Phenology and temporal niche overlap differ between novel, exotic- and native-dominated grasslands for plants, but not for pollinators

The temporal dynamics of ecosystem functioning and services are largely regulated by seasonal patterns, or phenology, of Earth’s biota. Recent evidence suggests that species in exotic-dominated systems may exhibit altered phenology relative to species in native-dominated systems. However, whether phenology of plants and pollinator communities differ between existing novel, exotic-dominated ecosystems and native-dominated systems remains poorly understood. We tested whether exotic- and native-dominated grasslands differed across a growing season for plant production, species and functional diversity, and bee pollinator abundances and diversity in the Northern Great Plains tallgrass prairie region, Iowa, USA. We found that niche overlap and some aspects of phenology in exotic-dominated systems were different from native systems for plants, but not for pollinators. Exotic dominated grasslands peaked earlier in biomass production, and exhibited consistently lower levels of species and functional diversity, including higher levels of proportion of C₃ biomass, than native grasslands. These results were related to higher levels of temporal niche overlap in exotic grasslands, where species were similar across the growing season in exotic, but not in native, grasslands. Surprisingly, bee pollinator communities did not differ between exotic and native grasslands despite higher forb:grass ratios in native sites, implying that factors other than exotic-native status may be important in structuring pollinator communities at this scale in highly fragmented landscapes. Our results imply that phenological differences between communities in novel and native grasslands can have important consequences to diversity and ecosystem functioning, and that exotic or native status of plant species should be considered in future studies of phenology.