Ecology
ECOLOGY
Definition: Ecology is the study of the relationship of living organisms to their surrounding environment.
Some common ecological terms:
BIOSHPERE: only a part of planet earth, its land, oceans and atmosphere
inhabited. The majority of organisms live in a narrow belt, from the
upper soil to the lower atmosphere. Or, if marine, many live near
the ocean surface. The restricted zone which living things inhabit is
called the biosphere.
ECOSYSTEM: an ecosystem is a stable, settled unit of nature consisting of a
community of organisms, interacting with each other and with their
surrounding physical and chemical environment. Examples of
ecosystems are ponds, lakes, woods or forests, sea shores or salt
marshes, grasslands, savanna or tundra. Ecosystems are necessarily
very variable in size.
POPULATION: a population consists of all the living things of the same species in a
habitat at any one time. The members of population have the
chance of interbreeding (assuming the species concerned
reproduces sexually). The boundaries of populations are often hard
to define, but those of aquatic organisms occurring in a small pond
are clearly limited by the boundary of the pond.
COMMUNITY: a community consist of all the living things in a habitat – the total
of all populations, in fact. So, for example, the community of a well-
stocked pond would include the populations of rooted, floating and submerged plants, the populations of bottom-living animals, the populations of fish and non-vertebrates of the open water, and the populations of surface-living organisms – typically a very large number of organisms, in fact.
HABITAT: the habitat is the locality in which an organism occurs. It is where
the organism is normally found. If the area is extremely small we
call it a microhabitat. The insects that inhabit the crevices in the
bark of a tree are in their own microhabitat. Conditions in a
microhabitat are likely to be very different from the conditions in
the surrounding areas.
NICHE: the nice of an organism is how it feeds where it lives. For example,
the sea birds known as the cormorant and shag, feed in the same
water and nest on the same cliffs and rocks, so they share the
same habitat. However the cormorant feeds on sea-bed-living fish,
such as flatfish, whereas the shag feeds on the surface-swimming
fish such as herring. Since these birds feed differently they have
different niches.
COMPETITION: resources of every sort are mostly limited in supply, and so
organisms must compete for them. For example, plants may
compete for space, light and mineral ions. Animals may compete
for food, shelter and a mate. Whatever resource is in short supply
and preventing unlimited growth is known a limiting factor.
Competition between individuals of the same species, intraspecific
competition, occurs when individuals compete for a mate, for
example. Competition between individuals of different species
interspecific competition, occurs when the red and grey squirrel
compete for hazel nuts in autumn, for example.
ENVIRONMENT: environment is a term we use for ‘surroundings’. We talk about
the environment of cells in an organism, or the environment of
organisms in a habitat. It is our term for the external
conditions affecting the existence of organisms, too, so
‘environment’ is a rather general unspecific term, but useful
none the less.
ENERGY FLOW THROUGH ECOSYSTEMS
PRODUCERS, CONSUMERS AND DECOMPOSERS.
Think of an ecosystem such as broad-leaved woodland in summertime, with its community of plants, animals, fungi and microorganisms, all engaged in their characteristic activities. The essence of survival is activity. To carry out these activities organisms need energy. The immediate source of energy in cells is the molecule adenosine triphosphate (ATP). A great deal of the ATP is required is produced by respiration. The energy that ATP contains has been transferred from sugar and other organic molecules, called respiratory substrates. These organic molecules are obtained from nutrition. The nutrition of the green plants in the habitat is autotrophic. Plants make their own organic nutrients from and external supply of inorganic nutrients, using energy from sunlight in photosynthesis. In contrast, animals can use only existing organic nutrients, and they must digest the food the food that they eat so that the organic molecules within it can be released and used. Animal nutrition is therefore dependant on plant nutrition, either directly or indirectly. Animal nutrition is heterotrophic. In ecology, organisms are often conveniently classified by their feeding relationships. Green plants are producers, animals are consumers, and, in the process of feeding energy is transferred. Some of the consumers, known as herbivores, feed directly and exclusively on plants. Herbivores are primary consumers. Animals that feed exclusively on other animals are called carnivores. Carnivores feeding on primary consumers are known as secondary consumers. Carnivores that feed on secondary consumers are called tertiary consumers, and so on. Eventually all producers and consumers die and decay. Organisms that feed on dead plants and animals, and on the waste matter of animals, are a category of feeders known a detritivores or decomposers. Feeding by detritivores releases inorganic nutrients from the dead organic matter. These inorganic nutrients are absorbed by green plants, sooner or later, and reused.
FOOD WEBS: is the interconnection of energy flow through different trophic levels the more complex the food web becomes the less energy will flow to the top of the trophic level
ENERGY LOSSES IN A FOOD CHAIN:
Leaves loose energy from the sun by reflection and heat energy, which is 90% of the total amount.
Total amount of sunlight, which strikes the leaf, is only 10 % fixed by the leaf.
The leaves then use this 10% for energy transfer, which equals then 100%.
Only 10% flows from the producers to the primary consumers.
And only 10% of the total amount of energy flows from the primary consumers to the secondary consumers.
At each level is lost by respiration, excretion and movement.
FOOD CHAINS AND FOOD WEBS
The feeding relationship describing which carnivore eats an herbivore that has eaten plants is called a food chain. Since the plant material at the start of the food chain may be either living or dead, the food chains are of two types. Herbivores feeding on living plants are said to be browsing or grazing, and the food chain of these herbivores is called a grazing chain. Herbivores feeding on dead plant material, degrading and decomposing the organic matter in the process, are part of a decomposer chain.
In fact, most food chains, both grazing and decomposer chains, interconnect with other chains. This is because most prey have more than one predator. The interconnected food chain forms a food web. Food chains and a food web for a deciduous woodland are shown in the figure below. Note that food chains tell us about the feeding relationships of organisms in an ecosystem, but they are qualitative relationships (we know which organisms are present) rather than quantitative (we do not know the number of organisms at each level).
PYRAMIDS OF NUMBERS, BIOMASS AND ENERGY
Ecological pyramids are a way of analyzing ecosystems. They can be used to express seasonal changes in one ecosystem, or to compare time, for example. It can be difficult, though, to decide at which trophic level every organism feeds. For example, omnivores feed as both primary consumers (level 2) and secondary (or higher) consumers (level 3). When first used as a way of analyzing ecosystems, ecological pyramids were considered as pyramids of number. To produce such an ecological pyramid diagram, all organisms at each trophic level within a given area are recognized and counted. This information could be relatively easily obtained, without destroying the organisms. At the time it was argued that this ‘numbers’ approach was correct since: Ecosystems typically contain a very large number of small animals and a progressively smaller number of larger animals. Predators are larger than their prey because they have to be able to overpower them easily Prey is never so small that it takes the predator a long time to catch sufficient numbers of them Small animals reproduce faster than larger animals, so maintaining their numbers despite predation.
The results were presented as rectangular blocks, stacked on top of each other, representing the numbers of organisms at each trophic level in a bar diagram. The problem with this approach was that no allowance was made for the difference in size of individual organisms. A giant oak tree and a single microscopic green alga both counted as one! Consequently, ‘pyramids’ of numbers sometimes created strange-shaped pyramids of limited meaning. This problem was overcome by producing pyramids of biomass. At the start of the grazing chain is a large biomass of green plant leaves. This supports a smaller biomass of primary consumers, which in turn supports an even smaller biomass of secondary consumers. An ecosystem pyramid diagram showing the structure of an ecosystem in terms of the biomass of the organisms at a given time at each trophic level is illustrated below. The fieldwork to do this involves estimating the numbers of organisms of each type at each trophic level, and then finding the biomass (dry weight) of a representative sample of each type of organism. The dry mass is found by heating a weighed sample to a temperature that is hot enough to drive off all the water (about 80˚C) but not hot enough to burn away any organic matter. Cooled samples were weighed and heated, cooled and reweighed. This process was repeated until two consecutive readings gave the same mass, showing that all the water had been driven off. A problem with accuracy of biomass measurements is that different tissues contain different amounts of energy. Consequently, a pyramid of energy is sometimes considered to be the most useful form of ecological pyramid. However to obtain the information needed for this, representative samples of the organism need to be burnt in a calorimeter in oxygen. The energy released raises the temperature of the water surrounding the combustion chamber. The mass of water in the calorimeter is known, so the amount of heat energy released from the rise in water temperature can be calculated. Carried out on all organisms at every trophic level, this is a time consuming and destructive activity. In practice, pyramids of biomass can be converted to pyramids of energy using published values of energy content, made in previous experiments in habitats that are compatible.
