Marine deforestation leads to widespread loss of ecosystem function

Trophic interactions can result in changes to the abundance and distribution of habitat-forming species that dramatically reduce ecosystem health and functioning. Nowhere may this be as dramatic as in the coastal zone of the Aleutian Archipelago, where overgrazing by herbivorous sea urchins that began in the 1980s resulted in widespread deforestation of the region’s kelp forests. Here we show that this deforestation resulted in decreased macroalgal and invertebrate abundance and diversity, increased benthic irradiances, and reduced rates of gross primary production and respiration by the ecosystem. These opposing metabolic processes remain in balance, however, which resulted in little-to-no changes to net ecosystem production. These patterns were consistent across nine islands spanning more than 1000 kilometers of the archipelago. In light of the worldwide declines in kelp forests observed in recent decades, our findings suggest that marine deforestation profoundly affects the health of coastal ecosystems and how they function. Significance statement Widespread marine deforestation results in reduced biodiversity and primary productivity throughout more than 1000 km of the Aleutian Archipelago.