Supplemental Lecture (97/02/11 update) by Stephen T. Abedon (firstname.lastname@example.org)
- Chapter title: Ecosystems
- A list of vocabulary words is found toward the end of this document
- "The trouble with ecology is that you never know where to start because everything affects everything else." - Robert A. Heinlein (from Farmer in the Sky)
- Ecology is the study of an organism's interaction with its environment.
- Though this definition sounds simple enough, it is deceptively so. In fact, the number of variables associated with an organism and it's (or, organisms and their) environment are all but endless making the actual interactions of even moderately complex organisms, in only moderately complex environments, intractably complex. Consequently, the sciences collectively referred to as ecology tend to be very, very, very difficult. Fortunately, ecological understanding, at least at a basal level, tends to be based on a finite number of broadly applicable principles. In this lecture we will consider these principles.
- Ecology is basically evolution playing in real time ("the study of all the complex interrelations referred to by Darwin as the conditions of the struggle for existence"; p. 3, Kormondy, 1996 citing Haeckel, 1870). Indeed, one facet of the complexity of ecological processes are the concomitant evolutionary processes. (Nevertheless, a great deal of the complexity seen with evolutionary processes are a consequence of the underlying ecology.) Thus, in gaining an appreciation for ecological complexities you should similarly gain further insight into the complexities of evolution, and vice versa.
- An understanding of ecological principles (as well as evolutionary biology, chemistry, geology, meteorology, planetary science, etc.) is also necessary to grasp the complexities of anthropogenic degradation of the environment. Indeed, ecology addresses such important issues as the determination of the role of the natural environment in sustaining human civilization. Is the natural environment robust enough to keep on taking our insults, or are we on the verge of a catastrophic environmental decline? It will be ecologists who are in the best position to answer such questions. It will also be ecologists, if anyone, who will be turned to pick up the pieces when humans as a race decide that the economic-progress-at-the-expense-of-the-environment party finally is over.
- The study of an organism's interaction with its environment including its interaction with other organisms (which, of course, also tend to be a part of an organism's environment).
- Doing ecology:
- "As an area of scientific study, ecology incorporates the hypothetico-deductive approach, using observations and experiments to test hypothetical explanations of ecological phenomena. . . . ecologists often face extraordinary challenges in their research because of the complexity of their questions, the diversity of their subjects, and the large expanses of time and space over which studies must often be conducted. Ecology is also challenging because of its multidisciplinary nature; ecological questions form a continuum with those from other areas of biology, including genetics, evolution, physiology, and behavior, as well as those from other sciences, such as chemistry, physics, geology, and meteorology." (p. 1061, Campbell, 1996)
- "Ecology concerns itself with the interrelationships of living organisms, plant or animal, and their environments; these are studied with a view to discovering the principles which govern the relationships. That such principles exist is a basic assumption---and an act of faith---of the ecologist. His field of inquiry is no less wide that the totality of the living conditions of plants and animals under observation, their systematic position, their reactions to the environment and to each other, and the physical and chemical nature of their inanimate surroundings . . . . It must be admitted that the ecologist is something of a chartered libertine. He roams at will over the legitimate preserves of the plant and animal biologist, the taxonomist, the physiologist, (the microbiologist), the behaviorist, the meteorologist, the geologist, the physicist, the chemist and even the sociologist; he poaches from all these and from other established and respected disciplines. It is indeed a major problem for the ecologist, in his own interest, to set bounds to his divagations." (p. 5, Kormondy, 1996 quoting Macfadyen, 1957)
- Living environment.
- Not living environment.
- A geographically defined entity . . .
- Interacting biotic components:
- . . . having one or more biotic component and one or more abiotic component.
- No matter how simple an ecosystem, it must still consist of more than one interacting component (e.g., an animal in a cage is an, admittedly artificial ecosystem since it contains a biotic component, the animal, and an abiotic component, the cage).
Ecosystems are the minimal unit of ecology.
Ecology is the study of the interaction of those components.
Non-arbitrary spatial boundaries:
The boundaries of an ecosystem are not arbitrarily defined.
Thus, an ecosystem might be a pond or a forest, or a real subset or conglomeration of one or more of such things.
However, an ecosystem is not, for example, one of two identical halves of a field, but instead the whole field or some non-arbitrary subset of the field (such as that shaded during the noon time sun versus all else).
"Ecosystems occur in space and exist in time---they have width, depth, and height, plus a past as well as a present and a future." (p. 14, Kormondy, 1996)
Ecosystems also have a strong tendency to vary across time and space.
Ecosystems are not closed, however. That is, things are always moving in and out of ecosystems.
"The spatial aspect of ecosystems is real, but precise delimitation is arbitrary, for one ecosystem is interrelated with other ecosystems. . . Ecosystems are not discrete entities delimited sharply from other ecosystems." (p. 14, Kormondy, 1996)
Ecosystems are hierarchical.
Note that also their exist numerous hierarchies associated with the interactions of species within an ecosystem, as will be discussed in part below under the guise of trophic structure.
Note that the absolute numbers of each component drops dramatically with each step up in the hierarchy.
Abiotic component of ecosystems
- At the base of an ecosystem's hierarchy are individual organisms.
- These make up species which in turn make up the assemblage of species found in the ecosystem (ecological community).
- There are also aggregations of ecosystems, with the largest aggregation of all being all of the ecosystems found on a planet.
- Abiotic components of ecosystems include all chemical and physical aspects of the system.
- Chemical aspects of ecosystems include:
- nutrients (organic and inorganic)
- Physical aspects of ecosytems include:
- chemical gradients
- currents (e.g., wind and water)
- degree of moisture
- Pattern of energy transduction:
- The trophic structure of an ecosystem is the pattern by which energy is transduced through the system.
- Ecosystems contain two broadly defined trophic members:
- An organism which produces its own food (e.g., carbohydrates).
- Plants, algae, and photosynthetic bacteria are all examples of autotrophs.
- An organism which acquires its energy by consuming either other organisms or the nonessential castoffs of other organisms.
- Generally, all organism which are not autotrophs (which includes most extant organisms) are heterotrophs and either eat autotrophs or other heterotrophs, or live off of their remains.
Remember, producers do not create energy. Instead, they transduce energy into stored carbohydrates.
This stored energy represents all the non-heat energy available to a the heterotrophic component of an ecosystem.
Primary production [primary productivity]
- The autotrophs found within an ecosystem.
- Carbohydrate producers:
- Producers produce their own carbohydrate utilizing energy obtained from abiotic sources.
- Plants as well as various types of bacteria are producers.
- A measure of the amount of energy transduced into reduced organic molecules by the producers within an ecosystem.
- The total weight of an ecological community.
- Biomass increases when solar energy is net stored and declines if solar energy stores are depleted.
Primary consumer [herbivore]
- Those organisms which consume living organisms to obtain nourishment.
- Consumers are one of the heterotrophic components of an ecosystem.
Secondary consumer [carnivore]
- A consumer which eats producers.
- A fish which browses on algae is a primary consumer (so is a cow).
- A consumer which eats primary consumers.
- A fish which browses on fish which browse on algae is a secondary consumer.
- A consumer whose diet consists, at least in part, of secondary consumers.
- A bird which browses on fish which browses on fish which browse on algae is a tertiary consumer.
Population density hierarchy:
- Hierarchy of consumers:
- The categories of consumers are known as trophic levels.
- Thus, a tertiary consumer inhabits a higher trophic level than a secondary consumer, which in turns inhabits a higher trophic level than a primary consumer, etc.
- Energy hierarchy:
- A basic principle of ecology is that it takes more environment derived energy (usually from the sun via photosynthesis) to produce 1 gram of tertiary consumer than it does to produce 1 gram of secondary consumer, etc. (i.e., in terms of sun light equivalent required to produce one gram: tertiary consumer secondary consumer primary consumer producer).
- In general, 90% of the energy is lost per step up in trophic level.
The cost of energy transduction is why there are so many more producers and primary consumers in this world than there are secondary consumers and, especially, tertiary consumers.
It takes many times more producers (or solar energy equivalents) to sustain a consumer for each trophic level higher that consumer inhabits.
Thus, secondary, and especially tertiary consumers tend to be rare relative to the abundance of their prey.
In general, only 10% as much mass can be present per each increasing trophic level. Thus, tertiary consumers should contain no more than about 0.1% (0.1*0.1*0.1*100) of the mass of the producers in a given ecosystem.
However, there is some question as to whether this 10% figure is completely generalizable. The Herbivore to Carnivore step, for example, may be more like 15% efficient.
Physiological considerations of consumers are important. Warm blooded animals, for example, use far more energy to maintain themselves than do cold blooded individuals, and swimmers tend to be far more efficient than are terrestrial walkers.
"Actually, the 10 percent figure is only a convenient approximation, since ecosystems vary in their levels of photosynthetic production and consumers differ in their efficiency at transforming food energy into biomass. The actual transfer varies from 5 to 85 percent in different food webs." (pp. 567-568, Benjamin et al., 1997)
Plug for vegetarianism:
This is also the basic reason vegetarianism is far more environmentally friendly than meat eating.
It takes far more wheat and soy bean (e.g., 10 times as much) to put a pound of muscle on you when indirectly consumed through a primary consumer (such as a cow) than it does to achieve the same result through the direct consumption of producers.
The higher the trophic level at which you eat, the increasingly less efficient your diet is in terms of utilization of the world's resources.
Note that if you must eat meat, at least in terms of efficiency of production, the cold blooded swimmers such as pond reared fish are preferable to mammals.
Hand-in-hand with energy dissipation through trophic levels is the concentration of non-metabolizable, difficult to excrete, fat-stored toxins.
This occurs because these toxins are not easily eliminated by animals in general. Since animals must eat many plants to sustain themselves, these toxins become more concentrated in primary consumers. In turn, secondary consumers consume a large number of primary consumers thus further increasing the toxin load in their own systems.
For omnivores, such as humans, consumption of secondary (and even tertiary consumers as well as scavengers) thus results in a greater toxin load than does consumption of producers, even if it is in/on the producers that the toxin originates (e.g., pesticides).
Higher level consumers, since they are fewer in number and often long lived, tend to have more difficulty evolving defences to these toxins than do lower level (or smaller, e.g., mosquitoes) consumers. Consequently novel toxins introduced into an environment tend to affect the more desirable animal species, especially over the long term, than the various "pest" species.
Examples of man-made toxins include DDT and PCBs. In addition, various radioisotopes and heavy metals can be analogously concentrated.
Decomposer [reducer, detritivore]
- A consumer whose diet includes both producers and consumers.
Bacteria decomposers are the chief means by which the discarded remains of dead animals are mineralized.
Fungi decomposers are the chief means by which the discarded remains of dead plants are mineralized.
- Distinct heterotrophic category:
- The second distinct heterotrophic components of ecosystems.
- Decomposers tend to consume dead organisms rather than living organisms or, more specifically, dead that they have not killed.
- Decomposers consist of bacteria and fungi.
- Decomposers accomplish their consumption of dead things chiefly through the use of exoenzymes thus allowing them to obtain their nutrients by absorbing predigested material.
- Decomposers are essential to ecosystems since through there actions organic matter is "mineralized," that is, converted, ultimately, into inorganic carbon (i.e., they decompose dead organisms).
- Eaters of parts:
- A second category of detrivores.
- Animals which are carnivores of cast off parts of other individuals.
- Scavengers include many kinds of invertebrates (such as worms, nematodes, insects, and crabs), vultures, and hyenas, as well as early genus Homo.
- Any time you see anything picking away at road kill, you are witnessing scavenging.
Movement of energy through ecosystems
- Complex trophic connections:
- In practice, consumers (except for the most specialized) tend to eat more than one kind of organism.
- Trophic connections consequently between organisms tend to resemble complex webs rather than showing one to one correspondences.
- Such complex trophic relationships in total for a given ecosystem are referred to as food webs.
- Energy tends to move not in simple lines from one kind of producer to one kind of primary consumer to one kind of secondary consumer, etc. Instead, many kinds of primary consumers tend to consume more than one kind of producer each, and so this continues up the trophic levels.
Movement of nutrients through ecosystems [chemical cycling, biogeochemical cycles]
- The movement (transduction) of energy through an ecosystem is by necessity non-cyclic. Instead, it is unidirectional.
- Energy is captured by producers, then passes through consumers, finally ends up the fodder for decomposers, and all the while is continuously lost as heat, a product of the inefficiencies of biochemical reactions.
- Net flow away from producers:
- Another way to consider this unidirectional flow of energy is that producers tend not to be consumers, and thus producer captured energy tends only to flow away from producers.
- Energy is captured, usually from the sun by producers, and then is lost to the environment as heat as it is employed within producers or as it travels up trophic levels.
- A total of about 2% of the sunlight energy striking earth is transduced into chemical energy by producers.
The water cycle is mostly driven by climate, meteorology, and geology (i.e., evaporation and movement down hills).
The biotic component of the carbon cycle is photosynthesis (net loss of gaseous CO2) and cellular respiration (net gain of gaseous CO2). Abiotic components generally bury or unbury carbon. In addition, a net gain of gaseous CO2 occurs through burning such as during forest fires, of fossil fuels (buried carbon stores), etc. Since decomposers often utilize cellular respiration, decomposition can produce as much CO2 from a dead organism as does burning. Accumulation of gaseous CO2 is thought to be associated with biospheric greenhouse warming.
Oxygen (O2) is produced through photosynthesis, lost to cellular respiration, and is also lost upon reaction with reduced materials, abiotic or biotic.
nitrogen and phosphorus cycling will be treated in more detail, below.
Loss and gains:
- Chemical cycling:
- Unlike energy, nutrients such as minerals tend to cycle through ecosystems. This is true for most ecosystems and assemblies of ecosystems up to and including the entire planet.
- A plant may take up a mineral from its roots. That mineral may be incorporated into a consumer which eats that plant as well as any subsequent consumers of that consumer. Finally, the mineral is liberated from the various producers and consumers when they die through the action of decomposers. The mineral is consequently or subsequently found in an appropriate state in the soil allowing plant uptake through roots.
- Biogeochemical cycles:
- The cycling and recycling of nutrients through ecosystems results from the actions of geology, meteorology, and living things.
- Various nutrient biogeochemical cycles include:
- water cycle
- carbon cycle
- oxygen cycle
- nitrogen cycle
- phosphorus cycle
- sulfur cycle
Minerals can be lost from ecosystems for example by the action of rain.
Nutrients can also can also be carried into ecosystems by, for example, the action of wind or migrating animals.
Movement of Salmon up rivers is an example of how nutrients might be delivered into an upstream ecosystem (e.g., from the oceans back to terrestrial forests).
A consequence of ecosystem disruption is an impaired ability to recycle nutrients which leads to nutrient loss and long term ecosystem impoverishment.
In general, a disturbed habitat probably loses (rather than recycles) nutrients to a much greater degree than an undisturbed habitat. The action of human activities are not necessarily rapidly nor readily reversible. A common consequence of human disturbance of ecosystems and the associated irreversible loss of nutrients is desertification.
Typical U.S. farming practices of the past century have profoundly disrupted of ecosystems. Consequently, substantial effort, often in the guise of the application of chemical fertilizers, is necessary to temporarily reverse ongoing nutrient loss.
Tropical rain forests are often examples of a nutrient-poor environment which survive only through rabid nutrient recycling. This is in part a consequence of the high amount of water (and therefore leaching) which cycles through these ecosystems. The cutting of trees, burning of bush, and subsequent erosion often results in a nutrient-depleted environment which is neither capable of supporting regrowth of rain forest nor even the farming which the forest was "slashed and burned" to support in the first place.
Nitrogen to air:
- Limiting nutrient:
- Nitrogen is a component of both amino acids and nucleic acids. In the absence of readily available nitrogen, ecosystems fail the thrive.
- This is why farmers fertilize plants with bioavailable nitrogen compounds.
- This is what carnivorous plants living in bogs get out of eating insects.
- This is why nitrogen fertilizers can disrupt ecosystems: they tilt the ecological balance towards those organisms (especially producers) with higher nitrogen needs.
- Nitrogen from air:
- The ultimate source of nitrogen in ecosystems is the atmospheric, elemetary nitrogen which makes up the majority of air.
- The conversion of gaseous nitrogen to a non-gaseous, more widely bioavailable form is called nitrogen fixing.
- Though extremely plentiful, the conversion of gaseous nitrogen into a form usable by organisms is very difficult due to the extremely high stability of gaseous nitrogen (molecular nitrogen consists of two nitrogen atoms triple bonded together). Consequently, nitrogen fixing is expensive, costing from 6 to 18 ATP per nitrogen atom fixed.
- Many kinds of bacteria, algae, and fungi are capable of fixing atmospheric nitrogen. The most significant source of fixed nitrogen is from nitrogen fixing bacteria (Rhizobium spp.) which inhabit nodules in roots of plants known as legumes (bean, peas, alfalfa, etc.).
Under anaerobic (no oxygen) conditions certain bacteria carry out a process called denitrification which is the conversion of fixed nitrogen into molecular nitrogen. Denitrification returns nitrogen to the atmosphere where it is once again available for fixation.
Though the removal and then return of nitrogen to the atmosphere is essentially a waste of energy, the alternative would be a gradual depletion of atmospheric nitrogen and therefore a loss of a source of readily obtainable nitrogen.
Of course, this is not the reason why some organisms employ denitrification, but an important consequence of them doing so.
An non-artificial means of regenerating the nitrogen content of farmland is to rotate crops such that a given piece of land is employed to grow net nitrogen users (such as corn) and then net nitrogen suppliers (such as the nitrogen fixing legumes, soy beans). After harvesting, the nitrifying plant may be plowed under thus supplying newly fixed nitrogen to the soil.
Crop rotation can reduce pesticide use by interfering with the yearly accumulation of pest species on mono-cropped land. It also reduces reliance on synthetic nitrogen fertilizers, which in addition to be expensive, also are not held well by the soil thus resulting in a requirement for frequent application as well as polluting run off.
Mineral source/loss to sea:
- Limiting nutrient:
- Phosphorous is found in nucleic acids, cellular membranes, and, of course, ATP.
- Phosphorous is second only to nitrogen as a limiting nutrient in ecosystems.
- This is why farmers fertilize their fields with phosphorous products.
- Because phosphorous tends to be so limiting in ecosystems, excess application of phosphorous can lead to excessive and destructive "blooms" of organisms.
- It is to avoid such blooms that one may feel compelled to avoid detergents which include added phosphates.
- No gaseous form:
- Unlike water, carbon, hydrogen, nitrogen, oxygen, and sulfur, phosphorus has not gaseous form.
- This limits its availability to ecosystems compared with these other nutrients.
- This also slows the phosphorus geochemical cycle as compared to these other nutrients.
Phosphorus is made available to the biotic component of ecosystems only as it is released (solubilized) from the rocks found in soil. Consequently it is often limiting in ecosystems.
Phosphorus not assimilated into organisms is ultimately lost to flowing water which transports the phosphorous to the sea. There it precipitates out of solution forming phosphate deposits which require geological processes (or man) to return to cultivatable land (i.e., the phosphate deposit must be geologically uplifted through the action of plate tectonics thus turning into land, a very slow process).
A method alternative to solubilization as a source for phosphorous is the transportation back onto land from the sea by various animals.
- Biotic component of ecosystem:
- Populations of individual species inevitably coexist with populations of other species. A grouping of populations constitutes an ecological community, and an ecological community is the biotic component of an ecosystem.
- The community associated with a well functioning ecosystem would include producers, primary consumers, secondary consumers, and decomposers.
- Each member of a community (i.e., species) possesses a unique ecological role within that community.
- Equivalent assemblages:
- The assemblage of species within a community tends to vary though the members of a community occurring in a given type of ecosystem tend to resemble (ecologically) members of similar ecosystems, rather than members of distinctly different types of ecosystems.
- You would not expect to see analogous biotic components between a midwestern pond and an desert plateau, but would not be surprised to observe ecologically analogous species occurring in both a pond in Kansas and a second in Nebraska).
- "We expect a priori that no two ecosystems would have precisely the same biological composition but rather would have ecological equivalents performing comparable functions." (p. 13, Kormondy, 1996)
Communities vary in their assemblage within ecosystems.
These variations can follow abiotic variation, though species variation has a tendency also to lead to further species variation.
The species ultimately present at a given time and place are often a function not only of the abiotic components but of the history of the ecosystem and of that particular spot as well.
- Increase in biotic complexity:
- Ecosystems have a tendency to change over time and, especially, to increase in complexity.
- Unless some new means is achieved for mining the abiotic component of an ecosystem, it is only the biotic component which does the changing.
- New = impoverished:
- An impoverished ecosystem would be one which is comparatively new and therefore lacking particularly in terms of numbers of species.
- Such impoverishment is often achieved through the activities of man. At this point in time there are few ecosystems left on earth which have not been impoverished to at least some extent by the actions of man.
- Agents of both local and widespread impoverishment can include:
- geological forces (uplifting, volcanism, earthquakes, etc.)
- meteorological forces (hurricanes, droughts, tornadoes, etc.)
- climatic forces (ice ages, glaciers, etc.)
- chemical forces (pesticides, toxic spills, crude oil, heavy metals, etc.)
- astrophysical forces (meteorites, etc.)
- other forces (agriculture, acid rain, forest fires, clear-cutting etc.)
Ecological succession is the means by which ecosystems increase in complexity.
Often this is accomplished through the migration of new species into an ecosystem.
However, an alternative means is the evolution of new species (speciation) within the ecosystem.
Loss of biodiversity = impoverishment:
Due to the actions of man, the total number of species available to participate in ecological succession is diminishing at an unprecedented rate. This loss of biodiversity implies that for the next few million years of history on this planet, many of the majority of worlds ecosystems (all?) are doomed to impoverishment.
Many ecosystems undergo ecological succession in a fairly predictable manner. Thus, following a disturbance:
- fast growing, quickly distributed organisms tend to dominate
- these are followed by less rapidly growing and/or dispersing, but better competing organisms
- ultimately the ecosystem comes to be dominated by those organisms able to enter the ecosystem which are most able to out compete existing flora and fauna over the long term.
- physical disturbances such as forest fires can set the succession clock back to zero.
- The movement of organisms into an essentially sterile environment. Such environments are essentially created from nothing (i.e., no biotic component).
- Primary succession often occurs as a consequence of radical environmental change such as:
- volcanic eruption on an existing land mass
- the formation of new islands (often also by volcanism)
- the retreat of glaciers
- the opening of new deep sea vents can.
- Secondary succession is the movement of new organisms into an environment already inhabited by organisms.
- The gradual replacement of meadow grasses by trees is an example of secondary succession.
- Given a stable environment, a lack of human interference, and a generally lack of nonhuman catastrophic phenomenon, an ecosystem will eventually reach a point where subsequent succession does not occur. Such an ecosystem is said to have climaxed and to contain, for example, climax vegetation.
- Abiotic components
- Bacteria decomposer
- Climax vegetation
- Ecological communities
- Ecological succession
- Energy movement
- Food web
- Fungal decomposer
- Nitrogen cycle
- Nitrogen fixing
- Nutrient movement
- Phosphorous cycle
- Primary consumer
- Primary production
- Primary productivity
- Primary succession
- Secondary consumer
- Secondary succession
- Tertiary consumer
- Trophic level
- Trophic structure
Practice question answers
- True or false, both energy and nutrients cycle through ecosystems (circle best answer). [PEEK]
- What consumes the energy associated with dead animals other than other animals (i.e., other than the predators and scavengers)? [PEEK]
- What occurs under anaerobic conditions which is necessary to complete an important nutrient cycle? [PEEK]
- Name three means by which a typical non-gaseous nutrient might be carried into an existing ecosystem. [PEEK]
- Name the typical anthropogenic effect on the species composition of ecosystems. [PEEK]
- Ecosystems contain two broadly defined trophic members: _________ and _________. [PEEK]
- ___________ is the means by which ecosystems increase in complexity. Often this is accomplished through the migration of new species into an ecosystem. [PEEK]
- False, energy moves through but does not cycle through ecosystems
- bacterial decomposers
- wind, rain, organismal migration
- impoverishment (genetic, total number of species, nutrients in ecosystem, etc.)
- autotrophs and heterotrophs, or producers and consumers
- ecological succession
- Benjamin, C.L., Garman, G.R., Funston, J.H. (1997). Human Biology. The McGraw-Hill Co., Inc., New York, pp. 556-586.
- Campbell, N.A. (1996). Biology. Fourth Edition. Benjamin/Cummings Publishing Co. Inc., Menlo Park, CA. pp. 1060-1092.
- Kormondy, E.J. (1996). Concepts of Ecology. Fourth Edition. Prentice Hall, Upper Saddle River, NJ. pp. 8-18.
- Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 486-504.