Do graminoid and woody invaders have different effects on native plant functional groups?
Abstract
While there are numerous studies of impacts by individual invader species, there are few syntheses of invader impacts on plant community structure and composition. We sought to identify native plant functional groups (PFGs) which are under-represented in post-invaded compared with non-invaded environments to determine whether different invader morphologies consistently suppressed native species richness within particular PFGs. We conducted a meta-analysis of 20 international studies to analyse the effects of graminoid (grass and grass-like species) and woody (shrub or tree species) invaders on species richness within growth form, longevity and seed mass native PFGs. Graminoid and woody invaders could not be distinguished by their effects on native PFGs: in most cases, native PFGs were negatively affected by both graminoid and woody invaders. However, within the growth form classification, we found that negative invader effects were greater for native graminoid than tree species. Perennial rather than short-lived species appeared more affected by invasion (particularly graminoid invasion). Species with small seed masses appeared more negatively affected than those with large seed masses, particularly by graminoid invaders. Synthesis and applications. Despite different morphologies of graminoid and woody invaders, they had similar negative effects on species richness within a range of native plant functional groups (PFGs). Further, the response of many PFGs to graminoid invaders in particular was highly variable. In consequence, managers cannot rely on invader morphology to distinguish impacts and prioritize control efforts. While graminoid and woody invaders had similar impacts, they had particularly adverse effects on small herbaceous (especially graminoid) and perennial species. Managers should focus monitoring activities on these potentially under-represented PFGs following control of both woody and graminoid invaders. Native species belonging to herbaceous and perennial PFGs may require focused reintroduction following control efforts.