The Floristic Status of the Aldermen Islands Compared with Other Offshore Islands of the Auckland Province

61 THE FLORISTIC STATUS OF THE ALDERMEN ISLANDS COMPARED WITH OTHER OFFSHORE ISLANDS OF THE AUCKLAND PROVINCE by D.J. Court* SUMMARY Data shows that the Aldermen Islands possess a reduced indigenous flora compared to that of other offshore islands. The Aldermen have few weeds, but these are more significant because of the reduced flora. A list of adventive weed species is given showing those most frequently found on offshore islands of the Auckland Province. DISCUSSION To obtain any valid comparison of island floras it is necessary to take into account the following influencing factors: (a) Distance from the mainland. (b) Direction of prevailing winds. (c) History of any modification of the vegetation. (d) Area suitable and available for development of "coastal forest". Data for 10 islands or small groups of islands (including the Aldermen Islands) has been collated. Only those islands of the Auckland Province between 70 and 200 hectares in area have been included (Table 1). Large islands such as Mayor, Little Barrier and Hen Island have therefore been excluded from this study. Small rock stacks, such as Old Man Rock and Sail Rock (Atkinson, 1962, 1972) are also excluded. The apparently low proportion of adventive weed species on the Aldermen Islands prompted a comparison based on numbers of weed species in relation to distance offshore (Figs 1 and 2), i.e. were there fewer weed species on islands a greater distance offshore? This was found to be so to a slight extent only, as Cuvier and Burgess Islands have been thoroughly disturbed by grazing stock. All the islands considered have been burned over to a greater or lesser extent by the Maori, except West Island of the Three Kings Group. Most are in an intermediate stage of forest regeneration. The composition of both adventive and indigenous floras of all the islands is therefore probably still changing. Some important questions arise: 1. Which are the islands now able to regenerate the original coastal forest seen as remnants on isolated rock stacks? 2. Do the Aldermen belong to this group? 3. Do weed species affect regeneration of indigenous species, and if so, which weed species are involved? 4. At what rate will regeneration to coastal forest take place? •Department of Botany, University of Auckland. 62 Fig. 1. Number of adventive species versus distance offshore.* 10 20 30 40 50 Km Fig. 2: Percentage adventive species in flora versus distance offshore.* !° 20 30 40 50 Km "Figures and associated plot represent each island (see Table 1) 63 The Aldermen Islands possess a depauperate indigenous flora (Fig. 3). Apart from the inhabited Burgess Island, only the Aldermen have fewer than 100 indigenous species. The island with next fewest species is Red Mercury, and this, with 118 species, has a 45% larger flora. It would be expected then that not enough species are present on the Aldermen to form a climax coastal forest similar to the original vegetation. However, canopy species are present in restricted numbers, and seedlings of these are also present. These include Heimerliodendron brunonianum, Corynocarpus laevigatus, Planchonella novo- -zelandica, and Paratrophis banksii. Pohutukawa, now common, would also contribute to this coastal forest. Weeds on the Aldermen Islands do not seem to include any noxious species such as are found on Red Mercury and other islands. Recently, summit areas of a number of offshore islands (including Ruamahua-iti in the Aldermen Group), have been damaged or destroyed to allow survey points or automatic navigation lights to be placed there. It is imperative that damage of this nature should not occur, as the clearings allow the introduction by starlings of such plants as boxthorn (Lycium ferocissimum); introduction of two Hakea species should also be prevented. These three spinose shrubs have been shown to be injurious to petrels nesting on offshore islands. Over 100 dicot and 30 monocot weeds are reported in the literature on the offshore islands. Many of these are derived from pasture land, and it would be of interest to survey the relationship between seed source and establishment of weeds with wind-distributed seeds on the offshore islands. The species most successful in establishing are listed below (each is found on 5 or more islands): 50- 3K 30 + 10 Dm 10 30 50 »u«S«» Fig 3: Histogram to show positive or negative deviation of the number of indigenous species on individual islands from the mean (128) for ten islands. 64 1. Phytolacca octandra (10)+ 7. Sporobolus spp. (6) 2. Erigeron spp. (9)+ 8. Stellaria spp. (6)+ 3. Cerastium glomeratum (8)+ 9. Cirsium spp. (6)+ 4. Hypochaeris radicata (8)+ 10. Orobanche minor (6) 5. Sonchus oleraceus (8)+ 11. Vulpia spp. (5) 6. Anagallis arvensis (7) + 12. Polycarpon tetraphyllum (5)+ Nine of the 12 "successful" species occur on the Aldermen Islands (marked +). The Aldermen Islands are therefore very similar to the majority of other islands in this respect. It is of interest to note that Hamilton and Atkinson (1961) considered that not more than 30 adventive species would survive on Little Barrier Island if grazed pasture was removed. If Burgess Island is disregarded, a mean value (for number of adventive species per island) of 31 is obtained from the present data. The Aldermen in fact have a low number of adventive species (19), but the percentage contribution of these is not so far below that for other islands because of the depauperate indigenous flora. It must be emphasised that floristically important weed species are not necessarily important components of the structure of plant communities, e.g. Solanum nigrum was reported from 4 island groups only, but, especially on the Aldermen, it may form an important ground cover. On the Aldermen, a loss of fern species seems to have occurred over the past 20 years. Sladden and Falla (1927) listed the following 11 ferns found in 1925: Adiantum aethiopicum Dryopteris decompositum A. affine ( = Ctenitis decomposita) ( = A. cunninghamii) Polypodium diversifolium Asplenium lucidum ( = Phymatodes diversifolium) Cheilanthes sieberi Polystichum richardii Cyclophorus serpens Pteridium aquilinum Doodia media Pteris comans Cochrane (1962) listed the fo owing 14 ferns found in 1952: Adiantum aethiopicum* Pellaea rotundifolia* A. cunninghamii* Polypodium diversifolium Adiantum hispidulum* ( = Phymatodes diversifolium) Asplenium lucidum Polystichum richardii* A. flaccidum Pteridium aquilinum Cheilanthes sieberi* Pteris comans * Ctenitis decomposita* Pyrrosia serpens Doodia media * *not found in 1972. The 1972 survey (Court et al, 1973) notes only 9 fern species, of which 4 had not been previously recorded. Of these 9, three species were recorded as single specimens. The species marked* in Cochrane's list are those not found in Shortest Island(s) Author(s) Distance Area Total Native Adventive % to (Ha.) No. of Species Species Adventives History* Mainland Species in total (Km.) ; 1 MOTUOROH1 Newhook et al. 1971. 2 100 159 125 34 21 B,G 20 (Coromandel) 2 WHALE Parris et al. 1971 7 160 167 127 40 24 B 19, G20 (Whakatane) 3 MOTUMUKA Jane & Beever 1965 11 140 193 161 32 17 ?B 19,G20 (Hen & Chickens) 4 WHATAPUKE Ritchie & Ritchie 1970 14 100 181 143 38 21 B 19,?20 (Hen & Chickens) 5 RED MERCURY Lynch et al. 1972 14 200 136 118 18 13 B19.20 0> (Mercury Group) 6 COPPERMINE Atkinson 1968 16 70 194 157 37 19 B 19 (Hen & Chickens) Ritchie & Ritchie 1970 7 ALDERMEN Court et al. 1973 19 85 100 81 19 19 B 19,20 (Coromandel) 8 CUVIER Beever et al. 1973 24 180 165 125 40 24 B 18,19,G 201 20 (Coromandel) 9 BURGESS Gillham 1960 45 200 128 70 58 46 B,G 20, I 20 (Mokohinau Group) 10 THREE KINGS Baylis 1958 53 100 195 174 21 11 B18.19 (G 20) (Far North) Table 1. Offshore islands arranged according to increasing distance from mainland. *B = Burning, G = Grazing, I = European inhabitants, i.e. B,G 20 = Burning, Grazing in 20th century. Note: where appropriate, species lists have been amended to follow Parris (1969, 1971). 66 1972. Although the ferns recorded are all coastal species it seems that some of those not seen in 1972, especially Ctenitis, Adiantum and Pteris, may not be as resistant to exposure as are the species remaining. It is envisaged that, with the regeneration from dense coastal scrub in 1925 to coastal forest in 1972, much cover for fern establishment has been lost, the now open understorey only allowing ferns that can withstand wind, damage by bird movements, and possibly drier soil conditions. Of all the offshore islands considered in this study, the Aldermen possess the fewest fern species. The nearest comparison is with Burgess Island which has 14 species; in this case the low level is probably due to grazing. All the other islands have 20 or more fern species. Overall, the flora of the Aldermen Islands is most similar to that of the outlying islands of the Mercury Group save for a reduction in total indigenous species number. ACKNOWLEDGEMENTS I wish to thank Mr J.E. Braggins for reading the manuscript and offering useful comments. Thanks are also due to Miss P.A. Lynch for typing the manuscript. REFERENCES Allan, H.H 1961: "Flora of New Zealand." Vol. 1. Government Printer, Wellington. Anon. 1969: "Standard common names for weeds in New Zealand." N.Z. Weed and Pest Control Society. Atkinson, I.A.E. 1962: The flora and vegetation of Old Man Rock, Mercury Islands Group. Trans. Roy. Soc. N.Z. 1: 285-7. Atkinson, I.A.E. 1968: An ecological reconnaissance of Coppermine Island, Hen and Chickens Group. N.Z. J. Bot. 6: 285-94. Atkinson, I.A.E. 1972: Vegetation and flora of Sail Rock, Hen and Chickens Islands. N.Z.J. Bot. 10: 545-58.

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