Spatial variability in a plant–pollinator community across a continuous habitat: high heterogeneity in the face of apparent uniformity

Abstract

Large‐scale spatial variability in plant–pollinator communities (e.g. along geographic gradients, across different landscapes) is relatively well understood. However, we know much less about how these communities vary at small scales within a uniform landscape. Plants are sessile and highly sensitive to microhabitat conditions, whereas pollinators are highly mobile and, for the most part, display generalist feeding habits. Therefore, we expect plants to show greater spatial variability than pollinators. We analysed the spatial heterogeneity of a community of flowering plants and their pollinators in 40 plots across a 40‐km2 area within an uninterrupted Mediterranean scrubland. We recorded 3577 pollinator visits to 49 plant species. The pollinator community (170 species) was strongly dominated by honey bees (71.8% of the visits recorded). Flower and pollinator communities showed similar beta‐diversity, indicating that spatial variability was similar in the two groups. We used path analysis to establish the direct and indirect effects of flower community distribution and honey bee visitation rate (a measure of the use of floral resources by this species) on the spatial distribution of the pollinator community. Wild pollinator abundance was positively related to flower abundance. Wild pollinator visitation rate was negatively related to flower abundance, suggesting that floral resources were not limiting. Pollinator and flower richness were positively related. Pollinator species composition was weakly related to flower species composition, reflecting the generalist nature of flower–pollinator interactions and the opportunistic nature of pollinator flower choices. Honey bee visitation rate did not affect the distribution of the wild pollinator community. Overall, we show that, in spite of the apparent physiognomic uniformity, both flowers and pollinators display high levels of heterogeneity, resulting in a mosaic of idiosyncratic local communities. Our results provide a measure of the background of intrinsic heterogeneity within a uniform habitat, with potential consequences on low‐scale ecosystem function and microevolutionary patterns.

Impacts of beekeeping on wild bee diversity and pollination networks in the Aegean Archipelago

“…For instance, in a protected Mediterranean habitat, Ropars et al (2020) showed that richness of large wild bee species was negatively affected by honey bee colony density, while communities of small wild bees were structured instead by local flowering communities. Conversely, in another protected Mediterranean natural park, Reverté et al (2019) failed to find any relationship between honey bee visitation rate and the composition of wild pollinators in an area dominated by honey bees. A few other studies conducted in agroecosystems also failed to find any effect of honey bee density on wild bee richness (Steffan‐Dewenter and Tscharntke 2000, Russo et al 2015).…”

“…All of this suggests that massive introductions of honey bees could potentially lead to depauperate wild pollinator communities, with important consequences for natural ecosystems and agricultural productivity (Winfree et al 2007, Aizen and Harder 2009, Potts et al 2010b, Breeze et al 2011, Garibaldi et al 2013). However, the effect of honey bee density on wild pollinator diversity has been little explored (Mallinger et al 2017) and the results are so far inconclusive, with some studies reporting negative effects on pollinator richness (Valido et al 2019, Ropars et al 2020), while others failing to find any relationship with wild pollinator richness or species composition (Steffan‐Dewenter and Tscharntke 2000, Russo et al 2015, Torné‐Noguera et al 2016, Reverté et al 2019). To our knowledge, no previous study has evaluated the relationship between the density of honey bees and wild bee richness at large scales.…”

Spatial variability of hosts, parasitoids and their interactions across a homogeneous landscape

“…Given that cavity‐nesting bees and wasps are highly mobile organisms (e.g., Gathmann & Tscharntke, 2002; Greenleaf et al, 2007; Klein et al, 2004, Guedot et al, 2009) one might expect a more uniform host distribution. However, previous studies in the same study area found high spatial variability in local community composition of bees (both cavity‐ and ground‐nesting) (Torné‐Noguera et al, 2014) and pollinators in general (Reverté et al, 2019). In our study, less than 30% of the variability in host composition can be attributed to foraging distance limitations (as evidenced by the weak correlation between host β‐diversity and geographic distances); the rest of the spatial variability might be attributed to other factors such as food and nesting resource distribution, immigration history, philopatry, and topoclimatic variation (Torné‐Noguera et al, 2014).…”

“…First, because our study organisms are highly mobile (Gathmann & Tscharntke, 2002, Klein, Steffan‐Dewenter, & Tscharntke, 2004, Greenleaf, Williams, Winfree, & Kremen, 2007, Guedot et al, 2009, Cronin & Reeve, 2005), we should expect low spatial variation in community composition. Yet, recent studies in the same study area reported high levels of spatial heterogeneity in pollinator (Reverté et al, 2019) and bee species composition (Torné‐Noguera et al, 2014), despite similar levels of abundance and richness. We, thus, hypothesize (H1) moderate to high spatial variation in species composition for both taxa, and we expect this variation to be mainly due to species turnover (as opposed to nestedness).…”

“…We selected 25 plots distributed across the study area (Appendix ). Distances between nearest plots ranged from 585 to 1,354 m. As mentioned, the study area is homogeneous at a landscape scale, being occupied by a single type of vegetation with similar physiognomic features (Reverté et al, 2019; Torné‐Noguera et al, 2014).…”