Parasitism a in parasitism interaction one organism

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2. Parasitism a. In parasitism (+/– interaction), one organism, the parasite , derives nourishment from another organism, its host , which is harmed in the process b. Parasites that live within the body of their host are called endoparasites ; parasites that live on the external surface of a host are ectoparasites c. Many parasites have a complex life cycle involving a number of hosts d. Some parasites change the behavior of the host to increase their own fitness E. Mutualism 1. Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species 2. A mutualism can be Obligate, where one species cannot survive without the other Facultative, where both species can survive alone F. Commensalism 1. In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected 2. Commensal interactions are hard to document in nature because any close association likely affects both species II. Concept 54.2: Dominant and keystone species exert strong controls on community structure 1. In general, a few species in a community exert strong control on that community’s structure 2. Two fundamental features of community structure are species diversity and feeding relationships 3. Two communities can have the same species richness but a different relative abundance 4. Diversity can be compared using a diversity index Shannon diversity index ( H ): H = –[( p A ln p A ) + ( p B ln p B ) + ( p C ln p C ) + …] 5. Determining the number and abundance of species in a community is difficult, especially for small organisms 6. Molecular tools can be used to help determine microbial diversity A. Trophic Structure 1. Trophic structure is the feeding relationships between organisms in a community 2. It is a key factor in community dynamics 3. Food chains link trophic levels from producers to top carnivores B. Food Webs 1. A food web is a branching food chain with complex trophic interactions 2. Species may play a role at more than one trophic level 3. Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community C. Limits on Food Chain Length 1. Each food chain in a food web is usually only a few links long 2. Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis 3. The energetic hypothesis suggests that length is limited by inefficient energy transfer 4. The dynamic stability hypothesis proposes that long food chains are less stable than short ones 5. Most data support the energetic hypothesis D. Species with a Large Impact 1. Certain species have a very large impact on community structure 2. Such species are highly abundant or play a pivotal role in community dynamics E. Dominant Species 1. Dominant species are those that are most abundant or have the highest biomass 2. Biomass is the total mass of all individuals in a population 3. Dominant species exert powerful control over the occurrence and distribution of other species 4. One hypothesis suggests that
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