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Organismal Ecology: Individual Responses to Environmental Challenge

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 various aspects of an organism's environmental tolerances help determine its ecological niche.

    The Ecological Niche

    The organismal ecologist studies organisms' physiological responses (both short term and evolutionary) to 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 all ecological aspects of that species:

  • the range of physical extremes (heat, salinity, pH, etc.) it can survive
  • the resources it uses (food, space, etc.)
  • the physical parts of the habitat it uses
  • its Circadian rhythm
  • etc.

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

    The ecological niche is the position a particular species occupies in an ecosystem, relative to the other species in that ecosystem.

    The fundamental niche is, essentially, the niche that a species potentially could occupy, given its environmental tolerances.

    Because of competition and other limitations, few species actually fill their fundamental niche.

    Instead, they occupy a subset of the fundamental niche, the realized 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: Interactions Among Species

    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.

    Ecological Succession

    Ecological Succession is the process of change in community structure of an ecosystem over time. It generally begins with some type of disturbance (e.g., wildfire, volcanic eruption, emergence of a volcanic island in the ocean, etc.), and can take decades or millions of years to proceed to a climax community state.

    Ecosystem Ecology: Communities Interacting with the Abiotic Environment

    The ecosystem ecologist studies interactions of living communities with the abiotic components of the ecosystem.
    The functional processes that maintain ecosystem structure and ecosystem services (<--required link!) include

    Trophic Structure

    The structure and dynamics of a community are, to a great degree, determined by the feeding relationships among its species.
    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.
    The multiple, interweaving food chains create a complex food web.

    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

    Food chains generally 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.

    Organisms are not efficient at utilizing the energy in the food they eat.

    Energy transfer efficiency can be 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.

    Efficiency with which trophic levels convert energy from the previous trophic levels varies across ecosystems, ranging between 5% - 20%.

    General Rule of Thumb: only 10% of the energy in organisms eaten by those in the next trophic level is converted to that consumer's biomass.

    The reduction in available energy at higher trophic level can be expressed with a trophic energy pyramid.

    This is reflected by the standing crop biomass of each trophic level
    The higher the trophic level, the lower the biomass (usually fewer individuals).

    In some ecosystems, there can be surprises.

    These principles are scientifically testable. The Dynamic Stability Hypothesis states that long food chains are less stable than shorter food chains.
    Implication: if there is a population crash at one level of a food chain, it will cascade up or down the chain and be magnified at other levels, possibly causing extinctions.

    High Impact Species

    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?

    example:: The Ochre Seastar (Pisaster ochraceus) is a keystone species in rocky intertidal ecosystems on the west coast of North America.
    The number of starfish strongly affects species diversity in the intertidal regions.

    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 Impacts: Bottom --> Up or Top --> Down?

    Consider the impact some species we have considered have on their ecosystems.
    When their numbers fluctuate, there can be strong effects on the entire community.

    Bottom Up
    When fluctuations in a low trophic level species' numbers has an impact on the community, the effect is said to move from the bottom (of the food chain), upwards, or "Bottom Up".

  • Example: The Leaf Litter Experiment described previously

    Top Down
    When fluctuations in a high trophic level species' numbers has an impact on the community, the effect is said to cascade from the top (of the food chain), downwards, or "Top Down".

  • Example: The starfish predator example described previously

    We can sometimes use these concepts to solve environmental problems via biomanipulation.

    Case Study: Lake Vesijarvi in Finland

  • By 1976, city sewage and industrial wastewater had turned this beautiful lake into a polluted mess.
  • Pollution controls improved water quality.
  • However, by the mid 1980s, blooms of cyanobacteria were starting to occur, despite improved water quality.
  • These blooms coincided with an increase in the population of Roach.
  • Roach feed on zooplankton (e.g., crustaceans, fish larvae, etc.).
  • Until the roach population exploded, zooplankton had kept the cyanobacteria in check.


    Here's what Finland did...

    With Roach populations controlled, the zooplankton rebounded, and the cyanobacterial blooms stopped.

    All because the Finns understood the food chain.

    Ecological processes and species interactions like those we have seen today determine the next level of ecological complexity, the flow of energy and matter through ecosystems.

    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.