Specialization of plant–pollinator interactions increases with temperature at Mt. Kilimanjaro

Abstract

Aim Species differ in their degree of specialization when interacting with other species, with significant consequences for the function and robustness of ecosystems. In order to better estimate such consequences, we need to improve our understanding of the spatial patterns and drivers of specialization in interaction networks. Methods Here, we used the extensive environmental gradient of Mt. Kilimanjaro (Tanzania, East Africa) to study patterns and drivers of specialization, and robustness of plant–pollinator interactions against simulated species extinction with standardized sampling methods. We studied specialization, network robustness and other network indices of 67 quantitative plant–pollinator networks consisting of 268 observational hours and 4,380 plant–pollinator interactions along a 3.4 km elevational gradient. Using path analysis, we tested whether resource availability, pollinator richness, visitation rates, temperature, and/or area explain average specialization in pollinator communities. We further linked pollinator specialization to different pollinator taxa, and species traits, that is, proboscis length, body size, and species elevational ranges. Results We found that specialization decreased with increasing elevation at different levels of biological organization. Among all variables, mean annual temperature was the best predictor of average specialization in pollinator communities. Specialization differed between pollinator taxa, but was not related to pollinator traits. Network robustness against simulated species extinctions of both plants and pollinators was lowest in the most specialized interaction networks, that is, in the lowlands. Conclusions Our study uncovers patterns in plant–pollinator specialization along elevational gradients. Mean annual temperature was closely linked to pollinator specialization. Energetic constraints, caused by short activity timeframes in cold highlands, may force ectothermic species to broaden their dietary spectrum. Alternatively or in addition, accelerated evolutionary rates might facilitate the establishment of specialization under warm climates. Despite the mechanisms behind the patterns have yet to be fully resolved, our data suggest that temperature shifts in the course of climate change may destabilize pollination networks by affecting network architecture.

Elevational and seasonal patterns of butterflies and hawkmoths in plant-pollinator networks in tropical rainforests of Mount Cameroon

“…The prevailingly non-apparent trends in the plant-lepidopteran network characteristics can be surprising because several other studies of pollination networks along elevation revealed strong patterns, although inconsistent among the studied areas and groups 1 , 51 . Nevertheless, we are not aware of any similar study on pollination by butterflies or sphingids along any tropical elevational gradient for comparison.…”

“…Recently, pollination research has shifted from detailed studies of single pollination systems to network approaches. Nevertheless, most of the complex studies of individual pollinator groups’ role in plant-pollinator networks have focused on bees or hoverflies 1 , 2 , whilst the other flower visitors have often been excluded or side-lined. Although some less abundant groups play important roles in pollination systems, as secondary pollinators, nectar thieves and competitors, or even as key pollinators of specialised plants 3 – 7 , their importance in plant-pollinator networks remains understudied, especially in tropical forests.…”

Role and patterns of butterflies and hawkmoths in plant-pollinator networks at different elevations and seasons in tropical rainforests of Mount Cameroon

“…The prevailingly non-apparent trends in the plant-lepidopteran networks characteristics can be surprising because several other studies of pollination networks along elevation revealed diverse strong patterns (e.g. Classen et al, 2020;Ramos-Jiliberto et al, 2010). Nevertheless, we are not aware about any similar study on pollination by butterflies or sphingids along any tropical elevational gradient for comparison.…”

“…Nevertheless, most complex studies of individual pollinator groups' role in plantpollinator networks have focused on bees or hoverflies (e.g. Classen et al, 2020;Klecka et al, 2018), whilst the other flower visitors have often been excluded or side-lined. Although some less abundant groups play important roles in pollination systems, as secondary pollinators, nectar thieves and competitors, or even as key pollinators of specialised plants (Hahn & Brühl, 2016;Martínez-Adriano et al, 2018;Mertens et al, 2020;Ollerton, 2017;Wardhaugh, 2015), their importance in plantpollinator networks remains understudied, especially in tropical forests.…”

The context dependency of pollinator interference: How environmental conditions and co‐foraging species impact floral visitation

“…High resource availability—measured in flower density or nectar volume—has been shown to decrease the duration of foraging trips for bumblebees (Westphal et al, 2006) and increase the floral visits (Thomson, 1988; Thomson et al, 1987). Insect pollinators also show changes in their interactions with plants due to temperature; higher temperatures have been documented to shorten the time spent on individual flowers relative to low temperatures for bumblebees (Heinrich & Raven, 1972) and to promote floral specialisation within an elevation gradient (Classen et al, 2020). Hence, studying the context in which interactions occur is as important as studying the interactions themselves.…”