Niche Complementarity and Resistance to Grazing Promote the Invasion Success of Sargassum horneri in North America

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.

The role of inputs of marine wrack and carrion in sandy‐beach ecosystems: a global review

“…pneumatocysts or buoyant blades) which are present in the native M. pyrifera and D. antarctica that are common in beach wrack in the region (Suárez‐Jiménez et al ., 2017 a ). However, the invasive brown alga Sargassum horneri is an annual species that can outcompete native kelp species and can seasonally dominate the wrack deposited on some Californian beaches (Marks, Reed & Holbrook, 2020 ). Beach‐cast of its congener S. muticum appears either to alter or to have no effect on invertebrate composition and densities compared to native macroalgal species (Rodil et al ., 2008 ; Cacabelos et al ., 2010 ).…”

Flotsam and jetsam: a global review of the role of inputs of marine organic matter in sandy beach ecosystems

“…pneumatocysts or buoyant blades) present in the native M. pyrifera and D. antarctica that are common in beach wrack in the region (Suarez-Jimenez et al, 2017a). However, the invasive brown alga Sargassum horneri is an annual species that can outcompete native kelp species and can seasonally dominate the wrack deposited on some California beaches (Marks et al, 2020). Beach-cast of its congeneric S. muticum appears to either alter or have no effect on the invertebrate composition and densities compared to native macroalgal species (Rodil et al, 2008;Cacabelos et al, 2010).…”

Feeding preferences and the effect of temperature on feeding rates of the graceful kelp crab in the Salish Sea

“…Increased populations of S. muticum have also been documented to in uence grazers within subtidal ecosystems. Experiments demonstrate that S. muticum serves as an additional food supply, shelter, and spawning habit for many crab and snail species (Seebach et al 2010; Britton-Simmons 2011), while for other species such as the green sea urchin, Strongylocentrotus droebachiensis, are deterred by S. muticum (Marks et al 2020).…”