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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)
- euryhaline - able to tolerate wide range of salinities
- stenohaline - can tolerate a relatively narrow range of salinities
- eurythermal - able to tolerate a wide range of temperatures
- stenobathic - can live at only a relatively narrow range of aquatic depths
- eurytopic - able to survive in a wide variety of habitats
...and so on.
Adaptation: Responding to External Change
- evolutionary adaptation is a change--occurring over generations--in the physiology, biochemistry, or morphology of a species in response to natural selection.
- individual adaptation is a short-term response by an individual organism to some environmental challenge.
Some species are able to gradually change their physiological 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
Behavioral adaptations allow an animal to respond relatively quickly to
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
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.
- Interactive Hypothesis (F.E. Clements)
An assemblage of closely linked species, locked into mandatory
association and biological interactions. The community functions as an
integrated unit, a superorganism (<--be sure to read this link).
- Individualistic Hypothesis (H.A. Gleason; Sir Arthur Tansley)
A chance assemblage of species found in the same area because they happen
to have similar biological requirements. Central evidence for this model is the phenomenon ofecological succession: ecosystem species composition can change in response to disturbance, and will change over time in response to community interactions.
(Be sure to review the symbiosis link and know the concepts and vocabulary.)
The variety of species found in an ecosystem comprise that system's
Two components of species diversity:
- species richness - the number of different species in the community
- relative abundance - the proportion of the community represented by each species
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 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
- primary productivity
- trophic interactions/energy flow
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
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.
- Energy, once it is captured by autotrophs, takes a one-way trip through ecosystems as it moves towards entropy.
(Second Law of Thermodynamics)
- Matter (chemical elements) are continually recycled through ecosystems
(Law of Conservation of Mass)
- is the total dry weight of organisms in a given area.
- can be calculated for an entire community
- ...but is more commonly expressed for a single species or type of organism (e.g., vegetation).
- standing crop biomass is the dry weight of given organisms in an ecosystem at
any given "snapshot moment" in time.
Gross Primary Productivity (GPP) - total primary productivity
Net Primary Productivity (NPP) - total primary productivity -
energy used for respiration
- can be expressed as energy per unit area per unit time (e.g., Joules/m2/year)
- or as biomass per unit area per unit time (e.g., kg/m2/year).
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
- Pt = productivity at level t (e.g., primary consumers)
- Pt-1 = productivity at the trophic level just below t (e.g., primary producers)
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.
- prediction: Removing energy from a community should decrease the length of the food chain.
- experiment: Researchers (Jenkins, 1992) removed leaf litter from tree-hole communities (in Queensland, Australia) and found that the number of trophic links decreased.
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.
- prediction: Communities that suffer fewer disturbances should have longer food chains.
Unpredictable, labile communities should have shorter food chains.
- experiment: Suggest one.
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.
example: Sugar Maples typically form dense stands that affect the soil nutrient composition and shade habitat, determining which other polant species (and hence, animal species) can live in the forest.
Why does a species become dominant?
A keystone species exerts strong influence on a community by means of its ecological role in that community.
- better at competing for limited resources
- better at avoiding predation or pathogens (Why are exotic invasive species so invasive? (Hypothesize!)
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.
Pisaster predation on mussels prevented the mussels from taking over the habitat, thus providing resources for a wider variety of species.
The number of starfish strongly affects species diversity in the intertidal regions.
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.
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
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...
- Ecologists removed about 1 million kg of Roach from the lake
- They stocked the lake with predatory Pike, which feed on Roach
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.
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
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.