Insect-flower interaction networks vary among endemic pollinator taxa over an elevation gradient

Abstract

Interaction networks are sensitive to elevation gradients through changes in local distribution of interacting partners. Here, we use plant-pollinator interaction network metrics to assess the effect of elevation on flowers and flower-visiting insect assemblages on a sentinel mountain used for monitoring climate change in the flower- and insect-rich Cape Floristic Region. We also use these interaction metrics to explain the effect of environmental factors on the interaction networks. We did this over four vegetation zones <1640m asl, as determined by former botanical studies. Overall, bees were the dominant flower visitors, followed by monkey beetles, and far behind were wasps and flies. The middle elevation zone (650-744 m a.s.l), which is also an ecotone between two distinct botanical zones, had the highest species richness and abundance of interacting plants and insects. Interaction frequency and size of network were also greatest in the middle zone, as were network diversity, generality, and linkage density, while lowest in the peak zone. In sum, there was distinct elevation zoning of flower-visiting insects. The greatest zonal change was between species at the middle compared with peak zone. Large-sized monkey beetles, bees and flies characterized the unique assemblage in the peak zone (1576-1640 m a.s.l.). The insect zonation tracked that of plant assemblages, with air temperature (lapse rate) being the primary driver of bee distribution, with lowest levels in the peak zone. In contrast, beetle distribution was driven mostly by flower assemblages as well as air temperature. In turn, wasp and fly interaction networks were not affected by any of the measured environmental variables. We conclude that increased elevation stress from reduced temperatures, changing abiotic weather conditions (e.g. strong winds at high elevations),and decline in flowering plant composition causes breakdown of interaction networks involving bees and beetles but not that of flies and wasps.

Fig 1. Mean species richness (±SE) of flower-visiting insects across elevation zones.

Means with similar letters are not significantly different at P>0.05.

Fig 2. Mean species richness (±SE) of flowering plants across elevation zones.

Means with similar letters are not significantly different at P>0.05.

Fig 3. Mean number of interactions (±SE) across elevation zones.

Means with similar letters are not significantly different at P>0.05.

Fig 4. Mean interaction network size (±SE) across elevation zones.

Means with similar letters are not significantly different at P>0.05.

Fig 5. PCO showing separation of insect activities at different elevation zones, and the direction of environmental variables.