NORTHWESTERN NATURALIST 88:85–94 AUTUMN 2007 DIET OF THE DEL NORTE SALAMANDER (PLETHODON ELONGATUS): DIFFERENCES BY AGE, GENDER, AND SEASON CLARA AWHEELER,NANCY EKARRAKER1,HARTWELL HWELSH,JR, AND LISA MOLLIVIER Redwood Sciences Laboratory, Pacific Southwest Research Station, USDA Forest Service, 1700 Bayview Drive, Arcata, California 95521 ABSTRACT—Terrestrial salamanders are integral components of forest ecosystems and the ex- amination of their feeding habits may provide useful information regarding various ecosystem processes. We studied the diet of the Del Norte Salamander (Plethodon elongatus) and assessed diet differences between age classes, genders, and seasons. The stomachs of 309 subadult and adult salamanders, captured in spring and fall, contained 20 prey types. Nineteen were inver- tebrates, and one was a juvenile Del Norte Salamander, representing the first reported evidence of cannibalism in this species. Mites and ants represented a significant component of the diet across all age classes and genders, and diets of subadult and adult salamanders were fairly similar overall. We detected, however, an ontogenetic shift with termites and ants becoming less important and spiders and mites becoming more important with age. These differences between subadults and adults can likely be attributed to the inability of subadults to consume larger prey items due in part to gape limitation. The diet of the Del Norte Salamander, like other plethodontids, consists of a high diversity of prey items making it an opportunistic, sit-and- wait predator. Key words: Del Norte Salamander, Plethodon elongatus, food habits, diet, northern California, southern Oregon Terrestrial salamanders represent a signifi- 2005), and may require ecological conditions cant component of vertebrate biomass in forest found primarily in late seral stage forests ecosystems (Burton and Likens 1975a) and (Welsh 1990; Welsh and Lind 1995; Jones and strongly influence nutrient dynamics and en- others 2005). However, Diller and Wallace ergy flow (Burton and Likens 1975b). They may (1994) observed dense populations of Del Norte play an important role in the regulation of food Salamanders in younger coastal redwood for- web dynamics and can greatly impact abun- ests and attributed this to the climatic differ- dances of soil invertebrates (reviewed by Davic ences between the habitats of coastal and more and Welsh 2004). Knowledge of the feeding inland populations. habits of terrestrial salamanders can provide Prior to June 2002, the Del Norte Salamander useful system-specific information on variation was listed as a Survey and Manage Species and in feeding behavior, food availability, prey spe- protected on federal forestlands under the cies diversity, and trophic dynamics. Northwest Forest Plan (USDA and USDI 1994). We examined the diet of the Del Norte Sala- Currently it is designated as a Species of Spe- mander (Plethodon elongatus), a species endemic cial Concern by the State of California (Jen- to northern California and southern Oregon nings and Hayes 1994) and a Species of Con- (Jones and others 2005; Welsh and Bury 2005). cern by the State of Oregon (Marshall 1992). This terrestrial plethodontid salamander is as- These categories provide limited or no protec- sociated with rocky substrates in low-elevation tions. Recent analyses suggest that this species mixed conifer-hardwood forests (Nussbaum and others 1983; Stebbins 2003; Welsh and Bury may be comprised of several genetically dis- tinct lineages. Our objectives were to describe the compo- 1 Present address: University of Hong Kong, Department sition of the Del Norte Salamander diet, to as- of Ecology and Biodiversity, Pokfulam Road, Hong Kong. sess differences in diet by age class, gender, 85 86 NORTHWESTERN NATURALIST 88(2) and season, and to evaluate any differences that TABLE 1. Common names for organisms in prey might occur based on capture method. types. Prey type Common name METHODS Araneae Spiders We captured and collected 309 Del Norte Sal- Chilopoda Centipedes Coleoptera Beetles amanders from 9 localities in Del Norte, Hum- Collembola Springtails boldt, and Trinity counties, California, in 1984 Diplopoda Millipedes and 1985. We captured 120 salamanders using Diplura Diplurans time-constrained searches (TCS) during spring Diptera Flies Formicidae (Order Hyme- Ants (March through May) and 189 salamanders us- noptera) ing pitfall traps (PF) during fall (October Gastropoda Snails and Slugs through November). These months corre- Hymenoptera (except Bees, Wasps, and Saw- sponded with the time periods when these sal- Formicidae) flies Isopoda Isopods amanders were active on the forest floor. On the Isoptera Termites day of capture, animals were transported to the Ixodidae (Order Acari) Ticks laboratory where they were euthanized in 0.2% Lepidoptera Butterflies and Moths chloretone, fixed in 10% formalin, and stored in Oligochaeta Earthworms 70% ethanol. We measured snout-vent length Orbatidae (Order Acari) Mites Orthoptera Grasshoppers, Crickets, (SVL) and total length to the nearest 1 mm and and Katydids mass to the nearest 0.1 g for each euthanized Plethodon elongatus (Order Del Norte Salamander specimen. Amphibia) In the laboratory, stomachs were removed Pseudoscorpiones Pseudoscorpions Trichoptera Caddisflies and contents were examined in ethanol using a 10x dissecting microscope. We examined the stomach contents of all salamanders captured: ϩ ϩ 126 males, 116 females, and 67 subadults-ju- ϭ (nxxx /N) (v /V) (f /F) Ix veniles. Gender and age class of specimens 3 were determined by dissection using maturity ϭ ϭ where nx number of a given prey type, N and condition of sex organs as indicators (see ϭ sum of numbers of all prey items, vx volume Ollivier and Welsh 2003). We combined juve- of a given prey type, V ϭ sum of volumes of all nile and subadult data (hereafter subadults) to ϭ prey items, fx frequency of a given prey type, obtain sufficient sample sizes. Because we and F ϭ sum of frequencies of all prey items. could not determine the gender of juvenile an- The importance value is an index that provides imals, we did not examine differences in diet a more complete analysis of an organism’s food between subadult males and females. We sort- habits by incorporating numbers, volumes, and ed and identified all prey items to order, or frequencies of prey types (Powell and others family when possible (Table 1), counted each 1990; Anderson and Mathis 1999). prey item, and measured the length, width, and Cumulative prey curves were constructed for depth of intact items of each prey type in a giv- each group to determine if adequate sample en specimen. sizes were obtained to describe the diet and We determined the number (total number of differences between age classes, genders, and a given prey type in a stomach), frequency seasons (Adams and Kay 2002). To generate (number of stomachs that contained each prey curves, the order in which stomachs were an- type), volume, and importance of each prey alyzed was randomized 100 times and the type by season for all salamanders, each age mean number of new prey species cumulated class, and adults of each gender. Volume of prey consecutively was plotted against the number type was calculated as V ϭ n ϫ l ϫ w ϫ d, of stomachs examined. An asymptotic relation- where n ϭ number, l ϭ prey length, w ϭ prey ship was considered to be an indication that a width, and d ϭ prey depth. We calculated the sufficient number of stomachs were analyzed importance value (Ix) for each prey type using to represent dietary habits (Cailliet and others the following equation (Anderson and Mathis 1986). 1999): We calculated prey richness (number of spe- AUTUMN 2007 WHEELER AND OTHERS:DEL NORTE SALAMANDER DIET 87 FIGURE 1. Cumulative prey curves for a) all salamander stomachs (n ϭ 207) containing identifiable prey items, representing the largest sample, and b) subadults collected in the fall (n ϭ 13), representing the small- est sample. Error bars denote standard deviations. Asymptotic cumulative prey curves for other groups are not shown. cies), prey evenness (equitability of species), compared with 2% and 3%, respectively, in the and Shannon index of prey diversity (a diver- spring. sity index that accounts for both evenness and Specimen stomachs contained a total of 2449 species richness) by season, age class, and gen- identifiable prey items (Table 2) representing 20 der (Magurran 1988). We compared diversity of prey types (19 invertebrate and 1 vertebrate). prey species between groups using t-tests (Ma- The mean number of prey items per salaman- gurran 1988) (␣ϭ0.017 after Bonferroni ad- der, for all age classes combined, was higher in justment). This allowed us to examine the pos- the spring (x¯ ϭ 13.7, s ϭ 24.5) than in the fall sible effects of capture method, season, and age (x¯ ϭ 4.5, s ϭ 17.0). Much of this difference be- class on diet diversity. We used t-tests to com- tween seasons was attributable to subadults. pare prey volume and number of prey items be- The number of adult stomachs that contained tween age classes, genders, and seasons (␣ϭ contents was comparable between the two sea- 0.017 after Bonferroni adjustment). Prey vol- sons (spring: n ϭ 73; fall: n ϭ 77), with ap- ume and number of prey items were log-trans- proximately 700 prey items observed in both formed prior to analysis. the spring and fall. However, the number of subadult stomachs that contained contents was RESULTS not comparable between seasons (spring: n ϭ Prey Items 13; fall: n ϭ 44). Approximately 3 times more Asymptotic cumulative prey curves indicat- prey items were observed in the spring than in ed that an adequate number of stomachs were the fall after correcting for the number of stom- examined to comprehensively describe and achs sampled (Table 2). Adult males contained compare food habits of all groups (Fig. 1a and almost twice as many prey items in the fall as 1b).