Trophic control of cryptic coralline algal diversity

Abstract

Understanding how trophic dynamics drive variation in biodiversity is essential for predicting the outcomes of trophic downgrading across the world’s ecosystems. However, assessing the biodiversity of morphologically cryptic lineages can be problematic, yet may be crucial to understanding ecological patterns. Shifts in keystone predation that favor increases in herbivore abundance tend to have negative consequences for the biodiversity of primary producers. However, in nearshore ecosystems, coralline algal cover increases when herbivory is intense, suggesting that corallines may uniquely benefit from trophic downgrading. Because many coralline algal species are morphologically cryptic and their diversity has been globally underestimated, increasing the resolution at which we distinguish species could dramatically alter our conclusions about the consequences of trophic dynamics for this group. In this study, we used DNA barcoding to compare the diversity and composition of cryptic coralline algal assemblages at sites that differ in urchin biomass and keystone predation by sea otters. We show that while coralline cover is greater in urchin-dominated sites (or “barrens”), which are subject to intense grazing, coralline assemblages in these urchin barrens are significantly less diverse than in kelp forests and are dominated by only 1 or 2 species. These findings clarify how food web structure relates to coralline community composition and reconcile patterns of total coralline cover with the widely documented pattern that keystone predation promotes biodiversity. Shifts in coralline diversity and distribution associated with transitions from kelp forests to urchin barrens could have ecosystem-level effects that would be missed by ignoring cryptic species’ identities.

Convergent evolution of niche structure in Northeast Pacific kelp forests

“…Because traits are generally simple, the evolution of novel features may not be particularly important in determining the habitat niche of macroalgae, and thus strong selection on quantitative, heritable traits may lead to divergence being common among close relatives. This hypothesis is supported by recent work on coralline algae, showing that intense grazing by urchins (analogous to environmental filtering) does not lead to phylogenetic clustering (Hind et al., 2019) as predicted by assumptions of niche conservatism. Regardless of the generality of our results to other marine macroalgae, we show that niche partitioning has been an important driver of kelp phenotypic evolution, highlighting the importance of divergent selection in the evolution of a lineage of marine foundation species.…”

“…Kelps (order Laminariales) are the largest and most productive macroalgae in the ocean and dominate approximately 25% of coastlines globally (Wernberg, Krumhansl, Filbee‐Dexter, & Pedersen, 2019). Kelps increase the productivity of cool, temperate nearshore ecosystems and their presence can substantially alter the composition of biotic communities (Graham, 2004; Hind et al., 2019; Steneck et al., 2002; Teagle, Hawkins, Moore, & Smale, 2017) by forming three‐dimensional habitats called ‘kelp forests’ (Wernberg & Filbee‐Dexter, 2019). In spite of their global importance, we still have a limited understanding of the processes underlying the evolution of kelps.…”

High diversity of coralline algae in New Zealand revealed: Knowledge gaps and implications for future research

“…influence on algal and invertebrate recruitment [9–11,89]), it is clear that taxonomic confusion can compromise otherwise excellent science and lead to false conclusions. The importance of species recognition in order to understand the community processes is demonstrated in the study of Hind et al [90] who found that although coralline algae are more abundant in urchin ‘barrens’, these communities are dominated by only a few species and have lower diversity compared to coralline assemblages under intact kelp forests. This work by Hind et al [90] is a good example of a study in which enhanced resolution and understanding of nearshore community dynamics was dependent upon reliable species identifications using molecular data.…”

Phylogeny and Evolution of the Brown Algae

“…Large brown algae form complex underwater forests that dramatically increase the structural complexity of marine ecosystems (Steneck et al, 2002;Teagle et al, 2017) and alter environmental factors such as light (Gerard, 1984;Connell, 2003a;Gattuso et al, 2006), fluid dynamics (Hurd and Stevens, 1997;Stephens and Hepburn, 2014), sedimentation (Connell, 2003b;Filbee-Dexter et al, 2016) and food availability (Duggins et al, 1989;Estes et al, 2016). Large brown algae also provide habitat for a wide range of other taxa (Steneck et al, 2002;Graham, 2004;Teagle et al, 2017;Hind et al, 2019), including many commercially important animals (Bologna and Steneck, 1993;Smale et al, 2013;Markel et al, 2017), and serve as essential nursery grounds for many species (Holbrook et al, 1990;Kitada et al, 2019). Besides habitat provision, brown algae are a key source of productivity along the coast (Mann, 1973;Pfister et al, 2019) and can significantly increase secondary productivity in nearshore ecosystems through direct herbivory and increased detrital production (Duggins et al, 1989;Krumhansl and Scheibling, 2012).…”