Native and Non-Native Supergeneralist Bee Species Have Different Effects on Plant-Bee Networks

Abstract

Supergeneralists, defined as species that interact with multiple groups of species in ecological networks, can act as important connectors of otherwise disconnected species subsets. In Brazil, there are two supergeneralist bees: the honeybee Apis mellifera, a non-native species, and Trigona spinipes, a native stingless bee. We compared the role of both species and the effect of geographic and local factors on networks by addressing three questions: 1) Do both species have similar abundance and interaction patterns (degree and strength) in plant-bee networks? 2) Are both species equally influential to the network structure (nestedness, connectance, and plant and bee niche overlap)? 3) How are these species affected by geographic (altitude, temperature, precipitation) and local (natural vs. disturbed habitat) factors? We analyzed 21 plant-bee weighted interaction networks, encompassing most of the main biomes in Brazil. We found no significant difference between both species in abundance, in the number of plant species with which each bee species interacts (degree), and in the sum of their dependencies (strength). Structural equation models revealed the effect of A. mellifera and T. spinipes, respectively, on the interaction network pattern (nestedness) and in the similarity in bee's interactive partners (bee niche overlap). It is most likely that the recent invasion of A. mellifera resulted in its rapid settlement inside the core of species that retain the largest number of interactions, resulting in a strong influence on nestedness. However, the long-term interaction between native T. spinipes and other bees most likely has a more direct effect on their interactive behavior. Moreover, temperature negatively affected A. mellifera bees, whereas disturbed habitats positively affected T. spinipes. Conversely, precipitation showed no effect. Being positively (T. spinipes) or indifferently (A. mellifera) affected by disturbed habitats makes these species prone to pollinate plant species in these areas, which are potentially poor in pollinators.

Fig 1. A) Abundance, B) Degree (number of interactions), and C) Strength of Apis mellifera (Am) and Trigona spinipes (Ts) in Brazilian weighted plant-bee networks.

There is no significant difference between the variables (t-test; P> 0.05). The horizontal line within each box is the median, and the lower and upper limits of the box define the 25th and 75th percentiles, respectively. The lower and upper whiskers define the 10th and 90th percentiles, respectively. Photo by Adrian Gonzalez and Sheina Koffler.

Fig 2. Conceptual model of the regulation of geographical variation in Brazilian weighted plant-bee networks tested by a structural equation model.

Only significant effects are shown: *P<0.05; **P<0.01; ***P<0.001. The arrow size is proportional to the effect size (the intensity of relationship). The dotted arrows indicate negative effects. “Natural” refers to an undisturbed area (see Materials and Methods).

Fig 3. The strength of Apis mellifera (Am) decreases with temperature at lower latitudes in Brazilian weighted plant-bee networks.

The strength of Trigona spinipes (Ts) was not included because it presented no significant relationship to temperature (see Fig 2) (DEM = Digital Elevation Model). Map was built using ArcGIS 10 (Esri Inc.).