Grasshoppers in the Rockies: Surmounting Alpine Challenges
When it is cold outside, we respond by dressing differently or finding warmer places to stay. Cartoon images often portray other animals coping the same way. Jiminy Cricket, for instance, wears a hat and boots. But if they don't have a winter wardrobe, I wondered, how do grasshoppers really cope with the cold?
There were two ways in which I could investigate the effects of cold in the field: changing latitude and moving closer to the poles, or changing altitude and going up. In late August, while on a summer course offered by a Canadian university, I had an opportunity to pursue the second route and traveled to the Kananaskis Valley of the Rocky Mountains in Alberta, Canada. Even in August at 1,980 meters, it snowed as I bushwhacked up the mountain through a dense fog to a subalpine site. But as soon as the sleet stopped, the grasshoppers began hopping.
Though the image of Jiminy Cricket appears harmless and lovable, some grasshoppers are major pests. From the time of Moses and the eighth plague upon Egypt, we have recognized their often-devastating effects. Grasshoppers, however, also have beneficial effects by feeding on noxious plants like snakeweed that harm mammals, and cycling nutrients in the ecosystem because they serve as food for flies, spiders, birds, and other animals. As I planned a method of investigating their cold-coping strategies, I was interested in how the answers might relate to pest as compared to non-pest species. Who was leaping around on those chilly mountain meadows, and why?
The answer was more complicated than I had imagined. Grasshoppers belong to the insect order Orthoptera, which has an estimated 1,200 species in the U.S. and Canada alone. The order Orthoptera is divided into two suborders, and the name "grasshopper" most commonly refers to individuals in the suborder Caelifera. Within this suborder, the best known grasshoppers belong to the family Acrididae, which represents about half of the species of Orthoptera in the U.S. and Canada. Acrididae are sometimes called short-horned grasshoppers because of their relatively short antennae (Capinera et al. 2004). I decided to narrow the scope of my research to the grasshopper family Acrididae.
While formulating a plan, I became concerned about another issue. On the one hand, some biologists say that "cold is the single most important enemy of life" (Franks 1985). On the other hand, climate warming was having some very visible effects in the Kananaskis Valley. According to some of the residents, the nearby Rae Glacier had receded 100 meters over the past 25 years. If cold is an enemy with which grasshoppers struggle, will climate warming help or hinder their survival?
Temperature is the most significant climactic variable at high altitudes, but it is not the only one. Insects experience other significant changes in environmental conditions along the elevational gradient, such as variations in solar radiation, precipitation, oxygen availability, air density, and wind turbulence (Hodkinson 2005; Martin 2001). Alpine zones are characterized by high winds, low temperatures, low effective moisture, short growing seasons, and intense ultraviolet radiation (Martin 2001).
These environmental conditions, and especially temperature, play a direct role on many aspects of an insect's life, including its growth and fecundity. As I conducted the background research to develop my hypothesis and methods, I learned that grasshopper responses to the severe alpine environment could take many forms: microhabitat selection, behavioral thermoregulation (such as basking in sunlight), cold-hardiness strategies (such as freezing tolerance or freezing avoidance), and life-cycle adjustments. I therefore decided to focus my inquiry on specific kinds of adaptations to increases in altitude that would affect a grasshopper's appearance. I scouted potential alpine sites and began to observe the grasshoppers more closely. Watching them jump across the snow, I noticed an interesting phenomenon: Why weren't they flying?
I hypothesized that the elevation gradients in the Kananaskis Valley of the Rocky Mountains, 50° N. latitude, would influence the morphology and phenotypes of the grasshoppers living there, and that they would therefore differ in appearance in response to the challenges associated with increased elevation. To explore this, I tested three predictions: at higher altitudes their color would be darker, their wings would be shorter, and an increase in leg size might occur. I also predicted that the severity of the environment would result in the presence of fewer and different species at higher levels of elevation.
Methods and Materials
I collected specimens from August 27-29, 2007, in the Kananaskis Valley of the Rocky Mountains, Alberta. I chose three meadow sites along an elevational gradient, from a montane region to a subalpine and an alpine. In an effort to ensure that my sample was representative, I further subdivided each site into three to four zones that were at approximately the same elevation, had similar but not identical vegetation, and were separated by a distance of between 20 and 45 meters. I measured elevation, latitude, and longitude using a GPS; aspect with a compass; and slope with a clinometer. Table 1 indicates the measurements I recorded at each zone.
At the montane site (Figure 1) on August 29, I collected 42 grasshoppers, including two nymphs over four zones. Temperatures during the collection period of three hours averaged 16°C, and I identified the characteristic vegetation as tall grasses (50 cm), horsetail, aster, tall yarrow, thistle, small trembling aspens, and willows.
At the subalpine site (Figure 2) on August 27, I collected 22 grasshoppers over three zones. It rained and snowed throughout the collection period of 2.5 hours, and temperatures averaged 7.5°C. I identified the characteristic vegetation as krummholz and other subalpine fir trees, hedysarum, willows, cow parsnip, tall grasses (25 cm), juniper, licapodium, wild strawberry, vibernum, Indian paintbrush, and a considerable amount of scree, so the entire slope was not covered with vegetation.
At the alpine site (Figure 3) on August 28, I collected 41 grasshoppers over the four zones. Temperatures during the 2.5-hour collection period averaged 0.19°C. I identified the characteristic vegetation as heather, low grasses (2 to 5 cm), yarrow, heather, wintergreen, western anemones, small subalpine firs, triangle-leaved ragwort, locoweed, and more bare soil than at the other sites.
I caught grasshoppers with two aquarium nets of different sizes while they were basking or hopping. I later transferred them to vials that I placed in the freezer. After 24 hours, I photographed them in black and white (Panasonic DMC-FX07, macro setting) in groups of five, arranged in uniform squares on a piece of cardboard covered with a uniform matte white paper. In one of the squares I placed a light blue piece of Styrofoam, which served as a constant in the measuring of the color of each grasshopper. Using Adobe Photoshop, I selected five specific points of the same pattern (the four corners of a square and the midpoint of the square) on each pronotum and measured for the percentage of gray, 0 being white and 100 black. I averaged the 5 percentage points for each grasshopper.
I assigned grasshoppers to morphospecies with the assistance of photographs, keys, and a hand lens, as well as dividing them by sex. I compared female grasshoppers to males caught in the same vicinity (Chapco & Litzenberger 2002) and examined the male genitalia as the primary distinguishing characteristic of theMelanoplus species (Knowles & Otte 2000). I took measurements after removing the hind legs and forewings. I measured the pronotum (at the widest part), femur length and width, and forewings with a caliper which measured to 0.05 mm, and also using a calibrated base line and the hypotenuse to measure the length of the specimen in between (Villet 2007).
With the adult grasshoppers, and independent of the final species identifications, I applied statistical analysis (Excel and SPSS) to the data I had collected. An Analysis of Variance (ANOVA) was used for color, and a Multiple Analysis of Covariance (MANCOVA) was used for wing length and leg size. I controlled for the pronotum size in the MANCOVA to avoid concluding that increases in wing length and leg size were only a function of having a larger grasshopper. Leg size was measured as the femur width multiplied by the femur length. Aspect was measured from 1 to 360 degrees and classified for the ANOVA for color in north-facing samples from 0 to 89 degrees and 270 to 360 degrees, and south-facing samples from 90 to 269 degrees.
To obtain a final identification, I was grateful for the assistance of Dr. Dan Johnson, at the University of Lethbridge in Alberta, to whom I sent the grasshoppers pinned in standard insect boxes.
I found 11 species in total among 105 grasshoppers (103 adults and 2 nymphs). I found 10 species at the montane site and one species at the subalpine and alpine sites (Table 2).
I observed an increase in melanization of the pronotum with increasing elevation (Figure 4). An ANOVA indicated that changes in color over elevation were significant and that the shade of the pronotum became darker as elevation increased (Table 3). The ANOVA also indicated that the shade of the pronotum of grasshoppers on the north-facing aspect was darker than those on the south-facing aspect (Table 3). The ANOVA further indicated that males were darker than females, and that this comparative difference was consistent along the elevational gradient (Table 3, Figure 5).
Most of the grasshopper species that I found in the montane region had long wings ( Arphia conspersa, Camnula pellucida, Aeropedellus clavatus, Melanoplus bruneri, Melanoplus borealis, Melanoplus sanguinipes, Melanoplus bivittatus, Melanoplus fasciatus ). Chloealtis abdominalis had no wing tissue other than their short forewings and were flightless. Chortippus curtipennis had short forewings but also had hind wings that provide lift for flight. At the alpine and subalpine sites, the grasshopper I found ( Melanoplus oregonensis triangularis) had short forewings, and like Chloealtis abdominali s it had no additional wing tissue (such as a hind wing) and could not fly (Figure 6). In total, wing length varied at the montane site, but was consistently low in the subalpine and alpine regions (Figure 7).
A MANCOVA indicated that changes in wing length over elevation were significant, and that wing length declined with increasing elevation (Table 4). Males had longer wings than females, a result maintained over the elevational gradient (Table 4). There also were significant changes (a decline) in wing length with an increase in slope (Table 4).
D. Leg Size
I found that the proportional size of the leg to the body declined with elevation (Figure 8). Proportional leg sizes were bigger for males than for females. A MANCOVA indicated significant changes in leg size with elevation, and that leg length declined as elevation increased (Table 5). Males had larger legs than females, a result maintained over the elevational gradient (Table 5).
My results confirm my hypothesis that the grasshoppers facing harsher climactic conditions associated with an alpine environment are different in appearance. As I predicted, their color was darker at higher elevations and also on a north-facing aspect. At higher elevations, their wings were shorter and lacked hind wings; they hopped because they could not fly. There were also fewer and different species. Contrary to my prediction, however, leg size declined at higher elevations.
My finding that grasshoppers are darker at higher altitudes and on a north-facing aspect—which in the Northern Hemisphere is colder than a south-facing aspect—is consistent with the results of other investigations. Melanization is a response to the challenge presented to alpine ectotherms by low temperatures, since the absorption of solar energy is affected by the degree of melanization. Fielding and Defoliart (2005) found that visible color was a reliable measure of relative absorption of incoming solar radiation, and that increased pigmentation in Alaskan grasshoppers enhanced their ability to thermoregulate. According to Samietz et al. (2005), at high elevations there is a strong selection for efficient thermoregulation, since the ability to elevate body temperature above ambient temperature is crucial. The benefits of increased temperature include accelerating developmental rates in an environment with a short-growing season, and enhancing fecundity and disease resistance (Forsman et al. 2002; Fielding and Defoliart 2005).
B. Wing Length and Flightlessness
My prediction that grasshoppers would have shorter wings at higher altitudes was borne out by my finding only flightless grasshoppers in the subalpine and alpine regions, and just one flightless grasshopper among the 10 species in the montane region. This result supports the conclusions of other studies that report a greater incidence of winglessness at high elevations (Darlington 1943; Haubold 1951; Strathdee and Bale 1998; Hodkinson 2005).
An increase in the incidence of flightlessness through wing reduction (brachyptery) or wing loss (aptery) is a response to the climatic pressures of higher altitudes (Hodkinson 2005; Strathdee and Bale 1998). Darwin (1876) suggested in the case of island beetles that natural selection reduced wings to a rudimentary and therefore harmless form; the beetles that flew the least would have the best chance of not being blown out to sea. Although his conclusion was criticized on the grounds that the selection pressure to develop winglessness would depend on other factors, such as the proximity to the sea, others have suggested that the increase in flightless insects at high altitudes with high winds results from the same concern: that winged individuals would be more likely to be blown or straggle away (Roff 1990).
Strathdee and Bale (1998) suggest that increased aptery and brachyptery at high elevations is explained by the fact that the development of wings and related muscles delays maturation. The delay could result in a failure to complete the summer's life cycle in polar environments where growing seasons are short. Melanoplus oregonensis triangularis , the flightless subspecies that I found at the subalpine and alpine regions, overwinter as eggs. They need to complete a summer life cycle in which they hatch in early June, develop five immature instars, and appear as adults in late summer.
The decline I observed in wing length with increased elevation could also be due to an effort to avoid the energy and resources costs of wings. According to Dillon et al. (2006), energetic costs on flying insects at high altitudes increase because low temperatures compromise flight by altering metabolism and muscle physiology, and the reduced air density increases the power required to offset body weight and lift the insect into the air. Since flight muscles cost a lot to build and maintain, the investment constrains other aspects of function, especially fecundity (Marden 2000). More specifically, the costs of wings have been shown to slow nymphal development, increase time to first oviposition, decrease numbers of developed ovarioles and the numbers of eggs per clutch, and ultimately decrease overall fecundity (Dixon and Wratten 1971; McAnelly 1986). There is therefore a tradeoff in females between macroptery and reproduction, and fecundity is favored over the growth and maintenance of flight muscles (Roff 1990; Roff and Fairbain 1993; Zera 2001).
My result that wing length declined as the slope increased could similarly be related to the greater energy cost from flight at an increased slope and the negative consequences on the energy budget for reproduction. Increased slope might also mean flight is less useful, since the likelihood of being blown or straggling away on a steeper slope would be greater.
C. Leg Size
My prediction that an increase in the leg size as a proportion of body size might occur at higher altitudes as a tradeoff between legs and flight, was not supported by my results. It is possible that shorter legs at the higher elevations are more adaptive, perhaps for the same reason that it is best to stay low to the ground in a windy environment. In addition, movement might not require that legs be as long because I found the height of the grasses at subalpine and alpine elevations to be lower. Furthermore, the energy resources argument against wing development as a means to ensure the completion of the short summer life cycle, and the preference for energy expenditure on fecundity, might similarly apply to larger leg development. My result may also have been affected by the fact that two of the species I found in the montane region, Melanoplus bivittatus and Chorthippus curtipennis , have long legs and inhabit generally lower elevations (Capinera et al. 2004; Pfadt 1994).
My finding of fewer species of grasshoppers at the subalpine and alpine levels is consistent with the reports of many biologists that there is lower species diversity at high elevations (Rahbek 1995; Lomolino 2001; Dillon et al. 2006) and the conclusion of Alexander and Hilliard (1969) that the severity of the alpine climatic environment limits the number of species of Orthoptera at high altitudes. My finding of different species at different levels of elevation is similar to the results of Claridge and Singhrao (1978), who demonstrated along Mont Ventoux a more or less continuous altitudinal zonation of species.
Six of the 11 species I found were from the subfamily Melanoplinae and the genus Melanoplus . Melanoplinae is the largest subfamily of acridid grasshoppers in North America, and almost half of the diversity occurs within the genus Melanoplus (Otte 1981; Knowles and Otte 2000). According to Knowles and Otte (2000), in western North America, Melanoplus species are restricted to particular mountain systems, with flightless populations isolated and distributed along mountaintops. Three of the 10 species I found at the montane level are among the top four most serious pest grasshoppers on the rangeland of 17 western American states: Melanoplus sanguinipes, Camnula pellucida and Melanoplus bivittatus. A fourth, Chorthippus curtipennis , is among the top 20 pest species (Dysart 2000) (Figure 9).
The relatively little-studied Melanoplus oregonensis triangularis , which I found at the upper elevations, is not considered to be a pest.
E. Climate Change
Given the pressures upon alpine grasshoppers as a result of low temperatures, it might seem that climate warming would be to their benefit. As I researched the specific adaptations of alpine grasshoppers to their harsh environment, however, I began to suspect that the opposite was true. Biologists confirm that the effects on invertebrates of climate warming will be greatest and most rapid at the coldest sites, and mountain species will be very vulnerable to climate change (Hodkinson et al. 1998). Grasshoppers at many elevations will move to higher altitudes as temperatures increase, and their ability to extend their ranges will grow as the challenges of surviving colder temperatures abate (White and Sedcole 1991). Alpine species will be jeopardized, however, by this immigration up the elevational gradient (Hodkinson and Jackson 2005). Since flightless subalpine and alpine grasshoppers have less dispersal capacity, their range may change, but they will not be able to colonize elsewhere or move above the mountaintops (Walther et al., 2002; Hodkinson and Jackson 2005). According to a recent article in Science , alpine species are already experiencing a loss of habitat because an increase in temperature is allowing trees to invade high-elevation meadows (Kerr 2007). As a result, some pest species will have an increased range and relative abundance as temperature increases, but some non-pest species will be threatened (Olfert and Weiss 2006). Therefore, though subalpine and alpine grasshoppers face hardships from environmental conditions such as low temperatures, climate warming is likely to increase rather than decrease the challenges of survival for these arthropods.
My data supports my hypothesis that the elevation gradients in the Kananaskis Valley of the Rocky Mountains, 50° N. latitude, influences the morphology and phenotypes of the grasshoppers living there. My results support my predictions that at higher altitudes:
- grasshoppers are darker in color
- their wings are shorter
- and there are fewer species.
My data also support the conclusions that
- the pigmentation of grasshoppers increases on a north-facing aspect and
- their wing length declines as the slope increases.
My data did not confirm my prediction that leg size increases with elevation. In addition, my findings support the view that climate warming will significantly affect the distribution and diversity of grasshoppers in the Kananaskis Valley of the Rocky Mountains, and pose an additional threat to the survival of grasshoppers such asMelanoplus oregonensis triangularis .
In the future, I would like to further explore the idea that flightless populations of the Melanoplus species are isolated and distributed along mountaintops. I was intrigued by the suggestion of Dr. Dan Johnson that the prevalence of Melanoplus oregonensis triangularis on the sites I found is likely the consequence of a historical event such as a bad winter or a fire, after which other species had difficulty recolonizing. So I would like to compare the grasshopper species that inhabit the alpine regions of other mountains in Alberta.
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More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2008 is from a New York 10th grader. Michelle investigated how grasshoppers cope with winter weather. Her essay includes:
- an introduction to her investigation and the opportunity she had to pursue it during a trip to the Rocky Mountains in Alberta, Canada;
- her hypothesis that elevation gradients would influence the morphology and phenotypes of the grasshoppers living there; and
- details about the materials and methods she used, along with an examination of her results and the conclusions she reached.
Supplement a study of biology with an activity drawn from this winning student essay.
- Discuss how mammals cope with winter weather. Ask students to speculate about how insects deal with the weather change.
- Send students to this online article, or print copies of the essay for them to read.
- Have them write a one-page reaction to the essay, focusing on what they learned about how insects' bodies adapt to cope with weather conditions.