Fish communities diverge in species but converge in traits over three decades of warming

Abstract

Describing the spatial and temporal dynamics of communities is essential for understanding the impacts of global environmental change on biodiversity and ecosystem functioning. Trait‐based approaches can provide better insight than species‐based (i.e. taxonomic) approaches into community assembly and ecosystem functioning, but comparing species and trait dynamics may reveal important patterns for understanding community responses to environmental change. Here, we used a 33‐year database of fish monitoring to compare the spatio‐temporal dynamics of taxonomic and trait structure in North Sea fish communities. We found that the majority of variation in both taxonomic and trait structure was explained by a pronounced spatial gradient, with distinct communities in the southern and northern North Sea related to depth, sea surface temperature, salinity and bed shear stress. Both taxonomic and trait structure changed significantly over time; however taxonomically, communities in the south and north diverged towards different species, becoming more dissimilar over time, yet they converged towards the same traits regardless of species differences. In particular, communities shifted towards smaller, faster growing species with higher thermal preferences and pelagic water column position. Although taxonomic structure changed over time, its spatial distribution remained relatively stable, whereas in trait structure, the southern zone of the North Sea shifted northward and expanded, leading to homogenization. Our findings suggest that global environmental change, notably climate warming, will lead to convergence towards traits more adapted for novel environments regardless of species composition.

Rebound in functional distinctiveness following warming and reduced fishing in the North Sea

“…These results are consistent with previous studies showing spatial differences between fish communities in the southern and northern North Sea for both species and traits (e.g. [37]). Here we also detected a spatial segregation of fish taxa according to their functional distinctiveness, associated with a north-south contrast in environmental conditions and fishing pressure.…”

“…Interestingly, the temporal changes varied spatially, as functionally distinct species increased in abundance everywhere in the North Sea but primarily in the south where they were initially less abundant, while functionally common species increased in abundance primarily in the north. We can hypothesize that reduced fishing pressure on K-selected species allowed them to progressively rebuild their populations (especially in the southern North Sea), while r-selected species likely increased in the north, through northward range expansions and immigration from the North Atlantic [37]. Thus, our results highlight that both functionally common and distinct species have increased in the North Sea in response to rising temperatures and decreased fishing since the mid-1990s; however, decreased fishing pressure appears particularly important to the rebound in functionally distinct species.…”

“…There is strong environmental and ecological heterogeneity between the northern and southern North Sea, particularly in terms of bathymetry and biodiversity [33]. The north is deeper, colder (most of the year) and has higher salinity, and the dominant fish species are Norway pout (Trisopterus esmarkii), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens) and whiting (Merlangius merlangus) [36,37]. The south is characterized by warmer and shallower waters with lower salinity, higher primary productivity, and greater seasonal fluctuations in environmental conditions [38].…”

“…The south is characterized by warmer and shallower waters with lower salinity, higher primary productivity, and greater seasonal fluctuations in environmental conditions [38]. The dominant fish species are dab (Limanda limanda), plaice (Pleuronectes platessa), and herring (Clupea harengus) [36,37].…”

Food web structure in relation to environmental drivers across a continental shelf ecosystem

“…The Celtic Sea Shelf located in the Northeast Atlantic Ocean supports some of the largest and most important demersal and pelagic trawl fisheries in the region (Mateo et al 2017; Moore et al 2019). The shelf has undergone recent declines in the abundance of the large cold‐water species (Atlantic cod Gadus morhua ) in the area and an increase of small, noncommercial pelagic species (such as the boarfish Capros aper ) has been documented in the area and elsewhere in the Northeast Atlantic (Pinnegar et al 2002; Blanchard and Vandermeirsch 2005; McLean et al 2019). To date, research into the Celtic Sea food web has predominantly focused on the trophic ecology of the fish community—demersal, benthic, and pelagic assemblages important in terms of biomass and fisheries in the Celtic Sea (Martinez et al 2013) on the upper continental slope (Kopp et al 2018), and along the shelf‐wide gradient (Pinnegar et al 2003; Rault et al 2017; Day et al 2019).…”