Optional Biology Notes. Dr. M. J. Butler. 1
Communities and Ecosystems (chapter 17)
Ecology is the study of the interactions of organisms with one another and with their
Some other definitions are commonly used by ecologists:
Biosphere – the zone of the earth's crust, atmosphere, and water that supports life.
Habitat – the place where a species lives; it is characterized by distinctive physical
features and vegetation.
Population –all of the members of the same species living in the same place at the same
Community - all of the populations in a given place at the same time.
The Niche of an organism is a subtle concept and is not just equivalent to its habitat, it is
the range of physical and biological conditions under which a species can live and
reproduce, often somewhat colloquially referred to as the “lifestyle” of an organism.
One of the axioms of ecology is that no two different organisms can occupy exactly the
same niche, if this was to happen, one would out-compete and eliminate the other
Ecosystems consist of one or more communities interacting with one another, and their
Energy flows in an ecosystem, and materials are cycled. The sun is regarded as the
ultimate source of energy in terrestrial and aquatic ecosystems (with the exception of
deep sea geothermal vents and their associated prokaryotic life forms that get chemical
energy from the vents), and all ecosystems require energy and nutrient input.
Ecosystems generate heat, a form of energy lost to the biology of the ecosystem because
it is not a form directly assimilated by organisms – it cannot be used, so ultimately,
energy is lost continually from ecosystems and must be replenished by sunlight, so
energy is not “perfectly” recyclable, while, in principle, matter is.
Structure of Ecosystems.
Organisms can be categorized within ecosystems according to the manner in which they
obtain energy and matter.
Autotroph’s – (Producers); these are “self feeders” they generate their own complex
organic materials from simple inorganic materials (carbon dioxide and water primarily)
using energy derived from light or chemicals. Thus they capture sunlight energy and Optional Biology Notes. Dr. M. J. Butler. 2
incorporate it into organic compounds. Examples are plants, algae, cyanobacteria, and
very deep sea bacteria that use chemical energy to form organic matter.
Heterotroph’s – (Consumers) obtain energy from tissues of other organisms, so
heterotroph’s eat each other, or they eat the producers.
The types of heterotroph’s are;
Herbivores - consume plants;
Carnivores - consume animals;
Parasites - extract energy and materials from living hosts.
Omnivores - consume a variety of organisms, vegetable, animal, fungal etc – humans are
Detritivores: feed on partly decomposed particles of organic matter, includes small and
microscopic creatures in soil and water.
Decomposers: extract energy from dead remains or products of organisms, these are
microbes such as bacteria and fungi and small protozoa.
As well as evolving in response to changes in their abiotic environment, organisms
clearly evolve in response to their relationship with other organisms and this just as
profoundly influences the structure of the ecosystem and the ecological relationships of
those organisms. This is called co-evolution and is classically found in predator-prey
The prey has to be faster or smarter to evade the predators and these properties evolve,
and the predators have to be smarter and faster too, in response to the changing
behaviours of the prey population. The species involved in co-evolution are thus acting as
agents of natural selection on each other.
Feeding Relationships in Ecosystems
Trophic Levels are steps along a feeding pathway; a hierarchy of energy transfers levels;
Primary Producers – secure energy from their physical environment, normally sunlight
and use it to convert simple inorganic materials like carbon dioxide, water and minerals
into organic form. This includes plants and algae.
Consumers feed on the biomass generated by the producers.
Ecologists create simplified models of the complex feeding relationships in an ecosystem
by constructing food chains, producers – eaten by primary consumers that are in turn Optional Biology Notes. Dr. M. J. Butler. 3
eaten by secondary consumers etc, this helps ecologists to understand feeding
relationships (technically called trophic relationships and levels).
In reality, feeding (trophic) relationships form complex Food webs of relationships
between producers, consumers and the recycling decomposers.
Energy moves up through the feeding webs and chains of an ecosystem, but it is lost at
each step “up” the web or chain.
Think of the old axiom – “it takes three pounds of corn to make one pound of chicken”.
Metabolism is inefficient, and ultimately, much energy is lost as heat through the levels
of the food chain, and the heat released is a form of energy no longer directly usable by
Energy is lost from ecosystems and must be replaced by new energy input, primarily
from sunlight, used to fix inorganic carbon dioxide and water and minerals into new
organic materials in plants, algae, and cyanobacteria.
This new biomass contains the new energy obtained from sunlight, in the form of the
bonds between the atoms that formed the new organic material, made by the plants and
algae and other primary photosynthesizing producers.
An example of a food chain?: algae => fish => shark => fisherman
Energy Flow Through Ecosystems.
Primary Productivity – this is the total rate of photosynthesis for the ecosystem during a
Net Primary Productivity – this is the rate of energy storage in plant tissues that is in
excess of the rate of plant respiration in a given area – in effect it amounts to the energy
used to produce more biomass by photosynthesis – it is the amount of energy captured by
plants and made available to consumers in an ecosystem.
Ecological Pyramids. Trophic feeding relationships can be diagrammed as a pyramid.
Producers form the base of the pyramid and higher levels are filled successively by tiers
of primary, secondary, tertiary consumers. There are 2 basic conceptual types of
Pyramid of Biomass – this diagrams the total biomass (as dry weight for instance) of
members in each trophic level at a particular time
Pyramid of Energy – the diagram reflects the rate at which energy in food moves
through each trophic level of an ecosystem. Optional Biology Notes. Dr. M. J. Butler. 4
Imagine that a farm of say 200 acres is abandoned, and that when it was abandoned most
of it was pasture – grass. Over the years the land reverted to its “wild” appearance,
bushes, shrubs and then trees took over the fields and wild animals populated the
This process is not random. Certain plants and animals will be the first to appear because
they can “use the land’ as they find it when it is first abandoned, and many other plants
and animals can’t use the land and grow immediately after farming is abandoned.
A common occurrence is that plants that can fix nitrogen from the air and incorporate it
in plant tissue are the first to colonize the abandoned land, especially when it is deficient
in nitrogen materials in the soil.
NB, The term “nitrogen fixing plants” is commonly used, even by biologists, but the
plants themselves do not actually fix the nitrogen, it is the bacteria that live in special
endosymbiotic relationship with their roots that fix the nitrogen. Only certain types
of bacteria can fix nitrogen, no eukaryotic cell can do this.
Those early nitrogen fixing plants in particular will grow and alter the soil by
adding nitrogen into it in chemical soluble form, and by adding nutrient to the soil
when they die and are decomposed, so that the soil is now suitable for the next and
different community of plants that can use the land in its changed form, and these
newer types of plants will support a new type of animal community.
It is possible to generally predict how one type of plant and animal will succeed
another in this process. This is called succession. Classic examples exist of studies
showing this process, a volcanic island was suddenly created near Iceland in the 1970’s
by a volcanic eruption and plants and animals began to populate it is a definite succession
pattern. The succession process in the devastated landscape left after the eruption and
explosive destruction of Mount St Helens in the USA is another example.
Eventually in this orderly succession a climax community will be reached - a stable
plant and animal community that changes little over time as long as the environment is
In Primary Succession changes begin when pioneer species colonize a barren habitat.
In Secondary Succession a community reestablishes itself toward a climax state after a
disturbance such as a forest fire.
Keystone species have a disproportionately large effect on a community, especially when
this is related to their low numbers. As an example, a population of relatively few beavers
have an enormous impact on their environment, they make dams that create ponds, these
ponds hold diverse ecologies of organisms that depend on maintenance of the pond by Optional Biology Notes. Dr. M. J. Butler. 5
beavers, and the damming and tree cutting activities of beavers causes major changes in
the environment. The African elephant is also a keystone species.
A community consists of coexisting populations of different species that occur
in the same area. In Chapter 29 you will learn about how these populations
interact with one another in an amazing myriad of ways (these
are symbiotic interactions, that may be beneficial, harmful, or seemingly
neutral). Some species, called keystone species, are key to maintaining
community structure (relative species diversity and abundance). The loss of