Are there predictable clines in plant–pollinator interactions along altitudinal gradients? The example of Cytisus scoparius (L.) Link in the Sierra de Guadarrama (Central Spain)

Abstract

Abstract. We analyse the hypothesis that predictable gradients occur in plant–pollinator interactions along altitudinal gradients due to thermal constraints on insect body size and floral traits being selected for by pollinators. Pollinators’ size should follow Bergmann's rule in mountains (larger body size at higher altitudes) and selection should, in turn, lead to larger flower size at higher altitudes. The study focuses on the flower and pollinator size variation in 11 Cytisus scoparius populations located between 680 and 1300 m a.s.l., and on the relationship between flower size and pollination success of plants in each population. Significant differences among populations were found in flower size, pollinator size and fruiting success (anova, P < 0.001 in all cases). Regression models show that (i) pollinator size increases significantly with altitude, (ii) a parallel quadratic increase in flower size and (iii) a quadratic decrease in pollination success with altitude. Moreover, there is a tendency of plants with smaller flowers in each population to have a higher percentage of flowers initiating development into fruits (ancova, P < 0.05). All these observations support the presence of the predicted gradient in the studied species and they suggest the possibility of it to be somewhat common among plants that occupy large altitudinal ranges due to the simple basis on which it could be rooted.

The implications of climate change for positive contributions of invertebrates to world agriculture.

“…Pollinator 1463 communities tend to change with increasing elevation (and decreasing temperature). At 1464 higher altitudes the average size of pollinators increases (Malo and Baonza, 2002), but 1465 pollinator abundance is lower (Arroyo et al, 1982). In addition, flies tend to become the 1466 dominant pollinator group, mainly because other groups are restricted to lower areas (Kearns, 1467(Kearns, 1992).…”

“…In the Rocky Mountains (United States of America), flies replace bees as the main 1468 pollinators (Kearns 1992), while in the Andes of South America, tachinid flies and 1469 hummingbirds replace bee flies (Bombyliidae) and bees at higher altitudes (Arroyo et al, 1470(Arroyo et al, 1982. In addition, flower size tends to increase (Malo and Baonza, 2002). The lower 1471 pollinator abundance results in lower pollinator to flower ratios and lower seed set in plants at 1472 higher altitudes (Arroyo et al, 1985;Malo and Baonza, 2002).…”

“…In addition, flower size tends to increase (Malo and Baonza, 2002). The lower 1471 pollinator abundance results in lower pollinator to flower ratios and lower seed set in plants at 1472 higher altitudes (Arroyo et al, 1985;Malo and Baonza, 2002). Temperature increases might 1473 lead to pollinators moving to higher altitudes and might change pollination levels in plants.…”

Variation of vegetative and floral traits in the alpine plant Solidago minuta: evidence for local optimum along an elevational gradient

“…2009; Takahashi and Yoshida 2009;Maad et al 2013), lower numbers of flowers (Kelly 1998;Baret et al 2004;Maad et al 2013) and lower numbers of flower heads (Asteraceae family) (Kelly 1998;Alexander et al 2009), but larger flower sizes (Kudo and Molau 1999;Malo and Baonza 2002;Herrera 2005;Maad et al 2013). In addition, along with an increase in elevation, plants possess higher plasticity of flower longevity (Trunschke and Stöcklin 2017), increase clonality, tend to be older (Št'astná et al 2012) and produce more flowering shoots (von Arx et al 2006).…”

“…However, plant species with wide elevational ranges are expected to be exposed to different selective pressures at low and high elevations (Frei et al 2014a), and this may result in non-linear responses in some traits along the elevational gradient. For example, Malo and Baonza (2002) in their study on plant-pollinator interactions along an elevational gradient found that the Cytisus scoparius flower size in the mountains of Central Spain had a unimodal relationship with elevation and the maximum size occurred at the intermediate elevation. This curvilinear, unimodal relationship may be explained as a result of the trade-off between pollinator selection for larger flowers in higher elevations (e.g.…”