Abstract
Invasive species are a growing threat to conservation in marine ecosystems, yet we lack a predictive understanding of ecological factors that influence the invasiveness of exotic marine species. We used surveys and manipulative experiments to investigate how an exotic seaweed, Sargassum horneri, interacts with native macroalgae and herbivores off the coast of California. We asked whether the invasion (i.e., the process by which an exotic species exhibits rapid population growth and spread in the novel environment) of S. horneri is influenced by three mechanisms known to affect the invasion of exotic plants on land: competition, niche complementarity and herbivory. We found that the removal of S. horneri over 3.5 years from experimental plots had little effect on the biomass or taxonomic richness of the native algal community. Differences between removal treatments were apparent only in spring at the end of the experiment when S. horneri biomass was substantially higher than in previous sampling periods. Surveys across a depth range of 0–30 m revealed inverse patterns in the biomass of S. horneri and native subcanopy-forming macroalgae, with S. horneri peaking at intermediate depths (5–20 m) while the aggregated biomass of native species was greatest at shallow (<5 m) and deeper (>20 m) depths. The biomass of S. horneri and native algae also displayed different seasonal trends, and removal of S. horneri from experimental plots indicated the seasonality of native algae was largely unaffected by fluctuations in S. horneri. Results from grazing assays and surveys showed that native herbivores favor native kelp over Sargassum as a food source, suggesting that reduced palatability may help promote the invasion of S. horneri. The complementary life histories of S. horneri and native algae suggest that competition between them is generally weak, and that niche complementarity and resistance to grazing are more important in promoting the invasion success of S. horneri.
Highlights
Marine ecosystems are increasingly threatened by invasive species as global trade expands and human-mediated introductions via commercial shipping occur at escalating rates [1,2,3,4,5]
Mean biomass (a) taxonomic richness all algae in biomass (a) and taxonomic richness (b) of(b) all of native algae measured in Sargassum biomass (a) and and taxonomic richness (b) of all native native algae measured measured in Sargassum horneri-removal (S−; grey bars) and non-removal (S+; white bars) years for horneri-removal (S−; grey bars)
We found that S. horneri displayed spatial complementarity with other subcanopy algae as it was most abundant at intermediate depths (5–20 m), while native algae were most abundant at shallower (20 m) depths
Summary
Marine ecosystems are increasingly threatened by invasive species as global trade expands and human-mediated introductions via commercial shipping occur at escalating rates [1,2,3,4,5]. Once an introduced species becomes established, biotic interactions with native species can play a major role in limiting population growth, spread and ecological impacts [8,9,10,11]. Diversity 2020, 12, 54 promote or inhibit “invasion”, here defined as the process by which an exotic species exhibits rapid population growth and spread in the novel environment [12]. Invasion success can be promoted when exotic species have functional traits or resource requirements that differ from the native biota, which allows them to take advantage of underutilized resources in space and time [16,18,19,20,21]
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