A CAUTIONARY NOTE: TEMPORAL EFFECTS ON THE CAPTURE OF MANGROVE CRABS BY PITFALL TRAPS. Tansy Boggon

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1 A CAUTIONARY NOTE: TEMPORAL EFFECTS ON THE CAPTURE OF MANGROVE CRABS BY PITFALL TRAPS Tansy Boggon Marine and Estuarine Ecology Unit, School of Life Sciences, University of Queensland, St Lucia, Queensland, 4072, Australia. ABSTRACT This study suggests an effect of temporal variation on the capture of mangrove crabs by pitfall traps. More crabs, and less consistently, more species were captured during a period with rainfall than a dry period. These results have important implications for methods used to sample mangrove crabs that are dependent on the activity of crabs such as pitfall traps, visual counts and hand-catches. Future studies that use these methods need to consider the effect of weather on the catchability of crabs and take into account these effects when interpreting data. INTRODUCTION Crabs are a dominant macrofaunal element in mangrove forests (Jones 1984) and consequently are often studied. However, many methods used to quantify crab assemblages such as visual counts, pitfall traps and handcatches are relative to crab activity, such that the numbers recorded by these methods provide data on the degree of activity rather than the actual population densities (Luff 1975; Adis 1979; Lee 1998). This is acceptable for comparing crab assemblages between space and time, providing biases can be assumed constant (Seber 1973; Caughley 1977). However, amongst other factors, differences in weather conditions over space and/or between times may alter crab activity and hence catchability. This difference in crab activity may be independent of population density and thus bias measures of crab assemblages. This study illustrates the effect of temporal variation on crab assemblages captured by pitfall traps. METHODS In a study comparing crab assemblages among sites of different levels of human activity (Boggon & Skilleter In prep) crab assemblage data were collected during spring tides of two consecutive months (August and September) in Unintentionally data were collected in a single wet (rainfall) and dry (no rainfall) period (Figure 1) and the differences between the wet and dry periods are presented in this study. During the wet periods rainfall was relatively constant throughout the entire period the pitfall traps were set. Seven sites were sampled at three locations (Boondall, Fisherman Islands and Wynnum) in Moreton Bay, Southeast Queensland, Australia. All sites were located in Avicennia marina forests that were tidally inundated daily. Sites were nested in these three locations based on their level of human activity (i.e. two sites at both Boondall and Wynnum and three at Fisherman Islands). At each site five pitfall traps were randomly located within three replicate 10x10m plots. Each pitfall trap consisted of a 2 litre plastic container, 15cm in diameter, reaching a 10cm depth, with 3mm holes drilled 32

2 Rainfall (mm) Wynnum Boondall Locations Wet Dry Fisherman Islands Figure 1: Rainfall (mm) for 48 hour periods at each location in the wet and dry periods in the two consecutive months sampled. Source: Bureau of Meteorology, Brisbane, into the sides and base to allow water to drain from the trap at low tide. The upper surface of the trap was comprised of a tight-fitting lid with a 5.5cm diameter opening and funnel (directed downwards) attached to the underside. The funnel acted as a barrier preventing crabs from crawling out of the trap or being washed out at high tide. Pilot studies found that pitfall traps without the lid and funnel did not capture large crabs. The traps were placed in a 10cm hole dug into the substratum so that the lid was just below the surface of the substratum. The traps were then covered in mud, except the opening, to reduce the risk of them floating away on the rising tide. After 48 hours the traps were removed from the substratum and the crabs were counted and identified to species level in situ where possible and then released. Those crabs that could not be identified in the field were taken to the laboratory and preserved in 70% ethanol for later identification. Differences in crab abundance and species richness between the wet and dry periods were determined by oneway fixed analysis of variance (ANOVA) for each site. The percent difference of abundance of each species between the wet and dry period was calculated to determine the magnitude of difference in species abundance between wet and dry periods. RESULTS AND DISCUSSION More crabs were captured in the wet than dry period (Table 1). All but the two sites at Wynnum had significantly more crabs in the wet than dry period (Figure 2a). No known previous studies have recorded an increase in crab abundance with rainfall. Crab abundance may be greater with rainfall because of a reduced risk of desiccation; thus increasing the amount of time crabs can be active on the forest floor. Increased moisture in the soil and atmosphere because of rainfall could reduce the need for crabs to return to their burrows or to puddles to acquire water for respiration (MacNae 1968; Hutchings & Saenger 1987). In addition, by maintaining soil moisture, rainfall may increase the time of feeding of species of crabs that require moist sediments for deposit feeding. Therefore, rainfall may allow crabs to be active on the forest floor for longer periods of time, thus increasing their catchability. Consistent with total crab abundance, more individuals of all species, except Helice leachii (Hess 1865), were captured in the wet than dry period. 33

3 Table 1: Number of crabs of each species captured in the wet and dry periods and the percent difference in number of crabs. Family # of crabs captured Sub-family % difference Wet Dry Species Grapsidae Grapsinae Ilyograpsus paludicola Sesarminae Helograpsus haswellianus Helice leachii Sesarma erythrodactyla Ocypodidae Macrophthalminae Australoplax tridentata Macrophthalmus punctulatus Paracleistostoma mcneilli Cleistostoma wardi Xanthidae Pilumnus sp Total Number Total Species 9 7 a number of crabs / pitfall trap wet dry b number of species / pitfall trap B2 W2 FI3 B1 W1 FI2 FI1 wet dry low human activity high human activity Figure 2: Mean (+ standard error) of a: crab abundance and b: species richness at all of the sites for the wet and dry periods; where W = Wynnum; B = Boondall; FI = Fisherman Islands and the number corresponds to the individual site sampled at each of these locations. Indicates that crab abundance or species richness was not significantly different between the wet and dry periods (p > 0.05) (n = 15 pitfall traps per site per period). 34

4 However, species differed in the magnitude of difference in abundance between the wet and dry period. Some species had considerably more individuals in the wet than dry period, whilst others were not substantially different. Ilyograpsus paludicola (Rathbun 1909) and Sesarma erythrodactyla (Hess 1865) had less than 13% more individuals in the wet than dry period, whilst Helograpsus haswellianus (Whitelegge 1899), Australoplax tridentata (Edwards 1873), Paracleistostoma mcneilli (Ward 1933), Cleistostoma wardi (Rathbun 1926) and Pilumnus sp. had over 50% more individuals captured in the wet than dry period. This may be because different species have different tolerances to desiccation (e.g. Omori et al. 1998). More species of crabs were captured during the wet than the dry period (Table 1) and four of the seven sites had significantly more crabs in the wet than dry period (Figure 2b). Overall nine species were captured in the wet period compared with seven species captured in the dry period (Table 1). Species richness may be greater during periods of rainfall because more crabs are active on the forest floor, and thus the catchability of rarer species is increased. Alternatively, these rare species may be more sensitive to desiccation. Although this study suggests an effect of rainfall on the abundance and species richness of mangrove crabs, these results were not consistent across all sites. This may be the result of differences among sites in the level of rainfall (Figure 1), the period of time of rainfall (e.g. day or evening, midday or dawn or dusk) and the coincidence of rainfall with low tide periods. Other factors such as tidal height and elevation that could influence soil moisture and thus the relative effect of rainfall on crabs. In addition, differences in the habitat structure among sites (e.g. canopy cover, pneumatophore density; see Boggon & Skilleter In prep) may influence the effect of weather (Honek 1997; Melbourne 1999). Changes in weather may be less dramatic at sites with complex habitat structure because the habitat structure may provide a buffer to large-scale differences in weather. For example, at a site with minimal habitat structure the humidity and temperature throughout a day may differ considerably more that in a forest with a dense canopy in (e.g. Honek 1997; Melbourne 1999). Therefore, the effect of weather on crab assemblages should be tested across sites that differ in habitat structure and other variables that may influence the effect of weather (e.g. elevation). Weather variables should also be measured at the site-scale rather than using broad-scale meteorological data because many sitespecific factors may influence the weather at the small-scale relevant to mangrove crabs. The effects of weather on measures of crab abundance and species richness is also potentially important for other methods for sampling mangrove crabs that are dependent of surface activity. Measures of crab abundance and richness using visual counts or handcatches may be more sensitive to weather than pitfall traps because visual counts and hand-catches may relate to differences in crab activity over smaller temporal scales. Crab activity is likely to differ within a single day (Warner 1977; Nakasone 1982), thus biasing measures of relative abundance. For example, diurnal differences in temperature and time since high tide influences the abundance of crabs measured by visual 35

5 counts (Nobbs 1999). Factors such as time since low or high tide, time of day, day length, temperature, soil and atmospheric moisture, rainfall, cloud cover and lunar phase may all influence crab activity (Golley et al. 1962; Palmer 1973; Crane 1975; Zucker 1978; Murai et al. 1982; Salmon 1984, 1987; Nobbs 1999; Macia et al. 2001) and should be considered when interpreting measures of crab abundance that relate to crab activity. The results of this study also have implications for seasonal studies. For example, more crabs may be captured during rainy seasons than in dry seasons because of greater crab activity with rainfall, even if population size is the same. Studies have shown that crabs are less active during winter than summer and have attributed this to factors such as temperature and humidity (Griffin 1968; Nakasone 1982). Therefore, studies should take caution when comparing seasonal differences in crab abundance. We will require a greater understanding of the effects of weather on crab activity to validate studies on the seasonal differences in crab assemblages. This study has shown differences between crab captures at two times. This suggests comparative studies of crab abundance based on capture methods where there is variation between sites in date of study need to be interpreted with caution. One hypothesis to explain the particular differences detected in this study is that there is an effect of rainfall on mangrove activity and/or catchability, such that weather may bias measures of crab abundance and species richness. The results highlight the view that we should consider the effect of weather on measures of crab assemblages that reflect crab activity such as visual counts, pitfall traps and hand-catches. Future studies that use methods for sampling mangrove crabs that are dependent on crab activity should consider or examine weather variables. The hypothesis that the observed effect was specifically due to rainfall requires further examination, both through further observation and if possible experimental manipulation (for example, by watering plots). REFERENCES Adis J. (1979) Problems of interpreting arthropod sampling with pitfall traps. Zoologischer Anzeiger 202: Boggon T. and Skilleter G.A. Does human activity relate to the habitat structure of mangrove forests and associated crab assemblages? In Prep. Caughley G. and Gunn A. (1996) Conservation biology in theory and practice. Blackwell Science Pty Ltd, Victoria, Australia. Crane J. (1975) Fiddler crabs of the world. Ocypodidae: genus Uca. Princeton University Press, New Jersey. Golley R., Odum H.T. and Wilson R.F. (1962) The structure and metabolism of a Puerto Rican red mangrove forest in May. Ecology 43:9-19. Griffin D. J. G. (1968) Social and maintenance behaviour in two Australian ocypodid crabs (Crustacea: Brachyura). Journal of Zoology (London) 156: Honek A. (1997) The effect of plant cover and weather on the activity density of ground surface arthropods in a fallow field. Biological Agriculture and Horticulture 15:

6 Hutchings P.A. and Saenger P. (1987) The ecology of mangroves. University of Queensland Press, St Lucia. Jones D.A. (1984) Crabs of the mangal ecosystem. In: Hydrobiology of the mangal (eds F.D. Por and I. Dor) pp Dr W. Junk Publishers, The Hague. Lee S.Y. (1998) The ecological role of grapsid crabs in mangrove ecosystems: a review. Australian Journal of Marine and Freshwater Research 49: Luff M.L. (1975) Some features influencing the efficiency of pitfall traps. Oecologia 19: MacNae W. (1968) A general account of the fauna and flora of mangrove swamps and forests in the Indo-west Pacific region. Advances in Marine Biology 6: Macia A., Quincardete I. and Paula J. (2001) A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco mangrove, Inhaca Island (Mozambique). Hydrobiologia 449: Melbourne B.A. (1999) Bias in the effect of habitat structure on pitfall traps: an experimental evaluation. Australian Journal of Ecology 24: Murai M., Goshima S. and Nakasone Y. (1982) Some behavioral characteristics related to food supply and soil texture of burrowing habitats observed on Uca vocans vocans and U. lactea perplexa. Marine Biology 66: Nakasone Y. (1982) Ecology of the fiddler crab Uca (Thalassuca) vocans vocans (Linnaeus) (Decapoda: Ocypodidae) I. daily activity in warm and cold seasons. Research in Population Ecology 24: Nobbs M. (1999) The behavioural ecology of fiddler crabs (Genus: Uca) that live in the mangrove forests of Darwin Harbour. Ph.D. Thesis, Northern Territory University Australia. Omori K., Irawan B. and Kikutani Y. (1998) Studies on the salinity and desiccation tolerances of Helice tridens and Helice japonica (Decapoda: Grapsidae). Hydrobiologia 386: Palmer J.D. (1973) Tidal rhythms: the clock control of the rhythmic physiology of marine organisms. Biological Review 48: Salmon M. (1984) The courtship, aggression and mating system of a 'primitive' fiddler crab (Uca vocans: Ocypodidae). Transactions of the Zoological Society of London 37:1-50. Salmon M. (1987) On reproductive behaviour of the fiddler crab Uca thayeri, with comparisons to U. pugilator and U. vocans: evidence for behavioural convergence. Journal of Crustacean Biology 7: Seber G.A.F. (1982) The estimation of animal abundance and related parameters. Charles Griffin & Company Ltd, London. Warner G.F. (1977) The biology of crabs. Elek, London. Zucker N. (1978) Monthly reproductive cycles in three sympatric hood building tropical fiddler crabs (Genus Uca). Biological Bulletin 155:

A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco Mangrove, Inhaca Island (Mozambique)

A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco Mangrove, Inhaca Island (Mozambique) Hydrobiologia 449: 213 219, 2001. J.P.M. Paula, A.A.V. Flores & C.H.J.M. Fransen (eds), Advances in Decapod Crustacean Research. 2001 Kluwer Academic Publishers. Printed in the Netherlands. 213 A comparison