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.

Keywords: Laminariales; climate change; coastal ecosystems; global change; kelp forest.

Fig. 1.

(A) Number of sites in the dataset (n = 1,454) by ecoregion (16). Gray shading indicates ecoregions where kelps are present but for which no data were available. (B) Range of dates within each study within each ecoregion, with line shading indicating the weight of studies within that range.

Fig. 2.

Modeled ecoregion slopes (instantaneous rate of change per year) and 90% credible intervals for the full dataset (n = 8,846 data points from 26 ecoregions), with variable temporal coverage in each ecoregion, and by decade (1983–1992, 1993–2002, and 2003–2012). Ecoregion means are colored red if the 90% credible intervals (i.e., high probability) do not overlap zero.

Fig. 3.

Modeled slopes (instantaneous rate of change per year) and associated probability (Bayesian probability cutoff) for each ecoregion (n = 8,846 data points from 26 ecoregions). Slope magnitudes are shown as colored shading of ecoregions, whereas probability is demonstrated using the thickness of ecoregion outlines.

Fig. 4.

Global distribution of modeled site level slopes (instantaneous rate of change per year) and 90% credible intervals (n = 8,846 data points from 26 ecoregions). The vertical black line shows the global mean slope, with the shaded band indicating the 90% credible interval, which does not overlap zero (i.e., >95% probability of decline).