From Plant Press Vol. 14, No. 3 from July 2011.
By Vinita Gowda
For over a century the Caribbean region, held between North and South America, has been an active area of research for people with interests in island biogeography, character evolution, speciation, as well as geology. Most research have invoked both dispersal and vicariance processes to explain the distribution of the local flora and fauna, while ecological interactions such as niche partitioning and ecological adaptations have been used to explain the diversity within the Caribbean region. One of the biggest challenges in understanding island colonization in the Caribbean, however, has been its complex, dynamic and variable geological history, which varies both along a North-South and an East-West axis.
The Caribbean region is divided into the Greater Antilles (Northern Islands) and the Lesser Antilles (Southern Islands). The Lesser Antilles archipelago, the focus of my research interests, is 850 km long with a radius of curvature of 450 km, and consists of 19 islands. The Lesser Antilles stretches from South American continental margin (eastern Venezuela) to the Anegada Passage, which marks its boundary with the Greater Antilles (Puerto Rico-Virgin Islands platform).
Geologically, the Caribbean region is estimated to have formed in the Cenozoic era (~65 mya), following the separation of North and South America during the Mesozoic era. The volcanic islands, today’s Lesser Antilles, are proposed to have emerged from the tectonically active Aves Arc after a series of subsiding volcanic islands migrated eastward after the Aves Ridge was formed to the West. Although, the Lesser Antilles is commonly referred to as a volcanically active chain of islands, not all of the Lesser Antilles is volcanic. Based on geological origin and elevation all the islands of the Lesser Antilles can be divided into two groups: a) Limestone Caribbees (outer arc: calcareous islands with a low relief, dating to middle Eocene to Pleistocene), and b) Volcanic Caribbees (inner arc: young volcanic islands with strong relief, dating back to late Miocene).
For over more than a decade John Kress, Ethan Temeles (Amherst College) and their team of researchers have been investigating mutualistic interactions between heliconias (Heliconia: Heliconiaceae) and their sexually dimorphic hummingbird pollinators the Purple-throated Caribs (Trochilidae: Eulampis jugularis) throughout the Eastern Caribbean Islands. Based on their studies they proposed the Caribbean Heliconia-hummingbird system as a case for adaptive evolution between the beak morphology of the Purple-throated Caribs and the floral morphology of the two native heliconias (Temeles et al. 2000 Science; Temeles and Kress 2003 Science). My involvement in this project started in September 2002, or more appropriately from July 2002 when I first met John at the Association of Tropical Biology and Conservation (ATBC) meeting in Bangalore, India. At the time, I was investigating a Mussaenda frondosa-insect interaction in the Western Ghats, India and I was ready for new and bigger research challenges.
On joining the graduate program at The George Washington University in Washington, DC, in the Fall of 2002, I decided to investigate adaptation in plant-pollinator interactions using a ‘multi-island’ comparative approach using the Caribbean Heliconia-hummingbird interactions as the study system. Since I was interested in understanding factors that could influence plant-pollinator mutualistic interactions between the geographically distinct islands, I chose three strategic islands: St. Kitts in the North, Dominica in the Center, and St. Vincent to the South of the Lesser Antilles, respectively.
On all the three islands only two native species of Heliconia occur in varying abundance: Heliconia bihai (L.) L. and H. caribaea Lam. . However, floral polymorphism and abundance of H. bihai and H. caribaea is completely reversed on the three islands of St. Kitts, Dominica, and St. Vincent. In the northern island (St. Kitts) H. caribaea is common and H. bihai is rare; in the central island (Dominica) both the species are common but are mostly allopatrically distributed (commonly H. bihai can be found above ~800 m and H. caribaea below ~ 800m with a small overlapping zone around ~800 m); and a reversed distribution is observed in the southern island of St. Vincent where H. bihai is common and H. caribaea is rare. Both species have distinct color polymorphisms that vary among islands and serve as the primary nectar source for the Purple-throated Caribs.
The Purple-throated Caribs are sexually dimorphic birds that vary in their body size and bill morphology: males are 25 percent heavier than females, yet females have bills that are 30 percent longer and 100 percent more curved than males (Temeles et al. 2000 Science). Males also display territorial behavior while females trap-line. The male Purple-throated Caribs allow only conspecific females to occasionally feed on their territories in exchange for mating.
Since the breeding system of the two heliconias was not known at the time one of the main focus of my dissertation research was to investigate pollination, breeding system, and phenology of the two heliconias on each island. To further understand the role of hummingbirds in promoting outcrossing within the two species on each of the islands I also measured the inbreeding rates in both H. bihai and H. caribaea on each island using microsatellite markers that were developed specifically for both the species, and also measured pollinator effectiveness using manipulative field experiments.
Some of the interesting results of my research on the three islands are as follows: a) Male Purple-throated Caribs were indeed found to be an important and effective pollinators of H. caribaea, however their territorial displays showed significant difference among the three islands, varying from a specialized interaction on Dominica to a more generalized interaction on St. Kitts, and almost no interaction on St. Vincent; b) Female Purple-throated Caribs were the sole pollinators of H. bihai on all the three islands; c) Inbreeding rates were significantly lower in H. bihai, which was pollinated by traplining females when compared to inbreeding rates in H. caribaea, which was pollinated by territorial males thus supporting the ecological hypothesis that traplining pollinators promote outcrossing while territorial pollinators assist in reproductive success at the cost of higher inbreeding; and d) Marked territorial male Purple-throated Caribs were observed to defend the same patch of heliconias for at least 5-6 years on the island of Dominica and St. Kitts which was not known for this species of hummingbirds or for any other territorial species of hummingbird.
Although there is much to say about each of the three islands where I conducted the majority of my dissertation work here, the northern island of St. Kitts (or St. Christopher and Nevis) warrants special discussion because, despite its small size and easy access, it is biologically under-explored and offers much in terms of biodiversity and evolutionary questions waiting to be asked. The Federation of Saint Kitts and Nevis is comprised of two volcanic islands that are separated by a 3-km-wide channel called The Narrows. Comparatively, the size of St. Kitts is 1.5 times that of Washington, DC. The island comprises of a wet, rainforest on the Atlantic side while the Caribbean side of the island tends to have a drier and more xeric habitat. The Mount Liamuiga (formerly Mount Misery) stratovolcano is the highest point on the island standing at 3,793 feet (1,156 m) and one of its summits is topped by a crater lake known as dos d’ane pond. The mountain sides above ~800 m and the top is covered in an elfin woodland that is laden with moss, epiphytes, orchids, bromeliads, and aroids. Some of the unique plants that can be seen on hiking to the top of the dos d’ane pond are: Podocarpus coriceus Rich. & A.Rich, Hillia parasitica Jacq., Prestoea montana (R. Graham) G. Nicholson, Miconia mirabilis (Aubl.) L.O. Williams, Miconia laevigata (L.) D. Don, Anthurium cordatum (L.) Schott, Philodendron giganteum Schott, Begonia retusa O.E. Schulz., and Heliconia bihai. The color form of H. bihai on St. Kitts and Nevis is unique to these islands, and a population was discovered on the edge of the dos d’ane pond in 2005 with the help of a local tour guide Mr. Gregory Pereira. Interestingly, the H. bihai on this island are restricted to the mountaintops and have a very narrow distribution. Only three accessible areas were found where they were present and even among these areas the number of individuals was restricted from four to about 27 individuals.
Despite the low population sizes of H. bihai, pollinator observations showed that the Purple-throated Carib females were visiting and pollinating these few, widely scattered plants at least 2-3 times a day and against all odds of the strong cloud cover at the mountaintops for most of the day. Often, the female Caribs managed to find the opportunity to visit the heliconias whenever the cloud cover broke; sometimes this meant a mere 5-10 minutes break between cloud covers. On the island of Nevis, the scattered and very restricted population of H. bihai was found on the trail leading to the top of the Nevis peak between 800 -1200 m. Both the climb to the top of dos d’ane pond and Nevis peak is a treacherously steep trail that can get quite slippery due to the moist cloud cover that is persistent throughout the day.
The largest distribution of H. caribaea on the island of St. Kitts was found in two localities around the town of Molineaux: the rainforest trail on the south-east side of Mt. Laiamuiga or the Phillips level and behind the Ottley’s plantation trail. On Nevis, H. caribaea were most easily found around the trail leading to the Nevis peak, although the plants were also reported from other sides of the Nevis peak. Heliconia caribaea are abundant on both islands and both red and yellow morphs are found on both St. Kitts and Nevis. Despite the small sizes of both these islands, the high relief of these islands (especially St. Kitts) and significant difference in weather from the Caribbean to the Atlantic sides results in a world of difference in flora and fauna that can be found on each side of these islands, and thus provides an ideal natural experimental garden where one can investigate adaptive differences in plants colonizing these islands.
Although my doctoral dissertation research focused on only three of the Lesser Antillean Islands, from 2004 to 2009, I along with John Kress and other colleagues also had the opportunity to collect and study heliconias on many other islands such as St. Eustatius, Saba, St. Kitts and Nevis, Montserrat, Guadeloupe, Dominica, Martinique, St. Lucia, and St. Vincent. In general, strong efforts were invested in collecting vouchers, observational data, population samples, and morphological data on all the heliconias from the other Caribbean Islands too. This has resulted in a very extensively collected, fine-scale meta data of variations in morphological characters in the two heliconias throughout the Caribbean Islands and has great potential in future investigations of character evolution (especially floral) and adaptations between plants and their pollinators.
Ecological studies of the kind that was carried out during my dissertation is not common among scientists in the Natural History Museum; however, the results from this study highlights the importance of exploring ecological studies along with population genetic and taxonomic studies to understand the diverse tropical interactions that define a tropical rain forest. One of the broader outcomes of this study has also been the research exchange between the local forestry divisions on the islands of St. Kitts, Dominica, and St. Vincent and Smithsonian Institution where the local hosts have been significantly important partners in facilitating our research in the Caribbean Islands. The local forestry divisions and concerned citizens not only view the unique Heliconia-hummingbird interaction as a source of national pride unique to their island but have also included it as part of their conservation programs in the wake of concerned developments within the islands.
Two websites proved to be useful in my research: (1) a catalogue of plants on each of the Caribbean Islands; (2) The Global Volcanism Program database for the Caribbean Islands, with thermal activity for 17 volcanoes.