Lecture 14 - Species Interactions, 11/02
Species Interactions or “The Balance of Nature”
(Two-way) species interactions
Community: a group of (actually or potentially) interacting species living in the same place.
Bound together by a network of influences
Mutualism: Species interaction where both benefit.
Facultative – interaction not essential for either’s survival.
Obligate – interaction essential for mutual survival.
Commensalism: One species benefits, the other is unharmed or unaffected.
Competition: When two species require the same resource.
Struggle for existence
Uniqueness of niche
Competitive exclusion (Gause’s Principle)
Species using resources in identical fashions cannot co-exist
One species per niche
Types of competition
Intraspecific – two populations of a single species (Indirect competition).
Interspecific - multiple species (direct competition, interference).
Competition coefficient, α
"dN1/dt=r1 N1 (1-(N1-α12N2)/K1)"
α = 1, individual of species 2 has the same effect on per capita growth rate as an individual of species 1.
α = 0, species 2 has no effect on per capita growth rate.
0< α < 1, intraspecific competition is stronger then interspecific competition.
α > 1, interspecific competition is stronger then intraspecific competition.
N1 = K1 – "α12N2" ,at equilibrium conditions – this is the equation of a straight line y=mx+b.
Net zero growth isoclines.
: For species 1
: For species 2
Combination of species 1 and 2 at which their populations are at equilibrium.
Only occurs when isoclines cross
Results of theory
Coexistence of competing species requires interspecific competition to be weaker than intraspecific competition.
Species will limit their own growth before the reach densities that can exclude competitors.
Time to equilibrate depends on r1 & r2.
Equilibrium point depends on α and carrying capacities.