Transcript LV. CNM 2010-09-28 3580

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