Relative Effectiveness of Insects Versus Hummingbirds As Pollinators of Rubiaceae Plants Across Elevation in Dominica, Caribbean L
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Plant Biology ISSN 1435-8603 RESEARCH PAPER Relative effectiveness of insects versus hummingbirds as pollinators of Rubiaceae plants across elevation in Dominica, Caribbean L. J. Lehmann1,2 , P. K. Maruyama3,4 , P. Joaquim Bergamo3,5, M. A. Maglianesi6, C. Rahbek1 & B. Dalsgaard1 1 Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, University of Copenhagen, Copenhagen Ø, Denmark 2 Section for Ecology and Evolution, Department of Biology, University of Copenhagen, Copenhagen Ø, Denmark 3 Departamento de Biologia Vegetal, Instituto de Biologia, Universidade Estadual Campinas, Campinas, SP, Brasil 4 Departamento de Biologia Geral, ICB, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brasil 5 Programa de Pos-Graduac ßao~ em Ecologia, Instituto de Biologia, Universidade Estadual de Campinas, Campinas, SP, Brasil 6 Vicerrectorıa de Investigacion, Universidad Estatal a Distancia, San Jose, Costa Rica Keywords ABSTRACT Environment; functional specialisation; plant–pollinator interactions; pollen • Most angiosperms rely on animal pollination for reproduction, but the dependence deposition; pollination syndromes; pollinator on specific pollinator groups varies greatly between species and localities. Notably, effectiveness; pollinator importance. such dependence may be influenced by both floral traits and environmental condi- tions. Despite its importance, their joint contribution has rarely been studied at the Correspondence assemblage level. L. J. Lehmann, Center for Macroecology, • At two elevations on the Caribbean island of Dominica, we measured the floral traits Evolution and Climate, Natural History and the relative contributions of insects versus hummingbirds as pollinators of plants Museum of Denmark, University of in the Rubiaceae family. Pollinator importance was measured as visitation rate (VR) Copenhagen, Universitetsparken 15, DK-2100 and single visit pollen deposition (SVD), which were combined to assess overall polli- Copenhagen Ø, Denmark. nator effectiveness (PE). E-mail: [email protected] • In the wet and cool Dominican highland, we found that hummingbirds were relatively more frequent and effective pollinators than insects, whereas insects and humming- Editor birds were equally frequent and effective pollinators at the warmer and less rainy A. Dafni midelevation. Furthermore, floral traits correlated independently of environment with the relative importance of pollinators, hummingbirds being more important in plant Received: 5 June 2018; species having flowers with long and wide corollas producing higher volumes of dilute Accepted: 13 February 2019 nectar. • Our findings show that both environmental conditions and floral traits influence doi:10.1111/plb.12976 whether insects or hummingbirds are the most important pollinators of plants in the Rubiaceae family, highlighting the complexity of plant–pollinator systems. 2009; Sch€affler et al. 2015). Furthermore, the role of a specific INTRODUCTION pollinator or functional group of pollinators may vary spatially The majority of angiosperms rely on animals for pollination along environmental gradients (Herrera 2005; Dalsgaard et al. (Ollerton et al. 2011; Rech et al. 2016), thus plant–pollinator 2009; Martın Gonzalez et al. 2009). In fact, pollinator relationships are crucial for the functioning of most terrestrial assemblages can vary considerably across an environmental gra- ecosystems (Kearns et al. 1998). However, not all flower visi- dient, even limiting plant reproduction (Chalcoff et al. 2012). tors are equally important as pollinators (Ollerton et al. 2007; When quantifying the importance of a pollinator or group King et al. 2013). At one extreme, some plants have ecologically of functionally similar pollinators for a given plant species, a and functionally generalised pollination systems (Ollerton et al. key issue is to distinguish flower visitors from pollinators (King 2007), being pollinated by numerous species and groups of ani- et al. 2013). Traditionally, visitation frequency, or visitation mal pollinators (Waser et al. 1996; Fumero-Caban & Melen- rate (VR), has been used as a measure of pollinator importance dez-Ackerman 2007; Dalsgaard et al. 2009), whereas other and dependency (Dalsgaard et al. 2013; Souza et al. 2018). plants are phenotypically, ecologically and functionally spe- However, VR might not correlate with pollinator importance, cialised, the extreme being plants adapted to pollination by one as a frequent flower visitor does not necessarily act as an effec- or a few functionally similar pollinators (Temeles et al. 2000; tive pollinator (Mayfield 2001; Watts et al. 2012). Thus, to Lindberg & Olesen 2001; Ollerton et al. 2007; Geerts & Pauw accurately quantify the importance of a given pollinator, a 738 Plant Biology 21 (2019) 738–744 © 2019 German Society for Plant Sciences, Royal Botanical Society of the Netherlands Lehmann, Maruyama, Bergamo, Maglianesi, Rahbek & Dalsgaard Floral traits and environment influence pollination of Rubiaceae measure of pollinator effectiveness (PE) – such as pollen depo- (Malhotra & Thorpe 1991). This generates a heterogeneous sition per visit – should be incorporated along with VRs (King environment and associated habitats, ranging from dry shrub- et al. 2013; Ballantyne et al. 2015). Although it is very time con- land along the western coastline to rain forest in wetter areas at suming to collect this kind of data, the inclusion of effective- midelevation to montane thicket and elfin forest in the cooler ness measures has led to some support of the ‘pollination and wetter highlands. We focused on two elevations covering syndrome hypothesis’, which may predict the most effective different vegetation types: rain forest (500–600 m a.s.l.) and pollinator for a given plant based on floral trait combinations montane thicket and elfin forest (750–975 m a.s.l.). At each (Fenster et al. 2004; King et al. 2013; Rosas-Guerrero et al. elevation, data were collected at two sites separated by a mini- 2014; Ashworth et al. 2015; Fernandez de Castro et al. 2017; mum of 1.5 km. Here, environmental variables were recorded Wester & Johnson 2017). However, such studies rarely consider during sampling and an ANOVA confirmed the expected differ- the variation in the relative importance of pollinators across ences between sites in humidity (F3,646 = 22.18, P < 0.01) and localities with contrasting environmental conditions. temperatures (F3.646 = 13.86, P < 0.01). A following post-hoc Here, we used plants in the coffee family (Rubiaceae) to test Tukey test showed that the sites were more alike within each how the relative importance – including per-visit effectiveness elevation than between elevations (Table 1). Thus, we found – of insects versus hummingbirds as effective pollinators relates higher humidity and lower temperatures at high elevation to both floral traits and environmental conditions. Specifically, compared to midelevation. The fieldwork was conducted from we tested: (i) whether the relative importance of insects and April 2015 to July 2015, covering the end of the dry season and hummingbirds as pollinators changes between elevations using beginning of the rainy season and thereby including the flower- VR, single visit pollen deposition (SVD) and a combination of ing period for different Rubiaceae species. All species in this the two expressing overall PE, and (ii) whether floral traits in study have flowers with relatively generalised morphological addition to elevation influence VR, SVD and PE. We tested this features and are visited by an array of distinct pollinator groups on the Caribbean island of Dominica where many plants from (Dalsgaard et al. 2009; Martın Gonzalez et al. 2009). the Rubiaceae have previously been shown to be visited legiti- mately by both hummingbirds and an array of insect groups Floral traits (Dalsgaard et al. 2009; Martın Gonzalez et al. 2009). We worked in two distinct elevation bands: mid-elevation rain for- All flowering Rubiaceae species in the two elevation bands were est habitat and high-elevation cloud forest and elfin habitat. identified and, whenever possible, a minimum of 20 individu- Due to the environmental differences between these bands, we als were sampled for homostylous (no distinct style morphs expected a shift in both plant species composition and in the among individuals) species. For distylous (distinct style importance of different groups of pollinators. Notably, as bees morphs among individuals) species, 10 individuals for each and most other insect pollinators, except flies, thrive better in morph were sampled and combined in the following analyses. warmer and drier habitats, we expected the floral traits to be For each flowering Rubiaceae species, floral morphological more specialised towards hummingbirds in the wetter and traits were measured with a digital calliper to the nearest cooler elfin/cloud forest in the highland, and the importance of 0.10 mm; corolla length as the internal distance from the tube insects, especially bees, to be higher in the warmer and drier opening to the base of the flower; internal width at the inside rain forest at midelevation (Cruden 1972; Dalsgaard et al. distal opening of the flower; and outer width as the longest dis- 2009; Martın Gonzalez et al. 2009). We also expected floral tance between the flower petals. We measured corolla length traits to impact the VR and importance of hummingbirds ver- and internal width as reflecting the morphological fit and nec- sus insects as pollinators (Dalsgaard et al. 2009; Bergamo