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Chapter 35
Important Ecological Terms
Niche: role, function or boundaries of an organism
Habitat: the place where a plant or animal normally lives
Population: a group of individuals of one species
in an area
Community: many populations of different kinds of organisms living in the same place
Ecosystem: assemblages of organisms together with
their physical and chemical environments
Biome: an ecosystem of a large geographic area in
which plants are of one formation
Population Dispersion
Basic Characteristics of Populations
The suitability of habitats influences the geographic distribution of a
species.
Insights can be gained by studying the spatial distributions of populations
within habitats.
Population Density
The total population per unit area.
Depends on:
Quality of habitat
Interactions within the environment
Density dependent factors
Density independent factors
 Capacity – the maximum number of organisms that can be
supported by an environment.
Controlled by limiting factors
Population size can be measured by several sampling techniques.
Quadrats
Indirect counting
Mark- recapture
Population Growth Models
Interactions within the Environment can effect population
Competition
Niche – organisms habitat, food source, activity time – role in
environment.
A fight within the niche
Predation
Symbiotic Relationships
Parasitism – one organism benefits, one is harmed
Mutualism – both organisms benefit
Commensalism – one organism benefits and the other is not harmed or
helped

Density-Dependent and Density-Independent Effects on Populations
In many habitats, the forces that limit population sizes are independent of
population density. For example, extreme weather events may decrease
populations.
For most species, density-dependent factors limit birth rates or increase death
rates at least some of the time. This type of population determination often is
referred to as “regulation.”
Disease outbreaks and starvation are two factors that may increase with
population density.
Communities Constantly Change
Whether change occurs naturally or it is influenced by humans, disturbances are
constantly occurring in ecosystems.
Ecological Succession
Natural, gradual changes in the types of species that live in an area.
The gradual replacement of one plant community by another through natural
processes over time
Two types:
Primary
Secondary
Primary Succession
Begins in a place without any soil
Sides of volcanoes
Landslides
Flooding
Starts with the arrival of living things such as lichens that do not need soil to
survive
Called PIONEER SPECIES
Secondary Succession
Begins in a place that already has soil and was once the home of living
organisms
Occurs faster and has different pioneer species than primary succession
Example: after forest fires
Secondary succession takes place following a major disturbance, such as a fire or
flood. Farmland that has been abandoned also can undergo secondary succession.

Primary succession always begins on a barren surface, while secondary
succession begins in an area that already has soil.

Climax Community
A stable group of plants and animals that is the end result of the succession
process
Does not always mean big trees
Grasses in prairies
Cacti in deserts