Invasive plants typically escape specialist herbivores but are often attacked by generalist herbivores in their introduced ranges. The shifting defense hypothesis suggests that this will cause invasive plants to evolve lower resistance against specialists, higher resistance against generalists, and greater tolerance to herbivore damage. However, the duration and direction of selective pressures can shape the evolutionary responses of resistance and tolerance for invasive plants. Two critical factors are (1) residence time (length of time that an invasive species has been in its introduced range) and (2) specialist herbivore reassociation (attack by purposely or accidentally introduced specialists). Yet, these two factors have not been considered simultaneously in previous quantitative syntheses. Here, we performed a meta-analysis with 367 effect sizes from 70 studies of 35 invasive plant species from native and invasive populations. We tested how the residence time of invasive plant species and specialist reassociation in their introduced ranges affected evolutionary responses of defenses against specialists and generalists, including herbivore resistance traits (physical barriers, digestibility reducers and toxins), resistance effects (performance of and damage caused by specialists or generalists) and tolerance to damage (from specialists or generalists). We found that residence time and specialist reassociation each significantly altered digestibility reducers, specialist performance, generalist damage, and tolerance to specialist damage. Furthermore, residence time and specialist reassociation strongly altered toxins and generalist performance, respectively. When we restricted consideration to invasive plant species with both longer residence times and no reassociation with specialists, invasive populations had lower resistance to specialists, similar resistance to generalists, and higher tolerance to damage from both herbivore types, compared with native populations. We conclude that the duration and direction of selective pressure shape the evolutionary responses of invasive plants. Under long-term (long residence time) and stable (no specialist reassociation) selective pressure, invasive plants generally decrease resistance to specialists and increase tolerance to generalist damage that provides mixed support for the shifting defense hypothesis.
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