For example: Osmoregulation

• Anadromous (fish that migrate from salt to freshwater habitats annually) and catadromous (fish that migrate from freshwater to marine habitats annually) maintain constant salt balance in their tissues via their renal systems, even when their environments vary. They are regulators

• Echinoderms, entirely lacking an excretory system, are strictly limited to marine environments, and their tissues have the same salinity as sea water. They are conformers

There is, of course, a continuum of tolerances to various environmental challenges within and among species. Beyond certain levels of any given factor, a lethal range exists.

Short-term responses to environmental changes are 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:

Some species are able to physiologically acclimate (gradually change their tolerance levels) in a slowly changing environment. This ability, too, is controlled by genes that have been selected over evolutionary time.

Morphology may change in response to environment
• fur coat changes: color and thickness change seasonally
• Daphnia carapace changes in response to water conditions or to presence of predators
  >
  • behavioral adaptation
    Behavioral adaptations allow an animal to respond relatively quickly to environmental challenge.

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    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 = 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.

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    Most natural populations exhibit logistic growth.

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    Life History Strategies

    The logistic growth model predicts population growth at both very high and very low population densities. Consider this in real populations...
    r-selected species
    
    
  • 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
    K-selected species
    
    
  • 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.
    

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    Population-limiting Factors

    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

    What are the actual factors that cause these phenomena?

    A limiting factor is any environmental condition that stops a population from unlimited growth at its intrinsic rate of increase.
    Limiting factors that increase in intensity as population density increases are known as density dependent factors. Example: Any limited resource.

    Limiting factors that do not increase in intensity with population density 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. But there are some unfortunate myths out there.
    • The Myth of the Suicidal Lemmings (from the Disney nature "documentary" White Wilderness, shown to thousands of elementary school kids in the 1960s). Here's the sad reality. But the falsehood persists to this day.

      Even real phenomena are controversial...
      

    • 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.

      The jury's still out on the actual cause(s) of the concurrent fluctuations.