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Angraecoid Orchids and Hawkmoths in Central Madagascar: Specialized Pollination Systems and Generalist Foragers

1987; Wiley; Volume: 19; Issue: 4 Linguagem: Inglês

10.2307/2388628

1744-7429

Leif Nilsson, Lars Johnsson, Lydia Ralison, Émile Randrianjohany,

Plant Parasitism and Resistance

Studies on hawkmoth pollination in a primary forest in central Madagascar indicated that at least five of six flowering, long-spurred angraecoid species exploited the same hawkmoth, Panogena lingens, as a pollen vector. Several other long-tongued hawkmoth species were present, but no evidence was found that they participated in orchid pollination. The pollinaria of Angraecum arachnites, A. compactum, Neobathiea grandidierana, and Jumellea teretifolia were deposited on the basal portion of the proboscis. The latter three species utilized the same dorsal portion of the proboscis, whereas attachment by A. arachnites was ventral. The pollinaria of Aerangis fuscata were deposited on the frons and palps. Hawkmoths frequently carried mixed loads, but at least ethological and mechanical mechanisms seemed to restrict interspecific pollination. The P. lingens-angraecoid orchid relationship may have reached a specialized state before progenitors of other long-tongued Sphingidae colonized or evolved in central Madagascar. THE HAWKMOTH FAUNA OF MADAGASCAR contains some 60 species and is highly endemic (Griveaud 1959, Carcasson 1967). Compared to other Old World faunas, it is especially rich in long-tongued forms. Tongues of at least six species are often longer than 7 cm, and two of these, Xanthopan morgani (Walk.) ssp. praedicta R. & J. and Coelonia solani (Boisd.), have tongues that often exceed 20 cm (Nilsson et al. 1985). In the Sphingidae, a long proboscis represents a plesiomorphic state, and all Malagasy long-tongued species belong to relatively archaic genera (Rothschild & Jordan 1903). If long tongues are traits produced through processes of coevolution or diffuse coevolution (sensu Janzen 1980) in relation to food plants of adult hawkmoths, then a corresponding guild of plants with long nectar spurs or corolla tubes should exist. About 10 percent ( > 70 species) of all Malagasy orchids have spurs 8 cm or longer (cf. Perrier de la Bathie 1939-1941); hawkmoth pollination is also suggested by the floral features of many members of other plant families. Darwin (1862) predicted that the Madagascar Star Orchid Angraecum sesquipedale Thou., with a spur often exceeding 30 cm, was pollinated by an unknown giant hawkmoth. One very long-tongued candidate, discovered 41 years later, was accordingly named X. morgani praedicta (Rothschild & Jordan 1903). Thus a considerable guild of Malagasy plants exists that exploit the fuel demands of long-tongued hawkmoths. We recently described the pollination system of A. arachnites Schltr. (Nilsson et al. 1985). The present paper reports further on relations between Malagasy long-spurred orchids and hawkmoths and focuses on specialized pollination systems, shared and partitioned pollinator utilization, and species-isolating mechanisms between plants at low densities in a tropical primary forest. STUDY SITE AND METHODS The study was carried out 13-30 November 1983 in the Ambohitantely forest, 25 km northwest of Ankazobe on the Ankazobe tampoketsa, an old granitic erosion plain at 1500-1600 m altitude in central Madagascar. The forest consists of patches that remain mostly in ravines and along watercourses (Bastian 1964). These fragments of primary forest display a rich epiphytic orchid flora. We explored about 4 km2 of forest for flowering orchids and collected floral reference material of long-spurred species. Every night when the weather permitted, hawkmoths were captured at the border of the forest with a 2 5 0-W mercury vapor lamp. Total collection time was 91 hr. Some hawkmoths were stored intact, but in the majority, the proboscis was cut at the base and stored in separate tubes according to species. The proboscis of each hawkmoth individual was unrolled along a ruler, and under magnification both sides were examined for pollinaria. Position and quantity of pollen load were recorded. Pollinaria, or parts thereof, that could not immediately be identified to species were moistened, removed from the proboscis, dissected in 70 percent ethanol, and compared with reference material. Voucher specimens of hawkmoths and plants are deposited in UPS. I Received 12 October 1985, revision accepted 18 June 1986. 310 BIOTROPICA 19(4): 310-318 1987 This content downloaded from 157.55.39.104 on Mon, 20 Jun 2016 05:29:56 UTC All use subject to http://about.jstor.org/terms TABLE 1. Long-spurred orchids in flower recorded in Ambohitantely forest, and some of their general characteristics. Digits underlined in columns 2 and 5 represent the most common condition; whereas those underlined in column 6 denote the mean spur length. Approximate level aboveground of flower-proFlowering Frequency in No. of ducing indiperiod in flower flowers Length of spur Species viduals (m) the areaa observed producedb (cm) A. arachnites Schltr. 1-2-5 X-XII Abundant 1-3 10-12.7-15.5 A. compactum Schltr. 0-1-7 X-XII Sparse 1-4 13-14.5-17 N. grandidierana (Reichb.f.) Garay 2 XI 2 IC 14 J. teretifolia Schltr. 2-4-7 X-XI Sparse 1 12-13 A. articulata Schltr. 1-2 XI 2 12-19 7.5-11.4d A. fuscata (Reichb.f.) Schltr. 1-2-3 X-XI Sparse Ie 14 a Estimated from the flowering stage of each species. b In those plant individuals that produced flowers. c We have seen a pressed specimen (leg. J. P. Peyrot, Nov. 1961) in P (Paris) with several flowers. d Proximal and distal flower of one 12-flowered rachis. e Large individuals can produce several flowers (J. Stewart, pers. comm.).