BIL 106 - Lecture 7

ORGANISMAL, POPULATION, COMMUNITY, AND ECOSYSTEM ECOLOGY

ORGANISMAL ECOLOGY: The Ecology of Individuals Homeostasis: An organism's ability to maintain a constant internal environment (in terms of temperature, water content, salt balance, etc.)

regulators - organisms able to metabolically (i.e., with internal chemical reactions) regulate their internal environments in response to environmental changes.

conformers - organisms whose internal conditions are controlled primarily by environmental conditions.

Anadromous (fish that migrate from salt to freshwater habitats annually) and catadromous (fish that migrate from freshwater to marine habitats annually) are REGULATORS. They maintain constant salt balance in their tissues via their renal systems, even when their environments vary.
Echinoderms (e.g., starfish, echinoderms, etc.), are CONFORMERS. THEY lack kidneys or any sort of excretory system. Because they can't regulate salt balance, they are strictly limited to marine environments, and their tissues have the same salinity as sea water. They can't survive in fresh water. (why?)

There is a continuum of tolerances to various environmental challenges within and among species. Beyond certain levels of any given factor, a lethal range exists. (Can you think of an example in our own species?)

Short-term responses to environmental changes are called ADAPTATIONS. These are governed by the homeostatic mechanisms in the individual, but their limits are set by the EVOLUTIONARY HISTORY of that individual.

Individual adaptations to change include:

physiological acclimation (change in internal chemical reactions) (examples?)

morphological change (change in shape) (examples?)

behavioral adaptation (change in behavior) (examples?)

An animal's ability to change in response to environmental pressure is limited and controlled by genes that have been selected over evolutionary time.

POPULATION ECOLOGY: How do Populations Change over time?

Two important population characteristics are

density (# of individuals/unit area or unit volume)

dispersion

clumped - individuals aggregated in patches (most common)
random - unpredictable, patternless distribution (fairly uncommon)
uniform - evenly spaced (relatively uncommon, due to intraspecific interaction)

Demography: the study of vital statistics that affect population structure, such as

birth rate ( = fecundity)
- number of offspring per unit time (in population or per female)
- this generally varies with age of individual
death rate
- depending on species, environment, etc., this is generally highest in juveniles and old individuals, but not always
generation time
- average time elapsed between birth of individuals and the birth of their offspring.
- generally, the shorter the generation time, the faster the population growth rate (all other factors being equal) (what might change this?)
- generally, the larger the organism, the longer the generation time
- age structure (how many individuals at each age)
  - This is the number of individuals of each age in the population
  - age structure affects population growth rate
- survivorship
  - the likelihood of death at any given age
  - a COHORT is all the individuals in a population of the same age
  - by following the death rate of a cohort, a survivorship curve can be generated, and by doing this for many cohorts, the general pattern for a population can be estimated
  - Note the difference between Type I, Type II and Type III curves

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 = r_maxN in which...

dN = the change in population size (in small increments)

dt = the time interval (change in time)

r_max = 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 = [r_maxN][K-N/K] in which...

dN = the change in population size (in small increments)

dt = the time interval (change in time)

r_max = 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 r_max)?

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

COMMUNITY ECOLOGY: How do populations interact with one another? All the populations of different species living in a particular area comprise that area's COMMUNITY--the living portion of the ecosystem. The number of different species found in an ecosystem comprise that system's SPECIES DIVERSITY. Diversity varies greatly among ecosystems, and hence, so do the interactions among populations in those ecosystems.

A COMMUNITY is any assemblage of populations in an area or habitat.

Individualistic Hypothesis (H.A. Gleason)
Interactive Hypothesis (F.E. Clements)

SYMBIOSIS - This term (from the Greek sym, meaning "together" and bios, meaning "life") refers to the members of two different species (i.e., two populations) having some sort of ecological interaction that affects both populations. Here are some of the theoretical types of interactions that can evolve over many generations. When two species evolve in response to each other's activities, the process is known as coevolution.

"+" means that the population benefits from the interaction

"-" means that the population is harmed by the interaction

"0" means that the population is not affected by the interaction

type of interaction	pop'n A	pop'n B	nature of effect
mutualism	+	+	obligatory; both populations benefit
EXAMPLES:
protocooperation	+	+	NOT obligatory; both pop'ns benefit
EXAMPLES:
competition	-	-	populations inhibit one another
EXAMPLES:
neutralism	0	0	populations don't affect one another
EXAMPLES:
predation	+	-	predator (A) kills & consumes prey (B)
EXAMPLES:
parasitism	+	-	parasite (A) exploits the host (B), but does not kill it outright
EXAMPLES:
parasitoidism	+	-	parasitoid (A) eventually kills host (B)
EXAMPLES:
commensalism	+	0	commensal (A) benefits; host (B) not affected
EXAMPLES:
amensalism	-	0	A inhibited; B unaffected
EXAMPLES:

Some of the results of predation (If you missed class, you didn't get to see the beautiful demo materials!)

Crypsis - camouflaging coloration

Aposematism - warning coloration (poisonous or venomous species)

Mimicry - a species has evolved the superficial appearance of something else

Batesian Mimicry - a harmless mimic looks like a poisonous model.

Mullerian Mimicry - several poisonous/distasteful species resemble one another.