Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Abstract

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell's neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.

Size, weapons, and armor as predictors of competitive outcomes in fossil and contemporary marine communities

“…Soft body parts, such as stolons, also have been demonstrated for a handful of species to redirect the growth of spatial competitors (Tzioumis 1994). Life history variation including fecundity and differential colony growth rates (Sebens 1986), and consequently colony size, might also influence competitive outcomes but little is known about these traits for bryozoans in general (see also Centurion andGappa 2011, Liow et al 2017). While some of the remaining Table 1).…”

“…Three disparate data sets met the criteria above. They are 308 overgrowth battles among 18 species encrusting pebbles and rocks from the Strait of Magellan (Centurion and Gappa ); 2,920 overgrowth battles among 15 species encrusting rocks from the Antarctic (Barnes and Rothery ); and 1,118 overgrowth battles among 15 species encrusting settlement panels in Scotland (Turner and Todd ).…”

“…In the same system, zooid size (i.e., the size of the individual modules of the colony) was shown to be a predictor of overgrowth outcomes: species with relatively large zooids have the upper hand (Liow et al 2017). However, not only is there variation in the outcomes of such spatial competition not fully explained by zooid size, but other traits are also thought to affect spatial competition (Centurion andGappa 2011, Taylor 2016). Hence, in the current study, we model overgrowth interactions (termed battles hereon to avoid confusion with explanatory variable interactions) among 82 cheilostome bryozoan species in the Wanganui Basin of the North Island of New Zealand (Rust and Gordon 2011).…”

Seasonal and multi-annual patterns of colonisation and growth of sessile benthic fauna on artificial substrates in the brackish low-diversity system of the Baltic Sea

“…Bryozoan colonies confined to lower, hidden surfaces have been observed widely in many temperate and sub-polar ecosystems, e.g. Irish coastal waters (Maughan, 2001), southern Argentina (Centurión & Gappa, 2011) and northern Chile (Pacheco et al, 2010). The cheilostome bryozoan exhibits a notable preference for hard substrates sheltered from sedimentation to avoid clogging (Maughan, 2001).…”

Competition between encrusters on marine hard substrates and its fossil record

“…Competitive hierarchies may be developed, whereby species A overgrows species B, B overgrows C, and A also overgrows C (e.g. Centurión & López Gappa ). Alternatively, competitive networks can occur in which A overgrows B, B overgrows C, but C overgrows A (Buss & Jackson ).…”