Great Basin Naturalist
Volume 55 Number 4 Article 7
10-31-1995
Grasshopper densities on grazed and ungrazed rangeland under drought conditions in southern Idaho
Dennis J. Fielding Arizona State University, Tempe
Merlyn A. Brusven Arizona State University, Tempe
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Recommended Citation Fielding, Dennis J. and Brusven, Merlyn A. (1995) "Grasshopper densities on grazed and ungrazed rangeland under drought conditions in southern Idaho," Great Basin Naturalist: Vol. 55 : No. 4 , Article 7. Available at: https://scholarsarchive.byu.edu/gbn/vol55/iss4/7
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 55(4), © 1995, pp. 352-358
GRASSHOPPER DENSITIES ON GRAZED AND UNGRAZED RANGELAND UNDER DROUGHT CONDITIONS IN SOUTHERN IDAHO
Dennis J. Fieldingl and Merlyn A. Brusven1,2
ABSTRAGJ:-Low-density grasshopper populations were sampled at 15 pairs of rangeland sites in south central Idaho. One site of each pair had not been grazed by livestock for at least 10 years. Grazed sites were managed under normal grazing regimes established by the Bureau ofLand Management. Mean grasshopper density was higher on ungrazed sites than on grazed sites. Proportions of Melanoplus sanguinipes were higher on ungrazed sites than on grazed sites and were higher on annual grasslands than on other vegetation types. Effects ofgrazing appeared to be independent of vegetation type. Proportions of Gomphocerinae, a subfamily of grasshoppers that feeds almost exclusively on grasses, were affected hy vegetation type, but not grazing. Crested wheatgrass seedings supported the highest proportions of Gomphocerinae. Proportions of Oedipodinae were affected by grazing and vegetation type. Higher proportions of Oedipodinae were found on grazed sites than on ungrazed sites, and on sagebrush/grass sites than on annual grasslands. Results indicate that livestock grazing during drought conditions tends to reduce grasshopper populations on southern Idaho rangeland.
Key words: Orthoptcm, Acrididae, Melanoplus sanguinipes, livestock grazing, drought, population density, range tfwnagement.
Grasshoppers are frequently the most abun livestock grazing regimes administered by the dant arthropods, in terms of biomass, in the Bureau of Land Management and rangeland intermountain sagebrush ecoregion of the that had not been grazed for at least 10 years. western United States. As primary consumers they may be important in energy and nutrient STUDY AREA cycling, and, at outbreak densities, they compete with livestock and wildlife for forage. Because The study area is located southeast of Sho of their ecologic and economic importance, shone, ID, within the Bureau of Land Manage the potential effects of range management ment's (BLM) Shoshone District, between practices on grasshoppers are a concern to longitude 114'30' and 114'00' Wand latitude those interested in the healtb of rangeland 42'37.5' and 43'00' N. This area receives an ecosystems. Several studies have addressed average of about 26 cm of precipitation annu the role of livestock grazing on grasshopper ally, most of it between October and May. populations (Coyner 1938, Smith 1940, Average annual temperature is about 9.0°C. Campbell et aJ. 1974, Holmes el aJ. 1979, The intermountain sagebrush ecoregion Capinera and Sechrist 1982, Jepson-Innes and was subjected to heavy grazing pressure in the Bock 1989, Quinn and Walgenbach 1990, late 19th and early 20th centuries, frequent Miller and Onsager 1991). Onsager (1987) sug fires, and subsequent invasion by cheatgrass gested that there is probably geographic varia and other exotic plant species (Pickford 1932, tion in grasshopper responses to grazing Stewart and Hull 1949, Mack 1981, Yensen among rangeland types and their constituent 1982). As a result, stands of grazing-intolerant grasshopper communities. To date no studies native grasses were greatly diminished over have investigated the relationship between much ofthe region, and cheatgrass has become livestock grazing and grasshopper densities on the dominant species on more than 40 million rangelands in the intermountain region. ha of the Intermountain West (Mack 1981, This study, conducted during years ofbelow Pellant and Hall 1994). The present vegetation normal precipitation and low grasshopper within the study area consists primarily ofcheat densities, examined differences in grasshopper grass, Bromus tectorum L., with sagebrush, densities between rangeland under normal Artemisia tridentata wyomingensis Beetle &
1J)cpilllmcnt of Ph",l, Soil (\nd Entomo1r)! 352 1995] GRASSHOPPERS AND LIVESTOCK GRAZING 353 Young and A. tridentata tridentata (Rydb.) a site was estimated by counting the number Beetle, where it has not burned recently. As of of grasshoppers flushed from 50, 0.I-m2 rings 1988, about 23% (ca 40,000 hal ofthe study area 5 m apart in a circular transect (Richards and consisted of crested wheatgrass, Agropyron Waloff 1954, Onsager and Hemy 1977). Species cristatum (L.) Gaertn., plantings (USDI-BLM composition on a site was determined by a 1984, 1990). "flush-capture" method (Capinera and Sechrist 1982). 'Thirty to 100 specimens were captured MATERIAL';, AND METHODS and identified at each site on each sampling date by slowly walking in a circular transect Ungrazed sites were selected on the basis of and, to avoid bias toward more conspicuous grazing history (not grazed for at least 10 years), species, counting only those grasshoppers en size (at least 16 hal, and shape (at least 100 m conntered directly in the path ofthe observer: across the narrowest dimension). Fifteen Vegetation was sampled on the same dates rangeland sites were found within the study as the grassbopppers by visually estimating area that met these criteria. Most sites were the percentage ground cover in 5% incre isolated tracts fenced to exclude livestock and ments by plant species in each of 40, 0.1-m2 to provide habitat for upland game birds. square quadrats in a circular transect. Plant Grazing by wildlife within the ungrazed tracts species unidentifiable in the field were col was negligible. Black-tailed jackrabbits were lected and identified later. The percent cover not abundant during the years in which sam of cryptogams, cattle dung, and bare ground pling took place, and populations of prong was also estimated. Vegetation data from the born antelope, the only other large vertebrate three sampling dates were combined for sub herbivore present in the summer, are quite low sequent analyses. and widely dispersed across the study area, ea DATA ANALYSIs.-We classified the sites into 300 individuals over 180,000 ha (J. Rnssell, three vegetation types based on dominant vege USDI-BLM, personal communication). tation on a site. Sites that had been seeded to A grazed site was selected to match each crested wheatgrass were categorized as re ungrazed site for a total of 30 sites. In most planted. Sites with sagebrush as the dominant cases grazed sites were adjacent to, and plant species were placed in the sagebrush shared a boundary with, ungrazed sites. For category, and the remaining sites, dominated six sites adjacent matehed pairs were not pos by cheatgrass without significant sagebrush sible; consequently, grazed sites were chosen cover, were categorized as annual grasslands. within 2 km. All grazed sites matched the Differences in percentage ground cover ungrazed sites in soil type, topography, vege among vegetation types were confirmed with tation, slope, and aspect. All grazed sites were a Kruskal-Wallis non-parametric one-way located within BLM-administered grazing analysis of variance (Zar 1984). Non-paramet allotments. Stocking rates for the grazed sites ric statistical tests were used with the ground varied !i'om 1.9 to 2.8 ha/AUM (USDI-BLM cover data because ofthe large number of zero 1990). Not all grazed sites were grazed each values involved. Comparisons between vege year, as prescribed by rest-rotation grazing tation types were made with a non-parametric management plans. analog ofTnkey's test (Dunn 1964, Zar 1984). Elevation of the sites ranged !i'om 1180 to Because paired sites were in close proximi 1320 m. Five pairs of sites were located on ty and ofsimilar vegetation, we used Wilcoxon areas replanted with crested wheatgrass, four paired-sample tests to identify differences in pairs were on annual grassland sites having lit percentage ground cover between grazed and tle or no sagebrush, and six pairs were located ungrazed sites. on sagebrush-grass sites. Grasshopper densities were too low to con GRASSHOPPEH AND VEGETATION SAMPLING. duct meaningful statistical comparisons sepa Grasshoppers were sampled on ] 9 Jnly-7 rately for all species. Accordingly, analyses were August 1990 (adult stage), 21-28 June 1991 conducted on densities oftotal grasshoppers, on (primarily nymphal stage), and 2---13 August proportions ofMe1mlOpIus sanguinipes (the major 1991 (adult stage). Corresponding sites of a pest species in the region), and on proportions grazed and ungrazed pair were always sam of the three subfamilies of Acrididae within the pled on the saIne day. Grasshopper density on region, Melanoplinae, GOInphocerinae, and 354 GREAT BASIN NATURALIST [Volume 55 Oedipodinae. Grasshopper densities were TABLE 1. Median (s.d.) percentage ground cover by veg transformed by log.,(x + 1) to normalize the etation type. data. The arcsin transformation was applied to Vegetation type the proportions (Zar 1984). Ground cover Annual Three-way analysis of variance (PROC variables Replanted Sagebrush grasslands GLM, SAS Inst.) was used to determine the (N ~ 10) (N ~ 12) (N ~ 8) significance of sampling date, vegetation type, Annual grasses 1.2b1 7.8b 20.0a and grazing treatroent effects on grasshopper (3.7) (8.1) (9.0) density and proportions. For the ANOVA, Annual forbs O.5a 1.2b 3.1b sites were not blocked by location; i.e., pairing (16) (3.9) (2.8) was ignored. Comparisons among vegetation types were made with least significant differ Agropyron 16.8a O.Ob O.Ob cri.swwm (7.2) (0.02) (0.03) ence mean separation tests (PROC GLM, SAS Inst.). Paired-sample t tests were used for Poo sandbergii 6.9. S.Oa 12.0a comparisons between grazing treatments. (4.9) (1.7) (5.4) Trends between habitat characteristics and Other perennial O.la O.6a l.la grasshopper densities/proportions were evalu grasses (0.4) (2.9) (2.8) ated by Spearman rank correlations (PROC Sageblush O.Ob 13.Oa O.Ob CORR, SAS Insl). Mean values from the three (2.8) (50) (2.4) sampling dates were used for the correlation analyses. Total vegetation 26.1a 32.9a 41.8b (86) (5.7) (8.9) RESULTS Cryptogams 1.& 7.0b 3.100 (3.2) (5.9) (4.3) The replanted vegetation type was domi Bare ground 40.5a 24.0ab 16.0b nated by crested wheatgrass and also had the (13.7) (6.5) (6.3) greatest amount of bare ground (Table 1). IMeMUTe., within rows followed by differenlletten are sIgnificantly different, Annual grasslands were dominated by annual P < .05, non,pllJw»etric analogofTub)"s te.,t (Dunn 1964, Zar 1984). plant species, primarily cheatgrass (Table 1). Sagebrush sites had the greatest shrub cover, Total density of grasshoppers was affected although other vegetation types had small by sampling date and grazing treatroent ('fable amounts of sagebrush (Table 1). Annual grass 3). No differences in density were detected lands and sagebrush sites had little perennial among vegetation types (LSD mean separa grass cover, other than Poa sandbergii. tion, P = .05; Table 3, Fig. 1). Grazing treatment did not greatly affect Proportions ofM. sanguinipes and all species most ground cover variables (Table 2), but sites within the subfamily Melanoplinae were grazed by livestock had more bare ground and affected by sampling date, vegetation type, cattle dung and less total vegetative cover and and grazing treatroent ('fable 3). Annual grass perennial grass cover than the permanently lands had the highest proportions of M. san ungrazed sites. guinipes and ofall species within the subfami Twenty-three species of grasshoppers were ly Melanoplinae (LSD mean separation, P = indentified from the 30 sites. Melanoplus san .05; Fig. 1). The proportion of grasshoppers gUinipes was found at all 30 sites and repre within the subfamily Gomphocerinae was sented 36% of all grasshoppers on the study strongly affected by sampling date and vegeta sites. Other common species included Oeda tion type, but not grazing (Table 3). Replanted leonotus enigma (Scudder), Ageneotettix deorum (crested wheatgrass) sites had the highest pro (Scudder), Aulocara elliotti (Thomas), Conozoa portions ofGomphocerinae (LSD mean separa sulcifrons (Scudder), and '[rar;hyrachys kiowa tion, P = .05; Fig. 1). Proportions of grasshop (Thomas). No species other than M. san pers within the subfamily Oedipodinae were guinipes comprised more than 10% of all significantly affected by sampling date, vegeta grasshoppers from all sites. Total density of tion type, and grazing, although F-values were grasshoppers on the three sampling dates not as great as for proportions of the other ranged from <0.2 to 2.6 per m2. subfamilies (Table 3). The mean proportion of 1995] GRASSHOPPERS AND LIVESTOCK GRAZING 355 TABLE 2. Comparison of median (s.d.) ground cover Total grasshopper density was not correlated behv8en grazed and ungrazed sites. with any ground cover variables. Proportion of Grazing treatment M. sanguinipes was negatively correlated with Grazed Ungrazed percentage bare ground and cover of perenni al grasses (rs ~ -.59 and -.62, respectively, N Annual grasses 10.0 7.0 ~ 30, P < .001), and was positively correlated (8.5) (12.3) with percentage ground cover of cheatgrass Annual Forbs 1.0 1.5 and annual forbs (r, ~ Al and .12, respective (1.9) (3.8) ly, N ~ 30, P < .05). Proportion of all All perennial grasses'" 3.0 4.3 Melanoplinae combined was correlated posi (6.4) (11.3) tively with cheatgrass (rs ~ .52, N ~ 30, P < .01) and negatively with perennial grasses and Faa sandhergii 8.7 7.1 (5.6) (49) percentage bare ground (-.70 and -.64, respectively, N ~ 30, P < .001). Sagebrush 3.0 3.0 As proportions of Melanoplinae declined (6.8) (7.1) with increasing cover of perennial grasses and Total vegetation· 32.5 39.9 bare ground, proportions of other species in (7.6) (6.3) creased. Gomphocerinae showed trends oppo Cattle dungl 0.7 0.0 site those of the Melanoplinae. Proportions of (0.5) (0.0) Gomphocerinae were correlated positively with perennial grasses and bare ground (.66 Cryptogams 4.6 6.2 ~ (5.7) (6.3) and .16, respectively, N 30, P < .01) and negatively with cheatgrass and annual forbs Bare ground** 31.0 23.0 (-.52 and -.12, respectively, N ~ 30, P < .05). (13.4) (10.2) Proportions of Oedipodinae were not signifi lBeC Grazing influenced both total density and Oedipodinae was greater on sagebrush sites species composition ofgrasshoppers. Members than on annual grassland sites (LSD mean sep of the subfamily Melanoplinae accounted for aration, P ~ .05; Fig. 1). most of the increase in total density on un The effect of grazing treatment was consis grazed sites. Although vegetation did not affect tent across vegetation types and sampling dates density in this study, it strongly influenced for all grasshopper variables (Table 3). Because species composition. Relative abundance of no significant interactions between date and grazing, or vegetation type and grazing, were Gomphocerinae increased, and Melanoplinae detected (Table 3), comparisons of grazing decreased, with increasing coverage of peren treatments were made across all vegetation nial grasses and bare ground, while total num types and sampling dates. bers of grasshoppers remained the same. Overall density ofgrasshoppers was greater Proportions ofM. sanguinipes, the primary pest on ungrazed than on grazed plots (paired-sam species in the region, were negatively associ ple t test, P < .001; Fig. 2). Proportions of M. ated with grazing, perennial grasses (primarily sanguinipes and of all species of Melanoplinae crested wheatgrass), and percentage bare combined were higher on the ungrazed sites ground. (paired-sample t test, P < .005; Fig. 2). Oedi Habitat preferences ofM. sanguinipes, or any podinae showed a trend opposite that of the organism, represent an integrated response to Melauoplinae, beiug found in greater propor many stimuli. Short-term changes in habitat tions on the grazed sites (paired-sample t test, due to grazing may include reduced quantities P < .001; Fig. 2). Proportions ofGomphocerinae of food, less escape space, increased amounts were not affected by grazing (paired-sample t of bare ground, altered host plant quality, and test, P > 0.10; Fig. 2). changes in microhabitat temperature and 356 GREAT BASIN NATURALIST [Volume 55 TABLJ~ 3. Summary of Type III F-values (and significance levels) from three-way ANOVA for densities of total grasshoppers, Melmwplus sanguinipes, MeIanoplinae other than M. sa.nguinipes, Gomphocerinae, and Oedipodinae. Percentage Grasshopper Mdanoplus Percentage Percentage Percentage Source d.f. density/m2 sanguinipes Melanoplinae Gomphocerinae Oedipoclinae Date (D) 2 12.9 «.01) 18.7«.01) 11.9 «.OJ) 21.3 (01) 6.1 «.01) Vegetation type (V) 2 1.2 (.30) 19.3 «.01) 29.3 «.01) 22.2«.01) 5.6 «.01) Grazing (G) I 5.6 (.02) 13.4 (<.01) 11.7 «.01) 0.5 (.50) 8.8 (<.01) YxD 4 2.1 (.09) 2.2 (07) 2.8 (.03) 1.9 (.13) 1.2 (.34) GxD 2 0.6 (.58) 0.3 (.77) 0.1 (.88) 2.1 (.13) 0.7 (.48) YxG 2 0.7 (.49) 0.1 (.89) 0.1 (.87) 0.8 (43) 0.9 (.43) DxVxG 4 0.3 (.91) 0.6 (.69) 0.5 (.76) 0.2 (.96) 0.2 (.93) 0.6 ...... ------ 0.5 E ..... 0.4 >...... - ~ 0.3 <1l Q c 0.2 ------ C O-L...- Replanted Sagebrush Annuals _ Oedipodinae ~ Gomphocerinae _ M. sanguinipes l@wl Other Melanaplinae Fig. 1. Mean density of Oedipodinae, Gomphoeerinae, Melanoplinac other than Melanoplus sanguinipes, and M. san guinipes by vegetation type. humidity. Differences in plant species compo southern Idaho. Perennial grasses are favored sition betwecn grazing treatments were mini food plants for Aulocm'a elliotti (pfadt 1949, mal, indicating that long-term alteration of the Fielding and Brusven 1992), the most common plant community composition was probably gomphocerine in the study area. Proportions not a fadm: of M. sanguinipes were also negatively corre~ Results of this study are consistent with lated with percentage bare ground. Nerney some previous studies. Proportions of M. san and Hamilton (1969) and Kemp and Sanchez guinipes were negatively correlated with crested (1987) reported that M. sanguinipes avoids ovi wheatgrass in this study. Fielding and Brusven position in bare soil, whercas A. eUiotti prefers (1992) demonstrated that crested wheatgrass to oviposit in bare ground (Kemp and Sanchez is not a preferred host plant for M. sanguinipes in 1987, Fisher 1992). High percentages of bare 1995] GRASSHOPPERS AND LIVESTOCK GRAZI"G 357 0.7 ,....------..., '" 0.6 E ...... 0.5 ....;., 'iii 0.4 <: OJ Cl 0.3 <: ~'" 0.2 0.1 oL-_- Grazed Ungrazed _ Oedipodinae ~ Gomphocerinae _ M. sangulnipes 111MI Other Melanopllnae Fig. 2. Mean density of Oedipodinae, Gomphocerinae, Melanoplinae other than Melanoplus sanguinipes, and M. san guinipes by grazing history. ground were associated with both grazing and populations will respond to livestock grazing crested wheatgrass seedings in the present during more favorable years when rangeland study (Tables 1, 2). productivity is high and grasshopper popula Previous studies that examined grazing tions are rapidly expanding. effects on grasshoppers reported results similar Rangeland grasshoppers have traditionally in some respects to those reported here. On the been viewed solely as destructive rangeland short-grass prairie of Colorado, Capinera and pests. However, in an ecosystem context they Secbrist (1982) reported that Oedipodinae were may have net beneficial worth during most most abundant on the most heavily grazed pas years as an important food source for at least a tures, while lightly grazed pastures supported part of the life cycle of many species of mam the highest total grasshopper densities. Quinn mals, birds, and reptiles. Results presented here and Walgenbach (1990) found Melanoplinae, may serve as a cautionary note regarding range particularly Melanoplus sanguinipes, to be dom land ecosystem management under drought inant on ungrazed sites on mixed-grass prairies conditions. Grazing during years of drought of South Dakota, even though total grasshop and low grasshopper populations could con per abundance was less than on grazed sites. ceivably add to the stress experienced by However, Miller and Onsager (1991) were insectivorous animals by reducing available unable to detect auy effect of different grazing food resources (Le., grasshopper populations), regimes on adult grasshopper populations, especially if other arthropods serving as alter including ~1. sanguinipes, in a crested wheat nate foods are also at low densities. grass pasture in Montana. Our observations were made under condi ACKNOWLEDGMENTS tions of low grasshopper density and drought in southern Idaho. Fielding and Brusven (1990) We thank the staff of the BLM Shoshone showed that grasshopper population density District office for their support. J. A. Onsager, in southern Idaho was positively correlated M. A. Quinn, and L. P. Kish reviewed earlier with precipitation. 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