The short‐term effect of sheep grazing on selected invertebrates (Diptera and Hemiptera) relative to other environmental factors in an alpine ecosystem

Abstract

Grazing by large herbivores is well-known to influence plant communities, while much fewer studies have been carried out on grazing effects on invertebrates. In Norway, some 2.2 million sheep graze on outlying pastures during summer, most of them in the alpine zone, but no study has reported the relative impact of sheep grazing on invertebrate communities relative to other environmental factors such as the plant community and altitude. A fully replicated landscape-scale experiment (2.7 km 2 ) was performed with no, low (25 per km 2 ) and high (80 per km 2 ) sheep densities in an alpine habitat of Norway (1050-1300 m a.s.l.). The increased vulnerability hypothesis (H 1 ) predicts that the more folivorous invertebrates, the higher the grazing pressure by sheep, as large herbivore grazing may stress the plants so they are more vulnerable to insect herbivory. The increased defence hypothesis (H 2 ) predicts increased levels of general anti-herbivore defences, and thus a lower abundance of invertebrates with increasing sheep densities. Contrary to both predictions, no evidence was found that sheep grazing affected invertebrate richness, or abundance of folivorous, predatory or detritivore invertebrates -in a community dominated by Diptera and Hemiptera. Demonstrating an effect will always be a function of sample size, but at least our study shows that other environmental variables (such as plant species richness and functional plant richness) are more important determinants than sheep grazing for the selected invertebrate groups. Our study was short-term (first year of grazing) mainly designed to test specific hypotheses related to induced plant defences; long-term effects are probably owing to the impact sheep may have on vegetation composition, primary production, litter cover and soil properties.

Does Long-term Grazing by Pack Stock in Subalpine Wet Meadows Result in Lasting Effects on Arthropod Assemblages?

“…Sweep nets have been shown to yield higher numbers of individuals, species, families, and orders, and capture higher levels of diversity than pitfall traps, light traps, or scented traps (Gadagkar et al 1990). Sweep netting is probably the most widely used method for sampling arthropods in vegetation (Southwood and Henderson 2000), and this technique has been used in other investigations of the effects of grazers on arthropods (Rambo and Faeth 1999;Mysterud et al 2005).…”

“…Rambo and Faeth (1999) reported that deer, elk, and cattle reduced abundance, but not richness or evenness, of insects in a pine-grassland assemblage. Mysterud et al (2005) found that sheep grazing in alpine pastures did not affect diversity or abundance of insects, and similarly Heske and Campbell (1991) and Bestelmeyer and Wiens (2001) discovered few differences in ant species richness, abundance, or assemblage structure as a function of livestock grazing. At the other end of the spectrum, Bock et al (2006) found that grazing of small ranches by horses, cattle, and sheep can increase grasshopper abundances, and Majer and Beeston (1996) found higher ant species richness in more heavily grazed areas.…”

Influences of forest harvest and environmental gradients on aquatic invertebrate communities of seasonal ponds

“…Sweep nets integrate terrestrial collections over a wide area and have a number of advantages for sampling remote wetlands (Holmquist et al, 2010). Sweep netting is the most common method used for sampling epigeal arthropods (Southwood and Henderson, 2000;Samways et al, 2010) and has been used in other investigations of arthropods in mountain vegetation (e.g., Wettstein and Schmid, 1999;Mysterud et al, 2005;Holmquist et al, 2010).…”

Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak

“…Managed livestock herbivory is a dominant land use in grasslands worldwide that often affects plant composition, quality, and abundance (Milchunas & Lauenroth, 1993;Milchunas et al, 1995). Although a number of studies have examined effects of managed vertebrate herbivores on invertebrate abundance or diversity, few studies have directly examined competition between managed vertebrate and invertebrate herbivores (Gomez & Gonzalez-Megias, 2002Kruess & Tscharntke, 2002;Joern, 2004;Mysterud et al, 2005).…”