Feedback effects between plant and flower-visiting insect communities along a primary succession gradient

Abstract

Primary successions of glacier forelands are unique model systems to investigate community dynamics and assembly processes. However, successional changes of plant and insect communities have been mainly analysed separately. Therefore, changes in plant-insect interactions along successional gradients on glacier forelands remain unknown, despite their relevance to ecosystem functioning. This study assessed how successional changes of the vegetation influenced the composition of the flower-visiting insect assemblages of two plant species, Leucanthemopsis alpina (L.) Heyw. and Saxifraga bryoides L., selected as the only two insect-pollinated species occurring along the whole succession. In addition, we investigated the links between reproductive output of these plants and pollinator abundance through experimental exclusion of pollinators. Plant community structure changed along the succession, affecting the distribution and the abundance of insects via idiosyncratic responses of different insect functional groups. L. alpina interacted with ubiquitously distributed pollinators, while S. bryoides pollinators were positively associated with insect-pollinated plant species density and S. bryoides abundance. With succession proceeding, insect assemblages became more functionally diverse, with the abundance of parasitoids, predators and opportunists positively related to an increase in plant cover and diversity. The reproductive output of both plant species varied among successional stages. Contrary to our expectation, the obligate insect-pollinated L. alpina showed a reproductive output rather independent from pollinator abundance, while the reproductive output of the self-fertile S. bryoides seemed linked to pollinator abundance. Observing ecological interactions and using functional traits, we provided a mechanistic understanding of community assembly processes along a successional gradient. Plant community diversity and cover likely influenced insect community assembly through bottom-up effects. In turn, pollinators regulate plant reproductive output through top-down control. We emphasise that dynamics of alpine plant and insect communities may be structured by biotic interactions and feedback processes, rather than only be influenced by harsh abiotic conditions and stochastic events. bryoides seemed linked to pollinator abundance. Observing ecological interactions and using 45 functional traits we provided a mechanistic understanding of community assembly processes 46 along a successional gradient. Plant community diversity and cover likely influenced insect 47 community assembly through bottom-up effects. In turn, pollinators regulate plant 48 3 reproductive output through top-down control. We emphasize that dynamics of alpine plant 49 and insect communities may be structured by biotic interactions and feedback processes, 50 rather than only be influenced by harsh abiotic conditions and stochastic events. 51 52

Plant interactions shape pollination networks via nonadditive effects

“…S5). This way we accounted for the response of pollinator abundance to flower density (Losapio et al 2016).…”

Facilitation between plants shapes pollination networks

“…Pollinator diversity was calculated with the Shannon index (Oksanen et al, 2017) at the plot level. As pollinator abundance responds to flower density (Losapio et al, 2016), visitation rate of each plant species was calculated by standardising the number of pollinators by the number of flowers and sampling hours at the plot level. Then, we considered the pollination networks at the treatment level.…”

Do carabids (Coleoptera: Carabidae) and chironomids (Diptera: Chironomidae) exhibit similar diversity and distributional patterns along a spatio-temporal gradient on a glacier foreland?

“…We selected the Vedretta d'Amola glacier foreland in Italian Alps which has been investigated previously from the glaciological, geomophological and ecological point of view (Losapio et al, 2015;Losapio et al, 2016;Tenan et al, 2016;Gobbi et al, 2017;Lencioni, 2018). The general aim was to investigate if global warming affects in the same way terrestrial and aquatic glacial communities trophically related, and to know which species and in which relative proportion colonize aquatic and terrestrial habitats left free under glacier retreating, so describing primary succession patterns.…”