Animal and Plant Responses to the Biotic Environment
Environmental factors can be classified in a different way (from simply biotic
and abiotic) They can be:
 Resources which are factors that may be competed for. In animals, these
may be food, mates and territory; in plants they may be light, water and
mineral ions.
 Conditions are factors that influence organisms without in any way being
used up, such as temperature, soil pH, rate of water current flow and so on.
Some factors eg light may be a resource for plants but a condition for animals.
Some biotic factors, such as competition, are neither resources nor conditions.
Coping with physical factors in the environment is only one aspect of survival.
Equally important are:
 For animals, obtaining food
 For all organisms, avoiding being used as food by other organisms such as
predators, herbivores and parasites.
In meeting - or failing to meet - the above two requirements, organisms interact with other
organisms in various ways. These interactions can be divided into four groups on the basis
of cost and benefit.
1. Competition. May be inter or intraspecific. Exists when the demand for a resource
exceeds the supply. All competitors are harmed to some degree.
2. Exploitation. One organism benefits at the expense of another. Includes predation,
parasitism and grazing (herbivory)
3. Cooperation. Both organisms benefit. Interspecific cooperation is called mutualism
but there are also cooperative interactions within a species.
4. Commensalism. A zero harm relationship in which members of one species benefit
but the members of the other are not affected.
Relationship
Competition
Exploitation
Cooperation
Commensalism
Species A
Species B
+
+
+
+
0
COMPETITION
Competition may be intraspecific or between members of the same species
Competition may be interspecific or between members of different species.
No two species can indefinitely occupy the same niche in the same habitat (Competitive
Exclusion Principle)
In species sharing the same habitat, competition is avoided by divergence in their niche
requirements.
Competition within animal species may be for food, mates, territory, living space, and in
plants, for light, minerals or water.
Competition for mates may be by fighting or displaying to females.
A territory contains a resource in limited supply such as food or nesting area.
The benefits of holding a territory must exceed the costs (time and energy ) of its defence.
When food is the resource in a territory there is an optimum territory size.
Social dominance is expressed by subtle signals that are often hard for an observer to
detect.
Competition is often avoided by dispersal of the young.
In plants, competition for light water and minerals has a measurable effect on growth rate.
EXPLOITATION – PREDATION, GRAZING AND PARASITISM
Exploitation is a plus-minus relationship in which one species obtains food from another,
which is always harmed to some degree.
Exploitation is a relationship in which each party has affected the other’s evolution :
exploiters have evolved adaptations to obtain more food and the prey, hosts and other
“exploitees” have evolved adaptations which resist being exploited.
Some adaptations, such as mimicry, are self-limiting , in that they only work when the
mimic is rare relative to the model.
Prey numbers place a limit on the numbers of their predators, and some predators exert a
powerful effect on the numbers of their prey.
Predators can only control prey numbers if the proportion of the prey eaten increases with
prey density.
Plants have a wide range of chemical and physical defences against animals.
It is in the interest of a parasite to maximise the food-producing capacity of the host, and
the best adapted parasites do little harm to the host.
COOPERATIVE INTERACTIONS AND COMMENSALISM
Cooperation may occur between members of the same species or between different
species.
Many species form groups – clusters of individuals that derive benefit from the presence of
others.
Mutualism may be ecological or physiological, and may between particular species or may
be much less specific.
Mutualism may be obligate for both partners, facultative for both, or obligate for one and
facultative for the other.
COURTSHIP, PARENTAL CARE AND LIFE HISTORY STRATEGIES.
In many species, reproduction is preceded by courtship, which can have several functions.
Organisms invest a great deal of energy in becoming successful parents, but they do this in
very different ways. One way is by parental care of a few, well-protected and wellnourished offspring.
One way of giving the offspring greater protection and nourishment is by viviparity.
In birds, some have larger numbers of less-dependent offspring, while others have smaller
numbers of totally dependent young, requiring more parental care.
A special case of reproductive care is by sterile sisters, which occurs in ants, bees and
wasps. This is correlated with the fact that these insect sisters are more closely related to
each other than parents are to offspring.
In quite a few fish and some other animals, offspring are cared for by their fathers.