ECOLOGY: The Study of Ecosystems
Ecology (from the Greek oikos meaning "house" or "dwelling", and logos meaning
"discourse") is the study of the interactions of organisms with each other and
their environment.
The hierarchy. Define each of the following.
- species -
- population -
- community -
- ecosystem -
- biosphere -
Ecology is a SCIENCE, not a sociopolitical movement (e.g., environmentalism).
The Ecologist engages in the hypothetico-deductive method to pose questions
and devise testable hypotheses about ecosystems. Often, this involves the
generation of complex mathematical models to simulate ecosystems. These
models represent idealized systems to which real systems can be compared
for their predictive value. Sometimes, when a very large scale project is
logistically impossible to perform, a computer model is used to predict
expected results.
An ecosystem consists of
- biotic components - the living organisms
- abiotic components - non-living factors, such as light,
temperature, water, nutrients, topography, etc.
Evolution by natural selection is driven by ecological interactions.
Levels of Ecological Study
- ORGANISMAL ECOLOGY - the study of individual organisms' behavior,
physiology, morphology, etc. in response to environmental challenges.
- POPULATION ECOLOGY - the study of factors that affect and change the
size and genetic composition of populations of organisms.
- COMMUNITY ECOLOGY - the study of how community structure and
organization are changed by interactions among living organisms
- ECOSYSTEM ECOLOGY - the study of entire ecosystems, including the
responses and changes in the community in response to the abiotic components
of the ecosystem. This field is concerned with such large-scale topics as
energy and nutrient cycling.
POPULATION ECOLOGY
Ecologists have devised several mathematical models to describe population
growth under various conditions and in various types of populations.
arithmetic growth - population increases by the same amount over each
time interval
exponential growth - population growth is very rapid, reflecting the
maximum intrinsic rate of growth. This is described by the equation:
dN/dt = rmaxN
in which...
dN = the change in population size (in small increments)
dt = the time interval (change in time)
rmax = maximum population growth rate (intrinsinc rate of
increase, equal to per capita birth rate minus per capita death
rate; (remember what is implied by the term RATE!))
N = population size
It plots out like SO.
The human population has been exhibiting exponential growth since it
dropped out of the trees. But how long can this last?
logistic population growth - exponential growth with environmental
resistance (carrying capacity of the environment = K) incorporated into the equation:
dN/dt = [rmaxN][K-N/K]
in which...
dN = the change in population size (in small increments)
dt = the time interval (change in time)
rmax = maximum population growth rate (intrinsinc rate of
increase)
N = population size
K = carrying capacity (maximum number of individuals the environment
can sustain indefinitely)
It plots out like SO.
Most natural populations exhibit logistic
growth.
LIFE HISTORY STRATEGIES
The logistic growth model predicts population growth at both very high and
very low population densities. Consider this in real populations...
What reproductive strategies would be advantageous at high population
densities (i.e., at or close to K)?
- ability to reproduce with few resources
- ability to compete well for limited resources
- ability to use resources very efficiently
- organisms showing this type of strategy are sometimes called
"K-selected", or equilibrial
- most likely to be found in stable habitats where population size
does not vary much once it has approached K.
What reproductive strategies would be advantageous at low population
densities (i.e., population is close to rmax)?
- early sexual maturity
- short generation time
- increased fecundity
- organisms showing this type of strategy are sometimes called
"r-selected", or opportunistic
- most likely to be found in unstable habitats where environmental
fluctuations result in large die-offs, or in newly colonized habitats
where resources are not a limiting factor
POPULATION LIMITING FACTORS
Limiting factors that increase in intensity as population size increases
are known as DENSITY DEPENDENT FACTORS. Example: Any limited resource.
Common results of high population density include
- reduced fecundity (why?)
- increased mortality (why?)
- increased rate of disease (why?)
- possibly increased per capita predation (due to altered predator
"search image")
- increased contact with toxic waste produced by the population
itself
Limiting factors that do not increase in intensity with population size
increase are known as DENSITY INDEPENDENT FACTORS. Example: sunlight,
rainfall, temperature, etc.; any factor that affects all individuals, no
matter what the population size.
- certain species undergo seasonal population crashes due to changes in
the climate/weather (freeze, drought, heat, etc.)
- periodic disturbances such as volcanic eruptions, major storms, fires
and other natural disasters can sometimes cause extreme population
fluctuations.
In most natural populations, both density-dependent and density-independent
factors play a role in controlling population size.
- climatic and seasonal factors can severely decrease populations,
but survivors of the disaster benefit from increased resource
availability, and may have a higher reproductive rate
- when a population is near K, small environmental changes can
sometimes magnify the effects of density-dependent factors, causing
large fluctuations in population size.
Some populations undergo regular "boom and bust" cycles.
- The Story of the Lemmings (a Disney myth)
- Snowshoe Hare and Lynx
The latter was once believed to reflect that Hare and Lynx populations
regulated each other. However, studies of populations of Snowshoe Hares
that were not subject to Lynx predation showed the same population
fluctuations. It is possible that Snowshoe Hare density regulates Lynx
density--or maybe not. Both populations may be exhibiting the effects of
density-independent factor magnification of density-dependent factors.
ECOSYSTEM ECOLOGY
You've all seen something like THIS, and know
what TROPHIC LEVELS are.
The Food Web reflects the flow of ENERGY and NUTRIENTS through
ecosystems.
Energy Flow begins with PRIMARY PRODUCTIVITY, the amount of light energy
converted to chemical energy (organic molecules) by an ecosystem's
autotrophs over a given period of time.
- GROSS Primary Productivity (GPP) - Total primary productivity
- NET Primary Productivity (NPP) - Total primary productivity -
energy used for respiration
NPP = GPP - R
Primary productivity (always expressed as a RATE) can be expressed as
- Energy per unit area per unit
time (e.g., Joules per square meter per year)
or
- BIOMASS, the
dry weight of vegetation added to an ecosystem per unit area per unit time
(e.g., grams per square meter per year).
STANDING CROP BIOMASS is the dry weight of vegetation in an ecosystem at
any given "snapshot moment" in time. It is not the same as primary
productivity: it's not a rate!
Energy flow is never 100% efficient,
and this results in the classic Pyramid of
Productivity.
The efficiency with which trophic levels convert energy from the previous
trophic levels varies greatly with ecosystem, and usually range between
5% - 20% (What does this mean in terms of how much is LOST at each trophic
level?), as reflected by the STANDING CROP BIOMASS of each level.
In a few ecosystems, there are some surprises!
Biogeochemical Cycles
A generalized cycle...
...can be superimposed on Biogeochemical Cycles for many different
nutrients
Your activities any day have profound effects on these cycles, and on
energy cycling in the biosphere. To test how much impact you have on our
biosphere, take the following test of your Ecological Footprint.
Go forth and spread the word.
