Introduction

Invasive alien trees have a variety of negative effects on a native community (La Maitre et al. 1996; Ashton et al. 2005; Brown et al. 2006). These effects particularly occur on oceanic islands where the ecosystems are vulnerable to alien species. Indeed, alien trees greatly disturb insular ecosystems by altering community structure (Meyer and Florence 1996; Hughes and Denslow 2005; Tuttle et al. 2009), reducing the regeneration success of native trees (Cordell et al. 2009), and changing nutrient cycling (Hughes and Denslow 2005; Tuttle et al. 2009). In the Ogasawara Islands, small oceanic islands in Japan, several invasive alien trees have altered both plant (Shimizu and Tabata 1985; Yamashita et al. 2003) and insect (Sugiura et al. 2008) communities.

On the other hand, eradication programs have begun in the Ogasawara Islands to protect endemic species from invasive alien species. Eradication of the invasive tree Bischofia javanica was started in 2002 on several islands and was completed in 2006 on Otouto-jima Island. Control of alien goats (Capra hircus) has been conducted since 1973, and complete eradication had been achieved on the islands of Minami-shima, Muko-jima, Yome-jima, Nakoudo-jima, and Ani-jima by 2009 (Government of Japan 2010). In such restoration efforts, scientific monitoring is important to assess the degree of success (Yoccoz et al. 2001; Nichols and Williams 2006).

Nishi-jima Island is undergoing eradication of alien species. The eradication of goats began in 2002 and was complete in 2007 (Government of Japan 2010). The eradication of alien black rats (Rattus rattus) began in 2007 and the population density is decreasing. Both alien mammals are listed among the world’s 100 worst invasive alien species (Lowe et al. 2000). This eradication program on Nishi-jima Island was the first trial of rat eradication in Japan. In addition, the government plans to undertake a process of integrated ecosystem management that includes the eradication of invasive plants (e.g. Casuarina equisetifolia and Leucaena leucocephala) on the island.

Previous studies have shown that eradication of herbivorous mammals enables rapid vegetation recovery (Hamann 1979), but alien plants sometimes increase unpredictably (Hata et al. 2006). Because records of Nishi-jima’s vegetation and flora are very limited, we investigated the island’s vegetation and flora before the 2006 eradication of feral goats and rats and also focused on recent vegetation changes. This information will be useful for future management of the ecosystem on Nishi-jima Island after eradication of the herbivorous mammals.

Methods

Study site

Nishi-jima Island (27°07′N, 142°10′E, 0.5 km2, 100 m a.s.l) is situated next to Chichi-jima Island and is uninhabited by humans. The first settlement of the Ogasawara Islands occurred in the 1820s, and Nishi-jima Island was also settled and cultivated in the Meiji era (1868–1912) (F. Nobushima, personal communication). There are only a few available records about the island’s biota and vegetation. For the status of the island in the 1970s, Okutomi et al. (1981) reported that the major vegetation of Nishi-jima was plantations of Pinus luchuensis and C. equisetifolia, and the remaining area was covered by a Zoysia tenuifolia community, which is secondary vegetation suppressed by goat grazing. The maximum population density of R. rattus in Nishi-jima was 90 individuals/ha before the eradication (Hashimoto 2009) and most fruits of native trees were damaged (Abe 2007).

Vegetation survey

The island’s flora was surveyed using a route census by walking the entire island. Route census was conducted 8 times during the study period between 2004 and 2007. Several routes were walked every time, and other routes were surveyed at least once (Fig. 1). On the basis of observations made while walking the census routes, the abundance of each species was categorized as abundant, common, or rare. For calculation of the proportion of endemic species and alien species in the flora, we used the number of indigenous species as denominator.

Fig. 1
figure 1

Topographic map of Nishi-jima Island. Circles represent the 10 seedling plots and squares represent the census plots of every tree in Casuarina forests. Solid lines represent the census routes surveyed every time, and dashed lines are census routes surveyed occasionally. The arrow indicates the photograph direction in Fig. 2a

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To clarify the tree composition of Casuarina forests, a census of every tree was conducted in three 400-m2 quadrats in September 2008. We measured diameter at breast height (DBH) for trees with a DBH of more than 10 cm. To describe the vegetation of the herbaceous layer, in May 2005 we surveyed the number of all seedlings of woody species and vegetation coverage (%) of herbaceous species in 40 quadrats (4 m2) on the forest floor and 10 quadrats (4 m2) in grassland. In addition, the number of seedlings of woody species less than 2 m high was counted in 10 circular plots with the diameter of 20 m (314 m2) because the number of seedlings within the 4 m2 quadrats was not enough to evaluate regeneration of woody species. This survey was conducted in May 2006.

An old vegetation map was drawn in 1979 (Environmental Agency 1979). The current vegetation map was drawn on the basis of field surveys and the satellite image from Google Earth, which was taken in the 2000s (probably 2001–2005). We made a vegetation map by walking census routes (Fig. 1) in 2006, and identified vegetation types and zones by their floristic composition and structure. There was no difficulty in identifying zones because the species composition on the island is simple. Then, vegetation boundaries of particular places where we could not check in the field because of the time constraint and because of dangerous areas, for example coastal cliff, were identified using the satellite image. The shooting date was estimated on the basis of the fact that Google Earth satellite images were taken during last 5 years in this area. In addition, satellite images are updated at specific intervals. The image was downloaded on 28 February 2008, and when we accessed Google Earth again on 15 October 2009, the image of Nishi-jima had been updated with a new image taken soon after typhoon no. 14 in 2006 (easily identified from intense forest damage over the whole island). So, we estimated that the image used in this paper was taken before 2006 at the earliest.

Areas of each vegetation type were measured using free software made by use of the macro-function on MS Excel (http://www.vector.co.jp/soft/win95/art/se312811.html).

Results

The forest vegetation of Nishi-jima Island is predominantly C. equisetifolia (Fig. 2a, b). Tree species composition was very simple in the forests, with most sites being dominated by C. equisetifolia and some Pandanus boninensis (Table 1). Native woody species were found in small forest patches, where most native trees were less than 10 m high and their crowns were not dense. A native Terminalia catappa (Combretaceae) forest was found on the southern part of the island. Other native forests were composed of Ochrosia nakaiana (Apocynaceae), Hibiscus glaber (Malvaceae), T. catappa, Livistona chinensis var. boninensis (Palmae), and Neolitsea sericea var. aurata (Lauraceae). These native forests were scattered around the central valley and occupied only approximately 3% of the island area. These patches of native forests seem to have been omitted from the 1979 vegetation map (Fig. 3), probably because their patch sizes were too small. Although the understory of native and Casuarina forests was not very dark, the vegetation cover of the herbaceous layer was usually low. In Casuarina forest, occasional seedlings of C. equisetifolia were found (Table 2) and alien Lagestroemia subcostata (Lythraceae) was found in the central valley and eastern part of the island. In the remaining native forests there were several seedlings of H. glaber, L. chinensis var. boninensis, N. sericea var. aurata, and T. catappa, but the most prevalent seedling was invasive C. equisetifolia (Table 2). Alien plants such as Acacia confusa and L. subcostata partially dominated, along with C. equisetifolia.

Fig. 2
figure 2

Various locations on Nishi-jima Island. a Landscape of Casuarina forests, b pure forest of Casuarina equisetifolia, c Zoysia tenuifolia grasslands, d grassland landscape on the southeastern peninsula, e seedlings of C. equisetifolia on remnant Z. tenuifolia, f a fruit of Pandanus boninensis predated by alien Rattus rattus

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Table 1 Results of every-tree census in Casuarina equisetifolia forests
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Fig. 3
figure 3

Vegetation changes that occurred over about 30 years on Nishi-jima Island. The vegetation map in 1979 was redrawn from Environmental Agency (1979). The vegetation map in 2006 was drawn on the basis of a field survey and a satellite image from Google Earth

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Table 2 Vegetation of the herbaceous layer in each vegetation type: (a) 4 m2 quadrat plots; mean (±SE) numbers of individuals in woody seedlings are converted to values per ha, and values of herbaceous plants are mean (±SE) vegetation cover (%); (b) 314 m2 circular plots; mean (±SE) numbers of individuals are converted to values per ha
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Grassland vegetation was found along the coastal edge and on the southwestern peninsula. This community was dominated by Z. tenuifolia (Graminaceae) (Fig. 2c), and Paspalum orbiculare (Graminaceae) also appeared at high frequency (Table 2).

According to the aerial photograph taken in 1974 there were several patchy grasslands or bare lands in the forest areas (Fig. 4), but these patches had been replaced by Casuarina forests in the 2006 vegetation map (Fig. 3). The 1979 vegetation map showed that P. luchuensis forests and Zoysia grasslands occupied a large area of the island. On the basis of comparisons of the positions of vegetation patches in the 1974 aerial photograph and the 1979 vegetation map, the forests in which the crown was not closed in the 1974 photograph would be Pinus forests. But these Pinus forests had nearly disappeared by 2006, and many had become Casuarina forests (Fig. 3). On the southwestern peninsula of the island, Pinus forests had become Zoysia grasslands and C. equisetifolia had invaded the northern slope of the peninsula (right side of Fig. 2d). Several area of Zoysia grasslands had been replaced by Casuarina forests (Figs. 2e, 3). The Pandanus community in the 1979 map had also been replaced by Casuarina forests. In the field survey, many diebacks of P. boninensis were found in this area. In total, the area of Casuarina forests increased 2.8-fold (from 4.6 to 17.6 ha) during 27 years, whereas Pinus forests and Zoysia grasslands decreased from 9.2 to 0.0 ha and from 16.1 to 12.2 ha, respectively (Fig. 5).

Fig. 4
figure 4

Aerial photograph of Nishi-jima Island taken in 1974. Broken circles indicate the areas where tree coverage had increased by 2006, and the dotted circle indicates the area where tree coverage had decreased by 2006

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Fig. 5
figure 5

Change of vegetation area between 1979 and 2006

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Sixty-nine angiosperm species were found on Nishi-jima (see Appendix). Alien angiosperm species accounted for 38% of the flora, whereas endemics accounted for 30%. Other species were common natives that are widely distributed in other regions. The most diverse taxa belonged to the Compositae (9 species), followed by Gramineae (8 species). Five endangered species listed in the Red List (Ministry of the Environment 2007) were found and populations of the alien Lagestroemia subcostata are only endangered in the original range (the Ryukyu Islands). Among the four native endangered species, Cirsium boninense, Fimbristylis longispica var. boninensis, Myrsine maximowiczii, and Psilotum nudum were rare on the island whereas Drypetes integerrima occurred frequently.

Discussion

The vegetation of Nishi-jima Island has changed greatly during the past 30 years. The two dominant vegetation types, C. equisetifolia forest and Z. tenuifolia grassland, are not natural vegetation. The great decrease noted for the alien P. luchuensis was probably caused by the pine wood nematode (Shimizu 1986), which resulted in quick colonization by alien C. equisetifolia that successfully invaded the open Pinus forest understory. Expansion by invasive trees generally requires disturbances killing crown trees in the previous forest (Fine 2002; Bellingham et al. 2005; Brown et al. 2006). Although detailed records of the original vegetation on Nishi-jima do not exist, one possible natural vegetation is likely to be dry forests composed of L. chinensis var. boninensis, O. nakaiana, and P. boninensis which are remnants of the L. chinensis var. boninensis–P. boninensis community. Zoysia grassland is secondary vegetation that has been maintained by goat grazing (Mueller-Dombois and Fosberg 1998). With the eradication of goats, Zoysia grassland would be likely to change into Casuarina forest. The vegetation zone with scarce Z. tenuifolia was often found in the Casuarina forest understory, which suggests that the previous vegetation was Z. tenuifolia grassland and that C. equisetifolia colonized the open grassland. Casuarina forests occupied a large area of the island and the number of woody species in the community was low. Although we surveyed only three quadrats for every-tree census, the result that species diversity was low in Casuarina forests agreed with previous studies (Parrotta 1995; Hata et al. 2009). Regeneration of native woody species would not be expected in Nishi-jima because competitive C. equisetifolia seedlings were the vast majority compared with seedlings of native woody species in all types of vegetation (Table 2). In addition, regeneration of native woody species in Casuarina forests is difficult probably because of lack of seed rain of native woody species and accumulation of thick litter of C. equsetifolia (Parrotta 1995; Hata et al. 2009). These facts suggest that native vegetation might not recover after eradication of alien herbivorous mammals.

Alien rats (R. rattus) are harmful seed predators and twig cutters on Ogasawara Islands (Fig. 2f; Abe 2007; Yabe et al. 2010; Abe and Umeno 2011). In addition, grazing pressure of feral goats has great effects on insular vegetation (Coblentz 1978; Courchamp et al. 2003; Hata et al. 2007). Few seedlings of native woody species were found despite rich fruit production, suggesting that these alien herbivorous mammals also have negative effects on the regeneration of native plants. The amount of endemic Nishi-jima flora (30%) was far less than that of the flora of the Ogasawara Islands as a whole (45%). The low species richness and low level of endemic Nishi-jima flora may indicate decline of native plant communities that can be caused by these alien herbivore mammals (Abe 2007; Hata et al. 2007) and expansion of invasive C. equisetifolia (Hata et al. 2009). However, several remaining endangered species enhance the value of conserving this island ecosystem. Endangered C. boninense remains on a cliff on the southwestern peninsula, where plants escaped the grazing pressure from goats. Endangered land snails of the genus Hirasea also survive under the leaves of L. chinensis var. boninensis (S. Chiba, personal communication), and patches of such environments remain on the island. Also, endemic insects that were extinct on neighboring inhabited islands are still found on Nishi-jima (Sugiura et al. 2009). But the abundance of these endangered species would have declined with the expansion of C. equisetifolia (Parrotta 1995; Hata et al. 2009). To conserve these endangered species, eradication of C. equisetifolia and rats and goats is a task of immediate importance.