Pollinators provide essential ecosystem services, and declines in some pollinator communities around the world have been reported. Understanding the fundamental components defining these communities is essential if conservation and restoration are to be successful. We examined the structure of plant-pollinator communities in a dynamic Mediterranean landscape, comprising a mosaic of post-fire regenerating habitats, and which is a recognized global hotspot for bee diversity. Each community was characterized by a highly skewed species abundance distribution, with a few dominant and many rare bee species, and was consistent with a log series model indicating that a few environmental factors govern the community.Floral community composition, the quantity and quality of forage resources present, and the geographic locality organized bee communities at various levels: (1) The overall structure of the bee community (116 species), as revealed through ordination, was dependent upon nectar resource diversity (defined as the variety of nectar volume-concentration combinations available), the ratio of pollen to nectar energy, floral diversity, floral abundance, and post-fire age.(2) Bee diversity, measured as species richness, was closely linked to floral diversity (especially of annuals), nectar resource diversity, and post-fire age of the habitat. (3) The abundance of the most common species was primarily related to post-fire age, grazing intensity, and nesting substrate availability. Ordination models based on agecharacteristic post-fire floral community structure explained 39-50% of overall variation observed in bee community structure. Cluster analysis showed that all the communities shared a high degree of similarity in their species composition (27-59%); however, the geographical location of sites also contributed a smaller but significant component to bee community structure.We conclude that floral resources act in specific and previously unexplored ways to modulate the diversity of the local geographic species pool, with specific disturbance factors, superimposed upon these patterns, mainly affecting the dominant species.

Abstract. 1. The habitat components determining the structure of bee communities are well known when considering foraging resources; however, there is little data with respect to the role of nesting resources.2. As a model system this study uses 21 diverse bee communities in a Mediterranean landscape comprising a variety of habitats regenerating after fire. The findings clearly demonstrate that a variety of nesting substrates and nest building materials have key roles in organising the composition of bee communities.3. The availability of bare ground and potential nesting cavities were the two primary factors influencing the structure of the entire bee community, the composition of guilds, and also the relative abundance of the dominant species. Other nesting resources shown to be important include availability of steep and sloping ground, abundance of plant species providing pithy stems, and the occurrence of pre-existing burrows.4. Nesting resource availability and guild structure varied markedly across habitats in different stages of post-fire regeneration; however, in all cases, nest sites and nesting resources were important determinants of bee community structure.Key words. Bees, community organisation, Mediterranean, nesting guilds, resource availability. IntroductionOrganisation of bee communities is closely related to the floral communities they forage upon, with several key characters having been identified, including floral diversity (e.g. Tepedino & Stanton, 1981;Gathmann et al., 1994), floral abundance (e.g. Banaszak, 1996), and availability of pollen and nectar resources (Petanidou & Vokou, 1990). However, few studies have attempted to quantify the combined effect of these structuring agents (but see Potts et al., 2003a).In turn, some drivers have been identified that impact directly upon bee communities, while others act indirectly through modification of floral communities or other habitat characteristics. These drivers include changing land use practices such as agricultural intensification (Banaszak, 1995), habitat fragmentation (Jennersten, 1988) and habitat isolation (Steffan-Dewenter & Tscharntke, 1999), grazing (Potts et al., 2003a), and agrochemical use (O'Toole, 1993). Other important drivers have also been recently identified: fire (Potts et al., 2003b); disease (Watanabe, 1994) and parasite spread (Schmid-Hempel & Durrer, 1991); climate change (Price & Waser, 1998); introduction of non-native plants (Brown & Mitchell, 2001;Chittka & Schu¨rkens, 2001); and competition with managed pollinators (Butz-Huryn, 1997;Steffan-Dewenter & Tscharntke, 2000).While the forage rewards provided by floral communities are generally accepted as the primary determinants of pollinator community structure, there is an increasing body of evidence suggesting that nest sites and nesting resources may

Measuring pollinator performance has become increasingly important with emerging needs for risk assessment in conservation and sustainable agriculture that require multi-year and multi-site comparisons across studies. However, comparing pollinator performance across studies is difficult because of the diversity of concepts and disparate methods in use. Our review of the literature shows many unresolved ambiguities. Two different assessment concepts predominate: the first estimates stigmatic pollen deposition and the underlying pollinator behaviour parameters, while the second estimates the pollinator's contribution to plant reproductive success, for example in terms of seed set. Both concepts include a number of parameters combined in diverse ways and named under a diversity of synonyms and homonyms. However, these concepts are overlapping because pollen deposition success is the most frequently used proxy for assessing the pollinator's contribution to plant reproductive success. We analyse the diverse concepts and methods in the context of a new proposed conceptual framework with a modular approach based on pollen deposition, visit frequency, and contribution to seed set relative to the plant's maximum female reproductive potential. A system of equations is proposed to optimize the balance between idealised theoretical concepts and practical operational methods. Our framework permits comparisons over a range of floral phenotypes, and spatial and temporal scales, because scaling up is based on the same fundamental unit of analysis, the single visit.