Invasive plant architecture alters trophic interactions by changing predator abundance and behavior

TLDR: The results indicate that invasive plants that change the architecture of native vegetation can substantially impact native food webs via nontraditional plant → predator →-consumer linkages.

Abstract: As primary producers, plants are known to influence higher trophic interactions by initiating food chains. However, as architects, plants may bypass consum- ers to directly affect predators with important but underappreciated trophic ramifications. Invasion of western North American grasslands by the perennial forb, spot- ted knapweed (Centaurea maculosa), has fundamentally altered the architecture of native grassland vegetation. Here, I use long-term monitoring, observational studies, and field experiments to document how changes in vegetation archi- tecture have affected native web spider populations and predation rates. Native spiders that use vegetation as web substrates were collectively 38 times more abundant in C. maculosa-invaded grasslands than in uninvaded grass- lands. This increase in spider abundance was accompanied by a large shift in web spider community structure, driven primarily by the strong response of Dictyna spiders to C. maculosa invasion. Dictyna densities were 46-74 times higher in C. maculosa-invaded than native grasslands, a pattern that persisted over 6 years of monitoring. C. mac- ulosa also altered Dictyna web building behavior and foraging success. Dictyna webs on C. maculosa were 2.9- 4.0 times larger and generated 2.0-2.3 times higher total prey captures than webs on Achillea millefolium, their primary native substrate. Dictyna webs on C. maculosa also captured 4.2 times more large prey items, which are crucial for reproduction. As a result, Dictyna were nearly twice as likely to reproduce on C. maculosa substrates compared to native substrates. The overall outcome of C. maculosa invasion and its transformative effects on vegetation archi- tecture on Dictyna density and web building behavior were to increase Dictyna predation on invertebrate prey C89 fold. These results indicate that invasive plants that change the architecture of native vegetation can substantially impact native food webs via nontraditional plant ? preda- tor ? consumer linkages.