Lost in urban space: assessment of the reproductive success of the native tree Parkinsonia aculeata (FABACEAE) across urbanization and isolation gradient in Bahía Blanca, Argentina

Abstract

Urbanization transforms natural environments into mosaics of permeable and impermeable surfaces, resulting in habitat loss, fragmentation, increased temperatures and pollution. For biotic-pollinated plants, these changes can negatively affect reproductive success. This study analyzed the reproductive success of a native tree, Parkinsonia aculeata, across a gradient of urbanization and reproductive isolation in Bahía Blanca, Argentina. Twenty sites with reproductive individuals were selected, where urbanization and reproductive isolation levels were calculated. During flowering phenology, pollination treatments, floral visitor surveys, and fruit collection were conducted. Based on the collected data, reproductive success, seed predation, floral visitor richness, diversity, and visitation frequency (distinguishing between native and exotic species) were estimated and related to the urbanization and isolation gradient using regression-based statistical models (linear, generalized linear, and generalized linear mixed models) and structural equation modeling. Bees accounted for 97.3% of floral visits. We recorded 19 bee species, with most visits performed by two species of the tribe Eucerini (Apidae) and six species/morphospecies of the tribe Megachilini (Megachilidae). Seed predation was caused by Penthobruchus germaini, a bruchine specialist beetle (Bruchidae). Urbanization did not affect floral visitor richness or diversity, but it was associated with a higher frequency of exotic bee visits. Moreover, it had no effect on fruit and seed production or seed predation rates. Reproductive isolation had negative effects on fruit and seed production, and positive effects on seed predation. Therefore, reproductive isolation presents the main challenge for the species, since isolated individuals exhibit the lowest levels of reproductive success and highest seed predation rates.