Eastern fox squirrels have been introduced to many regions within the western United States including areas within California, Idaho, Montana, Oregon, and Washington. In California, fox squirrels have been introduced to San Francisco, Fresno, . The geographic range of the eastern fox squirrel has expanded greatly both through natural dispersal and additional intentional introductions by humans. The fox squirrel has replaced the native western gray squirrel in certain habitats while the two species coexist in other habitats. Western gray squirrels exist by themselves in a third type of habitat even though eastern fox squirrels have occupied adjacent habitat for about 30 years. Habitat Suitability Models (HSMs) have been used to predict the presence and/or abundance of a particular species within a habitat. Analysis of 9 habitat variables using Discriminant Analysis produced a HSM where replacement, coexistence, and exclusion habitats can be identified using only 3 of the 9 habitat variables: average height of ground cover, percent canopy cover, and percent of total trees that are deciduous. A field study underway for over two years at Rancho Santa Ana Botanic Garden in Claremont, CA is testing the validity of the HSM with the study site predicted to be a long-term coexistence habitat. The ratio of western gray squirrels to eastern fox squirrels has gone from 3:1 shortly after the fox squirrel entered the habitat to 1:1, but after 2.5 years of coexistence both species are commonly observed in the study site. The HSM is also being tested through the analysis of additional habitats. Although coexistence of the two species is possible for many decades, the number of western gray squirrels in coexistence habitats is often small, coexistence habitats are geographically isolated from each other, and the probability of extinction of western gray squirrels from these habitats could be high. We are also looking for new habitats which could support western gray squirrels if individuals were moved to these habitats.

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.

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.