The impact of urban forest corridors on the richness of urban bird populations in Baltimore forest patches

Background/Question/Methods

Global habitat loss reduces biodiversity and disrupts ecosystem processes. The remaining fragmented habitat contains fewer niches and resources and reduces the capacity to support viable populations. The cumulative decline in habitat quality that accompanies fragmentation may further reduce species richness and abundance beyond the original habitat loss. While understanding the patterns and processes of habitat loss remain critical topics, research increasingly focuses on mitigating the consequences of habitat loss. Increasing landscape connectivity has long been thought to limit the negative effects of habitat loss and fragmentation, and corridors linking habitat patches are often considered a practical solution. However, empirical tests of whether corridors increase biodiversity in the patches they link remain rare. We tested the hypothesis that forest patches connected by corridors have higher forest-breeding bird species richness than unconnected, isolated patches. We surveyed birds with10-min point counts from May 20 to July 15, 2020 in Baltimore at 50 sites: 32 isolated forest patches and 38 patches connected by a corridor. To account for factors known to affect bird species richness, we calculated patch size and landscape-level variables within 100-m and 250-m of point count sites and included them as covariates in analyses. We fit candidate models for species richness using linear-mixed models and used AIC model selection to identify the best-fit models given the data.

Results/Conclusions

At the 100-m scale, the AIC top model indicated that bird species richness was lower at higher levels of impervious surface and was higher in larger patches. After accounting for these effects, bird species richness was comparable in connected (Tukey p = 0.893; mean = 9.99, 95% CI = 9.1, 11.0) and isolated patches (mean = 10.3, 95% CI = 9.4, 11.2). At the 250-m scale, the AIC best model showed that bird species richness was lower with higher coverage of impervious surface cover. After accounting for covariate effects, species richness was markedly higher in connected (t = -5.67, p < 0.001; mean = 14.9, 95% CI = 13.7, 16.0) compared to isolated patches (mean = 10.8, 95% CI = 9.9, 11.6). Our findings provide insights about connectivity that can inform conservation and management planning to enhance biodiversity in cities. For example, Baltimore already plans to expand tree canopy throughout the city. Because forest corridors increase canopy cover, existing corridors maintenance and creation of new ones should be a part of this initiative and will likely increase bird species richness.