The effects of cattle grazing on plant‐pollinator communities in a fragmented Mediterranean landscape

Abstract

2006. The effects of cattle grazing on plantpollinator communities in a fragmented Mediterranean landscape. Á Oikos 114: 529 Á 543.The main aims of this study were to assess grazing impacts on bee communities in fragmented mediterranean shrubland (phrygana) and woodland habitats that also experience frequent wildfires, and to explain the mechanisms by which these impacts occur. Fieldwork was carried out in 1999 and 2000 on Mount Carmel, in northern Israel, a known hot-spot for bee diversity. Habitats with a range of post-burn ages and varying intensities of cattle grazing were surveyed by transect recording, grazing levels, and the diversity and abundance of both flowers and bees were measured. The species richness of both bees and flowers were highest at moderate to high grazing intensities, and path-analysis indicated that the effects of both grazing and fire on bee diversity were mediated mainly through changes in flower diversity, herb flowers being more important than shrubs. The abundance of bees increased with intensified grazing pressure even at the highest levels surveyed. Surprisingly though, changes in bee abundance at high grazing levels were not caused directly by changes in flower cover. The variation in bee abundance may have been due to higher numbers of solitary bees from the family Halictidae in grazed sites, where compacted ground (nesting resource) and composites (forage resource) were abundant. The effects of grazing on plants were clearest in the intermediate-aged sites, where cattle inhibited the growth of some of the dominant shrubs, creating or maintaining more open patches where light-demanding herbs could grow, thus allowing a diverse flora to develop. Overall, bee communities benefit from a relatively high level of grazing in phrygana. Although bee and flower diversity may decrease under very heavy grazing, the present levels of grazing on Mount Carmel appear to have only beneficial effects on the bee community.

Disruption of a Belowground Mutualism Alters Interactions Between Plants and Their Floral Visitors

“…The importance of plant species richness as a predictor of bee species richness or abundance has been shown in several studies (Potts et al 2003, Hegland and Boeke 2006, Vulliamy et al 2006), but we are unaware of any study demonstrating the importance of evenness of the flowering-plant community for variation in insect-community visitation patterns. Visual inspection of the data suggests that the evenness of number of flowering stems of other species is positively related to the total number of visits received by a particular species.…”

“…However, in recent years there has been a significant paradigm shift associated with the realization that above and below ground processes can be of meaningful-effect sizes (Wardle 2002). For example, through direct effects on plant community composition (via consumption) and indirect effects on bee nest sites (via soil compaction), cattle grazing can increase bee abundance (Vulliamy et al 2006). Interactions within communities can also be trait-based, such as when plant competition for soil resources reduces shoot growth, which in turns impacts a plant's ability to compete for light (Cahill 1999), or when reductions in flower size due to foliar herbivory leads to reduced floral visitation (Strauss et al 1996).…”

Short‐term responses of native bees to livestock and implications for managing ecosystem services in grasslands

“…Several studies of livestock grazing and native bees have found significant effects, some negative (e.g., Kruess and Tscharntke 2002, Hatfield and LeBuhn 2007, Sjö din 2007, Xie et al 2008, Kearns and Oliveras 2009) and some positive (e.g., Carvell 2002, Vulliamy et al 2006, Yoshihara et al 2008. Whether livestock grazing has a positive or negative effect on bee communities may depend upon various factors including the species composition of the community, the intensity of grazing, the types of grazers (e.g., cattle, goat, sheep), how long grazing has occurred, land use history, and habitat type.…”

“…However, previous work on native bees suffers from some limitations common in many livestock grazing studies. First, most studies have been observational (i.e., they have not experimentally manipulated grazing level), and thus lack the ability to infer a causal relationship (e.g., Sugden 1985, Carvell 2002, Kruess and Tscharntke 2002, Vulliamy et al 2006, Hatfield and LeBuhn 2007, Sjö din 2007, Sjö din et al 2008, Xie et al 2008, Kearns and Oliveras 2009. Second, many studies have compared only the presence or absence of grazing (e.g., Sugden 1985, Xie et al 2008, rather than a gradient of grazing intensities.…”

Food Availability for Insectivores in Grasslands – Arthropod Abundance in Pastures, Meadows and Fallow Land

“…However, different aspects of grassland management are mainly discussed from the view of insect conservation (e.g. Carvell, 2002;Wetterstein and Schmid, 1999;Vulliamy, 2006;Sjödin, 2007;Radlmair and Dolek, 2002). Intensive application of fertilizers and frequent mowing or heavy grazing have indeed a negative impact on many arthropod species (von Nordheim, 1992; von Wingerden et al, 1992;Benton et al, 2003) and this may have consequences also for species depending on arthropods as food (Vickery, 2001).…”