Environmental Science: Toward a Sustainable Future Richard T. Wright

Ecosystems: Populations and Succession

PPT by Clark E. Adams

Chapter 4

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

Births Deaths

A dynamic balance between births and deaths.

Population Growth Curves

Population Growth Curves

Reproductive strategies:

Many offspring withlow parental care

Few offspring withhigh parental careJ-shaped growth curve

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

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?

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. 4-11 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

GENES ENVIRONMENT

ADAPTATIONS

NATURAL SELECTION: For? or Against?

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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