Timescale Analyses of Fluctuations in Coexisting Populations of a Native
and Invasive Tree Squirrel
Abstract
1. Competition from invasive species is an increasing threat to
biodiversity. In Southern California, the western gray squirrel (Sciurus
griseus, WGS) is facing increasing competition from the fox squirrel
(Sciurus niger, FS), an invasive congener. 2. We used spectral methods
to analyze 140 consecutive monthly censuses of WGS and FS within a 11.3
ha section of the California Botanic Garden. Variation in the numbers
for both species and their synchrony was distributed across long
timescales (> 15 months). 3. After filtering out annual
changes, concurrent mean monthly temperatures from nearby Ontario
Airport (ONT) yielded a spectrum with a large semiannual peak and
significant spectral power at long timescales (> 30
months). Squirrel-temperature cospectra showed significant negative
covariation at long timescales (> 35 months) for WGS and
smaller significant negative peaks at 6 months for both species. 4.
Simulations from a Lotka-Volterra model of two competing species
indicates that the risk of extinction for the weaker competitor
increases quickly as environmental noise shifts from short to long
timescales. 5. We analyzed the timescales of fluctuations in detrended
mean annual temperatures for the time period 1915-2014 from 1218
locations across the continental USA. In the last two decades,
significant shifts from short timescales to long timescales have
occurred, changing from less than 3 years to 4-6 years. 6. Our results
indicate that (i) population fluctuations in co-occurring native and
invasive tree squirrels are synchronous, occur over long timescales, and
may be driven by fluctuations in environmental conditions; (ii) long
timescale population fluctuations increase the risk of extinction in
competing species, especially for the inferior competitor; and (iii) the
timescales of interannual environmental fluctuations may be increasing
from recent historical values. These results have broad implications for
the impact of climate change on the maintenance of biodiversity.