Soil compaction and moisture status from large mammal trampling in Coleogyne (blackbrush) shrublands of southern Nevada.
The Red Rock Canyon National Conservation Area (RRCNCA) may seem rugged and desolate at first glance, but a closer look reveals an area teeming with wildlife (BLM 1999). A variety of wild mammal species live within the boundaries of Coleogyne ramosissima (blackbrush) shrublands, a major mid-elevation vegetation zone in the RRCNCA. During the peak of dry summer seasons, most large mammals prefer to be active from dusk to dawn hours where air temperatures are cooler compared to midday hours. These typical nocturnal mammals include Urocyon cineroargenteus (gray foxes), Vulpes macrotis (desert kit foxes), Canis latrans (coyotes), Lynx rufus (bobcats), Odocoileus hemionus (mule deer), and Felis concolor (mountain lion) (Eifert and Eifert 2000). Ovis canadensis nelsoni (desert bighorn sheep) and Equus asinus (burro) are active during the day (Eifert and Eifert 2000). Although Felis and Ovis inhabit high cliffs and canyons, they can be found occasionally in the Coleogyne shrublands. Equus asinus and E. caballus (wild horses) are often seen in the vicinity of Bonnie Springs and Spring Mountain Ranch State Park, the southernmost part of the RRCNCA. Equus caballus are also active during the day (BLM 1994).
Large mammals derive some moisture from their food but require drinking water periodically. Ovia c. nelsoni will not live more than two miles from a water source. They may expand their range after rains fill more potholes, but such expansions are only temporary (BLM 1999). Equus asinus are frequently seen on roadsides begging for food (Lei, personal observation 2001). Approximately 50 E. asinus and 70 Ovis live within the Conservation Area (Eifert and Eifert 2000). Odocoileus hemionus prefer foothills with low scrub growth or thickets along washes. By late evening, Odocoileus leave their daytime hiding places to search for water in seeps and springs (BLM 1999).
Many mammal species are faithful to their home ranges, although home ranges tend to be larger in fall and winter than in summer and spring (Douglas and Haitt 1987; Douglas and Hurst 1993). Home ranges are smallest during dry summer seasons because the proximity to a water source is crucial to survival. Cool winter air temperatures permit a greater dispersal of large mammals from water sources (Norment and Douglas 1977).
Since large, heavy mammals follow many paths over the ground surface, severe soil compaction may occur, especially near water sources. Soil compaction has been defined by Lull (1959) as the packing together of soil particles by instantaneous forces exerted at the soil surface. These forces (animal, human foot or vehicle traffic) can increase soil bulk density and reduce macropore space. The loss of macropores reduces water infiltration and water movement through the soil, thus increasing surface water run-off and fluvial erosion (Scholl 1989). A significant increase in bulk density has been the most common means of expressing soil compaction problems. Both penetrometer and bulk density are used to assess compaction effects of vehicles, humans, and animals in the Mojave Desert of southern California (Webb et al. 1986).
Studies of cattle trampling under semiarid conditions have shown detrimental effects on various soil properties (Reed and Peterson 1961; Orr 1975; Warren et al. 1986; Stephenson and Veigel 1987; and Van Havern 1983). Yet, soil compaction from large, heavy mammal trampling under arid conditions of southern Nevada have not been documented. The objective of this study is to quantitatively evaluate changes in soil attributes (compaction, bulk density, macropore, and moisture status) resulting from mammal trampling in Coleogyne shrublands of the RRCNCA.
Southern Nevada is in the Basin and Range physiographic province, a region characterized by annual weather extremes and a sparse vegetation cover (Brittingham and Walker 2000). Most precipitation in southern Nevada occurs in the winter, while summer storms are highly localized and unpredictable (Brittingham and Walker 2000).
Red Rock Canyon National Conservation Area (roughly 36[degrees]05'N, 115[degrees]15'W) of the Spring Mountains, Nevada, was chosen because it supports well-established, nearly monospecific Coleogyne shrublands, ranging from approximately 1,450 to 1,775 m in elevation. Coleogyne shrublands support at least nine relatively large mammal species (Table 1) as indicated by large numbers of dung, paw and hoof prints, and grazed vegetation. Blue Diamond Wash, Willow Spring, Pine Creek, Oak Creek, Calico Hills, First Creek, Lost Creek Canyon, and Ice Box Canyon are some of the major areas within the RRCNCA that can provide ample water for large mammals throughout much of the year. Water can be replenished with melting snow from adjacent high mountains in the spring, erratic precipitation during fall and winter, as well as with occasional monsoon thunderstorms in the summer. Summer thunderstorms often cause locally intense rainfall where intermittent washes throughout the RRCNCA can rapidly collect excess running surface water during and shortly after major storm events. Prolonged cloudbursts in the summer can create major flash floods in wash and depression areas.
Large, heavy mammals directly trample and turn up the fragile desert soil. Sandstone and limestone are both rock types that have shallow soils in which Coleogyne and other associated woody species grow well (Callison and Brotherson 1983; West 1983). Within Coleogyne vegetation zones, many common herbaceous species are members of the Asteraceae, Brassicaceae, Fabaceae, and Poaceae families.
Field Design and Sampling
Field studies were conducted in animal trails during Summer 2000 in the RRCNCA. A total of 49, 1-ha plots containing paw and hoof prints and fecal material was established within 7 km of intermittent springs and streams. Because animal trails were not conspicuous, sampled plots were randomly selected at each distance (1 to 7 km) from water courses, and individual piles of fecal material (fresh and dried) were counted.
Within each kilometer (1, 2, and 6) of water in the same plots, 60 soil samples containing clearly defined paw and hoof prints were randomly collected. Adjacent reference soils were collected beyond 7 km of water, with no clear evidence of paw or hoof prints, dung, and grazed vegetation. Soil samples were excavated approximately 10 cm in diameter to depths of 15 cm. Soil compaction was obtained in the field using a penetrometer inserted into the soil after removing the stony surface (Lei and Walker 1997; Lei 1999; Lei 2000). The compaction readings were taken at the point where the cone base reached the soil surface (point depth = 3.7 cm). Soil samples were sifted through a 2-mm mesh to remove plants roots and rocks > 2 mm in diameter. Soil bulk density measurements were performed on sifted soils that were oven-dried at 65[degress]C for 72 h.
For each sampled plot, soil surface characteristics of bare ground, gravel (2-64 mm in diameter), cobble (65-256 mm), and boulder (> 256 mm) were visually quantified using 10% increments.
Soil moisture characteristics were stratified by trampling severity (control, light, moderate, and heavy) and geomorphic surfaces (terrace and slope sites). Water infiltration rates were measured by using PVC pipe, 5.5 cm in diameter and 9.5 cm tall. This pipe was open at both ends and was gently tamped into the trampled and non-trampled soils to a depth of 2 cm to prevent leakage, and then 50 mL of water was poured into the pipe. Time taken for the water to soak completely into the soil was recorded with a stop-watch.
Approximately 1.5 L of water, acting as an artificial rain, was manually poured through a perforated 13-cm disk, with perforations being evenly spaced on a 0.1-cm grid. The disk was placed at 1.0 m aboveground. Total delivery time was 1 min for the water to be dispensed on the soil surface and to create precipitation at a cloudburst level (Brotherson and Rushforth 1983). A sudden heavy precipitation is significant due to its impact on surface-water runoff and fluvial erosion. Depth of water penetration was measured once the water had disappeared completely into the soil surface.
Surface-water runoff was measured by recording the downslope and across-slope spread of water that was artificially rained onto study sites (Brotherson and Rushforth 1983). The shape of surface-water runoff resembled an ellipse, thus was computed using the following formula: ([pi]ab) where a and b are radii of an ellipse (Larson et al. 1994). Since surface water runoff did not form a perfect elliptical shape, measured areas were likely to be overestimates.
Soil movement was assessed by estimating the amount of soil moved through fluvial erosion during a measured rain. The following index was used: 1 = no appreciable movement; 2 = moderate movement, up to 10 % of soil particles being displaced; and 3 = heavy movement, between 10 % and 20 % of soil particles being displaced (Brotherson and Rushforth 1983).
Laboratory and Statistical Analyses
A set of soil cores of known volume was carefully removed from the field. Fresh soil cores were oven-dried at 65[degrees]C until they reached constant mass. Soil bulk density was estimated by dividing dry mass by known volume. Average pore space was determined using the equation: per space (%) = 100 - ([D.sub.b]/[D.sub.p] * 100), where [D.sub.b] is bulk density of the soil and [D.sub.p] is average particle density, usually about 2.65 g [cc.sup.-1] (Hausenbuiller 1972; Davidson and Fox 1974).
One-way analysis of variance (ANOVA; Analytical Software 1994) was used to determine if fecal densities differed with respect to distance from water, and to compare physical property differences between trampled and adjacent reference soils. Tukey and Scheffe's multiple comparison tests (Analytical Software 1994) were then performed to compare site means when a significant trampling effect was detected. Linear regression analysis was performed to correlate fecal density with increasing distance from water courses.
Path analysis and Pearson's correlation analysis (Analytical Software 1994) were conducted to correlate trampling severity with soil compaction, soil bulk density, and macropore, as well as to intercorrelate among these three soil properties. Path analysis was used to examine proposed causal links between trampling and soil moisture attributes, and among the three soil physical properties.
Multivariate Analysis of Covariance (MANCOVA; Analytical Software 1994) was conducted on six soil moisture attributes, with trampling severity and geomorphic surface (terrace and slope) as main variables, and with rock size (gravel, cobble, and boulder) and rock abundance (percent rock cover) as covariate variables. Percent ground cover of gravel, cobble, and boulder was visually quantified using 10 % increments. Tukey and Scheffe's multiple comparison tests were then performed to compare site means when significant effects of trampling severity and geomorphic surface were detected. The presence of abundant rocks on the soil surface would influence various soil moisture regimes. Mean values were presented with standard errors, and statistical significance was determined at p [less than or equal to] 0.05.
Fecal density decreased significantly (p [less than or equal to] 0.001; Fig. 1) when moving away from water sources, and was negatively correlated ([R.sup.2] = -0.88; p [less than or equal to] 0.001) with increasing distance from water courses in the RRCNCA.
[FIGURE 1 OMITTED]
Moderate to heavy mammal trampling had a significantly lower macropore (percent pore space), and higher soil compaction and soil bulk density compared to light or no trampling (p [less than or equal to] 0.01 ; Table 2). However; significant differences were not detected (p > 0.05; Table 2) in soil compaction, soil bulk density, and percent pore space between moderate and heavy trampling, as well as between light and no trampling. Path analysis revealed that trampling severity was a significant positive predictor of macropore, soil compaction, and soil bulk density (Fig. 2). Soil compaction was a significant positive predictor of bulk density, and a negative predictor of macropore (Fig. 2).
[FIGURE 2 OMITTED]
Significant interaction (p [less than or greater than] 0.05; Tables 4 and 5) was detected between trampling severity and geomorphic surface for area of water spread (surface water runoff) only. Significant differences were observed in trampling severity and geomorphic surface for all measured soil moisture variables.
Excessive trampling by large mammals significantly altered a number of soil properties and moisture characteristics in Coleogyne shrublands of the RRCNCA. Location of water was a main factor in large mammal movement. The extent of trampling disturbances was a function of their distance from water sources. In this study, fecal density increased significantly when approaching water sources.
Dry summer seasons are the time of greatest soil disturbance as large herbivores congregate in large numbers near water to graze upon shrubs, perennial forbs and grasses. In summer, approximately 60 % of the entire E. asinus populations is found within a 2-km radius of water, and about 98 % of the E. asinus are restricted to within 4 km of water (Douglas and Hurst 1993). In contrast, about 80 % are seen at distances greater than 6 km of water in winter (Douglas and Hurst 1993).
Because mammal trails radiate in all directions from springs, soil disturbance is substantially greater near a spring in Death Valley of southern California (White 1980). Within 0.5 km of a spring in the Lake Mead National Recreation Area, the number of converging trails contributes to severe soil compaction (O'Farrell 1978). Animals have caused a moderate to severe soil compaction within 0.8 km of major water courses in the Las Vegas valley (Woodward 1976). Nevertheless, ecological impacts on soil and vegetation beyond 0.8 km of the spring has been minor in southern California (Woodward 1976).
Path analysis indicated that percent pore space, soil compaction, and soil bulk density were significantly directly influenced by the severity of mammal trampiing. Percent pore space and bulk density were also significantly directly affected by the severity of soil compaction. Short, unlabeled (residual) arrows shown in the path diagram (Fig. 2) indicated that these variables are also subject to additional biotic influences, which include small animal and human trampling (recreational) activities. After all, the RRCNCA is a popular place for year-round, outdoor recreational activities. Humans and their motor vehicles go off-trails periodically.
Significant increases in soil compaction and bulk density can lead to a significant decrease in the percentage of macropores in heavily trampled compared to lightly or non-trampled soils. Pore space consists of macropores that allow the ready movement of air and water (Davidson and Fox 1974). The increase in soil compaction, along with the subsequent increase in soil bulk density and decrease in macropore space in heavily trampled soils, reduces the amount of water that the soil can hold and the rate at which water can flow through the soil.
Furthermore, soil compaction resulting from heavy paw and hoof impacts significantly reduced water infiltration and water movement into the soil, and increased surface-water runoff in Coleogyne shrublands. Soil compaction greatly reduces water infiltration in alluvial soils of the Mojave Desert in southeastern California (Webb and Wilshire 1980; Vasek et al. 1975). Severely compacted animal trails are nearly impervious to penetration by water so that heavy precipitation tends to run-off compacted soils, leading to fluvial erosion and resistance to plant colonization (Carothers 1976).
A high percentage of rock cover was observed on the soil surfaces throughout much of the study site. By far, gravel was the most common type of rock compared to cobbles and boulders. In New Mexico, rock size and abundance may influence a variety of other soil attributes including infiltration, porosity, water-holding capacity, and erodibility (Carlson and Whitford 1991). Fluvial erosion and surface water runoff were significantly greater for slope than terrace site. At some slope sites, a small movement of soil particles occurred when water traveled rapidly downslope during a cloudburst, perhaps due to a lack of abundant rocks on the soil surface to reduce fluvial erosion and surface water runoff.
Excessive trampling by large mammals significantly altered a number of edaphic attributes where Coleogyne and associated woody taxa exist. Research plots containing heavily trampled soils included countless visible, overlapping paw and hoof prints on the soil surface. Some of the trampling herbivore (prey) species were a much more important source of soil compaction than others. Among the five large mammal species, E. asinus were most frequently seen during the course of study. In this study, wild E. caballus, by far, were the heaviest in weight, averaging 409 kg. Equus asinus and Ovis were the second and third heaviest mammals, weighing 182 and 105 kg, respectively (Table 1). Although Canis, Lynx, Urocyon, and Vulpes are considered as light-weighted mammals, having a high traffic volume contributed to the overall water infiltration rate, as well as to the overall severity of soil compaction and surface-water runoff. The predators, however, would have a minimal impact on soil compaction because they were smaller and considerably less abundant compared to those large prey species.
Soil compaction is an aspect of land degradation associated with excessive foot traffic from large animals. Heavily compacted soils through animal trampling can alter the composition of Coleogyne shrublands through time, favoring weedy, pioneer plant species. Proximity to water courses, along with preference for abundant food supply during dry seasons, appear to be the driving force in producing massive soil disturbance. Equus asinus, E. caballus, and Ovis are large, heavy opportunistic herbivores. Their generalized feeding behavior provide them with the ability to exploit and degrade many aspects of desert woody vegetation zones in southern Nevada and the southwestern United States.
Table 1. Nine large manunal species found within the boundaries of Coleogyne shrublands in the Red Rock Canyon National Conservation Area (Eifert and Eifert 2000). These mammal species are arranged by weight, from heaviest to lightest. Common name Species name Mean weight Type of track (kg) Horse Equus caballus 409.1 Hoof Burro Equus asinus 181.8 Hoof Desert bighorn sheep Ovis canadensis 105.0 Hoof nelsoni Mule deer Odocoileus 90.9 Hoof hemionus Mountain Felis concolor 63.6 Paw lion Coyote Canis lartrans 13.6 Paw Bobcat Lynx rufus 9.8 Paw Gray fox Uracyon 4.5 Paw cineroargenteus Desert kit Vulpes macrotis 2.1 Paw fox Table 2. Physical characteristics (mean [+ or -] SE; n = 60 per treatment per characteristic) of non-trampled (reference) and various levels of trampled soils in Coleogyne shrublands. Heavily, moderately, and lightly trampled soils are located within 1, 2, and 6 km of water, respectively. Mean values in rows followed by different letters are significantly different at P [less than or equal to] 0.05. Soil property Control Compaction (kg/[cm.sup.2]) 6.1 [+ or -] 0.4 a Bulk density (g/[cm.sup.3]) 1.24 [+ or -] 0.04 a Pore space (%) 53.2-2.4a 51.2 [+ or -] 2.5 a Trampling severity Soil property Light Compaction (kg/[cm.sup.2]) 6.3 [+ or -] 0.7 a Bulk density (g/[cm.sup.3]) 1.29 [+ or -] 0.05 a Pore space (%) 53.2-2.4a 51.3 [+ or -] 2.5 a Trampling severity Soil property Moderate Compaction (kg/[cm.sup.2]) 7.0 [+ or -] 0.5 b Bulk density (g/[cm.sup.3]) 1.41 [+ or -] 0.07 b Pore space (%) 53.2-2.4a 46.8 [+ or -] 2.3 b Trampling severity Soil property Heavy Compaction (kg/[cm.sup.2]) 7.3 [+ or -] 0.5 b Bulk density (g/[cm.sup.3]) 1.43 [+ or -] 0.06 b Pore space (%) 53.2-2.4 a 44.9 [+ or -] 2.2 b Table 3. Relationship between mammal trampling severity and three soil physical attributes and relationship among these three soil attributes in Coleogyne shrublands. The direct causal effect of each pairing of variables is the standard partial regression coefficient (path coefficient). Total causal influence (Pearson's r-value) sums all direct and indirect pathways. Heavily, moderately, and lightly trampled soils are located within 1, 2, and 6 km of water, respectively. All computed values are statistically significant at p [less than or equal to] 0.001. Pairing of variables Direct causal Indirect causal Compaction x trampling severity 0.85 0.10 Bulk density x trampling severity 0.88 0.09 Macropore x trampling severity -0.88 -0.09 Bulk density x compaction 0.94 0.05 Macropore x compaction -0.94 -0.05 Macropore x bulk density -0.98 -0.01 Pairing of variables Total causal Compaction x trampling severity 0.95 Bulk density x trampling severity 0.97 Macropore x trampling severity -0.97 Bulk density x compaction 0.99 Macropore x compaction -0.99 Macropore x bulk density -0.99 Table 4. Moisture characteristics (mean [+ or -] SE: n = 60 per treatment per characteristic) of nontrampled soils and various levels of trampling in Colegyne shrublands. Heavily, moderately, and lightly trampled soils were located within 1, 2, and 6 km radius of water respectively. Moisture parameter Control Infiltration (seconds) Terrace 181.2 [+ or -] 17.2 Slope 202.2 [+ or -] 12.4 Depth of water penetration (cm) Terrace 4.0 [+ or -] 0.01 Slope 3.6 [+ or -] 0.02 Downslope spread (cm) Terrace 69.0 [+ or -] 4.6 Slope 94.4 [+ or -] 5.4 Across slope spread (cm) Terrace 60.5 [+ or -] 3.9 Slope 85.1 [+ or -] 4.1 Area of spread ([cm.sup.2]) Terrace 3276.9 [+ or -] 498.1 Slope 6306.3 [+ or -] 523.4 Soil movement Terrace 1.1 [+ or -] 0.01 Slope 1.3 [+ or -] 0.01 Trampling severity Moisture parameter Light Infiltration (seconds) Terrace 197.3 [+ or -] 14.7 Slope 220.2 [+ or -] 15.1 Depth of water penetration (cm) Terrace 3.5 [+ or -] 0.02 Slope 3.2 [+ or -] 0.02 Downslope spread (cm) Terrace 80.1 [+ or -] 4.9 Slope 10.3 [+ or -] 5.2 Across slope spread (cm) Terrace 68.4 [+ or -] 4.9 Slope 94.4 [+ or -] 5.2 Area of spread ([cm.sup.2]) Terrace 4295.5 [+ or -] 455.7 Slope 7432.6 [+ or -] 701.2 Soil movement Terrace 1.2 [+ or -] 0.01 Slope 1.4 [+ or -] 0.01 Trampling severity Moisture parameter Moderate Infiltration (seconds) Terrace 236.7 [+ or -] 13.4 Slope 253.3 [+ or -] 12.1 Depth of water penetration (cm) Terrace 2.4 [+ or -] 0.01 Slope 2.3 [+ or -] 0.01 Downslope spread (cm) Terrace 90.8 [+ or -] 5.4 Slope 108.2 [+ or -] 5.7 Across slope spread (cm) Terrace 84.2 [+ or -] 5.8 Slope 110.4 [+ or -] 5.8 Area of spread ([cm.sup.2]) Terrace 6001.6 [+ or -] 612.7 Slope 9377.0 [+ or -] 689.4 Soil movement Terrace 1.3 [+ or -] 0.01 Slope 1.5 [+ or -] 0.01 Trampling severity Moisture parameter Heavy Infiltration (seconds) Terrace 270.6 [+ or -] 16.4 Slope 287.6 [+ or -] 13.9 Depth of water penetration (cm) Terrace 2.1 [+ or -] 0.01 Slope 1.9 [+ or -] 0.01 Downslope spread (cm) Terrace 98.2 [+ or -] 5.8 Slope 119.6 [+ or -] 6.2 Across slope spread (cm) Terrace 96.1 [+ or -] 5.9 Slope 123.5 [+ or -] 6.7 Area of spread ([cm.sup.2]) Terrace 7392.9 [+ or -] 659.1 Slope 1594.9 [+ or -] 750.3 Soil movement Terrace 1.4 [+ or -] 0.01 Slope 1.6 [+ or -] 0.01 Table 5. Summary from two-way ANOVA with trampling severity, geomorphic surface, and their interactions on various soil moisture characteristics. df = 1 for geomorphic surface; df = 3 for trampling severity and for the trampling severity x geomorphic surface combination. Trampling severity Geomorphic surface Moisture parameter F P F P Water infiltration 56.23 0.0000 26.23 0.0014 Depth of water penetr. 254.77 0.0000 15.93 0.0052 Downslope water spread 10.03 0.0063 31.49 0.0008 Across-slope water spread 35.02 0.0001 78.32 0.0000 Area of water spread 999.79 0.0000 275.16 0.0000 Soil movement 6.22 0.0219 16.80 0.0046 Trampling x surface Moisture parameter F P Water infiltration 0.65 0.6092 Depth of water penetr. 1.38 0.3317 Downslope water spread 0.13 0.9359 Across-slope water spread 0.05 0.9861 Area of water spread 6.42 0.0022 Soil movement 0.62 0.6193
I gratefully acknowledge Steven Lei, David Valenzuela, and Shevaun Valenzuela for valuable field assistance. Steven Lei assisted with statistical analyses, and David Charlet provided helpful comments on earlier versions of this manuscript. The Department of Biology at the Community College of Southern Nevada (CCSN) provided logistical support.
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Accepted for publication December 11, 2002.
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|Author:||Lei, Simon A.|
|Publication:||Bulletin (Southern California Academy of Sciences)|
|Date:||Dec 1, 2003|
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