Experimental Zoogeography of Islands: The Colonization of Empty Islands

Daniel S. Simberloff; Edward O. Wilson

Ecology, Vol. 50, No. 2. (Mar., 1969), pp. 278-296.

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http://www.jstor.org Tue Aug 28 10:40:06 2007 278 DANIEL S. SIMBERLOFF AND EDWARD 0. WILSON Ecology, Val. 50, No. 2 ACKNOWLEDGMENTS Hermannsson, S. 1967. Introduction. Surtsey Re- We thank the IJnited States Department of the Interior search Progress Report, I11 : 1. for permission to fumigate the islands of this experiment. LaRue, D. C., and T. T. Muzik. 1954. Growth, re- In particular, Jack Watson of the Great White Heron generation and precocious rooting in Rhizophora man- National Wildlife Refuge, Roger W. Allin and Willianl gle. Pap. Mich. Acad. Sci. Arts Lett. (I) 39: 9-29. B. Robertson of the Everglades National Park, and Lauck, D. R. 1965. Chloropicrin for fast action with Robert M. Linn of the National Park Service examined a Berlese funnel. Turtox News 43: 115. the plans of the experiment and concluded, with the au- Lindroth, C. H., H. Andersen, H. Bodvarsson, and S. H. thors, that the total mangrove faunas would be unaffected. Richter. 1967. Report on the Surtsey investigation Steve Tendrich, Vice-President of National Extermina- in 1966. Terrestrial invertebrates. Surtsey Research tors, Inc. (Miami) coped imaginatively and effectively Progress Report, 111 : 59-67. with the unique problems presented by the project. W. H. MacArthur, R. H., and E. 0.Wilson. 1963. An equi- Bossert and P. B. Kannowski assisted in the original librium theory of insular zoogeography. Evolution censuses. Maps and tables for all parts of this series 17: 373-387. were done by Mrs. Helen Lyman. The work was sup- MacArthur, R. H., and E. 0.Wilson. 1967. The theory ported by NSF grant GB-5867. of island biogeography. Princeton University Press. 203 p. Mainwaring, A. P. 1961. Some effects of methyl bro- Chisholm, R. D. 1952. Nature and uses of fumigants, mide on aphids and whitefly and their host plants. p. 358. Ift , The Yearbook of Agriculture 1952. Doctoral thesis, London Univ., London, England. Darlington, P. J. 1957. Zoogeography: the geographi- Monro, H. A. U., and R. Delisle. 1943. Further appli- cal distribution of . Wiley, New York. 675 p. cations of methyl bromide as a fumigant. Sci. Agr. Davis, J. H. 1940. The ecology and geologic role of mangroves in Florida. Papers from Tortugas Lab., 23: 546-556. Carnegie Inst. Wash. 32: 307-412. Page, A. B. P., and 0. I?. Lubatti. 1963. Fumigation French, R. A. 1964 (1965). Long range dispersal of of insects. Ann. Rev. Entomol. 8: 239-264. insects in relation to synoptic meteorology. Proc. Simberloff, D. S., and E. 0. Wilson. 1969. Experi- Int. Congr. Entomol. 12th (London). 6: 418-419. mental zoogeography of islands. The colonization of Fridriksson, S. 1967. Life and its development on the empty islands. Ecology 50: 278-296. volcanic island, Surtsey. Proc. Surtsey Research Wolfenbarger, D. 0. 1946. Dispersion of small orga- Conf., 1967: 7-19. nisms. Amer. Midland Naturalist 35: 1-152.

EXPERIMENTAL ZOOGEOGRAPHY OF ISLANDS : THE COLONIZATION OF EMPTY ISLANDS

DANIELS. SIMBERLOFF~AND EDWARD'0. WILSON The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138 (Accepted for publication December 16, 1968)

Abstract. We report here the first evidence of faunistic equilibrium obtained through controlled, replicated experiments, together with an analysis of the immigration and extinction processes of species based on direct observations. The colonization of six small mangrove islands in Florida Bay by terrestrial was monitored at frequent intervals for 1 year after removal of the original fauna by methyl bromide fumigation. Both the observed data and climatic considerations imply that seasonality had little effect upon the basic shape of the colonization curves of species present vs. time. By 250 days after defamation, the faunas of all the islands except the most distant one ("El") had regained species numbers and composition similar to those of untreated islands even though population densities were still abnorn~allylow. Although early colonists included both weak and strong fliers, the former, particularly psocopterans, were usually the first to produce large populations. Among these same early invaders were the taxa displaying both the highest extinction rates and the greatest variability in species composition on the different islands. Ants, the ecological dominants of mangrove islands, were among the last to colonize, but they did so with the highest degree of predictability. The colonization curves plus static observations on untreated islands indicate strongly that a dynamic equilibrium number of species exists for any island. We believe the curves are produced by colonization involving little if any interaction, then a gradual decline as inter- action becomes important, and finally, a lasting dynamic equilibrium. Equations are given for the early immigration, extinction, and colonization curves. Dispersal to these islands is predominantly through aerial transport, both active and pas- sive. Extinction of the earliest colonists is probably caused chiefly by such physical factors as drowning or lack of suitable breeding sites and less commonly by competition and predation. ' Present address : Department of Biological Science, Florida State University, Tallahassee, Florida 32306 Early Spring 1969 COLONIZATION OF EMPTY ISLANDS 279 As population sizes increase it is expected that competition and predation will become more important. Observed turnover rates showed wide variance, with most values between 0.05 and 0.50 species/day. True turnover rates are probably much higher; with 0.67 species/day the extreme lower limit on any island. This very high value is at least roughly consistent with the turnover equation derived from the MacArthur-Wilson equilibrium model, which predicts turnover rates on the order of 0.1-1.0 species/day on the experimental islands.

1NTRODUCTION intervals, as in our monitorings, every species In the first article of this series (Wilson and for which at least one propagule exists is desig- Simberloff 1x9) we showed how the recent for- nated a colonist. It is also designated an immi- mulation of mathematical biogeographic theory grant if it was not a colonist at the preceding has both intensified the need for studies of the count. Every immigrant is also, by definition, a entire colonization process and defined the mea- colonist. This definition says nothing about surements reauired-for such studies. The idea whether food and a breeding site exist; a species was conceived of approaching the problem experi- whose propagule lands on one of our islands is a mentally by the removal of entire faunas colonist even if it is doomed to quick extinction from series of small islands. Six very small man- for purely physical reasons (e.g., the absence of grove islands of the Florida Keys, each consisting a suitable nest site in the Rhizophora for a given of only one to several Rhizophora mangle trees species of ant). standing in shallow water, were selected. Elimi- Because of the relative nearness of our experi- nation of the faunas("defaunation") was achieved mental islands to source areas and, to a lesser through fumigation with methyl bromide; and extent, their small size and ecological simplicity, techniques were worked out for censusing the ambiguities concerning the state of colonization arthropod species during the recolonization pro- exist that would not arise if we were dealing with cess. truly distant and larger islands. Except for a few In the present article we discuss the criteria birds, any animal species for which a propagule we used for counting species and describe the is recorded either avoids leaving the boundary recolonization process on all six islands, from the of the island or (much more rarely) leaves and moment of defamation to the reattainment of equi- perishes in the sea. The island. therefore. is not librial numbers of species less than a year later. simply an extension in some sense of the mainland. In our experiment a small percentage of the animal species, less than 10% of all species sighted, behave as though the distances to the experimen- For the species counts and discussion which fol- tal islands are not qualitatively different from the low these definitions will be used: same distances overland. We wish to discount Propagule: the minimum number of indi- these species in our calculations, unless insularity viduals of a species capable of breeding becomes important in particular instances. Two and population increase under ideal con- classes of species can be recognized in this con- ditions for that species (unlimited food nection. Several kinds of insects (cicadas, odo- supply and proper habitat, no predators, nates, foraging bees and wasps) treat small man- etc.) grove islands as part of a fine-grained foraging Colonization : the existence of at least one area, traveling readily and frequently among sev- propagule of a species on an island eral islands and adjacent shore regions. A species Extinction : the disappearance of a species of the wasp genus Polistes forages regularly over from an island small mangrove islands but rarely nests there. Invasion: the arrival of one or more propa- When nesting does occur the wasps apparently gules on an island restrict their foraging largely to the nesting island. Immigration: the arrival of a propagule on Only an extant nest qualifies the Polistes as a an island unoccupied by the species colonist. In similar cases actual breeding, rather than just the presence of sufficient animals to breed The distinction between invasion and immigration under the most favorable conditions, was employed should be noted. It is incorrect to speak of an as the criterion for colonization. Transient adult immigration rate for one species, since an itnmigra- butterflies, particularly Ascia monuste and Plzoe- tion rate for an island is in units of species/time. bis agarithe (Pieridae), migrating over and be- A species can have an invasion rate on a given yond the experimental islands, occasionally skim island, however; this is simply the number of briefly through but do not stop to breed. These propagules of that species landing per unit time. will not be considered colonists for reasons similar In analyzing species counts made at discrete to those used to discount fine-grained foragers. 280 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON Ecology, Vol. 50, No. 2 A second difficulty associated with the nearness TABLE1. Arthropod community of intermittently sub- of our islands concerns intermittent breeding by merged mudbanks on small mangrove islands strong-flying insects combined with continuous foraging by adults. Females of the moth Auto- INSECTS CoUembola wzeris io (Saturniidae) fly frequently onto small Poduridae: gen. sp. mangrove islands and occasionally deposit eggs. Dermaptera Labiduridae: Labidura riparia The life cycle from egg to adult of Autovneris on Coleoptera Rhizophora is about 2% months, and if (as may Carabidae: Bembidion sp. nr. contractum have happened on E3) adults breed on an island Tachys occulator Corylophidae: Anisomerisles sp. at intervals greater than that, extinction rates Ptiliidae: Actinopteryx fucicola would appear to be high. For after a brood ma- Staphylinidae: gen. sp. tures, the survivors generally a11 disperse from Saldidae: Pentacora sphacelata the island and no new adults may breed there for a period. This is obviously not extinction in its OTHER Acarina classical sense-it is not caused by competition for Veigaiaidae: Veigaia sp. food or space, predation, climatic catastrophe, etc. Isopoda -but it does accord with our strict definition given Oniscoidea: Ligia exotica above. This situation was fortunately rare, almost entirely restricted to a few lepidopterans. Species only one whose energetic interaction with the man- of this type will be considered colonists, with one grove and its arboreal fauna is significant. All immigration only and no extinction, until a defini- tive and extended absence is recorded. The be- the others apparently feed on seaweed or washed- up detritus, or else prey upon those which do. havior of the very few nesting birds would place Consequently L. riparia alone is considered a them in this category but they were nevertheless colonist. discounted in our analysis. The tree snail Littorina angulifera and tree crab A few arthropods live in and among Rhizophora Aratus pisonii inhabit all but the upper canopy of roots at or below the water level, some foraging on mud at low tide. These include the isopod small mangrove islands but will not be counted for the following two reasons. Neither can be Ligia exotica, an unidentified amphipod, and three removed-Littorina is unaffected by 50 kg/1000 insects : Trochopus pluuvtbeus (Hemiptera : Velii- m3 of methyl bromide for 3 hr, and Arntlds simply dae) , Axelsonia littoralis (Collembola : Isotomi- drops to the water and may swim under the tent- dae) , and Anurida uvtaritiuvta (Collembola : Po- and both have planktonic larvae. Again, the in- duridae). These will be excluded from the teraction of these species with the remainder of species counts because they are essentially part of the arboreal community appears superficially not a surface marine community and apparently do to be significant. not interact significantly with the arboreal man- Our definition of a propagule dictates that ani- grove fauna. All are ubiquitous around small mals with zero reproductive value (e.g., a male mangrove islands and cannot be eradicated with ant landing on an island after a nuptial flight) certainty. not be considered colonists. The inlpermanent mudbanks on E8 and E9 Although birds were not counted here, bird (Wilson and Sinlberloff 1969, Fig. 2) that remain parasites which establish breeding populations on wet but above water for several weeks in calm the islands rather than wholly on the birds were weather harbor a characteristic marine arthropod listed as colonists. Specifically, the hippoboscid community, listed in Table 1. Most of the spe- flies Olfersia sordida and Lynchia albipennis cies are concentrated in washed-up and wet debris (whose puparia are con~monly found in tree and algae. That all but the earwig Labidurn crevices) and the tick Argus radiatus (all stages ripnria are virtually marine and do not breed on of which live under dead mangrove bark) were the islands is indicated first by their never having counted. been collected on Rhizophora (even when the mud is submerged and debris washed away by Finally, except for the rare larvae and pupae, wind-driven high tide), second by most species all Diptera were excluded. Monitoring of flies having been observed swimming from one patch proved too difficult to warrant faith in the accu- of mud or debris to another, and finally by the racy of species counts, and the extreme scarcity of swift recolonization by large populations of most immature stages indicates that small mangrove species observed after extinction caused by ex- islands rarely support breeding dipteran popula- tended flooding. L, riparia is the only species tions. found on the islands proper, arid it is probably the Deep-boring beetles were deemed valid colonists Early Spring 1969 COLONIZATION (IF EMPTY ISLANDS 281 in this series. The uncertainty of extinction dis- TABLE2. General rules applied in interpolation of colo- cussed in our first report (Wilson and Simberloff nists 1969) notwithstanding, the data imply that the Number of few initially surviving cerambycid larvae were cycles / Animal I destroyed by a delayed effect, and that the weevils interpolated Justification may have succumbed in the same way. In any Ants: 2 (previous) Conservative on physiological workers seen first event there are but four species involved, and I grounds rarely were more than two found on a single Ants: queen indefinite Obvious for short periods. Data island. seen first of E9 support for longer periccs. Acceptable colonists were counted conserva- Deep-boring 2+ Present in relatively low density tively. In all instances of uncertainty about the beetles and only 10% of twigs broken at number of species of a given taxon present, the each monitoring. minimum possible number is taken. For example, Small leaf 1-2 Inconspicuous, hut densities occasionally the records from one census revealed dwellers usually incresae rapidly. two thrips, Neurothrips magnafemoralis and Lio- Barkdw~llers 1 Often become dense quickly, and thrips sp. (both Tubulifera), as well as larval halitat examined completely. thrips identifiable only to the suborder Tubulifera. Araueids 1 Spiderlings may be minute, but Since the larvae could conceivably be ascribed to webs are conspicuous. one of the two species known present, only two 2 Position of webs makes less species are recorded for this period. Similarly couspicuous than araneids. the observations on El, (May 29, 1967) of a moth caterpillar Bema ?ydda (Phycitidae) and a small Salticids 2-3 Furtive, often inconspicuous, and , usually present in low densities. adult moth similar to Bema but seen too briefly to be so recorded with certainty yield a species Anyphaenids 1 Build up relatively high densities count of one only, since both individuals could quickly. belong to one species. Small 2 Usually low densities; difficult The presence of an immature animal need not crawlers to record. imply breeding on an island; spiderlings balloon Fliers 2 Frequently conspicuous, but may more readily than adults, and any larva be inactive because of weather. could be blown or rafted to an island, although for Caterpillars 1 (usually) NOgeneralization possible; large some, of course, the probability of this is quite onea are conspicuous. low. Similarly, an adult female does not consti- tute a propagule or part of one if she has not been fertilized, is not of a parthenogenetic species, and stead of March would the tables and derived no male is present. Nevertheless, we assume here curves be qualitatively different ? that an adult female, an adult of indeterminate This would obviously be so in much of the sex, and an immature animal each imply the pres- United States, where dispersal stages of most in- ence of a propagule. Adult males are not so sects and spiders occur at short, distinct periods counted. (usually in the summer). The Florida Keys are In the Appendix are given the complete re- subtropical, however, with the mean temperature corded histories of all of the colonists on El, E2, of the coldest month (20.g°C) only 7.7"C lower E3, ST2, E7, and E9, the six islands whose en- than that of the warmest month. Frost has never tire faunas were removed by fumigation (see Wil- been recorded. The mean humidity of the driest son and Simberloff 1969). These records are month is but 8% lower than that of the most based on direct observation in over 90% of the humid. Rainfall is less homogeneous, September- cases. In certain instances (hatched bars) ani- October averaging about 150 mm and December- mals were assumed present through one or more January only 38 mm. The precise amount for all motlitoring cycles when not actually observed. To months is quite variable, however, and there are ensure consistency in such interpolations the rules obviously no extreme dry or wet seasons. Wind given in Table 2 were followed. is also relatively constant over the year in both speed and direction: it averages about 18 km/hr from the eastern quadrant. It is not surprising, We must first discuss whether any aspects of therefore, that we found most of the mangrove the colonization depicted in the Appendix (and arthropod species active throughout the year. All Pigs. 1-3) are artifacts of the particular season life stages, including dispersal forms, of many and at which defaunation was performed. That is, if probably most species of insects and spiders were all the islands were fumigated in September in- present during every month. We can make no 282 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON Ecology, Vol. 50, No. 2 quantitative assertions, but it seemed to us that there were no striking seasonal decreases among the more abundant mangrove inhabitants in either population size or activity, including flight-ex- cept for mosquitoes, which were much more nu- merous in the summer. We had the good fortune to be present Feb- ruary 26, 1967,when the temperature at Key West fell briefly to 9.S°C, the lowest reading in several years. Our surveys during that cold spell re- vealed no apparent mortality or even great lessen- ing of activity of mangrove inhabitants other than a decrease in flight activity quite normal for the prevailing wind speed. Finally, and most importantly, the conclusion that propagules were constantly hitting our islands DAYS is incontrovertible from the data summarized in FIG.3. The colonization curve of island E9. the Appendix. Moreover, these data provide no clear indication of seasonality in the dispersal of any taxon. The colonization of E7, defaunated approximately half a year before the other islands,

'~~-defoun~tionsurveys . ~inaimonltorinp should certainly have manifested any strong sea- h - I sonal component. Yet in both form and specifics (Fig. 2 and Appendix) it is consistent with the colonization of the other islands.

As would be expected in a system involving but one plant,. succession in the usual sense, a progression of discrete and relatively stable com- munities, did not occur. This does not imply a lack of order in the time course of colonizatibn: indeed, the invasion of species was remarkably regular. But it was not accompanied by wholesale extinction of distinct animal associations.

DAYS Several broad patterns are nevertheless evident. FIG. 1. The colonization curves of the experimental First, although the earliest immigrants on all is- islands in series 1. In each curve the last and next-to- lands included both strong fliers (especially moths last census points are not connected by a line, only in and wasps) and weak fliers or nonfliers (particu- order to stress the greater period of elapsed time com- larly psocopterans, chrysopids, and spiders), the pared with the times separating earlier censuses. latter built up large populations more rapidly and became numerically dominant. That wind should transport many of the early invaders is not sur- prising. The first animal recorded on Krakatau after its eruption was a spiderling (Cotteau 1885), while psocopterans (including wingless nymphs) and spiders are prominent in aerial plankton sam-

Fmol mon>lormg ples (Glick 1939). But their success in coloniza- -+ -C tion deserves further comment. The food supply for psocopterans, algal and lichen growth on the mangrove itself, is evidently sufficient to allow much larger populations than one normally finds on undisturbed Rhizophora islands. This implies that on untreated islands there may be predation by animals not present on recently defaunated is- lands. In fact, we have observed the ants Pseudo- 60 120 I80 240 3W 360 420 480 510 DAYS vnyrmex elongatus and Pseztdovnyrwtex "flavidula" FIG.2. The colonization curve of island E7. (both of which colonized later) carrying appar- Early Spring 1969 COLONIZATION OF EMPTY ISLANDS 283 ently freshly killed psocopterans to their nests. and Galz~g+tna sp. were omnipresent and their Running and jumping spiders can, of course, eat populations long-lived. An apparent correlation the psocopterans. Maezhia vittata (Salticidae) exists in the spiders, mites, and wasps between was seen catching a Psocidus texanus adult on E7, mean length of persistence and number of islands while the webs of even small araneid spiderlings colonized. This relation, however, may be arti- commonly trapped transient flies, especially tipu- factual, for the following reason. If two species, lids and ceratopogonids, in the same size range as the first with a very high, the second with a very psocopterans. low initial probability of extinction (long expected Although it involves only a relatively small sub- persistence time), invaded all six islands with set of the entire Florida Keys fauna, colonization equal frequency, we would expect to see the former by these early weak fliers was more variable than on one or two islands only and the latter on most by other classes of arthropods, both in time of or all of them. arrival on individual islands and s~eciescom~osi- Thrips, lepidopteran's, orthopterans and ants dis- tion at any given time among islands. The pso- play a regularity of colonization in sharp contra- copterans were particularly unpredictable; from distinction to the relatively unordered patterns of the 2d month after defaunation there were usually colonization previously described. The last three 1-4 species on each island at any census period, groups, particularly the ants, mount the largest but over the course of the experiment a total of populations in undisturbed mangrove animal com- 24 species were involved. Although certain pso- munities. copterans, especially species of Psocidus and Only five species of thrips colonized, four of Peripsocus, were generally more prominent than which were widespread among the islands. Al- others, there was little correlation among the sets most all invasions occurred 4-5 months after de- of psocopterans found on different islands. Fur- faunation. Most thysanopteran colonizations en- thermore. as can be seen from the Corrodentia dured for at least 3 months. Large populations (= Psocoptera) sections of the Appendix, many were occasionally produced but rarely persisted- psocopterans persisted for less than a month. As the late colonizing ants may have attacked thrips. a group they invaded and multiplied readily, and There was no consistent order of colonization: became extinct almost as readily. On E7, only all species commonly immigrated about the same 5 species of the 15 colonists remained as long as 2 time. months. Only 8 lepidopteran species colonized the ex- Spiders as a group were less variable than perimental islands, of about 30 species known psocopterans in their colonization pattern but much from mangrove and a few hundred from the gen- more so than that of most later colonists. Al- eral Keys fauna. (This figure does not count the though the majority of the 36 spider species which 2 or 3 fine-grained foragers discussed earlier.) colonized the islands followed the pattern just All eight were recorded more than once, and six described for the psocopterans-that is, they im- were widespread. Most colonizations were sus- migrated readily and were extinguished quickly, tained and many involved sizable populations. and occurred on but 1 or 2 islands-a few species There was a somewhat predictable order of inva- behaved quite differently. Eustala sp., Tetragnatha sion, with Phocides baiabano, Bewta ?ydda, and sp.. Leucaztge venusta, Hentzia palwtaruwt, and Ecdytolopha sp. usually the first arrivals, Newta- Ayslza velox in particular, colonized most of the pogon sp. appearing somewhat later, and Alarodia islands and usually persisted for at least several slossoniae and Automeris io usually not seen until months. These include most of the spiders found about 200 days. on the islands before defaunation. The orthopteroids colonized still more pre- Wasps present a similarly heterogeneous picture dictably. Of approximately 25 species that occur of colonization, many species appearing on one in mangrove swamps and 60 or more that occur or two islands and vanishing rapidly while a few, in the Keys as a whole, only 9 invaded the experi- notably Pachodynerzts nasidens, ~cleroderwtamu- mental islands. If the two very near islands (E2 crogaiter, and ~alliephialtesferrugineus, coloniz,ed and E7) are discounted, only four species were many islands and persisted for long intervals. involved and all have multiple records. These Mites did not invade as early as did wasps and four were rarely extinguished; two, the green tree spiders, but displayed the sake pattern of many cricket Cyrtoxipha confusa and roach Latiblattella short-lived species, a few persisting and recurring. n. sp., produced large populations. All appeared Most of the 20-odd species of Acarina were re- capable of early invasion. corded from one or two islands only and dis- The ants displayed the most orderly pattern of appeared within a month, while Awzblyseius sp. colonization. These insects are also nun~erically, 254 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON Ecology, Vol. 50, No. 2 TABLE3. Colonization of experimental islands by ants The order of colonization by ants of the experi- mental islands (Table 3) provides a curious zoo- Island El E2 E3 E7 E9 ST2 geographic analog of Haeckel's biogenetic law. ------Ant species In almost every instance Crewzatogaster ashmeadi before was the first colonist, even on the two near islands defaunationa 5 3 5 1 4 5 (E2 and E7) where it was not present before 6 8 6 2 8 6 11 defaunation. On all but the most distant island 11 11 11 11 11 (El) Pseudowzyrwzex elongatus was an early and l3 l3 l4 l2 l3 prominent colonist. Moreover, the small subset 14 13 14 - - of the mangrove ants which comprised the re- Ant colonists mainder of the colonists was almost identical to in order of colonization 6 that found on small islands as one moves nearer to {I 1 1 { .b the source areas. only 3 extinctions were ob- 11 14 1 l1 served, and 2 were of species believed unable to [ nest in red mangrove forests. As a group, the 11 { {::14 14 13 ants colonized later than most other taxa and at 12 the close of the first phase of our study (April 1968) few species had built up populations nu- aSpecies are coded a? follows 1 = Camponotus~orz~nus 9 = Paracryptocerus varzans inerically similar to those on untreated islands. 2 = Camponotus plannt~s 10 = ParatreJlzna bourbontea 3 = Camponotus tortuganus 11 = Pseudomyrmez elongat~s 4 = Camponotus sp. 12 = Pseudomyrmex"~avzdula" 5 = Camponotus (Colobopszs) sp 13 = Taptnomn lzttorale COLONIZATIONCURVES G = Crematogaster ashmeadz 14 = XenomyrmexfEorzdanus 7 = Cremtogaster atkznsonz 15 = Brachymyrmex sp. Figures 1-3 show the original numbers of spe- 8 = Monomorzum Rorzcola 16 = Hypoponera opuczor b indicates later ext~netion. cies present and the colonization curves (number of species present vs. time) for the experimental and probably energetically, the dominant animals islands. on all small mangrove islands. Of the more than Estimation of the number of species present be- 50 species found in the Keys, 20 species inhabit red fore defaunation and after regular monitoring had mangrove swamps and about 12 of these normally ended is complicated by two factors. First, as occur on small islands. The pre-defamation sur- can be seen from the data presented in the Ap- veys revealed a highly ordered fall-off of ant spe- pendix, a few colonists are inferred present at each cies in two directions. First, on islands of equal regular post-defamation monitoring period with- size but varying distance from source area, the out actually having been observed because they most distant islands contain Crewzatogaster ash- were recorded at both preceding and subsequent meadi; those somewhat nearer, both the Crew- periods. Since there was but one pre-defaunation togaster and Pseud~wzyrwzex elongatus; those survey, its total species number must be increased nearer still, these two species plus Paracryptocerus by the mean number of species inferred present varians, Tapinol~zalittorale, and Camponotus ( Co- without having been observed for all regular post- lobopsis) sp. ; and on islands near shore, most or defaunation monitorings after an approximate all of the above species plus one or more species equilibrium had been reached. The last four of Camponotus, Pseudowzyrwzex "flavidula," Mo- regular monitorings were used for this purpose, nomoriuwz floricola, and Xenomyrmex floridanus. since the population structure then was probably If instead one fixes a distance (usually small) closest to that before defaunation. A similar cor- from the source area and examines islands of rection must of course be made for the final cen- increasing size, he generally finds on the smallest sus, after regular monitorings had ended. In bush (ca. 1 m high) Crewzatogaster ashwzeadi; addition to this correction, the pre-defamation on slightly larger bushes, C. ashmeadi and/or surveys of El, E2, E3, E6, and E7 were believed Pseudowzyrwzex elongatus; on small trees these deficient for leaf fauna. Since this habitat nor- two with perhaps two species drawn from among mally harbors 3 to 5 species on all islands, it was Paracryptocerus, Tapinovna, Colobopsis, Xeno- assumed that a total of 4 such species were present myrwzex, and Monowzoriuwz; and on islands the on each of these islands, though the actual num- size of our experimental ones, the full comple- ber seen was 1 to 4. Figures 1-3 do not include ment expected on an island of the appropriate a correction for unseen short-lived species, esti- distance from source. In short, the ability to mated at about 2 species per monitoring period but colonize increasingly smaller islands parallels with very high variance (Simberloff 1969). closely the ability to colonize increasingly distant The numbers of species recorded on the control ones. islands before and after the experiment were : Early Spring 1969 COLONIZATION OF EMPTY ISLANDS 285 before after time (Wilson and Taylor 1%7), but we will ne- glect this effect because the time course of such E6 30 28" a change is obviously beyond that of this experi- El0 20 23 ment. Also, the invasion rates for most species * plus 9 spp. in bird nests only on our islands are probably too high to allow sig- These cannot be corrected for unseen colonists, nificant genetic alteration to occur in populations but show conclusively that no important long-term on individual islands; they are not isolated in an effect was operating in the Keys that might have evolutionary sense. distorted the results from the defaunated islands. The curves of Figs. 1-3, except for that of El, Although the numbers of species on the control are believed best explained by the following equa- islands did not change significantly, the species tion, based on a model devised by W. H. Bossert colnposition varied considerably, implying that the and P. N. Holland: number of species, S, approaches a dynamic equi- librium value. 3. The most apparent implication of the coloniza- tion curves and other information presented so far is that this equilibrium .!? does exist. Three where E[S(t)] = the expected number of colo- lines of evidence are relevant. First is the fact, nists present at time t just mentioned, that S on the control islands did P = number of species in the pool not change greatly from the beginning to the end i, of the year-long period in which the nearby ex- = invasion rate of species a perimental islands were being colonized. Second e,= intrinsic probability of extinction for spe- is the static observation that untreated islands cies a with similar area and distance from source have The derivation of this equation and a discussion similar .$ (note E3 and ST2 before defaunation). of the concepts involved will be presented else- Perhaps the most convincing argument. for an where (Simberloff 1969). The important aspects equilibrium 3, however, is the increase of species of this theory for our immediate purpose, however, present on all our islands to approxin~atelythe are as follows : same number as before defaunation, and then The variance of S(t) is high: rough oscillation about this number. This S may be only a quasi-equilibrium-that is, the curve of P S versus time after S is reached may not be truly var [S(t)J = C E[Sa (t)] { 1 - E [S, (01} stationary-because of two long-term processes. a=l The first is that when an approximate S is first reached the population structure of the particular S, is a species indicator variable, which equals 1 set oi species on the island is still changing rap- when species a is present and 0 when species a idly; some species are represented b$ few indi- is absent. Also, viduals and hay have large r (intrinsic rate of increase). Others may have abnormally large populations. Population sizes are generally fluc- tuating much more rapidly than on untreated Once the number of species on these particular islands. We have alreadv indicated one mani- islands is between about 75% and 90% of the festation of this process in our experiment: pso- equilibrium value of copterans colonized early and built up immense populations before presumed ant predators ap- eared in number. The effect of this extreme population fluctuation on the colonization curve is that species may be eliminated and added at a the major premise of this stochastic version of the rate systematically different from that on an un- MacArthur-Wilson equilibrium theory, namely treated island. The concept of a dual dynamic non-interaction of species, would be invalidated. ecl~~ilibrium-of species number and population From this point onward S(t) declines slightly, at structure-will be mentioned briefly in this paper a slow rate which cannot yet be predicted well and described more completely and formally by stochastically. It equilibrates ultimately at an en- Simherloff ( 1969). during S partly determined by interaction and The joint evolution of the particular constella- close to the number that existed before defauna- tion of species on an island ought logically to tion. The decrease on all islands in number of raise S systematically over very long periods of species present for the final census (after regular 286 DANIEL S. SIMBERLOFF AND EDWARD 0. WILSON Ecology, Vol. 50, No. 2 monitoring had ended) is believed to be a mani- island habitat, and that the enduring 3 which will festation of this decline. ultimately be achieved on E7 may be very different El was so distant from its presumed source area from the pre-defamation figure. that a few early invaders were able to build up large populations before their probable competi- tors arrived. Interactions thus became important The agents of dispersal for specific immigra- before even a small fraction of the non-interaction tions can rarely be given with assurance, but the 3 was achieved. We predict that on El the colo- evidence implies that aerial transport, passive and nization curve will ascend slowly and irregularly active, is the major mode of invasion. to an equilibrium near the pre-defamation 3. This For the several parasites of vertebrates that enduring equilibrium will probably be the same breed on the islands proper, zoochorous transport one to which the colonization curve of El would is certain. These arthropods include the hippo- ultimatelv have descended had interactions not boscid flies Olfersia sordida and Lynchia nlbipen- become important until a large fraction of 3 had nis and tick Argas rndiatus on El and E9. All been achieved, as on the nearer islands. three species parasitize the cormorants and peli- As the distance of the island from the faunal caris which roost on these islands. A number of source increases, the non-interaction 3 should de- mite colonists could also have arrived on transient crease because df decreases in the i ,. Further- larger animals. This appears to be so for Ento- more, the time necessary to reach any given per- nyssus sp. on E2; it is an obligate parasite in centage of the non-interaction 3,though not readily snake lungs, a::d a iVatrix was recorded sloughing on E2. The invasions of El, E3, and E7 by the ex~ressedmathematicallv, <, can be shown to in- crease with increasing distance from source. If mite Crnitlzonyssits bursa, an avian parasite, were this time is sufficiently long the few early colonists probably ornithochorous. are able to produce large-enough populations to A more interesting ~~ossibilityinvolves phoresy interact significantly with later immigrants. This on birds, particularly by solile of the psocopterans in fact is what happened on El. prominent anlong early immigrants. Mockford The colonization curve of E7 (Fig. 2) must (1967) has hypothesized that phoresy may be a be considered in light of the fact that 85% of the more efficient means of insular inlasion than wind tree was killed by the fumigation (Wilson and transport, especially for smaller species. Among Simberloff 1969). The dead or ti on did not dis- four species found on Asian birds he lists Ecto- appear, but rather deteriorated until by the end of psocopsis crypto~~zeriae,which has also colonized one year the wood was brown and dry and much E3 and E7. Whether phoresy is a significant phe- of the bark was peeling. Thus the island was nomenon in the Keys remains to be determined. not a constant factor, and the relative proportions Other arthropods that may utilize phoresy are of the various microhabitats changed drastically the pseudoscorpion Tyrannochelifer sp. found on until ultimately there was far more dead wood and E7, and free-living mites on all islands. These bark and far less leafy canopy than on an untreated could have been transported by larger insects as island. If a consistent measure of area existed well as by birds. for these islands it would probably have remained A related transport mechanism utilizes nesting unchanged on E7, but the expected number of material carried by birds. We suspect without species would not because the different micro- proof that several mites in our experiment arrived habitats normally support different numbers of in this fashion. Two beetle larvae were found in species. In particular, dead bark shelters numer- deep excavations in Green Heron nest twigs so011 ous species while mangrove leaves rarely support after the nests were built: Chrysobothris tranque- more than four species on a single island. Before barica on E3 and ?Sapintux fulwipes on ST2 (the its gradual decline, the colonization curve of E7 latter in a twig not of Rlzisophora). It seems rose to a far higher percentage of the original 3 certain that both were in the twigs when the birds (135%) than did that of any other island. In constructed their nests. Meyerriecks (1960) has addition, it was still rising a year after defauna- observed green herons using twigs from other tion, when those of all other islands but El had trees and from the nest tree itself. It is our im- leveled off. Arachnids (excluding orb-weavers) pression that most nesting material that goes into and psocopterans were the animals largely respon- new nests on small mangrove islands is brought sible for the higher S near the end of the experi- from elsewhere. This was assuredly so for the ment. Both groups are primarily bark dwellers. non-~ilangrovetwig on ST2. All the above considerations imply that one of Hydrochorous transport, either free or on rafts, the determinants of the shape of the colonization was not as important as aerial transport in the curve on this island alone was variation of the invasion of our islands. Many mangrove colonists Early Spring 1969 COLONIZATION OF EMPTY ISLANDS 287 remain afloat almost indefinitely in salt water, but ing debris in Florida Bay-far less than the only the orthopteroids seem able to achieve sig- amount found at river mouths. nificant independent, oriented motion. Further- The preceding considerations lead, by elimina- more, most floating insects and spiders of all sizes tion, to the inference that aerial transport must are rapidly devoured by fingerling fishes which are be an important means of dispersal to our islands. immensely numerous about all mangrove islands. Several observations of actively flying propagules Even actively swimming crickets and earwigs fre~ support this hypothesis. Buprestid and ceramby- quently meet this fate, attracting more attackers cid beetles, lepidopterans, wasps, and a lacewing by the very vigor of their efforts. have all been seen to land on the islands after flight Nevertheless, at least one invasion (though not from an outside source. Many other mangrove immigration, since the species was already present) colonists are evidently carried passively by wind, occurred this way, and there are other pertinent especially psocopterans, thrips, neuropterans, and observations. An adult female Cyrtoxipha cricket most spiders. Many of these more or less pas- was seen floating from Upper Snipe Key to E2, sively dispersed organisms are minute, and direct a distance of only about 2 m. She successfully evidence on their invasion method is therefore climbed a root and disappeared into the lower scarce. Occasionally spiderlings and psocopterans canopy. That a winged individual fully capable were found in the air around our islands. In of oriented flight should actively choose to disperse general, they are usually important components by water is dubious, but at least over this short a of aerial plankton (Glick 1939), and it is known distance the method is feasible in case the animal from numerous anecdotal records (e.g., Bristowe accidentally lands in the water. 1958) that ballooning for the distances involved in Four Azttomeris io caterpillars were seen to our experiment is regularly achieved by some spe- float from one end of E9 to the other, a distance cies of spiders. So far, the small sizes of most of of about 8 m. Three of them lodged on roots and these animals has made it impossible to follow a eventually climbed up out of the water. The weak- flight visually from source to island. One sugges- ening of leaves attacked simultaneously by several tive record, however, is that of a spider dragline large Automeris caterpillars (a common occur- stretching the 2 m between Upper Snipe Key rence) might cause them to fall into water occa- and E2. sionally. Also, the caterpillars release their hold An attempt to correlate Figures 1-3 with hourly readily when the branch is shaken, apparently as wind data of the U. S. Weather Bureau station a defensive maneuver. It seems doubtful, how- at Key West was inconclusive, but this piece of ever, that a high proportion of those that land negathe evidence is hardly damaging to the thesis in the water could successfully travel between is- of dominance of aerial transport. For such data lands. Aside from fish attack, they would be give only timed readings at one nearby point, plagued by their inability to direct their motion while dispersal depends largely on specific gusts and to climb readily from the water. at odd times in highly circumscribed areas about Occasionally earwigs (Labidura riparia) were the islands. Furthermore the published wind data seen swimming from one root to another within are from a single fixed anemometer, while the an island. We do not know how well this species entire wind profile would be necessary to describe flies, but its swimming ability seems adequate for dispersal. aquatic travel of considerable distance, and it IMMIGRATIONAND EXTINCTIONRATES readily climbs out of the water. Fishes would still be a hazard, however. The intermittent nature of the monitoring had the unavoidable result that many immigrations and Our experiments resulted in no unequivocal evi- extinctions (perhaps two-thirds) were not ob- dence of invasions by rafting. In fact, several served, the species involved being obligate or near- considerations imply that rafting must play a obligate transients which immigrate and are ex- minor role in inangrove colonization. There is tinguished all within one interval between two rarely land on which rafts can lodge, since Rhizo- monitoring periods. For this reason absolute phora islands generally have no supratidal ground, immigration and extinction curves cannot be de- Drifting wood usually hits an island, gets trapped rived from the observational data. temporarily among roots, and eventually floats The observed immigration and extinction curves away. Even more importantly, green Rhizophora for all islands were highly variable, with no ap- wood sinks immediately and many dead twigs also parent pattern except for generally higher immi- fail to float, thus precluding rafting by much of gration rates on nearer islands during the first the wood-boring fraction of the fauna. Finally, 150 days. Rates were usually between 0.05 and except during hurricanes, there is very little float- 0.50 species/day. Employing a statistical method 285 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON Ecology, Vol. 50, No. 2 devised by Siinberloff (1969) it was determined other species they are doomed to swift ex- that the very least the expected error in immigra- tinction. tion and extinction rates could be is 0.67 species/ ii) Population sizes during most of the rise of day, and furthermore that the variance about the S from 0 to near S are uniformly low. "expected" immigration and extinction curves is very high. The observed curves are consequently The observed data from this ex~eriment~rovide of limited interest, since they yield only an extreme rough quantitative information on both the dis- lower limit for turnover rates and cannot be used tribution and sizes of the e, unlike with the i,. Whatever the i, and e, , the expected curve of to intuit shapes of the "expected" curves. On the immigration rate vs. time is represented during other hand, the values of the turnover rates are the rise of S from 0 to a large fraction of the non- of considerable interest even when they, can be ap- interaction S by: proximated only to the nearest order of magnitude. They are of course surprisingly high, in the vi- cinity of 1% of the equilibrial species number per day or higher. Yet this is at least roughly con- sistent with the MacArthur-Wilson ( 1967) model, which predicts that the turnover (= extinction) P rate at equilibrium is 1.15 (mean S/tO.gO, where = C i, - i,E[S, (t)] mean 3 is the average equilibrial species number a=l and to.go is the time (in days) required to reach 90% of the equilibria1 number. According to this while the expected curve of extinction rate vs. formula, which is based on the sim~lestnon-inter- time during the same period is : active version of the model, the turnover rates in our experimental islands should fall somewhere between 0.1 and 1.0 species/day. The relation be- tween this version of the model and the more precise stochastic form of the model will be treated later at length by Simberloff (1x9). The Mac- Arthur-IViIson formulation is a special case of the many cases covered by the stochastic version and it has the advantage of permitting this first rough Beyond this point accurately predicted curves (and approximately correct) prediction of turn- are impossible. It is nevertheless clear that during over rates. the slight decline of S to an enduring S, E(t) In testing such predictions with measurements must be, on the average, slightly greater than in the field there is reason lo expect that the inva- I(t), while after the equilibrium is reached the sions not observed will occur by different means two must remain approximately equal. On any than those which are recorded. The assumption real island, of course, the two curves would cross that most propagules arrived by air in the experi- and recross indefinitely. It also seems reasonable mental keys is therefore probably valid. The evi- that after interactions become important, the E(t) dence against a major seasonal component of dis- and I(t) curves still do not change much, since persal was given earlier. In sum, we have a large the contribution from their common major com- body of information which implies that i, (inva- ponent, the transients, does not change with time. sion rate of species a) is nearly constant through Whereas evidence on the specific agents of dis- time for all a. What we lack at present is quan- persal, and hence on immigration, has been plen- titative information on the sizes and distribution tiful during this study, observations on the causes of the i,. of extinction have been meager. Obviously the Extinction rates, at least during most of the rise probability of witnessing the death or disappear- of the colonization curve from 0 to a large fraction ance of the last member of a population is ex- of S are adequately represented by the unchanging, ceedingly low. Some inferences can be drawn species-characteristic e, , without an additional from observed means of population decrease. S-dependent or density-dependent factor included. Population decline should be most apparent The main arguments behind this assertion are: when associated with interaction, especially pre- dation, yet the small sizes of most populations dur- i) Most of the species in the Florida Keys pool ing our experiment reduced interactions enor- are obligate transients on these small man- mously. A few cases of predation have already grove islands. For a variety of reasons not been mentioned. Insectivorous birds, particularIy directly related to their own densities or to warblers and red-winged blackbirds, were fre- Early Spring 1969 COLONIZATION OF EMPTY ISLANDS 289 quently observed eating numerous insects of many Similarly, numerous mangrove colonists inhabit species. Wasps, both parasitic and nonparasitic, hollow twigs and even under normal conditions were seen destroying several insects and spiders. many of these fall into the water. Certainly such Some of these attacks may have led directly to events are multiplied during storms. The ant Caw- extinction, when the prey populations were small. ponotus floridanus and the oedemerid beetle Oxy- Examples include the parasitism on Autorneris io copis sp. seem particularly vulnerable in this re- caterpillars by Apanteles hemileucae (Braconidae) spect : they typically inhabit low, weakly anchored, and the destruction of the salticids Hentzia pal- hollow roots. mumand Stoidis aurata by Trypoxylon collinurn (Sphecidae) . ACKNOWLEDGMENTS Exclusion can also provide indirect evidence of We wish to thank Joseph A. Beatty and Edward L. extinction through interaction. A possible in- Mockford for identification of specimens and advice on stance is the apparent predation of crickets on ~9 experimental technique. Robert E. Silberglied performed the survey of Keys arthropods and aided in monitoring. by a large population of the centipede Orflhnaeus S. Peck, H. Nelson, and R. B. Root participated in island brasiLianus. E8, with no centipedes, had immense censuses. John Ogden cultured many arthropod colonists. populations of four cricket species. After defauna- The following specialists gave generously of their time tion of E9 removed Orfihnaeus, a large popula- to identify animals encountered during the experiment: tion of the cricket Cyrtoxipha confusa was rapidly D. M. Anderson, E. W. Baker, J. Re Barren, J- C. Be- quaert, L. Berner, B. D. Burks, J. A. Chemsak, K. A. established. Christiansen, W. J. Clench, C. M. Clifford, B. Conde, From observation of the pre-defaunation distri- R. E. Crabill, P. J. Darlington, G. W. Dekle, H. Dybas, bution of ant species' numbers and population H. E. Evans, J. G. Franclemont, R. C. Froeschner, A. B. sizes on the various islands, it seems probable that Gurney, J. L. Herring, W. Herrnkind, R. W. ~od~es, R. L. Hoffman, D. G. Kissinger, J. P. Kramer, J. F. One to when ant 'pecies is up large Lawrence, H. W. Levi, J. E. Lloyd, E. G. MacLeod, P. populations before invade, it can M. Marsh, R. Matthews, W. B. Muchmore, S. B. Mulaik, exclude one or more other species. However, L. Pinter, C. C. Porter, E. S. ROSS,L. M. Roth, L. M. direct evidence of aggressive behavior among the Russell, R. L. smile^, T. J. Spilman, L. J. Stannard, mangrove ants is lacking ; the observed exclusion R. L. USinge" B. D. Valentine, G. B. Vogt, T. J. Walker, L. M. Walkley, R. E. Warner, D. M. Weisman, F. G. may have been the of nest site P ~ ~ - Werner,~ ~ R,P E.~ White,~ ~S. L.~ Wood,. R. E. Woodruff, Most extinctions were probably not the result P. W. Wygodzinsky. The work was supported by NSF of interactions, at least during the initial rise of grant GB-5867. the colonization curves, but rather resulted from the inability of most species in the Florida Keys LITERATURECITED pool to colonize these tiny mangrove islands under Bristowe, W. S. 1958. The world of spiders. Collins, any conditions. Lack of proper food or nest site London. 308 p. and hostile physical conditions are probably corn- Cotteau, E. 1885. A week at Krakatau. Proc. Geogr. Soc. Australia, N.S.W. and Victorian branches, 11. causes' An was the Observation Of Glick, P. A. 1939. The distribution of insects, spiders, a dealate queen of BrachywYf'.l.1.zex sp. on E9. and mites in the air. U.S.D.A. Tech. Bull. 673. 151 p. Since this species nests in soil, which is lacking MacArthur, R. H., and E. 0.Wilson. 1967. The theory on the experimental islands, it is not surprising of island biogeography. Princeton University Press. 203 p. that workers were observed, de- Meyerriecks, A. 'J. 1960, Comparative breeding be- spite the fact that the queen landed in a totally ant- havior of four species of ~~~~h ~~~~i~~~ herons. free environment. Publ. Nuttall Ornithological Club, No. 2. 158 p. Even species which can survive on small man- Mockford, E. L. 1967. Some Psocoptera from plumage grove islands have high probabilities of extinction of birds. Proc. Entomol. Soc. Wash. 69: 307-309. Simberloff, D. 1969. Experimental zoogeography of not to interaction' A number of islands, 111. A theory of insular colonization. Ecol- have been found drowned during this experiment, ogy (in press). including entire small ant colonies, numerous Wilson, E. O., and D. Simberloff. 1969. Experimental lepidopterous larvae, beetles, and psocopterans. zoogeogra~h~of islands. Defaunation and moni- It seems probable that during the hurricanes which toring techniques. Ecology 50: 267-278. Wilson, E. O., and R. W. Taylor. 1967. An estimate periodically buffet these islands such deaths would of the evolutionary increase in species den- be commonplace. sity in the Polynesian ant fauna. Evolution 21: 1-10. 290 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON ~cology,Val. 50, No. 2

APPENDIX. The colon~stsof six experimentally defaunated islands. Headings identify the island and glve the number of days after defaunation for each census ; "Pre" IS the pre-defaunation census. Solid entries indicate thot a species was seen; shaded, thot it was inferred to be present from other evidence; open, that it was not seen and inferred to be absent.

THE COLONISTS OF ISLAND E 1 Q& ,.,.,.&,:,$\@,$.~I~'Lo+~~$$ o\4<401 gb0

L*P~Y~III~8p. uttblutlalle. n. sp. Cycioptiium sp. l'afirllscu lurldu DEllhlAPTERA Labiduridao 1ilbidul.a rlpn~.ia COLEOPTERA Anthicidar Snplnlus fulvtpes Curculionidae Crypturhynchus nllnutissimue Psrudoucalles sp. Gen. ~p.' T~sANOPTERA Phlaeolhripldae Neurothrips rnagnafemoraus Thripidae Psrudolhrips inequalis CORRODENTN Peripsocidae Ectopsmus sp. 4 Peripsmus pauliani Peripsmus stagnivsg~s Peripsmus sp. c Pssidae P80cidu texaus HEMIPTERA Aleyrcdidae Tetraleurcdes sp. Coccidae Ceroplastes sp. Miridae Psauus Consplrcahis NEUROPTERA Chrysopidae Chrysopa collaris Chrysopa rufllabris LEPmOPTERA Eucleidae Alarcdia slossoniae Hesperiidae Phocides batabano Lymantriidae Orgygia detrita Olethreutidae Ecdytolopha sp. Phycitidae Bema ydda Piineidae Nemapogon sp. Gen. sp. Saturniidae Automeris io DIPTERA Hippobscidae Lynchia albipennie Olfersia sordida HYMENOPTERA Bethylidae Sclercderma macrogaster Chalcidae Brachymeria psyche Eumenidae Pachodynerus nasidens Formicidae Camponotus toraganus Colobopsls sp. Crematogaster ashmeadi PseLdomyrmex elongatus Ichneumonidae Calliephialtes ferrugineus Camaria texana ORDER UNK. Fam. Unk Gen, sp. ARANEAE Araeidae EuSI!dla BP. I Nephila clavips Saltlcidae Hentzia palmarum Fam. Unk. Gen. sp. ACARINA Argasidae Argas radlatus Ascidae Arctoseius sp. Dermanyssldae Ornitnonyssus bursa Pellonyssus sp. Galumnidae Galumna sp. OriDatulidae Scheloribates sp. Phytoseiidae Amblyseius sp. LSOPODA FmyscoDus sp. DIPLOPODA Polyxeuidae Lophoproctinus bartschi

THE COLONISTS OF ISLAND E 2 8 acbq$.ixQ,O1eaw,ccec8 b 01 \ \ ,8@b@4bm@ -?' EMBIOPTERA Teratembiidae Gen. sp. ORTHOPTERA Blaitidae Aglaopteryx sp. Iatiblanella n. sp. Latiblanella rehnl Cycloptilum sp. Cyrtoxtpha sp. Orocharis gryllcdes Tafalisca lurida Tenigoniidae Turpilia rostrata LSOPTERA Kalotermitidae Kalotermes jartell

COLEOPTERA Anobiidae Anthlcldae Sapintus fulvlpee Buprestidae ActenOdes auronotata

Cerambycidae Ataxla sp. Styloleptus biustus Chrysomelidae Gen. sp. Cuculldae Gen. sp. Curculionidae Cryptorhyn~husminutissin Pseudoaoalle# sp. Lampyridae Micronaspis Iloridana Oedemeridae Copidita suturalla Oxacis sp. oxycopie sp. Scolytidae Poecilip rhizophorae Trischidias atoma Fam. Unk Gen. ap. .rly Spring 1969 COLONIZATION OF EMPTY ISLANDS APPENDIX

THE CC ' ISLAND E2 (con

THYSANOPTERA Phlaeothripidae Liothrips n. sp, Thripidae P~eudothripsinequalis Fam. Unk. Gen, sp. Caeciliidae caecilius ap. n-b Lepidopsocidae Echmepteryx hagenl b Lipascelidae Gen. sp. Peripsocidae Peripsccus sp. Pswidae Psoeidffi kxanus Acanaloniidae Acanalonia latifrons Aleyrodidae Aleurothrtrua ap. Paraleyrodea ap. 1 Paraleyrodes sp. 2 Tetraleurodes sp. Cicadellidae Scaphytopiua se. Cixiidae Gen. sp. Flatidae Flatoidinus punctatus Miridae PsaUus conspurcatus Pseudocwcidae Pseudococcus sp. Tropiduchidae Neurotmela breviceps Fam. Unk. Gen. sp. Chrysopidae. Chrysopa collaris Chrysopa externa Chrysopa rufilabris Geometridae Oxydia sp: Hesperlidae Phocldes bataba~o Noctuidae hlelipotis sp. Olethreutidae Ecdytolopha sp. Phycitidae Bema ydda Plineidae Nemapogon sp. Pyralidae Gen. sp. Saturniidae Aulomeris io Cecidomyidae Gen. sp. Culicidae Aedes laemorhynchus Sylphidae Gen. sp. Fam. bnk. Gen. sp. Bethylidae Ceohalonomia waterstoni

Braconidae

Heterospilus sp. Iphiaulax eplcus Chalcedectidae EU~hrysiasp. Chdcidae Gen. sp. Encyrtidae Ooencyrtus submetallicus Eumenidae Pachodynerus nasldens Eupelmldae Metapelma schwarzi Neanastatus sp. Gen. sp. Formicidae Camponotffi Uorldanus Camponotffi tortuganus Crematogaster ashmeadi Monomorlwn floricola Paracryptmerus varians Pseudomyrmex elungatus Pseudomyrmex "flavidula" Taphoma iittorde Xenomyrmex fjoridanus Caiiiephialtes ferrugineus Casharia texana Sphecidae Trypoxylon collinum Araneidae Arglope argentam Euslala sp. 1 Eustlla sp. 2 Gasterxantha ellipoides Gen, sp. Nephilia clavlpes ARANEAE Anyphaena sp. Aysha sp. Gen, sp. I Gnaphoaidae Gnaphosa sp. Linyphiidae hlelonetl sp. Saitic~dae Baiius sp ilentzia p.dmarum Yetac) rbn "Mala Geo. sp. I1 Ar~adnunrthuri I.eucauge venusta Trlrngmtha sp. Therldi~dae re", sp. Acandac Rhlzoglvphu~~dlllle Bdeilidae Gen. sp. Che, Ichdac Cheyletiil wrllsi. Dermanyssidae Gun, sp. Entunysu~dac Cntunissus sp. Galumn~dae Ciaiult~nasp. Orihaluiidae Sehelonbates sp. Phytusrildac Anih1)se~ussp. Totrunychidae Tetlanychus sp. Rhy~c~tu6 SL). Luphap rucltnus bartschi tiiiiiiiiiiiiiil 292 DANIEL S. SIMBERLOFF AND EDWARD O. WILSON Ecology, Vol. 50, NO.2 APPENDIX - PART 3 THE COLONISTS OF ISLAND E3

COLLEMBOLA Poduridae Gen, sp. EMBIOPTERA Teratembildae Diradlua carlbbeana OllTHOPTERA Biattidae Laliblaltelia n. sp. Oryllidae Cycloptilum apectabile Tafalisca lurlda ISOPTERA Kalotermilidae Kalotermea Joutell COLEOPTERA Anoblldae Tricorynus sp. Anthicidae Sanintus fulvivea Buprestldae Actenodea auron,tala Chrysobothris tranquebarica Cerambycidae Curculionidae ~;~~to;hynchusmlnutlssimua Paeudoacallea sp. Lathrldlldae Melanovhthalma floridana Oedemeridae Oxacis sp. mycopla sp. Scolytidae Trlschldlas atoma THYSANOPTERA Phlaeothripidae Haplothrips navlpes Liothrips n. sp. Neurothrips magnafemoralis Thripidae Pseudothrips ~nequalis Fam. Unk. Gen. SD. CORRODENTIA Liposcelidae Peripsocidae EC~O~~DCUSSP. c Peripsocus pauliani Psocidae Psocidus texiulus HEMIPTERA Aiey rodidae Tetraleurodes sp. ~aathocoridae Dufouriellus afer Orius sp. Cicadellidae scaphytopius sp. Lygaeidae Gen. sp. 2 hlembracidae Gen. sp. Mlridae Psalius conspurcatus Carthasis decoratus NEUROPTFR4 Chcys2pidae Cnrysopa collaris Chrvsow. . externa Chrysopa rufilabris LEPDOPTERA Eucleidae . Alarodia sl,ssoniae Geomelridae Gxydia sp. Hesperiidae Pnocldes batabnnq Oletnreutidae Ecdylolopha sp. Phycitidae Bema ydda Ptineldae Nemapogon sp. Saturniidae Automeris iu HYMENOPTERA Bethylidae Nesepyris floridanus Scieruderma nlacrogaster Gen. sp. Braconidae Heterospiius sp. Iphiaulax epicus Macrocentrus sp. Chalcidae Brachi merla psych6 Gen, sp. Encyrlidae Gen, sp. Euiophidae Entedontlni sp. Melitlobla chalybii Eumenidae pachodynerus nasidens Eupelmidae Gen, sp. i Gen. sp. 2 Formicidae Colobops~ssp. Crematogaster ashmeadi Monomorium floricola Paracryptocerus varians paratrechina bout.b',nica Pseudomy rmex elongalus Tapin,,ma lit lorale Xenomyrmox llaridnnw Ichneumonidae Callicphinites ferrugincus Caslnarin texana Scelianidae Probaryconus sp. Tolonemus sp. Sphecidae Trypoxylon coiirnum Vespidae Gen. si, ARANEAE Armcidae Eustala sp. I Gnslerslcanlnd elli~)soides Ciubionidae Anyphnena sp. Aytiha sp. Gnnphasidae Scrgiolw 81) Salticidae HontzIa palmarum Sepstriidne Aridnn arlhuri Totrngnalnirlue L&ucauge venusla Telraywlha 3Ulillana Theridiidne Cen. 81, Acaridao Tyrq,hagus pulresccnliae Ascidae Asca 8p. DermmysslrlPe Cdn. YP. Eupodidae Eu~mcssp, nr. fusrfer Galumnidae Gklurnna 81,. Oribalullrlne Schcluriba'es Sp. PnyiosciWac Am1,lyaciub' Sp. I'oiyxenidae lapnbproclinuw bsrtschi llhysculus Sp. .rly Spring 1969 COLONIZATION OF EMPTY ISLANDS APPENDIX - PART 4 THE COLQNlSTS OF ISLAND ST2

EMBlOPTERA Teratemblldae Diradlus caribbeana ORTHOPTERA Blattidae Latlblattella n. sp. Gryllidae Cycloptilum ap. Cyrtoxlpha sp. Tafallsca lurida ISOPTERA Kalotermitidae Kalotermea jmteli COLEOPTERA Anobiidae Trlcorynus sp. Anthicldae Gen, ap. Buprestidae Gen. sp. Cerambycidae SIyloleptus biuatua Curcullonldae Cryptorhynchua sp. Pseudoacalles ap. Lathrldiidae Melanophthalma floridana Oedemeridae Oxycopis sp. THYSANOPTERA Phlaeothripidae Haplothrlps flavlpes Llothrlps n. sp. Thripldae Paeudothrips inequalis CORRODENTIA Llposcelidae Lipascelis bastrychophilus Peripsocidae Ectopsocus sp. Perlpsocus pauliani Peripsocus stagnivagus Psocidae Psocldus texanus HEMIPTERA Aleyrcdidae Tetraleurodes sp. Lygaeidae Blissus insularls Gen sp. 2 NEUROPTERA Chrysopidae Chrysopa collaris Chrysopa externa Chrysopa rufilabris Eucleidac Atarodia slossoniae Geometridae Gen. ap. Hesperiidae Phocides batabano Lymantriidae Orgygia detrita Olethreutidae Ecdytolopha sp. Phycitidae Bema ydda Ptineidae Nemapagon sp. Pyraustidoe Gen. sp. DIPTERA Fam. Unk. I Gen. sp. Pam. Unk. 2 Gen. sp. HYMENOPTERA Bethylidae Sclercderma macrogaster Braconidae Iphiaulav epicus Macrocentrus sp. Gen. sp. Chalcldae Brachymeria psyche Gen. sp. Eumenidae Pachcdynerus nasldens Eupelmidae Gen. sp. Formicldae Camponotus floridanus Colobopsls sp. Crematngaster asluneadi Crematogaster atkinsoni ParacryQtocerus varims Pseudomyrmex elongatus Tapinoma littorale Xenornyrmex floridanus lchneumonidac Calliephialtes ferrugineus Casinaria texana Sphecidae Trypoxylon collinum Apocrita Gen. sp. ARANEAE Araneidae Eustala sp. I Eustala sp. 2 Gasteracantha ellipsoides Nephila clavipes Clubionidac Anyph a+ na sp. Aysha velox Salticidae Hentzia palmarum Segestriidae Ariadna arthuri Tetragnathidae Uucauge venusta Tetragwtba sp. Theridildae Thcridion sp. ACARINA Ascidae Arctoseius sp. Asca sp. Bdellldae Bdella sp. Eupodidae Eupodes sp. nr. fusifer Orlbatulidae Scheloribates sp. Phytoseiidae Amblyseius sp. Veigaiidae Veigaia sp. Fam. Unk. Oen. sp. CHE LONETHIDA Cheliferidac Tyrmnochelilc> n, sp. PAUROPODA Gen. sp. D1I'I.OPODA laphoproctinus bartschi DANIEL S. SIM ERLOFF AND EDWARD 0. WILSON Ecology, Vol. 50. No. 2 APPENDIX - PART 5 THE COLONISTS OF ISLAND E 7 6 cl.8e.39 ow %O r2& .\"'$\!+%% % 444' 4' w e EMUIOPTERA TeralemblldPC Cen. sp. ORTHOPT~A ulaltidac LBtlblattella n, sp. Gvyllidtte cyol~ptilulllsp. Tafdlscl lurldil DERMAPTERA Lab!dur~dar Labldula rlpnrla COLEOPTERA Anobiidne Trloorynus sp. GB". 8P. Arlenodes auronolntu Ch~ysok)th~.lsl~anqlieburica Gen. sp. Lcptostylus up. Styloleptus btlslus CucuJldne Gen. sp. Curculiunidue I'seuduacdles sp. Oedemerldne Gen, sp, Soydm~~sdd~~eGen. sp. TmSANOPTERA Phlneothripldae Haplothrips llavlpes Neurothrlp mapafemo

APPENDIX - PART 6 THE COLONISTS OF ISLAND E9

ORTHOPTERA Gryllidae Cy~loptiiumsp. Cyrtoalpha eonfusa Orocharis sp. DERMAPTERA Labidundae Labidura riparia COLEOPTERA Anobi~dae Cryptorama minutum Tricoriilus sp. Anlhicidae Saplntus fulvlpes VBCUSUBvicinu~ Buprestidae Actenodes auronorata Chrysobathns tranquebarira Canthar~dae Chauliognathus marginatus Cerambyctdae sgloleptus blustu~ Cur~Ylionldae Cryptorhynchus mlnutlssmus Pseudoacalles sp. Lathridiidae Holoparamecus sp Oedemendae OXaciB Sp. Fam. ,Unk. Gen, sp. THYSANOPTER~ Phiaeothr~p~dar Haplothrips nav~pes Neurothrips magnafemoralls Pseudothrips ~nequalls Cae~~llussp. np Lachesllla n sp Echmeptenx hagem b Belaphotrmtes okalensls Emb~dopsmuslarmeps Uposcel~ssp. not bostrychophilus Ectopsmus sp tv Peripswus stagmvagus Psocldus tenanus Psoc~dussp. I Troglomorpha Gen sp HEMIPTERA Anthocortdae Dufounellus afer Cmldae Ollarus sp M~ndae Psallus conspurcatus Pentatomldae Oebaius pug"= Fam. Un* Gen sp. NEUROPTERA Chrysopldae Chrysopa collarls Chrysopa externa Chrysopa rufllabr~s LEPmOPTERA Eucleldae Alarodm slossomae Olethreul~dae Ecdltolopha sp. Phycitidae Bema ydda Ps)cludae olket~cusabbottu Ptlneidae Nemapogon sp Pyral~dae Tholeria reversall8 Sahlrm~dae Autome~16io Fam Un*. Gen, sp. DlPTERA Hippoboscldae Oliers~asordida Fam. Unk Gen, sp. HYMENOPTEM Bracomdae Apanteles hem~leucae ~pantelesmargimventrls Calllhorm~usbiiasclatus ~cphylusn, sp nr, chramrsl lphlaulu epicus Gen, sp. I Gen. sp. 2 Gen. sp 3 Gen, sp. 4 Eulophidae Euderus sp Eumenidae Pachodperus nae~dens Euplmidae Gen, sp. Formicidae Brachymyrmex sp. camponotus nor~danus Camponotus sp. Crematogaater aehmeadl hlonomorlum noncola Paracr)vtmerus varians pseudomyrmex elongatus Pseudomyrmex "ilavldula" Tapinoma l~ttorale Xenomjrmex floridanus Gen. sp. Callieptdaltes ferrqineus Cashana lexana Pteromalidae Uroleple ruiips Spheculae Tr)pxvlvn colllnum Vespidae Polisles sp ARANEAE Araneidae Argtope argentam Eriuphora sp EusLala sp. Gasterwantha elllpst~ldas Metepeira lab, I lnlhru Nephlla clnvi]~i~s Clublonidae Aysha sp. Dictynldae Dictyna sp Gnaphosldae Serglolus sp Linyphlldae Meioneta sp Lycosidae Mrata sp. salricldae HenCzia palmarum Scyidldae Scytdes sp. 296 DANIEL S. SIMBERLOF~ Ecology, Vol. 50, No. 2 APPENDIX - PART 7 COLONISTS OF ISLAND (cont.) THE E9 q& $ p @% g.\$ $ $\Q$$$?$?$~ ,$' Leucauge venusta ~~t~qnatha antlllana Tetragnatha sp. 2 Theridiidae Cen. BP. Uloborldae mobrue sp. ACARMA Argasidae Argae radlatua Ascidae Laslo$eiu6 8P Meilcnai'es sp. Bdells aP. carpglyphus lactia sphaerolophuli sp. CalumM SP. Amblyaelus sP ISOPODA RhgacON8 sP CHLLOPODA Orphnaeus braadlanus

EXPERIMENTAL ZOOGEOGRAPHY OF ISLANDS : A MODEL FOR INSULAR COLONIZATION DANIELS. SIMBERLOFF~ Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138 (Accepted for publication December 22, 1968)

Abstract. A distinction is made between immigration rate (in spp./time) for an island, and invasion rate (in propagules/time) for a species and an island. An analogous distinction is drawn between an island extinction rate and a species extinction rate (or intrinsic prob- ability of extinction in a given time interval). It is claimed that the most objective definition for "propagule" is any animal or group capable of population increase under any conceivable circumstances. Immigration and island extinction curves are unique only if plotted against time, not against number of species. A model for non-interactive colonization is discussed, and its equilibrium number of species, S derived. It is shown that data from the defaunated Florida Keys can be interpreted as arising from non-interactive colonization to an S near the non-interactive S (and to a point above a more enduring interactive S), followed by a slow decline in S (as population sizes and interaction increase) to an enduring S near that obtaining before defaunation. The effect on this scheme of increasing distance from source area is shown, and a simula- tion of the non-interactive part of the scheme was performed which corroborates that part of the model. Explicit equations for the "expected" colonization, immigration, and island extinction curves are given for non-interactive colonization, and predictions are given about the general changes in these curves as interaction becomes significant.

SYMBOLSUSEDIN THE TEXT i ,(a) probability that species a invades in time period of length 7. C(t) colonization rate, in species/time, at time t i , invasion rate of species a, in propagules/ time e,(a) probability that species a, if present, is immigration rate for island, in species/ extinguished in time period of length 7. I(t) time, at time t e, species extinction rate of species a, de- lim I,(t) contribution to immigration rate of spe- fined analogously to i, (q.v.) ; cies a; defined in text + e,(a) lim P number of species in species pool S(t) number of species on island at time t 3 equilibrium numher of species for island If species a were replaced immediately S, (t) species indicator variable for species a upon extinction, e ,would be the frequency = 1 if species a present on island dur- of extinctions/time. ing time period t E(t) extinction rate for island, in species/time, =0 otherwise at time t E, (t) contribution to island extinction rate of INTRODUCTION species a ; defined in text A discussion of theoretical zoogeography must Present address : Department of Biological Science, be based on clear definitions of the following often Florida State University, Tallahassee, Florida 32306 ambiguous terms :