Rising temperatures due to global climate change can alter the phenology of plants, affecting their mutualistic partners, pollinators. These changes affect plant flowering time, which affects the amount and types of pollinators which will visit them. Rafferty and Ives (2011) examined how changes in flowering onset affected visitation by pollinators in fourteen perennial plants native to Wisconsin. Six of the 14 species demonstrated a change to sooner flowering in the past 70 years, the other eight did not exhibit any changes in flowering. The researchers experimentally advanced and delayed flowering of these species. The results showed that when flowering was artificially advanced, five of the six species with historically advanced flowering received more pollinator visits, while seven of the eight species with historically unchanged flowering received fewer visits from pollinators. Rafferty and Ives concluded that in their study species, phenological shifts to earlier flowering were not disrupting mutualistic relationships with pollinators. — Isabelle Heilman
Rafferty, N., Ives, A.R., 2011. Effects of experimental shifts in flowering phenology on plant-pollinator interactions. Ecology Letters 14, 69-74.
When two organisms have a mutualistic relationship, changes in the life events of one organism directly affect the other. Life events are dependent on environmental cues, and changes in temperature could signal an organism to begin a life event earlier in the year than is historically expected. For plants, temperature is a cue to begin flowering and attract pollinators. In the mutualistic relationship between plants and pollinators shift in the flowering time of the plant may lead to changes in pollinator visitation. Rafferty and Ives (2011) investigated how shifts in flowering related to climate change affect pollinator visitation. The researchers used 14 perennial plant species native to Wisconsin and experimentally advanced or delayed their flowering time. Six of the plants had advanced flowering time already, and eight had historically unchanged flowering time.
To test the relationship between flowering time and pollinator visitation, the authors first raised plants of 14 species controlling for factors such as fertilizer and pot size. In order to experimentally advance flowering, plants were put into a greenhouse between 24ºC and 27ºC. Delayed flowering was achieved by putting the plants into greenhouses between 18ºC and 21ºC. To measure changes in plant composition which could affect pollinator attraction, the nectar and sucrose content were sampled one day before putting the plants into the field to observe pollinator interactions. This could only be performed for eight of the 14 plants. Once plants were put into the field, pollinator visitations were recorded. A visitation was defined as touching anthers, stigma and/or nectar of the flower. Daily weather variables and the number of flowers present on each plant were also recorded.
To analyze the number of pollinators visiting, and visits received the authors used a general linear mixed model. Species of plant, time of day, maximum daily temperature, and precipitation were accounted for as fixed effects. A general linear mixed model was also used to assess the difference in composition of pollinators during the different weeks of experimentation. Analysis of the differences between historically advanced flowering species and historically unchanged flowering species in pollinator visitation was done using a chi square analysis. To account for the phylogenetic relationships between the fourteen species, a parallel analysis was used. Nectar and sucrose content were examined using linear mixed models.
These statistical tests showed that there was a significant change in number of pollinator visits in 12 of the 14 species. The historically advanced flowering species showed a trend of fewer pollinator visitations over time, compared to the historically unchanged flowering species. However, historically advanced flowering species were visited by more pollinators when their flowering was artificially advanced. The historically unchanged flowering species saw more pollinator visitations when flowering was artificially delayed. Nectar volume and sucrose concentration significantly changed for some plant species throughout the experiment, but there was no constant relationship when all of the species were considered.
The two categories of perennial plants that Rafferty and Ives examined showed different interactions with pollinators under manipulated flowering time. These results show that changes in flowering phenology can affect visitation from pollinators. However, the authors did not find an expansion of mismatch between flowering onset and visits by pollinators. They explain that despite the earlier flowering caused by climate change, these plants still have a variety of pollinators available to visit them. The coordination of this phenological reaction between plants and pollinators is optimistic for the future of their mutualistic relationship under the rising temperatures of climate change.