Flower colour and size-signals vary with altitude and resulting climate on the tropical-subtropical islands of Taiwan

Abstract

The diversity of flower colours in nature provides quantifiable evidence for how visitations by colour sensing insect pollinators can drive the evolution of angiosperm visual signalling. Recent research shows that both biotic and abiotic factors may influence flower signalling, and that harsher climate conditions may also promote salient signalling to entice scarcer pollinators to visit. In parallel, a more sophisticated appreciation of the visual task foragers face reveals that bees have a complex visual system that uses achromatic vision when moving fast, whilst colour vision requires slower, more careful inspection of targets. Spectra of 714 native flowering species across Taiwan from sea level to mountainous regions 3,300 m above sea level (a.s.l.) were measured. We modelled how the visual system of key bee pollinators process signals, including flower size. By using phylogenetically informed analyses, we observed that at lower altitudes including foothills and submontane landscapes, there is a significant relationship between colour contrast and achromatic signals. Overall, the frequency of flowers with high colour contrast increases with altitude, whilst flower size decreases. The evidence that flower colour signaling becomes increasingly salient in higher altitude conditions supports that abiotic factors influence pollinator foraging in a way that directly influences how flowering plants need to advertise.

Keywords: colour contrast; flora; flower colour; green contrast; insect; islands; vision.

Figure 1

(A) Overview of location of Taiwan, and (B) Location of sampling sites and elevational range within Taiwan Solid circles, squares and triangle represents the different sampling sites. Maps are prepared using package Maps in R version 3.4.4.

Figure 2

Example of flowering plants from five different elevational zones in Taiwan. (A) Trachelospermum lanyuense (15mm); (B) Alpinia x ilanensis (16mm); (C) Trochodendron aralioides (15mm); (D) Anaphalis morrisonicola (7mm); (E) Spiraea hayatana (4.5mm); (F) Ipomoea imperati (37.5mm); (G) Begonia longifolia (22mm); (H) Adenophora morrisonensis (31.5mm); (I) Veratrum shuehshanarum (12mm); (J) Veronica morrisonicola (11mm); (K) Bretschneidera sinensis (45mm); (L) Tricyrtis formosana (45mm); (M) Rhododendron formosanum (50mm); (N) Gentiana arisanensis (19.5mm); (O) Viola adenothrix var. tsugitakaensis (17.5mm). We show mean flower size (diameter, mm) in parenthesis for each species. Image credit: King-Chun Tai.

Figure 3

Left panel: phylogenetic relationship for 714 species. Terminal branches are visually presented as the flower colour of each species for human vision, which was generated based on the reflectance spectrum using the function ‘spec2rgb’ in R package PAVO. Solid circles at the tip represent the elevational zone for the respective plant species (dark green: foothills, light green: submontane, gold: montane, orange: upper-montane, beige: alpine). Right panel: pruned sub-tree from the whole phylogeny (see red square on the left panel) that shows the elevational zone, flower colour, and flower size for each example plant species. Solid squares at the tip represent the elevational zone. The solid green, solid red, and open white circles represent the green contrast, colour contrast, and flower size for each species, respectively, while the size of the circles indicates the magnitude of each variable.

Figure 4

Flower size (i.e., flower diameter, mm) at different elevational zones.

Figure 5

Distribution of green contrast (panels A-E, solid bars), colour contrast (panels F-J, hatched bars), and size (panels K-O, cross hatched bars) values for the flowers of 714 species at five elevational zones, n= in the island of Taiwan: Foothills (< 500 m a.s.l., first column, n=183), Submontane (500– 1,499 m a.s.l., second column, n=258), Montane (1,500–2,499 m a.s.l., third column, n=170), Upper-montane (2,500–2,999 m a.s.l., fourth column, n=67), and Alpine region (>3,000 m a.s.l., last column, n=36).

Figure 6

Mean and 95% confidence intervals for the (A: green contrast, GC), (B, colour contrast, CC), and (C: size) observed at each elevation: Foothills (1), Submontane (2), Montane (3), Upper-montane (4), and Alpine (5).

Figure 7

Floral reflectance spectra based on elevational zones (first column; A, C, E, G, I), and floral spectra in bee colour hexagon model in Taiwan (second column; B, D, F, H, J). The colour bar at the x-axis in the first column shows the human colour. The red circles in the hexagons show the achromatic region where bees do not reliably perceive chromatic signals. Colour solid circles (second column B, D, F, H, J) in bee hexagon represents the human flower colours that falls at different colour region (UV:Ultraviolet, B: and G: Green) of bee colour space.

Figure 8

Graphical representation of the linear models describing the relationships between colour contrast, flower size, and green contrast for flowers present at five different elevations on the island of Taiwan: Foothills (A), Submontane (B), Montane (C), Upper-montane (D), and Alpine (E). Significance of the relationship is indicated by asterisks: α = 0.05 (*), α = 0.01 (**), and not significant (NS). Markers on each panel indicate the values for the significant predictor and green contrast for the plant species present at each elevational zones.