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 Years pass –
More land: more trees, birds, squirrels, frogs etc.
Development: concrete Communities change
Organism and/or habitat changes
Population Growth and Decline
Reproduction: birth rate 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 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:
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: 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 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: Temperature, availability of water or
minerals, wind
Ecosystems change over time
Succession: the gradual change in an ecosystem 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
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 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 Can forest, wetland, coastal, ocean community Can happen over tens or hundreds of years
Pattern is still the same: Community of producers is established Followed by decomposers and consumers, then more producers, etc.
Pioneer species can either: Help other species to grow
Alders have N-fixing bacteria on their roots, improving soil quality Also good for shade and nutrients when they die
Or prevent species from getting established Plants may release chemicals to keep other plants from takeing roots New species may outcompete others by using up resources or better
resisting disease