Giant kelp, Macrocystis pyrifera , increases faunal diversity through physical engineering

Abstract

Foundation species define the ecosystems they live in, but ecologists have often characterized dominant plants as foundational without supporting evidence. Giant kelp has long been considered a marine foundation species due to its complex structure and high productivity; however, there is little quantitative evidence to evaluate this. Here, we apply structural equation modelling to a 15-year time series of reef community data to evaluate how giant kelp affects the reef community. Although species richness was positively associated with giant kelp biomass, most direct paths did not involve giant kelp. Instead, the foundational qualities of giant kelp were driven mostly by indirect effects attributed to its dominant physical structure and associated engineering influence on the ecosystem, rather than by its use as food by invertebrates and fishes. Giant kelp structure has indirect effects because it shades out understorey algae that compete with sessile invertebrates. When released from competition, sessile species in turn increase the diversity of mobile predators. Sea urchin grazing effects could have been misinterpreted as kelp effects, because sea urchins can overgraze giant kelp, understorey algae and sessile invertebrates alike. Our results confirm the high diversity and biomass associated with kelp forests, but highlight how species interactions and habitat attributes can be misconstrued as direct consequences of a foundation species like giant kelp.

Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities

“…Carbon is exuded as DOC from kelp tissues, elevating DOC concentrations within the kelp bed, thus providing an organic carbon resource for heterotrophic microbial processes (Kirchman 1994, Butturini andSabater 2000). The rich animal life within a kelp bed (Harvell 1984, Bodkin 1986, Miller et al 2018) is likely responsible for the increased ammonium that we documented inside kelp beds. The rich animal life within a kelp bed (Harvell 1984, Bodkin 1986, Miller et al 2018) is likely responsible for the increased ammonium that we documented inside kelp beds.…”

“…Foundational primary producers support higher trophic levels, physically stabilize substrate, provide biogenic structure, and enhance species diversity (Dayton 1972). They create critical habitat in temperate coastal environments worldwide, supporting animal productivity and diversity (Estes and Palmisano 1974, Bodkin 1986, Miller et al 2018. Canopy kelp are foundational species that form extensive underwater forests.…”

Loss of foundation species: disturbance frequency outweighs severity in structuring kelp forest communities

“…, Miller et al. ). However, giant kelp also structures reef communities through positive interactions by facilitating a diversity of epiphytes (Yoshioka ), epifauna (Coyer ), and other organisms living above the seafloor that were not measured in our study (e.g., fishes and invertebrates within the giant kelp canopy).…”

“…, Miller et al. ). Bottom‐dwelling mobile invertebrates include several herbivores that feed upon giant kelp and other algae (e.g., sea urchins, gastropods; Leighton , Foster et al.…”

“…As such, the loss of giant kelp can change local biodiversity (Graham , Miller et al. ) and productivity (Reed et al. , ).…”