Abstract
Kelps dominate rocky shores in temperate latitudes, acting as important foundation species. Recently, kelp forests have received growing attention for their decline around the world. Climate change is one of the main drivers of their deforestation, both through abiotic stress and/or distortion of biotic interactions. In NW Spain, golden kelp (Laminaria ochroleuca) forests have recently declined in some areas due to excessive grazing. To further investigate the mechanisms behind this decline, a thorough 1-year seasonal demographic study was carried out on eight golden kelp reefs: four healthy and four degraded by herbivory. Kelps covered the full range of size classes throughout the year on healthy reefs. By contrast, only small, newly recruited kelps were found on degraded reefs in spring-summer, because excessive herbivory in autumn-winter caused a total collapse of kelp populations on a reef-scale (i.e., all kelps reduced to inviable stubs with no blade and no growth meristem). Bite marks in kelps covering all size classes showed that fishes, instead of invertebrates, were the culprits of the excessive herbivory. Recent studies, mostly conducted in the Indo-Pacific, also found that fish herbivory, alone or in combination with abiotic stressors, can maintain a canopy-free state in kelp reefs. To the best of our knowledge, this is the first time that fish is also shown to cause the collapse of entire kelp forests on a reef scale in the temperate Atlantic. The implications of this finding for the management and potential recovery of degraded reefs are discussed.
Highlights
Climate change is behind the redistribution of many species on a global scale, and shifts community structure and ecosystem functioning (Pecl et al, 2017; Gilson et al, 2021)
It is overgrazing that is preventing some kelp beds from regaining the healthy forests with large adult kelp that used to occupy those reefs some years ago. This is clearly evidenced by our Herbivory Pressure Index, which shows how the herbivory damage gradually increased in autumn and, especially in winter, on degraded reefs relative to healthy reefs
This study reveals, for the first time, that herbivorous fish are able to lead to an annual collapse of kelp forests in the temperate Atlantic
Summary
Climate change is behind the redistribution of many species on a global scale, and shifts community structure and ecosystem functioning (Pecl et al, 2017; Gilson et al, 2021). Poleward range shifts have been documented for several species in response to rising seawater temperatures (Lima et al, 2007; Sorte et al, 2010; Poloczanska et al, 2013). These range shifts are especially noticeable in temperate-subtropical transition zones where species of both climatic regimes co-exist (Horta e Costa et al, 2014; Troast et al, 2020; Smith et al, 2021). In mid-latitudes away from the range limits, kelps may be more susceptible to biotic pressures such as herbivory (Steneck et al, 2002)
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