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52 Community Ecology Lecture Outline I. Community structure and functioning In a community, populations interact in a variety of ways Within a community, no species exists independently of other species The niche is a species role in the community The niche describes all aspects of an organism’s existence The niche describes not only the habitat, but much more The fundamental niche is the niche that an organism would occupy in the absence of competition The realized niche is the niche that an organism occupies in the presence of competition Limiting resources restrict the ecological niche of a species Limiting factors as varied as soil composition, climatic extremes, or population interactions may influence the niche Any environmental resource that is scarce or unfavorable is called a limiting resource Biotic and abiotic factors may influence a species' ecological niche Barnacles in the intertidal zone is an example Competition is intraspecific or interspecific Intraspecific competition exists between members of the same species Interspecific competition exists between members of different species Competition between two species with overlapping niches may lead to competitive exclusion Competitive exclusion results in the exclusion of one species due to interspecific competition Coexistence occurs when overlap between niches is reduced Gause performed the first experiments that showed the effect of competition on fundamental and realized niches Character displacement reduces interspecific competition Darwin’s finches exemplify character displacement Competing species evolve slightly different niches to avoid competition Natural selection shapes the body forms and behaviors of both predators and prey Predation is the consumption of one species, the prey, by another, the predator Pursuit and ambush are two predator strategies Predators that chase their prey are pursuit predators Some predators are camouflaged to avoid notice by their prey; others may attract their prey like the anglerfish Chemical protection is an effective plant defense against herbivores Plants use spines, tough leaves, and chemicals to detract herbivores Milkweeds produce chemicals that are poisonous to all but a few insects Monarch caterpillars graze on milkweeds and accumulate chemicals in their bodies that make them unpalatable to birds and other predators Animals possess a variety of defensive adaptations to avoid predators Animals may flee, hide, live in groups, or have mechanical defenses to avoid being preyed upon Warning coloration results in avoidance by experienced predators Batesian mimicry is the resemblance of a harmless organism to a harmful or unpalatable organism Müllerian mimicry is the similar morphology of a group of harmful or unpalatable organisms Monarch and viceroy butterflies are examples of Müllerian mimicry Symbiosis involves close associations between species Symbiosis is any intimate relationship or association between members of two or more species In mutualism, benefits are shared Nitrogen-fixing bacteria and legumes are mutualists Zooxanthellae and reef-building coral are mutualists Mycorrhizae facilitate mutualistic relationships between fungi and the roots of a wide variety of plants Commensalism is taking without harming Epiphytes living on tropical trees benefit from the habitat of the host, but the host is not harmed or benefited Parasitism is taking at another's expense The parasite benefits, while the host is harmed A well-adapted parasite does not kill the host A parasite that causes the death of the host is a pathogen Keystone species affect the character of the community Keystone species have a great effect on other species in the community Keystone species are commonly top predators Dominant species influence a community as a result of their greater size or abundance Trees are the dominant species of forests because they change the local environment; similarly, cordgrass in salt marshes, prairie grasses in prairies, and kelp in kelp beds Animals can also be dominant species, such as coral in coral reefs and cattle in overgrazed rangelands II. Community biodiversity Species richness is the number of species within a community; species diversity is a measure of the relative importance of each species Ecologists seek to explain why some communities have more species than others Isolated communities are typically less diverse than continental communities Species diversity typically increases in less stressful habitats High latitude communities have lower species diversity Ecotonal zones have high species diversity, known as the edge effect Geologically older habitats have higher species diversity Species richness probably causes community stability The traditional view stated that stability was a result of community complexity Newer mathematical models suggest that complexity does not lead to stability III. Community development A. Succession is the process of community development over time, with one species being replaced by another Primary succession occurs when a community develops in a "lifeless" environment During primary succession on bare rock, the rock is eventually transformed into soil Primary succession occurs on newly formed volcanic lava and recently glaciated rock Secondary succession occurs when a community develops where a previous community existed Secondary succession occurs where soil already exists, such as areas that have been denuded or modified by fire or agriculture Abandoned farmland has been extensively studied Disturbance influences succession and species richness The idea of a climax community has fallen out of favor; rather, communities exist in a state of continual disturbance The intermediate disturbance hypothesis says that species richness is greatest at moderate levels of disturbance Ecologists continue to study community structure Clements developed ideas about communities, now called the organismic model, which emphasizes the interdependence of species within the community Gleason developed ideas, called the individualistic model, which emphasizes species individuality Most research supports the individualistic model Research and Discussion Topics How is diversity related to successional stages? Is diversity greatest at early, middle, or climax stages? Investigate the wide variety of secondary compounds found in plants. Look at plants that produce chemicals used as drugs by humans, plants that produce chemicals that we use for their taste (e.g. horseradish and various spices), and plants that produce noxious chemicals, such as poison ivy and poison oak. Interestingly, although it was long believed that persons indigenous to hot climates used more spices than persons living in cold climates because the spices would mask the foul taste of food that had "gone bad," recent research shows that many spices, herbs, and condiments like garlic and onion, actually act to keep bacterial numbers low. The components of the Asian five-spice powder, or the combination traditionally used to spice a salsa (garlic, salt, pepper, chilies, and cilantro) have been shown to greatly prevent the growth of potentially harmful bacteria in food.