Trait‐ and Density‐Mediated Indirect Interactions Initiated by an Exotic Invasive Plant Autogenic Ecosystem Engineer

Indirect interactions are important for structuring ecological systems. However, research on indirect effects has been heavily biased toward top‐down trophic interactions, and less is known about other indirect‐interaction pathways. As autogenic ecosystem engineers, plants can serve as initiators of nontrophic indirect interactions that, like top‐down pathways, can involve both trait‐mediated indirect interactions (TMIIs) and density‐mediated indirect interactions (DMIIs). Using microcosms, I examined a plant → predator → consumer interaction pathway involving the exotic autogenic ecosystem engineerCentaurea maculosa; nativeDictynaspiders (which exhibit density and trait [web‐building] responses toC. maculosa);Dictyna’s insect prey,Urophora affinis; andUrophora’s host plant (a secondary receiver species) to quantify DMIIs and TMIIs in an autogenic engineered pathway. Both DMIIs and TMIIs were strong enough to reduceUrophorapopulations, but only DMIIs, which were 4.3 times stronger than TMIIs, were strong enough to also reduceUrophora’s fecundity and increase the fecundity of its host plant. Prior field studies support these results, suggesting that the differences between DMIIs and TMIIs are even stronger in nature. This study illustrates that autogenic ecosystem engineers can initiate powerful indirect interactions that generally parallel predator‐initiated interactions but also differ in important functional ways.