Chapter 4 Ecosystems: How They Change Copyright © 2008 Pearson Prentice Hall, Inc. Environmental Science Tenth Edition Richard T. Wright

Fire and Succession

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 Births Deaths A dynamic balance between births and deaths.

Population Growth Curves

Reproductive strategies: Many offspring with low parental care Few offspring with high parental care J-shaped growth curve 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 No regulatory control (predation) on herbivores Went into exponential growth pattern Overgrazed habitat Massive die-off of herbivores Reindeer on St. Matthew Island

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?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.

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 Statesgypsy moths Water hyacinth, kudzu, spotted knapweed, purple loosestrife (see Fig 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 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 GENES ENVIRONMENT 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

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 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