Organismal Ecology

An organism's ability to survive extremes of its environment reflect its evolutionary history. Reproductive success is affected by ability to metbolically meet environmental challenges

  • regulators use metabolic means to maintain homeostasis in response to environmental changes. (e.g. - homeotherms, endotherms)

  • conformers are less able to metabolically maintain homeostasis. Their internal environment is governed primarily by the external environment. (e.g., poikilotherms, ectotherms)

    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. Their tissues have the same salinity as sea water. They are conformers

    Limits of Tolerance There is a continuum of tolerances to various environmental challenges within and among species. But beyond certain levels of any given factor, a lethal range exists. Up to that point, an individual organism may be able to adjust its metabolism or behavior to better tolerate a changing environment.

    Eury- vs. Steno-

  • Organisms that tolerate a wide fluctuation of a given environmental parameter are known as eury-(parameter).

  • Organisms that do not tolerate wide fluctuations of a given environmental parameter are known as steno-(parameter) Examples:

    Adaptation: Responding to External Change

    Individual Adaptation: Natural or Induced?

  • acclimatization is a physiological, biochemical, or anatomical change that occurs within the lifetime of an individual organism. It results from chronic exposure to a naturally occurring environmental challenge, and usually involves a change in gene expression (reflected in phenotype).

  • acclimation is a physiological, biochemical, or anatomical change that occurs in an invidual organism in a manipulated situation (e.g., experimental or other captive situation) as a result of exposure to an environmental challenge.

    The Ecological Niche The organismal ecologist studies the physiological responses (both short term and evolutionary) of organisms generated by environmental challenges.

    The sum of a species' use of the biotic and abiotic resources in its environment--as dictated by its evolutionary adaptations--is called that species' ecological niche.

    The ecological niche includes everything about that individual species in relationship to its ecology:

  • the range of physical extremes (heat, salinity, pH, etc.) in which it can thrive or survive
  • its diet
  • the physical parts of the habitat it uses and lives in
  • the time it's active during a 24-hour cycle

    In short, everything a species is and does determines its ecological niche.

    Each species has its own niche, but that does not mean that niches can never overlap or interact. When that happens, we enter the realm of community ecology.

    Community Ecology All the populations of different species living in a particular area comprise that area's community--the living portion of the ecosystem.

    You are already familiar with the concept (and some results of) symbiosis, the product of two interacting populations' coevolution.
    (Be sure to review the symbiosis link and know the concepts and vocabulary.)

    Species Diversity The variety of species found in an ecosystem comprise that system's species diversity.

    Two components of species diversity:

    The two forest communities below have the same species richness (four species), but the relative abundance of the four species is different between the two communities.

    As species diversity varies among ecosystems, so do the interactions among populations in those ecosystems.

    Communities with higher species diversity are often more resistant to disruption by invasive exotic species than less diverse communities. This may be because a more varied community, consisting of many species with different ecological requirements, uses a greater diversity of the available resources. This leaves fewer resources for an intruding newcomer with specific needs.

    Trophic Structure The structure and dynamics of a community are, to a great degree, determined by the feeding relationships among the species in that community. This is known as the trophic structure of the community. (Greek troph means "nourishment"; think of other words with this root, such as atrophy)

    The transfer of energy from primary producers to consumers to decomposers is known as a food chain.

    Energy transfer in real ecosystems is almost never a simple chain, however.
    Most organisms feed on (or are eaten by) more than one other type of organism, creating a complex food web.

    Food chains tend not to be composed of more than three to four steps. Why?

  • Energetic Hypothesis - inefficiency of energy transfer between trophic levels limits the length of the food chain
    (only about 10% of energy is coverted to biomass with each trophic level)
  • Dynamic Stability Hypothesis - long food chains are less stable than shorter food chains. (If there's a population crash at one level of the chain, it will cascade up the chain and be magnified at the highest levels, possibly causing extinction of top carnivores.)
    Species with Impact The dominant species in a community are those that are the most numerous or have the highest total biomass in that community. They tend to exert a strong influence on community structure and dynamics. Why does a species become dominant?
    A keystone species exerts strong influence on a community by means of its ecological role in that community.

    What happens when you remove the keystone?

    Pisaster predation on mussels prevented the mussels from taking over the habitat, thus providing resources for a wider variety of species.

    A foundation species (sometimes called an "ecosystem engineer") is one that causes dramatic alterations to the habitat, thus affecting the community structure and diversity.

    The effect can be positive on some species, and negative on others, depending on those other species' ecological requirements.

    Species interactions like these determine the next level of ecological complexity, the flow of energy and matter through ecosystems.

    Ecosystem Ecology Food Webs drive the flow of energy and nutrients through ecosystems.

    Energy Flow begins with primary productivity, the amount of light energy converted to chemical energy (organic matter) by community autotrophs over a given period of time.

    biomass productivity
  • Gross Primary Productivity (GPP) - total primary productivity
  • Net Primary Productivity (NPP) - total primary productivity - energy used for respiration

    Efficiency of Energy Transfer Energy transfer 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 across ecosystems, ranging between 5% - 20%, as reflected by the standing crop biomass of each level.

    Transfer efficiency is expressed as

    E = Pt/Pt-1



    Copepods, tiny planktonic crustaceans, feed on crunchy phytoplankton.

    If the standing crop biomass of copepods is 25g/m3, and the standing crop biomass of the plankton upon which they feed is 150g/m3, then the transfer efficiency of that part of the system is:

    25g/m3 divided by 150g/m3 = 0.17

    The efficiency of that trophic level is 17%, meaning that 17% of the phytoplankton biomass is converted into copepod biomass.

    In some ecosystems, there can be surprises.

    Biogeochemical Cycles All these trophic interactions result in a cycling of various nutrients through the biosphere in systems we call 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 quiz of your Ecological Footprint.

    Go forth and spread the word.