Global patterns of kelp forest change over the past half-century

Abstract

Kelp forests (Order Laminariales) form key biogenic habitats in coastal regions of temperate and Arctic seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. Although local evidence suggests that kelp forests are increasingly threatened by a variety of stressors, no comprehensive global analysis of change in kelp abundances currently exists.Here, we build and analyze a global database of kelp time series spanning the past half-century to assess regional and global trends in kelp abundances. We detected a high degree of geographic variation in trends, with regional variability in the direction and magnitude of change far exceeding a small global average decline (instantaneous rate of change = −0.018 y −1 ). Our analysis identified declines in 38% of ecoregions for which there are data (−0.015 to −0.18 y −1 ), increases in 27% of ecoregions (0.015 to 0.11 y −1 ), and no detectable change in 35% of ecoregions. These spatially variable trajectories reflected regional differences in the drivers of change, uncertainty in some regions owing to poor spatial and temporal data coverage, and the dynamic nature of kelp populations. We conclude that although global drivers could be affecting kelp forests at multiple scales, local stressors and regional variation in the effects of these drivers dominate kelp dynamics, in contrast to many other marine and terrestrial foundation species. A ssessing ecosystem change on a global scale has been instrumental in highlighting the magnitude of human impacts on natural ecosystems. For example, awareness of global declines in fish populations (1), coral reefs (2), and tropical rainforests (3) has substantially increased public interest and subsequent political motivation for environmental conservation. In some cases, global assessments have highlighted complex patterns of change (4, 5), which often reflect variable trajectories among regions (4). SignificanceKelp forests support diverse and productive ecological communities throughout temperate and arctic regions worldwide, providing numerous ecosystem services to humans. Literature suggests that kelp forests are increasingly threatened by a variety of human impacts, including climate change, overfishing, and direct harvest. We provide the first globally comprehensive analysis of kelp forest change over the past 50 y, identifying a high degree of variation in the magnitude and direction of change across the geographic range of kelps. These results suggest region-specific responses to global change, with local drivers playing an important role in driving patterns of kelp abundance. Increased monitoring aimed at understanding regional kelp forest dynamics is likely to prove most effective for the adaptive management of these important ecosystems.

Climate Change Impacts on Seagrass Meadows and Macroalgal Forests: An Integrative Perspective on Acclimation and Adaptation Potential

“…Seagrass beds and kelp forests are increasingly threatened by a variety of stressors (Orth et al, 2006;Waycott et al, 2009;Krumhansl et al, 2016). The combined effect of multiple climatechange related stressors on the extinction risk and productivity of macrophytes can be additive, synergistic, or antagonistic (Wahl et al, 2011(Wahl et al, , 2015, and may not be predicted from the individual effect of each variable operating in isolation (Darling and Côté, 2008).…”

“…For macroalgae, the magnitude and direction of abundance changes vary strongly between geographic regions (Krumhansl et al, 2016), but macrophyte losses are concentrated in warm-temperate to tropical regions Fraser et al, 2014). Physiological, genetic, and modeling data predict, and already document, that rising temperatures cause massive die-offs of genetically unique populations along warmtemperate distribution limits and open up new thermally suitable habitat in polar regions (Wernberg et al, 2011;Jueterbock et al, 2013Jueterbock et al, , 2016Brodie et al, 2014;KrauseJensen and Duarte, 2014;Valle et al, 2014;Olesen et al, 2015;Assis et al, 2016aAssis et al, , 2017aHyndes et al, 2016).…”

The importance of phenotypic plasticity and local adaptation in driving intraspecific variability in thermal niches of marine macrophytes

“…Warming mediated range shifts have been observed for marine macrophytes across the world (Marba and Duarte 2010, Wernberg et al 2011, 2013, 2016a, b, Smale and Wernberg 2013, Thomson et al 2015, Krumhansl et al 2016 and are predicted to continue as warming progresses (Martínez et al 2012a, b, Jueterbock et al 2013. Perhaps the most notable is a ~ 1250 km range contraction of the intertidal fucoid, Fucus vesiculosus, from Morocco polewards to the mid Iberian Peninsula over a 30 yr period (Nicastro et al 2013).…”

Bubble Curtains: Herbivore Exclusion Devices for Ecology and Restoration of Marine Ecosystems?

“…The potential of bubble curtains as herbivore exclusion devices is relevant given the rate of loss of coastal marine vegetation globally (Waycott et al, 2009;Krumhansl et al, 2016) and the profound influence herbivores can have on this process . Indeed, Port Gregory where the current study took place, experienced 100% loss of kelp forests following a marine heatwave in 2011 that wiped out over 900 km 2 of kelp forest in the region .…”

“…Moreover, abrupt changes in herbivore abundances and over-grazing have contributed to catastrophic regime shifts from vegetated to denuded states in temperate systems (Ling et al, 2009;Filbee-Dexter and Scheibling, 2014;Bennett et al, 2015;Vergés et al, 2016), while a decline of herbivores (e.g., scrapers, excavators and browsers; sensu Bellwood and Choat, 1990) on coral reefs has been closely associated with undesirable regime shifts from coral to seaweed dominated states (Bellwood et al, 2004;Mumby et al, 2006;Hughes et al, 2007). In addition to the direct effects of herbivores, many vegetated marine ecosystems such as seagrass meadows and kelp forests are in decline as a result of anthropogenic disturbances from eutrophication, climatic warming and extreme events, sediment-loading, salinity changes and invasive species (Orth et al, 2006;Waycott et al, 2009;Wernberg et al, 2013Wernberg et al, , 2016Bennett et al, 2016;Krumhansl et al, 2016). This has prompted efforts to actively revegetate coastal marine systems to curb losses and restore habitats where the initial cause of the loss has been reduced (Bastyan and Cambridge, 2008;Statton et al, 2013;Campbell et al, 2014).…”