Abstract
Non-native species having high per capita impacts in invaded communities are those that modulate resource availability and alter disturbance regimes in ways that are biologically incompatible with the native biota. In areas where it has been introduced by humans, American beaver (Castor canadensis) is an iconic example of such species due to its capacity to alter trophic dynamics of entire ecosystems and create new invasional pathways for other non-native species. The species is problematic in several watersheds within the Southern California-Northern Baja California Coast Ecoregion, a recognized hotspot of biodiversity, due to its ability to modify habitat in ways that favor invasive predators and competitors over the region's native species and habitat. Beaver was deliberately introduced across California in the mid-1900s and generally accepted as non-native to the region up to the early 2000s; however, articles promoting the idea that beaver may be a natural resident have gained traction in recent years, due in large part to the species' charismatic nature rather than by presentation of sound evidence. Here, we discuss the problems associated with beaver disturbance and its effects on conserving the region's native fauna and flora. We refute arguments underlying the claim that beaver is native to the region, and review paleontological, zooarchaeological, and historical survey data from renowned field biologists and naturalists over the past ~160 years to show that no evidence exists that beaver arrived by any means other than deliberate human introduction. Managing this ecosystem engineer has potential to reduce the richness and abundance of other non-native species because the novel, engineered habitat now supporting these species would diminish in beaver-occupied watersheds. At the same time, hydrologic functionality would shift toward more natural, ephemeral conditions that favor the regions' native species while suppressing the dominance of the most insidious invaders.
Highlights
Ecologists generally agree that biological invasions rival other factors like climate change, pollution, nutrient loading, and habitat conversion in determining contemporary ecosystem structure and function (Vitousek, 1990; Strayer, 2012; Koel et al, 2019; Liu et al, 2020)
Using 16 years of field data (2003–2018) collected by the U.S Geological Survey (USGS) in three watersheds on Marine Corps Base Camp Pendleton, at least one of which supports introduced beaver (Santa Margarita: Ervin, 2017), we developed occupancy models to identify factors that influence breeding habitat occupancy and colonization-extinction dynamics of arroyo toad in coastal San Diego County (Supplementary Table 1)
We argue that the existence of a word for beaver in the Yuman–Cochimí linguistics is best explained by the geographic distribution of the language itself, which spans from the lower Colorado River Basin into coastal and northern Baja California (Laylander, 2010)
Summary
Ecologists generally agree that biological invasions rival other factors like climate change, pollution, nutrient loading, and habitat conversion in determining contemporary ecosystem structure and function (Vitousek, 1990; Strayer, 2012; Koel et al, 2019; Liu et al, 2020). The presence of beaver has implications for protecting native aquatic species and riparian woodland in the Southern California/Northern Baja California Coast Ecoregion of North America (hereafter SC-NBC: Griffith et al, 2016; Figure 1).
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