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Transcript
Environmental Science: Toward a Sustainable Future Richard T. Wright Chapter 4 Ecosystems: How They Change PPT by Clark E. Adams Factors That Contribute to Ecosystem Change Dynamics of natural populations Mechanisms of population equilibrium Mechanisms of species adaptation Ecosystem response to disturbance Lessons to learn Dynamics of Natural Populations Population growth curves Biotic potential versus environmental resistance Density dependence and critical number Biotic Potential and Environmental Resistance Population Equilibrium A dynamic balance between births and deaths. Births Deaths Population Growth Curves Population Growth Curves Reproductive strategies: Many offspring with low parental care J-shaped growth curve Few offspring with high parental care S-shaped growth curve Population Dynamics Environmental resistance: combination of biotic and abiotic factors that may limit population increase Predators, competitors, disease Adverse weather, limited food/nutrients Biotic Potential and Environmental Resistance Density Dependence and Critical Numbers Factors of environmental resistance are either: density-independent: effect does not vary with population density; e.g., adverse weather density-dependent: effect varies with population density; e.g., infectious disease Critical number: the lowest population level for survival and recovery Mechanisms of Population Equilibrium Predator–prey dynamics Competition Interspecific Intraspecific Introduced species Predator–Prey Balance: Wolves and Moose Lessons to Be Learned about Predator–Prey Balance Absence of natural enemies allows a herbivore population to exceed carrying capacity, which results in overgrazing of the habitat. The herbivore population subsequently crashes. The size of the herbivore population is maintained so that overgrazing or other overuse does not occur. Plant–Herbivore Dynamics Reindeer on St. Matthew Island No regulatory control (predation) on herbivores Went into exponential growth pattern Overgrazed habitat Massive die-off of herbivores Mechanisms of Population Equilibrium: Plant–Herbivore Compare the predator–prey with plant–herbivore methods of controlling the size of the herbivore population. How would the herbivore population growth curve look if diseases or predators were used as the control mechanism? Keystone Species A single species that maintains biotic structure of the ecosystem Pisaster ochraceus: a starfish that feeds on mussels, keeping them from blanketing the rocks http://www.marine.gov/ Competition: Intraspecific Territoriality: defense of a resource against individuals of the same species Examples of wolves and songbirds Results in priority access and use of resources How do wolves and songbirds establish territory? Competition: Interspecific Grasslands contain plants with both fibrous roots and taproots Coexist by accessing resources from different soil levels Introduced Species Rabbits in Australia (next slide) Chestnut blight in United States Japanese beetles, fire ants, gypsy moths in United States Water hyacinth, kudzu, spotted knapweed, purple loosestrife (see Fig. 4-13 in text) in United States Rabbits Overgrazing in Australia Introduced Species Why have introductions of nonnative and exotic species resulted in a degradation of ecosystems? (Think in terms of environmental resistance and biotic potential.) An example of the answer to this question is given in the next slide. Introduced Species: Rabbits in Australia Introduced into Australia from England in 1859 No natural enemies – rabbit population exploded Overabundant herbivore population devastated natural vegetation (see Fig. 411 in text). Using disease as control measure – why will this procedure fail in the long term? Mechanisms of Species Adaptation Change through natural selection Selective pressure determines which organisms survive and reproduce and which are eliminated. Recipe for Change + ADAPTATIONS NATURAL SELECTION: For? or Against? Adaptations to the Environment The Limits of Change Adapt Move (migrate) Die (extinction) Vulnerability of different organisms to environmental changes Prerequisites for Speciation Original population must separate into smaller populations that do not interbreed with one another. List some ways this might happen. Separated populations must be exposed to different selective pressures. Example: arctic and gray fox (next slide) Speciation: Foxes Speciation: Galápagos Finches Ecosystem Responses to Disturbance Ecological succession Disturbance and resilience Evolving ecosystems Equilibrium Theory Ecosystems are stable environments in which the biotic interactions among species determine the structure of the communities present. Succession and Disturbance Ecological succession: transition between biotic communities Primary: no previous biotic community Secondary: previously occupied by a community Aquatic: transition from pond or lake to terrestrial community Primary Succession Primary Succession Mosses invade an area and provide a place for soil to accumulate. Larger plants germinate in the new soil layer, resulting in additional soil formation. Eventually shrubs and trees will invade the area. Secondary Succession Aquatic Succession Disturbance and Resilience Removes organisms Reduces populations Creates opportunities for other species to colonize Fire and Succession http://www.fs.fed.us/photovideo/ Ground Fire Fire and Succession Fire climax ecosystems: dependent upon fire for maintenance of existing balance; e.g., grasslands, pine and redwood forests What significance does this have for humans and where they live? Resilience in Ecosystems Resilience Mechanisms after a Forest Fire Nutrient release to soil Regrowth by remnant roots and seeds Invasions from neighboring ecosystems Rapid restoration of energy flow and nutrient cycling Lessons to Learn Managing ecosystems The pressure of population Managing Ecosystems Protecting and managing the natural environment to maintain the goods and services vital to human economy and survival. The Pressures of Population What is the carrying capacity for the human population on Earth? How will the human ecological footprint impact on nature’s goods and services? Carrying Capacity and Overshoot End of Chapter 4