Floristic diversity of the Cagarras Islands Natural Monument, Rio de Janeiro, Brazil

were undertaken by R. Oliveira in 1980 and A.S. Rodrigues in 2000, with sporadic collecting on the archipelago islands (see Rodrigues et al. 2007). The present study presents a species list of the terrestrial flora of the islands of the recently created Cagarras Islands Natural Monument, to increase our knowledge of the vegetation growing there and provide subsidies for regional conservation planning.


Introduction
Islands have an enormous importance in terms of global biodiversity conservation, as approximately one quarter (~70,000) of all known plant species are endemic to islands. Species richness is principally determined by an island's size, its degree of geographical isolation, and present climatic conditions (Kreft et al. 2008). Unfortunately, island biotas are now seriously threatened by habitat losses and climate change, and are particularly sensitive to biological invasions (Serafini et al. 2010).
Research projects focusing on island vegetations have been relatively rare in Brazil, whether coastal (e.g., Barros et al. 1991;Menezes-Silva 1998;Oliveira 2002;Kemenes 2003;Bovini et al. 2013) or oceanic (e.g., Batistella 1996;Alves 1998Alves , 2006Gasparini 2004) -in spite of the fact that the identification and description of island plant communities are of fundamental importance to evaluations of their conservation statuses. The low resilience (and consequent fragility) of these environments demands special consideration and specific management policies to guarantee their conservation.
Rocky islands along the coast of Rio de Janeiro State are important landscape elements that contribute to both marine and terrestrial diversity (e.g., Bastos and Callado 2009;Moraes et al. 2013). These islands were linked to the mainland during the last glacial period (Wisconsin), whose maximum occurred approximately 17,000 years ago, when the sea level was approximately 110 m lower than today (Tessler and Goya 2005), which allowed greater migratory flux.
The Cagarras Archipelago is an important ecological refuge situated just off the coast from Rio de Janeiro, but its fauna and flora have been little studied until quite recently (Moraes et al. 2013). The islands (and surrounding marine areas) have important roles in regional tourism and receive many visitors during the summer months (Aguiar et al. 2013), as well as fishermen throughout the year (Moraes et al. 2013a). The first studies of the local flora expeditions to the four main and two smaller islands between July 2011 and February 2013, using the "walking" survey method (Filgueiras et al. 1994). Fertile plant material was dried using traditional botanical methods and incorporated into the herbarium at the Instituto de Pesquisas Jardim Botânico do Rio de Janeiro (RB). The plants were identified using the literature, as well as by comparisons with illustrations in the specialized literature and with collections deposited in the RB, R (National Museum), and GUA (INEA, Instituto Estadual do Ambiente) herbaria and, when necessary, by consulting specialists. The APG III (2009) classification system was followed.
The vegetation of the CINM demonstrated variable physiognomies on different islands, with plant heights and densities being related to factors such as landscape declivity, substrate type, and exposition -with vegetation formations varying from herbaceous to low forests. In many areas the vegetation was typical of "restinga" (sandy, open, nearshore vegetation), showing the clumped associations typical of this environment, and abundant species such as Abutilon esculentum A.St.-Hil., Clusia fluminensis Planch. & Triana, Hylocereus setaceus (Salm-Dyck) R.Bauer, and Neoregelia cruenta (R.Graham) L.B.Sm. One of the most prominent species in island forests was Syagrus romanzoffiana (Cham.) Glassman (common name "jerivá"). This palm tree was widely distributed in southeastern and southern Brazil (Lorenzi et al. 2004) and in the coastal mountains of Rio de Janeiro State, and is common on the higher areas of the Palmas and Redonda islands (Figure 3).
The island vegetation grows under environmental conditions distinct from those found on the continent, as the archipelago is relatively distant from propagule sources and is not influenced by the orographic rains occurring along the mainland coast.
The numbers of species on each island was very variable (Table 1) and did not appear to be directly dependent on the sizes of the islands or on the heterogeneity of available habitats. According to Bovini et al. (2013), the presence of nesting frigatebirds and brown boobies and the consequent excesses of phosphorus in the island soils contribute to strong reductions in the floristic diversity on Cagarra and Redonda islands. As such, Comprida Island had the largest number of species (98) ( Table 1), which is probably related to its relatively large size and complete absence of nesting marine birds; the second most species rich island (Palmas) likewise had no nesting birds. Cagarra Island, which has with the highest concentrations of nesting sites in the CINM, had the lowest floristic richness (23 species).
Fifteen of the species encountered were classified as being threatened with extinction to some degree according Dubard, and Cissus serroniana (Glaz.) Lombardi. Following literature searches, and our consultations of herbarium collections, it was discovered that Gymnanthes nervosa had not been collected since the 1940s, and was only found at the highest point in the CIMN on Redonda Island. Some CIMN species may be seen in figure 5.
Most of the species encountered in the CINM are shared with the coastal mainland restinga ecosystems (to which they were directly connected in the recent past). However, the occurrence of Syagrus romanzoffiana, which is uncommon in restinga areas, is apparently related to forest formations growing on latosols on the islands not used for nesting by marine birds. The species richness of each island appears to be principally related to the presence or absence of nesting colonies of marine birds (and the resulting accumulations    of guano and its very high concentrations of phosphorus).
The presence of species threatened with extinction on islands that are currently relatively well-protected from direct anthropogenic impacts makes this conservation area rather unique within the municipality of Rio de Janeiro.