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Transcript
Richard T. Wright Environmental Science Tenth Edition Chapter 4 Ecosystems: How They Change Copyright © 2008 Pearson Prentice Hall, Inc. Fire and Succession http://www.fs.fed.us/photovideo/ One Year Later Thirteen Years Later Ecosystems: How They Change • • • • • Dynamics of natural populations Mechanisms of population equilibrium Evolution as a force for change Ecosystem response to disturbance Living beyond our means Dynamics of Natural Populations • Population growth curves • Biotic potential versus environmental resistance • Density dependence and critical number 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. • Biotic potential: combination of biotic and abiotic factors that enhance population increase. 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 an 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 ochraeceus a starfish that feeds on mussels, keeping them from blanketing the rocks. http://www.marine.gov/ Community Stability Species Diversity and Community Stability 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 • Grassland contains plants with both fibrous and tap roots • Coexist by accessing resources from different soil levels Introduced Species • Rabbits in Australia and on Phillip Island (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 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? Rabbits Overgrazing in Australia Phillip Island 1978 Largely devoid of vegetation Phillip Island 1988 After Rabbit Removal The Third Principle of Ecosystem Sustainability • The size of the consumer population is maintained so that overgrazing or other overuse does not occur. Zebra Mussels Ctenophores: Eat Fish Larvae Water Hyacinths Kudzu Evolution as a Force for Change • Adaptations through natural selection • Drifting continents 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 for Survival and Reproduction Lesson Activity • Examine how protective coloration is a common adaptation of insects on plants. • Can you find the spider? The Limits of Change • Adapt • Move (migrate) • Die (extinction) Vulnerability of different organisms to environmental changes. Which Species Will Survive in a Rapidly Changing Environment? • Panda Narrow distribution Small population Low genetic variation Large size Small # of offspring Limited migration • Mouse Wide distribution Large population High genetic variation Small size Large # of offspring High migration 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: Galapagos Finches Earth’s Crust: 14 Tectonic Plates Ecosystem Responses to Disturbance • Ecological succession • Disturbance and resilience • Evolving ecosystems? Equilibrium Theory • Ecosystems are stable environments in which species interact constantly in well balanced predator-prey and competitive relationships. • Balance of nature idea derived from this theory. Succession and Disturbance • Ecological succession: the orderly and progressive replacement of one community type by another until a climax stage is reached. – 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 Aquatic Succession This used to be a lake! Disturbance and Resilience • Removes organisms • Reduces populations • Creates opportunities for other species to colonize 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 Living Beyond Our Means • 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. Pressures on Ecosystems • What is the carrying capacity for the human population on Earth? • How will the human ecological footprint impact on nature’s goods and services? End of Chapter 4 PPT by Clark E. Adams