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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 Moving on • Chapters 2 and 3:Structure – Structure and Function of Ecosystems as Sustainable Units in Natural Landscapea • Chapter 4 : Ecosystem Dynamics – What does it mean for an ecosystem to be in balance? – What happens after a major natural disturbance such as a fire? – Changes are the only constant in ecosystems – The notion of balance must be carefully defined KEY TOPICS • • • • Dynamics of natural populations Mechanisms of population equilibrium Evolution as a force for change Ecosystem response to disturbance 4.1 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 Exponential Growth • Growth by a factor (by multiplication, by a percentage) • Unrestricted Growth under ideal conditions • Doubling time is constant • Population grows faster and faster in numbers • Leads to population explosion Genera Number of Rabitts -tion 0 2 = 2 x 100 1 2 20 = 2 x 101 200 = 2 x 102 4 2000 = 2 x 103 5 20000 = 2 x104 Exponential Growth Rabbits Population Growth Curve Polulation 250000 200000 y = 2e 150000 2.30x Series 1 100000 Ex pon. (Series1) 50000 0 0 1 2 3 4 5 6 Months Biotic Potential Doubling Time y = 2 × e 2.30 x e = 2.72 P (t ) = P (0)e rt DT ~ 70 %r r : growth rate Fate of Exponential Growth (J-curve) • Population crash due to starvation and disease • Natural control mechanisms return the population into the S-curve balance Dynamic Equilibrium • In natural ecosystems, undisturbed by human activities, populations are in a dynamic equilibrium represented by the Scurve • J-curves occur in cases of disturbances or sudden alteration of a habitat • Environmental resistance limits population growth Population Dynamics • Environmental resistance: combination of biotic and abiotic factors that may limit population increase. • Biotic potential is the number of offspring that a species may produce under ideal conditions • Recruitment is survival to reproducing age • Replacement level is a recruitment level that just replaces the parents Biotic Potential and Reproductive Strategies • Two Reproductive Strategies: Many offspring (high biotic potential) withlow parental care (low recruitment) Few offspring (low biotic potential) With high parental care J-shaped growth curve S-shaped growth curve Factors of Environmental Resistance • Abiotic factors – – – – Temperature, Moisture, Light Salinity, pH Lack of Nutrients Fire • Biotic factors – Predation – Competition – Parasites Carrying Capacity • The carrying capacity is maximum population of a species that a given habitat can support sustainably Population dynamics are regulated by the interplay between Biotic Potential and Environmental Resistance • Biotic Potential is a constant for any given species • Therefore, it is ER that determines growth or decline of population • Density dependence of ER factors limit the population size 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, parasites • Density dependent environmental factor keep the population within a limited range – As population density increases, environmental resistance increases too leading to increase in mortality • Critical number: the lowest population level for survival and recovery Critical Number and Extinction • Endangered species: population size near critical number • Threatened species: rapidly declining population 4.2 Close Look at Specific Mechanisms of Population Equilibrium • Top-down regulation is control of population by predation • Bottom-up regulation is control of population by scarcity of resources -----------------------------------------------------------------• Predator-prey dynamics • Competition – Interspecific – Intraspecific • Introduced species 4.2.1 Predator-Prey Dynamics Herbivore Population is Regulated by their Predators Simple ecosystem Isle Royale Moose-Wolves : Top-down or Bottom-up Population Regulation? • Both • Top-down for the moose • Bottom-up for the wolves • Parasites, like predators, can also regulate the population of their host in a density dependent fashion 4.2.2 Plant-Herbivore Dynamics Reindeer on St. Matthew Island • No regulatory control (predation) on herbivores • Population grew, exploded, overgrazed and crashed • Overgrazing: eating plants faster than the plants grow. 4.2.3 Keystone Species • A single species that plays a crucial role in maintaining the biotic structure of the ecosystem • Pisaster ochraeceus a starfish that feeds on mussels, keeping them from blanketing the rocks. http://www.marine.gov/ 4.2.5 Competition: Intraspecific • Territoriality: defense of a resource against individuals of the same species – Results in priority access and use of resources – Breeding is restricted to those who can claim and defend their territory – Survival of the fittest! • How do wolves and songbirds establish territory? 4.2.5.Competition: Interspecific • Grassland contains plants with both fibrous and tap roots • Coexist by accessing resources from different soil levels 4.2.6 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.) 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). Rabbits Overgrazing in Australia Phillip Island 1978 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. • First? • Second? 4.3 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 GENES + EN V IRO NM EN T ADAPTATIONS NATURAL SELECTION Adaptations for Survival and Reproduction 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 Drifting Continents: Pangaea 225 Million Years Ago Earth’s Crust: 14 Tectonic Plates 4.4 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 • 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 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 (MA) • Managing ecosystems • The pressure of population • We have already exceeded the earth’s carrying capacity 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?