Anthony Joern

Grasshopper Food Web Analysis: Use of Carbon Isotope Ratios to Examine Feeding Relationships Among Terrestrial Herbivores

Twenty-five species of grasshoppers and 61 plant species were collected from a west Texas study site. Stomach contents of grasshoppers and carbon isotope ratios for plants and grass- hoppers were determined and correlated to test the hypothesis that herbivores have the same carbon isotope ratio as their food. The results of this study include: (1) grasshopper carbon isotope ratios were similar to the carbon isotope ratios of the diet; (2) d d and Y Y of the same species differed little in their feeding on CQ and C4 plants as measured either by stomach content data or carbon isotope ratios; (3) in 7 specialist species which fed on either C3 or C4 plants, individual grasshoppers generally differed in their carbon isotope ratios by 1.6%. or less, 3 generalist species which fed on substantial amounts of both C3 and C4 plants showed larger amounts of individual variation; (4) both the average carbon isotope ratio for a grasshopper species and the variability of carbon isotope ratios among individuals of the species are useful in predicting grasshopper diet in terms of mixtures of Ci and C4 plants.

Coexisting generalist herbivores occupy unique nutritional feeding niches

A mainstay of ecological theory and practice is that coexisting species use different resources, leading to the local development of biodiversity. However, a problem arises for understanding coexistence of multiple species if they share critical resources too generally. Here, we employ an experimental framework grounded in nutritional physiology to show that closely related, cooccurring and generalist-feeding herbivores (seven grasshopper species in the genus Melanoplus; Orthoptera: Acrididae) eat protein and carbohydrate in different absolute amounts and ratios even if they eat the same plant taxa. The existence of species-specific nutritional niches provides a cryptic mechanism that helps explain how generalist herbivores with broadly overlapping diets might coexist. We also show that performance by grasshoppers allowed to mix their diets and thus regulate their protein–carbohydrate intake matched optimal performance peaks generated from no-choice treatments. These results indicate the active nature of diet selection to achieve balanced diets and provide buffering capacity in the face of variable food quality. Our empirical findings and experimental approach can be extended to generate and test predictions concerning the intensity of biotic interactions between species, the relative abundance of species, yearly fluctuations in population size, and the nature of interactions with natural enemies in tritrophic niche space.

Feeding patterns in grasshoppers (Orthoptera: Acrididae): Factors influencing diet specialization

SummaryDiets of grasshopper species from two arid grassland communities in Trans-Pecos, Texas, were determined by gut analysis. Species-specific food plant choice and niche breadths are presented for each of these species. As a group, grasshoppers range from monophagous to polyphagous feeders although most species fall in the oligophagous to polyphagous group. Phylogenetic constraints are evident such that gomphocerinae are primarily grass feeders while melanoplinae feed predominantly on forbs; the oedipodinae show less clearcut tendencies.Feeding patterns are remarkably constant from site to site and overall, community niche breadth distributions between sites do not differ greatly. Individual species tend to eat the same plant species at various sites and maintain similar niche breadths. Species with relatively specialized diets tend to feed on predictable plant species such as grasses and long-lived perrenial forbs.Grasshopper feeding patterns present some problems to the current theory of herbivore diet specialization since forb feeding melanoplines tend to be polyphagous (contrary to predictions). Life history patterns unrelated to tracking host plants may explain some aspects of diet breadth since diet selectivities are presumably adjusted according to the probability of finding suitable food plants.

Importance of dietary nitrogen and carbohydrates to survival, growth, and reproduction in adults of the grasshopper Ageneotettix deorum (Orthoptera: Acrididae)

Abstract Key demographic traits in insect herbivores (survival, growth, and egg production) are often responsive to variation in diet quality, especially to dietary nitrogen (N) levels. Soluble carbohydrates may also be limiting. Using defined diets under controlled laboratory conditions, we examined survival, growth, and egg production in response to a range of diet qualities in adult females of a grass-feeding grasshopper Ageneotettix deorum (Scudder). Diets varied factorially within naturally occurring ranges of total N (1–7%) and carbohydrate (4–27%) levels. N concentrations significantly impacted weight gain, egg production rate, the elapsed time until the first egg pod, and the time between the first and second egg pod. These responses were typically quadratic in nature with a maximum response near 4–5% total N. The rate of pod production rather than number of eggs per pod best explained changes in reproductive rate. Dietary carbohydrate levels seldom exerted a significant impact on demographic parameters except when interacting with N on survival, egg weight, and the period between egg pods. Clearly, factors that alter the availability of quality diet, especially total N levels, can contribute to demographic responses in A. deorum.

Disturbance by fire frequency and bison grazing modulate grasshopper assemblages in tallgrass prairie

Understanding determinants of local species diversity remains central to developing plans to preserve biodiversity. In the continental United States, climate, grazing by large mammals, fire, and topography are important ecosystem drivers that structure North American tallgrass prairie, with major impacts on plant community composition and vegetation structure. Frequency of fire and grazing by bison (Bos bison), through effects on plant community composition and altered spatial and structural heterogeneity of veg- etation in tallgrass prairie, may act as bottom-up processes that modulate insect community species richness. As previously seen for plant species richness, I hypothesized that grazing had more impact than fire frequency in determining species richness of insect herbivore communities. I examined this prediction with grasshoppers at Konza Prairie, a representative tallgrass prairie site in which fire frequency and bison grazing are manipulated over long terms with landscape-level treatments. Topographic position (upland vs. lowland) and fire frequency (1-, 2-, 4-year intervals, and unburned) did not significantly influence grasshopper species richness or indices of diversity, while grazing had significant effects. On average, I found ;45% more grasshopper species and significantly increased values of Shannon H9 diversity at sites with bison grazing. Species abundances were more equally distributed (Shannons Evenness Index) in grazed sites as well. No significant interactions among burning and grazing treatments explained variation in grasshopper species diversity. Grasshopper species richness respond- ed positively to increased heterogeneity in vegetation structure and plant species richness, and negatively to average canopy height and total grass biomass. Variation in forb biomass did not influence grasshopper species richness. A significant positive relationship between grasshopper species richness and overall grasshopper density was observed. Species rich- ness increased marginally as watershed area of treatments in grazed areas increased, but not in ungrazed areas. Disturbance from ecosystem drivers operating at watershed spatial scales exhibits strong effects on local arthropod species diversity, acting indirectly by mediating changes in the spatial heterogeneity of local vegetation structure and plant species diversity.

Ten Suggestions to Strengthen the Science of Ecology

Abstract There are few well-documented, general ecological principles that can be applied to pressing environmental issues. When they discuss them at all, ecologists often disagree about the relative importance of different aspects of the sciences original and still important issues. It may be that the sum of ecological science is not open to universal statements because of the wide range of organizational, spatial, and temporal phenomena, as well as the sheer number of possible interactions. We believe, however, that the search for general principles has been inadequate to establish the extent to which generalities are possible. We suggest that ecologists may need to reconsider how we view our science. This article lists 10 suggestions for ecology, recognizing the many impediments to finding generalizations in this field, imposed in part by the complexity of the subject and in part by limits to funding for the study of ecology.

Plasticity and overcompensation in grass responses to herbivory

Several hypotheses predict defoliation-induced increases in individual plant fitness. In this paper we examine three such hypotheses: the Herbivore Optimization Hypothesis (HOH); the Continuum of Responses Hypothesis (CRH); and the Growth Rate Model (GRM). All three have in common predictions based on responses of defoliated individuals with the objective of explaining community and higher level phenomena. The latter two extend theory by specifying conditions for overcompensatory responses. They differ in whether overcompensation is sensitive to conditions external (CRH) or internal (GRM) to the plant. We tested these hypotheses with field experiments in a grassland system in which two native, perennial grass species replace each other along a short topographic/resource gradient. We detected positive, neutral, and negative changes in plant mass in response to partial defoliation. Patterns of responses to the edaphic and competitive environment combinations were unique to each species and neither the CRH nor the GRM were able to consistently predict responses in these grasses. Predictions of the HOH were fully supported only by the species naturally limited to lower-resource environments: overcompensation occurred in natural environments and it occurred at herbivory levels these plants experience naturally. Thus, the overcompensatory response can be important for the maintenance of local plant population distributions. However, new mechanistic theory is needed to account for the trend common to both species: overcompensatory responses to herbivory were greater in the edaphic environment in which each species was naturally most abundant.

LANDSCAPE VS. LOCAL HABITAT SCALE INFLUENCES TO INSECT COMMUNITIES FROM TALLGRASS PRAIRIE REMNANTS

Habitat loss and fragmentation currently threaten ecosystems worldwide, yet remain difficult to quantify because within-fragment habitat and landscape-scale influences often interact in unique ways. Although individual species respond to fragmentation dif- ferently, large-scale conservation planning must unavoidably target multiple species. Al- though information on a populations response to fragmentation is critical, and measure- ments of species richness provide useful insights, exclusive reliance on these responses may mask important information about the taxonomic composition of assemblages in re- sponse to fragmentation. The North American tallgrass prairie ecosystem is one of the most threatened and fragmented ecosystems in the world, and insects are significant contributors to its biodiversity. In remaining grassland fragments, we evaluated within-fragment influ- ences in conjunction with landscape-scale responses of representative insect communities from four feeding guilds: generalists, specialists, multiple life stage habitat use, and pred- ators. Fragment-specific attributes capable of influencing insect diversity include plant species composition, plant biomass, abundance of blooming flowers, and vertical habitat heterogeneity created by the vegetation. Landscape-scale factors expected to influence pat- terns of insect species diversity include fragment size and shape as well as the spatial configuration of fragments. Ordination techniques were used to summarize composition of each feeding guild assemblage of each fragment, and structural equation modeling was used to examine the direct and indirect effects of fragmentation with influences from local habitats. Generalists (Orthoptera), mixed-modality feeding that changes with life stage (Lepidoptera), and specialist herbivores (Curculionidae) all responded directly to within- site characterizations of the plant community. Site management from large ungulate grazing or mowing for hay production consistently had an indirect effect on the insect community through influences on plant community composition. The predator assemblage (Coccinel- lidae) was influenced directly by fragment shape. To maintain insect biodiversity in tallgrass prairie fragments, these results indicate that conservation practices should focus on com- munities in order to maintain insect biodiversity in tallgrass prairie fragments. Landscape- scale factors must also be considered when making conservation decisions, primarily be- cause predators (top trophic level organisms) are more likely to respond to regional changes.

Impact of diet quality on demographic attributes in adult grasshoppers and the nitrogen limitation hypothesis

1. Various formulations of the nutrient stress hypothesis predict that insect herbivore populations will respond positively to increased nutrient quality of host plants, especially dietary nitrogen. Survival and reproduction by adult females of two grasshopper species [Acrididae;Melanoplus sanguinipes (Fabricius) and Phoetaliotes nebrascensis (Thomas)] were evaluated in response to defined diets that varied factorially in both total nitrogen (1–7%) and total soluble carbohydrate (4.3–26.7%). These grasshopper species coexist naturally but are typically shifted phenologically so that specific developmental stages normally encounter host plants of different nutritional quality under natural conditions.

Experimental study of avian predation on coexisting grasshopper populations (Orthoptera: Acrididae) in a sandhills grassland

An experiment was performed to examine the impact of avian predation on populations and assemblages of insect herbivores in a grassland system. Here, I examine the effect of avian predators on grasshopper populations in regard to overall density, densities of individual species, species diversity, and overall taxonomic composition. Predation pressure was examined by comparing each of these attributes of grasshopper populations in 10 m x 10 m exclosures where birds were excluded with equally sized plots where birds were allowed to forage normally. In general, avian predation significantly affected grasshopper populations in a sandhills grassland. Overall densities were reduced 27.4% in the predation treatment, most species occurred in lower numbers (though some exceptions were observed), and fewer species coexisted. Specific differences in overall taxonomic composition between the treatments were not detected using MANOVA. The relevance of these results to understanding community organization of herbivorous insects is discussed; the compensatory mortality hypothesis is not supported although other plausible hypotheses cannot be further evaluated with the present data.