1. 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

2. 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

3. 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

4. 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

5. 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).

6. 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

7. 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

10. 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

11. 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…

12. 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

13. 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