Plant-pollinator networks along an altitudinal gradient in the Drakensberg Mountain Centre – implications for climate change

Abstract

Biotic pollination underpins ecosystem stability and biodiversity maintenance. However, pollinators and their interactions with plants are increasingly threatened by climate change, habitat loss and degradation. This is particularly important in montane ecosystems, which are characterised by high diversity and unique elevation-dependent species assemblages. Yet, local-scale assessments of pollination interactions in these highly diverse systems remain scarce and necessary to predict the future resilience of these systems. This study provides baseline data on plant-pollinator interactions in the northern Maloti-Drakensberg, South Africa’s highest-elevation region. Observations were conducted in three altitudinal zones — lower montane (~1350 m a.s.l.), upper montane (~2100 m a.s.l) and alpine (~3000 m a.s.l) during early, mid and late summer (2023/2024). I analysed interaction network properties (connectance, nestedness, dependence asymmetry, modularity and specialisation). Additionally, plant species (flowering) and insect functional group richness and diversity as well as insect order abundance across these three altitudes and over the summer. Plant diversity decreased with altitude in early summer but rose later in the season. Insect functional group diversity declined with altitude but increased over the summer. Networks shifted from bee- to fly-dominated with increasing altitude. Connectance peaked at low altitude, where invasive species were present and was lowest at mid-altitude where species richness was highest. High-altitude networks were most nested, reflecting adaptation to high environmental variability. Low dependence asymmetry at mid-altitude indicated frequent specialist-specialist interactions. At low altitude, pollinators relied more on specific plants and vice versa for high altitudes. Modularity and network specialisation did not vary across altitude or over the summer. Insect functional group specialisation peaked where the landscape was most heterogeneous. Plant species specialisation decreased with altitude, reflecting a typical response to high altitude conditions. The low species richness, habitat degradation and network property scores suggest that low-altitude grasslands are currently the most vulnerable to climatic changes. The mid-altitude networks are buffered by high species richness but may face long-term background species losses. While high-altitude networks currently appear to be the most resilient lack of upslope refugia and potential competition from lower-elevation species pose long-term risks to these networks and may eventually lead to ecological tipping points in the community.