Abstract
Wetland habitats are becoming increasingly scarce worldwide while experiencing exceptionally high levels of plant invasion. Invasive plant species affect ecosystems through numerous avenues, including acting as ecosystem engineers, contributing unique plant functional traits, and altering trophic dynamics. We examined the impacts of the invasive weed Lepidium latifolium on soil‐dwelling and canopy‐dwelling invertebrate communities in a brackish marsh of northern California. Invertebrate abundance, species richness, diversity, and community composition were measured in both invaded and non‐invaded areas of the marsh in four time periods that correspond with different L. latifolium phenologic phases. We observed these different stages of L. latifolium, and we recorded alterations in the habitat structure provided by L. latifolium as the plant entered senescence. L. latifolium had differential impacts on the two invertebrate communities studied. The presence of L. latifolium increased abundance of soil‐dwelling invertebrates and decreased the species richness of canopy‐dwelling invertebrates in the tidal marsh‐terrestrial ecotone where L. latifolium ameliorates harsh physical conditions. No changes occurred in the tidal marsh plain and fringing tidal marsh where L. latifolium's physical structure is more similar to existing non‐L. latifolium structure. During full senescence, L. latifolium reduced canopy‐dwelling invertebrate abundance and altered trophic dynamics, shifting composition to a predator‐dominated community. Our results demonstrate that L. latifolium can impact brackish marsh ecosystems, and we hypothesize L. latifolium's unique functional traits (large inflorescence and seasonal senescence) enable it to act as an ecosystem engineer that alters community and trophic dynamics. Mechanisms through which plant invaders alter ecosystem processes are dynamic in space and time, thus, future research will require the use of manipulative experiments to inform site‐specific management strategies.
Highlights
Between 1970 and 1989, 90% of California’s naturally occurring tidal wetlands were destroyed by anthropogenic influences (Dahl 1990)
By examining multiple communities at the same time, this study revealed the L. latifolium invasion is impacting canopy-dwelling invertebrates to a greater degree than soil dwelling invertebrates
The changes observed in invertebrate assemblages may be due, predominantly, to the physical structure provided by the plant and the way this structure changes through the course of L. latifolium’s phenology
Summary
Between 1970 and 1989, 90% of California’s naturally occurring tidal wetlands were destroyed by anthropogenic influences (Dahl 1990). Ehrenfeld (2010) outlines three mechanistic explanations which inform the study of invasive plant-ecosystem relationships: plant species functional traits, strong effects, and trophic interactions. Invasive plants often possess a suite of unique functional traits not present within the non-invaded plant community (Ehrenfeld 2010). In the context of invasion, Drenovsky et al (2012) defines plant functional traits as, ‘‘the readily measurable morphological, chemical, physiological, and phenological attributes of plants that interact with surrounding biotic and abiotic factors’’, and we will use this as our working definition. Strong effects of invasive plants are characteristics exerting wide-ranging impacts, such as the ability to act as ecosystem engineers. In addition to altering floral composition, wetland plant invasions have been demonstrated to change trophic structure either by altering food availability, accessibility, or the exposure of prey to predators (Zedler and Kercher 2004)
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