Types of species interaction include:
Competition: each species inhibits the population of the other.
Predation or Parasitism: one species benefits, the other is harmed.
Several types of symbiosis (living together), which often evolve from
parasitism to commensalism to a mutualism from which both species benefit.
Outcomes of species interactions include niche subdivision and specialization.
TYPES OF INTERACTION
In this chart,
+ means a benefit (or population increase)
- means a net harm (or population decrease)
0 means no net effect on population size
|
|
| INTERACTION
| Effect on
species 1
| Effect on
species 2
| |
| Competition |
- | -
| Both species are inhibited |
| Amensalism |
0 | -
| Only one species is inhibited |
| Neutralism | 0
| 0
| Neither species is inhibited |
| Predation | +
| -
| One species increases,
the other is harmed |
| Parasitism | +
| - |
| Commensalism | +
| 0
| Benefits one species, neutral to
the other |
| Mutualism | +
| +
| Interaction benefits both species |
Competition occurs when each species reaches a smaller population size
than it would by itself. Most often, both species are
competing for a limited resource like food or space. The
mathematical equations of Lotka & Volterra describe competition:
dN1/dT for species 1 = r1 N1 ( K1 -
N1 - aN2 ) / K1
dN2/dT for species 2 = r2 N2 ( K2 -
N2 - bN1 ) / K2
where N1 and N2 are population sizes and
K1 and K2 are carrying capacities.
Depending on conditions, the values of coefficients a and b
determine one of four possible outcomes:
- Species 1 always wins (species 2 becomes locally extinct)
- Species 2 always wins (species 1 becomes locally extinct)
- Either species can win, depending on which is initially more abundant
- Both species will coexist indefinitely
Predation and parasitism: In these interactions, the prey
or host has a beneficial effect on the predator or parasite,
but the latter reduces the population of the prey or host. (Parasites are
generally much smaller than their hosts, but predators are often similar
in size to their prey.) Many simple predator-prey interactions
(one predator, one prey species) are unstable: predators that kill too
many prey will starve. Some interactions result in fluctuating cycles
of abundance, but most are stabilized if predators can switch to
different prey species, choosing whichever is most abundant.
Symbiosis: two species living in close contact. The one that houses
the other (provides habitat) is called the host.
- Parasitism: The parasite benefits from the interaction, but
the host is harmed. Most parasites are r-selected; many have
hooks, suckers, or other adaptations for holding on. Highly evolved
parasites often have complex life cycles; they are usually very small or
very flat because these adaptations facilitate absorptive nutrition and
minimize detection, dislodging, and harm to the most.
Organs that are dispensible (especially sense organs, nervous systems,
and organs of digestion, respiration, or excretion) are lost in highly
evolved parasites.
- Commensalism: one species benefits; the host receives equal
benefit and harm, so its population size is unaffected.
- Mutualism: both species benefit from the interaction.
Consequences of species interactions:
- Species often specialize, subdividing niches into smaller ones,
each dominated by a different species.
- The interaction may evolve to be more favorable (e.g., from parasitism
to commentalism to mutualism).
- Species that are less successful may change to new ways of life (new niches).
- Species that can do none of these things become extinct.
|