The North American Prairie Ecosystem Biology Essay


With the passage of time, the North American prairie ecosystem is being altered by the progress in agriculture. Rangelands consist of annual or perennial grasses and forbs (Mueggler and Stewart 1980). The grazing time for all types of rangelands is variable. Some are grazed whole year while others in summer, winter, fall or spring. The northern parts of United States have mostly cool season grasses while southern areas dominate with C4 grasses. However some areas have both types of grasses (Hewitt and Onsager 1983). Irrespective of the type of grass, grasshoppers occur in all types of rangeland. However grasshopper densities are of higher economic importance where annual precipitation does not exceed 60 cm (Hewitt and Onsager 1983).

Beef production is the most valuable and persistent agriculture industry in Nebraska with approximately $7 billion in sales (Nebraska Studies 2000-2024). Beef accounted for 5.4 $ billion dollars in sales in 2002 compared to only $ 3.1 billion dollars in sales of all grains combined in the same year (Veneman et al. 2004). The western parts of Nebraska receive relatively less rainfall (Dow 1932) than eastern parts (Johnsgard 2001) and remain largely rangeland (Veneman et al. 2004). Because of Nebraska's relatively dry climate, the state's vegetation is primarily grassland and is unsuitable for majority of row crops like corn and soybean while western parts of the state are largely rangeland devoted for cattle production.

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Livestock grazing pose the most wide-ranging impact on natural ecosystems of western North America (Crumpacker 1984), while cattle grazing is ample in this region. An approximate of 70% of the 11 western states of United States is grazed by livestock (Longhurst et al 1983). In west, Majority of federal lands have been used for grazing, including most of the areas of the U.S Bureau of Land management (BLM) and US forest Service.

Grasshoppers (Orthoptera: Acrididae) are notable native herbivores in the rangelands of western United States. Excessive feeding by insect herbivores makes the range plant unsuitable for grazers. Feeding by insect herbivores disturbs the plant physiology and nutritive composition. Replacing native plant species with introduced species also affect the overall health of rangelands (Gillespie and Kemp 1995). Majority of the western states encounter grasshopper as a serious pest of rangeland and cause losses to forage and few states including Minnesota, Iowa, Missouri, Arkansas and Louisiana do not seems to have consistent problem of grasshoppers (Hewitt and Onsager 1983). In North America, there are however, some grasshopper species which may occur both in row crops and rangelands. Among these, the most damaging species in both areas include Melanoplus sanguinipes (Fabricius), Melanoplus bivittatus (Say), Melanoplus packardii Scudder, and Camnula pellucida (Scudder) (Brooks 1958, Edwards 1964)

A modification in prairie system either by human activities or natural process is likely to bring changes in grasshopper species complex (Kemp et al. 1990). The distribution and abundance of grasshoppers has been related to several factors. Vegetation, temperature, precipitation and geographic are some of the important factors that play an important role in grasshopper distribution. Temperature and precipitation are important for plant growth thus grasshoppers are also affected (Clark 1949) due to changes in plant conditions (Anderson and Wright 1952).

It is critical to manage rangelands from the damaging effect of grasshoppers. Grasshoppers have been documented as economically important pest in western US (Pfadt 2002) that can consume about 21-23% of available forage (Hewitt and Onsager 1983). Sometimes the damage to these rangelands and other crops is wide ranging (Hewitt and Onsager 1982). Both crops and grasslands were severely impacted by grasshopper damage during late 1800s and early 1900s (Hewitt and Onsager 1983). The grasshoppers also occasionally compete with livestock and wildlife for forage (Hewitt 1977, Hewitt and Onsager 1983). Humans also suffer competition with grasshoppers for food (Pfadt 1985). Although grasshoppers are abundant in tall grass prairie of North America but their damage is not as significant as in shortgrass prairies in western rangelands and are considered serious pests in shortgrass prairie (Pfadt 1977). While, the impacts of grasshopper feeding in rangelands is mostly miscalculated. Many species of grasshoppers has been known to cause outbreaks in Nebraska (Hauke 1953). The economic damage caused by the rangeland grasshoppers requires prediction of the damaging species. The long-term control of grasshopper populations is not evident (Hewitt and Onsager 1983, Schell and Lockwood 1997) but short-term efforts were fruitful and have enhanced the interest of researchers to study the biology of grasshoppers for long term control.

Grasshopper Biology and Ecology

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In United States, more than 600 species of grasshoppers exist. However, not more than a dozen species that occur in higher numbers and among these, one or more than one species is present in every rangeland (Hewitt 1977). Some grasshopper species like Hypochlora alba (Dodge) and Hesperotettix viridis (Thomas) are considered beneficial because of their feeding on unwanted plants (Hewitt and Onsager 1983) but majority of the rangeland grasshoppers species are destructive. Grasshoppers have only one generation per year. Majority of egg laying by economically important grasshopper species start in late summer, especially in fall. The moist surface with warm fall is the favorable conditions for egg laying. In the following year of egg laying, hatching starts in late spring and early summer. The growth of key indicator plants could also be useful for predicting the hatching time. Hatching period consist of several weeks. Aeropedellus clavatus (Thomas) hatches first while Phoetaliotes nebrascensis (Thomas) among last to hatch (Hewitt and Onsager 1983). Most rangeland grasshoppers have 5 nymphal instars with one week of each instar. Adults take about 1-2 weeks to become sexually mature and retain this ability for about 3 weeks.

There are many mortality factors for the newly hatched nymphs and do not cause much damage to forage while the later developmental stages have greater potential of forage consumption (Davis et al 1992). The 4th, 5th and adult stages in grasshopper are important to seasonal growth of grasses because these stages are responsible for greater consumption and destruction of foliage (Hewitt 979). For instance, in Montana, most growth of cool season grasses takes place when grasshopper are in a stage where they can cause maximum damage to plant material (Hewitt 1979).

There is a decrease in the number of grasshopper densities as they move from nymphal stage to adult in summer. While higher temperatures mostly speed up their developmental rates. Despite the fact that several factors are very important in the life cycle of grasshoppers but under ideal conditions, the grasshopper can live up to 14-16 weeks after hatching. There has not been found a consistent pattern and rate of forage consumption in grasshoppers, it depends on grasshopper species, densities, life stage and synchrony with the forage growth (Onsager 1983).

Feeding Preference

The rangeland grasshopper species vary in their food specialization. Grasshoppers graze in a similar fashion as that of livestock except that their feeding results in additional loss of foliage during which they cut the plant but do not consume and that clipped foliage becomes a part of litter on ground (Mitchell and Pfadt 1974). The economically important species especially in their early instars are likely to feed on grasses or behave as omnivorous. However, there are species which prefer to feed on certain sets of plants including Hesperotettix viridis (Scudder), Hesperotettix alba (Dodge) and Hesperotettix speciosus (Scudder) (Mulkern et al. 1969, Joern 1983, Pfadt 2002, Sword et al. 2005) while some prefer grasses, some feed on forbs and others as omnivorous (Mulkern 1967). Although H. viridis feed on many forb species but more preferred is the snakeweed (Gutierrezia spp.) (Pfadt 2002). The members of the subfamily Melanoplinae have broader diet breadth relative to Oedipodinae which are mostly grass feeders with narrow diet breadth while Gomphocerinae mostly feed on grasses and sedges (Craig et al. 1999).

The damage caused by grasshoppers to forage increases with their increasing developmental stages (Hewitt 1978). Thus the first two instars do not cause much damage because of their presence during the periods which are more favorable for plant growth. The third instar of grasshoppers is critical due to its noteworthy consumption as many of C3 grasses mature at the appearance of 3rd instar and thus consumption of foliage by grasshoppers restricts the plants to regrow. Similarly third instar of grasshoppers becomes less susceptible than early instars to various mortality factors (Hewitt 1979).

Host Specificity and Genetic Variation

Majority of grasshopper species are polyphagous and feed on a number of plants (Otte and Joern 1977). Because of their polyphagy, most grasshopper populations do not experience a change in population genetics related to host shifts. There are some grasshopper species which have limited host range while others behave strictly host specific (Otte and Joern 1977, Sword and Dopman 1999). Differences of developmental rates, prolonged existence and size have been observed in host specific grasshoppers (Traxler and Joern 1999). The grasshoppers H. viridis and Schistocera lineata have been cited for their host associated genetic differences (Sword et al. 2005). The role of natural selection in promoting the reproductive isolation is important and serves as basis for speciation. A number of insect specialist herbivores are monophagous or feed on a number of closely related plant groups (Bernays 1998). Among populations of few insect species host specific genetic variation has been observed (Prokopy et al. 1988).

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Morphological studies have been used to study the taxonomic status of many insect species but with the passage of time, recent advances in molecular techniques using mitochondrial and DNA polymorphisms have contributed about the life history and speciation process (Hoy 2004). DNA markers proved helpful in revealing the population genetics of a number of insect species (Reineke et al. 1998). Restriction Fragment Length Polymorphisms (RFLP) have been used to study genetic diversity. Random Amplified Polymorphic DNA (RAPD), microsatellite (SSR) are some examples being used for genetic variation studies (Gocmen and Devran 2002). Recently, Polymerase Chain Reaction (PCR) based Amplified Fragment Length Polymorphism (AFLPs) has been extensively used for genetic differentiation within and among populations (Vos et al. 1995). Polymerase chain reaction use small amount of DNA and makes thousands of copies. The AFLP have the advantage over other techniques that it does not require prior knowledge of the specific sequence (Vose et al. 1995). AFLP has been proved very useful comparing individuals and populations (Muller and Wolfenbarger 1999). This technique has the ability to detect point mutations, insertions, deletions and other genetic arrangements. Several researchers have used this technique in grasshopper studies. Brust et al (2010) used AFLP procedures for Melanoplus packardii studies. Tatsuta and Butlin (2001) used this technique to study the interspecific genetic differentiation of grasshopper species. Sword et al. (2005) studied the snakeweed grasshopper, H. viridis for its host plant associated differentiation. Ullah et al. (2012) also studies the genetic differentiation among the host specific forms of grasshopper Melanoplus bowditchi (Scudder).

Grasshopper Sampling

There is no standard method for grasshopper sampling (Larson et al. 1999) and all methods produce different results. Among the various methods used for the estimation of grasshopper densities, sweep sampling is the most frequently used (Gardiner et al. 2005). Although sweep sampling gives a poor estimation of the grasshopper density (Evans et al. 1983, Larson et al. 1999) especially during the early season when nymphs are sampled (Evans et al. 1983). Sampling should be done according to activity of the common species. Low and slow sampling is good for slow moving species and nymphs while high and fast sampling is suitable for more active grasshopper species. Ideally it is suggested that a mingling of both methods obtains the best results (Foster and Reuter, 1996-1999).

In Nebraska, the method used by United States Department of Agriculture (USDA) to estimate grasshopper densities is the visual estimation method (USDA-APHIS 2006). Because of estimation errors between surveyors, several authors suggest to either change this method or to use some alternate method for estimation (Legg et al. 1996, Larson et al. 1999). Sweep sampling is most common method used because of its low cost and quick assessment of densities captured (Larson et al. 1999). Similarly, sweep sampling is less labor intensive compared to quadrat sampling, ring estimations, pan trapping, night trapping and visual estimation (Legg et al. 1996, Olfert and Weiss 2002). Despite the shortcomings of sweep net sampling (Foster and Reuter 1966-1999), it is still considered the best means by which to obtain generally accurate estimates of grasshopper community composition (Mulkern et al. 1969, Evans et al. 1983, Larson et al. 1999) and the only cost effective way to get species level information.

Grasshopper Damage

Grasshoppers have been found to feed on approximately 269 million hectare area of western rangelands and compete directly with livestock for available forage. There are many factors which contribute to the intensity of the damage caused by rangeland grasshoppers. Similarly, the geographic variation in damage and yearly damage also depends on these factors. These factors include weather pattern, grasshopper and available plant species (Hewitt et al. 1976). However, researchers (Morton 1936, Pepper et al. 1951, Anderson and wright 1952, Nerney and Hamilton, 1966; 1967) measured the forage losses caused by rangeland grasshoppers based on their numbers but measuring forage losses solely based on the number of grasshoppers was declined by Anderson (1961) who raised the question of food preference in grasshopper species. Laboratory studies on grasshoppers species for food consumption had also been carried out by Parker (1930) and Smith (1959) but such laboratory studies are not always compatible for field conditions.

Heavy grazing by grasshoppers has detrimental effect on the health of grasslands which results in the loss of plant or portion of plant. The consequences of this loss appear in the form of reduced photosynthetic rates, inhibiting vegetative production (Burleson and Hewitt 1982). Hinkle (1938) and other researchers have documented the forage loss by grasshoppers in northern rangeland of Montana and Colorado, less information is available about northern rangelands in relation to growth of plants, phenology and environmental factors. Hinkle (1938) found maximum forage consumption by grasshoppers occurred during peak period of grass production. Pfadt (1949) measured the damage to rangeland vegetation by known numbers of grasshoppers in cages and found that a number of 15 grasshopper /square yard cause a damage up to 66%. Rubtzav (1932) also conducted cage study to measure grasshopper damage and he found that 10 grasshopper/square yard ate about 275 kg of grass per acre. Langford (1930) measured leaf areas before and after feeding the grasshoppers and found that even populations within same species vary in their daily consumption. Nerney (1957) used percentage of leaves and plant parts that were eaten by grasshoppers to measure the damage. Anderson and Wright (1952) did not used cages and measured the grasshopper damage on large field where they sprayed insecticide on half of the area while other half untreated with grasshopper populations and concluded that depending only on the grasshopper numbers to evaluate losses is not valid unless the production of the grasses is considered (Anderson 1961). Vegetation density plays an important role in determining grasshopper density and distribution (Anderson 1964). In Nebraska there are only a dozen species which are economic pests including Melanoplus bivittatus (Say), Melanoplus femurrubrum (DeGeer), Melanoplus differentialis (Thomas), and Melanoplus sanguinipes (Fabricius) (Hauke 1953, Pfadt 2002, Brust 2008). Aulocara elliotti (Thomas), Eritettix simplex (Scudder), Mermeria bivittata (Serville), Trachyrhachys kiowa (Thomas), and Xanthippus corallipes (Haldeman) are also important but their damage is relatively less (Pfadt 2002).

Temperature and Precipitation

Because of the threat posed by grasshoppers to rangeland forage, numerous studies have examined the factors responsible for triggering outbreaks. Temperature appears to play an important role as Smith (1954) and Edwards (1960) found positive correlations between grasshopper densities and temperature. Numerous studies have examined the effects of precipitation on grasshopper numbers with variable results. Some studies found positive correlations between grasshopper abundance and precipitation (Nerney, 1960; Nerney, 1961; Fielding and Brusven, 1990), while others found negative correlations (Parker 1933, Smith 1954, Edwards 1960, Gage and Mukerji 1977, Skinner and Child 2000). A detailed study by Nerney (1960; 1961) and Nerney and Hamilton (1969) have examined precipitation from October to March best predicted grasshopper densities in Arizona. The studies of relationship between grasshopper numbers and precipitation came out with numerous models in an attempt to predict grasshopper outbreaks. Carter et al. (1998) created a model for Melanoplus sanguinipes in Colorado using both precipitation and temperature as variables. This model predicts high egg mortality during years with above normal precipitation. Others (Gage et al. 1976, Hardman and Mukerji 1982, Hilbert and Logan 1983, Johnson and Worobec 1988) also use above-normal precipitation as a negative factor for grasshopper survival. Despite previous studies and models developed, grasshopper outbreaks remain difficult to predict (Lockwood and Lockwood 1991, Edwards 1960).

Switchgrass Biofuel

A fuel made from plant sources in the form of solid, liquid or gas with renewable characteristics is termed as biofuel. The primary sources of first generation biofuels, ethanol and biodiesel are the food crops (Dufey 2006, Reiinders and Huijbregts 2007, Plieninger and Bens 2008). There are many plant sources which have the ability to produce biofuel such as sugarcane (Saccharum spp.), sugar beet (Beta vulgaris L.). Several starchy crops like corn (Zea mays L.), wheat (Triticum spp.), potato (Solanum tuberosum L.) and sorghum (Sorghum bicolor L.). Several plant oil sources are also used to produce biodiesel. For example soybeans (Glycine max (L.) Merr.), coconut (Cocos nucifera L.), palm (Elaeis spp.), sunflowers (Helianthus annuus L.), and jatropha (Jatropha curcas L.).

Lignocellulosic biomasses from crop residues, woody crops are also a potential source of biofuel (McLaughlin and Walsh 1998). The lignocellulosic biomass used for biofuel is extracted potentially from non-food plants and is termed as second generation biofuel (Gwehenberger et al. 2007, Himmel et al. 2007, Plieninger and Bens 2008).

Although sugarcane is being used for biofuel production in Brazil since 1975 but in the recent past a rise in petroleum products, and environmentally based concerns of using fossil fuels (Dufey 2006) have increased the global importance of biofuels. Brazil is at top in domestic use of biofuels while the U.S. stands second where biofuel is mainly generated from corn. Other countries from Europe, Asia, South Asia also produce biofuel from corn, sugarcane, wheat, cassava (Manihot esculenta C.), rice (Oryza sativa L.), and straw. Australia, Africa and South American countries have also imitated producing biofuels (Dufey 2006, Gwehenberger et al. 2007, Larson 2008).

In United States, several herbaceous plants such as alfalfa (Medicago sativa L.), Miscanthus (Miscanthus x giganteus) and bermudagrass [Cynodon dactylon L. (Pers.)] are the potential perennial feedstocks based on climatic and land (Heaton et al. 2004) but switchgrass is the only North American native that is well adapted to marginal croplands. Maize and switchgrass have gained interest for biofuels in United States. There have been several concerns for larger scale cultivation of both crops for biofuel production. Biofuel production competes with food and fiber production and also with the resources for biofuel and fiber. These resources include light, nutrients and water. Nutrients have gained attention due to increasing prices of chemical fertilizers and run-off pollution but competition for limited water resources is of most importance in U.S. (Kiniry et al. 2008)

Switchgrass, Panicum virgatum L., is a perennial warm-season grass native to North America and has wide geographical distribution (McLaughlin and Walsh 1998). It is approximately 0.5 to 3 m tall with extensive and deep root system (Surrency et al. 2003, Parrish and Fike 2005, Jensen et al. 2007). Being C4 grass species, switchgrass has higher photosynthetic rates and thus efficient in water and nitrogen use and can tolerate the water deficiency (Parrish and Fike 2005). Dense foliage and deep root system in switchgrass further make it useful to control erosion (Gettle et al. 1996, Parrish and Fike 2005), soil conservation and providing organic matter to soils (McLaughlin and Walsh 1998, Surrency et al. 2003).

Ecological Regions

The large units of land and water are termed as ecoregions because of several biotic and abiotic factors responsible for controlling the structure and function within ecosystem where a general similarity of type, and quantity of resources is observed. The relative characteristics of vegetation, soils, wildlife and land use varies among ecoregions. Two major maps for ecoregions of United States have been developed. These maps were developed by U.S. Forest Service and (Bailey et al. 1994) and U.S. Environmental Protection Agency (Omernik et al. 1987). The ecological regions are designated by numeral roman hierarchical structure. There are 15 regions of North America in level I and 52 regions in level II while level III has 104 regions (US-EPA 2000). The level III and level IV were further subdivided and revised at lager scale as level III was based at small scale (Omernik 1987; US-EPA 2000).

Management of Economically Important Grasshoppers

Conventional control of grasshopper at large scale is dependent on chemical insecticides (Pfadt and hardy 1987). Although chemical control at large scale is not possible but Malathion, carbaryl chemicals are usually used to control grasshoppers. The arsenic bait efficacy for grasshoppers was proved in 1855 but it was not commonly used until 1913. After that, Chlorinated hydrocarbons and Malathion became popular (Blickenstaff et al. 1974). Using chemicals had adverse effects on natural enemies, birds, animals and environment. Reduced agent and area treatment (RAATs) was developed by Lockwood and Schell (1997) to minimize the costs and negative effect of insecticides on environment. This method proved efficient with lesser effect on non-target insects and birds. In year 2003 approximately 400,000 acres were treated by USDA using RAATs for grasshopper control in Wyoming .There are several biological control agents including Nosema locustae which have been extensively used for grasshopper control but with limited control (Blickenstaff et al. 1974, Hewitt and Onsager 1983, Schell and Lockwood 1997). In this method grasshopper ingest the spores and become infected.


My dissertation include the work on rangeland grasshoppers for their distribution at different levels of Nebraska ecoregions, feeding preferences study, the additional loss by grasshoppers in the form of generating forage clippings and morphological and genetic study of Melanoplus bowditchi. The objectives of studies include: 1) to predict the hotspots for economically important rangeland grasshopper species; 2) to check the feeding preference of different subfamilies of grasshoppers for switchgrass used for biofuel and big bluestem; 3) the examining the clipping behavior and quantification of the clipped vegetation by rangeland grasshoppers with respect to different moisture level of grasses and 4) to find the genetically and morphologically differences among the host specific form of M. bowditchi.