Ecology can be described as the study of interactions which determine the distribution and abundance of organisms. Changes in distribution and abundance occur both in space and time.
Factors which influence the distribution and abundance of organisms can be biotic or abiotic. So, an organism's niche, potential distribution, and realised distribution are the result of interactions between the organism's own characteristics and external factors.
Most interactions with other living things in an ecosystem will fall into one of the following categories;
Symbiosis; which can be further subdivided into commensalism, mutualism, or parasitism.
Predation is density dependant. If the predator's prey is very close, the predator can more easily reach the prey before dying. Thus, the predator survives and grows, producing more predators.
Competition refers to competition for resources, when organisms' niches overlap. Success is usually measured in numbers of offspring. Competition can be either;
- intraspecific; within the same species
- interspecific; between different species
Responses to predation
Defence - plants will use physical defences such as spines, or chemical defences to make themselves unpalatable.
Warning colouration - the toxic Monarch butterfly is brightly coloured to alert predators.
Batesian mimicry - non-toxic animals develop the same colours as toxic animals.
(Refer Fig 43.14 pg 962 Knox et al)
There are an estimated 400 000 species in Australia, of which only 80 000 have been described to a high standard.
Organisms can be divided into three different types of distribution;
- Found only in Australia
- Found all over the world (including Australia)
- Found only in Australia and other southern continents
These groupings can tell us something about the antiquity of the organisms.
1. Found only in Australia
These are called endemic, or indigenous organisms.
Among mammals we have;
- lay eggs.
- includes the echidna & platypus
(NB - the echidna also occurs in New Guinea)
- young leave the uterus early, a marsupium is used to provide milk.
- includes quoll, thylacine, wombat, kangaroo etc
- long retention of young in uterus where they are nourished through a blood exchange system called a placenta.
2. Found around the world
- Ferns, especially low bracken
- Cockroaches, flies, mosquitoes
3. Found in Australia and southern continents
A large number of organisms fit into this category.
Some examples are;
- Flightless birds such as the cassowary, ostrich, emu, kiwi, rhea
An explanation for such distribution can be found in plate tectonics - the movements of plates that make up the Earth's crust. Organisms found around the world date back from the time of Pangaea (about 245 million years ago), when all the continents were joined. Those found in Australia and southern continents originated in the time of Gondwana (around 200 million years ago), when Australia and the southern continents were a single landmass. Organisms found only in Australia date from some time after Australia separated from the rest of Gondwana (c 145 million years ago).
(Refer Fig 31.4 pg 682 Knox et al)