Is there a disease‐free halo at species range limits? The codistribution of anther‐smut disease and its host species

While disease is widely recognized as affecting host population size, it has rarely been considered to play a role in determining host range limits. Many diseases may not be able to persist near the range limit if host population density falls below the critical threshold level for pathogen invasion. However, in vector‐ and sexually transmitted diseases, pathogen transmission may be largely independent of host density and theory demonstrates that diseases with frequency‐dependent transmission may persist in small populations near the range limit. Empirical studies of disease at species range limits have lagged behind the theory, and to date, no previous study has tested the hypothesis that vector or sexually transmitted diseases can be maintained at host range limits. We studied the distribution of anther‐smut disease, a sterilizing pollinator‐transmitted disease, on four alpine plant species to determine whether disease was present at the host range limits. We found that host abundance declined towards the elevational range limits, and disease extended to the most extreme elevational range limits in three of the four host species. Maximum likelihood estimation of the magnitude of the disease‐free halo showed that it was small or nonexistent for all host species. Moreover, disease prevalence within populations was often higher nearer the host's range limit than in the range centre and was independent of host density. Synthesis . Our results show that diseases where transmission is frequency‐dependent have the potential to affect host distributions not just in theory, but also in real world populations.