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