Fish Assemblages and Habitat Gradients in a Rocky ...

Transactions of the American Fisheries Society 120:319-332, 1991

Fish Assemblages and Habitat Gradients in a Rocky Mountain-Great Plains Stream: Biotic Zonation and Additive Patterns of Community Change FRANK J. RAHEL Department of Zoology and Physiology, University of Wyoming Laramie, Wyoming 82071, USA

WAYNE A. HUBERT U.S. Fish and Wildlife Service, Wyoming Cooperative Fish and Wildlife Research Unitl University of Wyoming Abstract.—We examined the importance of zonation and species additions in explaining longitudinal changes in the fish assemblage of a Rocky Mountain stream that descends onto the Great Plains of Wyoming. Community changes along an elevational gradient from 2,234 to 1,230 m above mean sea level reflected a combination of zonation and downstream addition of species. Zonation was evident on a broad spatial scale as a result of stream temperatures. A coldwater trout (Salmonidae) assemblage dominated headwater reaches but was replaced by a warmwater minnow-sucker (Cyprinidae-Catostomidae) assemblage below 2,000 m. Within the warmwater zone, fish community change was due mainly to the addition of new species downstream. Headwater sites were dominated by members of the insecti vore feeding guild, and other trophic guilds were added downstream. The major gradient of habitat change downstream consisted of a decrease in pool habitat and increases in stream width, depth, current velocity, turbidity, and proportion of the channel consisting of run habitat. Minor gradients of habitat change involved streambank condition and substrate particle size. Contrary to streams in forested regions, habitat diversity did not increase downstream, suggesting that increased living space and moderating environmental conditions contributed to the downstream increase in species richness. Local habitat modification due to cattle grazing or alterations in streamflow caused minor changes in fish assemblages but did not disrupt the dominant longitudinal patterns. Broad-scale zonation based on temperature regime and additive patterns within zones should typify other streams originating in montane regions. Identifying environmental gradients that influence community structure has been a major focus of stream ecology (Minshall 1988; Power et al. 1988). Studies of stream fish assemblages have shown that abiotic factors such as temperature, current velocity, and substrate can determine the distribution and abundance of individual species as well as influence community-level properties such as species richness, production, and guild composition (Lotrich 1973; Gorman and Karr 1978; Matthews 1985). The influence of environmental gradients can be detected on many spatial scales including regional patterns in species occurrence (Hawkes et al. 1986; Hughes et al. 1987), longitudinal changes in community composition within a stream (Sheldon 1968; Hughes and Gammon 1987), habitat use along pool-riffle sequences (Finger 1982; Schlosser 1982), and microhabitat ———— ' The Unit is jointly supported by the University of Wyoming, the Wyoming Game and Fish Department, and the U.S. Fish and Wildlife Service.

segregation within a pool or riffle (Winn 1958; Mendelson 1975; Gorman 1987). Longitudinal changes in community composition have usually been attributed to one of two processes: biotic zonation or continual addition of species downstream. Biotic zonation refers to relatively distinct communities that exist within flowing waters as a result of discontinuities in stream geomorphology or temperature. These community types or biotic zones provide a useful basis for classifying stream reaches for fishery management purposes. Descriptions of biotic zones were first formulated by European workers, who named the zones after dominant fishes (discussed by Hawkes 1975). Thus the trout zone occurs in small, cold, headwater streams, whereas the bream zone is in large, warmwater, downstream reaches (Huet 1959). Transitional reaches between these zones are designated as the grayling zone or the barbel zone Similar « Patterns of stream zonation have been reported elsewhere (Balon and Stewart 1983; Moyle and Herbold 1987).

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In contrast to the advocates of zonation, many North American workers have viewed changes in fish assemblage as reflecting a continual downstream increase in community complexity due to the addition of species (Sheldon 1968; Jenkins and Freeman 1972; Evans and Noble 1979). According to this model, consistent changes in stream habitat lead to predictable changes in fish assemblages downstream. For example, headwater streams are thought to be dominated by small, invertivorous fishes, with large-bodied, piscivorous, and pool-dwelling fish species joining the assemblage downstream (Evans and Noble 1979; Schlosser 1982; Power et al. 1988). Also, headwater reaches are thought to be dominated by a colonizing community capable of reinvasion after the environmental pertubations common to these areas, whereas downstream reaches are dominated by a stable community in which biotic interactions are important (Schlosser 1987). The gradual accumulation of species downstream is often attributed to a downstream increase in habitat diversity (Gorman and Karr 1978;

Schlosser 1982). An alternative explanation maintains that periodic stressful physicochemical conditions are more common in upstream sites, where only a few vagile or physiologically hardy species persist, whereas many species can exist in more benign downstream areas (Horwitz 1978; Matthews and Styron 1981). Much stream ecological theory is based on studies of temperate zone forest streams, and there is a widely recognized need to apply this theory to other areas to test its generality (Angermeier and Karr 1983; Moyle and Herbold 1987; Matthews 1988; Wiley et al. 1990). In addition, recent emphasis on hierarchical views of stream community organization suggests that zonal and additive changes in stream communities may operate simultaneously but at different spatial scales (Naiman et al. 1988). With these ideas in mind, we examined relations between fish assemblages and habitat conditions in a western North American stream to address the following questions. (1) What changes are evident in fish assemblages along a longitudinal gradient in a stream that descends from the Rocky Mountain foothills onto the shortgrass prairie of the Great Plains? (2) How are fish assemblages related to environmental changes along this longitudinal gradient? (3) How do these changes in fish assemblage and habitat relate to general theories of stream community organization? In particular, what are

the relative importances of longitudinal zonation and continual addition of species downstream? Study Area This study was conducted on Horse Creek in Laramie and Goshen counties, southeastern Wyoming (Figure 1). The stream originates in the Laramie Range 2,440 m above mean sea level. It flows northeasterly and is joined by several tributaries before entering the Platte River near Lyman, Nebraska. We sampled fish at 17 sites (numbered 4-20) ranging in elevation from 1,234 to 1,591 m. Headwater sites had predominantly gravel substrates and riparian zones of willows Salix spp., sedges Scirpus spp., and grasses. Downstream sites had predominantly sand substrates and riparian zones of mostly grasses. Sampling was done from June to August 1986, when the stream was at or near summer base flow. We supplemented our data set by adding three headwater sites (1,2, and 3 at elevations 2,234, 2,030, and 2,015 m) on Horse Creek and a site (21) on the Platte River near the mouth of Horse Creek (elevation 1,230 m), giving 21 sites with fish community data. Data for the headwater sites were from summer electrofishing surveys by Eifert and Wesche (1982) and included some habitat information. Data for the Platte River site were from unpublished seine and electrofishing records of the Ichthyology Museum of the University of Wyoming. The addition of these sites enabled us to examine fish community patterns upstream and downstream from our sampling sites, thereby increasing the length of stream over which longitudinal changes in fish assemblages could be examined.

Methods Sampling of fish populations.—Estimates of population size for each fish species, excluding larvae and young-of-the-year fish, were made at each site the day after habitat sampling. For reasons related to habitat measurement, the length of stream sampled was 30 times the average wetted width. Each site was blocked with small-mesh seines at the upper and lower end, and three depletion passes were made in an upstream direction with a 110-V DC electrofishing unit. Specimens were preserved in formalin and identified in the laboratory with the keys of Baxter and Simon (1970). Common and scientific names of the fish species captured are given in Table 1. Program CAPTURE Model M(bh), which allows for variation in behavior due to the first capture attempt,

FISH ASSEMBLAGES IN A WYOMING STREAM

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Horse Creek Drainage

FIGURE 1.—Study sites along Horse Creek, Wyoming. Site 21 was on the Platte River near the mouth of Horse Creek. The cross-hatched reach between sites 17 and 19 conveys irrigation water.

was used to estimate population size for each species (White et al. 1982). Because the length of each sampling site was a function of stream width (see methods for habitat measurements), the population estimates were for various stream lengths. To standardize the data for comparisons, we expressed the abundance of each species as the proportion of all fish of all species estimated to be present in the stream reach. Habitat measurements.—The 17 study sites were selected to represent the range of habitat conditions in Horse Creek. The length of stream sampled at each site—30 times the average wetted width—was well above the length (10-14 times the average wetted stream width) recommended to ensure adequate sampling of stream riffle-pool sequences (Leopold et al. 1964; Bovee and Milhous 1978). Each site was divided by 30 equally spaced transects, and there were seven equally spaced sampling points along each transect. At each sampling point, we measured or noted water depth, substrate, presence of aquatic vegetation, and habitat type. Habitat types (Table 2) were based on the categories of Bisson et al. (1981). The dominant substrate was visually classified according to a scale of increasing particle size (Table 2).

Current velocity was measured with a Pygmy current meter at 0.6 times the water depth at three equally spaced points along each transect. Bank features were measured at both ends of each stream transect and included vegetation covering the banks, overhanging (undercut) bank, and overhanging vegetation that could serve as cover for fish (Table 2). Only banks and vegetation that extended at least 10 cm wide over water at least 15 cm deep were considered to be effective cover for fish (Wesche 1980). We measured discharge at each site in mid-August 1986, following the procedure of Platts et al. (1983), Turbidity (in Jackson turbidity units) also was measured during the mid-August sampling (Table 2). Data analysis. —We examined gradients of community change among the 21 sites for which fish data were available by using both direct gradient analysis and ordination. Direct gradient analysis (Whittaker 1978) was done by examining fish assemblage changes along a longitudinal gradient in the main stem of Horse Creek from near its origin in the Laramie Range (elevation 2,234 m) to its confluence with the North Platte River (elevation 1,230 m). Ordination was done with

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TABLE 1.—Fish species collected in Horse Creek, Wyoming. Family and species

Salmonidae Salvelinus fontinalis Salmo trutta Cyrpinidae Notropis dorsalis Hybognathus hankinsoni Cyprinus carpio Luxilus cornutus* Semotilus atromaculatus Pimephales promelas Rhinichthys cataractae Cyprinella lutrensis* Notropis stramineus Campostoma anomalum Phenacobius mirabilis Catostomidae Catostomus catastomus Carpiodes cyprinus Moxostoma macrolepidotum Catostomus commersoni Ictaluridae Noturusflavus Cyprinodontidae Fundulus zebrinus Percidae Etheostoma nigrum Centrarchidae Lepomis cyanellus Micropterus salmoides a

Common name Brook trout Brown trout

Bigmouth shiner Brassy minnow Common carp

Common shiner Creek chub Fathead minnow Longnose dace Red shiner Sand shiner Central stoneroller Suckermouth minnow Longnose sucker Quillback

Shonhead redhorse White sucker

Stonccat Plains killifish

Johnny darter

Green sunfish Largemouth bass

These species were formerly in the genus Notropis.

detrended correspondence analysis (DCA; Gauch 1982). Input data were the proportional abundances of species at each of the 21 sites transformed to an octave scale as suggested by Gauch (1982). Detrended correspondence analysis ordinates sites and species simultaneously and thus provides insight into which species are contributing to the major gradients of community change. To identify major environmental gradients in our data set, we performed a principal component analysis (PCA) on habitat features for the 17 sites for which complete habitat data were available. To improve normality, variables expressed as percentages (Table 2) were arcsme-transformed; values for turbidity and the coefficients of variation for water depth and velocity were Iogi0-transformed. All other variables were normally distributed. All variables were then standardized to zero mean and unit variance (achieved by extracting principal components from the correlation matrix) to compensate for different units of measurement. Interpretation of the principal components as environmental gradients was facilitated by an orthogonal rotation of the axes based on the varimax criterion. We also examined habitat

TABLE 2.—Habitat characteristics measured at 17 stream sites in the Horse Creek drainage, Wyoming, JuneAugust 1986. Percentage values represent the frequency of transect points for a habitat feature. Observed Variable name

General site description Site elevation (m) Mean wetted width (m) Mean depth (m) Coefficient of variation (SD/mean) in water depth Mean water velocity (m/s) Coefficient of variation in velocity Turbidity in August (Jackson turbidity units) Aquatic vegetation present (%) Discharge in August (mVs) Riparian zone characteristics (in %) Grass cover along bank Shrub or tree cover along bank Bare ground along bank Overhanging (undercut) bank

Overhanging vegetation Habitat type (in %) Riffles or rapids Backwater pools Main channel run Main channel pool Sand or gravel bar Substrate type (in %) Silt Sand Gravel Rubble Bedrock

range

1,591-1,234 2.3-12.9 0.05-0.41 0.41-0.98 0.002-0.540 0.25-2.65 22-475 0-78 0.01-4.46 15-99 0-36 0-83 1-40 5-86 0-10 0-5 38-94 4-39 0-19 7-99 0-57 1-66 1-12 0-12

changes along the same longitudinal sequence of sites in Horse Creek for which fish community changes were examined. To determine if major gradients of fish community and habitat change were related, we examined correlations among site scores along biological axes (based on DCA) and habitat axes (based on PCA). High correlations indicated that fish community changes were related to independently identified habitat changes. The direct gradient analysis of sites along the Horse Creek main stem also provided insight into correlated changes in fish community composition and habitat conditions. Zonation and additive patterns of change in fish assemblages were examined by calculating the similarity (based on the percent similarity index; Legendre and Legendre 1983) between fish assemblages of adjacent stream sites. These similarities were then plotted from upstream to downstream sites. Regions of low similarity were taken to indicate major changes in the fish fauna suggestive of a transition from one zone to another. We av-


eraged community data for two upstream sites and compared community similarity with a similar average for the next two downstream sites. This procedure of moving averages helps distinguish true ecological discontinuities from sampling variability in noisy data sets (Ludwig and Cornelius 1987). To test the hypothesis that habitat diversity increases in a downstream direction, we examined correlations between measures of habitat diversity and site position along the longitudinal gradient in Horse Creek. Four measures of habitat diversity were calculated: the coefficients of variation for current speed and water depth were used as measures of the diversity of current velocity and water depths available to fish, and a ShannonWeiner diversity index based on proportional abundance data was used to measure variability in habitat and substrate types (German and Karr 1978; Schlosser 1982). Results

Fish Assemblage Patterns Fish community changes evident by direct gradient analysis were complex, showing elements of both zonation and species additions (Table 3). Zonation was evident on a broad spatial scale. Headwater sites (1-3) were characterized by a simple assemblage dominated by trout, whereas middle to downstream sites (4-21) supported more complex assemblages dominated by minnows and suckers. Within the middle to lower reaches of Horse Creek, fish community changes followed an additive pattern. Minnows and suckers dominated the middle reaches, and large-bodied species (common carp, quillback) and sunfishes (green sunfish and largemouth bass) were added in downstream reaches. Four species (stonecat, suckermouth minnow, central stoneroller, and johnny darter) did not fit this pattern but rather were absent at headwater sites, most abundant in middle reaches, and then rare or absent in downstream reaches. The detrended correspondence analysis also identified an upstream-to-downstream pattern as the major gradient of community change (Figure 2). Brook trout, brown trout, longnose dace, and white suckers were characteristic of upstream sites, whereas suckermouth minnows, plains killifish, and red shiners were the most distinctive species at downstream sites. A major community transition was evident by the separation of the three upstream sites (1-3) from the remaining sites along

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the first axis. Sites 4, 5,6, 8, and 9 formed a minor group distinguished from upstream sites by the absence of salmonids and distinguished from downstream sites by the absence of minnow species such as sand shiner, bigmouth shiner, and common shiner that are associated with moderate currents and sand or gravel substrates. The appearance of these species at site 10 probably reflected the enhanced streamflow and current velocities at this site (Figure 3). A second axis represented a less extensive gradient of community change that distinguished midreach sites (where johnny darters, central stonerollers, and common shiners reached peak abundance) from downstream sites (where species characteristic of larger streams occurred—quillback, common carp, and green sunfish).

Environmental Gradients Together, the first three principal components accounted for 73.5% of the variance in habitat features among the 17 sites and could be readily interpreted as general habitat gradients. The dominant gradient (principal component I) reflected longitudinal changes in stream habitat from upstream to downstream sites (Table 4). A low score on this axis was associated with high-elevation sites that were shallow, narrow, and nonturbid and that had abundant backwater pools and main channel pools but little main channel run habitat. At the opposite end of this axis were low-elevation sites that were wide, deep, and turbid with predominantly main channel run habitat. Average current speed also tended to increase with downstream position. The limited habitat data available for the three upstream sites (sites 1-3) indicated these sites fit into the upstream end of this gradient in terms of being narrow and shallow with low current velocities (Figure 3) and abundant pool habitat. The second principal component axis (PC II) reflected the condition of the streambank and riparian zone (Table 4). Sites with grass-covered, undercut banks and much overhanging vegetation were at the high end of this axis, whereas sites with bare ground along the bank and little undercut bank or overhanging vegetation were at the low end. The third principal component (PC III) was a minor gradient that distinguished sites with silt or sand substrates from sites with gravel or rubble substrates (Table 4). A plot of the 17 sites along the first three principal component axes (Figure 4) revealed several sites where cattle grazing and flow alterations due to irrigation influenced local habitat conditions.

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TABLE 3.—Fish community changes along a longitudinal gradient in Horse Creek, Wyoming, to North Platte River, Nebraska (site 21). Sites are arranged upstream to downstream. Data (except species numbers) are the percentages of all individuals at a site belonging to a particular species. A plus sign (+) means present but less than 1%. Site number and elevation (m)

Species (1) Brook trout (2) Brown trout (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)

(1) (2) (3) (4) (N)

White sucker Longnose dace Longnose sucker Creek chub Sand shiner Bigmouth shiner Fathead minnow Common shiner Brassy minnow Common carp Plains killinsh Red shiner Green sunfish Quillback Largemouth bass Shorthead redhorse

Stonecat Johnny darter Central stoneroller Suckermouth minnow Number of species

11 14 12 13 1 2 3 4 7 8 9 5 6 10 2,234 2,030 2,015 1,591 1,559 1,524 1,510 1,490 1,470 1,423 1,383 1,369 1,352 1,332

100

79 21

Upstream species 6 + + + Species showing an additive pattern 4 42 34 13 26 51 52 23 18 6 2 19 12 13 1 27 27 20 6 47 1 12 + 6 + 8 72 24

2 35 33 1 28

39 12 2 24 14 4 + 4 +

25 12 9 49 3 1 +

Species abundant in middle reaches + 3 + + 4 19 3

7

6

For example, the site (18) with the highest score on the first principal component was not the site farthest downstream. This site was in an 18-km section of Horse Creek used to convey irrigation water (Figure 1). As a result of the increased flow (1 mVs from a nearby irrigation canal from May 1 to June 30 and 2 mVs from July 1 to September 10), this section of Horse Creek was wider and had faster currents than adjacent upstream or downstream reaches (Figure 3). Sites 7 and 8 were

7

10

11

10

10 35 + 8 5 18 + 6 8

5 +

10 49 6 2 3

8 9 + 22 48 + + 2

+ +

+ +

10

4 21

11

12

14

also low on the third principal component axis, indicating a predominance of silt or sand substrates caused by low streamflow and trampling

of the streambank by cattle. Habitat diversity did not increase downstream; none of the measures of habitat diversity were significantly correlated (P > 0.05) with stream position (Table 5). The trend actually was for diversity of current velocities and habitat types to decrease downstream. This reflected the tendency

at the low end of the second principal component

for downstream sites to consist primarily of main

axis, indicating poor streambank conditions. At site 7, water removal for irrigation caused the stream to occupy only a portion of its normal channel. As a result, unvegetated silt or sand bars separated the stream from its normal riparian zone of undercut banks and overhanging vegetation. At the next downstream site (8), low streamflow continued and the streamside zone was further degraded by heavy cattle grazing. Streamflow and habitat conditions returned to normal at site 9 because of the inflow of a major tributary stream, Little Horse Creek (Figure 1). Sites 7 and 8 were

channel run habitat with sand or gravel substrates. The riffle-pool sequences found at headwater sites were absent at these lower sites.

Relations between Fish Assemblages and Habitat Conditions To determine if gradients of community change corresponded to environmental gradients, we examined correlations among site scores for the two DCA axes and the three PCA axes. We also examined correlations between site elevation and scores along these axes to identify which axes most

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Brook Trout Brown Trout

upstream

Long Nose Dace

Suckermouth Minnow Plains Killlfish

Red Shiner

FIGURE 2.—Detrended correspondence analysis ordination of the 20 study sites along Horse Creek, Wyoming, and 1 site on the North Platte River, Nebraska, based on fish community composition (see Figure 1 for site locations). Axis I is a longitudinal gradient from a simple, coldwater assemblage to a more complex, warmwater assemblage downstream. Axis II distinguishes middle-reach sites from the sites farthest downstream. Also shown are the species that distinguish each end of these gradients of community change.

along the upstream-downstream gradient (Figure 5). The major change, between sites 3 and 4, was a transition from a simple trout assemblage to a more complex minnow-sucker assemblage. A second, less-drastic community change occurred between sites 18 and 19 and represented the influence of flow enhancement. This community change primarily reflected the abundance of the suckermouth minnow, which was common only in and near the section of Horse Creek with enhanced flows.

The major community transition between sites 3 and 4 occurred over a 424-m drop in altitude. Our data do not allow us to determine how sharp the transition zone is. Propst (1982) reported that the transition between trout-dominated and minnow-sucker-dominated assemblages occurred over a short distance (usually less than 2 km) in the Rocky Mountain foothill streams he studied. The sharpness of community transitions should reflect the sharpness of environmental discontinuities in streams and warrants further study. Our data indicate assemblage patterns during the summer. Discussion How the extent and location of the transition zone Longitudinal changes in the fish fauna of Horse change on a seasonal basis remain to be examined. Creek reflected a combination of zonal and adReplacement of a trout assemblage in the headditive patterns. Zonation was evident when faunal waters by a minnow-sucker assemblage downchanges were viewed on a broad spatial scale. stream is most likely due to a change in thermal Headwater reaches (altitudes above 2,000 m) were conditions. Magnuson et al. (1979) proposed that dominated by brook trout and brown trout, and fish can be classified into coldwater, coolwater, corresponded to the trout zone identified by Eu- and warmwater thermal guilds with optimal temropean workers (Huet 1959). Below this head- peratures centered at 11-15°C, 21-25°C, and 27water zone, trout were replaced by minnows and 31°C. Our limited field measurements indicated suckers. Within the minnow-sucker zone, fish that summer midday stream temperatures were community changes were due mainly to the ad- 15-18°C at headwater sites and 22-27°C at downdition of new species downstream. Local condi- stream sites. This temperature gradient corretions—flow diversions, flow enhancement, and sponds to a transition from a coldwater to a warmcattle grazing—caused minor shifts in community water thermal guild, which is what we observed composition but did not disrupt the major pattern in the fish fauna. The importance of thermal conof species additions along a longitudinal gradient. ditions in determining zonation was discussed by


327

Row Enhancement

Flow Diversion

B

)>=- .2

20

I

. . . . . . IWX. 5

10

Site Number (downstream—*) FIGURE 3.—Longitudinal changes in habitat characteristics for the study sites in Horse Creek, Wyoming. Flow was diverted between sites 6 and 7 and enhanced between sites 17 and 19.

Hawkes (1975), who considered the four classic fish zones used by European biologists to represent only two fish faunistic regions: an upper coldwater region (trout and grayling zones) and a lower warmwater region (barbel and bream zones). Most workers who report an additive pattern in stream fish assemblages have studied systems lacking major thermal or geomorphic transitions. For example, Evans and Noble (1979) observed a longitudinal addition of fish species in a Texas stream and concluded that this pattern would be appropriate for most geographic areas. The elevation drop along their stream was only 64 m, however, and even the upstream fish assemblage was dominated by warmwater species. Little change in thermal conditions would be expected in such a system, and an additive pattern reflecting the addition of habitats and enhanced stability with longitudinal distance downstream seems logical. Similar examples of additive patterns along

longitudinal gradients that involve warmwater fish associations were provided by Jenkins and Freeman (1972), Whiteside and McNatt (1972), Lotrich (1973), and Schlosser (1987). Community changes also proceed by species additions in coldwater streams, provided that altitude or geographic conditions do not change enough to cause warmwater conditions. Gard and Flittner (1974), for example, observed an additive pattern along 21 km of Sagehen Creek, California. Only brook trout occurred at the uppermost stations, whereas three species of trout, along with sculpins, suckers, whitefish (Salmonidae), and two coldwater minnow species, occurred at lower elevations. Despite a drop in elevation from 2,103 to 1,783 m, thermal conditions remained suitable for coldwater species throughout Sagehen Creek, and the greater diversity of species downstream probably reflected a moderation of environmental conditions and the addition of new microhabitats

327

FISH ASSEMBLAGES IN A WYOMING STREAM Flow Diversion

5

Flow Enhancement

10

15

20

Site Number (downstream

FIGURE 3.—Longitudinal changes in habitat characteristics for the study sites in Horse Creek, Wyoming. Flow was diverted between sites 6 and 7 and enhanced between sites 17 and 19.

Hawkes (1975), who considered the four classic fish zones used by European biologists to represent only two fish faunistic regions: an upper coldwater region (trout and grayling zones) and a lower warmwater region (barbel and bream zones). Most workers who report an additive pattern in stream fish assemblages have studied systems lacking major thermal or geomorphic transitions. For example, Evans and Noble (1979) observed a longitudinal addition of fish species in a Texas stream and concluded that this pattern would be appropriate for most geographic areas. The elevation drop along their stream was only 64 m, however, and even the upstream fish assemblage was dominated by warmwater species. Little change in thermal conditions would be expected in such a system, and an additive pattern reflecting the addition of habitats and enhanced stability with longitudinal distance downstream seems logical. Similar examples of additive patterns along

longitudinal gradients that involve warmwater fish associations were provided by Jenkins and Freeman (1972), Whiteside and McNatt (1972), Lotrich (1973), and Schlosser (1987). Community changes also proceed by species additions in coldwater streams, provided that altitude or geographic conditions do not change enough to cause warmwater conditions. Card and Flittner (1974), for example, observed an additive pattern along 21 km of Sagehen Creek, California. Only brook trout occurred at the uppermost stations, whereas three species of trout, along with sculpins, suckers, whitefish (Salmonidae), and two coldwater minnow species, occurred at lower elevations. Despite a drop in elevation from 2,103 to 1,783 m, thermal conditions remained suitable for coldwater species throughout Sagehen Creek, and the greater diversity of species downstream probably reflected a moderation of environmental conditions and the addition of new microhabitats

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TABLE 5.—Correlations between measures of habitat TABLE 4.—Factor loadings of principal components derived from environmental characteristics of 17 sites diversity and position downstream for 17 sites in Horse along Horse Creek, Wyoming. Only values significantly Creek, Wyoming. Correlations are based on ranked data. (P < 0.05) correlated with a principal component are Correlation shown. Principal component Statistic or variable Variance accounted for (%) Single component Cumulative Environmental variables Mean wetted width Mean depth Mean water velocity % Backwater pools % Main channel run % Main channel pool Turbidity

Habitat measurement

Coefficient of variation for depth I II III Coefficient of variation for current velocity 30.6 26.8 16.1 Diversity of habitat types 30.6 57.4 73.5 Diversity of substrate types 0.77 0.68 0.77 -0.72 0.83 -0.77 0.56

with position downstream

P

-0.083

0.751

-0.453 -0.451 0.340

0.068 0.069 0.182

observed in foothill streams of the Sierra Nevada by Moyle and Nichols (1973). A trout association in cold, high-elevation streams was replaced by a minnow-sucker association in warmer, low-eleElevation -0.78 vation streams. Temperature-related zonation was % grass cover along bank 0.87 % bare ground along bank -0.86 also reported for benthic invertebrate assemblages in a Rocky Mountain stream spanning a longitu% overhanging bank 0.80 % overhanging vegetation 0.92 dinal gradient from alpine tundra to shortgrass % vegetation in stream 0.68 prairie (Ward 1986). A faunal discontinuity was % riffles 0.88 evident in this stream between the lower foothills % gravel-rubble substrates 0.89 % silt-sand substrate -0.89 and the prairie. The shift in community composition corresponded to a large increase in annual average water temperatures. The foothills marked as observed in warmwater systems. Similar pat- the downstream limit of some taxa (e.g., Plecopterns involving the downstream addition of spe- tera), whereas other benthic taxa were restricted cies within coldwater stream systems were noted to the prairie reaches (e.g., Amphipoda, Gastropby Platts (1979) and Beecher et al. (1988). oda, Odonata). A major change in the fish comTransitions in biotic communities due to chang- munity was also evident at the foothills-prairie ing thermal conditions should be expected in transition. Foothill reaches were dominated by mountainous regions where average temperatures coldwater species (brook trout and cutthroat trout typically increase as altitude decreases. For ex- Oncorhynchus clarki), whereas the prairie reaches ample, zonation related to thermal conditions was contained 16 warmwater fish species (mainly minnows, suckers, and sunfishes) but lacked trout (Propst 1982). Shifts in stream geomorphology (particularly in the gradient) also can cause faunal zonation, even when thermal conditions remain unchanged. For example, distinct biotic communities were associated with three current zones in a warmwater Louisiana stream (Guillory 1982). Of 57 fish species, 2 were restricted to intermittent headwater sites, 12 to high-gradient middle reaches, and 24 to slow-flowing lower reaches on the Mississippi River floodplain. Only eight species were collected across all three zones. Abrupt transitions in community composition corresponded with the transitions in stream gradient. Similar examples of faunal zonation due to major changes in stream FIGURE 4.—Principal component (PC) ordination of geomorphology were reported for streams in the 17 study sites in Horse Creek, Wyoming, for which Pennsylvania (Hocutt and Stauffer 1975), Arizona complete habitat data were available. See Figure 1 for (Barber and Minckley 1966), and Zaire (Balon and site locations. Stewart 1983). In all cases, zonation was due pri-

329


TABLE 6.—Correlations among scores along the three principal component analysis (PCA) axes, scores along the two detrended correspondence analysis (DCA) axes, and elevation for 17 sites along Horse Creek, Wyoming. All correlations based on ranked data. Asterisks denote P < 0.05* or P < 0.01**. Elevation or score Site elevation PCA I score PCA II score PCA m score DCA I score

PCA I score

PCA II score

-0.75**

0.24 0.14

manly to changes in gradient, because thermal conditions remained relatively constant. Zonation and species additions are thus complementary processes that operate on different spatial scales to influence stream biota. Zonation is most likely over large spatial scales where changes in temperature regimes, landforms, or vegetation types result in different ecological conditions within lotic systems. Zonation often reflects interactions of factors, as when altitudinal changes result in different thermal conditions within the stream and different terrestrial vegetation communities, which in turn change detrital and nutrient inputs to aquatic systems. For example, Gulp and Davies (1982) observed zonation in benthic invertebrate communities in relation to three altitudinal and terrestrial vegetation zones along a Canadian stream, and Perry and Schaeflfer (1987) attributed sudden shifts in benthic community composition in a Colorado stream to discontinuities in stream geomorphology. Within zones, biotic communities can be ex-

5

PCA III score

DCA I score

DCA II score

-0.04 -0.03 -0.09

-0.66** -0.51* -0.35 -0.10

-0.21 0.17 0.08 -0.15 0.21

pected to show a pattern of downstream addition of species. This pattern is the result of increased living space downstream (streams tend to get bigger and deeper), increased habitat diversity (e.g., the addition of pools and backwater sloughs), and greater environmental stability (e.g., reduction in flow variability; Horwitz 1978). Recognizing that zonation and species addition may operate on different spatial scales should help alleviate some of the controversy over which model of community change is the most appropriate for stream systems (Balon and Stewart 1983; Minshall et al. 1985; Statzner and Higler 1985). Habitat diversity in Horse Creek did not increase downstream, as has been reported for other stream systems (German and Karr 1978; Schlosser 1982,1987). In those streams, upstream reaches were shallow and structurally simple, whereas downstream reaches had complex channels and large pools. In contrast, downstream reaches in Horse Creek were dominated by a uniform run habitat with a substrate of shifting sand. Pool hab-

10

15

20

Site Number (downstream—»)

FIGURE 5.—Community similarity for pairs of adjacent sites along a longitudinal gradient in Horse Creek, Wyoming. The low similarity between sites 3 and 4 corresponds to the transition from a salmonid to a suckerminnow assemblage. Low similarity between sites 18 and 19 reflects the local abundance of suckermouth minnows in a reach with enhanced flows due to conveyance of irrigation water.

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itats and cover associated with woody debris were absent. This monotony of habitat is typical of prairie streams (Fausch et al. 1984), indicating that Great Plains streams may experience a decrease, rather than an increase, in habitat diversity in moving from foothill regions onto the prairie. The downstream increase in species richness in Horse Creek (Table 3) despite no increase in habitat diversity (Table 5) provides support for the idea that downstream increases in community richness can result from moderation of environmental conditions and increased living space, not strictly from increased habitat diversity. In summary, both zonation and downstream species addition were evident in fish assemblages in Horse Creek, but they appeared on different spatial scales. On a broad scale, zonation into a simple, coldwater trout assemblage upstream and a more diverse, warmwater assemblage downstream was evident. On a smaller scale, community change in the warmwater zone was primarily due to the downstream addition of species. Local habitat modifications caused minor changes in fish community composition but did not disrupt the dominant longitudinal patterns. This pattern of broad-scale zonation and species addition within

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Received June 6, 1990 Accepted November 9^ 1990