Relationships of the Galapagos flora

Transcription

1 Biological Journal of the Linnean Society ( 1984) 21: With 2 figures Relationships of the Galapagos flora DUNCAN M. PORTER Department of Biology, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061, U.S. A. Joseph Dalton Hooker s pioneer 1847 paper on Galapagos plants and their relationships is a classic in the field of phytogeography. It was the first study of its kind to be published, comparing the islands flora with island and continental floras elsewhere, hypothesizing on the dispersal mechanisms of the plants, and pointing out anomalies in the inter-island distributions of the native species. These are still three of the primary concerns of contemporary phytogeographers, and the present paper contrasts Hooker s findings with those of today. Despite the accumulation of a large amount of data since his time, many of Hooker s conclusions regarding Galapagos phytogeography remain valid. KEY WORDS:-Island Darwin. biogeography - Galapagos Islands - Joseph Dalton Hooker - Charles CONTENTS Introduction Phytogeographic relationships,..,.,,,,,,,,,. 244 Dispersal Inter-island relationships Acknowledgements References INTRODUCTION Our knowledge of the vascular plants of the Galapagos Islands and their relationships has, not surprisingly, markedly increased since Charles Darwin s six-week visit on H.M.S. Beagle in Darwin was not the first to collect plants in the archipelago, having been preceded by the British naturalists Archibald Menzies in 1795, David Douglas, Dr John Scouler and James McRae in 1825, and Hugh Cuming in The first flora of the islands (Hooker, 1847a) was based on their collections, except for those of Menzies which were labelled Sandwich Islds., and a few others. Of these collections, Darwin s was by far the largest, not being surpassed until the California Academy of Science s year-and-a-day expedition in Darwin s plant collections also are the most important for typification of Galapagos endemics (Porter, 1980b). Although he played down his prowess as a plant collector while on the Beagle, Darwin did admit to his mentor the Rev. John Stevens Henslow, Professor of Botany in the University of Cambridge, that he had collected everything that he /84/ S03.00jO The Linnean Society of London

2 244 D. M. PORTER saw in flower in the islands. The 210 collections of 173 taxa that Darwin made in 1835 represent a sample of about 24% of the presently known flora. PHYTOGEOGRAPHIC RELATIONSHIPS Not only did Joseph Dalton Hooker write the first floristic study of the Galapagos Islands, he soon produced the first phytogeographic study as well (Hooker, 1847b). As with the former, the latter was also primarily based on Darwin s collections. After making the point that about half of the species are endemic, Hooker (184713: ) continued as follows. The results of my examination have been, that the relationship of the Flora to that of the adjacent continent is a double one, the peculiar or new species being for the most part allied to plants of the cooler parts of America, or the uplands of the tropical latitudes, whilst the non-peculiar are the same as abound chiefly in the hot and damper regions, as the West Indian islands and the shores of the Gulf of Mexico; also that, as is the case with the Fauna, many of the species, and these the most remarkable, are confined to one islet of the group, and often represented in others by similar, but specifically very distinct congeners. A few pages later in the paper, Hooker (1847b: 239) slightly modified his remarks on the relationships of the flora. Here, as in other countries, the vegetation is formed of two classes of plants; the one peculiar to the group, the other identical with what are found elsewhere. In this there are even indications of the presence of two nearly equal Floras, an indigenous and introduced, and these of a somewhat different stamp; for the introduced species are for the most part the plants of the West Indian islands and of the lower hot parts of the South American coast; whilst the peculiar Flora is chiefly made up of species not allied to the introduced, but to the vegetation which occurs in the Cordillera or the extra-tropical parts of South America. Here, Hooker is using indigenous or peculiar for endemic, a term not widely used in a geographical context until a few years later (DeCandolle, 1855). He uses introduced to mean non-endemic, rather than those species introduced by human activity. A further clarification is made in his introduction to a more detailed discussion of the archipelago s phytogeographical relationships (Hooker, 1847b: 250). In this second part of the essay I propose to treat of the Flora of the Galapagos as divisible into two types: these are the West Indian (including Panama), to which the plants common to other countries and the dubious species almost universally belong; and the Mexican and temperate American type, or that under which the great majority of the peculiar species will rank. These relationships, recognized by Hooker on the basis of about 25% of today s known vascular plant flora, still hold. The endemic plants have their closest relatives primarily (56%) in the Andean region (Table 1). In addition, 52% of the original introductions that have given rise to the present endemic flora presumably have come from the Andean region. The Neotropical region, which overlaps with much of the Andean region, accounts for 27% of endemic taxa and of introductions. The third-largest element is the endemics with Pantropical affinities. Hooker did not think that many species with Pantropical relationships were represented in the islands, clearly because of a lack of knowledge of the archipelago s entire flora and of these species distributions

3 8 Table 1. Geographical relationships of the endemic vascular plants of the Galapagos Islands Mexico & Neotropical Pantropical Andean C. America South America Caribbean North America Total Pteridophytes Monocotyledons 4 I Dicotyledons Total 61 (27%) 18 (8%) 128 (56%) 6 (3%) 4 (2%) 9 (4%) 3 (1%) 229 Number of single original introductions from each area 31 (27%) 10 (9%) 60 (52%) 5 (4%) 1(1%) 6 (5%) 3 (3%) 116 Geographical areas are defined as follows: Neotropical (distributed generally in the American tropics); Puntropical (distributed in both the Old and New World tropics); Andean (occurring only in western South America from Venezuela to Chile, generally or in part); Mexico and Central America (occurring only in Mexico and/or Central America, and in one case also in northern Colombia and Venezuela); South America (occurring only in extra-andean South America); Caribbean (occurring in the West Indies and often also on the edges of the surrounding continents); North America (occurring in the south-western United States and ajacent northern Mexico).

4 246 D. M. PORTER elsewhere. Those that were present he thought to be derived from South America, not from the westward. Pantropical relatives apparently provided 9% of the original introductions, which evolved into 8% of the presently known endemics. The indigenous non-endemics have their greatest numbers occurring elsewhere in the Neotropics (Table 2). Here are to be found 48% of the indigenes; 25% are Pantropical, and 21 yo Andean. The Neotropical, Pantropical, and Andean groups all overlap in adjacent South America, so it is possible that 88% of the introductions that have given rise to the endemic flora, and 94% of introductions of indigenes, have come from this nearby land mass. The importance of the Pantropical element in the Galapagos flora was not recognized until the publication of the second floristic study by N. J. Anderson (1855). Andersson s work was based on his own collections, made during a 10- day visit to the islands in Interestingly, the first to indicate the flora s relationship to adjacent South America was Charles Darwin (1839). Th is was done before Hooker had seen Darwin s collections, although Henslow had examined them. However, this insight came about through Darwin s own field observations (see Barlow, 1933), and not from information supplied by Henslow. Henslow did a superb job in handling Darwin s collections, shipped to him from South America, but he failed in his promise to identify the plants (Porter, 1980a). Subsequent studies on Galapagos phytogeography (Robinson & Greenman, 1895; Robinson, 1902; Stewart, 1911, 1915; Svenson, 1935, 1942, 1946; van Balgooy, 1960; Porter, 1976, 1979, 1983, in press) have continued to provide evidence for Hooker s hypotheses of relationship. The close relationship of the vascular flora with that of adjacent South America was first documented by Svenson (1935, 1942, 1946), and is supported by my own research (Porter, 1976, 1979, 1983, in press). Hooker s estimate of about 50% endemism still holds if all subgeneric taxa are considered. Given 541 indigenous species, subspecies, and varieties, 229 endemics and 312 indigenes, endemism would be 42%. However, at the species level, this drops to 34%, 170 endemic species out of a total native flora of 497 species. If flowering plants only are considered, species endemism is 41 yo, while that for species, subspecies, and varieties rises to 51%. The relationships of each endemic taxon are discussed in detail elsewhere (Porter, 1979, 1983), as are those of the entire flora (Porter, 1983). The first section of Hooker s paper concludes with a family-by-family discussion of geographical relationships. In it he makes the observation that the Table 2. Geographical relationships of the indigenous, non-endemic vascular plants of the Galapagos Islands Mexico & Neotropical Pantropical Andean C. America Caribbean Total Pteridophytes Monocotyledons Dicotyledons Total Percentage 48% 25% 21% 1 % 5%

5 RELATIONSHIPS OF THE GALAPAGOS FLORA 247 flora is basically a disharmonic one. That is, one comprised of a skewed sample of easily dispersed taxa and not of a random sample of all taxa in the flora of the adjacent continental area: the more an island is indebted to a neighbouring continent for its vegetation, the more fragmentary does its flora appear, migration being effected by the transport of isolated individuals, generally in no wise related, while an independent flora is generally made up of groups, the lowest order of which we call genera. (Hooker, 1847b : 247). The Galapagos flora is indeed disharmonic, as is the fauna, providing further biological evidence that it has been derived by long-distance dispersal. DISPERSAL Hooker s paper was not only a pioneering study in the geographical relationships of island floras, it also was the first to speculate on how such a flora might be derived. Several pages were devoted to a family-by-family discussion of adaptations for dispersal. Hooker (1847b : 253) concluded that The means of transport which may have introduced these plants are, oceanic and aerial currents, the passage of birds, and man. Subsequent botanists (Andersson, 1855; Robinson, 1902; Stewart, 1911; Svenson, 1942) added comments on dispersal mechanisms, but none provided as detailed observations as those of Hooker. More recently, in a study based on Stewart s (1911) flora, Carlquist (1967) determined that the flowering plants were derived as follows: 73% by bird dispersal, 23% by oceanic drift, and 4% by wind currents. With the publication of a modern flora for the islands (Wiggins & Porter, 1971), it became possible to more accurately portray the relationships of the plants and their methods of dispersal. For the vascular plants, this revealed that 40% had been derived through birds, 32% by human carriage, 21% by wind, and 6% through drift (Porter, 1976). Natural means of dispersal were 60% by birds, 31 yo wind, and 9% drift. Since the publication of the Flora of the Galapagos Islands, a series of papers have appeared, adding to and subtracting from the flora (see Schofield, 1973, 1980, for references). Addition of this information does not significantly change my previously calculated figures (Table 3). If only the flowering plants are calculated, then the overwhelming importance of bird dispersal for this group is revealed. Of the total successful introductions that have given rise to the presently known angiosperms, 48% have been by birds, 39% by humans, 7% by drift, and 6% by wind. However, if Table 3. Original introductions that have resulted in the present vascular plant flora of the Galapagos Islands Birds Humans Wind Oceanic Drift Total Pteridophytes Monocotyledons I20 Dicotyledons I Total 243 (40%) 195 (32%) 134 (22%) 36 (6%) 608 Total for natural introductions 243 (59%) 134 (32%) 36 (9%) 413

6 248 D. M. PORTER only natural agencies are considered, this changes to 79% of the introductions having arrived on or in birds, 12% floating in on oceanic currents, and 9% wafted on the wind. Taxon by taxon discussion of dispersal mechanisms is published elsewhere (Porter, 1979, 1983). Hooker was the first botanist, and the only one until recently, to recognize the importance of human impact on the islands flora. He wrote that currents, winds, and birds played their various roles in the derivation of the plants, then added (Hooker, : 254). Man is the last agent to which I alluded: that he has been already active is very perceptible from the fact, that Charles Island, the only colonized island, contains the smallest proportion of peculiar plants, and numerically far the most of these common to and probably introduced from the coast with cultivation. So far as is known at present, humans have introduced 195 weeds and escapes from cultivation that reproduce themselves in the islands. Indeed, humans are now the most important dispersers of plants, both to and within the archipelago. For example, the Pantropical (originally tropical Asian) weed Cleome uiscosa was first collected on the island of Baltra in Personal observation in 1977 showed it to be common in the vicinity of the Baltra airport, and in 1978 it formed large populations across the island. In 1981 I found it along the trail above Tagus Cove on Isla Isabela (Fig. I). Tagus Cove is usually visited by Figure 1. Cleome aiscosu (Capparidaceae), a pantropical weed introduced into the Galapagos Islands through human activity. The fruits are thickly covered with sticky glandular hairs and readily stick to clothing. It was first collected on Isla Baltra in 1963 and has since spread to Isla Isabela.

7 RELATIONSHIPS OF THE GALAPAGOS FLORA 249 tourists soon after their arrival in the islands at the airport on Baltra. Obviously, Cleome uiscosa, easily dispersed because of the many sticky hairs on the fruits, is now a prime candidate for distribution throughout the islands through the inadvertent intervention of humans. A more alarming example is that of the endemic grass, Cenchrus platyacanthus. Like other members of its genus, the fruits are enclosed in a burr-like involucre that is provided with numerous retrorsely-barbed spines, which make them easily dispersed. Presumably, they are naturally dispersed by birds. However, I know by personal experience that these disseminules are as likely to be found attached to one's trousers or socks as to the feathers of a bird (Fig. 2). In 1978 I found a specimen of Cenchrus platyacanthus growing along a path on Isla Plaza Sur, a small, well-collected island much visited by tourists, from which it had not hitherto been reported. Thus, humans are not only introducing new elements into the flora of the islands from elsewhere, they are interfering with the distributions of the endemic plants as well. Their effects on the vegetation, and those of their introduced animals, also are profound (see Hamann, 1975; van der Werff, 1979). Figure 2. Cenchrus plulyucanthus (Poaceae), an endemic grass that occurs on a number of islands in the archipelago. The fruit is surrounded by a burr-like structure that aids in dispersal. Natural dispersal is presumably by birds, but dispersal by humans is also possible. In 1978 it was found along a trail frequented by tourists on Isla Plaza Sur, a small, well-known island on which it had not hitherto been seen.

8 250 D. M. PORTER INTER-ISLAND RELATIONSHIPS Another subject which fascinated Hooker was the apparently restricted ranges of various species from island to island within the archipelago. He stated (Hooker, 1847b : 239), In the third place, I shall allude to the most singular feature in the botany of the group, the unequal dispersion of the species, the restriction of most of them to one islet, and the representation of others by allied species in two or more of the other islets. Hooker reckoned that 112 of 128 (88%) of his peculiar (that is, endemic) taxa occurred on only a single island. Subsequent investigators, particularly Robinson (1902) and Kroeber (1916), also have commented on the number of endemics in the islands with restricted distributions. This line of investigation begun by Hooker has been extended by several recent studies, which attempted to discover correlations between species numbers on the various islands and such parameters as island area, elevation, distance to the nearest island, distance to the center of the archipelago, etc. Island elevation, the area of the adjacent island, and the log of the island area were found to be significant variables in predicting species numbers (Hamilton et al., 1963; Johnson & Raven, 1973; Simpson, 1974). However, Connor & Simberloff (1978) repeated the calculations of the former investigators and, adding more precise information on island areas, elevations, and species numbers, found that only area contributed significantly to explaining variance in species numbers from island to island, a point made by Kroeber in Connor & Simberloff (1978 : 219) also showed that, The number of botanical collecting trips to each of the Galapagos islands is a better predictor of species numbers than are area, elevation, or isolation. As knowledge of species distributions has advanced through further collecting and observation, many of the single-island taxa found by Hooker have disappeared as well. According to my latest calculations, no pteridophyte, two monocotyledon, and 65 dicotyledon taxa (30% of the endemics) occur on only a single island. A majority of these taxa of restricted distribution are subspecies or varieties, and some are questionably distinct entities. These presumably are recently evolved taxa, and they provide biological evidence for the hypothesis that the islands themselves are geologically young (Porter, in press), which is confirmed by geological evidence discussed in another paper at this symposium. ACKNOWLEDGEMENTS Field trips to the Galapagos Islands in 1977, 1978, and 1981 were made courtesy of Harvard University s Friends of the Museum of Comparative Zoology. I was able to present the gist of this paper at the Linnean Society symposium Evolution in the Galapagos Islands through a travel grant from the Society. Both organizations are gratefully acknowledged. REFERENCES ANDERSON, N. J., Om Galapagos oarnes Vegetation. Kongelige Vetenskaps-academiens Handingar, 1853: BALGOOY, M. M. J. VAN, Preliminary plant-geographical analysis of the Pacific as based on the distribution of Phanerogam genera. Blumea, 10:

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