversity of the community. Otherwise the area would be dominated by Enteromorpha and Chondrus.
The significant refinements added to the keystone hypothesis by the work of Lubchenko are as follows:
• Her work precisely explained the influence of consumer keystone species on a community.
• The effect of keystone species depends on the feeding preferences by the species.
• The effect of keystone species is influenced by the population of the food species.
• The relative competitive ability of the two or more prey can lead to significant change in the structure of web, which is influenced by the keystone species.
Thus the work by Lubchenko clearly explained that consumers can be effective keystone species, and their influence on any community is affected by many other factors. This added to further clarity in the keystone hypothesis.

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Winemiller (1990) deleted "weak" trophic links from one set of food webs that he described for fish communities in Venezuela (see fig. 17.3). What was his criterion for designating weak interactions? Earlier, Paine (1980) suggested that ecologists could learn something by focusing on "strong" links in communities. How did Paine's criterion for determining a strong link differ from Winemiller's?
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Kirk Winemiller described the feeding relationships between the species of fresh water. The food web gave a very complex structure. As a measure to reduce the complexity of the food web, he included the strong tropic links and thereby excluded the weakest tropic links from the structure.
The criteria by which he determined whether a link is weak or strong are on the base of the percentage of consumption. If a species on a higher tropic level consumed less than 1% of the diet species, then it was considered as a weak interaction and thus, excluded from the web. However, still the webs gave a very complex appearance.
Robert Paine was another ecologist who studied the food web relationships of various ecosystems. He suggested that some species have a very strong influence on the community. The interaction by such species was considered as the strong interactions. He also suggested that the identification of strong interactions in a food web or a community by any ecologist would enable them to understand the effect and influence of the species on the community. More precisely, the elimination or alteration of a strong link would lead to more significant effects on any community than the elimination of the weaker interactions.
Paine's criteria for determination of a "strong link" were as follows:
• Degree of influence of a link on the community.
• Quantity of energy flow.
The criteria for determination of the "strong link" by Paine differed from that of Winemiller because Winemiller only determined the "strong interaction" on base of energy flow. Winemiller determined the energy flow by on base of the percentage of diet being consumed. On the other hand, Paine considered the effect of a species on the community and energy flow for determination of a "strong link".

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Using Tscharntke's food web (1992) shown in figure 17.5, predict which species would be most affected if you excluded the bird at the top of the web, Parus caeruleus. What species would be affected less? Assume that P. caeruleus is a keystone species in this community.
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The food web is based on the interactions between the members of various tropic levels. This can be represented as follows:
img If the blue tit ( Parus caeruleus ) is considered to be the key species in this food web, then the other members of the community would be affected by its abundance. If the blue tit is excluded from the food web, the following species would be affected:
• Aprostocetus calamarius : They are parasites that prove to be a direct food for the blue tits. The removal of blue tits reduces the predation on these parasites, and they show an increase in population
• Torymus arundinis: They are parasites that prove to be a direct food for the blue tits. The removal of blue tits reduces the predation on these parasites, and they show an increase in population
• Giraudiella inclusa (large gall clusters): The parasites Aprostocetus and Torymus feed on these large gall clusters. As their population increases, the predation pressure on the large gall clusters also increases. As a result, the population of gall clusters show a decline
• Main shoots of common reed ( Phragmites australis ): The large gall clusters feeds largely on the main shoot of the common reeds. The decrease in the population of gall clusters reduces the predator pressure on common reeds, and their population shows an increase.
As a consequence of exclusion of blue tit ( Parus caeruleus ), the following species would be moderately affected:
• Aprostocetus orithyia :
• Eurytoma crassinervis
• Eudecatoma stagnalis
• Platygaster szelenyii
They are the comparatively less preferred feed for the blue tits. The elimination of blue tits from the food web; moderately affect the population of the parasite Aprostocetus. Even though the population would not show a remarkable difference, but some reduction would be seen.
As a consequence of exclusion of blue tit ( Parus caeruleus ), the following species would be least affected:
• Aprostocetus gratus : These parasites are least preferred by the population of blue tits. Their population would not show any significant reduction due to the elimination of blue tits
• G. inclusa (small gall clusters): The population of small gal clusters would be least affected as all its predators, would not show any significant reduction in population. However, it is possible that the population of gall bladders increase due to the increase in the population of the common reeds, which are the main source of food for them.
• Archanara geminipuncta (stem boring): They may not be directly affected by the elimination of blue tits. However, their population may show some increase due to the abundance of their food resource that is common reeds.

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What is a keystone species? Paine (1966, 1969) experimented with two sea stars that act as keystone species in their intertidal communities along the west coast of North America and in New Zealand. Describe how the intertidal communities in these two areas are similar. Describe the differences between these two communities and the differences in the design of Paine's experiments in these two areas.
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Some paleontologists have proposed that overhunting caused the extinction of many large North American mammals at the end of the Pleistocene about 11,000 and 10,000 years ago. The hunters implicated by paleontologists were a newly arrived predatory species, Homo sapiens. Offer arguments for and against this hypothesis.
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When Power (1990) excluded predaceous fish from her river sites, the density of herbivorous insect larvae (chironomids) decreased. Use the food web described by Power to explain this response.
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How would increasing the level of confidence from 0.95 to 0.99 affect the range of values included in the confidence intervals for the abundance of
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All the keystone species work we have discussed in chapter 17 has concerned the influences of animals on the structure of communities. Can other groups of organisms act as keystones? What about parasites and pathogens?
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Why do larger sample sizes improve the ability of a researcher to detect statistical differences between populations?
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