Enhancing biodiversity: historical ecology and biogeography of the Santa Catalina Island ground squirrel, Otospermophilus beecheyi nesioticus

Abstract

People have influenced Earth's biodiversity for millennia, including numerous introductions of domestic and wild species to islands. Here, we explore the origins and ecology of the Santa Catalina Island ground squirrel (SCIGS; Otospermophilus beecheyi nesioticus), one of only five endemic terrestrial mammals found on California's Santa Catalina Island. We synthesized all records of archaeological/palaeontological SCIGS, conducted radiocarbon dating and stable isotope analysis of the potentially earliest SCIGS remains and performed genetic analysis of modern SCIGS. Squirrels were not identified in island palaeontological deposits, but at least 12 island archaeological sites contain SCIGS bones, including some that are butchered or burned. All directly dated SCIGS bones are Late Holocene in age and younger than approximately 1290 cal BP. The first mitochondrial genome for modern Otospermophilus beecheyi and 15 modern SCIGS mitogenomes document at least one introduction of squirrels. Stable isotope data indicate variable SCIGS diets and potential subsidies from marine environments to terrestrial plants consumed by some individuals. We cannot rule out a natural overwater dispersal, but the earliest SCIGS remains post-date the earliest evidence for people by several millennia and, along with other lines of evidence, support a human-assisted translocation of squirrels during the Late Holocene. These data illustrate the important role of Indigenous people in shaping and enhancing island biodiversity and ecology around the world.

Keywords: California; Islands; human-assisted dispersal; translocation; zooarchaeology.

Figure 1.

Location of archaeological sites (blue circles) with SCIGS bones and modern genetic samples (green triangles and squares) on Santa Catalina Island. All sites contain direct radiocarbon dates on SCIGS bones except CA-SCAI-29, CA-SCAI-32, CA-SCAI-564 and CA-SCAI-CWC. Inset of western United States with box around area containing Santa Catalina Island.

Figure 2.

Santa Catalina Island ground squirrel (photo by Julie King).

Figure 3.

Radiocarbon age distributions for SCIGS specimens reported in table 2. Each date listed by radiocarbon sample number and archaeological site. These are 95% confidence intervals for each date and the unmodelled calibrated age using the IntCal20 terrestrial calibration curve.

Figure 4.

Bulk bone collagen δ¹³C and δ¹⁵N values of archaeological and modern Santa Catalina Island ground squirrels (SCIGS; Otospermophilus beecheyi nesioticus). Filled circles are isotopic results for archaeological SCIGS specimens from eight Santa Catalina Island archaeological sites: (1) CA-SCAI-118, (2) CA-SCAI-27, (3) CA-SCAI-106, (4) CA-SCAI-137, (5) CA-SCAI-CCR (camp cactus road), (6) CA-SCAI-17, (7) CA-SCAI-26 and (8) CA-SCAI-45. Open circle represents mean δ¹³C and δ ¹⁵N values of modern ground squirrels (n = 9) and black rats (n = 4) from Newsome et al. [42]; error bars show standard deviation around the mean. These data come from bulk tissue analysis of bone collagen extracted from remains found in bald eagle nests on Santa Catalina Island. Following Dombrosky [51], we have corrected the modern SCIGS/black rat δ¹³C data for temporal shifts in δ¹³C values of atmospheric CO₂ (i.e. the Suess effect) by adding 1.8 ‰.

Figure 5.

Genetic analysis of Santa Catalina Island ground squirrels (SCIGS). (a) Median joining network analysis of mitochondrial genomes from present-day SCIGS. The star-shaped pattern supports a single introduction. However, the divergent haplotype, Ob_10, may represent an additional introduction. (b) Maximum likelihood phylogeny with IQTree of partial cytochrome b gene (approx. 795 bp) with comparative data from [48] and [49]. Nodes with UFboot values 95% and above and SH-aLRT values above 80% are labelled with stars to indicate strong support. Following [48], the tree was rooted with Callospermophilus lateralis (NC_031210 not shown). SCIGS fall within the southern O. beechyi group and are closely related to O. beechyi sampled in southern California and Baja.