14.4: Interactions within Communities
pg. 676
Community: all populations in a given ecosystem at a given time.
Interspecific Competition
Interspecific Competition: are interactions between individuals of different
species for an essential common resource that is in limited supply.
Interference Competition: interspecific competition that involves
aggression between individuals of different species who fight over the same
resource(s).
Exploitative Competition: interspecific competition that involves
consumption of shared resources by individuals of different species, where
consumption by one species may limit resource availability to other species.
Occurs between individuals of different populations and restrict population
growth.
It may occur in tow ways:
a) Interference Competition – fighting between 2 different species
over l limiting factor (resource).
b) Exploitive Competition – occurs when 2 different species depend
on one limiting factor to survive.
These competitions increase in intensity when the species have the same
niche or overlap.
Gause’s Principle or the principle of competitive exclusion …
G. F. Gause concluded that if resources are limited, no two species can
remain in competition for exactly the same niche indefinitely.
Impact;
- the population size for the weaker competitor species could
decline in size.
- one species could change its behaviour, so that it is able to
survive using different resources.
- individuals in one population could migrate to another habitat
where resources are more plentiful.
In any one of these cases, competition declines.
Figure 5: G.F. Gause performed experiments to observe interspecific competition in
Paramecium. a) and b) When each species was raised alone, both species reached a stable
population size. c) When the species were raised together, Paramecium aurella persisted
while the population of Paramecium caudatum declined.
To avoid or reduce competition between species for similar resources,
resource partitioning may occur.
Resource Partitioning: is the avoidance of, or reduction in, competition for
similar resources by individuals of different species occupying different nonoverlapping ecological niches.
Interspecific competition is a driving force for populations of species to
evolve adaptations that enable them to use resources for continual survival.
Predation:
- predator population will increase
- prey population will decrease
When the prey population increases, there is more food for the predator
population. The predator population will increase and cause the prey
population to decrease over time. As the prey population decreases the
predator begins to decrease because of lack of food. There is a lagging effect
in response to a decrease or increase of the prey.
Figure 4: a) A model of the predator-prey cycle.
Symbiosis
Symbiosis: various interactions in which two species maintain a close,
usually physical, association; including parasitism, mutualism, and
commensalisms.
Mutualism: a symbiotic relationship in which both organisms benefit; as
neither is harmed, it is categorized as a +/+ relationship.
Commensalism: a symbiotic relationship in which one organism benefits
and the other organism is unaffected; it is categorized as a +/0 relationship.
Parasitism: a symbiotic relationship in which one organism (the parasite)
benefits at the expense of another organism (the host), which is often harmed
but usually not killed; it is categorized as a +/- relationship.
Disruption of Community Equilibrium
Stability in biological communities exists when the resources for survival
are sustained, populations do not exceed their environment’s carrying
capacity, and interspecific interactions contribute to biodiversity.
Interspecific interactions help maintain the necessary equilibrium within
complex and dynamic natural systems that sustain communities. A variety of
disturbances can affect this equilibrium in drastic ways such as;
a) Natural Disasters
b) Exotic Non-indigenous species
The complex interactions among interdependent species of a community are
subtle yet essential for sustaining biodiversity. Often the intricacies of these
complex interactions are revealed only when conditions in ecosystems
deteriorate. Disruptions of such interactions can pose serious threats to the
ecosystem and the species that inhabit it.
Humans are by no means exempt from interventions into biological
interactions. As human populations expand, their activities affect ecological
communities and increasingly their own.