Ground-dwelling Invertebrate Community Composition Changes between Coastal Sage Scrub Community of San Diego and Tijuana with Urbanization

Invertebrates are some of the most diverse and abundant groups of terrestrial organisms. They provide important functions within an ecosystem and can rapidly respond to environmental changes, such as urbanization, making them excellent ecological indicators for ecosystem function and health. Urbanization, and the disturbance and introduction of exotic plants often associated with it, affects invertebrate taxa, changing community composition, structure, diversity, and abundance of invertebrates. Some invertebrate taxa can exploit these changes and increase in abundance; while others exhibit sensitivity and are at risk of extirpation. The San Diego and Tijuana region has been increasing in urbanizing pressure for decades. Tijuana, in particular, has experienced exponential population growth over the past 30 years, and urbanization threatens the integrity of much of the remaining coastal sage scrub, an endangered and rare ecosystem unique to the coasts of southern California and northwestern Mexico composed of many coastal deciduous shrubs and annual wildflowers. However, the invertebrates currently residing in these semi-disturbed landscapes have rarely been sampled. In this study, ground-dwelling invertebrates from a native plant nursery in an urbanizing area of Tijuana at Vivero Hormiguitas and a protected site in Border Field State Park in San Diego were sampled to compare the changes in coastal sage scrub ground-dwelling invertebrate community composition in response to urbanization. Beetle abundance and richness decreased in the urbanizing site in Tijuana, while spider abundance and richness increased in Tijuana compared to the protected site in San Diego. Particularly, we noted decreases in Tenebrionidae abundance and failed to detect Silphidae, suggesting slower decomposition rates in the urbanizing Tijuana site. Although spider abundance in general increased, mostly due to increased prevalence of one genus, Xysticus, Gnaphosidae abundance decreased in the Tijuana site. Lastly, non-native invertebrate taxa, especially isopods, showed dramatic increases in abundance at the urbanizing Tijuana site, contributing to overall increases in invertebrate abundance in Tijuana compared to the protected site in San Diego. These results indicate that urbanization has negatively affected ecosystem functions in coastal sage scrub communities and that it has drastically impacted the diversity and health of this rare ecosystem, much of which is already fragmented, and actions need to be taken to protect this rare habitat as much of its range continues to become more urbanized. KEYWORDS: Beetle; Coastal Sage Scrub; Community Composition; Disturbance; Invasive Species; Invertebrates; San Diego; Tijuana; Spider; Urbanization