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Biological Sciences
Mary Olaveson

Biology – Chapter 55 Community: The species that live an interact in an area Ecological communities are not an aggregate unit of organisms that move together when environmental conditions change. Rather, each separate unit independently interacts with its biotic and abiotic environment Trophic Level: Consists of the organisms whose energy source has passed through the same number of steps to reach them The organisms in a community can be divided into different trophic levels based on their source of energy: 1. Primary Producers: Consists of plants and other photosynthetic organisms which gain their energy from the sun (consists the tropic level: photosynthesizers) 2. Primary Consumers: Non-photosynthetic organism that consume the energy rich organic molecules of the primary producers (constitute the trophic level: herbivores) 3. Secondary Consumers: Organism that eat primary consumers 4. Tertiary Consumers: Organism that eat secondary consumers 5. Decomposers: Organisms that eat the dead bodies of organisms or their waste products a.k.a. detritivors 6. Omnivores: Organisms that receive their food source from more than one trophic Level Because so many organisms are omnivores, the boundaries between trophic levels are fuzzy Food Chain: The process by which a primary producer is eaten by a primary consumer, which in turn is eaten by a secondary consumer, and so on Food Web: An interconnection of food chains because communities eat/are eaten by more than one species Biomass: Weight of living matter There is a loss of energy and biomass as you rise in trophic levels In most cases, the distribution of energy and biomass within trophic levels in all ecosystems are similar, where one trophic level usually dominates the others in terms of energy and biomass storage In most terrestrial ecosystems, it is the photosynthetic plants which dominate because of their abundance as well as their ability to hold energy for long periods of time The next stage up from the primary producers is herbivores. The amount of energy/biomass herbivores have vary and depend on the ecosystem i.e. in forests, where wood dominates as the primary producer, the herbivores share of energy/biomass is very small because wood contain very difficult-to-digest energy, therefore not many herbivores eat it whereas in grasslands, where there is little hard-to-digest food sources, much of the grasslands will be eaten by herbivores, therefore they will have a larger share of energy/biomass In aquatic conditions, the energy distribution is the same as the terrestrial ecosystems however the actual biomass distribution is inverted. Herbivores, instead of photosynthesizers, contain the majority of the biomass This is because in aquatic ecosystems, the primary photosynthesizers are protists and bacteria (which have very small biomass). However, these organisms have such high rates of cell division that they can feed a much larger biomass of herbivores, even though their own biomass is so small Much of the energy/biomass ingested by organisms is eventually transferred to decomposers when they die and are eaten by detritivores They convert this energy/biomass into free-mineral nutrients that can be taken up by plants again; completing the cycle The way organisms interact with one another can also be divided into categories: 1) Predation/Parasitism: Interactions in which one participant is harmed, but the other benefits (+/- interactions) 2) Competition: Interactions in which two organisms use the same resources and those resources are insufficient to supply their combined needs (-/- interactions) 3) Mutualism: Interactions in which both participants benefited (+/+ interactions) 4) Commensalism: Interaction in which one participant benefits but the other is unaffected (+/0 interactions) 5) Amensalism: Interactions where one participant is harmed but the other is unaffected (-/0 interactions) These 5 types of interactions plus the physical environment determines population densities Predation/Parasitism Predation and parasitism are universal processes as all organisms are eaten by at least one other organism or fall victim to pathogens and parasites Parasites and pathogens (parasitism) are often smaller than their hosts and typically do not kill the host as they eat them Predators/prey interactions (predation) are different in that predators are often bigger than their prey and typically kill them Predator/Prey interactions The relationship between predator and prey is complex. In most cases, predators do not only depress the population of their prey, but they may also increase population densities, an even increase prey population There is typically more prey than there are predators. As the predator population grows, they reduce the population of the prey because they eat most of the population. This results in too little food and too much predators therefore the predator population crashes which increases the population of the prey This cycle causes the prey/predator interaction to oscillate The presence of predator also has an effect on the habitat and geographic distribution of prey. I.e. they may avoid living somewhere with a high population of predators Throughout Evolution, prey have developed methods to avoid capture (i.e. toxic bristles, tough spines, camouflage etc.) however one of the most effective is mimicry. 2 kinds: Bastesian Mimicry: When a palatable species mimics an unpalatable one Mullerian Mimicry: When two or more unpalatable or noxious species may converge to resemble one another Batasian mimicry works because predators of an unpalatable or noxious species learns to avoid other prey individuals of similar appearance Individuals of an unpalatable species are more likely to get attacked if they have Bastesian mimickers, which some prey
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