DOI: 10.5061/dryad.w6m905qqv
title: Data and MATLAB files for: Timescale analyses of fluctuations in coexisting populations of a native and invasive tree squirrel

description: 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
 (&gt; 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 (&gt; 30 months). Squirrel-temperature cospectra
 showed significant negative covariation at long timescales (&gt; 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.
description: For methods, see the following paper:
 Desharnais, R.A., Muchlinski, A.E., Ortiz, J.L., Alvidrez, R.I.,
 and Gatza, B.P. 2021. Timescale analyses of fluctuations in coexisting
 populations of a native and invasive tree squirrel. Ecology and Evolution
 (submitted).
description: DATA FILE:  SquirrelCensus.csv Summary:  These
 are the 140 consecutive monthly census data for two tree squirrels, the
 native western gray squirrel (WGS) and the non-native fox squirrel (FS).
 The data are saved as comma separated values. Each row contains an
 observation for one monthly census. Variable names appear in the first row
 and are explained below. MONTH - the numeric value for
 the month (1= Jan, 2=Feb, ..., 12=Dec)<br>YEAR - the numeric value
 for the year (2009, 2010, ..., 2021).<br>WGS - the number of western
 gray squirrels observed in the study area<br>FS - the number of fox
 squirrels observed in the study area Refer to the
 original publication for details on the study area and data
 collection.