On the very edge: faunal and functional responses to the interface between benthic seagrass and unvegetated sand assemblages

Abstract

Changes in macrobenthic spatial structure were investigated across the precise 0.5 m wide boundary zone between intertidal seagrass and unvegetated sand in Moreton Bay, Queensland, Australia. Faunal abundance and species density in the marginal seagrass were only slightly reduced relative to local non-boundary areas. Although gradual diminution in seagrass faunal abundance occurred towards the interface and a few locally dominant species were absent from the boundary zone, most transition was far from gradual, instead being exactly coincident with the specific 0.1 m interface between the 2 habitats. Over this distance, faunal abundance and species density fell by 56 to 60% (from 1412 to 625 m −2 and from an estimated 42 species per sampling horizon to 17, respectively). Dominant seagrass species were abundant right up to the interface itself, but their densities had fallen by 82% a further 0.1 m into the adjacent sand. Functional-group density and diversity mirrored these changes, decreasing over the same distance by 30 to 36%. Spatial analysis showed that adjacent cores along lattice sampling horizons, although spaced further apart, were more similar to each other than were those oriented across horizons, even though these were contiguous. Characteristic high animal species richness in the seagrass relative to adjacent sand appears to be not so much related to characteristics of the seagrass habitat as a whole rather than to the presence or absence of individual plants. Asymmetrically, distributions of species characteristic of the sand stopped somewhat in advance of the interface, leaving a marginal sand zone dominated by generalists occurring at equal (low) species and population density across both habitats.

Are seaward pneumatophore fringes transitional between mangrove and lower-shore system compartments?

“…The North Stradbroke intertidal samples yielded a subset of of the species known to occur in intertidal soft sediments in Moreton Bay, at densities within the ranges of those previously recorded from the habitats concerned (e.g. Morgan and Hailstone, 1986;Camilleri, 1992; Barnes and Barnes, 2012;Barnes and Hamylton, 2016)…”

“…0.01m -2 ). These were: Mictyris longicarpus, a wandering crab found, especially in the form of juveniles, across the whole mangrove/sandflat boundary zone but in locations changing with each tide (see Barnes and Hamylton, 2016), and Velacumantus australis, a relatively large gastropod that likewise occurs across, and beyond, the whole seagrass/mangrove belt but in the form of dense aggregations separated by areas from which it is absent (see, e.g., Barnes and Hamylton, 2015). In each case, randomly encountering a dense patch in some samples and missing them in others had dramatically confounding effects on the results.…”

“…With reference to relative abundance at habitat interfaces, , however, have proposed a contextual model essentially for individual species that can nevertheless be extended to multi-species assemblages if responses of the various species to the interface are additive . This does seems generally to be the case in respect of the reactions of abundance and diversity of local seagrass and sandflat assemblages to their interface, albeit that this boundary can be abrupt rather than gradual (Barnes and Hamylton, 2016). In the model, only where one habitat M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT supports more of the same types of resource (in the widest sense of that word) than the adjacent habitat, i.e.…”

“…Because it lay outside the scope of their model, none of the scenarios predicted a situation in which abundances (and, in parallel, species density/richness) are least at the interface, as in case c 1 above. Such was the outcome, however, not only at the seagrass/unvegetated-sand boundary along the North Stradbroke intertidal (Barnes and Hamylton, 2016) but also in the experimental intertidal turf habitats of Matias et al (2013). The cause of this state was not specifically investigated but, for example, it is likely to occur when a key structural habitat resource in each compartment declines to zero at that point to be replaced by an incompatible alternative.…”

“…Inter-connectivity, complementarity, and passage of materials and organisms between these system compartments have received considerable recent attention (Bouillon et al, 2007;Unsworth et al, 2009;Lee et al, 2014), much of it motivated by the perceived general importance of the ecological and economic roles played by coastal interface zones (Levin et al, 2001;Waycott et al, 2011;Boström et al, 2011). Patterns of change in biodiversity and in relative abundance of faunal assemblages across mosaics of mangrove, seagrass and intertidal-flat habitats, however, constitute an important gap in current knowledge (Waycott et al, 2011), although the interface zones between seagrass and adjacent unvegetatedsediment systems have recently been investigated in some detail (Ollivier et al, 2015;Barnes & Hamylton, 2016). Seagrass-associated studies have been conducted within the context of its recent worldwide decline and fragmentation (Waycott et al, 2009;Fourqurean et al, 2012) and resultant concern that the alternative denuded habitat state may support lesser animal abundance and biodiversity (Pillay et al, 2010;Barnes & Barnes, 2012) thereby impacting significantly on the ecosystem services otherwise provided by seagrass beds (Costanza et al, 1997;Barbier et al, 2011).…”