Detrital supply suppresses deforestation to maintain healthy kelp forest ecosystems

Abstract

Herbivores can reach extraordinary abundances in many ecosystems. When herbivore abundance is high, heavy grazing can severely defoliate primary producers and, in some cases, even drive ecosystem to undergo regime shifts from a high productivity state to a denuded, low productivity state. While the phenomenon of herbivore-driven regime shifts is well documented, we only partially understand the mechanisms underlying these events. Here, we combine herbivory experiments with 21 years of long-term monitoring data of kelp forest ecosystems to test the hypothesis that herbivores drive regime shifts when herbivory exceeds primary production. To test this hypothesis, we quantified how the foraging habits of an important group of marine herbivores-sea urchins-change with increases in sea urchin biomass and trigger regime shifts to a foundation species, giant kelp (Macrocystis pyrifera). Using experiments, we quantified how the grazing capacity of urchins increases as urchin biomass increases, then we combined these estimates of urchin grazing capacity with estimates of kelp production to predict when and where urchin grazing capacity exceeded kelp production. When grazing capacity exceeded kelp production, sea urchins caused a 50-fold reduction in giant kelp biomass. Our findings support the hypothesis that the balance between herbivory and production underlies herbivore-driven regime shifts in Southern California kelp forests and provides insight into when and where urchins are likely to force regime shifts in kelp forest ecosystems.

Variation in body size drives spatial and temporal variation in lobster–urchin interaction strength

“…We placed a single lobster in an experimental arena, and fed each lobster one of three size classes of urchin at six different densities ( N = 2,3,5,10,16,26 ind.arena −1 ). We selected urchin densities such that the highest density in experimental trials was representative of that in urchin‐dominated areas (Rennick et al, 2022). We conducted all foraging trials for 48 h in 200 L arenas.…”

Patterns and drivers of benthic macroinvertebrate assemblages in the kelp forests of southern Patagonia

“…productivity [62][63][64], kelp production outweighing herbivore consumption [65], and structural complexity in habitat [60], to name a few. A better understanding of the kelp forests of southern Patagonia may help to tease out some of the drivers of these ecosystems owing to the minimal anthropogenic impacts within this region.…”

The legacies of land clearance and trophic downgrading accumulate to affect structure and function of kelp forests

“…Theory predicts that grazing distributed along gradients in primary production can result in sharp thresholds for persistence as grazing pressure exceeds vegetative regeneration (Oksanen et al, 1981). The effect has been observed in kelp forest systems where increases in the supply of detrital organic matter from kelps have suppressed deforestation from grazing sea urchins (Rennick et al, 2022). Physical oceanographic conditions in the region are dominated by tidal flows that result in high retention of neritic water and larvae in the inner Sounds and in persistent retention of sediment loads in eddies within Tasman Bay (Heath, 1976;Stevens et al, 2021).…”