An Evaluation of Some Marking and Trapping Techniques Currently Used in the Study of Anuran Population Dynamics

Abstract

-Many techniques are used in the study of amphibian breeding populations despite lack of knowledge of their inherent biases. I examined trapping and marking techniques for Rana temporaria and Bufo bufo. Panjet dye marking and PIT (passive integrated transponder) tags had negligible effects on survival and body condition, while persisting up to two years. Pitfall trapping was a reliable technique with <1% of individuals escaping from experimental traps; however inaccuracies were found when pitfall traps were used in association with partial barriers. The proportions of species and sexes trapped varied between partial barriers within a site. Many species of amphibian are declining world-wide (Wake, 1991; Griffiths and Beebee, 1992). However, the status of some populations has been inferred from anecdotal observations or assessed using a wide variety of techniques (see review in Pechmann et al., 1991). If comparisons among studies are to help us understand world-wide trends, then research techniques have to be standardized. Heyer et al. (1994) have produced a guide aimed at standardizing amphibian survey techniques. The authors of this guide acknowledge that we have an incomplete understanding of the biases associated with some techniques. I attempted to quantify these biases for some commonly used techniques, using data from a three-year, mark-recapture study of the breeding behavior of common frogs (Rana temporaria) and common toads (Bufo bufo) in the northeast of Scotland. The objectives were to examine potential errors associated with two marking techniques and some capture techniques (pitfall trap escape rates, the use of partial barriers, and visual surveying). In field studies, individual or cohort marking allows estimation of population size, growth rates, behavior, and other population attributes. For techniques to be valid, marks must be permanent and have no detrimental effect on the animal's growth, reproduction or survival (Krebs, 1989). Marking techniques used on anurans include skin branding, jaw tagging (Raney, 1940), and waist banding (Woodbury, 1956), all of which have associated problems (Woodbury, 1956). Toeclipping is the most widely used marking technique (see Donnelly et al., 1994). Some studies have found toeclipping did not influence survival or behavior (Dodd, 1993; van l Address for correspondence. Gelder and Strijbosch, 1996), however a small number of studies have found increased mortality, inflammation, or change in behavior (e.g., Humphries 1979 In Lemckert, 1996; Golay and Durrer, 1994; Lemckert, 1996). Two relatively recent techniques, PIT (passive integrated transponder) tags and intracutaneous injection of dye using a 'Panjet Inoculator, have been used on anurans (e.g., Wiesniwski et al., 1980; Sinsch, 1992; O'Neill, 1995). Although permanence of both has been examined for a number of amphibian, reptile, and mammal species (e.g., Hart and Pitcher, 1969; Camper and Dixon, 1988; Sinsch, 1992), few studies have examined possible effects on survival and growth. Elbin and Burger (1994) reviewed studies (over all vertebrate groups) which examined the use of PIT tags. Most of them examined only short-term infection at the implantation site (e.g., Sinsch, 1992; Schooley et al., 1993). Apparently panjet dye-marking had no long-term effect on fish growth (Mcllwain and Christmas, 1975; Hill and Grossman, 1987; Herbinger et al., 1990), but the technique has not been evaluated for anurans. In the present study I examined the durability of PIT tagging and dye marking on captive anurans. I also considered possible effects on body condition and survival in the field. If dye marking affects survival, then individuals with a higher number of dye marks should suffer higher mortality than those with fewer marks. Recapture rates of marked animals were used to examine survival rates. Techniques commonly used to capture amphibians include visual surveying, artificial coverboards, and pitfall traps in association with drift fencing (see Heyer et al., 1994, for review of all techniques). Drift fencing and pitfall trap systems are used both to monitor breeding populations (e.g., Gittins, 1983; Pechmann and Semlitsch, 1986; Ryser, 1989; Dodd, 1995) and to This content downloaded from 157.55.39.212 on Sun, 09 Oct 2016 05:20:31 UTC All use subject to http://about.jstor.org/terms FIELD TECHNIQUE EVALUATION sample amphibians in terrestrial habitats away from the breeding sites (e.g., Jones, 1988; Raphael, 1988; McComb et al., 1993; Pasanen et al., 1993). However, no single capture technique will catch all species, sexes, and age classes in proportion to their true abundance (Williams and Braun, 1983; Corn, 1994). Many terrestrial habitat studies have examined the efficiency of different capture techniques (Campbell and Christman, 1982; Vogt and Hind, 1982; Bury and Corn, 1987, Block et al., 1988; Friend et al., 1989; Greenberg et al., 1994; see Corn, 1994 for design recommendations) and studies now generally apply more than one technique (e.g., Douglas, 1995; Hadden and Westbrooke, 1996; Wiles and Guerrero, 1996). However, the potential biases of using pitfall capture techniques have been less considered for breeding studies, especially within Europe (e.g., Ryser, 1988; Elmberg, 1990; Merharg et al., 1990; cf., Dodd, 1991; Lizana et al., 1994; Arntzen et al., 1995). Capture success may be influenced by variation in the size and drainage of pitfall traps. It is likely that trap depth is more directly related to the ability of anurans to escape than is the volume. Hence, traps of the same volume may not have the same capture success because they differ in depth. Traps of the same depth and volume may also differ in capture rate if they vary in their ability to drain water (McComb et al., 1991). Interspecific or intersexual differences in pitfall escape, or barrier avoidance, could lead to misinterpretation of behavior or survival rates (Gill, 1985; Dodd, 1991). In the present study I examined the escape rates of frogs and toads for traps that differed in size and drainage. Partial barriers (where a small section of the pond is lined with a drift fence and pitfall traps) may be used when breeding sites are too large to enclose completely. However, unknown variation in the use of the movement corridors by different species, sexes and age classes could complicate the interpretation of results obtained from partial barriers (Dodd and Scott, 1994). For this reason, I compared the total numbers of animals, proportions of species or sexes, and different size classes captured between several partial barriers. Individual length of stay at the breeding site may also be examined by visual surveying. Wisniewski et al. (1980) caught common toads by hand at night on a road surrounding a lake and assigned them specific date marks. Marks were given only on first capture and they assumed that each individual was captured only twice; once each on the inward and once on the outward migration. This assumption is crucial for estimating the length of stay at the breeding site; if individuals are also caught during their residence at the pond, then the assumption will be invalid and length of stay will be underestimated. In the present study, I tested this ass mption. MATERIALS AND METHODS Marking Techniques. -I marked animals by one of two techniques. Individuals were marked with alcian blue dye injected with a Panjet innoculator; each was given a date code using a combination of eight possible marking locations on the ventral surface of the limbs (Wisniewski t al., 1980). The other individuals were marked with individually coded PIT tags, which were inserted by pinching a flap of skin on the dorsal surface and injecting the tag under the skin with a sterile needle. The tag was then gently rubbed down the back until it lay beneath the skin at the base of the spine between the back legs. I assessed durability of marks in three experim nts. In the first, ten common frogs (four alcian blue Panjet, four PIT tag, two unmarked control) were held in a large indoor vivarium (2 x 0.5 m) from February 1994, until January 1996. Two frogs died (one unmarked and one dy marked) and a further four were added on May 1994 giving a total of six dye, five PIT tag, one control. Animals were provided with terrestrial and aquatic habitats and were fed brown crickets. From November to March in 1994-1995 and 1995-1996, the frogs were placed in a darkened cold room held at 1-5 C to simulate natural winter conditions and to induce hibernation. Marks and injection sites were examined weekly for the first month of observation, and monthly thereafter. In the second experiment, I gave 30 male common toads both PIT tags and Panjet marks (alcian blue) in October 1994 and placed them in a 4 X 4 m outdoor enclosure containing natural vegetation, a pond and cover. The enclosure was searched monthly, except November-February, until June 1995. In the third experiment, I gave a further 14 t ads Panjet marks using purple Indian ink. They were held in a 1 x 0.5 m outdoor tank that was thoroughly searched every 2-3 d for 3 weeks. A field mark-recapture study examined survival of marked animals and effects of marking on body condition. The study was undertaken at two ponds 3 km apart. Boggerfool (2?56'W 57?06'N) is a natural wetland pool surrounded by a mosaic of moor and bog, deciduous and coniferous woodlands. Logie (2056'W 57?07'N) is a man-made pond which has become naturalized over time and is surrounded by conife ous woodland and rough-grazing pastures. Individual frogs and toads were captured, either by searching with a spotlight at night (vi411 This content downloaded from 157.55.39.212 on Sun, 09 Oct 2016 05:20:31 UTC All use subject to http://about.jstor.org/terms