Marine deforestation leads to widespread loss of ecosystem function

Abstract

Trophic interactions can result in changes to the abundance and distribution of habitat-forming species that dramatically reduce ecosystem functioning. In the coastal zone of the Aleutian Archipelago, overgrazing by herbivorous sea urchins that began in the 1990s resulted in widespread deforestation of the region's kelp forests, which led to lower macroalgal abundances and higher benthic irradiances. We examined how this deforestation impacted ecosystem function by comparing patterns of net ecosystem production (NEP), gross primary production (GPP), ecosystem respiration (Re), and the range between GPP and Re in remnant kelp forests, urchin barrens, and habitats that were in transition between the two habitat types at nine islands that spanned more than 1000 kilometers of the archipelago. Our results show that deforestation, on average, resulted in a 24% reduction in GPP, a 26% reduction in Re, and a 24% reduction in the range between GPP and Re. Further, the transition habitats were intermediate to the kelp forests and urchin barrens for these metrics. These opposing metabolic processes remained in balance; however, which resulted in little-to-no changes to NEP. These effects of deforestation on ecosystem productivity, however, were highly variable between years and among the study islands. In light of the worldwide declines in kelp forests observed in recent decades, our findings suggest that marine deforestation profoundly affects how coastal ecosystems function.of the region's river systems and its riparian plant communities [4,5]. Similarly, large marine algae, such as kelps, can form subtidal forests whose biogenic structures alter hydrodynamic, nutrient and light conditions, modify patterns of biodiversity, enhance primary production and carbon sequestration, and provide food and habitat for numerous other species [6][7][8][9]. Consequently, the loss of these forest-forming kelps and the benthic communities they support can have dramatic impacts to how nearshore ecosystems function, especially if they occur over large geographic areas. Indeed, kelp deforestation has occurred in numerous areas worldwide in recent decades due to a variety of forcing factors [10,11], and the subtidal rocky reefs of the Aleutian Archipelago serve as a model system to investigate the broader impacts of such deforestation. These forests have historically been dominated by dense populations of the surface canopy-forming kelp Eualaria fistulosa, several species of understory kelps such as Laminaria spp. and Agarum spp., the brown alga Desmarestia spp., and numerous species of fleshy read algae. However, the collapse of sea otter (Enhydra lutris) populations led to large increases in their primary prey, herbivorous sea urchins (Strongylocentrotus polyacanthus), which subsequently resulted in overgrazing and widespread losses of the region's kelp forests [12]. This collapse began in the late 1990s, likely in response to a dietary shift by killer whales toward sea otters, and by 2000 sea otter densities had declined throughout the ar...

Trophic downgrading reduces spatial variability on rocky reefs

“…Specifically, we sampled Adak, Chugidinadak and Tanaga in 2016, and Amchitka, Atka, Attu, Kiska, Nizki, and Yunaska in 2017. Previous analyses of community data by our research group did not identify any differences in community structure between the two sample years 12,16 . Two 6-8 m deep sites belonging to each habitat type (kelp forest and urchin barren) were sampled at each island using SCUBA.…”

“…Downgrading often results in the formation of alternate stable states 4,6,8 in which herbivores become hyper-abundant and autotrophs become rare. When this affects the distribution and abundance of ecosystem engineers, such as forest-forming trees and kelps that create habitat and supply energy for their communities, ecosystem function can be altered 12 and biodiversity reduced 16 . Such ecosystem changes, however, can vary among different spatial scales due to a variety of forcing factors 21,24,32,33 .…”

Variation in Photosynthetic Performance Relative to Thallus Microhabitat Heterogeneity in Lithothamnion australe (Rhodophyta, Corallinales) Rhodoliths

“…2020) and in dissolved oxygen (DO), and induce dynamic fluctuations of inorganic carbon (DIC) in the coastal zone (Edwards et al. 2020). Generally, changes in all components of DIC and total alkalinity ( A T ) are regulated, in part, by photosynthesis and calcification, respectively (Kim et al.…”

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).…”