The effect of sterilizing diseases on host abundance and distribution along environmental gradients

Abstract

This study analyses the effect of host-specific pathogens on range restriction of their hosts across environmental gradients at population margins. Sterilizing diseases can limit host range by causing large reductions in population size in what would otherwise be the central area of a species range. Diseases showing frequency-dependent transmission can also pull back a population from its disease-free margin. A wide range of disease prevalence versus abundance patterns emerge which often differ from the classical expectation of increasing prevalence with increasing abundance. Surprisingly, very few empirical studies have investigated the dynamics of disease across environmental gradients or at range limits.

Figure 1

Population size and disease prevalence along a gradient of increasing death rate. Solid thick line, total population size; solid thin line, number diseased; dashed line, prevalence (scaled from 0 to 100% on y-axis); thin dotted line, disease-free deterministic population size. (a) Classical model with density-dependent transmission and disease induced mortality; α=0.25, b=2, β=0.3. (b) Sterilizing disease with density-dependent transmission; b=1, β=0.3. (c) Sterilizing disease with frequency-dependent transmission; b=1, β=3. Throughout: d (min)=0.5, m=0.2, K=100, lattice=10×50 cells.

Figure 2

Population size and disease prevalence in a sterilizing disease with frequency-dependent transmission, along a gradient of increasing death rate. Parameters as in figure 1c, except for (a) K=1000, i.e. larger cell population numbers, (b) lattice=50×50 cells, i.e. wider area.

Figure 3

Lattice view of simulation in figure 2b. Relative number of (a) healthy and (b) diseased individuals; black is high. The mortality gradient goes from low on the left to high on the right.

Figure 4

Population size and prevalence of disease along a gradient of decreasing birth rate. Lines as in figure 1. (a) Sterilizing disease with frequency-dependent transmission; β=0.3. (b) Sterilizing disease with density-dependent transmission; β=0.03. Throughout: b (max) =1, d=0.5, m=0.2, K=100, lattice=10×50 cells.