Example: osmoregulation - the internal regulation of salt balance

• 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:

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

• 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

Population Growth

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?

It plots out like SO.

LIFE HISTORY STRATEGIES

• 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.

• 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?