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These are limiting factors that increase their resistance the higher the density gets ... 2) Specialization: Koala v. Crow. 3) Genetic variation in the gene ...

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Chapter 4 l.jpg

Chapter 4

Ecosystems: How they change

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Yellowstone fires 1988

  • Started Naturally - burned like crazy

  • Burned from May/June till September

  • Recovery was quicker than expected.

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Exponential Growth

  • Constant percent rate of growth

  • Increasing numbers of growth

  • Doubling time stays steady

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Population change

  • Most Populations approximate a S-curve

  • Some become J-curves

    • 1) repeated

    • 2) stabilize into S curve

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Carrying Capacity

  • The maximum in a particular population that can be sustained in an environment

  • A realized S-curve tends to fluctuate around the carrying capacity

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Density Dependent

  • These are limiting factors that increase their resistance the higher the density gets

    • Water

    • Food

    • Territory

    • disease

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Density Independent

  • Do not depend on the density of a population

    • Floods

    • Fires

    • Temperature extremes

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Critical Number

  • Below a certain # some populations experience an extreme change in survival

    • Flocks, packs, etc….

  • Threatened: rapid population decline

  • Endangered: near or below critical number

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Two problems

  • Pop grow too fast

    • J-curve boom and bust

  • Population decreases

    • Approaches critical number and is threatened or endangered

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Top-Down v. Bottom-up

  • Top-Down population regulation

    • Control of a prey species by its predator: Lions eat zebras

  • Bottom-up

    • Control of a species by a limiting factor or resource - often a food supply, especially with primary consumers

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How come all of the Moose did not get eaten?

  • Wolves cannot take down the large and healthy moose

  • As we’ll see, herd thinning can be good for the moose, as well.

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  • “A parasite can work in conjunction with a predator to control a given herbivore population: Parasitic infection breaks out in a dense population of herbivores; individuals weakened by infection are more easily removed by predators, leaving a smaller, but healthier, population (p 87).”

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Plant-Herbivore dynamic

  • Overgrazing: St. Matthew Island

  • 29 Islands introduced in 1944 with no predators

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Keystone species

  • A species with a crucial role in maintaining ecosystem biotic structure

    • Ex. Seastars eat mussels, when removed in a study the mussels were able to decrease the diversity by eating everything.

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  • 2 kinds

  • Interspecifc: between two different species

  • Intraspecific between members of the same species

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Intraspecific competition


For food, mates, limits mating, but can encourage dispersal

Self Thinning

causes growth to be spread out. Can happen to animals too


Population regulation, natural selection

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Interspecific Competition

Consider Plant Distribution in riparian woodlands

Sycamore and red maple along riverbanks

Oaks and pines and higher elevations

OR - Plants might get nutrients from different depths

Bottom line is:drives diversity in communities

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Interspecific Animals

  • Competitive Exclusion principle

    • Consider: Paramecia in flasks

    • Resource partitioning

    • Barnacle ranges in tidal regions

  • Bottom line: elimination of competitive species and driving of natural selection

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Introduced Species

  • Rabbits:

    • Introduced to Australia in 1859.

    • No Natural enemies led to overgrazing

    • Controll has been attempted with a virus, but adaptation occurred.

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Introduced Species

  • American Chestnut

    • Chestnut blight accidentally introduced with some chinese chestnut trees

    • American chestnut almost decimated

    • Ecologically oak filled the niche, but much economic damage

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Introduced Species

  • Pests

    • Agricultural - japanese beetles, fire ants, gypsy moths

    • Cats can help with rodents, but also remove songbirds

  • Bottom line - Globalization is increasing introduction

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  • Selective pressure - includes all factors of environmental resistance

  • Reminder (p84):

    • Lack of food, water, habitat,

    • Adverse weather

    • Predators, disease, parasites, competition

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Adaptations to the environment

  • Dealing with the climate

  • Obtaining food or water

  • Avoiding becoming food

  • Finding or attracting mates

  • Migration/dispersal

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Limits of change

  • When the environment changes

    • Adapt - Different than human response of the same name

    • Migration

    • Extinction (can be local)

  • MAD

    • Move, adapt, or die

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Keys to Survival

  • 1) Geographic distribution: more widespread=more likely to survive (as a species)

  • 2) Specialization: Koala v. Crow

  • 3) Genetic variation in the gene pool: affects ability to respond to change

  • 4) Reproductive rate Mammals v. insects

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Evolution of Species

  • New Species results from

    • Mutation - the “raw material” of natural selection

    • Reproductive Isolation

    • Natural Selection/Genetic Drift

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Ecosystems Disturbed

  • Ecological Succession

    • Primary: On bare rock - Lichen, moss, bigger, bigger

    • Secondary: after a disturbance such as fire, humans, flooding, hurricane - Already has a nutrient layer and usually seeds and nearby animals

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Landscapes and disturbance

  • Disturbance encourages diversity

  • Disturbance is often localized and uneven

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Fire Climax Ecosystems

  • Fire now recognized as a natural part of many ecosystems cycle of disturbance and renewal

    • Thins underbrush

    • Results in less damaging fires

    • Renews soil

    • Germinates seeds (Like lodgepole pine)

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Humans in this context

  • Ecosystem Management

    • Integrated ecosystem view

    • Revised as new research is available

    • Incorporates human element

    • Has goal of sustainibility

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Human stress on ecosystems

  • Big list on page 110

  • We all depend on nature

  • Humans cause change - may improve human lives at expense of ecosystems: soil, air, water, biodiversity, food supply

  • Pressures likely to increase

  • Technology may be part of the solution