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Dietary overlap of invertivorous fishes and macroinvertebrates in the Gila River, New Mexico.

Macroinvertebrates are potentially important predators in aquatic ecosystems, impacting density (Wooster, 1994), distribution (Peckarsky and Dodson, 1980), and herbivory rates (Obernborfer et al., 1984) of their prey. Fish also are considered important predators of benthic invertebrates but may be less important in moderating densities of prey than are predacious macroinvertebrates (Wooster, 1994). Characterizing use of resources by fish and macroinvertebrates could be useful in elucidating their differing roles in food webs of streams, yet, surprisingly, few studies have assessed dietary overlap between these two important groups (Fuller and Hynes, 1987). Thus, our objectives were to quantify the diets of two common predaceous aquatic invertebrates that occur in riffle habitats of the Gila River and compare them to the diets of native riffle-dwelling fishes.

We collected nymphs of stoneflies (Perlodidae) and larval hellgrammite (Corydalidae) from four sites in the Gila River, New Mexico, in February and June during 2008-2010. Specimens were collected using Surber and core samplers and stored in 10% formalin. Using a dissecting scope, we extracted contents of the guts and identified the remains to family if possible. Due to difficulty of identifying partially digested remains, Ephemeroptera and Trichoptera were not identified to family. Pieces of invertebrates that were not identifiable were placed in a category of unknown invertebrate, and nonchitinous organic material was placed in a category of amorphous detritus. Amorphous detritus represents organic precipitates in food webs of streams (Bowen, 1984). We determined percentage of area of each taxon in the diet by spreading contents of guts on 0.25-[mm.sup.2] grid paper, counting the number of squares occupied by each group, and dividing it by the total number of squares occupied by the contents. Dietary data for speckled dace (Rhinichthys osculus), longfin dace (Agosia chrysogaster), and desert sucker (Catostomus clarkii) are from Pilger et al. (2010), who collected these fish from the same locations as our sites for sampling of macroinvertebrates during June 2008-2010. These three species of fish were selected because they occur in the same riffle habitats as the predaceous macroinvertebrates.

Breadth of the diet of a species was quantified using Levin's normalized index (1968), whereas dietary overlap between species was calculated following Pianka (1973). To help visualize overlap in contents of the gut, we used nonmetric multidimensional scaling on a Bray-Curtis distance matrix. Analysis of similarity calculated with Bray-Curtis distance matrices was used to test the statistical significance of dietary overlap among predacious invertebrates and fishes. Type I error rates were controlled using the Bonferroni adjustment (P = 0.05/ 10; [alpha] = 0.005). We excluded from analyses dietary items that occurred in <5% of sampled individuals across all species and sampling periods.

In total, we analyzed guts of 29 hellgrammites and 57 stoneflies, with guts of 25 hellgrammites and 31 stoneflies containing measurable contents. Rare taxa were not included in analyses and included Elmidae (consumed by both predators), Crambidae (consumed by a stonefly), and Ceratopogonidae (consumed by a hellgrammite). Six common (i.e., occurred in >5% of samples) dietary categories were identified: chironomid larvae; ephemeropteran nymphs; simuliid larvae; trichopteran larvae; unknown invertebrates; amorphous detritus.

[FIGURE 1 OMITTED]

All predators relied heavily on ephemeropterans (Table 1). Further, simuliids represented >5% of the diet of all predators except longfin dace. Stoneflies had relatively broad diets (B = 0.81; Table 1) but consumed more chironomids and simuliids than did other predators. Desert suckers and hellgrammites had relatively narrow diets (B = 0.36 and 0.42, respectively), predominantly composed of amorphous detritus. A two-axes configuration for the nonmetric multidimensional scaling yielded a stress value of 0.12, suggesting accurate presentation of Bray-Curtis dietary relationships in bivariate nonmetric multidimensional scaling ordination space (Fig. 1a). Results of analysis of similarity indicated differences in diet among taxa (P < 0.005). Pair-wise comparisons indicated that diet of hellgrammites did not differ from the diet of desert sucker (r = 0.07; P = 0.150) or longfin dace (r = 0.05; P = 0.120; Fig. 1b), butall other comparisons were different. These results are consistent with the degree of dietary (niche) overlap found among these species (Table 2).

These results indicate a high degree of dietary overlap of hellgrammites with two species of native fish, with moderate overlap among other taxa. The overlap was attributed to amorphous detritus, but it is unclear if this resource is limiting. Nevertheless, all taxa relied heavily on ephemeropterans and may compete for resources at some level.

Dietary overlap among native fishes and invertebrates might have broader implications to the food web of streams. For example, the presence of nonnative species might modulate competitive interactions among the riffle-dwelling taxa evaluated in this study. Nonnative predators such as smallmouth bass (Micropterus dolomieu), rainbow trout (Oncorhynchus mykiss), and yellow bullhead (Ameiurus natalis) are present in the community of fishes in the Gila River (Propst et al., 2008) and consume native fishes and predacious aquatic invertebrates (Pilger et al., 2010). If there is competition between native fishes and predacious aquatic invertebrates for herbivorous insects such as mayflies, there is potential for competitive release (Rodriguez, 2006) of native fishes by nonnative fishes. Manipulative studies would be necessary to clarify competitive interactions between co-occurring taxa.

We thank the New Mexico Department of Game and Fish and the Department of Education Graduate Assistance in Areas of National Need fellowship for funding to conduct this research. D. L. Propst, Y. M. Paroz, E. Gilbert, and J. Rogosch provided assistance collecting dietary data, and A. D. L. Mota-Peynado translated the abstract into Spanish.

Submitted 14 December 2013. Acceptance recommended by Associated Editor Robert J. Edwards 17 March 2014.

LITERATURE CITED

BOWEN, S. 1984. Evidence of a detritus food chain based on consumption of organic precipitates. Bulletin of Marine Science 35:440-448.

FULLER, R., AND H. HYNES. 1987. Feeding ecology of three predacious aquatic insects and two fish in a riffle of the Speed River, Ontario. Hydrobiologia 150:243-255.

LEVINS, R. 1968. Evolution in changing environments. Princeton University Press, Princeton, New Jersey.

OBERNBORFER, R.,J. MCARTHUR, J. BARNES, AND J. DIXON. 1984. The effect of invertebrate predators on leaf litter processing in an alpine stream. Ecology 65:1325-1331.

PECKARSKY, B., AND S. DODSON. 1980. Do stonefly predators influence benthic distributions in streams? Ecology 61:1275-1282.

PIANKA, E. R. 1973. The structure of lizard communities. Annual Reviews in Ecology and Systematics 4:53-74.

PILGER, T. J., K. B. GIDO, AND D. L. PROPST. 2010. Diet and trophic niche overlap of native and nonnative fishes in the Gila River, USA: implications for native fish conservation. Ecology of Freshwater Fish 19:300-321.

PROPST, D. L., K. B. GIDO, AND J. A. STEFFERUD. 2008. Natural flow regimes, nonnative fishes, and native fish persistence in arid-land river systems. Ecological Applications 18:12361252.

RODRIGUEZ, L. F. 2006. Can invasive species facilitate native species? Evidence of how, when, and why these impacts occur. Biological Invasions 8:927-939.

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JOSIAH J. MAINE, * JAMES E. WHITNEY, AND KEITH B. GIDO

Cooperative Wildlife Research Lab, Southern Illinois University, Carbondale, IL 62901 (JJM)

Division of Biology, Kansas State University, Manhattan, KS 66506 (JEW, KBG)

* Correspondent: jjmaine@siu. edu
TABLE 1--Percentage of composition of diet and niche breadth of six
dietary categories for two predaceous macroinvertebrates and three
species of riffle-dwelling invertivorous fish in the Gila River, New
Mexico.

Taxa                                 Dietary category

                      Chironomidae   Ephemeroptera   Simuliidae

Corydalidae                   3.3            10.7          9.8
Perlodidae                   17.3            19.7         32.1
Agosia chrysogaster           1.1            36.8          2.8
Catostomus clarki             7.0            17.6         11.3
Rhinichthys osculus           4.7            53.3         25.5
Mean                          6.7            27.6         16.3

Taxa                                Dietary category

                      Trichoptera     Unknown      Amorphous    Niche
                                    invertebrate   detritus    breadth

Corydalidae                  4.0           12.0        60.1      0.42
Perlodidae                   9.7            7.6        13.6      0.81
Agosia chrysogaster          3.7           16.8        38.7      0.53
Catostomus clarki            0.5            0.1        63.5      0.36
Rhinichthys osculus          9.3            6.5         0.7      0.46
Mean                         5.4           10.6        35.3

TABLE 2--Niche overlap of two predaceous macroinvertebrates and three
riffle-dwelling invertivorous fish in the Gila River, New Mexico.

Taxon                                    Taxon

                 Corydalidae   Perlodidae      Agosia      Catostomus
                                            chrysogaster     clarki
Perlodidae          0.53
Agosia              0.84          0.60
  chrysogaster
Catostomus          0.99          0.58          0.87
  clarki
Rhinichthys         0.26          0.77          0.65          0.33
  osculus
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Author:Maine, Josiah J.; Whitney, James E.; Gido, Keith B.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2014
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