Mule deer were first introduced to Santa Catalina Island, California, in the early 1930s and persist today. Other feral, non-native ungulates have been eradicated (goats, pigs) or significantly reduced in numbers (bison) over the past two decades. Effective management of the deer population is necessary to protect the island’s biodiversity but is dependent upon reliable estimates of population density and demography. We used annual summer spotlight counts, conducted in eight of the past ten years, to estimate deer densities in the island interior. In 2021, we also surveyed transects in the area around Avalon, the largest town on the island. Distance sampling (Program DISTANCE) was used to model density based on line transect data. Island-wide densities varied from 6.3 to 16.9 deer per km2, with an average of 10.2 per km2, and were positively correlated with July-June rainfall during the preceding year. Most (77-96%) of the identifiable deer were adults and most adults were does (58-75%). Deer were spotted most frequently in island chaparral and coastal sage scrub vegetation, the most common vegetation cover types along transects, whereas the use of grasslands and man-made and non-native habitats varied among years. The estimated deer density in Avalon (65.7 per km2) was six to nine times higher than estimates for the interior transects in 2021. The high density of deer near Avalon suggests that gardens, landscaping, golf courses, and intentional feeding subsidize deer numbers in town that have the potential to spill over into the more natural areas of the island.

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.

Ear mites (Otodectes cynotis) and ear canal tumors are highly prevalent among federally endangered Island foxes (Urocyon littoralis catalinae) living on Santa Catalina Island off the coast of Southern California. Since studies began in the 1990s, nearly all foxes examined were found to be infected with ear mites, and ceruminous gland tumors (carcinomas and adenomas) were detected in approximately half of all foxes ≥ 4 years of age. We hypothesized that reduction of ear mite infection would reduce otitis externa and ceruminous gland hyperplasia, a risk factor for tumor development. In this study, we conducted a randomized field trial to assess the impact of acaricide treatment on ear mite prevalence and intensity of infection, otitis externa, ceruminous gland hyperplasia, and mite-specific IgG and IgE antibody levels. Treatment was highly effective at eliminating mites and reducing otitis externa and ceruminous gland hyperplasia, and mite-specific IgG antibody levels were significantly lower among uninfected foxes. Ceruminous gland hyperplasia increased in the chronically infected, untreated foxes during the six month study. Our results provide compelling evidence that acaricide treatment is an effective means of reducing ear mites, and that mite removal in turn reduces ear lesions and mite-specific IgG antibody levels in Santa Catalina Island foxes. This study has advanced our understanding of the underlying pathogenesis which results in ceruminous gland tumors, and has helped inform management decisions that impact species conservation.