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Biology Exam Notes.docx

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McMaster University
Richard B Day

Biology Exam Notes Chapter 53 Mutualisms  Are +/+ interactions that involve a wide variety of organisms and rewards.  Most critical of all mutualisms occur between bacteria that fix nitrogen and certain species of plants.  Mutualism benefits both species, but the interaction does not involve individuals from different species being altruistic.  The benefits received are a by-product of each individual pursuing its own self-interest by maximizing its ability to survive and reproduce. Keystone Species Structure Communities  The structure of a community can change dramatically if a single species of predator or herbivore is removed from or added to a community.  A keystone species is a species that has a much greater impact on the surrounding species than its abundance would suggest.  For example, the sea star Pisaster is a keystone species in some intertidal areas. When Pisaster was removed from experimental areas, the number of species present and the complexity of the habitat changed radically. Disturbance and Change in Ecological Communities  Community composition and structure may change radically in response to changes in abiotic and biotic conditions.  A disturbance is any event that removes some individuals or biomass from a community.  The important feature of a disturbance is that it alters some aspect of resource availability.  The impact of disturbance is a function of 3 factors: type of frequency, frequency of disturbance and severity of disturbance.  Most communities experience a characteristic type of disturbance; in most cases disturbances occur with a predictable frequency and severity. This is called a community’s disturbance regime.  Ecologist’s use 2 approaches to determine the pattern of disturbance: inference of long-term patterns from data obtained in short-term analysis and reconstruction of the history of a particular site.  Biologists are now better able to manage these forests by allowing, monitoring and controlling burns in them. The Development of Communication after Disturbance  Succession is the recovery that follows a severe disturbance  Primary succession occurs when a disturbance removes the soil and its organisms, as well as organisms that live above the surface.  Secondary succession occurs when a disturbance removes some or all of the organisms from an area but leaves the soil intact.  The specific sequence of species that appears over time is called the successional pathway.  Biologists focus on 3 factors to predict the outcome of succession in a community: the traits of the species involved, how species interact; and historical
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