Ecologists have quantified Orthoptera (grasshoppers and crickets) density in a wide variety of conservation studies. Objective determination of Orthoptera population size is possible using mark-release recapture techniques but these are time-consuming and of little use for all but the smallest scale studies. Therefore, a wide range of sampling techniques have been devised to quantify population density and the most commonly used methods include sweep netting and quadrat counts. It is the aim of this paper to critically review studies that have used these techniques and to provide useful suggestions for non-specialists on which method may be most applicable to their study site. This paper reviews a selection of the extensive literature reporting studies estimating the abundance of grasshoppers (Acrididae) in a wide range of grassland ecosystems. Where possible, studies on bush-crickets (Tettigoniidae) and crickets (Gryllidae) are included reflecting their overall contribution to assemblage diversity in grassland ecosystems and to highlight the need for further investigations of sampling efficiency on these two under-researched families.

Review of the Methods Frequently Used to Estimate the Abundance of Orthoptera in Grassland Ecosystems

Herbivore and pollinator responses to grassland management intensity along experimental changes in plant species richness

Land use and corresponding habitat loss are major drivers of local species extinctions. Orthoptera as important grassland herbivores showed different responses to land-use intensity in different studies, and the susceptibility of this group remains unclear. We sampled annually for seven years 150 temperate grassland sites across three regions in Germany, for which land-use gradients were quantified as mowing, grazing, and fertilization intensity. We analyzed the effects of land-use intensity on orthopteran diversity and community abundance. To describe species-specific responses to environmental gradients, we employed a new approach termed “niche model,” coupled with a randomization procedure, which is sensitive even for rare species for which trends may otherwise be difficult to detect. Based on abundance-weighted means for each species, we quantified the species' occurrence along land-use gradients and identified potential losers and winners of intensive land use. Overall, high land-use intensity negatively affected orthopteran diversity across years and regions, corresponding to decreases with high fertilization, mowing, and grazing intensity. Intensive mowing and grazing negatively affected abundance. Diversity and abundance increased with the time after the last cut. The niche model detected 15 of 29 Orthoptera species as losers of land use, showing significantly higher abundance in grasslands with low-intensity land use. Two species were winners of high land-use intensity, whereas the remaining 12 were assigned as opportunists. Most species were losers of high fertilization intensity, followed by frequent mowing. Grazing intensity was least detrimental at the species level. Omnivorous, herbivorous, and graminivorous species did not differ in their response to land-use intensity, whereas bryovorous/lichenivorous Tetrix species showed consistently negative responses to intensive land use. Our highly replicated, long-term and large-scale survey suggests that further land-use intensification threatens many Orthoptera and causes a consistent diversity loss. Low intensity of fertilization, infrequent mowing, and variable grazing will help to maintain a high diversity of orthopterans. The generality of our niche model approach advances studies on species' susceptibility in various study systems.

https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1002/ecs2.1545

Losers, winners, and opportunists: How grassland land‐use intensity affects orthopteran communities

1. Generalist insect herbivores occupy a variety of habitats that differ in food plant composition. Dietary mixing has been proposed as a possibility for generalists to overcome nutritional deficiencies of single plant species, but only a few studies have investigated herbivore feeding and fitness for diets that resemble natural scenarios. We studied feeding behaviour, survival, and reproduction of the generalist grasshopper Chorthippus parallelus raised on food plants of four typical habitats.



2. Grasshopper diet consisted of grasses (92.5%), legumes (6.7%) and, in small quantities, other forbs (0.8%). Diet selection differed between the four food plant mixtures, and depended on grasshopper sex and developmental stage. There was no correlation between the relative abundance of plant species in the field and the fraction of these species in the grasshopper diet.



3. Grasshoppers survived on average for 40.4 ± 1.0 days before maturity, grew 106.8 mg until maturity moult, and females laid 4.1 ± 0.4 egg pods, each of which contained 8.5 ± 0.4 eggs. However, despite the differences in feeding behaviour, grasshopper fitness was the same in all of the four food plant mixtures. While the digestibility of ingested food was similar in the four different treatments, indices indicated differences in the conversion efficiency to body mass.



4. Our results show that C. parallelus is a plastic feeder with no fixed preferences in diet composition. The results emphasise that generalist herbivores can counteract putative quality deficiencies of single food plants by selective dietary mixing.

Being a generalist herbivore in a diverse world: how do diets from different grasslands influence food plant selection and fitness of the grasshopper Chorthippus parallelus?

Theory predicts negative effects of increasing plant diversity on the abundance of specialist insect herbivores, but little is known about how plant diversity affects the performance and abundance of generalist insect herbivores. We studied oviposition rates and offspring numbers in females of the generalist grasshopper Chorthippus parallelus that were collected in 15 montane grasslands in 2005 and 2007 along a gradient of plant species richness in central Germany. In addition to plant species richness, we determined evenness and plant community composition in the grasslands and measured aboveground plant biomass and other habitat variables such as leaf area index, vegetation height, and solar radiation. There was substantial variation among sites in grasshopper fecundity and the number of nymphs that hatched from the egg pods. Both fitness measures were positively influenced by plant species richness at the sites, while female fitness did not correlate with any of the other habitat parameters. Abundance of C. parallelus in the grasslands was positively correlated with plant species richness, plant community composition, and incident solar radiation of the sites. There were no phenological differences between grasshoppers from the different study sites. Our results suggest that decreasing biodiversity threatens the persistence not only of specialist, but also of generalist insect herbivores via a variety of mechanisms including a decrease in diversity of the generalists' food plants.

Plant species richness in montane grasslands affects the fitness of a generalist grasshopper species

Biomass and energy budgets and food utilization efficiencies of laboratory and wild populations of three Central European grasshopper species, Chorthippus parallelus (Zetterstedt), C. biguttulus (L.), and Gomphocerus rufus (L.), were studied between 1979 and 1984. Larval consumption is relatively low in C. biguttulus (C=4.3 kJ/ind.) compared with C. parallelus and G. rufus (4.9 kJ/ind.). In the adult phase (maturity) consumption rates of C. biguttulus and G. rufus (6.9 kJ/ind.) are similar, but higher in C. parallelus (7.3 kJ/ind.). The energy content decreases from the egg (23.2–24.3 J/mg dw) and body tissue (22.1–23.2 J/mg dw) to faeces (16.6–18.1 J/mg dw). The energy contents of faeces differ significantly between the species, indicating different rates of food conversion (on Dactylis glomerata). On average, the assimilation rates are about 30%, slightly lower in G. rufus. Approximate digestibility (A/C) ranges from 28.2 (G. rufus) to 35.7 (C. biguttulus) without great differences between larvae and adults. In contrast, the efficiency of conversion of ingested food (P/C) differs significantly between larval (10–11) and adult stages (3.4–3.6), and so to a much higher degree does the efficiency of conversion of digested food (P/A), from 30.3–33.5 in the larval to 9.5–14.9 in the adult period. Based on 5-year (C. parallelus) and 2-year studies (G. rufus) of the population dynamics and life tables, the energy budgets of wild populations were calculated and summarized into diagrams. Depending on the annually fluctuating densities, in both populations about 1%–2% of net primary production was consumed, and another 5%–9% was cut and dropped. The proportion of production of the grasshopper populations (body tissue) used for egg production differs in C. parallelus (28%) and G. rufus (44%). The energy cycling owing to energy storage in the living eggs amounts to 2%–3% of the total energy consumed by the population. The results are compared with the available data for grasshoppers in the literature.

G. Schäller

Papers

Ecological energetics of Central European grasshoppers (Orthoptera: Acrididae).