Temporal variation in plant–pollinator networks from seasonal tropical environments: Higher specialization when resources are scarce

Abstract

1. The temporal dynamics of plant phenology and pollinator abundance across seasons should influence the structure of plant-pollinator interaction networks.Nevertheless, such dynamics are seldom considered, especially for diverse tropical networks.2. Here, we evaluated the temporal variation of four plant-pollinator networks in two seasonal ecosystems in Central Brazil (Cerrado and Pantanal). Data were gathered on a monthly basis over 1 year for each network. We characterized seasonal and temporal shifts in plant-pollinator interactions, using temporally discrete networks. We predicted that the greater floral availability in the rainy season would allow for finer partitioning of the floral niche by the pollinators, i.e. higher specialization patterns as previously described across large spatial gradients.Finally, we also evaluated how sampling restricted to peak flowering period may affect the characterization of the networks.3. Contrary to our expectations, we found that dry season networks, although characterized by lower floral resource richness and abundance, showed higher levels of network-wide interaction partitioning (complementary specialization and modularity). For nestedness, though, this between-seasons difference was not consistent. Reduced resource availability in the dry season may promote higher interspecific competition among pollinators leading to reduced niche overlap, thus explaining the increase in specialization. There were no consistent differences between seasons in species-level indices,indicating that higher network level specialization is an emergent property only seen when considering the entire network. However, bees presented higher values of specialization and species strength in relation to other groups such as flies and wasps, suggesting that some plant species frequently associated with bees are used only by this group.5. Our study also indicates that targeted data collection during peak flowering generates higher estimates of network specialization, possibly because species activity spans longer periods than the targeted time frame. Hence, depending on the period of data collection, different structural values for the networks of interactions may be found.6. Synthesis. Plant-pollinator networks from tropical environments have structural properties that vary according to seasons, which should be taken into account in the description of the complex systems of interactions between plants and their pollinators in these areas. K E Y W O R D SCerrado, functional diversity, modularity, nestedness, network sampling, Pantanal, resource availability, seasonality | 2411Journal of Ecology SOUZA et Al.

Seasonal variation of plant-pollinator networks on oceanic island: lower specialization when resources are more abundant

“…In our study, the pollination network was more specialized during rainy season, characterized by more plant species in full bloom with higher floral resources availability, than during dry season. This result was opposite to a recent study in the Central region of Brazil [41] which found networks were more specialized during dry season when floral resource availability was lower. Previous studies in binary temporal networks suggested that nestedness, modularity and connectance vary significantly within a single year [e.g.…”

“…However, when we considered four seasonal networks with shorter observation periods all together, we found that the network-wide specialization and modularity were reduced with the increasing number of plants in full bloom among the seven networks. Similarly, Souza et al [41] found higher specialization occurred in the networks when resources are scarce in tropical environments. One explanation is that considerably lower availability of floral resources, coupled with less plant species in bloom, still leads to changes in animal foraging behavior owing to higher competition, ultimately resulting in overall higher specialization and modularity [47].…”

Functional diversity mediates macroecological variation in plant–hummingbird interaction networks

“…A global analysis of nectarivorous birds, including hummingbirds, honeyeaters and sunbirds (Zanata et al, ), has previously reported a positive relationship between plant richness and network specialization. Interestingly, the association between plant richness and network structure was not mirrored by covariation between network structure and plant functional diversity (see also Souza et al, ). One possible reason for this is that, in contrast to birds, interaction frequencies of plants usually do not reflect their abundances (Vizentin‐Bugoni et al, ; Weinstein & Graham, ).…”

“…Besides MST and FD is, we included the following environmental predictors: temperature (MAT), precipitation (MAP), temperature seasonality (TS), precipitation seasonality (PS), topography (Topo), temperature anomaly (AnomT), precipitation anomaly (AnomP) and insularity (Insu) to the set of predictors. Finally, we included plant species richness (Plants), because this has been shown to influence resource partitioning in flower–bird networks (Zanata et al, ) and might potentially act independently of floral functional diversity (Souza et al, ). For the subset of 28 networks with plant FDis measures, we used linear models to test whether plant FDis and hummingbird FDis predicted network level specialization < d ′>, calculated for plants and hummingbirds separately, and Δ Q .…”

Are native bees and Apis mellifera equally efficient pollinators of the rupestrian grassland daisy Aspilia jolyana (Asteraceae)?

“…We were interested in the role of different floral visitors as pollinators for this plant. Moreover, the role of the invasive honeybee in pollination is rarely assessed with experiments (Hung et al 2018), even though A. mellifera is often a dominant floral visitor in diverse Neotropical communities (Souza et al 2018). Thus, we asked whether invasive Apis mellifera and native bees have different effectiveness as pollinators of a generalist native plant.…”