Ecological aspects of plant colonisation of the Krakatau ... - Springer Link

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GeoJournal 28.2 201-211 © 1992 (Oct) by Kluwer Academic Publishers

Ecological Aspects of Plant Colonisation of the Krakatau Islands Whittaker, R.J., Dr., University of Oxford, School of Geography, Mansfield Rd, Oxford OX1 3TB, UK; Bush, M. B., Dr., Duke University, Dept. of Botany, Durham, NC 27706, USA; Partomihardjo, T., Herbarium Bogoriense, Jalan Raya Juanda 22-24, Bogor, Indonesia; Asquith, N. M., University of Oxford, School of Geography, Mansfield Rd, Oxford, OX1 3TB, UK (Now at Department of Biological Sciences, University of Illinois at Chicago, IL 60680, USA); Richards, K., P. T. Robertson Research (Utama Indonesia), Building 108C, Cilandak Commercial Estate, Jakarta 12560, Indonesia ABSTRACT: Species assemblage data from the Krakatau Islands, Indonesia, are presented for the period 1883 to 1989 (including previously unpublished data from the 1989 sdrvey). Since 1934, 16 additional families of higher plants have colonised. Recent arrivals at the family level are mostly of zoochorous species of forest tree, indicating (subject to the effects of disturbance) a continuing increase in potential niche space within the island interiors. The data for Rakata (an uninterrupted prisere) conform to a successional explanation in which identifiable ecological groups of plants exhibit differing colonization and turnover patterns. Animal-dispersed canopy tree species and species which are widespread within the group, exhibit a very low probability of extinction once they have colonized successfully. There are, however, several constraints on the rapid spread of species within the group, in particular those connected to local dispersal (eg lack of large terrestrial mammals). In respect of dispersal to the group, partial survey data for the island of Sebesi from 1921 (revised) and 1989 provides the basis for comment as to the changing biogeographical circumstances of the Sunda Straits and the role of Sebesi as a 'stepping stone' island. The varied data discussed in the paper indicates that with the exception of the strand-line, no component of the Krakatau flora or vegetation has yet approached a stable composition. Both floral and faunal diversification are argued to be proximally controlled not only by dispersal opportunities but also by the dynamics of the dominant life-forms of the system, ie, the forest trees. Such hierarchical links, across trophic boundaries, should receive greater recognition in the construction of island biogeographical theory.

Introduction The pattern of a c c u m u l a t i o n and turnover of plant species on Krakatau is of considerable theoretical and practical interest. It constitutes a classic case study in both tropical succession, and island biogeography (Whittaker et al. 1989; T h o r n t o n et al. 1990). In the latter field, much interest has focused a r o u n d MacArthur and Wilson's (1967) attempt to introduce a science of model building into a previously rather ad hoc descriptive discipline. However, the elegant simplicity of their approach is not matched by a good fit with the biogeographical data for plants, nor for several animal groups from the Krakatau Islands (Whittaker et al. 1989; T h o r n t o n et al. 1990; Bush & Whittaker 1991; Whittaker & Bush 1992). In previous publications we have argued that successional processes, habitat factors and dispersal m e c h a n i s m s of plants are

critical to an u n d e r s t a n d i n g of species turnover, observations that would not have come as much of a surprise to m a n y of the early workers o n Krakatau (Ernst 1908; D a m m e r m a n 1922, 1948; Docters van Leeuwen 1936). Bush and Whittaker (1991) develop the a r g u m e n t further in discussing the hierarchical links between mobile and relatively short-lived animals (specifically birds and butterflies), and the turnover, population and c o m m u n i t y dynamics of the d o m i n a n t organisms of the system - f o r e s t trees. Our aim here is to e x p o u n d on these ecological themes with reference to revised assemblage data based on field survey in 1989. This will serve the additional purpose of establishing the degree to which our knowledge of the flora has changed since the 1983 datum. We also present and assess limited survey data from the nearby 'steppingstone' island of Sebesi, the first published since Docters van L e e u w e n ' s (1923a,b) 1921 study.

GeoJournal 28.2/1992

202

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with the Krakatau Research Project in Oxford. Herbarium

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is in progress and is being fed into a data-base 1°s%°'TRA / o5%o' work containing all species records for each island, together with limited auto-ecological details such as dispersal mechanism and growth form. For details of nomenclature see appendix 1.

Results New Records and Re-Appearances

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Sertung//

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Anak~ Krakatau

Rakata

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Fig 1

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Location of the islands of Krakatau and Sebesi

The Data Set A number of floristic surveys of Krakatau were made in the first fifty years, but between 1934 and 1979 only one partial survey was undertaken. One of the principal aims of the Krakatau Centenary Expeditions of 1979 and 1983 was thus to make as complete as possible a floristic survey of each Krakatau island (Flenley & Richards 1982; Bush et al. 1986). The complete 1883 to 1983 floral data set is given by Whittaker et al. (1989), who provide detailed analyses of colonization and turnover patterns and of habitat and dispersal characteristics of assemblages through time (see also: Whittaker & Bush 1992). In 1989 we undertook the collection of further voucher material from Rakata, Sertung and Panjang, concentrating on species not known from the 1979-1983 surveys. A brief collecting trip was also made to Sebesi (Fig 1) and vouchers collected. Specimens were preserved by the wet (ethanol) method prior to treatment in the Herbarium Bogoriense. Duplicate sets (where sufficient material was available) have been lodged at Herbarium Bogoriense, Royal Botanic Gardens, Kew and

In 1989 a total of 235 numbered vouchers were collected, 139 from Rakata, 23 from Panjang, 37 from Sertung, and 36 from Sebesi. An additional set of single, unnumbered vouchers was collected by T.P. of which there were respectively 3, 4, 5, and 6 for each of the above islands, and a further 34 from Anak Krakatau. Names have so far been assigned to 214 specimens. In addition, for each island, several species were recorded as present, but without a voucher having been collected. In most cases these were species known from the island concerned from the 1979 to 1983 survey but in a few instances they were not. This is unfortunate as even with experienced field botanists the possibility of mis-identifications arises. However, all new species (ie new to Krakatau) were collected, with the exception of Cynometra cauliflora (Leguminosae), the specimen of which was too small to be sampled without jeopardising its survival. It is of great interest, if correctly identified, as it is the first zoochorous legume known for Krakatau (Bush & Whittaker 1991). It has a plum-like pod and its seeds are known to be dispersed by bats. However, other species with which it may have been confused include sea-dispersed taxa such as the strand-line Cynometra ramiflora (Ridley 1930; Pijl 1982). The voucher material is variously unidentified, provisionally identified and confirmed. The following details are therefore provisional in nature. Trends at the Family Level: Bush & Whittaker (1991) point out that from the point of view of animals having specialist feeding relations with plants, habitat diversity should be measured in terms of availability of food-plants. Most butterflies are relatively host-specific, but if a female cannot find a "first-choice" foodplant she may oviposit on a chemo-taxonomically related species which is often a congenor or confamilial species. One crude index of the general increase in niche space afforded by the vegetation may thus be provided by the increase in plant families represented within these islands. Fig 2 displays the data for plant families found within the islands (including those introduced by humans). The discrepancy between the lines for spermatophytes-present (S-pres) and all spermatophytes (S-all) is accounted for by families which were recorded between 1886 and 1951 but which were not found between 1979-1989. Seven of these "failures" were of species introduced by people, at least two were associated with the lagoonal habitat on Sertung, since lost to the sea (Whittaker et al. 1989), and none was of families

GeoJoumal 2 8 . 2 / 1 9 9 2

203

represented by more than two species. The various "kinks" may too easily be sampling artefacts b u t the data clearly show that in simple terms, the rate of diversification at the family level has slowed compared to the first fifty years. Although fern t a x o n o m y is s o m e t h i n g of a minefield, it appears that a "plateau" was reached at an earlier point than for the flowering plants, if indeed the latter can be described as having done so. The n u m b e r of higher plant families recorded on the islands for the first time since 1934 was given as n i n e by Whittaker& Bush (1992 - for the 1983 datum). Our 1989 data add a further seven families, five of t h e m in the form of zoochorous trees (Tab 1). In the cases of Knema cinema and Crypteronia paniculata, they are moreover large canopy trees. Unfortunately, the first m e m b e r of the Ebenaceae ever recorded on the group (an individual specimen of Diospyros cf. maritima) had already been felled by tree poachers before we discovered it, but was re-sprouting from the base. The fern is an interesting case, as it is an exotic to the region. New genera recorded on the group in 1989, of families previously k n o w n from Krakatau were: Axonopus (A. compressus, G r a m i n e a e ; A n a k Krakatau), Brachiaria (B. distachya, G r a m i n e a e ; Anak Krakatau), Cenchrus (C. brownii, G r a m i n e a e ; A n a k Krakatau); Cynometra (C. cauliflora, L e g u m i n o s a e ; Rakata), Elephantopus (E. molis, Compositae; Anak Krakatau), Gigantochloa (G. cf. latifolia, G r a m i n e a e ; Sertung), Flickingeria sp. (Orchidaceae; Rakata), Peltophorum (P. inerme, L e g u m i n o s a e ; Anak Krakatau), Phoenix sp. (Palmae; Anak Krakatau), Pterocarpus (P. indicus, L e g u m i n o s a e ; Anak Krakatau), Salacca (S. sumatrana, Palmae; Rakata),

Tamarindus (T. indicus, Leguminosae; Anak Krakatau),

Tab 1 Families of flowering plant first recorded after 1934

12o7

Krakatau Plant Families .......

S-pres

----

P-pres

S-all 90

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J

P-all All

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60

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30

0

1883

1903

1923

i

i

i

'i 943

1963

1983

Year

Fig 2

The cumulative number of families of higher plant recorded within the Krakatau Islands. All = all higher plant families known from the group; S-all = all spermatophytes; S-pres = all spermatophyte families for which representatives were found within the 1979-1989 sample; P-all and P-pres = equivalent classes for pteridophytes. From Whittaker et al. (1989) and unpublished data from the 1989 KRP expedition.

Tridax (T. procumbens, Compositae; Anak Krakatau and Sertung). Most of those listed for Anak Krakatau were introduced accidentally by h u m a n s (Partomihardjo et al. 1992 b), testimony to the increasing n u m b e r of occasional visitors to that island. In addition, a n u m b e r of new species and new records for particular islands were obtained (work

In each case only one species has been found, except for Commelinaceae, in which one other speeies was introduced by humans in 1929, but has since disappeared. All are endozoically dispersed species except C. smilaeifolia and A. trapeziforme which are anemochorous. Annona muricata (Sour sop) may have been introduced bY humans, as it was found near a well-used

Family

Representative

Growth Form

Year & island recorded

Rosaeeae Sapotaceae Elaeo~aceae Myrtaeeae Elaeagnaceae Ranuneulaceae Commelinaceae

Maranthes corymbosum Planchonella ducIitan Elaeocarpusgtaber Syzygium polyanthum Elaeagnus latifotia Clematis smilacifolia

Bursoraceae

Canarium hirsutum Begonia isoptera Crypteroniapaniculata Adiantum trapeziforme Annona muricata Flacourtia rukam Knema cinema Mutingia calabura Capparts sp.

Large tree Large tree Large tree Small tree Shrub ....... Herb Large tree Herb Large tree Fern Small tree Small tree Large tree Small tree Shrub

Rakata 1951 Rakata 1951 Rakata 1979 Rakata 1979 Rakata 1979 Rakata 1979 Rakata 1979 Panjang 1982 Rakata 1983 Rakata 1983+ Rakata 1989 Rakata 1989 Panjang t989 Rakata 1989 Panjang 1989 Anak 1989

Begoniaeeae Crypteroniaceae Adiantaeeae Annonaceae Flacourtiaceae Myristicaceae Tiliaeeae Capparidaeeae

Pollia secundiflora

(+ observed in 1983, but not collected and identified until 1989 survey.) Anak = Anal