Untangling biodiversity interactions: A meta network on pollination in Earth's most diverse tropical savanna

Abstract

Pollination is vital for ecosystem functioning, especially in biodiversity-rich regions like the Brazilian Cerrado. Our research establishes a comprehensive meta network of pollinator-plant interactions within this biome. We quantified the importance of different pollinator groups, identifying keystone species. We examined potential biases in sampling effort and the spatial behavior of interactions within the heterogeneous Cerrado plant physiognomies. Our investigation uncovered 1499 interactions among 293 plant species and 386 visitor species, with legitimate pollination accounting for 42.4% of the interactions. The network exhibited modularity, driven by bees and insects, with vertebrates bridging diurnal and nocturnal modules. While a generalized pattern emerged, high specialization existed within modules due to habitat diversity. Bees, particularly Apis mellifera (exotic) and Trigona spinipes (native), played central roles as network hubs. Hummingbirds and bats, engaged in specialized interactions showing strong connectivity within and between modules. Interestingly, invertebrate-vertebrate modules were more prevalent than expected in the meta network. However, a bias was evident, primarily within specific biogeographical districts with fragmented landscapes and intrusion from other biomes. Variations in plant species and endemism rates influenced pollinator occurrence and the Cerrado network topology. Our study offers valuable insights into pollinator-plant interactions within the Cerrado, encompassing both invertebrates and vertebrates. The modeled network represents a significant step in understanding the structural complexity of pollination networks, integrating partial networks from diverse pollination systems within heterogeneous habitats. Nevertheless, a biogeographical bias could limit a comprehensive understanding of network functionality across the Cerrado.

Keywords: Brazilian Cerrado; conservation hotspot; floral visitors; modularity; mutualistic network; trait matching; zoophilia.

FIGURE 1

Species‐level interaction network between plants and floral visitors in the Brazilian Cerrado. Species clusters correspond to modules optimized by the Barber algorithm (see Section 2: Methods). Black nodes correspond to plants, and colored nodes represent different groups of floral visitors. Node size is proportional to species degree (number of partners). Interactions are classified as either legitimate pollination (blue edges) or floral visitation (gray edges), where pollination was not assessed.

FIGURE 2

Variations in the scores of species‐level indices of the various floral visitor groups in the Cerrado meta network. A: degree (d), B: closeness centrality (CC), and C: betweenness centrality (CC). Pollinator groups: hummingbirds (hum), bats (bat), bees (bee), beetles (bet), ants (ant), lepidopterans (lep), flies (fly), wasps (was), and others (oth). Insets contain the results of statistical analyses, and the letters above boxplots represent significance groups as determined by the post hoc analysis. The outlying and central bees Apis mellifera and Trigona spinipes were highlighted in all graphs.

FIGURE 3

The distribution of interactions reported by the reviewed articles according to vegetation categories: savannas, intermediate vegetations related to savannas, open vegetation, forests, and anthropogenic areas. Specific physiognomies: Css—cerrado sensu stricto; Cer—rupestrian cerrado; Cao—cerradão (arboreal savanna); Pal—palm swamps; Suj—campo sujo (grassland with a sparse occurrence of bushes and trees); Cac—campo cerrado (grassland with a high occurrence of bushes); Car—campo rupestre (grassland with rocky outcrops); Lim—campo limpo (regular grassland); Gal—gallery forest; Sde—semidecidual forest; Rip—riparian forest; Ant—Anthropogenic (deforested areas, farms, or cities).

FIGURE 4

Binary bipartite interaction network between floral visitors and plants in the Brazilian Cerrado, with the subdivision of plant species according to their occurrence in the major vegetation categories found in the biome. Node size corresponds to degree, and edge size to the number of times that the interaction was recorded in the literature. Inset values between vegetation cores correspond to the connectance (C) and mean degree (d) for the subnetworks containing only the pair of vegetation types in question.

FIGURE 5

Records of flower‐visiting/pollinator–plant interaction studies in the seven biogeographic districts of the Cerrado biome in Brazil, determined by Françoso et al. (2020)), based on the differences in plant species composition, endemism, and climatic conditions among biogeographic districts. CE—Central; CW—Central‐west; NE—North‐East; NW—North‐west; S—South; SE—South‐East; SW—South‐west; ExS—Extreme south; and ExN—Extreme north. The interactions were obtained through a literature survey, where each study reviewed may account for different interactions.