Disease-induced chaos, coexistence, oscillations, and invasion failure in a competition-model with strong Allee effect

Biological invasions have impacts on diverse social, ecological, and economic issues. Among others, invasion success can be determined by epidemiological aspects, intraspecific dynamics as, e.g., Allee effects, and interspecific interactions as, e.g., competition. In this study, a process-based model describing competitive eco-epidemiological dynamics of two species, which are both subject to an Allee effect, is developed. Only one of the species can be infected by an infectious disease which is transmitted both, horizontally and vertically. The local dynamics of the disease-free competition model, the competition-free SI-model, and the full eco-epidemiological model are considered. In particular, it is shown that an outbreak of a disease is more likely in the absence of a competitor. Thus, competition and species richness can increase disease resistance of particular species in a community. The complete partial differential equation model is investigated both, analytically and numerically in order to determine possible impacts of the disease on the invasion dynamics. It is shown that in case of strong competition, invasion fronts are always slowed down or even reversed due to the infection for parameter regimes in which the invader is the stronger competitor in the absence of the disease while in case of weak competitive pressure, the dynamics are more complex. Besides slowing down of the invasion front, disease-induced chaos, coexistence (i.e., coexistence in a regime in which coexistence without disease would not be possible), and oscillations can occur. Furthermore, spatial spread can temporarily prevent an infected population from going extinct with potentially detrimental impacts for the resident. This happens via a (replicating) traveling pulse which pushes the competitor out of the domain. The results are discussed in order to enhance the understanding of mechanisms underlying biological invasions and to develop better management strategies for biological invasions as, e.g., selective infections.

Pathology and Epidemiology of Ceruminous Gland Tumors among Endangered Santa Catalina Island Foxes (Urocyon littoralis catalinae) in the Channel Islands, USA

In this study, we examined the prevalence, pathology, and epidemiology of tumors in free-ranging island foxes occurring on three islands in the California Channel Islands, USA. We found a remarkably high prevalence of ceruminous gland tumors in endangered foxes (Urocyon littoralis catalinae) occurring on Santa Catalina Island (SCA)—48.9% of the dead foxes examined from 2001–2008 had tumors in their ears, and tumors were found in 52.2% of randomly-selected mature (≥ 4 years) foxes captured in 2007–2008, representing one of the highest prevalences of tumors ever documented in a wildlife population. In contrast, no tumors were detected in foxes from San Nicolas Island or San Clemente Island, although ear mites (Otodectes cynotis), a predisposing factor for ceruminous gland tumors in dogs and cats, were highly prevalent on all three islands. On SCA, otitis externa secondary to ear mite infection was highly correlated with ceruminous gland hyperplasia (CGH), and tumors were significantly associated with the severity of CGH, ceruminous gland dysplasia, and age group (older foxes). We propose a conceptual model for the formation of ceruminous gland tumors in foxes on SCA that is based on persistent, ubiquitous infection with ear mites, and an innate, over exuberant inflammatory and hyperplastic response of SCA foxes to these mites. Foxes on SCA are now opportunistically treated with acaricides in an attempt to reduce mite infections and the morbidity and mortality associated with this highly prevalent tumor.