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Resource partitioning in two stream salamanders, Dicamptodon tenebrosus and Rhyacotriton cascadae, from the Oregon Cascade Mountains.

INTRODUCTION

Resource partitioning in reptiles and amphibians may result from an interaction of mechanisms including competition, predation, and factors that operate independently of interspecific interactions, such as physiological constraints (Toft, 1985; Shine, 1991; Sih and Christensen, 2001). Salamanders generally partition food with prey size more than food type due to limitation of gape size (see Lynch, 1985). In particular the size-frequency distribution of food available to hatchling salamanders may be the primary selective force in the evolution of reproductive strategies in salamanders (Nussbaum, 1985).

Aquatic amphibians are the predominant vertebrate predators in many smaller streams of western Oregon. Stream amphibian density and biomass are greater than those reported for salmonid fishes (Bury et al., 1991). The Coastal giant salamander, Dicamptodon tenebrosus, and the Cascade torrent salamander, Rhyacotriton cascadae, often occur together in Oregon Cascades streams (Nussbaum, 1976; Hayes, 2005; Jones and Welsh, 2005). However, they often are in different parts of stream basins. In general R cascadae occur in high gradient reaches, waterfalls or seeps, whereas D. tenebrosus are associated with wider streams and often are in pools (Welsh and Lind, 1996; Olson and Weaver, 2007; Welsh, 2011). Larval D. tenebrosus attain much greater size than R cascadae larvae (see Good and Wake, 1992) so this may reduce food competition, if it occurs, between the two taxa. Still, smaller D. tenebrosus may compete for resources, including food (Toft, 1985; Bury, 1988). Dietary overlap might be expected to be greatest between R cascadae and D. tenebrosus larvae of similar size.

Prey items of adult Rhyacotriton included amphipods, small snails, worms, springtails, larval flies and stoneflies, and beetles (Bury and Martin, 1967; Bury, 1970; Nussbaum et al., 1983). Although Hayes (2005) and Welsh and Karraker (2005) reported that no data exist on larval food for any of the four species of Rhyacotriton, Nussbaum et al. (1983) describe larvae feeding on a variety of aquatic invertebrates (but not quantified). Recently, O'Donnell and Richart (2012) reported larval R. kezeri consumed a wide variety of invertebrates, including many copepods and flies; whereas, adults fed mostly on arachnids, flies, springtails, and beetles.

Larvae of D. tenebrosus feed on a wide variety of aquatic invertebrate (e.g., stoneflies, caddis flies, and mayfly larvae) as well as an occasional small aquatic vertebrates such as fish, tadpoles, and salamander larvae (Johnson and Shreck, 1969; Antonelli el al., 1972; Parker, 1993a). Esselstyn and Wildman (1999) report larval D. tenebrosus (n = 40) in the Oregon Cascade Mountains had mayflies in highest occurrence in their guts in one stretch of stream and snails (genus Juga) in a different stretch.

Although aquatic amphibians are major predators in small Pacific Northwest streams, the roles they perform in aquatic or riparian communities are poorly understood (Bury et al., 1991). Food habits of D. tenebrosus and R cascadae in the Cascade Mountains of western Oregon are little quantified nor tested against food availability. Here, we: (1) contrast differences in the diets of both species in two geographic areas; (2) compare stomach contents of salamander larvae with available prey and microhabitat use in one stream; and (3) examine the potential for food competition based on dietary overlap.

METHODS

STUDY AREAS

We collected salamanders in the Western Hemlock (Tsuga heterophylla) Zone of the Western Cascades Physiographic Province (Franklin and Dyrness, 1973). We sampled permanent creeks occurring in mature closed-canopy stands dominated by Douglas-fir (Pseudotsuga menziesii) in western Oregon. We sampled six creeks in each of two study areas in Oregon: (1) Mount Hood National Forest, Multnomah and Clackamas counties; and (2) Willamette National Forest, Lane and Linn counties. Specific locations are in Table 1.

SAMPLING PROTOCOL. AND HABITAT MEASUREMENTS

We found larval D. tenebrosus in approximately equal numbers at each study area, but we caught most larval and adult R. cascadae (85%) at Mt. Hood. We sampled 10 m lengths of creeks intensively for salamanders using small hand-held seines and dip nets (see Bury and Corn, 1991). We noted microhabitat of each salamander upon capture: riffle, pool, splash zone or seep. All salamanders were relaxed in MS-222. They then were preserved in 10% buffered formalin immediately after capture and later transferred to 70% ethanol. Specimens were deposited at the Charles R. Conner Museum, Washington State University, Pullman, Washington. Stomach contents were examined in water with the aid of a dissecting microscope and percent volume was visually estimated for each item. We summarize data as percent volume (total percent volume contributed by that item) and percent frequency (percent of stomachs containing that item) separately for large larval (snout-vent length [greater than or equal to]56 mm) and small larval (SVL <56 mm) D. tenebrosus, larval R cascadae, and adult R. cascadae. We employed a maximum SVL of 56 mm for small D. tenebrosus larvae because we found no R cascadae >55 mm SVL. Thirteen of 250 (5.2%) D. tenebrosus stomachs and 16 of 131 (12.2%) R cascadae stomachs were empty and were omitted from the analysis.

We sampled Mack Creek, H.J. Andrews Experimental Forest, Oregon, for both salamanders and available food. We collected benthic organisms at 25 randomly placed, 0.1 [m.sup.2] quadrats in Mack Creek with a Surber-type bottom sampler. We removed invertebrates from each bottom sample, preserved in 70% ethanol and identified major items to family. We estimated percent volumes for each item in the sample and then summarized data for comparison with stomach content data.

STATISTICAL ANALYSES

We determined dietary overlap between species using the similarity index discussed in Horn (1966), which ranges from no overlap (zero) to complete overlap (1). Values >0.60 indicate similar diets (Zaret and Rand, 1971). Marshal et al. (2012) reported that this index has relatively low bias and is appropriate for studies in which selection of resources is reported as a proportion of total resources used by the animal. Chi-square analyses were used to test for differences among diets and microhabitat use and to compare diets to food availability.

RESULTS

Both large and small D. tenebrosus larvae fed primarily on mayfly (Ephemeroptera), stonefly (Plecoptera), and caddis fly (Trichoptera) larvae (Table 2). These prey items accounted for 58.3% of the food volume of large salamander larvae and 71.3% of the volume in small salamanders. The only other items contributing more than 3% volume included adult beetles, spiders, and millipedes for large salamander larvae and chironomid larvae, ostracods, and spiders for small salamander larvae. Large salamander larvae fed more on allochthonous (terrestrial in origin) materials (35.6% by volume) than small larvae (10.2%).

Diets of R. cascadae larvae and adults did not differ significantly ([chi square] = 7.9, P = 0.54, df = 9) and were combined (Table 2). They fed heavily upon mayfly, stonefly, and caddis fly larvae (69.7% volume), but in different proportions than D. tenebrosus larvae. While D. tenebrosus larvae fed on these insect larvae in approximately equal proportions, R cascadae fed much more on caddis fly larvae and less on mayfly larvae than did larval D. tenebrosus. Stonefly larvae consumption was similar between the two species. For R. cascadae, mayflies (particularly Heptageniidae) were unimportant in the diet, whereas caddis flies of the family Philopotamidae accounted for approximately 30% of total volume. Chironomid larvae, ostracods, and small aquatic snails were also important foods. As with small D. tenebrosus larvae, allochthonous materials were relatively unimportant in the diet of R. cascadae, accounting for about 10% of total volume.

Patterns in the diets of salamanders collected at Mack Creek were similar to those observed for all samples combined (Table 3). Larval D. tenebrosus fed primarily on mayfly, stonefly, and caddis fly larvae in approximately equal amounts while R. cascadae fed more heavily on caddis fly larvae. Notable deviations of Mack Creek salamanders from overall diets were the abundance of baetid mayflies and peltoperlid stoneflies in the diets of all salamanders, the abundance of ostracods in the diet of R. cascadae, and the absence of small aquatic gastropods in the diets of all salamanders. Baetid mayflies (16.8% volume) and peltoperlid stoneflies (6.1% volume) were abundant in benthic samples (Table 3). Although ostracods did not contribute significantly to total volume of benthic samples (1.2%), they occurred in every sample and usually in high numbers (an average of 42.4 per sample). Small aquatic gastropods did not occur in Mack Creek benthic samples.

Small and large D. tenebrosus larvae were most similar in their diets (overlap index = 0.707). The occurrence of allochthonous items such as beetles, spiders, and millipedes in the diet of large larvae accounted for the majority of the difference. Diet of R. cascadae yielded overlap index of 0.389 compared to large D. tenebrosus, well below 0.60, suggesting dissimilar diets. Overlap index for Rhyacotriton and small larval D. tenebrosus was 0.503, which indicates some similarities in diet.

Diets of both large and small D. tenebrosus larvae at Mack Creek did not differ significantly from available foods as measured by benthic samples ([chi square] = 19.0, P = 0.52, df = 20, [chi square] = 18.9, P = 0.46, df = 19, respectively), suggesting that they opportunistically take food items in proportion to their availability. In contrast R. cascadae diets differed significantly from available foods ([chi square] = 68.3, P < 0.01, df = 19), indicating that foods are not being taken in proportion to their availability. Further, R. cascadae at Mack Creek consumed philopotamid caddis flies and ostracods at levels well above their abundance, and heptageniid mayflies and perlid stoneflies well below their abundance (Table 3).

Analysis of microhabitat use by larval Dicamptodon and Rhyacotriton at Mack Creek found difference in their use of the four measured microhabitats ([chi square] = 26.0, P < 0.01, df = 3). Larval D. tenebrosus occurred most commonly in pools (49.5% of individuals) and riffles (41.2%) while larvae and adults of R. cascadae occurred most commonly in riffles (57.5%) and seepage areas (22.6%) (Fig. 1).

DISCUSSION

Larval D. tenebrosus feed on a wide variety of prey and appear to consume whatever is most abundant in their habitats (Antonelli et al., 1972; Parker, 1993a; Steele and Brammer, 2006). Similarly, we found opportunistic feeding by larval D. tenebrosus because their diets and available foods did not differ significantly. We found many invertebrates but no vertebrate prey in the diets of 237 larval D. tenebrosus in the Oregon Cascade Mountains, which was the same pattern found for 362 larvae in the Cascade Mountains of Washington (Steele and Brammer, 2006) and 219 in the Coast Range of Oregon (Graff, 2006).

In contrast to our study, others have shown that larger-sized larval Dicamptodon may eat small vertebrates. For example, Johnson and Shreck (1969) reported consumption of larval Ambystoma gracile by D. tenebrosus and Metter (1963) found tailed frog tadpoles, Ascaphus montanus, in stomachs of D. atemmus. Antonelli et al. (1972) reported trout, Salmo gairdneri, and sculpin, Cottus tenuis, in the stomachs of larval D. tenebrosus. They stated that all fish were taken during summer low water periods when concentration of fish is greatest in streams. Similarly, Parker (1993) found young-of-year steelhead, Oncorhynchus mykiss in larger individuals of larval D. tenebrosus. Further, Parker (1994) reported that large (>100 mm TL) larval D. tenebrosus had four cases of cannibalism on single young-of-year (<65 mm TL) congeners plus an additional three juvenile trout. As larvae increased in size, they incorporated larger prey into their diet. Esselstyn and Wildman (1999) found two cottid fishes in 40 large-sized larval D. tenebrosus. Collectively, these studies include examination of approximately 1877 stomachs, and vertebrate prey constitutes only 4% of the diet (n = 75 items) in several species of larval Dicamptodon. Thus, range-wide and for all species examined, larger-sized larvae of Dicamptodon prey infrequently on some fishes, tailed frog tadpoles, other salamander larvae, and, rarely, smaller congeners. Although infrequent, these items likely provide large amounts of high protein food. Further, capture of small vertebrates appears to occur most often in periods of low water when food items (e.g., fish) are more likely to be confined with salamanders. Although different-sized Dicamptodon often occur together in the wild, cannibalism is rare (Bury, 1972; Parker, 1994).

We found small D. tenebrosus larvae to be opportunistic and dietary generalists. In contrast diets of R. cascadae were not closely tied to available foods where we sampled. There are two possible explanations for this apparent selectivity: (1) R. cascadae is more specialized in its feeding habits and has adapted to a more selective feeding strategy or (2) R. cascadae occupies slightly different microhabitats than D. tenebrosus and food availability in these microhabitats differs from that of the entire stream. Although we had insufficiently detailed measurements to document different microhabitat use by invertebrates, there is some evidence that D. tenebrosus and R. cascadae are spatially separated in streams (Corn et al., 2003; Welsh and Lind, 1996, 2002). We found that these two species varied in their use of four possible microhabitats with D. tenebrosus occurring most commonly in pools and riffles whereas R. cascadae frequented riffles, splash zones, and seeps. Bury et al. (1991) also described differences in microhabitat use by these species with more D. tenebrosus occurring in pools and fewer on wet stream banks and shallow seeps than Rhyacotriton.

We found little difference in the diets of larval and adult R. cascadae. However, O'Donnell and Richart (2012) reported that larval R. kezeri consumed many Copepoda and Diptera whereas adults were feeding mostly on Arachnida, Diptera, Collembola and Coleoptera, which was similar to adult R. variegatus in northern California (Bury and Martin, 1967). Overall, there are insufficient studies available to compare geographic or temporal patterns in the four species of Rhyacotriton, especially for the larvae.

Although both D. tenebrosus and R. cascadae feed on a wide variety of the same food items, D. tenebrosus is more of a generalist than R. cascadae. The two species may be in partial competition by feeding on many of the same items but in different proportions. Differences in food habits, possibly due to the differential use of microhabitats, may contribute to the ability of these two species to co-exist. We found no R. cascadae in the diet of D. tenebrosus, even when sympatric. This may be partly due to unpalatability of R. cascadae. despite being bitten and engulfed, all R. variegatus were rejected by D. tenebrosus in experimental trials (Rundio and Olson, 2001). However, approximately 25% of the attacked R. cascadae did not survive. Thus, avoidance of large larval Dicamptodon would be advantageous to the persistence of Rhyacotriton.

Aquatic amphibian larvae generally partition season of activity before habitat or foods (Toft, 1985). However, most studies have examined anuran rather than salamander larvae and few have investigated food partitioning directly. We do not know if R cascadae and D. tenebrosus larvae are feeding at different times of day. In northern California, both R variegatus and D. tenebrosus are active at night (Ashton et al. 2006). There is apparently no difference in seasonal use of creeks by larvae. Rhyacotriton spends its entire life cycle in seeps, headwaters, or splash zones (Nussbaum and Tait, 1977), while Dicamptodon larvae spend 1-3 y as larvae in streams (Nussbaum and Clothier, 1973). Some adult Dicamptodon remain in water as neotenes. Resource partitioning by these aquatic salamander larvae seems to follow the general pattern in which food and habitat are partitioned first.

Acknowledgments.--We thank F. F. Gilbert, J. O. Whitaker, Jr., C. A. Pearl and two anonymous reviewers for their helpful comments on the manuscript. At Washington State University R. Zack confirmed our insect identifications and C. Elstad identified several miscellaneous invertebrates. We had a Scientific Study permit from the State of Oregon. Field work was funded by the U.S. Forest Service and this paper represents Old Growth Wildlife Habitat Program Contribution No. 48. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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WYNN W. CUDMORE (1), Life Science Department, Chemeketa Community College, Salem, Oregon 97309; AND R. BRUCE BURY, U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, Oregon 97331.

(1) Corresponding author: e-mail: wynn.cudmore@chemeketa.edu

TABLE 1.--Specific locations of study streams

Mount Hood National            Willamette National Forest
Forest

45.48[degrees]N,
  122.02861[degrees]W   44.225[degrees]N, 122.16389[degrees]W
45.43[degrees]N,
  122.17361[degrees]W   44.26667[degrees]N, 122.17778[degrees]W
45.51306[degrees]N,
  122.03833[degrees]W   44.1753[degrees]N, 122.4370[degrees]W
45.26889[degrees]N,
  121.93667[degrees]W   44.2637[degrees]N, 122.1563[degrees]W
45.45694[degrees]N,
  122.10306[degrees]W   44.225[degrees]N, 122.19167[degrees]W
45.49917[degrees]N,
  121.97083[degrees]W   44.11639[degrees]N, 122.19972[degrees]W

TABLE 2.--Percent of foods of large (SVL [greater than or equal
to] 56 mm) and small (SVL <56 mm) Dimmptodon tembrosus larvae and
Rhyacotriton cascadae from 12 streams in the Oregon Cascade
Mountains. V = volume; F = frequency

                                     Dicamptodon         Rhyacotriton

                                 Large         Small          All
                                (n = 88)     (n = 149)     (n = 115)

Food types                    % V    % F    % V    % F    % V    % F

Ephemeroptera
  Ephemerellidae              2.1    6.8    1.9    4.7    0.6    2.6
  Heptageniidae               8.4   46.6   16.9   48.3    1.2    6.1
  Leptophlebiidae             4.1   14.8    3.9   12.1    2.6    6.1
  Baetidae                    8.0   40.9    9.1   37.6    1.8   11.3

Plecoptera
  Chloroperlidae              6.0   33.0   18.9   47.0   14.9   39.1
  Peltoperlidae               1.5    9.1    2.4   14.1    2.2   12.2
  Perlidae                    7.3   18.2    4.2    9.4    0.0    0.0
  Perlodidae                  0.8    4.5    1.7    7.4    2.4    8.7
  Miscellaneous adults        0.6    3.4    0.9    2.7    0.4    0.9

Trichoptera
  Brachycentridae             4.5   26.1    3.0   17.4    3.2   12.2
  Hydropsychidae              3.7    9.1    2.3    4.7    7.4   13.0
  Leptoceridae                0.6    4.5    0.1    0.7    0.0    0.0
  Limnephilidae               4.4   20.4    2.2   10.1    2.0    5.2
  Philopotamidae              4.8   27.3    4.2   19.5   31.4   58.3
  Polycentropidae             1.5    5.7    0.5    2.0    0.0    0.0
  Miscellaneous adults        8.8   20.5    1.8    5.4    1.3    3.5

Diptera
  Chironomidae                2.5   25.0    9.4   55.7    4.5   42.6
  Miscellaneous larvae        0.7    2.3    1.3    4.0    0.5    2.6
  Miscellaneous adults        1.2    4.5    0.6    2.7    2.8    8.7

Orthoptera (adults)           1.0    2.3    0.2    0.7    0.0    0.0
Coleoptera (adults)           6.6   11.4    1.6    4.7    0.2    0.9
Lepidoptera (adults/larvae)   2.2    4.5    0.3    0.7    0.0    0.0

Hymenoptera
  Miscellaneous adults        1.0    2.3    0.2    0.7    0.0    0.0
  Symphyta (larvae)           2.6    9.1    0.4    1.3    0.0    0.0
  Shed skin                   0.0    0.0    0.0    0.0    1.5    3.5

Ostracoda (Podocopa)          0.2    5.7    3.1   30.2    6.6   32.2
  Copepoda Canthocamptidae    0.0    0.0    0.8    4.7    1.7   13.0
  Gastropoda Amnicolidae      1.0    6.8    2.5    8.7    4.2    9.6

Aranae                        5.5   17.0    3.1    8.7    1.6    7.0
Platyhelminthes (Dugesia)     0.0    0.0    0.9    2.0    1.8    2.6
Diplopoda (Polydesmida)       3.2    6.8    0.0    0.0    0.0    0.0
Plant (Pseudotsuga)           2.9   23.9    1.0    5.4    0.9    3.5
  Miscellaneous               2.2   11.2    0.5    3.4    1.6    6.9

Totals                       99.9          99.9          99.3

TABLE 3.--Diets and available foods for Dicamptodon tenebrosus
and Rhyacotriton cascadae collected from Mack Creek, H.J. Andrews
Experimental Forest, Lane Co., Oregon. V = volume; F = frequency

                              D. tenebrosus     D. tenebrosus
                                  (LL)              (SL)

                                 (N = 31)          (N = 39)

        Food type             % V      % F      % V      % F

Ephemeroptera
  Ephemerellidae              5.9     19.4      7.4     17.9
  Heptageniidae               6.2     64.5     19.1     71.8
  Leptophlebiidae             2.7     16.1      7.6     25.6
  Baetidae                   16       71       18.5     79.5

Plecoptera
  Chloroperlidae              4.7     35.5     10.1     41
  Peltoperlidae               0.6     12.9      3.3     17.9
  Perlidae                    3.6     19.4      3        5.1
  Perlodidae                  0        0        0        0
  Miscellaneous adults        0        0        0        0

Trichoptera
  Brachycentridae             7.6     35.5      2.3     23.1
  Hydropsychidae              1.8      6.5      0.6      2.6
  Leptoceridae                0.5      6.5      0        0
  Limnephilidae              10.4     35.5      3.7     20.5
  Philopotamidae              5.1     45.2      5.1     35.9
  Polycentropidae             3.5     12.9      1        5.1
  Miscellaneous adults        7.5     22.6      0.9      5.1

Diptera
  Chironomidae                0.6     12.9      8       59
  Miscellaneous larvae        0        0        0.5      2.6
  Miscellaneous adults        0.1      3.2      0.9      2.6

Neuroptera
  Corydalidae                 0.8      3.2      0        0

Coleoptera (adults)           7.7     12.9      0.3      2.6
Ostracoda (O. Podocopa)       0.1      6.5      2.8     38.5
Copepoda Canthocamptidae      0        0        0        0
Aranae                        2.3      6.5      0        0
Acarina                       0        0        0.1      2.6
Platyhelminthes (Dugesia)     0        0        3.6      7.7
Diplopoda (O. Polydesmida)    7.6     16.1      0        0
Miscellaneous                 2.4     16.1      0.9      7.7

Totals                       99.7              99.7

                                R. cascadae        Benthic
                                                   samples

                                 (N = 20)          (N = 25)

        Food type             % V      % F      % V      % F

Ephemeroptera
  Ephemerellidae               0       0        3.6      40
  Heptageniidae                0.3     5       15.6     100
  Leptophlebiidae              1.5     5        2.7      64
  Baetidae                     9.2    50       16.8     100

Plecoptera
  Chloroperlidae              15      50       11.1     100
  Peltoperlidae                7.2    30        6.1      80
  Perlidae                     0       0        8.1      52
  Perlodidae                   0.8     5        0.1       4
  Miscellaneous adults         2.5     5        0         0

Trichoptera
  Brachycentridae              3.3    15        4.7      96
  Hydropsychidae               1       5        0.8       8
  Leptoceridae                 0       0        0.1       4
  Limnephilidae                4       5       13.5      88
  Philopotamidae              29      55        5.2      84
  Polycentropidae              0       0        1.3      24
  Miscellaneous adults         0       0        0         0

Diptera
  Chironomidae                 8.2    60        4.4     100
  Miscellaneous larvae         1       5        0.1       4
  Miscellaneous adults         3       5        0         0

Neuroptera
  Corydalidae                  0       0        1         4

Coleoptera (adults)            0       0        0         0
Ostracoda (O. Podocopa)       13.9    60        1.2     100
Copepoda Canthocamptidae       0.1     5        0.2      20.9
Aranae                         0       0        0         0
Acarina                        0.1     5        1.1     100
Platyhelminthes (Dugesia)      0       0        1.2      56
Diplopoda (O. Polydesmida)     0       0        0         0
Miscellaneous                  0       0        0.3      12

Totals                       100.1             99.2
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Article Details
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Title Annotation:Notes and Discussion
Author:Cudmore, Wynn W.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1U9OR
Date:Jul 1, 2014
Words:4806
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