Ecosystems and Ecosystem Management

Basic Characteristics of Ecosystems

• Sustained life on Earth is a characteristic of ecosystems, not of individual organisms or populations

• Structure: – Living (Ecological Communities)– Non-living

• Processes– Cycling of chemical elements– Flow of energy

• Change (Succession)

Ecosystems vs. Community Structure

• Ecosystem:– Biotic and Abiotic Components

• Community:– Biotic relationships only:

• Trophic levels;• Competitive interactions;• Symbiotic relationships.

What is an Ecological Community?

• Composed of the interactive species in an ecosystem.

• Does not include the abiotic variables.• The organisms of a community must

interact in some way to be considered part of the same community.– Direct: Predatory relationship– Indirect: Predator’s prey is outcompeted by

a third organism and both the prey and predator populations are affected.

What is an Ecological Community?

• Communities are defined by the population under study and the geographic area in which they exist.

Dominance in an Ecological Community

• Dominance: The degree that one or two species is abundant in a community.

• Keystone species:– The reduction or removal of a

keystone species will cause a large change in community structure.

Abiotic Factors• These include

temperature, water availability, pH, light, geology, nutrient capacity, etc.

• All organisms have a range of tolerance for the various abiotic factors that influence their success.

Abiotic Limiting Factors

• Too much or too little of a single biotic or multiple abiotic factors can limit or prevent the growth and reproduction of a population, even if all other conditions are optimal.– EX.: A plant has enough light and

water, but growth is limited by 1 or 2 nutrient needs.

Abiotic Limiting Factor Concept Applied to Global Ecosystems

• The abiotic characteristics of different regions of the planet lead to predictable groups of plants and animals that inhabit these regions.

Abiotic Impacts on the Biomes of the World

Ecological Communities and Food Chains

• Ecological Communities are defined in 2 ways:– 1. A set of interacting species that

occur in the same place• Food chains, food webs, trophic levels

(autotrophs)

– A set of species found in one area

Food Chains and Food Webs

• A Terrestrial Food Chain– 4 trophic levels:

• Autotrophs• Herbivores• Carnivores • Decomposers

– People are omnivores

• An Oceanic Food Chain• The Food Web of the Harp Seal

How Many Trophic Levels Are Possible?

• Typically, it is common to find 5 trophic levels in an ecosystem.

• There is usually a upper limit of 7 levels possible in a ecosystem.

The Community Effect

• Species interact directly and indirectly• Community-level interactions• Keystone species

– Have large effects on it’s community or ecosystem

– Its removal changes the basic nature of the community

• Holistic View

Holistic vs. Individualistic View of Communities

• There is a disagreement among ecologists over whether comminities are holistic or individualistic.

• Individualistic:– All species occur together by chance

and they just happen to survive because they are all lucky.

Holistic vs. Individualistic View of Communities

• Holistic:– Communities act like superorganisms.

• If any species is removed, the community will no longer function as a whole.

• These views are extremes on a continuum.

Holistic Individualistic

Direct Interaction Example

Community-Level Interactions

Holistic View

Are Watersheds Ecosystems?

• A watershed is a good way of defining an ecosystem on land.– Nutrients, water, and soils enter surface

water systems in the area by rainfall, erosion, air transport and groundwater infiltration.

– Biotic communities are less constrained by watershed boundaries, but they often reside in a single watershed their entire lives.

Ecosystem Management

• By controlling the inputs and outputs of certain abiotic factors, and by planting species, humans have learned to manage ecosystems for their benefit.– Agriculture can be viewed as an

example of ecosystem management.

Dealing With Environmental Change

• Change=– Changing abiotic conditions– Introduction of a new species– Change in food supply.

• Options:– Adaptation– Migration– Extinction

Dealing With Environmental Change• The ability to adapt will depend on the

species and how rapidly the change occurs.– Human induced changes often occur faster than

adaptation can occur = species lost.

Population Growth

• Populations of organisms in natural communities are usually “balanced”.

Population Growth

• Biotic Potential: Maximum number of offspring an organism can produce under ideal conditions.

• Example: Bacteria cell that divides every 20 minutes:– 0: 1 cell– 20min: 2 cells– 40min: 4 cells– 5 h: 32,000 cells– 7 h: 7 million cells– 36 h: 1 foot of bacteria covers surface of the planet

• BUT (Luckily): No organism maintains exponential growth indefinitely.

Environmental Resistance

• Combination of biotic and abiotic factors that limit population growth.

– Density-dependant factors (related to population size):• Competition for resources• Disease• Predation• Emmigration

– Density-independent factors (not related to population size):

• Atypical abiotic conditions• Natural disasters

Population Growth Curves

• J-shaped curve: Exponential growth– Characteristic of:

• A population introduced into a new area with plenty of resources and few/no predators.

• “r”-strategist species: cockroaches, weeds, etc. – Large number of young, low recruitment– Low/no parental care– Little ability to compete– Often the first to invade a disturbed ecosystem– Adapted to unstable environments

• No population can maintain exponential growth

Population Growth Curves

• S-shaped curve: Initially a sharp increase, but then population stabilizes around the carrying capacity.

• The various aspects of environmental resistance establish the carrying capacity.

• Characteristic of:– “k-strategist species”: Humans, elephants, some

trees• Low number of young/high recruitment

• Usually parental care

• Adapted to stable environments

Population Growth Curves

Carrying Capacity• Maximum number of individuals of a species

that can be supported in a system.• This limit may be exceeded, but not without

decreasing the ability of the ecosystem to support life in the long term.

Data from a White-tail Deer Population:

Examples of Imbalanced Population Densities

• Nutrient addition to water leading to algal “blooms”

• Land management approaches that cause population “explosions” of deer

• Introduced species– Zebra mussels– Rabbits in Australia