Abstract
Foundation species (FS) regulate ecological processes within communities often facilitating biodiversity and habitat complexity. Typically FS are dominant structure-forming taxa; but less dominant taxa having disproportionate ecological impacts to the community can also be FS. Mesophotic coral ecosystems (MCEs) are deep coral reef (∼30–150 m) communities, often dominated by emergent sponges in the Caribbean Basin. Despite the potential competitive advantage of sponges on MCEs, gorgonians are also common constituents of these reefs. Data from the Bahamas demonstrate increased biodiversity and densities of sponges on mesophotic reefs with gorgonians relative to reefs without these species. Drawing upon fifteen years of field surveys at five sites in the Caribbean Basin we assessed in situ interactions between gorgonians and sponges to quantify outcomes consistent with competition (i.e., tissue necrosis and overgrowth). Gorgonians were effective competitors against a variety of sponges, and two allelochemicals produced by Ellisella elongata were mechanistically important in interactions with Agelas clathrodes. We also examined invertebrate recruitment patterns near gorgonians to assess their role in facilitating MCE biodiversity. Our results indicate that live gorgonians, Antillogorgia bipinnata and E. elongata, facilitate biodiverse recruitment into MCEs, indicating that this process is governed by more than passive hydrodynamics. Collectively, these data indicate that these gorgonians exhibit both positive and negative ecological interactions (i.e., facilitation and competition, respectively) with sponges, and other taxa. Thus, these gorgonians are FS of MCE communities within the Caribbean Basin that display several traits contributing to the ecological structure of these understudied communities.
Highlights
Foundation species (FS) are critical constituents of a community that promote the success of other species by regulating ecological processes (Dayton, 1972); they are typically dominant structureforming taxa that occupy low trophic positions within an ecosystem
Gorgonian abundance and diversity on Caribbean Mesophotic coral ecosystems (MCEs) are lower than nearby shallow coral reefs, in contrast to the Red Sea where these metrics are higher at mesophotic depths (Shoham and Benayahu, 2017)
Sponge abundance is high at MCE depths in the Red Sea and eastern Mediterranean (Idan et al, 2018; Eyal et al, 2019), their morphology is encrusting like most sponges on Indo-Pacific MCEs (Slattery and Lesser, 2012)
Summary
Foundation species (FS) are critical constituents of a community that promote the success of other species by regulating ecological processes (Dayton, 1972); they are typically dominant structureforming taxa that occupy low trophic positions within an ecosystem (e.g., red mangroves: Ellison et al, 1996; giant kelp: Lamy et al, 2020). The roles of increasing algal cover on coral reefs has been well documented (McCook et al, 2001; Hay and Rasher, 2010), but all three structural groups are active competitors with corals, and with each other (Chadwick and Morrow, 2011) Ecological processes such as overgrowth and/or shading are common amongst these taxa, contact-mediated or water-borne delivery of allelopathic compounds is another competitive option for some species of algae, sponges, and gorgonians (Granéli and Pavia, 2006; Slattery and Gochfeld, 2012). We test the hypotheses that: (1) gorgonians are the primary FS of MCEs, and (2) the gorgonian Ellisella elongata structures MCEs with allelopathic compounds
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