Pollination in a new climate: Assessing the potential influence of flower temperature variation on insect pollinator behaviour

Abstract

Climate change has the potential to enhance or disrupt biological systems, but currently, little is known about how organism plasticity may facilitate adaptation to localised climate variation. The bee-flower relationship is an exemplar signal-receiver system that may provide important insights into the complexity of ecological interactions in situations like this. For example, several studies on bee temperature preferences show that bees prefer to collect warm nectar from flowers at low ambient temperatures, but switch their preferences to cooler flowers at ambient temperatures above about 30° C. We used temperature sensor thermal probes to measure the temperature of outdoor flowers of 30 plant species in the Southern regions of the Australian mainland, to understand how different species could modulate petal temperature in response to changes in ambient temperature and, potentially, influence the decision-making of bees in the flowering plant’s favour. We found that flower petal temperatures respond in different ways to changing ambient temperature: linearly increasing or decreasing relative to the ambient temperature, dynamically changing in a non-linear manner, or varying their temperature along with the ambient conditions. For example, our investigation of the difference between ambient temperature and petal temperature (ΔT), and ambient temperature, revealed a non-linear relationship for Erysimum linifolium and Polygala grandiflora that seems suited to bee temperature preferences. The temperature profiles of species like Hibertia vestita and H. obtusifolia appear to indicate that they do not have a cooling mechanism. These species may therefore be less attractive to bee pollinators in changing climatic conditions with ambient temperatures increasingly above 30° C. This may be to the species’ detriment when insect-pollinator mediated selection is considered. However, we found no evidence that flower visual characteristics used by bees to identify flowers at close range, such as colour or shape, were straightforward modulators of floral temperature. We could not identify any clear link to phylogenetic history and temperature modulation either. Mapping our test flower distribution on the Australian continent however, indicates a potential clustering that suggests different flower responses may constitute adaptations to local conditions. Our study proposes a framework for modelling the potential effects of climate change and floral temperature on flower pollination dynamics at local and global scales.

Global warming and plant–pollinator mismatches

“…However, this preference for warmer flowers may be context-dependent as shown by Australian stingless bees (Trigona carbonaria) that preferred warmer nectars at lower air temperatures (from 23 to 30°C), but changed their behaviour and selected ambient temperature nectar over warmer nectar when the air temperature reached 34°C [95]. While some plant species appear to modulate their temperature in a way consistent with bee preferences, other appear to lack a cooling mechanism, suggesting that they may be less attractive to bee pollinators under higher temperatures [96]. Regarding all these effects on sensory cues used by pollinators for host attraction, global warming is probably already disrupting the interactions between plants and pollinators since all these modifications might alter the attractiveness of flowers to pollinators and impact their foraging behaviour [89].…”

Floral Humidity in Flowering Plants: A Preliminary Survey

“…Transpiration and nectar evaporation impact floral temperature, perhaps acting as a cooling mechanism (Baker, 1977;Patiño and Grace, 2002;Seymour and Schultze-Motel, 1998). In this manner, the presence of elevated floral humidity may indicate temperature control measures of the flower (Shrestha et al, 2018). Control of floral temperature is important as it influences floral metabolism, pollen and ovule viability (van der Kooi et al, 2019) and pollinator responses to the floral display (Dyer et al, 2006;Rands and Whitney, 2008;Whitney et al, 2008;Harrap et al, 2017).…”

Floral temperature patterns can function as floral guides

“…The lack of improvement in bee incidence of failed visits when presented with Hot temperature patterns may be due to the excess heat discouraging bees. Although bees prefer higher floral temperature, there is a point where flowers will become too hot and will deter bees (as demonstrated in Australian stingless bees Norgate et al 2010;Shrestha et al 2018). It is possible the Hot temperature patterns we used were sufficiently hot to deter naïve bumblebees, meaning that they would have been less likely to search the rewarding feeder when they encountered it-further experimentation is required to explore this suggestion.…”

“…These preferences are thought to cause the guided behaviours pollinators show at flowers with these patterns (Johnson and Dafni 1998;Goodale et al 2014). However, temperature patterns can also occur without dark pigmented colour patterns, as floral temperature can be influenced by flower structure (Miller 1986), epidermal textures (Whitney et al 2011), floral transpiration (Tsukaguchi et al 2003), environmental temperature (Shrestha et al 2018), or selfgenerated (thermogenic) heat (Seymour and Schultze-Motel 1997). Due to these associations between floral colour and temperature, pollinators visiting natural flowers with coloured guides are likely to encounter a range of temperature patterns that overlap in various ways with colour patterns-a multimodal display involving two modalities (temperature and colour, which are received by the pollinator using two different sensory modalities).…”