The critical scale of small-scale spatial variation in ecological patterns and processes in intertidal macrobenthic seagrass assemblages

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<ul><li><p>arc</p><p>ustra</p><p>seagrassspecies co-occurrence</p><p>una</p></li><li><p>surface and near-surface benthic fauna were identied to speciesand counted. Previous work on the Dunwich seagrass macrofaunaat this tidal level has shown it to be numerically dominated bygastropod molluscs, polychaete annelids, and decapod and tanaidcrustaceans, these four groups comprising &gt;90% of individuals and70% of species (Barnes, 2010b; Barnes and Barnes, 2011). Thepolychaetes, however, are a difcult group in that the local fauna isincompletely known (Davie and Phillips, 2008) and many of thealleged identities of common species are dubious (Wilson et al.,2002). For convenience, therefore, the present study wasrestricted to the other three dominant taxa, these target groupsrepresenting &gt;70% of the total individuals likely to be present.Numbers of individual members of non-target groups present insamples, however, were also recorded for assessment oftotal density. Generic and specic names are as per the WorldRegister of Marine Species (WoRMS )unless otherwise specied.</p><p>2.2. Statistical analyses</p><p>Faunal assemblages were compared multivariately usingpermutational analysis of multivariate dispersions (PERMDISP),permutational analysis of variance (PERMANOVA) including topartition the total variance into the components due to each nestedsampling scale, similarity percentage analysis (SIMPER) and hier-</p><p>Coasrelationship of abundance, diversity and control of assemblagestructure of dwarf-eelgrass macrofaunas with latitude.</p><p>As yet, however, there is little appreciation of what spatial scalesare most critical to the small-scale ecological changes that havebeen shown to occur within seagrass meadows. Previous studies ofthe intertidal seagrass beds along the northwestern coast of NorthStradbroke, one of the Moreton Bay barrier islands in Queensland,Australia, have shown marked effects of spatial scale on bothecological pattern and process in what otherwise appears to bea rather homogeneous system overwhelmingly dominated by thedwarf-eelgrass Nanozostera muelleri capricorni. As also appears tobe the norm on shallow-water hard substrata (Fraschetti et al.,2005), macrofaunal assemblage variance in these soft sedimentswas greater at a scale of 1 m than at either 150 m or 2 km (Barnesand Barnes, 2011). These spatial intervals are, however, relativelycoarse and the present study aimed to investigate variation inassemblage structure in more detail over the potentially criticalrange of 0.5e90 m to try to locate more closely the point/s at whichpatterns and processes change.</p><p>2. Methods</p><p>2.1. Sample collection and processing</p><p>Samples of the assemblages of smaller macrobenthic inverte-brates were taken between November 2010 and January 2011 atDunwich on North Stradbroke, a sand-dune barrier island in therelatively pristine Eastern Banks region of Moreton Bay (Dennisonand Abal, 1999), and within the same intertidal Nanozostera muel-leri capricorni bed that formed the subject of the earlier study ofchanges in assemblage structure with spatial scale (Barnes andBarnes, 2011). Barnes and Barnes (2011) provide a detaileddescription of the site; the areas sampled in this study being partsof the same ne/medium-grained siliceous sandat with (at thetime) 100% seagrass cover. A total of 216 cores, each of 54 cm2 areaand 5 cm depth, was taken over a point-to-point distance ofz1 km(from 2729.30S; 15324.40E to 2729.70S; 15323.90E) in the form oftwo 130 m transects (one at each locality e see below) parallel tothe shoreline and approximately at LWN tidal level, 20e30 m infrom the landwards limit of the seagrass bed to avoid edge effects(Tanner, 2005; Warry et al., 2009). Sampling along each transectwas structured in a nested design of four spatial scales. Tworeplicate localities, with their centres some 900 m apart, wereselected one to the north and one to the south of the One MileHarbour Channel (One Mile and Polka respectively); two sites,with their centres 90 m apart, were then sampled at each locality;two stations, with their centres 30 m apart, were sampled at eachsite; three loci, with their centres 5 m apart, were sampled at eachstation; and nine cores, distributed in a regular 3 3 grid withinan area of z1 m2 and with the centres of adjacent cores 0.5 mapart, were taken at each locus (Fig. 1). The centres of each block ofnine cores along the 130 m transects were therefore located at 0, 5,10, 30, 35, 40, 90, 95, 100, 120, 125 and 130 m. A 5 cm depth wasselected because most benthic macrofauna in seagrass beds isknown to occur in the top few mm of sediment (e.g. 98% in the top5 mm in the study by Klumpp and Kwak, 2005). The core size wasadopted after preliminary work had indicated that nine samples ofthat surface area would yield an acceptable standard error&lt; 20% ofthe arithmetic mean in estimation of local seagrass macrofaunaldensity (Elliott, 1977). As size of basic sampling unit is in itselfa potential confounding variable (e.g. Parravicini et al., 2009),adopting the same sized core as used in the earlier study of spatialvariability at the site (Barnes and Barnes, 2011) also ensuredcomparability of the smaller and larger spatial-scale results. This</p><p>R.S.K. Barnes, M.D.F. Ellwood / Estuarine,120sampling regime collects the smaller andmost numerous membersof the macrofauna that constitute the large majority of the biodi-versity (Albano et al., 2011).</p><p>Each core sample was collected at low tide, soon after tidal ebbfrom the site, and was gently sieved through 710 mm mesh. Allretained material (a) was placed in a large polythene bag ofseawater and all seagrass was shaken vigorously to dislodge all butsessile animals and then discarded (earlier testing having shownthat no detectable motile species remained within the seagrassafter such treatment); (b) was then re-sieved and transported tothe laboratory, where (c) it was placed in a large white tray fromwhich the living animals were extracted by eye, extractioncontinuing until no further animal could be seen after a 3 minsearch, which preliminary trial runs had indicated was sufcient topermit the removal of all live individuals. Target members of the</p><p>Fig. 1. Diagrammatic representation of the nested set of sampling scales within eachlocality, with detail of a single locus to show the arrangement of the individual corestaken within.</p><p>tal and Shelf Science 98 (2012) 119e125archical clustering analysis using S17 BrayeCurtis and Euclidean</p></li><li><p>distance similarity measures, carried out via PRIMER 6 [PrimerELtd: Plymouth Routines in Multivariate Ecological Research,Version 6]. PERMANOVA was conducted on similarity matriceswith untransformed, square-root and fourth-root transformedversions of the data using 9999 permutations; neither similaritymeasure nor transformation materially affect any results and onlyBrayeCurtis and fourth-root transformed data are presented below.Observed patterns of species co-occurrence were compared withstatistical randomisations of the original species occurrence datausing ECOSIM simulations (Gotelli and Entsminger, 2010). As rec-ommended by Fayle and Manica (2010), 30,000 random matriceswere simulated for each analysis to avoid Type 1 errors. ECOSIMsrandomisation algorithmmaintains xed sums for row and columntotals so each matrix generated had the same number of speciesand samples as the original. Differences between simulated andobserved co-occurrence patterns were tested using the Stone andRoberts (1990) checkerboard score (C-) index that indicatesrandom assemblage structuring if Cobs is not signicantly differentfrom Csim.</p><p>Univariate correlations were tested via Spearmans rank, andunivariate comparisons of numbers of individuals and of speciesper sampling unit used one-way ANOVA (the data concerned notdeparting from homoscedasticity; Bartletts test P&gt; 0.3 in all cases).Departures from random of the dispersion patterns of individualspecies were tested by Morisitas Index of Dispersion c2 statistic(Morisita, 1959). Species diversity was measured as the linear Hills(1973) N2 or the effective number of species (Jost, 2006, 2007),and dominance by the BergereParker Index; Constancy Indicesgiven are percentage frequencies of occurrence in core samples;categories of faunal rarity are sensu Colwell and Coddington (1994).</p><p>3. Results</p><p>The 2454 target faunal individuals collected by the core samplesfrom the Dunwich seagrass beds comprised 77% of the total mac-rofaunal individuals obtained and represented a total of 42 species(indicating a total target-group species richness of 70 as estimatedby themethod of Chao (1984)). As expected from earlierwork at thesite (Barnes, 2010a; Barnes and Barnes, 2011), the fauna was sparseand numerically dominated by gastropod molluscs (30 speciescomprising 62% of the total faunal individuals) and particularly byrissooideanmicrogastropods (54% of the total fauna). Overall faunaldensity was 2725 ind.m2. The three most abundant target-groupspecies were all small to minute: the </p></li><li><p>together with spatial scale (Table 5). The result was also consis-tent even when the analysis was repeated with core data pooledfor each locus to reduce the variance at that smallest-scale spatiallevel.</p><p>Fig. 3. Numbers of total target macrofauna and of the three most numerous species(the microgastropods Calopia imitata and Pseudoliotia micans, and crab Enigmaplaxlittoralis) present in the nine core samples at the spatial scale of locus in sequencealong the two transects. Loci at the same station are joined by a line.</p><p>Table 1Density m2 (N), species richness (S), species diversity (D) and BergereParkerdominance (d) of the target macrofaunal taxa, and proportion of the total macro-faunal densities comprised by those target taxa (P), within the four intertidal sea-grass sites at Dunwich.</p><p>Site N S D d P</p><p>One Mile (N) 1900 22 2.13 0.67 0.72One Mile (S) 1818 24 2.38 0.63 0.76Polka (N) 1632 26 4.55 0.36 0.71Polka (S) 3066 23 2.33 0.63 0.86</p><p>Coastal and Shelf Science 98 (2012) 119e125(ANOVA F(1,22) 1.85, P 0.19; F(1,22) 2.43, P 0.13 respectively) norper station (ANOVA F(1,6) 1.06, P 0.34; F(1,6) 2.56, P 0.16 respec-tively). Number of individuals per locus was not uniform across thefour sites (ANOVA F(3,20) 4.65, P 0.01), however, although this wasentirely consequent on only one of the six component comparisons,that between the two sites at the Polka locality (Tukeys HSD posthoc test P 0.02). Species richness per locus did not differ betweenthe four sites (ANOVA F(3,20) 1.40, P 0.28). Coefcients of variationfor groups of core samples within an individual locus averaged&gt;0.5for numbers of target-group individuals and&gt;0.4 for numbers of allmacrofauna. There were no signicant differences in the levels ofthis variation between the two localities nor between that withinthe target groups and that within the total fauna at either locality(Tukeys HSD post hoc test P&gt; 0.07).</p><p>Even though Calopia occurred in 92% of samples, its dispersionpattern showed marked patchiness at all spatial scales 30 m(station c2&gt; 64, df 26; site c2&gt;177, df 53; location c2&gt; 604,df 107; all P&lt; 0.00003), although at a spatial scale of 5 m (seeFig. 3), the pattern did not depart from random at 38% of loci(c2 0.05). Nevertheless, its overall densities at thetwo localities were very similar (1209 and 1257 m2). Less abun-dant though far from rare species, however, did vary between thetwo: Enigmaplax numbers, for example, were signicantly (2.8)larger at Polka (ManneWhitney Z 5.9; P 300; df 23; P&lt; 0.00001). Therewere no signicant correlations between the numbers of thedominant Calopia and either of the two other most abundantspecies, nor between the total numbers of target-group individualsand those in non-target groups in the samples (Spearman rs 0.01to 0.07; P&gt; 0.3). The relative abundance of the above species ofEnigmaplax, Calopia and Pseudoliotia, and the occurrence of those ofLongiagrum and Alpheus, together with the abundance of thebuccinoid gastropod Nassarius burchardi, accounted for 60% of thedissimilarity between the two localities (SIMPER). Various generalecological features of the faunal assemblages at the four sites aresummarized in Tables 1 and 2.</p><p>Despite their signicantly different assemblages, analysis ofcomponents of variance at the two localities produced compa-rable results (Table 3), slightly complicated by the need to poolthe data from two spatial scales at the Polka locality because ofthe occurrence of an illogical negative component at the scale ofstation (Underwood, 1997). In each case, however, the componentat the smallest spatial scale of 0.5 m was, at 67e69% of the total,markedly larger than that at the next smallest scale of 5 m. Theoverall relationship between spatial scale and components ofassemblage variance within the Dunwich beds is shown in Fig. 4,nested PERMANOVA indicating that components at and above thespatial scale of station were not signicant (P&gt; 0.1). C-scoreindex analysis showed that assemblage organisation did notdepart from random at any spatial scale between 0.5 and 900 m.This result was apparent both when a xed number of sampleswere randomly selected from different scales (Table 4), thuskeeping the statistical power similar, and when samples were</p><p>R.S.K. Barnes, M.D.F. Ellwood / Estuarine,122amalgamated as scale increased, thus increasing sample size</p><p>Overall 2104 42 2.63 0.59 0.77</p></li><li><p>4. Discussion</p><p>The challenge still facing ecologists considering spatial patternsis to match these to the spatial scales of the processes causing suchpatterns (Burrows et al., 2009). Within coastal marine systems,</p><p>in that organisation over the spatial scales at which variancecomponents changed from signicant to non-signicant. Althoughthe random nature of the Dunwich assemblages accords withLeighs (1981) expectation that such a situation will prevail in lowdensity systems, the observation that it extends over horizontaldistances of up to 1 km is unexpected. Yet is very robust since notonly consistent numbers of samples selected randomly fromdifferent spatial scales yielded that result, but so did increasingnumbers of samples from loci to stations to sites across the twolocalities. This practice in itself may have been expected to lead toType I errors and false reporting of deterministic species co-occurrence patterns (Fayle and Manica, 2010) but no such resultswere obtained. This suggests that the null model analyses are</p><p>Table 2Frequencies of occurrence and rarity in the target macrofaunal taxa at the fourintertidal seagrass sites at Dunwich. Percentage of species with Constancy Indices(CI) 50 and 10, mean faunal CI (N 54 per site), and percentage of singleton plusdoubleton species (s/d).</p><p>Site 50 10 Mean CI s/dOne Mile (N) 9.1 63.6 14.6 54.5One Mile (S) 4.2 58.3 14.3 45.8Polka (N) 7.7 69.2 13.7 38.5Polka (S) 8.7 52.2 17.8 34.8</p><p>Overall 4.8 81.0 8.5 38.1</p><p>R.S.K. Barnes, M.D.F. Ellwood / Estuarine, Coastal and Shelf Science 98 (2012) 119e125 123these patterns have generally been investigated at two differentranges of spatial scale, those greater and those less than 10 km, atwhich dif...</p></li></ul>


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