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# The $P^*$ rule in the stochastic Holt-Lawton model of apparent competition

The author is supported by U.S. National Science Foundation grant DMS-1716803

• In 1993, Holt and Lawton introduced a stochastic model of two host species parasitized by a common parasitoid species. We introduce and analyze a generalization of these stochastic difference equations with any number of host species, stochastically varying parasitism rates, stochastically varying host intrinsic fitnesses, and stochastic immigration of parasitoids. Despite the lack of direct, host density-dependence, we show that this system is dissipative i.e. enters a compact set in finite time for all initial conditions. When there is a single host species, stochastic persistence and extinction of the host is characterized using external Lyapunov exponents corresponding to the average per-capita growth rates of the host when rare. When a single host persists, say species $i$, a explicit expression is derived for the average density, $P_i^*$, of the parasitoid at the stationary distributions supporting both species. When there are multiple host species, we prove that the host species with the largest $P_i^*$ value stochastically persists, while the other host species are asymptotically driven to extinction. A review of the main mathematical methods used to prove the results and future challenges are given.

Mathematics Subject Classification: Primary: 92D25, 60J05.

 Citation: • • Figure 1.  Stochastic persistence and the $P^*$ rule for the host-parasitoid model (1). In A, there is $k = 1$ host species and the condition for stochastic persistence is met. In B, there are $k = 4$ host species which only differ in the variance of their $R_i(t)$ terms. Parameter values: $a_i(t) = 0.1$ for all $i$ and $t$, $R_i(t) = 0.9+1.1\beta^R_i(t)$ where $\beta_i^R(t)$ are $\beta$ distributed with both scale parameters $= k+1-i$, and $I(t) = 0.1+0.9\beta^I(t)$ where $\beta^I(t)$ are $\beta$ distributed with both scale parameters $= 2$

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