Ecology class 2: Homeostasis, acclimation and developmental response:

a. Which of the following mechanisms of adaptation would better be employed by plants and which by the animals living in a creek vs a forest?

b. Give 1 example used by humans (you) for each of the types of responses listed below

c. Balance the costs with the advantages of being a generalist ( a jack-of -all trades organism such as a weed or pest animal)? do the same for a specialist? think through this carefully before answering.

The environment fluctuates, whether in regular ( predictable) or chaotic (non-predictable) pattern, and each organism must respond in kind. The way in which that being responds is dependent on:

• the response type required and,
• the period of that fluctuation.
  
  
  
  

Response type can include:

* physiological or internal change to maintain stasis: for example an increase in RBC's or production of appropriate isozymes to deal with changes in oxygen levels or changes in temperature.

* behavioral or external change: as food types decrease the organism may forage or predate on another food item.

---

The type of response will also depend on the time period involved and whether the period is predictable or not.

I. Short-term responses: homeostasis is generally dependent on a negative feedback response;

• *if you are cold your metabolism can "pick up" to generate heat or *with loss of fluids ( sweat), your body will readjust its osmotic concentrations by adding more solutes to the blood stream.

Time span involved is rapid--- on the order of seconds, minutes or hours but it is expensive solution in many cases....

To fine tune every body response with every fluctuation in the environment would be prohibitive energetically; only those bodily changes which are critical to body functioning can work in this way.
Next class will discuss endothermy vs. ectothermy ( control of body temperature); endothermy in which the body maintains a constant temperature requires a high metabolism which means most of the energy taken in the body is used for this purpose.

Another example is plants and CO2 levels- plants are finely tuned to carbon dioxide levels in the surrounding atmosphere- they will close their stomata ( energy-requiring reaction) when levels drop in order to conserve moisture- however even here there is a limit....

II. Behavioral modifications: Less expensive but a longer-term means of adapting is through the selection of microhabitats that minimize the body-environment gradient- these involve behavioral modifications, though behavior must be taken broadly when speaking of plants.

Desert birds modify their foraging behavior and nest building according to the temperature of the day.
Cactus wren, a desert insectivore chooses its microhabitats according to temperature----> cool hours it will use all microhabitats available: exposed ground, shrubs, under trees etc.
---> hot hours it will use only shaded areas to forage

Planktonic forms- unicellular diatoms, dinoflagellates etc. have a diurnal cycle of movement up and down the water column to track the phytoplankton or escape their own predation.

III. Acclimation involves reversible changes in either structure or biochemical pathway ( enzyme induction)- such changes require a longer period ( days- to weeks) to initiate but are effective in long-lived organisms in response generally to seasonal changes.

Structural examples are easy to find:
* Color:
In the tundra, many birds, small (hares) & large mammals (bears) change fur, feather and even skin pigmentation to mimic the brown vegetation of summer and the white snows of the winter

* Texture of fur and feathers: lightweight vs. heavyweight fur layers in summer vs. winter

* Biochemical changes in metabolism to accommodate need for body temperature regulation and fat metabolism.

Other examples along this line include:
Salinity responses in fish-
Plant growth and thermal changes- fall changes

Problem with this approach:
it reduces adaptability if the environment reverts once more....- if the organism is transported back to the original conditions or still other conditions then they suffer in performance.

• thus the environment must stay the same for the organism's lifetime ( if the individual has a relatively short-life span) or
• its must go through different developmental phases which allow change for it to be successful
  * Example: locust and grasshoppers
  Seasonal ppt. leads to camouflaging
  
  When vegetation is green, lush individuals develop into green grasshoppers
  When the vegetation gets brown at the end of the season, you find brown grasshoppers
  
  Pigment
  develops in epidermis ------> green
  biochemical variants ------> yellow ( additional carotenes )
  additional single pigment molecules -------> brown--------> black ( melanin)
  
  The colors can change with molts, but not between molts.
  Thus with drought, followed byincreasing moisture, vegetation status will change, but so can the organism to an extent.. what color would expect the grasshoppers to be right now? catch one if you can and bring it to class.

  ---

  * Changes in sexuality in red maples:
  
• female trees -------> hermaphrodites ( both male & female flowers) if the previous year was wet
• -------> female only flowers if previous year was dry
  male trees -------> can only be male

  ---

  However, not only is the length of time change important, but also its predictability
  
  Cyclical changes whether seasonal, tidal, diurnal are predictable and allow genetic polymorphisms which can match that environment. Probably the easiest to handle
  
  Directional changes over a long period of time ( erosion, siltation, glaciation) also allow the selection of individuals which match the environment without that much cost.
  
  Erratic or non-predictive events are probably the hardest to accommodate: storms, fires(?) etc. don't allow slow selection of types which are best type for the new condition.
  So either a mixture of genotypes set for different conditions exist with only a portion of the population doing its best at any one time or high early mortality to get rid of types that are not maximal forms at that time.
  
  Another important consideration is the length of the organism's life cycle
  

If short lived they are more likely to only experience one environment, so they can become specialists

If long-lived the environment they will most likely change during their life time, and they must accommodate by being generalists.




---