An introduced plant is associated with declines in terrestrial arthropods, but no change in stream invertebrates

Abstract

Riverine systems often spread non-native species, yet the co-occurring impacts of introduced riparian vegetation on aquatic- and terrestrial-derived resources are unknown. We compared aquatic and terrestrial arthropod communities and their flux into and out of streams in riparian reaches invaded and uninvaded by Robinia neomexicana, a woody plant introduced to a western Colorado watershed. We found that invaded reaches had fewer terrestrial arthropods collected off foliage, conceivably because of the plant’s later leaf-out phenology. Overall, seasonal and annual factors best described terrestrial and aquatic arthropod communities. However, when we evaluated vegetation and stream characteristics in lieu of season and year, we found terrestrial arthropod biomass and richness were negatively related to cover of R. neomexicana and positively related to vegetative cover, forb cover, and vertical vegetation structure. Our results suggest ecosystems respond to landscape variation differently, where directly related food web components (i.e., terrestrial arthropods on introduced vegetation) respond stronger than more distally related constituents (i.e., aquatic insects).

An introduced plant affects aquatic-derived carbon in the diets of riparian birds

“…Significant shifts in the diets of two songbirds lends support to our hypothesis that some bird species would rely more on aquatic insects at invaded sites ( Fig 2 ; Table 1 ). These species, Virginia’s warbler and warbling vireo, are foliage gleaners that may be more impacted by the reductions in terrestrial insects associated with invasion [ 21 , 22 , 35 , 48 ]. Although we did not detect significant diet shifts associated with trophic position (i.e., driven by δ 15 N) for any bird species evaluated, this is consistent with findings of Riedl et al [ 35 ], which found no significant difference in the abundance of predaceous or parasitic arthropods between sites.…”

“…In our system, delayed timing of R . neomexicana leaf-out likely provides less foliage to support arthropod production in late spring [ 35 ]. Our study was the first to use SIA to detect songbird diet shifts in response to a riparian plant invasion.…”

“…We considered the two species of flycatchers, Cordilleran flycatcher ( Empidonax occidentalis) and dusky flycatcher ( Empidonax oberholseri) , as a single unit (flycatcher). Analyses were conducted separately for each year to account for known annual variation in arthropod communities in this watershed [ 35 ].…”

“…We predicted that songbird fecal samples in invaded sites would have δ 13 C signatures more similar to aquatic-derived prey signatures, indicating decreased reliance on terrestrial insects. Non-native vegetation often supports depauperate terrestrial arthropod communities compared to native plants [ 21 , 22 , 35 ], and songbirds often forage in proportion to prey availability [ 36 , 37 ]. Therefore, we hypothesized that fecal samples from invaded sites would be less enriched in δ 15 N because vegetation invasions often disproportionately reduce higher trophic level arthropods compared to lower trophic level taxa [ 22 , 38 ].…”