Comparative sampling methods for subtidal ...

J. Exp. Mar. Biol. EcoL, 164 (1992)87-101 © 1992 Elsevier Science Publishers BV. All rights reserved 0022-0981/92/$05.00

87

JEMBE 01861

Comparative sampling methods for subtidal epibenthic gastropods T.R. McClanahan

a

and N.A. Muthiga b

" Wildlife Conservation hJternational. Coral Reef Conservation Project, Mombasa: b Kenya Marine & Fisheries Research hlstitute, Mombasa, Kenya

(Received 14 April 1992; revision received 3 July 1992; accepted 24 July 1992)

Abstract: A comparative survey of patchily distributed prosobranchs inhabiting seagrass and hard substratum (live, dead or Pleistocene coral) in both the Florida Bay and Hawk Channel environments of the Florida Keys was undertaken to compare a l-h search versus a quadrat method (5 m -~) of sampling. We tested the hypotheses that (1)there are differences in observer search ability and calculated community structure parameters, t2)large-bodied specie~ will be over-sampled compared to small-bodied species, (3)abundant species will be under-sampled due to observer habituation towards abundant species, (4)individuals with cryptic or nocturnal habits will be under-sampled during daytime sampling, and (5) there are differences in search efficiency among habitats. Two independent observers using the search method had less than 20°~o variation in all community structure parameters and 10°,o variation in community composition similarity (Bray-Curtis Index) suggesting that observer-bias is small for experk:,~ced observers, in three habitats, Hawk Channel seagrass, Florida Bay seagrass and hard substratum, there was no evidence of over-samplipg large-bodied species or of habi~iaation to abundant species. In Hawk Channel hard substratv, m sites Strombus gigas L. (Queen Conch) appeared to be the single species over-sampled due to its large body size, and some evidence suggests that Cerithium literatum Born, which buries itself in the sand, was under-sampled by the search method. Nocturnal sampling indicated that two species C. ao'atmn Born and Marginella apicina Meake n;ay have been under-sampled in the Hawk Channel seagrass habitat during the daytime while no species appeared to be under-sampled in the Florida Bay site. These nocturnally active species were patchily distributed, produce population estimates with high variation, and, therefore, day-night population density compa-'isnns were not statistically different. The search method missed cryptic juveniles found by quadrat sampling. Search t, ata displayed a greatcr pallern of central tendency, low coefficients of variation and more species encount,,rcd per unit effort in habitats with low population densities. Quadrat sampling data were right-skewed for low population density sites and had high coefficients of variation suggesting that estimates of population means by sample means may not be accurate when sampling only 5 m 2 per replicate. Search sampling is cost effective in terms of data collected per unit of labor and appears to produce fairly reliable population estimates but data are in units of time spent searching versus preferred units of 2-dimensional space. Key words: Gastropod; Population estimate; Quadrat; Search sampling; Spatial distribution; Subtidal population

INTRODUCTION

For the sake of expediency marine ecologists often focus their research on abundant species that are easily counted in small areas (i.e. < 10 mE). Yet, the majority of subtidal Correspondence address: T.R. McClanahan, Wildlife Conservation International, Coral Reef Conservation Project, PO Box 99470, Mombasa, Kenya.

88

T.R. McCLANAHAN AND N.A. MUTHIGA

tropical animals exist at low population densities which makes traditionally accepted sampling techniques (i.e. 1 m 2 quadrats) of questionable value. Tropical intertidal and subfidal prosobranchs are an assemblage which frequently exists at low population densities and may have cryptic behaviors that make sampling difficult. In order to sample in a time-efficient manner in intertidal and shallow subtidal areas some researchers (Kohn, 1968; Nybakken, 1978; McClanahan, 1989, 1990, 1992)have used a search-sampling technique which enumerates the number of individuals and species found in a certain search interval. Some potential weaknesses of this method are (1) differences in observer search ability may make results difficult to replicate over time and between observers, (2) large-bodied species may be over-sampled relative to smallbodied species, (3) very abundant species may be under-sampled due to observer habituation, (4) individuals with cryptic or nocturnal habits may be under-sampled, and (5) differences in sampling efficiency among habitats may make realistic between-habitat comparisons difficult. This paper explores the above concerns (i.e., hypotheses) about search sampling and compares quadrat versus search sampling techniques in four Florida Keys habitats (Fig. 1 in McClanahan, 1992). This comparison is then used to make su~ggestions aimed at improving sa~npling methods for low population density subtida[ animals. METHODS

A quadrat versus search sampling comparison was made in each of four habitats. These included both si~allow (0.3-1.5 m) seagrass (SG) and hard substratum areas (H5) in the Florida Bay and Hawk Chamlel (referred to by first letters of name, i.e., HCHS-~ Hawk Channel Hard Substrate), described by McClanahan (1992). Within the same habitat, area, and water depth one observer used the search-sampling procedure (McCianahan, 1989, 1992) while a second observer haphazardly tossed a 5-m 2 quadrat, identified species, and attempted to count all prosobranchs observed in the quadrat. Search sampling was completed using a diving mask, fins, and snorkel in shallow water (water depths greater than 1.5 m were avoided due to reduced visibility of the seaflo~r beyond this depth) while recording the number of individuals of each encountered species (Abbott, 1974) onto an underwater slate. Three replicate 1-h search periods were performed in each habitat. The search observer would maintain a constant but slow swimming speed while floating on the water's surface in order to avoid lingering in areas either with or without high prosobranch nopulation densities. The 6bserver would frequently change swimming directions (i.e. zigzag) and would occasionally ( ~ 5 min) descend to the bottom and search through seagrass or algae. Boulders were overturned and examined for prosobranchs as they were encounter~d. The quadrat observer spent between 5 and 10 min per quadrat and attempted to count all visible epibenthic prosobranchs in quadrats. Quadrats had :~ central focus with a 1 cm thick rope fastened to the focus. The observer (using a mask and snorkel but no fins) would bend or lie over the quadrat and start observations in one quadrat

SAMPLING EPIBENTHIC GASTROPODS

89

section, starting the search at the quadrat's focus and searching out away from the focus along the rope to the edge of the quadrat. The rope was then moved in a clockwise direction and the procedure repeated until the entire quadrat was covered. The observer would pick through algae and seagrass as required to identify and count enco~_t.,,,tcred prosobranchs. Although the observer attempted to count all prosobranchs (i.e. ground truth), it should be appreciated that in quadrats with thick seagrass and algal cover it was difficult to insure that all gastropods present were counted. Data from this sampling study were used to compare assemblage composition indices calculated from the two methods. Sampling methods allow for calculations of density (numbers/h or numbers/5 m2), diversity and species richness, and species and abundance similarity. Diversity was calculated using a modification of the Simpson's Index (D = 1 -(,~i/Nt)2; Simpson, 1949) which produces a number between 0 and 1; 0 being the lowest and 1 being the highest possible diversity. Similarity between samples was calculated using Sorensen's (1948) and the Bray & Curtis (1957) indices. Sorensen's Index (Similarity = 2C/(A + B); where C = the number of species common to both sites A and B, A = total species in site A, and B--total species found in site B) was referred to as species similarity. The Bray-Curtis Index (Similarity=2ZIminxi; xjl/ Y(xi + xj); where xi and xj are the abundance of each species found at sites i and j, was referred to as abundance similarity. Additionally, a time-space conversion was calculated for each species (i.e. number/h/number/m 2-- m2/h) and used to calculate species and an average species space-time conversion for each habitat. This calculation was also used to correlate with the species' median body lengths reported by Abbott (1974)to test tile hypothesis that larger species are encountered at a greater rate relative to smaller bodied species. To determine if observer habituation occurred towards the most abundant species correlations between relative abundance (i.e. ~ of the total faunal composition on a population density basis) determined for search and quadrat sampling were plotted against each other. If habituation was occurring then scatter-plots and correlations should result in bowed (i.e saturation curves) rather 'than straight best-fit lines. A day-night comparison of assemblage composition was made in a Florida Bay hard substratum and a Hawk Channel seagrass site (Long Key State Park). The searchsampling procedure was used during the daylight (n = 3 h per site) and then repeated in the same location at night (2100-.2400) using a flashlight with a halogen bulb. Previous cursory comparisons of the search-sampling method (McClanahan, 1989) comparing two observers suggests oniy small and statistically insignificant differences in observer searching ability for total prosobranch abundance and spccies richness of coral-reef prosobranchs. Yet, this research did not compare individual species population densities or total assemblage composition determined by independent observers. Consequently, during 6 of the search-sampling hours, a second less experienced observer used the same procedure while working independently in the same area. Both hard substratum and seagrass sites were sampled. An analysis of species composit;,on, abundance, diversity and similarity determined by the two observers is presented.

90

T.R. M¢CLANAHANAND N.A. MUTHIGA

Distribution patterns of population density (data taken from McClanahan, 1992, and unpubl, data) were tested for normality using cumulative frequency distributions for sample sizes of > 50 and the Rankits method for samples of 1; repulsed, CD < 1). Coefficients of variation (V - 100 s/~) of individual species' population estimates were correlated with estimates of their population density to determine the reliability of population density estimates by search and quadrat methods as a function of the species' abundance.

RESULTS

Search versus quadrat sampling comparisons of species ranks, diversity indices, coefficients of variation, species numbers, and species and abundance similarity at the four sites are presented in Table I. Spearman's correlations of species ranks indicate medium to high correlations for all sites (HCSG rs ---0.96, FBSG r s - 0.76 and FBHS r~ = 0.75) with the exception of HCHS (r~ - 0.30). In Florida Bay sites and in HCSG there was < 20% variation in the diversity index by the two sampling methods. In HCHS, however, the quadrat ~ampling method produced a lower diversity index than search sampling ,---/o) tan °~ largcl2~ attributable to the re',atively high density of Cerithium literature Born found by quadrat sampling. C. literature were found in large groups (5-21 individuals) either partially or completely buried in sand. Typically, the quadrat obseiver would notice a few individuals partially buried and by sifting through the sand by hand additional individuals would be ancovered. Many of these C. literature individuals may not be considered strictly epibenthic. This difference in measured C. literatm, abundance also resulted in low abundance similarity (427Jo) between the two sampling methods, Abundance similarity calculated by comparing the two sampling methods was > 85?'0 in all sites, except HCHS. In the HCHS site more species were found by search sampling than by quadrat sampling, although 6 h were required to complete quadrat sampling compared to 3 h for search sampling. FBHS and HCSG sites had less than a one species difference between the two sampling methods. Quadrat-sampling found two more species in the FBSG site than the search-sampling technique although it took ~ 4 h to sample 38 quadrats. Similarity in species encountered (Sorensen's Index)by the two techniques was > 0.89 at FBIIS arid tlCSG sites and between 0.89 and 0.67 for the FBSG and HCHS sites. Space-time conversion factors calculated frcm quadrat vs. search methods varied from 73 to 151 m2/h in the FBHS, HCSG, and FBSG sites. Quadrat-sampling uncovered many small, often cryptic, juveniles (i.e. estimated shell lengths < 1.5 cm) that were not found by the search-sampling procedure. Cryptic juveniles (particularly Astraea americana Gmelin), which were notably abundant in the FBSG site, largely account

SAMPLING EPIBENTHIC GASTROPODS

09

Researchers are frequently interested in finding the most accurate estimate of population density for the least effort. Since the effort per replicate is unequal for the two methods, we estimate from field work a 6-10/1 ratio of quadrat/search effort (i.e. 5-10min per 5 m-" quadrat), with seagrass and areas covered by thick algae (i.e. HCSG, FBSG and FBHS) requiring more time per quadrat than sparsely covered areas (i.e. HCHS). Therefore, for the same desired confidence interval the search methoa is, on average, somewhat more cost or effort effective than quadrat sampling. Quadrat sampling variation is likely to be reduced and data distributions should show greater central tendency with larger quadrat sizes (i.e. 50-100 m2). Search efficiency per unit area may be reduced for large quadrats and the effort per replicate will increase. Additional work is required to determine the optimal quadrat-sampling design for subtidal epibenthic prosobranchs which is both efficient in its use of time and produces accurate estimates of population means. Population density estimates of rare species, particularly for the quadrat method, are likely to remain inaccurate unless large sample sizes are obtainable. The above analysis is based on average normalized standard deviations of individual species. Since standard deviations of population densities appear to depend on population densities, we would recommend the search method for sampling rare species and habitats with low population densities, particularly when the number of species (i.e. species richness) is desired for the least sampling effort, and the quadrat method for sampling abundant species or habitats with high population densities. When different habitats and species comparisons are desired the method which is less likely to bias stated hypotheses should be chosen (i.e. McClanahan, 1992)realizing the limitations of each method in each habitat. When quadrat sampling is per'formed in habitats with low population densities, large replicates are desirable and ,lonparametric statistics should be used. Although it was hypothesized that between-habitat differences in the search method may make between-habitat comparisons cfifficult, it should also be appreciated that quadrat sampling has the potential for inaccuracies in comparing habitats. For irkstance, due to the high -lensity of stems and rhizomes in seagrass habitats it is difficult to find or observe prosobranchs partially or wholly buried in the sand, while in mo;e open hard substratum sites it is much easier (i.e.C. literature in the HCHS site). Conseqt~ently, perhaps the most precise comparisons would require dredging large area~, of seagrass and hard substratum and then sieving through dredgings. This would demand a greater expense of time and money, cause greater environmental damage, and, given the low population density and high variability of the assemblage, would require a large number of replicates or large replicate sizes. Due to the greater space covered per replicate, search-sampling replicates have lower levels of variation and a closer to normal data distribution, but the units are in time spent sampling rather than in area measured. Square root transformations of search data may be necessary when one is concerned about significance levels derived from parametric statistics. Search-sampling may also overestimate the relative abundance of very large prosobranchs (i.e.S. gigas), underestimate the abundance of cryptic juve-

100

T.R. McCLANAHAN AND N.A. MUTHIGA

niles, and search efficiency between different habitats may make betweep.-habitat comparisons difficdlt. Nonetheless, overestimating large individuals may not be a great detriment to sampling as these individuals, if measured on a biomass or energetic basis (i.e. respiratory biomass), would be far more dominant than density-based estimates of abundance. Conversely, cryptic juveniles may contribute greatly to a density-ba~ed measure of abundance bur would frequently be less important on a biomass or energetics basis. If one is interested in studying population structure or the abundance of juveniles, the search-sampling technique should not be used. Over- or underestimations by an observer may actually be a form of observer compensation to account for abundance based partly on individual body sizes, when differences are la~'ge. Despite these potential weaknesses, there was a high level of correspondence between the two observers in this study which suggests that fairly accurate replication is possible (~ 5-20~o variation due to observers) and both observers were making similar observations or errors. Further, we suggest that rather than differences in betweenhabitat search efficiency, differences in habitat-specific time-space conversions are largely due to (1) poor estimates of population means by the quadrat-sampling method, and to a greater extent, by (2) higher population densities of cryptic juveniles in seagrass and FBHS and FBSG than in HCHS sites. Excluding cryptic juveniles from quadratsampling data would reduce time-space conversion variation among habitats and greatly increase time-space conversion factors for most species. Consequently, the search sampling procedure seems sufficient when the exclusion of cryptic juveniles is desired or does not detract from sampling objectives or stated hypotheses. The search-sampling procedure could be improved If the observer used a measuring stick (i.e. carrying or attaching a measuring stick to their slate)of known length (i.e. 1-2 m) and recorded only those individuals found under the measuring stick during the specific time interv~ll. This would allow the observer the freedom and efficiency of scare'h-sampling while increasing the accuracy of population density estimates. If labor is sutticJent then laying transect lines (i.e. 50-100 m) and counting individuals within a given distance from the line (i.e. 1-2 m) could also improve the accuracy of population estimates, but this is likely to double the effort per replicate. It should be recognized that as size of the sample area is increased reliability of population estimates should decrease due to errors associated with an increased possibility of missing individuals in transects. Consequently, differences between search and quadrat sampling may be small when quadrat sampling is performed on large quadrats. If affordable in terms of cost and labor, quadrat sampling is preferred simply because the estimate is areal units rather than time. Some of the poor correspondence between the search and quadrat method particularly in the HCHS sites may be attributable to problems of defining the sampled assemblage. Some species such as cerithids may be partially or wholly buried depending on the time of day and other variables. After sampling we realized that the search observer did not include partially buried individuals in samples while the quadrat observer included individuals in the first few centimeters of sand. Consequently, a clear

SAMPLING EPIBENTHIC GASTROPODS

101

statement concerning c6teria for choosing species is desirable for ce~mparative purposes. The search method should only attempt to sample epibenthic species which have more than half of their bodies exposed. The quadrat method should clearly state the criteria for inclusion of species and the depth of sampling into the substratum. The time of the day when sampling is completed may, in some instances, affect population estimates. Some species are nocturnally active and bury themselves in sedime.nts during daylight hours. This occurred, though patchily, in HCSG sites. It may be that in areas where predation is more intense (i.e. HCHS, McClanahan, 1992) prosobranch nocturnal activity increases (Vermeij, 1987). This requires further testing. Nonetheless, although nocturnal sampling may ofter: be impractical for extensive sampling, it is good practice to visit sites at night to determine the degree of correspondence between day and night faunas. ACKNOWLEDGEMENTS

Research received partial financial support from Conchologists of America, the Florida Institute of Oceanography while employed at the Keys Marine Laboratory, and Wildlife Conservation International through grants from The Pew Charitable Trust and Conservation, Food & Health Foundation. The logistical support and advice of R.T. Abbott, K. Clark (for the use of his research vessel), and the staff of the Keys Marine Laboratory, Long Key are appreciated.

REFERENCES AbboU, R.I., 1974. American SeasheiL~. Van Nostrand Reinhold Co., New York, 663 pp. Bray, J. R. & J.T. Curtis, 1957. An ordination of the tq~land fi~rest communities c~l"southern Wi:~consin. Ecol. Monogr., Vol. 27, pp. 325-349. Kohn, A.J., 1968. Microhabntats, abundance and food of (,'omts on atoll reefs in the Maldive and Chagos Islands. Ecology, Vol. 49, pp. 1046- 1062. McClanahan, T.R., 1989. Kenyan c~,ral reef.associaled gastropod fauna: a comparison between protected and unprotected reefs. Mar. Ecol. Prog. Ser., Yol. 53, pp. 11-20. McClanahan, T.R., 1990. Kenyan coral reef-associated gastropod assemblages: distribution and diversity patterns. Coral Reef~, Yol. 9, pp. 63-74. McClanahan, T.R., 1992. Epibenthic gastropods of the Middle Florida Keys: the role of habitat and environmental stress on assemblage composition. J. Exp. Mar. Biol Ecol., Vol. 160, pp. 169-190. Nybakken, J., 1978. Abundance, diversity and temporal var ation in a Caiifornia intertidal nudibranch assemblage. Mar. Biol., Voi. 45, pp. 129-146. Simpson, E.H., 1949. Measurements of diversity. Nature, Vol. 163, p. 688. Sokal, R.R. & F.J. Rohlf, 1981. Biometrv. Freeman, New York, 859 pp. Sorensen, T., 1948. A method of establishing groups of equal amplitude in plant society based on similarity of species content. K. Danske Vendensk. SeL~k., Voi. 5, pp. 1-34. Vermeij, G.J., 1987. Evohaion and Escalation: An Ec~,iogical Histo O' ~71Li./e. Princeton University Press, Princeton, N J, 527 pp.