I. Introduction
Altieri et al. (2009) points out that species evenness, in addition to the more heavily researched species richness, is an important component of biodiversity that can influence ecosystem functions and services. The researchers manipulated varying densities of consumers in natural marine ecosystems and discovered that through herbivory, consumers can control the species evenness and thus species diversity of these ecosystems. Altieri et al. found that, “both algal species evenness and biomass-specific productivity were higher in tide pools with herbivorous snails than pools where snails were absent” (p. 3). Hence, future studies on ecosystem biodiversity should be evaluated in terms of trophic interactions and species evenness in addition to the more conventional component, species richness.
The authors understand the importance of evenness on ecosystem function, and their paper addresses “ecological interactions such as herbivory that generate natural patterns of evenness and richness” (p. 1). Their experimental design differs from previous studies on the effects of evenness on ecosystem function. Such studies were primarily conducted on experimental plant communities consisted of direct manipulation of species evenness and subsequent studies of its effect on primary productivity. However, instead of supplementing previous studies by conducting a study on natural terrestrial ecosystems, Altieri et al. conducted a study in a natural marine ecosystem. Trophic interactions of terrestrial systems may differ from those of marine systems, as demonstrated by a study that found insect herbivory decreased plant evenness provided they grazed on moderately abundant species (Mulder et al., 1999). However, the study was conducted on seminatural grassland instead of a completely natural plot, which while capable of producing useful results, is not ecologically realistic.
Results of ecologically realistic experiments on terrestrial ecosystems to investigate the effect of insect herbivory on the primary productivity and biodiversity could be different from results obtained by Altieri et al. However, I predict that in terms of biodiversity, a terrestrial study on similar concepts will yield observations comparable to those seen in the natural marine ecosystems. This of course depends on the specific species consumed during consumer grazing and its relative importance in the ecosystem of interest. In terms of primary productivity, I suggest that grasshopper influences on nutrient cycling could act to increase productivity as a function of grasshopper density. This study is designed to elucidate the relationships between insect herbivore density, primary productivity and biodiversity as measured by species evenness.
II. Purpose/ Significance
The purpose of this experiment will be to investigate the effect of insect herbivory on the primary productivity and biodiversity of a terrestrial ecosystem in a manner similar to that which was used in the study conducted by Altieri et al. The question addressed will be whether insect grazing will increase species evenness and productivity. This experiment will manipulate the densities of grasshoppers, an insect herbivore common to grassland ecosystems. Through nutrient cycling, grasshoppers play an important role in the functioning of shortgrass prairie ecosystems (Mitchell & Pfadt, 1974). Nutrient cycling has been an important phenomenon associated with biodiversity (Tilman et al., 1996). Taken with the results of the study conducted by Mulder et al., grasshopper grazing should be expected to impact the biodiversity and productivity of grassland ecosystems.
This experiment will allow for greater insight into the intertrophic processes that impact grassland productivity and biodiversity. While there are numerous studies that have been conducted proving the validity of investigating species evenness as a component of biodiversity (Wilsey & Potvin, 2000, Wilsey et al., 2005, Kirwan et al., 2007), few studies examine the effect of insect herbivory on primary productivity through species evenness as a measure of biodiversity. Grasslands are not only habitats for a large number of species, but they are also an important resource in grazing economies (Branson, 2006). Worldwide, grasslands are also listed as one of the most critically endangered ecosystems (Noss, 1995). Thus understanding the ways in which consumers such as grasshoppers affect grassland productivity and diversity is important for economic as well as conservation reasons.
III. Hypothesis
While grasshoppers are voracious herbivores whose grazing results in considerable damage to grasses, which they feed on predominantly, grasshoppers often harvest more plant biomass than they consume (Beckerman et al., 1997, Belovsky et al., 2000). Nutrients from the unconsumed plant mass as well as grasshopper waste contribute to the availability and distribution of litter in the grassland ecosystem (Belovsky et al., 2000). Based on this mechanism of nutrient cycling, grasshoppers would contribute positively to grassland productivity. Herbivory due to grasshopper foraging could affect grasslands by the selective feeding of grasshoppers on specific plant species. The resulting effect would be based on characteristics such as the relative dominance and disease susceptibility of the preferred plant species (Belovsky et al., 2000). If the preferred species is a dominant grass, its targeted consumption would remove some of the competitive pressure exerted by the dominant species and could lead to greater species evenness (Hillebrand et al., 2008).
IV. Methods
The experimental design will examine the influence of grazing Lubber grasshopper (Brachystola magna) abundance on diversity and productivity of shortgrass grassland in one square meter plots of the northern Grama-Buffalograss Prairie in northwestern Kansas. The effect of grazing by Lubber grasshoppers, native to the western Great Plains prairie (Smith, 1954), on the ten following native grasses and forbs will be studied: blue grama (Bouteloua gracilis), buffalograss (Buchloë dactyloides), western wheatgrass (Agropyron smithii), rosin weed (Grindelia squarrosa), prairie phlox (Phlox andicola), wild alfafa (Psoralea tenuiflora), prairie coneflower (Ratibida columnifera), scarlet globemallow (Sphaeralcea coccinea), sand dropseed (Sporobolus cryptandrus), and needle grass (Stipa comata), of which blue grama and buffalograss are the two dominant species (Küchler, 1974).
Lubber grasshopper field densities will be determined by counting grasshoppers in quadrants and fifty 1 m2 experimental plots will be randomly assigned to each of the three grasshopper densities: 0 individuals per m2, 1x field density, and 2x field density. Grasshopper densities will be established maintained by direct manual removal and addition of similarly sized insects. To minimize grasshopper immigration and emigration, roofless cages of fine stainless steel mesh 2 m high will be constructed and secured around the plots using washers and bolts. The experiment will be carried out during the period of one year to collect data in the most ecologically realistic manner possible.
Staggered biweekly measurements of species evenness will be taken to determine the effect of grasshopper density on biodiversity. Primary productivity will be quantified with aboveground net plant production (NPP), to be measured with a radiometer.
V. Interpretation of Results
A. Null Hypothesis
The null hypothesis for this experiment in terms of diversity is that there is no variation in species evenness between plots with 0 individuals per m2 and 2x field density compared to the control of 1x field density. This might occur if the grasshoppers fed in a general manner without displaying preferential feeding for any specific species or if the grasshoppers fed more heavily on the non-dominant species of grasses and the shift in numbers did not result in a change in species evenness. In terms of productivity, the null hypothesis for this experiment is that there is no difference between NPP of each of the different plots.
B. Alternative Hypothesis
If a correlation were found between grasshopper density and diversity, there would be differences in species evenness between the three different plots. If the grasshoppers fed more heavily on blue grama or buffalograss, a positive correlation between consumer density and biodiversity would be expected. If the grasshoppers fed more heavily on the other species of grass, a negative correlation between consumer density and biodiversity would be expected. This would occur presumably by reducing the numbers of less competitively dominant species, freeing resources and space which give the dominant buffalograss and blue grama and opportunity to increase in number, reducing species evenness and biodiversity. If a correlation were found between grasshopper density and productivity, there would be differences in primary productivity between the three different plots.
VI. Bibliography
Altieri A. H. et al. (2009). Consumers control diversity and functioning of a natural marine ecosystem. PLoS ONE 4(4): 1–5.
Beckerman, A. P. et al. (1997). Experimental evidence for a behavior-mediated trophic cascade in a terrestrial food chain. Proceedings of the National Academy of Sciences (USA) 94: 10735–10738
Belovsky G. E. et al. (2000). Grasshoppers—plus and minus: The grasshopper problem on a regional basis and a look at beneficial effects of grasshoppers. Pages VII.16.1–VII.16.5 in Cunningham G. L., Sampson M. W., eds. Grasshopper Integrated Pest Management User Handbook. Washington (DC): US Department of Agriculture, Animal and Plant Health Inspection Service. USDA/APHIS Technical Bulletin 1809.
Branson, D. H. et al. (2006). Sustainable Management of Insect Herbivores in Grassland Ecosystems: New Perspectives in Grasshopper Control. Bioscience 56(9): 743–755.
Hillebrand, H. et al. (2008). Consequences of dominance: a review of evenness effects on local and regional ecosystem processes. Ecology 89(6): 1510–1520
Kirwin, L. et al. (2007). Evenness drives consistent diversity effects in intensive grassland systems across 28 European sites. Journal of Ecology 95: 530–539.
Küchler, A. W. (1974). A new vegetation map of Kansas. Ecology 55(3): 586–604.
Mitchell, J. E. & Pfadt, R. E. (1974). The role of grasshoppers in a shortgrass prairie ecosystem. Environmental Entomology 3: 358–360.
Mulder, C. P. H., et al. (1999). Insects affect relationships between plant species richness and ecosystem processes. Ecology letters 2: 237–246.
Noss, R. F. et al. (1995). Endangered ecosystems of the United States: a preliminary assessment of loss and degradation. U.S. Dept. of the Interior, National Biological Service
Smith, R. C. (1954). An analysis of 100 years of grasshopper populations in Kansas (1854 to 1954). Transactions of the Kansas Academy of Science 57(4): 397–433.
Tilman, D. et al. (1996). Productivity and sustainability influenced by biodiversity in grassland ecosystem. Nature 379(22): 718–720.
Wilsey, B. J. & Potvin, C. (2000). Biodiversity and ecosystem functioning: importance of species evenness in an old field. Ecology 81(4): 887–892.
Wilsey, B. J. et al. (2005). Relationships among indices suggest that richness is an incomplete surrogate for grassland biodiversity. Ecology 86(5): 1178–1184.
