Characterizing reservoirs of infection and the maintenance of pathogens in ecosystems

Abstract

We use a previously published compartmental model of the dynamics of pathogens in ecosystems to define and explore the concepts of maintenance host, maintenance community and reservoir of infection in a full ecological context of interacting host and non-host species. We show that, contrary to their current use in the literature, these concepts can only be characterized relative to the ecosystem in which the host species are embedded, and are not 'life-history traits' of (groups of) species. We give a number of examples to demonstrate that the same (group of) host species can lose or gain maintenance or reservoir capability as a result of a changing ecosystem context, even when these changes primarily affect non-hosts. One therefore has to be careful in designating host species as either maintenance or reservoir in absolute terms.

Keywords: ecology; epidemiology; maintenance community; maintenance host; reservoir of infection.

Figure 1.

An illustration of the possible effects of ecosystem changes on the epidemiology of a pathogen. (a) All host types belong to the maximal maintenance community. There are three minimal maintenance communities outlined with rectangles. There are two sources of infection for the target. (b) Two non-host and two host species have been removed from the ecosystem. Some host species no longer support the pathogen, and there is just one source of infection for the target. Red, infected host species; magenta, uninfected host species; green, non-hosts; yellow (t), target; red (s), source. Thin lines indicate ecological interactions. Black and grey arrows indicate active and inactive transmission pathways, respectively. Blue rectangles enclose minimal maintenance communities. (Online version in colour.)

Figure 2.

The prey–predator ecosystem Ω discussed in §4.1 and its 14 sub-systems. (a) The static perspective: those shown with a red border have ${\mathcal{R}}_{{\mathit{\Omega }}_m}>1$; those shown with a green border have ${\mathcal{R}}_{{\mathit{\Omega }}_m}<1$. (b) The dynamic perspective: those shown with a red border have ${\mathcal{R}}_0({\mathit{\Omega }}_m)>1$; those shown with a green border have ${\mathcal{R}}_0({\mathit{\Omega }}_m)<1$. (Online version in colour.)

Figure 3.

The ecosystem Ω discussed in §4.2 and its 20 ecologically feasible sub-systems. (a) The static perspective: those shown with a red border have ${\mathcal{R}}_{{\mathit{\Omega }}_m}>1$; those shown with a green border have ${\mathcal{R}}_{{\mathit{\Omega }}_m}<1$. (b) The dynamic perspective: those shown with a red border have ${\mathcal{R}}_0({\mathit{\Omega }}_m)>1$; those shown with a green border have ${\mathcal{R}}_0({\mathit{\Omega }}_m)<1$. The sub-system Ω_m = {1, 2, 3, 5} is ecologically feasible, as all population densities are positive, but ecologically unstable. Hence it is shown with a magenta border. (Online version in colour.)

Figure 4.

(a) The ecosystem Ω discussed in §4.3 and its 537 ecologically stable feasible sub-systems. White, non-hosts only; green, ${\mathcal{R}}_{{\mathit{\Omega }}_m}<1$ and ${\mathcal{R}}_0({\mathit{\Omega }}_m)<1$; yellow, ${\mathcal{R}}_0({\mathit{\Omega }}_m)<1<{\mathcal{R}}_{{\mathit{\Omega }}_m}$; blue, ${\mathcal{R}}_{{\mathit{\Omega }}_m}<1<{\mathcal{R}}_0({\mathit{\Omega }}_m)$; red, ${\mathcal{R}}_{{\mathit{\Omega }}_m}>1$ and ${\mathcal{R}}_0({\mathit{\Omega }}_m)>1$. (b) A detail showing the relationship between some selected sub-systems highlighted as square symbols. These are referred to in the text by: moving left to right at level 8 Ω_8g, Ω_8y, Ω_8r1, Ω_8r2; at level 7 Ω_7y, Ω_7b1, Ω_7r, Ω_7g, Ω_7b2; at level 6 Ω_6r1, Ω_6y, Ω_6g, Ω_6r2, Ω_6b; and at level 5 Ω_5g1, Ω_5b, Ω_5g2, Ω_5y. For more details, see the electronic supplementary material, table S3. Thick blue lines show changes due to adding or subtracting a non-host species; thick black lines show changes due to adding or subtracting a host species. Labels on the vertical axes indicate the number of species in the sub-system. Labels on the horizontal axes indicate the species number ((a) only). (Online version in colour.)