Trophic downgrading reduces spatial variability on rocky reefs

Abstract

Trophic downgrading in coastal waters has occurred globally during recent decades. On temperate rocky reefs, this has resulted in widespread kelp deforestation and the formation of sea urchin barrens. We hypothesize that the intact kelp forest communities are more spatially variable than the downgraded urchin barren communities, and that these differences are greatest at small spatial scales where the influence of competitive and trophic interactions is strongest. To address this, benthic community surveys were done in kelp forests and urchin barrens at nine islands spanning 1230 km of the Aleutian Archipelago where the loss of predatory sea otters has resulted in the trophic downgrading of the region’s kelp forests. We found more species and greater total spatial variation in community composition within the kelp forests than in the urchin barrens. Further, the kelp forest communities were most variable at small spatial scales (within each forest) and least variable at large spatial scales (among forests on different islands), while the urchin barren communities followed the opposite pattern. This trend was consistent for different trophic guilds (primary producers, grazers, filter feeders, predators). Together, this suggests that Aleutian kelp forests create variable habitats within their boundaries, but that the communities within these forests are generally similar across the archipelago. In contrast, urchin barrens exhibit relatively low variability within their boundaries, but these communities vary substantially among different barrens across the archipelago. We propose this represents a shift from small-scale biological control to large-scale oceanographic control of these communities.

Landscapes shaped from the top down: predicting cascading predator effects on spatial biogeochemistry

“…Nevertheless, characterization of landscape‐scale biogeochemical and vegetation patterning is still commonly framed by conceptualizations of traditional bottom–up controls. Recent examples (le Roux et al 2018, 2020, Edwards and Konar 2020, Mackay et al 2021) show why bottom–up conceptual frameworks alone are insufficient to explain landscape spatial patterning. Indeed, our emerging understanding of zoogeochemistry complicates the classical dichotomous view of top–down and bottom–up controls on ecosystems entirely, suggesting that we should instead take a more holistic view of how the interplay among animal–plant–soil interactions and feedbacks together shape landscapes.…”

Tropicalization of temperate reef fish communities facilitated by urchin grazing and diversity of thermal affinities

“…Ecoregional species pools with diverse thermal affinities (large SD s of thermal midpoints) and narrow thermal ranges are more sensitive to large‐scale restructuring under temperature change, as observed through shifts in CTI values (Burrows et al., 2019). To establish theoretical expectations for a possible habitat effect, we used aggregated ecoregional fish species lists, representing all species that could occur in any given habitat or site within an ecoregion, in a simulation of differences in CTI across barrens and kelp under four scenarios of CTI sensitivity (following the approach of Edwards & Konar, 2020): scenario (a), species pool with high diversity of STIs and narrow STRs; scenario (b), high diversity of STIs and wide STRs; scenario (c), low diversity of STIs and narrow STRs; and scenario (d), low diversity of STIs and wide STRs. Sensitivity to CTI change is high in scenario (a), medium in scenarios (b) and (c), and low in scenario (d).…”

“…We expect a warming signature (CTI increase) for barrens fish communities relative to adjacent kelp beds, driven by an influx of warm-affinity species typical of kelp-free lower latitudes. Assuming, on average, lower species diversity at barrens (Edwards & Konar, 2020;Pinna et al, 2020), we expect lower community thermal diversity (diversity of warm-and cold-affinity species) and a shift towards communities dominated by more widespread thermal generalists (with broad thermal ranges; greater CTR). Given that patterns in thermal diversity might, for example, link to differences in feeding strategy or water column position, we run a supplementary analysis to test whether thermal composition changes are associated with, or occur independently of, overall community changes (change in species abundance and species richness) or functional richness.…”

“…The CTI is calculated as the average thermal affinity (STI) of all species in a local community. The CTDiv (Edwards & Konar, 2020) score indicates the variation of thermal affinities among species in a community and is calculated as the SD of STIs. Communities with a mix of cold‐ (blue) and warm‐affinity (red) species have a high score, and those containing species with similar thermal affinities have a low score.…”

Biodiversity loss leads to reductions in community‐wide trophic complexity

“…These urchin barrens can occur, and be maintained, in close proximity to kelp forests (Konar and Estes 2003, Konar et al 2014) and have persisted for decades in some locations (Filbee‐Dexter and Scheibling 2014, Konar et al 2014). Some consequences of reduced sea otter abundances and the phase shift from kelp forests to urchin barrens are a reduction in the abundance and biodiversity of macroalgae (Metzger et al 2019), reductions in ecosystem productivity and respiration (Edwards et al 2020), decreases in spatial variability in community structure (Edwards and Konar 2020), reductions in kelp particulate organic matter and bivalve growth (Duggins et al 1989), increases in sea star predation on invertebrates (Vicknair and Estes 2012), declines in fish abundance like rock greenling Hexagrammos lagocephalus (Reisewitz et al 2006), and shifts in the diets of eagles from fish to birds (Anthony et al 2008).…”