Chance Dispersal - Plant Geography - Handout | BOTANY 422, Study notes of Geography

Download Chance Dispersal - Plant Geography - Handout | BOTANY 422 and more Study notes Geography in PDF only on Docsity! ‘Sherwin Carlquist nN «| Instances of chance dispersal have been observed and reported from time to time, and many biogeogra- phers believe that this phenomenon has played an important role in the natural distribution of plants and ! animals. Yet other scientists doubt | that such occurrences as seeds or snails clinging to birds’ feathers can account for the development of the floras and faunas of entire islands, viewing these events as freak hap- penings of no significance. This paper will discuss examples of chance dis- persal; and will outline another prominent theory for the spread of organisms. Charles Darwin was interested in the phenomenon of dispersal to islands because his theory of evolution dic- tated that a given species orginated only once, and then spread thereafter. The alternate idea that a species could originate independently in various places throughout the world tequired an essentially Lamarckian explanation. Thus in Darwin’s 1859 correspondence with Sir Joseph Hooker, for example, he mentions with delight Milner’s discovery that nestling petrels on the Scottish island Sherwin Carlquist is Horton Professor of Botany at Claremont Graduate Schnol and Pomona College, and is a Research Associate 2 the affiliated Rancho Sante Ana Rotanic Garden. The author of numerous books and Papers on island biology and the comparative Anatomy af flowering plants, he has been *qually involved in botanical exploration in “ar:ous parts of the world, especially the Southern’ Hemisphere, and in taboratory Study of the materials he has collected. Ad- tress; Rancho Santa Ana Botanic Garden, Claremont, CA 9I7IE Unance Uispersal Long-distance dispersal of organisms, widely accepted as a major cause of distribution patterns, poses challenging problems of analysis of St. Kilda had West Indian seeds in their crops. In this case it seems unlikely that the petrels themselves carried the seeds from the West Indies to St. Kilda. Rather, the adult petrels may have picked up the seeds along Scottish beaches and fed them to juveniles; the Gulf Stream deposits seeds from the West Indies on the shores of the British Isles. However, this example does show the tendency of marine birds to ingest seeds, and could also explain how seeds deposited on a beach by ocean currents could reach an upland locality more suitable for their growth. Other examples of ma- rine birds consuming seeds and fruits appear in Ridley’s wonderful com- pendium of instances of seed disper- sal (1930), which lists so many ob- served occasions of dispersal of ail kinds that the chance events seem to add up to a genuine phenomenon, that of long-distance dispersal. Guppy (1906) devoted most of a vol- ume to his observations on plant dis- persal in the Pacific. He noted dis- tributions of plants in the region and attempted to find a correlation with means of dispersal. The easiest method of dispersal to test is that of flotation in seawater, and Guppy’s beok contains an overwhelming amount of data showing which seeds and fruits float, and for how long. One can only imagine that Guppy lived for years surrounded by jars of floating seeds. His experiments predictably showed that most beach plants are dispersed by seawater, and revealed similar dispersal possibilities for some inland plants, such as the vines Ent- ada and Mucuna, which must drop seeds into rivers or in some places overhang the seacoast. However, Guppy’s studies focus mainly on the dispersal of beach plants, and we must therefore look elsewhere for examples of other kinds of chance dispersal. Other documented instances of dis- persal show the surprising range and variety of the phenomenon. Small snails have been found adhering to bird feathers in a number of cases (Vagvolgyi 1975), and seeds discov- ered in bird feathers and in the mud on birds’ feet have been successfully germinated (Wallace 1895). Rand (1955) noted that the purple gallinule, known as a chance visitor to Tristan. de Cunha for many years, has finally formed breeding colonies there, and similar instances of dispersal of bird species followed by establishment have been recorded by MacDowall (1978). Wheeler (1916) reported that ants survived a journey of about 5 km in a floating log from the Brazilian coast to Sao Sebastiano Island. Monarch butterflies, which occa- sionally migrate over the Pacific, were observed to establish colonies on Canton Island by Zwaluwenberg (1942), who also noted the simulta- neous establishment of the food plants they needed. When I visited Pearl and Hermes Reef in Hawaii, 1 noticed that numerous Mucuna seeds had floated ashore. A few were ger- minating, but these soon withered in the hot sun, since Mucuna grows successfully only in wet forest. This example, like many of the preceding ones, indicates that, rare as events of chance dispersal are, adverse ecolog- ical conditions may be a greater ob- stacle to the establishment of a species in a new location than trans- port itself. The ideal site for the study of chance 1981 September-October 509 north cold temperate 4% Indo-Pacific 44% North and South America 22% HAWANT pantropical 13% ~ MARQUESAS TANT south cotd temperate 17% Figure 1. Evidence suggests that there were 272 hypothetical ancestors of Hawaii’s flowering plants, arriving from various regions. Biologists agree that both the patterns of distribution and dispersal is an island, because an island represents the ultimate test of long-distance transport. If a seed transported by wind fails to the ground, it can be picked up again, but if it falls into the sea, the dispersal event usually ends. An ideal island would be one newly emerged from the sea, not previously in contact with any land mass, and completely devoid. of any life at its time of origin. It should be large enough and high enough to support various types of life, and should be in a climatically favorable part of an ocean. The island should be well removed from conti- nents and other islands: if dispersal is too easy, one learns little. Though no such ideal site is now available for study, the island of Surtsey, near Iceland—too close to Iceland and in too unfavorable a cli- mate to have shown a great deal since its emergence in 1963—has never- theless illustrated some dispersal phenomena well (Einarsson :1967). Beach plants dispersed by drift, such as sea rocket (Cakile), were the first to arrive. aie The island closest to the ideal in-the the relationship of Hawaiian angiosperms to other angiosperm species point to long-distance dispersal as the source of the island’s flora. (Data from Zimmerman 1948.) past was Krakatau, in the strait be- tween Java and Sumatra. In 1883 Krakatau erupted with extraordinary violence. Very few species, if any, survived. Fortunately, the signifi- cance of the island was appreciated, and studies were begun soon after the eruption. Reports covering fifty years of recoyery of life on the island have been prepared, for animals by Dam- mermann (1948) and for plants by Docters van Leeuwen (1936). These studies show that both plant and animal species appeared slowly in the years following the eruption; then, as soils and plant cover formed, the rate of immigration accelerated. Later, when the number of species began to approach what might be expected on a small island in this re- gion, there were fewer new coloniza- tions. Although the’ distances tra- versed:were small—Krakatau is only 45 km from Java, and an even shorter distance from othér islands—the or- ganisms that appeared could all be said to have good dispersal mecha- nisms. The studies of Krakatau firmly support the idea that chance dispersal over long distances occurs, but since the istand is not on the scale of, say, the Hawaiian Islands, some scientists have been reluctant to concede that this phenomenon is capable of ex. plaining the colonization of larger areas. Experiments with chance dispersal Neither random observations nor data on the forms, attachment de. vices, and viability of seeds, fruits, and eggs offer as broad a picture as one could wish. Hence some biologists have attempted to test possibilities of dispersal in such a way that quanti- fiable and repeatable data can be obtained. For example, experiments have been carried ‘out on the dispersal of seeds by shorebirds. Vlaming and Proctor (1968) have shown that shorebirds.confined in pens will eat seeds, retaining them in the case of the killdeer for as long as 120 hours. Killdeer fly at speeds of 80 to 100 kph, and could thus disperse seeds over a range of more than 8,000 km in non- stop flight. Shorebirds retain large seeds longer than small seeds, so that long-distance dispersal of seeds the size of olive pits is not at all in conflict with the observed biology of these birds. Although the transport of seeds over distances as great as 8,000 km is rare, we should remeimber that the rare does occur. If such events were com- mon, the flora and fauna of each cli- matic’ zone would long since have been homogenized the world over— which is not the case. The fact that so many: Hawaiian: plant and animal species have evolved new character- istics. distinguishing: them from species in the source areas indicates that most of them were introduced in Hawaii only once, then never rein- troduced by a'second natural event of long-distance dispersal. Williams and Williams (1978) have used sources of evidence such as radar to document the large numbers o! individuals that participate in bird migrations. At Palo Alto Marsh in California as many as 11,700 migrat- ing shorebirds have been counted on a peak day; 4,000 to 5,000 individuals is not. an unusual number (Jurek and Leach 1971, 1972). Given these large bird feathers noteven: necessary,. invoke ‘numerous agents; only a:few believable. It is this rate, to ies of birds as ié8 of birds and terrestrial mammals that live at yund level near the seeds. However, mammals obviously cannot tra- yerse long oceanic distances, so the feathers of migratory birds provide a substitute animal surface where dis- rsal to islands is involved. Any plant with these adaptations that grows near the nesting or foraging grounds of shorebirds or marine birds is a likely candidate for dispersal in this fashion. Since most of these nesting or foraging areas tend to be located in lowlands rather than in wet upland forest, externally transported seeds and fruits in the Hawaiian flora are mainly found at lower eleva- tions. Herbs of North and South America Long-distance dispersal must also have operated in the case of a series of herbs—mostly annuals—found in Chile and California or nearby regions (Fig. 3). Constance (1963) and others have documented this series of plants (more than 100 species are involved), in which closely related spectes—or in afew instances the same species— occur in small pockets of Chile and North America, with enormous dis- tances in between. The great simi- larity of the species in the two regions can only mean that they have been distributed recently—probably less than 5 million years ago at most— since as time elapses the herbs will naturally evolve into quite different. species or genera. Not only must this disjunction be the tesult of a rather recent dispersal; the dispersal agent must be a good one. Humans, who are notoriously good dispersal agents, can be ruled out here, because the species involved do Rot follow the patterns of weeds or other plants known to be carried around the world by humans. Nor does tectonic plate movement seem to be a possible explanation: the pattern 's too recent, the distances too great, and the crustal movements of the farth have, in fact, worked contrary ‘othis distribution. North and South America have been moving slowly toward each other, not. diverging. The fonnection of South America to orth America via Panama has come into existence only recently, and the £ap of ocean between the two conti- "ents in the Pliocene and earlier “ould have made migration more difficult. However, at least seventeen species of shorebirds and marine birds are known to migrate over this route every year. Of these, ten shorebird species have been seen to feed on seeds, berries, or other plant material (Collins 1974). Two of the shorebird species are often seen well inland, so that distribution of inland as well as coastal plants can be attributed to birds. Moreover, during wetter peri- ods of the Pleistocene, swampy, low- lying areas attractive to shorebirds were widespread in both Chile and California. Plants now more re- stricted in distribution probably grew along the muddy shores of ponds, swamps, or even inland seas such as. the one then occupying the San Joa- quin Valley in California. Transport of seeds and fruits on or in birds can account for virtually all of the disjunctions of the Chile-Cali- fornia herbs (Fig. 4). Only a single instance, the morning glory Calyste- gia soldanella, requires another ex- planation. Although the seeds of this plant float in seawater, it would have been virtually impossible for them to have floated from North to South America or vice versa, because the currents in the intervening area would have swept floating seeds westward into the equatorial Pacific. However, the seeds could have drifted from a Pacific locality to both Nerth and South America; Calystegia soldanella also occurs in Australia, New Zealand, and Japan, as well as in the Atlantic. Figure 4. The presence in temperate South America of 106 herb species either identical with or closely related to North American species has been attributed to the migratory patterns of birds. Herb seeds or fruits with various adaptations that enhance the possi- bility of transport by birds are shown here. Species with barbed or bristly seeds or fruits, such as Cardionema ramosissima {(tup), ac- count for 42.4% of the 106 cases of dispersal. Scirpus nevadensis is a member of a group of seeds or fruits consumed by birds as fodder and later excreted in new locations; of the 106 species, 19.9% are believed to have traveled in this fashion. Viscid seeds or fruits thal become attached to birds’ feathers, here represented by Carpobrotus inequilateris, account for an ad- ditional 18.9%, while small seeds like Ortho- carpus attenuatus, easily carried in mud or sticky substances on birds’ feet or on their feathers, account for 15.1%. Less important methods of transport account for the remaining 3.7%. The fruits and seeds shown are respec- lively about 4, 2, 1.4, and 1.6 mm in size. (Per- centages are frum Carlquist, unpubl.) 1983 September-October 513 Vicariance biogeography The nonreproducible nature of chance dispersal events makes their analysis by use of statistical methods difficult, if not impossible. However, some biogeographers who have felt the need for rigorous precision have invented a methodology known as vicariance biogeography. Does this method indeed solve problems of distribution, or is it merely applicable to cases where organisms move only short distances over long periods of time? Vicariance biogeography assumes that. patterns of distribution follow geographical and climatic events such as the breaking apart of land masses—continental drift—and major shifts of wet and dry, cold and warm. If several groups of plants or animals independently show the same distribution patterns, the proposed explanation is seen as more likely. Replicate patterns have been appre- ciated since the time of Hooker (1860), who thought that the southern continents must have once been in- terconnected because of the many organisms they share. However, vi- cariance biogeography as a method- ology may be said to have begun with the writings of Croizat (1958, 1962). A lucid account of the methods of vi- cariance biogeography has been of- fered by Wiley (1980), on whose ac- count the folowing summary of pro- cedure is based. . The biogeographer first collects data on the distribution of particular plant and animal groups, and these data are then plotted on maps.to produce patterns of distribution called “tracks.” Tracks that simulate or replicate each other are now sought. A common pattern of distribution for several different groups would be called a “generalized track.” This type of activity, known as “track synthesis,” is considered a reduction of basic data. Next, the biogeographer identifies areas of endemism, or the restriction of species to particular areas. By grouping species restricted to these areas, it is possible to analyze the phylogenetic relationships of the various species in an attempt to form probable evolutionary trees for the organisms. The investigator asks two questions: What is the scheme of re- lationships among species of the or- fish © 1 2 3 1 2 3 Figure 5. In this area cladogram, the phyloge- netic relationships of two groups, fish and moss, confined to three areas of endemism are analyzed. After the ranges of the two groups have heen determined (top), possible evolu- tionary trees are formulated and compared (middie). The area occupied by each species is then substituted for the species name (bottom). in this imaginary example, there is a complete match between the phylogenetic tree of each group and that for the areas in which they ‘occur, suggesting that common factors affected the evolution and distribution of the two groups. (From Wiley 1980.) ganisms occupying the endemic areas? Do the interrelationships of the organisms reflect the geologic histories of the areas? The biogeographer then formulates various possible evolutionary trees, trying to find a match with the pat- tern of areas inhabited. To do this, the area in which a species is found is substituted for the species name in the evolutionary hypothesis. Such a hypothesis is called an “area clado- gram.” Figure 5 analyzes the phylo- genetic relationships of two groups, fish and moss, confined to three areas of endemism. Patterns will be similar to the extent that common or general factors affected the evolution and distribution of two or more groups of organisms. [n order to find such sim- ilarity, the biogeographer must be able to separate unique factors found in a single group from the common factors present in the evolution of all the groups considered. Does this scheme work in practice? Separating “unique” from “common” factors might be a troubling task. possibly involving arbitrary or intui- tive assessments. However, propo- nents of vicariance biogeography point to the compelling nature of replicated patterns, once these have been located. Even supposing that an objective or less subjective method is used to lo- cate unifying patterns, does this work for all situations? Chance dispersa: over time, widening the area occupiec by a species or suddenly causing « species to skip to new areas, woul destroy identity of patterns. If one o the species of moss in Figure 5 begin: to colonize new areas by dispersal whereas the fish do not, the general ized track would at first be altered and eventually might be obscured t« the point where an original patter: could no longer be discerned. A weed: species would, in fact,.be expected ts spread in just suth an irregula fashion. The majority of vicariance biogeo graphers concede that long-distanc dispersal exists, but they feel that i creates at most a minor “noise leve} superimposed on the basic pattern dictated by geological history, such a tectonic plate movements or the ris ing of a mountain chain. They believ that most species respond to geolog ical and climatic changes similarly and that species leave traces of thei responses in their distribution pai terns. The sum of similar response: in the opinion of these scientist: amounts to statements of statistic: likelihood and attendant verificatio or falsification. But is dispersal b plants or animals to new areas so slo or infrequent that basic patterns a) not obscured? Can this “noise leve! become so loud that no basic patter other than long-distance dispers: itself is evident? Vicarianee biogeography seems i come closest to explaining the situ tion when an organism is capable - dispersing only over very short di tances, and therefore shifts to ne ecological zones very rarely or e tremely slowly. Freshwater fish n adaptable to brackish water or se water are such organisms. It is no a ident that the most vocal proponent { vicariance biogeography, Rosen 1975), studies such fish. Those who westigate organisms capable of ing-distance dispersal and those who wdy islands or other areas that were robably populated by long-distance ispersal have not adopted the xethods of vicariance biogeo- raphy. ‘ceanic islands and areas such as wse colonized by the disjunctive erbs we have described above are not ie only places where long-distance ispersal may prevail. Many groups ft organisms characteristically excel + dispersal and are found in multiple avironments. Algae, fungi, and pro- woa are among groups with little ademism: a species found in Europe often also found in Australia or outh Africa and at any suitable lo- ation in between. Can these organ- ams, or relatively dispersible groups ich as flowering plants, ferns, in- acts, and Jand snails, illustrate the ovement of tectonic plates? Only a :w families of flowering plants have istributions that suggest continental tift (Thorne 1972), indeed, only Ider groups of flowering plants, be- ause of their time of origin, could be xpected to show such distribu- ons. : is also apparent that we may find jues to the reason for a particular istribution pattern by taking into ecount factors other than phyloge- etic trees and endemic areas. A good xample of this is seen in the plants of ye African volcanoes, which are sainly related to European plants, nd those of the Andean volcanoes, hich show a strong affinity with tants of temperate North America. ‘oth of these high-altitude environ- tents require plants adapted Lo cold. ‘ropical plants are apparently unable » make this adaptation readily be- ause of the long period of time nec- ssary for the numerous changes re- uired, whereas Europe and temper- te North America are rich in plants dapted to cold. The plants of these ¥o areas, moreover, tend to have volved good dispersal mechanisms ecause of the mountainous, discon- nuous terrain in which they live. ‘he appearance of these plants in the ew African and Andean locations iggests that these mechanisms are tpable of operating over very long istances as well as over fairly short nes. A new synthesis? Most modern investigators combine tectonic plate movement with chance dispersal in some fashion to explain present-day distributions {e.g., Raven and Axelrod 1974). The problem is not whether chance dispersal occurs, but the extent to which the patterns we see have been caused by it, and how, if such patterns occur fre- quently, we may analyze them. One possibility involving both tec- tonic plate movement and long-dis- tance dispersal should be mentioned: Was there interchange among conti- nents while they were moving apart? ‘Too often, it seems to have been as- sumed that interchange essentially ceased when the rifts were initiated. However, suitable habitats for a given organism are often distributed in a scattered fashion, and there is not much difference between habitats scattered on a single continent and those scattered on two continents still close to each other. Most families of flowering plants have patterns in which dispersal among continents after the split has been a major factor. if such a small target area as the Ha- waiian Islands has received so many successful colonists in such a short time, could not a continent receive many more? Unfortunately, the tools available to analyze and identify this phenomenon are poor, and the results are not in any range of statistical sig- nificance. The desire of vicariance biogeogra- phers for methods that can operate with precision is understandable, since biogeography is a field with lit- tle unanimity in interpretations. The refinements of geology are now mak- ing interpretations a little more cer- tain: the hypothetical land bridges once erected or obliterated with abandon by some biogeographers cannot be entertained in the hard light of newer geological evidence. However, our knowledge of any unique event in the history of life is still limited, especially if subsequent complexities destroy the pattern created by the original event. For groups like the primary-division freshwater fish, the use of the meth- ods of vicariance biogeography seems defensible, since—at least in the clearest cases—patterns are not erased or destroyed. On the other hand, patterns which circumstances tell us have probably resulted from long-distance dispersal, such as the distributions of Hawaiian organisms and of the herbs in disjunctive loca- tions, seem unlikely to yield to the methods of the vicariance biogeo- graphers—who indeed have not en- tered those areas. The biogeographer who deals with patterns probably created by long-distance dispersal must use evidence that is circum- stantial, indirect, and subjective, and therefore vulnerable. However, if that kind of evidence leads to plausible answers, we cannot afford to rule it out of court. Long-distance dispersal, although annoyingly difficult to study or take into account, appears to be a persistent theme which will not go away, and it is to be hoped that future biogeographers will find a way to in- corporate it with skill. References Carlquist, S. 1974. Island Biology. Columbia Univ. Press. Unpubl. intercontinental dispersal. Collins, R. 1974. Mechanisms for long-distance dispersal in some amphitropically disjunct. plant species. M.A. thesis, Claremont Graduate School. Constance, L. 1963. Amphitropical relation- ships in the herbaceous flora of the Pacitic Coast of North and South America: A sym- posium. Introduction and historical review. Quart. Rev. Biol. 38:109-16. Croizat, L. 1958. Panbiogeogrephy. Caracas: published by the author. 1962. Space, Time, and Form: The Biological Synthesis. Caracas: published by the author. Dalrymple, G. B., E. A. Silver, and E. D. Jack- gon. 1973. Origin of the Hawaiian Istands. Am, Sci. 61:294-308. Dammermann, K. W. 1948. The fauna of Krakatau 1883-1933. Verh. Kon. Ned. Akad. Wetensch. Afd. Nauurk., 2d ser., 44:1 594. Docters van Leeuwen, W. M. 1936. Krakatau, 1883-1933. Ann. dard. Bot. Buitenzorg 4647:2ii, 506. Einarsson, E. 1967. The colonization of Surt- sey, the new valeanic island, by vascular plants. Aquilo, Ser. Bot. 6:172-82. Gressitt, J. L., and S. Nakata. 1958. Trapping of air-borne insects on ships in the Pacific. Proc. Hawaiian Entomol. Soc. 16:363-65. Gressitt, J. L., J. Sedlacek, K. A. Wise, and C. M. Yoshimoto. 1961. A high-speed airplane trap for air-borne organisms. Pacific Insects 5:549.55. Guppy, H. B. 1906. Observations of a Natu- ralist in the Pacific between 1896 and 1899. Vol. 2: Plont-Dispersat. 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