Transcript 12.3: Ecosystems are always changing

Chapter 12:
Interactions Within
Ecosystems
12.1: Groups of living things interact within
ecosystems
12.2: Organisms can interact in different ways
12.3: Ecosystems are always changing
Ecosystems are always
changing
 Review:
 a relationship in which two organisms
both try to get the same thing
 Competition
 A relationship in which both the
organisms benefit
 Mutualism
 The role a species fills in a habitat
 niche
Populations change over time
 Think about a park you may have visited
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Years pass –
 More
land: more trees, birds, squirrels, frogs
etc.
 Development: concrete
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Communities change
 Organism
and/or habitat changes
Population Growth and
Decline
 Reproduction: birth rate
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Can increase a population or keep it stable
Can merely measure the number of births
or also be a measure of stability
 Ex:
black bears reproduce once every two
years
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If there is not enough food available, the
female’s reproductive cycle is delayed and the
bear population will not increase
Population Growth and
Decline
 Predator-prey interaction impacts
population size
 Moose vs wolves
 Graph:
 Simple models assume:
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1) the prey population will grow
exponentially when the predator is
absent
2) the predator population will starve in
the absence of the prey population (as
opposed to switching to another type of
prey)
3) predators can consume infinite
quantities of prey
4) there is no environmental complexity
(in other words, both populations are
moving randomly through a
homogeneous environment).
Population Growth and
Decline
 Limiting factor: any factor or condition
that limits population growth
 Predators may be a limiting factor for
prey
 Prey may be a limiting factor for
predators
 Biotic or abiotic factors serve as
limiting factors

Food, water, light, soil, nutrients
 Complex:
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lack of nutrients in soil
Farmers add fertilizer
Fertilizer runs off into a lake (river)
Increases algae population
Algae uses up oxygen needed by
fish
Fish population is limited
Eutrophication
Maintaining a Balance
 Carrying capacity: maximum number of
individuals that an ecosystem can support
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without considering predators
Different for each population
 Ex: Isle Royale supports many more moose
than wolves

Moose is a primary consumer of plants and
is lower on the energy pyramid
 Limiting biotic factors: interactions between
populations

Competition, predation, parasitism
 Limiting abiotic factors:
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Temperature, availability of water or
minerals, wind
Ecosystems change over time
 Succession: the
gradual change in an
ecosystem
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One biological
community is
replaced by another
Field to a forest,
farmland to plants,
shrubs, and trees
Primary Succession
 The establishment of a new biological community
 Plants move into an area that was previously barren
 Pioneer species: first living things to move into a barren environment
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Glacial retreats: mosses and lichen move in
As they grow, they weaken the rock surface
New soil is formed and a variety of small plants and shrubs can take root
And so on…
Secondary Succession
 Takes place after a major disturbance to a biological
community in a stable ecosystem
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Ex: natural event: fire, flood, etc; or human activity: forest cleared,
farmland abandoned
Soil remains, and seeds and plant roots survive
Patterns of Change
 Ecosystems go through successions
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Can forest, wetland, coastal, ocean community
Can happen over tens or hundreds of years
 Pattern is still the same:
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Community of producers is established
Followed by decomposers and consumers, then more producers, etc.
 Pioneer species can either:
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Help other species to grow
 Alders have N-fixing bacteria on their roots, improving soil quality
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Also good for shade and nutrients when they die
Or prevent species from getting established
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Plants may release chemicals to keep other plants from takeing roots
New species may outcompete others by using up resources or better
resisting disease