Research Article |
Corresponding author: David C. Lightfoot ( dlightfo@unm.edu ) Academic editor: Tim Gardiner
© 2018 David C. Lightfoot.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Lightfoot DC (2018) The effects of livestock grazing and climate variation on vegetation and grasshopper communities in the northern Chihuahuan Desert. Journal of Orthoptera Research 27(1): 35-51. https://doi.org/10.3897/jor.27.19945
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Grasshoppers are important herbivores of North American semi-arid grasslands and shrublands, and vegetation and climate are key factors controlling their species compositions and population dynamics. Domestic livestock grazing is a historic and a current landscape-scale ecological perturbation that has caused reductions of perennial grasses and increases in woody shrubs and weedy annual herbs in desert grassland communities. Climate variation also affects vegetation and grasshopper production, and the combined effects of livestock grazing and climate variation on vegetation and grasshoppers have not been adequately studied in the American Southwest. I measured vegetation and grasshoppers for five years at a series of five semi-arid sites in the northern Chihuahuan Desert to evaluate the interactive effects of short-term livestock grazing and climate variation on plant and grasshopper community structure and species abundances. The study sites ranged from shrub dominated to grass dominated landscapes, with livestock fence lines separating land that was grazed at 30% annual forage utilization, and lands on the other sides of the fences excluded from grazing for at least 20 years. I assigned grasshopper species to life-form guilds based on their ecomorphologies and their microhabitat substrate uses that I observed. A wet spring/dry summer El Niño event occurred at the beginning of the study, and a dry spring/wet summer La Niña event occurred at the end of the study. Livestock grazing changed plant and grasshopper species compositions and abundances significantly during those wet years, further favoring annual forbs, annual grasses and non-graminicole grasshoppers on grazed lands during wet years, while favoring perennial grasses and graminicoles on non-grazed lands also during wet years. The biotic communities at all sites probably supported more perennial grasses and more graminicoles prior to European settlement and livestock grazing that began over a century before this study.
Acrididae , desertification, ecological disturbance, guilds, life-forms
Grasshoppers are important primary consumers in semi-arid regions throughout the world (
Convergence or divergence in grasshopper species ecologies and specializations are likely driven by the evolution of ecological traits (e.g.
Desertification is the anthropogenic environmental degradation of semi-arid grasslands from long-term excessive and unsustainable domestic livestock grazing, that has occurred extensively throughout the semi-arid regions of the world, including the semi-arid regions of North America (
Livestock grazing typically causes changes to herbaceous vegetation composition and structure that in turn cause shifts in grasshopper species compositions and population densities in savanna, shrub-steppe and desert grassland environments (e.g.
Variation in weather or long-term climate is known to be a key factor affecting grasshopper populations (
Given that grasshoppers are key primary consumers in semi-arid ecosystems across the Southwest, and given that grasshoppers are known to be affected by variation in vegetation caused by livestock grazing and variation in climate, what effects do domestic livestock and climate have on vegetation and grasshoppers in the Southwest? I conducted this research project to address the following questions: 1) Does short-term livestock grazing alter the species compositions, plant life-form (i.e. grass, forb, shrub, tree) and grasshopper life-form guild structures, and abundances of rangeland plants and grasshoppers? 2) Does annual and seasonal variation in precipitation interact with livestock grazing to affect plant and grasshopper species assemblages and grasshopper guild structure? 3) Which grasshopper species and guilds are most sensitive to the impacts of short-term livestock grazing and climate variation?
This research was conducted as part of the U.S. Department of Interior (USDI), Bureau of Land Management (BLM), Global Change Research Projects program, 1991–1996, which was intended to support long-term research on the ecological impacts of global climate change to natural resources. However, in 1996, the program was terminated due to politically motivated USDI administrative research program changes. This article presents the findings of the five-year vegetation and grasshopper grazing response research that was conducted from 1992–1996 as part of the Chihuahuan Desert Subproject. This research was intended to be a long-term (decades) study to document biotic community responses to climate change, but the entire Global Change Research Program was terminated, so the long-term goals were not accomplished.
Study sites for this research were subjectively located where BLM lands within the Chihuahuan Desert in southern New Mexico were adjacent to lands under other ownership and/or management that excluded livestock grazing, and shared a common boundary with a standard 5 strand barbed-wire livestock fence. Livestock grazing was present on the BLM side of the fence, but not on the other side. From those potential locations, site selection then depended upon obtaining permission from the other landowner/agency to conduct the study, and then depended upon finding a 1 km long section of the boundary fence that had relatively homogeneous topography, soils, and vegetation, so that the presence of livestock grazing on the BLM side of the fence, but not on the other side, was the only primary factor that differed along the potential fence line. The grazed side of each fence line was BLM public land that was currently grazed by domestic cattle, and had been historically grazed for at least 20 years. The non-grazed side of the fences had been excluded from cattle for at least 20 years. Grazing intensity at all sites was year-round, approximately 30% utilization of available plant foliage by domestic livestock, the standard stocking rate for BLM public rangelands in the region. Each site consisted of semi-arid grassland or shrubland that was grazed by domestic cattle, and adjacent non-grazed land on the other side of the barbed-wire livestock fence line. All sites were further chosen to be situated at the same approximate elevation (~1,500 m above sea level), and all on similar topographic landscapes; lower piedmont slopes with silty to sandy loamy soils. All sites supported Chihuahuan Desert grassland or shrubland vegetation communities. Sites ranged from shrub-dominated to grass-dominated, but all sites had both grass and shrub elements.
The study sites were located in the northern Chihuahuan Desert (Chihuahuan Deserts Level III Ecoregion,
Sampling at each site was systematic, not random or subjective. Two 600 m, paired, grazed and non-grazed sampling transects were permanently installed at each of the five study sites. Each of the paired 600 m measurement transects were located parallel to, and each 20 m from the fence line between the two, to avoid roads and/or livestock trails along some of the fence lines. Each 600 m transect was partitioned into thirty, 20 m segments. All transects and segments were permanently marked and labeled with 0.5 m steel rods that were hammered into the soil.
Study site name | Location | Elevation | Level IV Ecoregion* |
---|---|---|---|
Bosque del Apache National Wildlife Refuge | N33°24', W106°45' | 1,520 m | Chihuahuan Basins and Playas 24a |
Jornada Experimental Range | N32°28', W106° | 1,340 m | Chihuahuan Basins and Playas 24a |
Otero Mesa | N32°29', W105°46' | 1,540 m | Chihuahuan Desert Grasslands 24b |
Phillips Hills, White Sands Missile Range | N32°27', W106°06' | 1,490 m | Chihuahuan Basins and Playas 24a |
Sevilleta National Wildlife Refuge | N34°24', W106°36' | 1,610 m | Chihuahuan Desert Grasslands 24b |
Weather data were obtained from the nearest long-term U.S. National Weather Service weather station to each of the five study sites. Monthly precipitation amounts and ambient temperatures were summed and averaged respectively over each year of this study. Table
U.S. National Weather Service weather stations that provided weather data for this study. Each of the five study sites was represented by one nearest weather station.
Study site name | Weather station name | Location | Elevation |
---|---|---|---|
Bosque del Apache National Wildlife Refuge | Bosque del Apache | N33°46', W106°54' | 1,445 m |
Jornada Experimental Range | Jornada Experimental Range | N32°37', W106°44' | 1,440 m |
Otero Mesa | Orogrande | N32°23', W106°06' | 1,270 m |
Phillips Hills, White Sands Missile Range | Carrizozo | N33°39', W105°53' | 1,650 m |
Sevilleta National Wildlife Refuge | Bernardo | N34°25', W106°50' | 1,085 m |
Vegetation was measured from a 1 m2 quadrat located at the start (north or west end) of each of the thirty, 20 m segments per transect. The same permanent quadrats were repeatedly sampled over the five-year study period. A 1 m2 vegetation measurement frame made of 0.5 inch PVC pipe with an internal string 10 by 10 grid of 100, 1 decimeter2 subunits, was used to measure vegetation canopy cover by species. The PVC frame was attached to 1 m tall legs with height adjustments on each corner to keep it elevated immediately above the plant foliage canopies. The total foliage canopy cover of each plant species, and the maximum foliage height of each plant species per quadrat were recorded. Vegetation was sampled twice each year, at the end of the spring growing season in late May (especially for spring annual C3-photosynthetic pathway plants), and at the end of the summer growing season in late September for most other largely C4 plants. Vegetation was measured over a period of five years; 1992, 1993, 1994, 1995, and 1996. Plant species classification, common names and Latin names, life-histories and growth-forms follows
Many different field sampling methods have been utilized to count grasshoppers (
The substrate was the physical surface that each grasshopper was first observed on, including soil surfaces, and different species of plants. I watched grasshoppers as they hopped and/or flew ahead of me and did not recount any individuals that I had already counted. Grasshoppers were sampled twice each year during the five-year study period, at the same time that vegetation was measured. Several species of grasshoppers in the region hatch from eggs in the late summer/fall, over-winter as juveniles and become adults in the late spring (e.g. Psoloessa spp., Cibolacris parviceps, Arphia conspersa, Xanthippus spp.). Also, one of the most common grasshoppers in the region, Trimerotropis pallidipennis has two distinct generations each summer in the region of this study, one early and one late (
I assigned grasshopper species to ecological life-forms following the morphological descriptions of
Vegetation data were entered on field data forms and then transferred to a Microsoft Excel spreadsheet for management and error checking, then converted to a text file for analysis. Grasshopper data were entered from field audio-recordings to an Excel spreadsheet and converted to a text file for analysis. All data were quality checked and verified. The vegetation, grasshopper, and climate data resulting from this study were summarized and analyzed using SAS analytical software (Version 9.4, SAS Institute, Carey, North Carolina, USA). I used hierarchical group-average cluster analysis (SAS; PROC CLUSTER, PROC TREE) utilizing Euclidean distance for similarity measures of species composition or grasshopper substrate use to evaluate entire assemblages of species from different locations, each year and season. Vegetation data were mean canopy covers and heights of each species/quadrat over all 30 quadrats per site, by control and treatment sides of the fence (control vs. grazed; 30 quadrats each). I used paired t-tests (SAS; PROC TTEST) to test for significant differences in vegetation canopy cover and heights between grazed and non-grazed paired fence side locations within sites. I used Chi-square goodness of fit tests (SAS; PROC FREQ) to test for differences in grasshopper counts, summed by species, and categorized by life-forms, from each paired 600 m transect (non-grazed vs. grazed) at each site and year/season. I used a standard statistical test level of alpha (p) = 0.05. The relationships between grasshopper life-form counts from individual grasshopper species counts, and available plant life-form and bare soil cover values that were measured from the 1 m2 quadrats, were evaluated with non-parametric Spearman-rank correlation analysis (SAS; PROC CORR).
Annual total precipitation summed over 12 months of each year from 1992–1996 across all five sites, ranged from 10 cm/year to 40 cm/year, with an overall decline trend over time, especially in 1995 (Fig.
A listing of all 151 plant species observed, their life-histories, and life-forms is presented in Suppl. material
Plant species counts or richness ranged from about 15 to 30 species over the study sites and years, with most sites showing declines in 1994 and increases in 1995, and slightly more species were present during late summer/fall sampling than during the early summer/spring (Suppl. material
Cluster analysis of the five study sites and their control vs. grazed sides of the fences, and based on similarities of plant species compositions summed over the five-year period, revealed that each of the sites supported very distinct plant species compositions both in the spring and fall (Fig.
Analysis of the major plant life-forms forbs, grasses, and shrubs, revealed that livestock grazing primarily affected grasses and forbs, but not shrubs (except for broom snakeweed). Across all five sites, forbs and grasses tended to have significantly more cover on the non-grazed sides of the fences than on the grazed sides, especially in association with the 1991/1992 El Niño event, and the 1996 La Niña event (Suppl. material
Overall, the canopy cover and abundance of annual forbs and annual grasses varied considerably in response to variation in rainfall over the five sites and five years, especially the late summer of 1996 when annual sixweeks threeawn grass had higher cover and height than perennial grasses at two of the five sites. Perennial grasses tended to be less variable in cover and height over time, but typically with consistently greater cover and height in non-grazed vs. grazed areas over the five years. Forb and grass canopy cover and height either did not significantly differ between grazed and non-grazed areas, or was significantly greater in non-grazed areas than grazed areas. Shrub cover tended to vary little over time, and generally was not significantly different between grazed and non-grazed locations, except for the small, short-lived shrub broom snakeweed that had greater cover in grazed areas following wet periods at the Sevilleta site. The only common exotic weed species, prickly Russian thistle (Salsola tragus), was typically more abundant on grazed than non-grazed lands.
A total of 54 grasshopper species were observed across the sites and years; their names, life-form and life-history status are presented in Table
Examples of each grasshopper life-form type; A. Arbusticole; Bootettix argentatus on Larrea tridentata; B. Graminicole; Paropomala pallida on Bouteloua eriopoda; C. Terri-graminicole; Phlibostroma quadrimaculatum; D. Herbicole; Tropidolophus formosus on Spharalcea hastulata; E. Terricole; Trimerotropis pallidipennis.
Grasshopper species observed across the 5 study sites. Taxonomic classification and names follow
Species | Family | Subfamily | Code | Life-form | Life history |
---|---|---|---|---|---|
Acantherus piperatus | Acrididae | Gomphocerinae | ACPI | G | SU |
Acrolophitus maculipennis | Acrididae | Gomphocerinae | ACHI | T | SU |
Ageneotettix deorum | Acrididae | Gomphocerinae | AGDE | TG | SU |
Amphitornus coloradus | Acrididae | Gomphocerinae | AMCO | G | SU |
Arphia conspersa | Acrididae | Oedipodinae | ARCO | T | SP |
Arphia pseudonietana | Acrididae | Oedipodinae | ARPS | T | SU |
Aulocara elliotti | Acrididae | Gomphocerinae | AUEL | TG | SU |
Aulocara femoratum | Acrididae | Gomphocerinae | AUFE | TG | SU |
Bootettix argentatus | Acrididae | Gomphocerinae | BOAR | A | SU |
Brachystola magna | Romaleidae | Romaleinae | BRMA | H | SU |
Campylacantha olivacea | Acrididae | Melanoplinae | CAOL | A | SU |
Cibolacris parviceps | Acrididae | Gomphocerinae | CIPA | T | SP |
Conozoa texana | Acrididae | Gomphocerinae | COTE | T | SU |
Cordillacris crenulata | Acrididae | Gomphocerinae | COCR | TG | SU |
Cordillacris occipitalis | Acrididae | Gomphocerinae | COOC | TG | SU |
Dactylotum bicolor | Acrididae | Melanoplinae | DABI | H | SU |
Eritettix simplex | Acrididae | Gomphocerinae | ERSI | G | SU |
Hadrotettix trifasciatus | Acrididae | Oedipodinae | HATR | T | SU |
Heliaula rufa | Acrididae | Gomphocerinae | HERU | T | SU |
Hesperotettix viridis | Acrididae | Melanoplinae | HEVI | A | SU |
Hippopedon capito | Acrididae | Oedipodinae | HICA | T | SU |
Hypochlora alba | Acrididae | Melanoplinae | HYAL | A | SU |
Lactista azteca | Acrididae | Oedipodinae | LAAZ | T | SU |
Leprus wheelerii | Acrididae | Oedipodinae | LEWH | T | SU |
Ligurotettix planum | Acrididae | Gomphocerinae | LIPL | A | SU |
Melanoplus regalis | Acrididae | Melanoplinae | MERE | H | SU |
Melanoplus aridus | Acrididae | Melanoplinae | MEAR | A | SU |
Melanoplus arizonae | Acrididae | Melanoplinae | MEAR2 | H | SU |
Melanoplus bowditchi | Acrididae | Melanoplinae | MEBO | A | SU |
Melanoplus flavidus | Acrididae | Melanoplinae | MEFL | H | SU |
Melanoplus gladstoni | Acrididae | Melanoplinae | MEGL | H | SU |
Melanoplus lakinus | Acrididae | Melanoplinae | MELA | H | SU |
Melanoplus occidentalis | Acrididae | Melanoplinae | MEOC | H | SU |
Melanoplus sanguinipes | Acrididae | Melanoplinae | MESA | H | SP, SU |
Melanoplus thomasi | Acrididae | Melanoplinae | METH | H | SU |
Mermiria texana | Acrididae | Gomphocerinae | METE | G | SU |
Mestobregma terricolor | Acrididae | Oedipodinae | METE2 | T | SU |
Opeia obscura | Acrididae | Gomphocerinae | OPOB | G | SU |
Paropomala pallida | Acrididae | Gomphocerinae | PAPA | G | SU |
Phlibostroma quadrimaculatum | Acrididae | Gomphocerinae | PHQU | TG | SU |
Phrynotettix robustus | Romaleidae | Romaleinae | PHRO | T | SP |
Psoloessa delicatula | Acrididae | Gomphocerinae | PSDE | TG | SP |
Psoloessa texana | Acrididae | Gomphocerinae | PSTE | TG | SP |
Schistocerca nitens | Acrididae | Cyrtacanthacridinae | SCNI | A | SU |
Syrbula montezuma | Acrididae | Gomphocerinae | SYMO | G | SU |
Trachyrhachys aspera | Acrididae | Oedipodinae | TRAS | T | SU |
Trachyrhachys kiowa | Acrididae | Oedipodinae | TRKI | T | SU |
Trimerotropis californica | Acrididae | Oedipodinae | TRCA | T | SU |
Trimerotropis pallidipennis | Acrididae | Oedipodinae | TRPA | T | SP, SU |
Trimerotropis pistrinaria | Acrididae | Oedipodinae | TRPI | T | SU |
Trimerotropis latifasciata | Acrididae | Oedipodinae | TRLA | T | SU |
Tropidolophus formosus | Acrididae | Oedipodinae | TRFO | H | SU |
Xanthippus corallipes | Acrididae | Oedipodinae | XACO | T | SP |
Xanthippus montanus | Acrididae | Oedipodinae | XAMO | T | SP |
Examination of the morphology of each species relative to
Cluster analysis of each grasshopper species based on observed specific substrate use by all individuals of each grasshopper species, over all five sites and all five years, revealed distinct groupings of species based on specific observed substrate use (Fig.
I further examined the relationships between grasshopper life-forms, plant-life forms, and bare soil, by performing a second cluster analysis of observed grasshopper species substrate use, with plant species specific substrates pooled into the plant life-form categories instead of plant species; forbs, grasses or shrubs, along with bare soil. The resulting dendrogram (Fig.
Cluster analysis dendrograms of grasshopper species similarities based on substrate use among all grasshopper species over all sites, years and seasons; A. Based on specific substrate use to the plant species level and bare soil; B. Based on substrates categorized to forbs, grasses, shrubs and bare soil.
Spearman rank correlation analysis compared the total numbers of individual grasshoppers observed across all species, and assigned to grasshopper life-forms, with available plant life-form and bare soil cover measured from 1 m2 quadrats and averaged over all sites, transects, years and seasons. Correlation analysis revealed significant relationships between grasshopper life-forms and substrate availability (Table
Cluster analysis of grazed vs. non-grazed sites in the spring and in the fall over all years revealed that, like vegetation, grasshopper species assemblages were unique to each site. Branch lengths in the dendrograms were not as long as for plant assemblages, demonstrating the site to site variation and differences in grazed vs. non-grazed in grasshopper assemblages was less than it was for plant assemblages (Fig.
Spearman-rank correlation coefficients (rS) and significance values (P) from testing relationships between grasshopper life-forms and the available cover of substrate categories measured on the grasshopper and vegetation transects at each study site, over all years and seasons. Correlation coefficients are listed first, above significance values within each life-form by substrate set of cells. Significant (P<0.05) correlations are in bold text, positive correlations are in regular font and negative correlations are in italic font. Sample size for all tests was 96.
Substrate Categories | ||||
Grasshopper life-forms | Bare Soil | Grasses | Forbs | Shrubs |
Arbusticoles | -0.24058 | -0.24703 | 0.14819 | 0.61254 |
0.0182 | 0.0153 | 0.1496 | <.0001 | |
Graminicoles | -0.31407 | 0.57682 | 0.12125 | -0.26054 |
0.0018 | <.0001 | 0.2393 | 0.0104 | |
Terri-graminicoles | 0.13191 | 0.47328 | -0.1248 | -0.57136 |
0.2002 | <.0001 | 0.2257 | <.0001 | |
Herbicoles | -0.13658 | 0.23129 | 0.38279 | -0.26019 |
0.1845 | 0.0234 | 0.0001 | 0.0105 | |
Terricoles | -0.28696 | 0.44601 | 0.17449 | -0.18596 |
0.0046 | <.0001 | 0.0891 | 0.0697 |
Analysis of the grasshopper life-form guilds revealed that livestock grazing primarily affected graminicoles and terri-graminicoles, which tended to be significantly more abundant on non-grazed than grazed areas, and especially at the Bosque, Otero, and Sevilleta sites, both in the spring and in the fall seasons (Suppl. material
Arbusticoles were mostly associated with one or a few species of perennial woody shrubs. Bootettix argentatus (Gomphocerinae) was associated only with creosote bush at the Jornada, Otero, and rarely at the Phillips sites. Campylacantha olivacea (Melanoplinae) and Ligurotettix planum (Gomphocerinae) were found only on tarbush (Flourensia cernua) at the Jornada and the Phillips sites, and Hesperotettix viridis (Melanoplinae) was only associated with broom snakeweed across the sites. Hypochlora alba (Melanoplinae) was associated primarily with sand sage (Artemisia filifolia), but also some forbs at the Bosque site (Suppl. material
The most abundant graminicoles were species in the subfamily Gomphocerinae; Paropomala pallida which was highly associated with black grama grass on the non-grazed side of the fences at the Bosque, Otero and Sevilleta sites, and less associated with bush muhly grass along with Acantherus piperatus, at the Jornada and Phillips sites, and Eritettix simplex and Opeia obscura that tended to be associated with galleta and tabosa grasses (Pleuraphis spp.) and burro grass (Sceropogon brevifolius) across all of the sites (Suppl. material
Abundant terri-graminicoles also were mostly in the subfamily Gomphocerinae; including Aulocara femoratum, Cordillacris occipitalis, Ageneotettix deorum, and Phlibostroma quadrimaculatum that were associated with blue grama and burrow grasses at the Otero and Sevilleta sites in the fall. Psoloessa delicatula was a terri-graminicole associated with fine soils and grasses at the Sevilleta, Bosque and Otero sites, while P. texana was a terricole associated with coarse gravelly soils at the Jornada and Phillips sites (Suppl. material
The most abundant herbicoles were species in the family Melanoplinae; the fall species Melanoplus arizonae, M. lakinus, and M. gladstoni at the Otero and Sevilleta sites, M. flavidus at the Bosque site, and M. aridus at the Jornada and Phillips sites (Table
Terricoles were mostly in the subfamily Oedipodinae; the most abundant terricole was Trimerotropis pallidipennis across all sites and years, especially in the fall of 1995 and 1996 at the Sevilleta site, and T. pallidipennis was represented by two cohorts each year, one in the spring, and another in the fall; the spring cohort was affected positively by the El Niño event in 1992 and the fall cohort by the La Niña event in 1996 (Table
The findings from this study demonstrate that short-term domestic cattle grazing and short-term climate variation did affect the species and life-form compositions and foliage canopy cover and height of vegetation, and the species and life-form guild compositions and abundances of grasshopper communities across a series of five study sites over five years. Grazing effects on vegetation and grasshoppers were significant during years with high rainfall, plant production and grasshopper abundance, but not years when rainfall, plant production and grasshopper abundance were all low. These results were similar to the findings of other research in North America (
The effects of livestock grazing on grasshoppers in this study were more pronounced in desert grassland environments than in desert shrubland environments. The Bosque, Otero and Sevilleta sites were desert grassland or shrub steppe and supported relatively high perennial grass cover on the non-grazed sides of the fences. The Jornada and Phillips sites were creosote bush shrublands, and most of the perennial grass at those sites was bush muhly which grew within the shrub canopies, while the soil surfaces between shrubs were primarily bare and gravelly. Livestock grazing at the desert grassland sites reduced the canopy cover and heights of perennial grasses on the grazed sides of the fences, while relatively higher perennial grass cover and canopy heights were present on the non-grazed sides of the fences. In spring 1992 and in fall 1996 grasshopper densities were high, and terricoles and terri-graminicoles were abundant along with annual grasses and forbs on the more open bare grazed fence sides, while graminicoles were more abundant on the denser perennial grasses on the non-grazed sides of the fences. Arbusticoles showed relatively little response to livestock grazing, because the perennial shrubs that they lived and fed on also did not change much over the five-year period.
Climate variation resulting primarily from opposing ENSO events over a five-year period further interacted with livestock grazing to amplify or reduce the effects of livestock grazing on vegetation and grasshoppers. Increased winter and spring precipitation from an El Niño event in 1992 positively affected both annual herbaceous vegetation and grasshoppers in the spring of 1992 and 1993, more so on grazed areas than non-grazed areas. The La Niña event of 1996 positively affected annual herbaceous vegetation and grasshoppers in the late summer of 1996, but not in the spring of that year, and that effect was more pronounced on grazed lands than non-grazed lands. Grasshopper responses to annual and season variation in precipitation were similar to the findings of
Grasshopper species and life-form guilds that were affected positively by livestock grazing and climate variation were those that preferred bare soil microhabitats, and also responded to increases in rainfall and annual forb and grass production on bare soils disturbed by livestock. Oedipodinae and Gomphocerinae species that tend to be terricole or terri-graminicole species also tend to be mixed grass and forb feeders with relatively broad diets (
In this study, terricoles that preferred bare soil tended to show the greatest responses to increased production of annual herbaceous vegetation in disturbed grazed areas that also had bare soil substrates, especially Trimerotropis pallidipennis, Trimerotropis californica, and Trachyrachis kiowa. Although terricoles used bare soil surfaces almost exclusively as substrates, and are known to utilize bare ground as a microhabitat, correlation analysis revealed that they were negatively associated with available bare ground across locations, years and seasons, but were positively correlated with spatially and temporally variable annual grass cover. These results indicate that while terricoles require long-term availability of bare soil for a microhabitat substrate, over time and space, their densities vary positively over the short-term with the availability of annual grass and forb canopy cover as a food resource.
Terri-graminicoles also preferred microhabitats with sparse, low-growing grasses such as blue grama and burro grass, and spent much of their time on bare ground substrates (bare soil), and responded to increases in grasses as correlation analysis revealed. Those terri-graminicoles included the Gomphocerinae species Aulocara femoratum, Ageneotettix deorum, Psoloessa delicatula, Psoloessa texana, Cordillacris occipitalis and Phlibostroma quadrimaculatum, most of which were more abundant on the grazed sides of fencelines, but primarily at the Otero and Sevilleta sites that had short and patchy perennial grasses like blue grama and burrow grass.
Graminicoles were affected negatively by livestock grazing, apparently due to the reduced cover and heights of the perennial grasses that they lived and fed on, which were often significantly taller and had greater canopy cover on the non-grazed sides of fencelines at the grasslands Sevilleta and Otero sites. Graminicoles increased with increased rainfall and perennial grass production which occurred mostly in non-grazed areas where perennial grass cover was higher and not affected by current livestock grazing. Graminicoles primarily used grass plants as substrates, and were positively correlated only to available grass canopy cover over space and time. Common graminicoles such as Paropomala pallida and Acantherus piperatus were highly associated with black grama and bush muhly grasses respectively, which experienced reduced canopy cover when grazed, and increased canopy cover and height under high precipitation conditions. Other graminicoles appeared to be less associated with particular grass species, but Eritettix simplex, Amphitornus coloradus, Syrbula montezuma and Opeia obscura were associated with dense, tall perennial grasses that provided adequate structural microhabitats in ungrazed areas, compared to structurally less robust annual grasses (e.g. sixweeks threeawn) that dominated grazed areas. Unlike terri-graminicoles that also feed on and are associated with grasses, but are adapted to live on bare soil, graminicoles have morphological adaptations (elongate bodies and antennae and short legs with grasping tarsi and arolia and camouflage patterns and colors) for living on the stems and leaves of tall dense grasses as resting and feeding substrates (
Herbicoles were composed largely of Melanoplinae, including several species of Melanoplus, and most appeared to be host-plant generalists except for the oedipodine Tropidolophus formosus that specialized on Spharalcea plants. Many of the common Melanoplussuch as M. arizonae, M. lakinus and M. sanguinipes are known to have broad diets and have not evolved to specialize on any particular plants. Such generalization on leafy forbs may be attributed to low plant apparency in space and time, and the diversity of acutely toxic plant secondary chemical defenses such as flavonoids and glycosides that limit herbivores from specializing on those plants as food resources (
The arbusticoles also were strongly associated with plants, not soil; all were host-shrub specific species except for the shrub generalist Schistocerca nitens. Each arbusticole species was strictly associated with its host shrub species, and unlike the other grasshopper guilds that shared grasshopper species across sites, arbusticoles tended to be site-specific based on shrub species distributions. Bootettix argentatus only occurred at the Jornada, Otero and Phillips sites where creosote bush was present, and was not affected by grazing. Campylacantha olivacea and Ligurotettix planum were restricted to tarbush, which only occurred at those same three sites, while Hypochlora alba was restricted to sand sage at the Bosque site, the only site where sand sage occurred, along with the more generalist Melanoplus flavidus. Broom snakeweed occurred at all sites, and supported not only Hesperotettix viridis which is monophagous on broom snakeweed, but also Melanoplus bowditchi and M. aridus which occurred on a variety of shrubs in the plant family Asteraceae. While terricoles, terri-graminicoles and graminicoles were more closely associated with the microhabitat structure than particular plant species, arbusticoles also were associated with particular plant microhabitats, but those present on particular shrub species with particular morphologies and chemistries. For example Bootettix argentatus is a leaf and small stem mimic of cresosote bush foliage, and Ligurotettix planum is a stem mimic on tarbush. Each shrub species also has unique foliage chemistry, apparently driving the evolution of monophagy in arbusticoles as the result of plant apparency and the evolution of specialization on highly apparent host plants with different secondary plant chemistries and different substrates for camouflage from predators (
The application of life-form guilds as grasshopper indicators to environmental change has world-wide utility and allows for global comparisons of grasshopper life-form guild structure across continents in relation to landscape features and ecological patterns and processes. As with any attempt by humans to classify species into ecological categories, not all species fit well into grasshopper life-form guilds such as some mentioned above. However, most grasshopper species addressed in this study did correspond to particular life-form guilds, or some combination of more than one guild (e.g. terri-graminicoles). Based on these findings, the grasshopper life-form guild concept does have merit for understanding resource use and structure of semi-arid and arid environment grasshopper communities.
Livestock grazing is prevalent and often ecologically unsustainable across semi-arid regions around the world, as is desertification, the long-term result of unsustainable livestock grazing (
Given the global extent of semi-arid landscapes that have been and continue to be negatively impacted by livestock grazing (see Introduction), understanding the effects of grazing on vegetation and grasshoppers is key to understanding how to manage natural resources of such lands (
The U.S. Department of Interior, Bureau of Land Management, funded this research as part of the BLM Global Change Research Projects Program, the Chihuhuhan Desert Subproject. Laura F. Heunneke (then of New Mexico State University, presently at Northern Arizona University) was instrumental in obtaining the funding for this research and assisted in study site selection. A special thank you to Karen S. Lightfoot for assisting me in the field and with manuscript preparation. Thank you anonymous reviewers for improving this manuscript. The Museum of Southwestern Biology, Biology Department, University of New Mexico, provided resources for me to work on this project.