Avian cholera, a threat to the viability of an Arctic seabird colony?

Abstract

Evidence that infectious diseases cause wildlife population extirpation or extinction remains anecdotal and it is unclear whether the impacts of a pathogen at the individual level can scale up to population level so drastically. Here, we quantify the response of a Common eider colony to emerging epidemics of avian cholera, one of the most important infectious diseases affecting wild waterfowl. We show that avian cholera has the potential to drive colony extinction, even over a very short period. Extinction depends on disease severity (the impact of the disease on adult female survival) and disease frequency (the number of annual epidemics per decade). In case of epidemics of high severity (i.e., causing >30% mortality of breeding females), more than one outbreak per decade will be unsustainable for the colony and will likely lead to extinction within the next century; more than four outbreaks per decade will drive extinction to within 20 years. Such severity and frequency of avian cholera are already observed, and avian cholera might thus represent a significant threat to viability of breeding populations. However, this will depend on the mechanisms underlying avian cholera transmission, maintenance, and spread, which are currently only poorly known.

Figure 1. Female common eider carcasses following an avian cholera outbreak, East Bay colony, Southampton Island, Canada (photo: S. Descamps).

Figure 2. Population model for Common eiders breeding at the East Bay colony, Southampton Island, Canada.

A. Life cycle of common eiders (Southampton Island, Nunavut, Canada) based on three age classes. The population matrix A contains the vital rates and projects the population from time t to t+1. The fertility parameter was calculated as the product between the breeding probability (BP), the average breeding success (BS) and the average number of hatchlings per breeding female (f). S_A represents adult survival (survival from 2 years of age onwards), S_Y survival of yearlings (from 1 to 2 years of age) and S_H survival of hatchlings (from hatching to 1 year of age). We considered four different periods based on cholera severity; demographic parameters for each period are shown in B.

Figure 3. Long term stochastic growth rates of the East Bay common eider population (Southampton Island, Canada).

The stochastic growth rates (log-λ_s) is described as a function of the frequency (number of epidemics per decade) and duration (number of years of epidemics in a row) of avian cholera epidemics. The growth rates were calculated from a stochastic model with two states: no cholera outbreak and cholera outbreak. For the state “cholera outbreak”, we considered different severities (i.e., different level of adult mortality) of epidemics: a low severity as observed in 2005 (A), a moderate severity as observed in 2007 (B) and a high severity as observed in 2006 (C). The black lines denote log-λ_s = 0; colors correspond to different growth rates ranging from blue (positive growth rate, log-λ_s>0) to red (negative growth rate, log-λ_s<0). Areas to the right of the black lines indicate combinations of epidemic frequency and duration that are not sustainable for the population (log-λ_s<0). The white areas represent impossible combinations of epidemic frequency and duration.

Figure 4. Risk of quasi-extinction for the East Bay common eider colony (Southampton Island, Canada).

Figure 5. Winter aggregation of common eiders in North Atlantic (photo: H.G. Gilchrist).