The Importance of Generalist Pollinator Complexes for Endangered Island Endemic Plants
Total Page：16
File Type：pdf, Size：1020Kb
Arquipelago - Life and Marine Sciences ISSN: 0873-4704 The importance of generalist pollinator complexes for endangered island endemic plants JULIE A. WEISSMANN & HANNO SCHAEFER Weissmann, J.A. & H. Schaefer 2017. The importance of generalist pollinator complexes for endangered island endemic plants. Arquipelago. Life and Marine Sciences 35: 23-40. To investigate whether endangered endemic plants of the Azores are threatened by pollinator limitation, we studied the insect pollinator communities of Azorina vidalii, Euphrasia azorica, Myosotis azorica and Solidago azorica on Corvo Island. We found no evidence for dependence on a specialised pollinator. Instead, we found five to 21 mostly generalist insect pollinators per plant species, six of them probably introduced species. Diptera, with at least 12 species, and Hymenoptera, with at least nine species, are the most important insect orders and also most important in visitation frequency. The relatively high pollinator diversity for each of the studied plants and the high proportion of generalists indicate that the pollination networks of the four study plant species are rather resilient, i.e. the loss of a species would not constitute an immediate threat. Seed counts and numbers of juvenile plants indicate that reproductive success of all four species is stable. Altogether, our results suggest that there is no pollinator limitation in the four study species. Conservation measures should therefore focus on other threats, on Corvo mainly on grazing pressure. Key words: generalist pollinators, island endemic plants, pollinator limitation, pollinator networks. Julie A. Weissmann (email: [email protected]) and Hanno Schaefer (email: [email protected]), Technical University of Munich, Dept. Ecology & Ecosystem Management, Plant Biodiversity Research, Emil-Ramann Str. 2, DE-85354 Freising, Germany. INTRODUCTION multitude of threat factors have been identified for them (Cardoso et al. 2008). Because of this, Endemic plants on oceanic islands constitute an recent studies highlight the importance of holistic important proportion of global biodiversity and at approaches (Caujapé-Castells et al. 2010; Silva et the same time are one of the most threatened al. 2015; Kueffer & Kaiser-Bunbury 2013), groups of organisms in the world (Kreft et al. studying a broad range of factors relevant for a 2008; Caujapé-Castells et al. 2010). The flora of population's survival, especially mutualistic the Azores, an archipelago of nine volcanic interactions with insects, vertebrates, and fungi islands close to the Middle-Atlantic ridge in the (Kaiser-Bunbury et al. 2010). Northern Atlantic Ocean (Fig. 1), comprises c. 70 Pollination, one of these key mutualisms, is endemic flowering plant species, about one third thought to differ fundamentally on islands of the total indigenous flora (Schaefer 2005b, compared to continental systems, because of Silva et al. 2010). Nine of these endemics are often reduced numbers of potential pollinators listed among the top 100 conservation priority (Valido & Olesen 2010; Olesen et al. 2012). In plant and animal species of Macaronesia and a some islands, there is evidence for adaptation to 23 Weissmann & Schaefer Fig. 1. Map of the Northern Atlantic islands (left), Azores archipelago (right) (modified from Schaefer et al. 2011) specialist pollinators including birds (Valido et al. archipelago. Specifically, we test for pollinator 2004), lizards (Olesen & Valido 2003) and locusts limitation indicated by low number of pollinator (Micheneau et al. 2010). In other islands, visits and reduced seed set. generalist pollinator complexes seem to dominate (Olesen et al. 2002). Insular pollination systems MATERIAL AND METHODS in general are thought to be particularly vulnerable to extinction of species, habitat Study site fragmentation, and invasion of introduced species Fieldwork was carried out by the first author from (Kaiser-Bunbury et al. 2010; Valido & Olesen June 20 to August 30, 2015, mainly on Corvo 2010). It has been suggested that the decline of island, with a few additional observations on the some endemic plant species on islands could be a neighbouring island Flores. Corvo is the smallest result of the loss of their specialist pollinators inhabited Azorean island with a size of only c. 17 (Robertson et al. 1998; Cox & Elmqvist 2000; km2 and 450 inhabitants, all living in a single Montgomery et al. 2001; Wheelwright et al. settlement at the southern tip. The island mainly 2006). consists of one big crater, with steep cliffs on the Knowledge on the pollination biology of north and west sides, pastures covering most of Azorean endemics is very limited (but see Pereira the land surface and a few small forest spots. Like 2008). An annotated checklist with some in most of the Azores archipelago, most natural ecological information is only available for the habitats on Corvo have been replaced by pasture bees (Weissmann et al. 2017), while knowledge land and cattle farming is the most important on other important clades like Diptera is still very economy on the island. We chose Corvo for our limited for the archipelago (Borges et al. 2010b). study because it is the last refuge for some of the Expanding knowledge in this area is crucial, rarest and most endangered endemics of the because the many ex-situ and in-situ conservation Azores, including Myosotis azorica, Veronica measures by the Azorean government, universities dabneyi, Euphrasia azorica, Euphorbia stygiana and NGOs will fail if lack of pollinators limits the and also hosts a large population of the endemic reproductive success of the study plants. Here we Azores bellflower (Azorina vidalii). test the hypothesis that the decline of Azorean endemic plants is partly due to loss of specialist Study species pollinators during the past five centuries of Out of the c. 40 endemic angiosperms existing on human-mediated environmental changes in the Corvo (Silva et al. 2010), we shortlisted ten 24 Pollinators of Azores Endemics threatened insect-pollinated plants as potential azorica, Myosotis azorica and Solidago azorica, study species: Azorina vidalii (Wats.) Feer, because they were found to be in flower and grew Centaurium scilloides (L. f.) Samp., Cerastium in accessible areas. Two of the study species, E. azoricum Hochst., Euphorbia stygiana Wats., azorica and M. azorica, are restricted to higher Euphrasia azorica Wats., Myosotis azorica Wats., elevation habitats, while the other two are mainly Platanthera pollostantha R.M. Bateman & M. coastal species. For three of the species, the study Moura, Ranunculus cortusifolius Willd., Solidago period overlapped with peak flowering, for S. azorica Hochst. ex Seub. and Veronica dabneyi azorica, only parts of the population were in Hochst. ex Seub. After arrival on the island, we flower during our stay and peak flowering was chose the four species Azorina vidalii, Euphrasia later in September. Fig. 2. Myosotis azorica in steep coastal cliff with the invasive Hydrangea macrophylla on the upper left; insert shows single flower with blue petals and orange-white scales limiting access to the corolla tube. Photos: J. Weissmann, Corvo 2015. Myosotis azorica, Boraginaceae, is endemic to Euphrasia azorica (Fig. 3), Orobanchaceae, is Corvo and the neighbouring island Flores and also endemic to the western group (Yeo 1973). It grows at 400-600 m altitude in steep and humid is an annual species (Vitek 2015) found in steep cliffs (Fig. 2). Its radially symmetric flowers are slopes, cliffs and waterfalls at 500 to 800 m up to 7 mm in diameter and arranged in groups of altitude. Its bilaterally symmetric white flowers 30-50 in dense umbel-like cymes (Schaefer with yellow spots are nototribic, i.e. place pollen 2005a). Five tiny scales narrow the entrance to the on the back of the insect. Like in most large- corolla tube, thereby reducing access for larger flowered species of Euphrasia, flower morphology insects (Weryszko-Chmielewska 2003). Like in does not support self-pollination (Liebst & many other species of the genus, self-pollination Schneller 2005; French et al. 2005). The third probably plays a certain role in this species species, Solidago azorica (Fig. 4), Asteraceae, despite its very showy attractive inflorescences occurs on all islands of the archipelago, mostly in (Robertson & Lloyd 1991). The second species, coastal cliffs and lava fields, but also in disturbed 25 Weissmann & Schaefer habitats like roadside slopes and landfills. sempervirens (Innes & Hermanutz 1988; Schaefer Restricted to coastal habitats in most islands, it 2015). Among the predominantly European flora can be found at altitudes up to 900 m on Corvo of the Azores, it is one of the few examples of and Flores. Its inflorescences are composed of up American origin (Schaefer 2015). Finally, to 400 capitulae, with about ten yellow ray and Azorina vidalii (or better: Campanula vidalii H.C. disk florets each. Solidago azorica is most likely Wats.) (Fig. 5), Campanulaceae, is another self-incompatible, like its close relative S. endangered endemic with a particularly enigmatic Fig. 3. Euphrasia azorica in natural high altitude grassland at the rim of the volcanic crater; insert shows the white two-lipped flower with yellow marking on the lower lip and anthers close to the upper lip. Photos: J. Weissmann, Corvo 2015. pollination biology: while its large and robust, species (Olesen et al. 2012) and self-compatibility pink campanulate flowers would fit best to bird is known from other Campanula species on islands