Resource Competition Tilman (1982) developed a model for how plants compete for a single resource-for instance, nitrogen. If B(t) denotes the total biomass at time t and R(t) is the amount of the resource available at time t, then the dynamics are described by the following system of differential equations:
dtdB=B[F(R)−M]dtdR=a(S−R)−CBF(R)
The first equation describes the rate of change of biomass, where the function f(R) describes how the species growth rate depends on the resource, and m is the per capita plant mortality rate. The second equation describes the resource dynamics; the constant S is the maximal amount of the resource in a given habitat. The rate of resource supply (dR/dt) is assumed to be proportional to the difference between the current resource level and the maximal amount of the resource; the constant a is the constant of proportionality. The term cBf(R) describes the resource uptake by the plants; the constant c can be considered a conversion factor.
f(R)=k+RdR
In what follows, we assume that f(R) follows the Monod growth function
where d and k are positive constants.
(a) Find all equilibria. Show that if d>m and S>mk/(d−m), then there exists a nontrivial equilibrium.
(b) Sketch the zero isoclines for the case in which the system admits a nontrivial equilibrium, and sketch the direction of the vector field on the isoclines and in the regions between them.