Biodivers Conserv DOI 10.1007/s10531-014-0791-6 ORIGINAL PAPER

The importance of novel and hybrid habitats for plant conservation on islands: a case study from Moorea (South Pacific) Jean-Yves Meyer • Robin Pouteau • Erica Spotswood Ravahere Taputuarai • Marie Fourdrigniez

•

Received: 19 March 2014 / Revised: 18 August 2014 / Accepted: 27 August 2014 Ó Springer Science+Business Media Dordrecht 2014

Abstract Biotic homogenization caused by the invasion of non-native (alien) species is recognized as one of the main drivers of biodiversity erosion in island ecosystems. Conservation on islands is also often challenged by limited data on the distribution of remaining native habitats and threatened native and endemic species. We combine botanical survey data and habitat mapping through remote sensing and GIS to illustrate how multiple data sources can be used together to provide a synoptic view of the spatial distribution of native, naturalized and invasive alien plant species at an island scale, using the case study of Moorea (French Polynesia), a small (135 km2) tropical high volcanic island in the South Pacific. Results reveal that (i) 42 % of the rare and threatened native and endemic plant species are currently found in native habitats representing only 6 % of the island (8 km2); (ii) 49 % of these species occupy ‘‘hybrid’’ habitats covering 45 % of the island area (60 km2) where native species co-occur with naturalized non-native species; and (iii) 9 % of these species occur in 17 % of the island (23 km2) considered ‘‘novel’’ habitats that are highly invaded by alien plants that form dense monotypic stands. We conclude that conservation efforts and priorities should not neglect these novel and hybrid

Communicated by Dirk Sven Schmeller. J.-Y. Meyer (&) De´le´gation a` la Recherche, Government of French Polynesia, B.P. 20981, Papeete, Tahiti, French Polynesia e-mail: [email protected] J.-Y. Meyer
R. Pouteau
E. Spotswood
R. Taputuarai
M. Fourdrigniez Moorea Biocode Project, Richard B. Gump South Pacific Research Station, University of California at Berkeley, B.P. 244, Papetoai, Moorea, French Polynesia R. Pouteau Institut de Recherche pour le De´veloppement (IRD), UMR AMAP, Laboratoire de Botanique et d’E´cologie Ve´ge´tale Applique´es, Herbarium NOU, 98848 Noume´a, New Caledonia E. Spotswood Department of Environmental Science, Policy and Management, University of California, 130 Mulford Hall, Berkeley, CA 94720-3041, USA

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habitats on Moorea where many rare and threatened plants are found. Innovative strategies adapted to current conditions should be emphasized on small islands where conservation resources are often limited. Keywords Anthropogenic disturbances
French Polynesia
Habitat mapping
Naturalized flora
Plant invasion
Prioritization

Introduction Oceanic islands are often celebrated as natural laboratories of evolution and favored sites for biogeographical and ecological studies (Carlquist 1974; Vitousek et al. 1995; Grant 1998; Whittaker and Fernandez-Palacios 2007). Conservation is a high priority in these ecosystems harboring some of the most unique biota on Earth (Cronk and Percy 2009; Bramwell and Caujape´-Castells 2011) but also some of the most vulnerable to increasing human pressures and disturbances including biological invasions. Invasive non-native (alien) plant species can have especially dramatic effects on islands (Sax and Gaines 2008; Kueffer et al. 2010a, Kueffer et al. 2010b), where they have altered trophic interactions, resource availability, disturbance regimes and habitat structure (Vitousek et al. 2011) and caused species extinctions (Paulay 1994; Cowie 2001; Steadman 2006), leading to a global biotic homogenization (Olden 2006; Castro and Jaksic 2008). The concept of novel (also called emerging) ecosystems, dominated by non-native species, is becoming a new paradigm in ecology (Hobbs et al. 2006; Lugo 2009; Hobbs et al. 2013). It is particularly relevant to small, remote oceanic islands with depauperate and disharmonic biota (Loope and Mueller-Dombois 1989) where high rates of species introductions and invasions on small land areas may make novelty the norm (Ewel et al. 2013). On remote islands where barriers to conservation include a lack of information on the distribution of converted habitats, the novel ecosystems paradigm may provide a decision making framework that can guide the effective use of limited conservation resources. Key steps in applying the framework include evaluating existing historic and current information on the distribution of native and non-native species, identifying historic baselines for habitats of interest, and deciding whether ecosystems are native, hybrid or completely dominated by non-native species, warranting identification as novel ecosystems (sensu Hulvey et al. 2013). Setting goals also requires that ecological and social barriers to recovery must be identified, as a key step towards deciding what management activities might be most successful in each habitat type (Hulvey et al. 2013). The first step toward applying the novel ecosystem framework (Mascaro et al. 2013) is to quantify the extent of current distributions of native and alien species. On the remote and small oceanic islands of the Pacific, limited human capacity (both in governments and NGOs) and poor financial resources and infrastructures are serious obstacles to effective biodiversity conservation (Kingsford et al. 2009; Woinarski 2010; Keppel et al. 2012). Thus, prioritization of scarce resources depends on accurate information about the location of rare and threatened species as well as reliable mapping of the extent of habitat conversion (Buchanan et al. 2010; Callmander et al. 2007; Good et al. 2006). The lack of financial resources is often a barrier to acquiring this information and good data are lacking for many small Pacific islands. While the World Conservation Union (IUCN) Red List is

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being used increasingly in conservation goal setting (IUCN, The World Conservation Union 2003), the deficiency of data, particularly for plants on the islands of the Pacific is a limitation to its utilization in the region. Mapping of habitat conversion using remotely sensed imagery can be a time saving and cost effective approach to addressing this challenge (Buchanan et al. 2010). Biotic surveys and herbarium or museum databases can provide key data in the form of species occurrence records documenting the localities of populations of rare and threatened species (Graham et al. 2004). Here, we use the island of Moorea (Society Islands, French Polynesia, South Pacific) to highlight emerging conservation challenges on oceanic islands, and to evaluate whether the novel ecosystems framework can serve as a guide to conservation goal setting. On Moorea, like on many small islands in the Pacific and Indian oceans, the few remaining native forests are restricted to the highest mountains or very remote areas, and thus protected from direct human impacts. The remaining native habitats represent between 3 and 20 % of the total surface in many islands of the Indo-Pacific (Table 1); all other areas are either disturbed by anthropogenic activities or invaded by non-native plants and/or feral ungulates. The objectives of this paper are (1) to use remotely sensed imagery to quantify the spatial distribution of invaded, mixed, and native forests, and (2) to combine habitat mapping with botanical records to evaluate which habitat types currently support populations of rare and threatened native plants.

Methods This study is based on five years of intensive field surveys conducted on Moorea through the Moorea Biocode Project, a research program with the goal of barcoding the entire marine and terrestrial biota, both native and non-native, of the island (http://www. mooreabiocode.org). Moorea (Fig. 1) is a small (about 135 km2) and young (between 1.15 and 2.45 My old) tropical high volcanic island with a rugged topography. It is divided into 57 main valleys (or watersheds) separated by knife-edged ridges and high peaks, reaching 1,207 m (Mt Tohiea). Located about 18 km to the north-west of Tahiti, the largest (1,045 km2) and most populated island in French Polynesia (178,000 inhabitants in 2007, ISPF, Institut de la Statistique de Polyne´sie franc¸aise 2007), Moorea is now a favourite destination for foreign and local tourists. The island has experienced recent rapid demographic growth, with a population increase of 35.5 % between 1996 and 2007 (ISPF, Institut de la Statistique de Polyne´sie franc¸aise 2007) to a current population of ca. 17,000 inhabitants (density of 126 per km2). Rapid growth has led to increasing anthropogenic impacts, such as urbanization, deforestation for agriculture, accidental fires, and biological invasions. Moorea has a long history of anthropogenic impact, and excavation in the Opunohu valley domain (Fig. 1), which covers 1,500 ha of low lying forest, has revealed evidence of extensive forest clearing, terracing for agriculture, tree planting, and fire beginning as early as 600 AD (Lepofsky et al. 1996). Much of the island was densely settled by Polynesians before European arrival in the eighteenth century, and a crash in the indigenous population following the introduction of European diseases led to abandonment of many of the low lying areas formerly used for agriculture. Land was recolonized by secondary forest including many species introduced by Polynesians (reviewed in Lepofsky and Kahn 2011). A second wave of introductions occurred following European arrival, and introduced species have steadily spread into lowland forests during the past 150 years.

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123 South Pacific South Pacific South Pacific South Pacific Indian North Pacific Indian Indian

‘Uvea (Wallis and Futuna)

Moorea (Society Is., French Polynesia)

Tutuila (American Samoa)

Tongatapu (Tonga)

Mayotte (Comoros)

Lanai (Hawaii)

Mauritius (Mascarenes) La Re´union (Mascarenes)

361

354

268

ca. 138

135

78

56

3,070

823

1,027

660

1,030

653

1,207

151

ca. 200

650

Highest summit (m)

193.4 (7.7)k

92.8 (5)j

41.4 (11.5)i

1.1a(3.2)h

8.6a(3.2)g

25.6a(18.6)f

0.8 (6)e

0.9a(11.6)d

\6 (10)c

5 (13)b

Native habitats in km2 (%)

Meyer (2011); c Franklin and Steadman (2010); d Meyer, unpublished report (2007); e This study; fcalculated after Liu et al. (2011); g Wiser et al. (2002); h Pascal (2002); i calculated after Hobdy (1993); j Safford (1997); k Lagabrielle et al. (2011)

b

Mangroves and wetlands are excluded

2,512

South Pacific

Lakeba (Fiji)

a

1,865

South Pacific

Rapa (Austral Is., French Polynesia)

40

Ocean

Island (archipelago)

Area (km2)

Table 1 Percentage of remaining native forests in selected small high volcanic islands of the tropical Indo-Pacific region

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Fig. 1 Location of the tropical oceanic island of Moorea (Society Islands, French Polynesia) in the Pacific Ocean and topography with the main summits

Botanical surveys We conducted an assessment of all native and non-native vascular plant species in 32 major watersheds on Moorea representing an area of over 10,000 ha, and corresponding to about 75 % of the island surface (Table 2). Valleys were surveyed on foot between 2007 and 2010. Surveys were not systematic, without the use of transects or plots, and time spent in each valley varied from one to five days of field work depending on the size of the watershed. Therefore, data are more representative of natural history collections, in which survey efforts are not standardized (Graham et al. 2004). We tailored our analyses accordingly considering only presence information, since we could not verify whether the absence of a species is a true absence (Hirzel et al. 2002).We identified all endemic or native plant species considered of high conservation value in Moorea. These include all the legally protected species as well as species considered rare and threatened occurring in ten or fewer locations on the island. Some species are listed CR (critically endangered) and VU (vulnerable) on the IUCN Red List of Threatened Species (IUCN, The World Conservation Union 2003), but most are classified as DD (data deficient) or not evaluated by IUCN. Locations were defined as a geographically isolated individual or population. Scientific names of plants are those adopted by the Nadeaud plant database (Florence et al. 2007). For both botanical surveys and habitat mapping, native and non-native species were categorized using information from literature and expert opinion. Alien species were considered naturalized if they are established in the wild and reproduce without human intervention. Species were classified as invasive only if they have known impacts on ecosystem function or species richness, and include only species that are naturalized and form dense or monotypic stands. Classification categories for non-native species are taken Meyer and Fourdrigniez (2011). Habitat mapping The vegetation of Moorea was mapped by classifying a set of multi-source remotely sensed and GIS data with support vector machines (SVM) (Pouteau and Stoll 2012). The use of

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Biodivers Conserv Table 2 Survey effort for the locations of native and endemic plants on Moorea (excluding the small calcareous islets)

Number of watersheds Completely surveyed Partially surveyed

Area (ha)

29

9,162

3

861

% Island 70.5 6.6

Not surveyed

25

2,974

22.9

Total

57

12,997

100

multiple complementary images allowed us to overcome challenges associated with tropical high volcanic islands often characterized by persistent cloud cover, complex terrain and dense vegetation (Mueller-Dombois and Fosberg 1998). Remotely sensed data combined both multispectral and synthetic aperture radar (SAR) data. Multispectral data were taken from a 0.60 m-resolution scene from the satellite Quickbird-2 (2006). SAR data included a mosaic of eight 2.75 m-resolution StripMap TerraSAR-X scenes and four 5 mresolution JPL/AirSAR scenes. GIS data for environmental layers were extracted from a digital elevation model (DEM) and included elevation, slope, aspect, windwardness and a wetness index. Ground surveys were also conducted to supervise the vegetation classification. We searched and geo-located 255 circular regions of interest measuring 125 m2 distributed among 17 identified vegetation types (Papy 1951–1954; Florence 1993; and authors’ observations). Vegetation types were labelled with regard to the dominant species and clustered depending on whether the dominant species were native or non-native. Here dominant refers to species reaching the canopy which have their own spectral response, and is therefore related to the height of the vegetation rather than to its density. Each data source was classified separately using a SVM then outputs were upscaled to 5 m to match the DEM resolution and merged using a consensus SVM. We ran a contextual analysis on the final vegetation map using a moving window of 3 9 3 pixels (15 9 15 m) to detect heterogeneities. A pixel was categorized as native if it was surrounded only by native pixels, as novel (i.e., highly invaded) if it was surrounded only by non-native pixels and as hybrid where it was within a matrix of native and non-native pixels. Hybrid habitats were defined as non-urbanized and non-cultivated areas covered by a mixture of native and nonnative plants (Table 3) including Miconia calvescens (Melastomataceae), a short statured tree that is difficult to identify in satellite imagery but possible to map using species distribution models (Pouteau et al. 2011). Anthropogenic converted habitats include areas cleared for agriculture, where the dominant species were crops, urbanized areas and forestry plantations. Native wetlands, coastal vegetation and littoral forests have not been considered further because their surface area is typically too small to be detected on remotely sensed images.

Results Botanical surveys A total of 300 native vascular plants, including 122 ferns (Nitta et al. 2011) and 178 flowering plants (Angiosperms) were identified during the five years of botanical surveys. Among the latter, 150 were already recorded in the Nadeaud plant database associated with the Herbarium of French Polynesia (Florence et al. 2007) and 28 are new island records. Seventeen flowering plants are endemic to French Polynesia and to the phytogeographical

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Houses, roads, gardens, cultivated lands (banana, pineapple), forest plantations (excluding pine plantations)

Dense almost monotypic stands of the alien trees Spathodea campanulata, Falcataria moluccana, Leucaena leucocephala, Syzygium spp., Psidium spp., Pinus caribaea

low-elevation dry-mesic forests, valley and slope mid-elevation rainforests, montane cloud forests and shrublands on ridges and at peaks

Urbanized and cultivated lands

Novel (highly invaded) habitats

Native habitats

Total area (%)

43 (32)

Matrix of native and alien trees, including the invasive small tree Miconia calvescens

Hybrid habitats

135 (100)

8 (6)

23 (17)

60 (45)

Description

Habitat type

Area in km2 (%)

Table 3 Habitat classification on Moorea based on remote sensing and ground surveys. Native wetlands, coastal vegetation and littoral forests have been excluded because their areas is too small to be detected on remotely sensed images

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subregion of Southeastern Polynesia (comprising the Cook and the Pitcairn Islands), 47 are endemic to the Society Islands, six are restricted to the two islands of Tahiti and Moorea, six are island endemics, and a final six species remain undescribed and/or new to science. A total of 767 non-native species were recorded, including 433 new island records compared to the Nadeaud plant database. Among them, 327 are naturalized, including 182 species introduced as ornamentals or cultivated plants, and 145 agricultural weeds, adventives, and ruderals (mainly found on road and trail sides). The naturalized alien flora represents about 30 % of the total vascular flora on Moorea. A total of 30 rare and threatened flowering plant species were recorded, representing nearly all plant life forms, including epiphytic orchids and herbs, vines and lianas, terrestrial woody erect herbs and woody herbs, subshrubs, shrubs and trees (Table 4). Habitat mapping The habitat map (Table 3 and Fig. 2) quantified native habitats as restricted to 6 % of the island (i.e., 8 km2), isolated primarily in lowland dry to mesic forests, valley and slope rainforests, montane cloud forests and ridges and peak shrublands. The remaining 94 % of the island was classified as highly invaded or novel habitats (sensu Hobbs et al. 2013), hybrid or anthropogenic habitats. Anthropogenic conversion via urbanization, agriculture, forestry plantations accounted for 32 % of the island (43 km2), mainly in the coastal areas and the flat lowlands. Highly invaded habitats cover 17 % of the island (23 km2), where several naturalized alien trees form monotypic stands such as Falcataria (syn. Albizia) moluccana (Mimosaceae), Leucaena leucocephala (Mimosaceae), the African tulip Spathodea campanulata (Bignoniaceae), the Java plum (Myrtaceae), and the common guava Psidium guajava (Myrtaceae), or were extensively planted with Caribbean pines Pinus caribaea (Pinaceae). The remaining 45 % (60 km2) of the island is partially invaded with mixed native and non-native plant communities (classified as ‘‘hybrid’’) including 25 % (ca. 34 km2) invaded by the small tree Miconia calvescens which forms dense stands in the subcanopy forest layer (Meyer 2010a). On Moorea, the proportion of land occupied by non-native plants is at a maximum on the littoral plain, where 40–50 % of land between 0 and 100 m above sea-level is invaded. The proportion non-native plants declines to 0 % at 900 m as elevation increases. Native habitats are highly fragmented between 0 and 700 m, whereas the area above 900 m remains un-fragmented (Pouteau et al. 2013). This high elevation forest, representing 1 % of the island (i.e., 1.4 km2), is comprised mainly of native forest and vegetation restricted to the summit of mount Tohiea (1,207 m). Distribution of species of conservation concern Among the 30 rare and threatened native species found in the 32 surveyed watersheds, 9 % of their populations were located in novel habitats, 42 % were in native habitats which represent less than 6 % of the island surface, and almost half (49 %) were located in hybrid habitats (Fig. 3). The frequency of locations of these species of high conservation value differed significantly from what would be expected by chance (Chi square test p \ 0.001), with more locations occurring in hybrid and native habitats compared to highly invaded habitats (Fig. 4). Moreover, a total of 14 species were restricted to hybrid and/or novel habitats, ten were found exclusively in native habitats, and six were shared across all three habitat types.

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Bidens cf. mooreensis

End. Soc

End. Pol

End. Pf

End. Mo & Ta

Liparis clypeolum

Liparis revoluta

Taeniophyllum elegantissimum

Ind.

Ind.

Corymborkis veratrifolia

Moerenhoutia plantaginea

Ind.

End. Mo & Ta

Sclerotheca forsteri

Leptecophylla tameiameiae

Leptecophylla cf. pomarae

End. Mo

End. Mo & Ta

Fitchia sp. nov.

Lepinia taitensis

Shrubs

End. Mo & Ta

End. Mo & Ta

Ophiorrhiza spp.

Woody herbs & subshrubs

End. Mo & Ta

Calanthe tahitensis

Terrestrial orchids

Ind.

Dendrobium crispatum

Ind.

Corybas minutus

Epiphytic orchids

Decaisnina forsteriana

Epiphytic woody herb

End. Mo

Ind.

Terrestrial erect herb

Ind.

Stephania japonica var. japonica

Biogeogr. status

Pachygone vitiensis

Vines & lianas

Species name

CR(p)

(p)

(p)

(p)

(p)

(p)

(p)

(p)

(p)

(p)

DD

VU

Conserv. status

3

1

2

1

4

9

5

4

1

1

1

3

1

1

1

1

2

1

Total locations number

0

1

2

1

4

5

2

0

1

0

0

1

1

1

0

1

0

0

Native habitats

3

0

0

0

0

2

2

4

0

1

1

2

0

0

1

0

1

1

Hybrid habitats

0

0

0

0

0

2

1

0

0

0

0

0

0

0

0

0

1

0

Novel habitats

Table 4 Locations of 30 rare, threatened and/or protected native and endemic flowering plants on Moorea according to their life form. Biogeographical status according to the Nadeaud plant database (Florence et al. 2007) and pers. obs

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123

End Soc

End. Mo

End. Mo & Ra

Ind.

Ind.

Psychotria cf. franchetiana

Psychotria sp. nov.

Santalum insulare var. raiateense

Schleinitzia insularum

Tabernaemontana pandacaqui

Ind.

Streblus anthropophagorum

LR

6

1

10

32 (42)

16 (54)

2

0

5

0

0

0

0

3

0

0

1

1

Native habitats

38 (49)

19 (64)

4

0

4

2

2

1

0

4

1

2

0

0

Hybrid habitats

7 (9)

6 (20)

0

1

1

0

0

0

1

0

0

0

0

0

Novel habitats

Ind.indigenous; End.endemic (Mo Moorea, Ta Tahiti, Ra Raiatea, So Society Islands, Pf French Polynesia, Pol Southeastern Polynesia, ? taxonomical uncertainty); Conservation status according to IUCN Red List (IUCN, The World Conservation Union 2003), the Nadeaud plant database and Florence 1997, 2004: CR Critically endangered, EN Endangered, VU Vulnerable, LR Low Risk, DD data deficient; (p) French Polynesia legally protected species

30 (100)

Ind.

Sapindus saponaria

(p)

2

2

77 (100)

End. Soc

Planchonella tahitensis

DD(p)

(p)

1

1

7

1

2

1

1

Total locations number

Total of locations (%)

End. Mo & Ta

Nesoluma nadeaudii

(p)

DD(p)

(p)

LR

Conserv. status

Total of species (%)

End. Mo & Ta

Christiana vescoana

Trees

Ind.

End Mo

Melicope sp. nov.

Biogeogr. status

Maoutia australis

Species name

Table 4 continued

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Fig. 2 Map of native versus anthropogenic habitats on Moorea based on SVM classification of remotely sensed and GIS data, with the locations of 30 rare, threatened and/or protected native and endemic plant species

Watershed prioritization was remarkably different depending on whether the conservation criteria was defined as the maximal area of native habitat (Fig. 5a), the highest proportion of native habitat (Fig. 5b), or the largest number of rare and threatened native species or populations of them (Fig. 5c, 5d), both of which yielded comparable outcomes. These results highlight that a large proportion of rare and threatened native plants are found in hybrid and novel habitats.

Discussion Evaluating the extent of habitat change Our results from the island of Moorea show that the naturalized flora currently exceeds the native flora in terms of both species richness (327 vs. 300 species respectively) and total land area occupied. Areas dominated by non-native species including novel and hybrid habitats represent 62 % of the island, and up to 90 % of the forested area if urbanized areas and cultivated lands are excluded. Over half of the 30 focal rare and threatened species are currently found outside of native habitats. Many of these plant species may be considered anthropogenic endemics (Gentry 1986) or species with highly restricted distributions resulting from human intervention and habitat destruction or alteration. Native habitats represent less than 6 % of the island surface which is consistent with findings on other islands in the Pacific and elsewhere (Table 1). Other low elevation small volcanic islands

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Fig. 3 Occurrence of naturalized alien plants (N = 308) and native and endemic plants (N = 158) according to habitat types on Moorea in the 32 surveyed watershed. A plant species can be found in one, two or all the three habitats

Fig. 4 Number of locations of the 30 rare, threatened and/or protected native and endemic plants according to habitat types on Moorea. A plant species can be found in one, two or all the three habitats

in French Polynesia such as Maupiti (13 km2, 380 m elev.) and Maiao (8.3 km2, 154 m) in the Society Islands, Rimatara (8.6 km2, 84 m) in the Australs or Mangareva (15.4 km2, 441 m) in the Gambier have lost almost all their native habitats (J-Y.M., unpublished data). Native habitats comprise 42 % of the populations of rare and threatened species on Moorea, whereas hybrid habitats harbor 49 % of populations of these same species. Rare and threatened native plant species are not found consistently in all habitat types. For example, the sole species found only in novel habitats is the dry forest tree Sapindus saponaria (Sapindaceae) and the coastal shrub Schleinitzia insularum (Fabaceae). These trees could be relicts of periods before invasion, and may not be able to successfully reproduce in these areas because of competition for light, alteration of litter and soil properties, seed predation, extirpation and/or extinction of dispersal agents. Therefore,

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Fig. 5 Conservation prioritization of the 32 surveyed watersheds on Moorea according to native habitats (a,b) versus rare or threatened species (c,d). a Total area of native habitat (ha), b % of native habitat, c total number of rare, threatened and/or protected species, d total number of locations of rare, threatened and/or protected species

monitoring the population biology of these rare and threatened native species in all habitat types is critical to determine whether populations will persist in the long term. Applying the novel ecosystems framework We have very little information about what native species were present in these areas before Polynesian arrival (A. Lepofsky, P. Kirch and J. Kahn pers. comm.). Restoration to a historic baseline, if it were possible, would be challenging without adequate knowledge of prior species composition. Much of the novel and parts of the hybrid habitats are most likely secondary forest re-growth following abandonment of land cultivated several hundred years ago (Bayliss-Smith et al. 2003). Habitat mapping indicates that most of the early successional native species, with the exception of the pioneer tree Hibiscus tiliaceus (Malvaceae), are not dominant species in lowland habitats, possibly because they were outcompeted by alien species in colonizing lowland forests after abandonment. Persistent secondary vegetation and invasion by non-native species despite hundreds of years of abandonment indicate that ecological thresholds are likely to have been crossed (Hobbs et al. 2006; Hulvey et al. 2013). Additionally, introduced rats (Rattus spp.) are now widespread at all elevations, and seed predation inhibits the regeneration of some native trees (Meyer and Butaud 2009).

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The novel ecosystems framework emphasizes a focus on maintaining ecosystem function in areas where restoration to historic baselines is not possible (Hobbs et al. 2006, Hallett et al. 2013). However, a barrier to implementation on remote oceanic islands is that ecosystem functions performed by native species may be unknown. With little reference for comparison, functions performed by introduced species may or may not act as replacements for those performed by native species. For example, introduced birds on Moorea and Tahiti consume fruit and disperse seeds, but do not act as replacements for endemic birds because they tend not to consume native fruit (Spotswood et al. 2012). Additionally, the focus on ecosystem functions may be misguided in areas where species of greatest conservation concern have such small populations that any services they provide are likely minimal due to their low abundance. Due to these obstacles, a pragmatic solution may be to focus on native species found in novel ecosystems, and conservation of rare species wherever they exist regardless of what ecosystem functions they provide. Defining new conservation strategies and priorities Conservation of the last remnants of native habitats and the most threatened species remains a priority in many international, regional and national conservation policies and agendas for island ecosystems, especially in the Pacific Ocean (Atherton et al. 2007). Strategies classically focus on the recovery of endangered endemic taxa (sometimes called salvage operations), often characterized by very localized and small populations (Williamson 1981), and the legal protection and management of native areas of high ecological value which harbor many threatened species (Given 1995; Myers et al. 2000). This focus on remaining habitats considered to be native is often expensive and increasingly unrealistic in the face of rapid habitat conversion by invasive non-native plants (Hobbs et al. 2009). Habitat restoration projects are often conducted in low elevation dry forest remnants (Cabin et al. 2000), and include invasive species management and control (e.g., rat eradication, weeding, fencing), endemic species reintroduction, and population reinforcement (Godefroid et al. 2011). However, the high number of endangered species on islands makes restoration efforts difficult and expensive, and feasible only within small manageable areas, such as the special ecological areas in the Hawaiian National Parks (Tunison and Stone 1992). The best recognized strategy for plant and forest conservation is in situ within protected areas such as natural parks and reserves (see Dudley 2008). However, in the Pacific islands, most of the land is privately owned by families or clans with multiple beneficiaries that may not be capable of reaching consensus regarding establishment of protected areas (Keppel et al. 2012). On Moorea, the only substantially sized public land is the Opunohu valley domain which we identified as one of the most important watersheds due to the high number of populations of rare and threatened species (Figs. 5c, 5d). Most of the domain is covered by potential hybrid habitats dominated by a matrix of the native tree Hibiscus tiliaceus with many non-native trees, including some species introduced during the Polynesian period (Whistler 2009; Larrue et al. 2010). These ‘‘socio-ecological’’ habitats (Vitousek 2006) are witnesses of the past human occupation and of strong cultural heritage. The Opunohu valley which has been recently proposed as a natural and cultural protected area with the approval of the island County Council in July 2012 may represent a new type of management area, not fully recognized by classical protected areas categories. Ecological barriers to conservation on Moorea are also numerous. Invasive plant control in novel and hybrid habitats is a challenge with a total of 327 introduced naturalized species including widespread plants such as the fast-growing pioneer trees Spathodea

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campanulata and Falcataria moluccana. Biological control for the most aggressive plant invaders may be considered as a tool for habitat restoration (Meyer and Fourdrigniez 2011). However, altered species interactions and mutualistic networks, e.g., between avian frugivorous and fruit-bearing plants may have modified forest dynamics by favouring invasive non-native plants and compromising native and endemic plant dispersal (Spotswood et al. 2012). Moreover, the future of the small cloud forest and subalpine areas on Moorea is under the potential threat of climate change, including an increase in air temperature (Pouteau et al. 2010) also documented on other Pacific islands (Loope and Giambelluca 1998; Halloy and Mark 2003). Management options We have shown that habitats being invaded on islands can support a higher proportion of plants of conservation concern than native habitats. Clearly, hybrid habitats should not be disregarded but included in conservation plans. Restoration of highly invaded novel habitats may not be possible (Hobbs et al. 2006) and, on Moorea, persistent secondary vegetation in lowland areas may indicate ecological thresholds have been crossed. Within these areas, realistic conservation goals could focus either on improving ecosystem function, or on management techniques that might increase biodiversity (Hallett et al. 2013; Kueffer et al. 2010b). For example, control of Miconia calvescens using a fungal pathogen (Colletotrichum gloeosporioides f. sp. miconiae) increases seedling recruitment of endemic plants in the understory of dense stands (Meyer and Fourdrigniez 2011). The bio-control agent does not kill adult trees, but creates holes in leaves that increase light availability at the forest floor. This pragmatic solution is unlikely to eradicate the invasive species, but it may enhance species coexistence in the understory which could improve conservation outcomes for rare and threatened native plants (see e.g., Lugo and Helmer 2004; Wolton and MacAlpine 2013) as well as other organisms. For example, the large blocks of mixed plant species composition on Moorea provide breeding and foraging sites for the two endemic land birds, the Grey-green fruit dove Ptilinopus purpuratus frater (Columbidae) and the Tahitian kingfisher Todiramphus (syn. Halcyon) veneratus youngi (Alcedinidae), which are only found below 600 m elevation. On Moorea, remnants of native habitats are restricted to elevations above 900 m, primarily in montane cloud forests and shrublands on ridges and at peaks. These areas were protected from anthropogenic disturbances such as housing development, road construction, agriculture, forestry plantations and fires because the steep topography made these locations difficult to access by humans. Additionally, few non-native species are able to invade montane habitats on islands (Daehler 2005). Tropical montane cloud forests constitute important refuges for rare and threatened native plant and animal species in the Pacific, including French Polynesia (Merlin and Juvik 1995; Meyer 2010b). On Moorea, the few remaining native animals including arthropods, tree snails (island endemic Partula spp., Partulidae), and breeding sea birds (Tahiti petrel Pseudobulweria rostrata and Wedge-tailed shearwater Puffinus pacificus), are currently restricted to high elevation habitats. An arborescent Asteraceae Fitchia sp. nov. (an endemic genus restricted to French Polynesia and the Cook islands) was recently discovered on mountainous peaks and steep slopes. The species was previously known from pollen records in sediments of lake Temae (Parkes 1997), a wetland located at sea-level, indicating that this rare species was much more common in the past. However, a key question is whether native habitats are static or in transition i.e., whether the encroachment by non-native species may be already changing species composition within high elevation habitats.

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Conclusions In spite of challenges, the recently developed novel ecosystems framework (Mascaro et al. 2013) does provide guidance on Moorea, where a history of disturbance and species introduction have altered most of the island, a phenomenon observed in many other IndoPacific islands (see e.g., Safford 1997; Franklin et al. 2006; Franklin and Steadman 2010; Kueffer et al. 2010a). We have shown that native habitats occupy only 6 % of the island encompassing half of the rare and threatened plant species, and conservation that would focus only on these areas will miss opportunities to protect all the other plants of high conservation value that are currently found in hybrid and novel habitats. A first step towards implementation of the novel ecosystems framework is to obtain recent data characterizing the distribution of ecosystem states in the area of interest. This can be achieved using multiple data sources, which when harnessed together, can complement one another. Here, the combination of occurrence records from botanical surveys with habitat mapping enabled an assessment of the current distribution of rare and threatened species in native and disturbed or invaded habitats, as well as an identification of priority areas for conservation. The island of Moorea is experiencing increasing human pressures associated with non-native species introductions and invasions, and constitutes a true observatory of global changes where innovative and more effective conservation strategies can be designed that could act as a model for other islands in the Pacific and elsewhere. Meeting this challenge requires developing the right tools and frameworks for conservation in a variety of different types of habitats. Acknowledgments We thank Richard Hobbs (University of Western Australia, Crawley, Australia) and Lauren Hallett (University of California, Berkeley, USA) for helpful discussions about the novel ecosystem concept, Lloyd L. Loope (formerly at USGS, Haleakala National Park, Maui, Hawaii, USA) and Christoph Kueffer (Institute of Integrative Biology, Zu¨rich) for a critical review of an early draft of this paper, Benoıˆt Stoll (Universite´ de la Polyne´sie franc¸aise) and Didier Lequeux (Service de l’Urbanisme, Government of French Polynesia) for providing remotely sensed and GIS data, and the two anonymous reviewers for their useful corrections and constructive comments. The study was funded by the Moorea Biocode Project in collaboration with the Government of French Polynesia. We dedicate this paper to the staff of the Gump Biological Research Station on Moorea (including director Neil Davies, managers Hinano and Franck Murphy, secretary Valentine Brotherson, laboratory technicians Reo Terai and Vetea Liao) for their great hospitality and precious logistical support during the five years of this study.

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