ORIGIN AND DISTRIBUTION OF ADVENT1VE ALIEN FLOWERING PLANTS IN HAWAI`I Lyndon Wester ABSTRACT Hawai`i has more than 800 known species of alien plants that are reproducing without direct human assistance. During this century, new plant species have been arriving at the rate of about five per year. Patterns of plant collecting in time and space suggest that other species await discovery and that data on the ranges of known species are far from satisfactory. The New and Old World tropics have supplied most of the adventive alien plants in Hawai`i, although other places, particularly those with which Hawai`i has close economic and cultural ties, have contributed significant numbers in recent years. INTRODUCTION The nature and derivation of native floras have long attracted the attention of biologists, but the rearrangement and wholesale homogenization of the world's biota caused by humans in the last 10,000 years has aroused comparatively little interest until recently. Contributing reasons for this are that alien plant invasions are generally short-lived, and that populations depend upon continued disturbance, are unstable, and are determined to a large extent on chance dispersal. Furthermore, there is a built-in preference for what is natural and rare and a prejudice against the commonplace and alien among biologists, except perhaps where economic factors are involved. To protect natural ecosystems, agriculture, and even urban areas from undesirable invasion from alien plants, reliable data about the alien species present are needed. Information about how plants were introduced is helpful to plan means to limit further additions to the alien flora. Understanding generalized patterns of establishment and spread is also useful in managing problems that already exist. 99 Wester \ Origin and Distribution of Alien Plants 100 DEFINITIONS AND DATA SOURCES Considerable confusion exists about terminology applied to introduced plants and weeds (Frenkel 1984; Smith 1985), so it is important from the outset to define, in exact terms, the subject of this study. The focus of the work here will be plants introduced to Hawai`i that have shown the ability to reproduce themselves, either sexually or vegetatively, and to spread without direct human assistance. Excluded, therefore, are the introduced plants cultivated as crops or ornamentals, although it must be recognized that new recruits for the naturalized alien flora often come from the plants cultivated intentionally by humans. Weeds may be categorized according to the degree of human disturbance they require for survival (Harlan and de Wet 1965; Hart 1976). Slight disturbance, such as a small opening in the canopy of a forest, may be sufficient to ensure the establishment of some species. However, ruderals, the plants of waste places, or the weeds of crops depend upon continual disturbance. Some species may depend for their survival on humans to supply water, to act as local dispersal agents, to set or extinguish fires, or to eliminate competitors by mowing, trampling, or spraying. All alien plants that are reproducing and actively spreading, or that have done so in the past, were considered in this survey. The degree of direct or indirect dependence on humans for the survival of the populations was not taken into account. Excluded, however, were plants confined to cultivation or persisting without showing any tendency to spread after abandonment. Opinions as to whether species are disliked, welcomed as valued additions to the flora, or simply tolerated were not taken into consideration. There is no infallible way to determine whether a plant is indigenous or alien by its external characteristics. However, if a plant was not recorded in early botanical surveys, has no closely related indigenous relatives, seems incapable of dispersing over great stretches of open ocean without human assistance, and favors disturbed habitats created by humans, it seems reasonably safe to assume the species is an alien. There is substantial agreement of opinion about the provenance of most species in Hawai`i; however, there is some disagreement about a small number of species that might be indigenous, Polynesian introductions, or very early European introductions. Because many of these species have spread well beyond cultivation, this question must be addressed in more detail. Exact and reliable information on the time and circumstances of an introduction is rarely available. Furthermore, many adventive aliens were first introduced as crop or ornamental species and only later did they escape from cultivation. It is rare for escapes to be recorded. In fact, a plant may spread for many years and build up a large population before it is noticed. For example, Oxyspora paniculata was observed growing as a planted species in a forest reserve in 1958 when a collector commented, "this plant does not spread, does not seem to be a pest" (B.P. Bishop Museum herbarium specimen 58128). Only in 1984 was a serious infestation found in the same area (Obata 1985). In the course of the present study, the earliest record of each alien species was sought, regardless of the situation where it was first found. Sources for this information included Wester \ Origin and Distribution of Alien Plants 101 published floras, checklists, other written accounts, accession records of arboreta or botanical gardens, and botanical specimens. The majority of first records (72%) are from herbarium specimens. The B.P. Bishop Museum herbarium was the most important source. However, other herbaria containing the collections of 19th century botanists were also searched for plants believed to be early introductions to Hawai`i. The collections consulted were U.S. National Herbarium, Gray Herbarium of Harvard University, British Museum of Natural History, Royal Botanical Gardens at Kew, Museum National d'Histoire Naturelle, Paris, Universitatis Florentinae, Instituto Botanico, Academy of Sciences of the U.S.S.R. in Leningrad, Harold L. Lyon Arboretum at University of Hawai`i, and Department of Botany, University of Hawai`i. EARLY ALIEN INTRODUCTIONS Most of the main food and fiber crops used by the Hawai`ians lack mechanisms of dispersal that would enable them to reach Hawai`i by natural processes. Handy and Handy (1972) believed that only 14 species were actually introduced by the Polynesians (Table 1), whereas St. John (1973, 1978) and Nagata (1985) have a rather longer list of candidates. Included are weedy species, which may have been introduced accidentally, and strand trees of widespread distribution in the Indo-Pacific (Merrill 1946). The status of several other plants, conventionally considered aliens introduced by Europeans, must be reconsidered in the light of the rediscovery of collections made by the first botanists to visit Hawai`i. As there is considerable controversy surrounding these species, each will be discussed separately. `Ahu`awa, (Mariscus javanicus) The first record of ahu`awa in Hawai`i comes from the collection by Lay and Collie, botanists on the Beechey voyage, in 1827 (Hooker and Arnott 1965). The plant is commonly found on sandy beaches, in taro (Colocasia esculenta) patches, and in marshes and may be one of the indigenous, pan- tropical wetland species whose seeds were widely dispersed by waterbirds; or it could have been carried as an accidental passenger on taro corms. Kili`o`opu (Kyllinga nemoralis) Mann (1866) was apparently the first botanist to collect kili`o`opu, which was regarded as indigenous by Hillebrand (1965) and "long established" by St. John (1973). It was reported in Tahiti in 1769 by Banks and Solander (Merrill 1954) and, like Mariscus javanicus, may either be indigenous or one of those species spread accidentally by the Polynesians. Kukaepua`a (Digitaria setigera) The botanist David Nelson, who accompanied Cook to Hawai`i, was the first to collect botanical specimens of this plant in 1779 (St. John 1978). Since it is frequently found growing around pig pens and its Hawai`ian name, kukaepua`a, implies an association with pigs, it may have been carried along accidentally when the Polynesians brought the pig to Hawai`i (Degener 1962). Wester \ Origin and Distribution of Alien Plants 102 Table 1. Plants associated with Hawai`ian culture and land use. Handy & St. John St. John Nagata Handy 1972 1973 1978 1985 MAIN ECONOMIC PLANTS Aleurites moluccana * Alocasia macrorrhiza * Artocarpus altilis * Broussonetia papyrifera * Cocos nucifera 9 Colocasia esculenta * Cordyline fruticosa * Curcuma longa * Dioscorea alata * Dioscorea bulbifera * Dioscorea pentaphylla * Ipomoea batatas * * Lagenaria siceraria * * Musa spp. * Nasturtium sarmentosum * Piper methysticum * * Saccharum officinarum * * Schizostachyum glaucifolium * * Syzygium malaccense * Tacca leontopetaloides * * Tephrosia purpurea * Zingiber zerumbet * STRAND TREES AND SHRUBS Calophyllum inophyllum * * Cordia subcordata * * Morinda citrifolia * * Thespesia populnea * * ACCIDENTAL INTRODUCTIONS Digitaria setigera Indigofera suffruticosa * Ludwigia octovalvis * Merremia aegyptia * Oxalis comiculata * Urena lobata * Waltheria indica OTHER POSSIBLE POLYNESIAN INTRODUCTIONS Kyllinga nemoralis Lycopersicon pimpmettifolium Lythrum maritimum Mariscus javanicus Solanum americanum Vigtia adenantha Wester \ Origin and Distribution of Alien Plants 103 `Iniko (Indigofera suffruticosa) Hillebrand (1965) noted that Iniko was one of the most common weeds in the Islands but believed that it had been introduced in 1836 for the production of indigo. Nelson, the botanist