Data and MATLAB files for: Timescale analyses of fluctuations in coexisting populations of a native and invasive tree squirrel

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.