Chp 4 PowerPoint

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Transcript Chp 4 PowerPoint

A.P. Environmental Science

Objective: The Learner will be able to: 1) Explain the dynamics of natural populations.

2) Discuss the mechanisms of populations equilibrium and mechanisms of species adaptation. 3) Explain ecosystem responses to disturbance.

4) Identify several invasive species, explain how they arrived and the impact they have had on ecosystems.

Yellowstone – June 1988

  Lightning started fire Burned thousands of acres of forest  Park Service Manages Yellowstone  Smokey Bear Policy – put out fires before they get started!

 Park Service policy -burn itself out unless near human habitation  Largest fire-fighting effort in U.S. History  Finally put out by snowfall

The Yellowstone fire was a “crown fire”.


 Fires marked in red with infrared satellite image.

 10% of park area was burned

Yellowstone - Recovery

Here is a picture of Yellowstone July 1986 before the fire

Yellowstone 1989

Same Site August 1989

 Same site in July 1999  Recovery was immediate. Within two weeks grasses began sprouting  Herbivores fed on the new growth  25 years later the diversity of plants and animals in the burned area have completely recovered  Predators – prey reacted positively to disturbance

Dynamics of Natural Populations

 Each species exists in a population  Over time most species remain more or less constant in size and geographic distribution  Deaths = Births – This balance is called Population equilibrium

Growth Curves – Two Types

J or S

 Exponential growth results in population explosion    Rule of 70

to find the doubling time of a quantity growing at a given annual percentage rate, divide the percentage number into 70 to obtain the approximate number of years required to double. For example, at a 10% annual growth rate, doubling time is 70 / 10 = 7 years.

This results in a J curve graph.

Dynamics of Natural Populations

 Population growth curves  Biotic potential (the ability to increase population numbers) versus environmental resistance (the combination of all the biotic and abiotic factors that limit a population’s increase.)  If recruitment is at replacement level, then the population will remain constant.

 Carrying capacity – the upper limit to the population of any particular organism that an ecosystem can support

Density Dependence And Critical Number  Environmental resistance factors can be density dependent.

– If population density increases, environmental resistance becomes more intense and causes in increase in mortality.

– If population density decreases, environmental resistance lessens, allowing the population to recover.

 Environmental factors that cause mortality can be density independent – A sudden deep freeze in spring – A fire that may kill all small mammals

Biotic Potential and Environmental Resistance

 Biotic Potential – Reproductive rate – Ability to migrate (animals) or disperse (seeds) – Ability to invade new habitats – Defense mechanisms – Ability to cope with adverse conditions  Environmental resistance – Lack of food or nutrients – Lack of water – Lack of suitable habitat – Adverse weather – Predators – Disease – Parasites – Competitors

Critical Number

 The survival and recovery of a population depends on a certain minimum population base – this is known as the population’s critical number.

Reproductive Strategies

 Reproduce in massive quantities  Few offspring  Leave nature to raise the  Spend a lot of time young nurturing their young  “low recruitment’ = high mortality rate  Organisms – Large  Favors quickly changing environments – Long life spans  – S – curves Organisms are usually – Small – Rapid reproductive rate – Short life spans – J – curves – Instinct important – Learned behavior

Environmental Resistance

 Population explosions seldom occur in nature  Biotic and abiotic factors cause mortality in populations  Biotic factors – Predators – Parasites – Competitors  Abiotic factors – Changes in temperatures – Moisture – Salinity – pH

Carrying Capacity

 Upper Limit to population number  Maximum population that a given habitat can support without the habitat being degraded over time  (sustainable system)

Density Dependence & Critical Number

 The size of a population remains within a certain range when environmental resistance factors are density dependent  As population density (the number of individuals per unit area) increases, Environmental resistance becomes more intense and causes an increase in mortality, resulting in the population decreasing.

Critical number

 The survival and recovery of a population depends on a certain minimum population base (critical number)  If population is depleted below this number surviving members become more vulnerable, breeding fails, and extinction is almost inevitable

Human impact activities

 Altering habitat  Pollution  Over-hunting  Intro alien species  Pet exploitation  Collectors

Alien Plants

Mechanisms of Population Equilibrium

 Predator – Prey – A rise in the moose population – Rise in the wolf population – Higher predation – Lower moose population – Lag time from year to year


 Worms, bacteria, viruses, protists, fungi  Similar to predator – prey relationship  Most parasites do not cause the host to die  Population density is high – parasites and vectors find hosts easily  Population density is low – parasites have trouble finding hosts

Plant – Herbivore Dynamics

 Overgrazing  Herbivore population is high, feed on grasses to the roots  Causes loss of secondary consumers  May take long time to recover food chain  Lead to erosion

Keystone species

 An organism that plays a crucial role in maintaining ecosystem biostructure


 Interspecific – No individual lives in isolation from other members – competition between two or more species for some limiting resource. – have identical requirements – Niches (roles) over-lap – Competition from members of the same species




the struggle between members of a population for scarce resources. As in intraspecific cooperation, there are two basic types of competition: 1.



Adapted or programmed


results from aggressive social behavior such as dominance hierarchies and territoriality. Only certain individuals high in the peck-order, or holding territories, succeed in breeding. This is sometimes called contest competition because it involves aggressive contests between competing individuals. Unadapted or incidental


results from the accidental interaction between individual organisms utilizing the same resources, for resources used by one are unavailable to others. This is sometimes called scramble competition because everybody is involved in a mad scramble for the scarce resources.

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

Rabbits Overgrazing in Australia

Mechanisms of Species Adaptation (selective pressure)

Selective Pressure: How it works

 All Individuals of a species have differences  If one difference is able to be inherited and it allows for the individual to survive, and it lives to reproduce, its young will have the gene to survive and over time adapt to changes in the environment  Red & black in nature means “leave me alone”

Limits of Change

 Organisms facing extreme selective pressure can  Adapt – Population of survivors may adapt to the new conditions through natural selection  Migrate – Surviving populations may migrate and find an area where conditions are suitable to them  Extinction – Failing the first two choices, the third is inevitable

Four Keys to Survival

 Geographical distribution  Specialization to a given habitat or food supply  Genetic variation within the gene pool of the species  The reproductive rate relative to the rate of environmental change

Vulnerability of different organisms to environmental changes

Prerequisites for Speciation

 Original population must separate into smaller populations that do not interbreed with one another   Reproductive isolation Unable to interbreed and produce fertile offspring  Separated populations must be exposed to different selective pressures


Darwin’s Finches

Ecosystem Response to Disturbance

 Equilibrium theory – ecosystems are stable environments in which species interact constantly in well-balanced predator prey and competitive relationships  Biotic interactions – Determine the structure of living communities within ecosystems

Ecological Succession

 Ecological succession: transition between biotic communities – Primary: no previous biotic community •

Ex: Newly formed island

– Secondary: previously occupied by a community •

Ex: Mount St. Helens blast area

– 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

Disturbance and Resilience

 Removes organisms  Reduces populations  Creates opportunities for other species to colonize

Fire and Succession

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  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