Community Ecology, Population Ecology and Sustainability

Chapter 4


General Types of Species


•Non-native (exotic or alien)




General Types of Species

•Keystone species

•Species that play roles affecting many other organisms in an ecosystem.

•Removal of that species greatly changes the entire ecosystem

•Examples: top predators, pollinators

Organism Interactions

•Predation  - One animal kills/eats another.

–Predator benefits from food.

–Prey species may benefit by eliminating non-adaptive genes from the gene pool.

Interspecific competition

•Two organisms of different species compete to obtain the same limited resource.

•In general, the more similar the competing species, the more intense the competition.

Resource Partitioning and Niche Specialization


•Symbiotic relationship  - Special, close, physical relationship between two different species.  At least one species benefits from the interaction. 

•Parasitism - One organism (parasite) living in or on another organism (host), from which it derives nourishment.

•Mistletoe is a

parasitic plant


Commensalism  - One organism benefits - other is not affected.

–epiphytic plants like ferns, mosses and orchids find a home on trees

–the epiphyte benefits

–the tree neither benefits

nor is harmed


Mutualism - Both species benefit - in many cases neither can exist without the other.


•mutualism between algae and a fungus

Acacia trees and ants

•ants raise young in swollen thorns of acacia

•feed on nectar and protein-rich Beltian bodies produced by acacia

•in return, ants protect acacia from attacking insects or competing plants


Ecological Succession:  Communities in Transition

•Disturbance – an event that alters an ecosystem, either significantly changing it, or wiping it out entirely.

•Examples include fire, flood, volcano, serious insect or disease outbreak, drought, glacier…

•Primary Succession

•Begins with bare mineral surfaces or water. 

•Nothing remains of the previous ecosystem. 

•Follows severe disturbance.

•Hundreds or thousands of years



Primary Succession

Secondary Succession

•Occurs when an existing community is disturbed or destroyed, but some parts are left behind:

•soil and organic matter

•seed bank

•rootstock or bulbs for resprouting

•maybe some living plants

•snags, logs


•Tends to be more rapid than primary succession.


Secondary Succession

Ecological Stability and Sustainability

•How predictable is succession?

•Pioneer Community

•Climax Community

•Balance of nature

•Concept of climax is less important


•Population - Group of individuals of the same species (humans or any other species) inhabiting the same area simultaneously. 

•Natality and Mortality

•Natality - Number of individuals added through reproduction

•Birth Rate

•Mortality - Number of  individuals removed via death

•Death Rate




Population Density and
Spatial Distribution

•Population Density - Number of individuals per unit area. 

•Spatial Distribution - Describes where the population is found, such as even or clumped.

•Dispersal - Movement of individuals into new areas.

•Emigration - Out movement

•Immigration - In Movement


Population Dynamics

•Biotic Potential - “r”

•intrinsic rate of increase

•Reproductive capacity, or ability of a population to produce offspring.

•Usually higher than replacement level.

•Leads to exponential growth curve.


Carrying Capacity

•Carrying Capacity - Number of individuals of a species than can be indefinitely sustained in a given area.

•“K” stands for carrying capacity


Exponential and Logistic Growth

Population Density Effects

•Environmental Resistance - Any factor in the environment limiting carrying capacity. Four main types:

•Raw material availability

•Energy availability

•Waste accumulation and disposal

•Organism interaction


•Density-dependent factors - get more limiting as population density increases

•Density-independent factors – do not vary by density

Reproductive Patterns and Survival

•Not all species reach a stable carrying capacity.

•Species can be broadly lumped into two categories:

•K- selected species (competitors)

•r- selected species (opportunists)

Reproductive Patterns and Survival

The Role of Predation in Controlling Population Size

Human Impacts on Ecosystems

Learning from Nature