Plant Communities at the Periphery of the Atlantic Rain Forest: Rare-Species Bias and Its Risks for Conservation

Biological Conservation 142 (2009) 1201–1208 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon Plant communities at the periphery of the Atlantic rain forest: Rare-species bias and its risks for conservation Fabio Rubio Scarano * Universidade Federal do Rio de Janeiro, CCS, IB, Depto. de Ecologia, Caixa Postal 68020, cep21941-970, Rio de Janeiro, Brazil Instituto de Pesquisas Jardim Botânico do Rio de Janeiro, Diretoria de Pesquisa Científica, Rua Pacheco Leão 915, cep22460-030, Rio de Janeiro, Brazil article info abstract Article history: Initiatives that establish species rarity as an indicator of conservation priority might be biased if they dis- Received 17 September 2008 regard important evolutionary and adaptive processes taking place in lower diversity communities and Received in revised form 15 February 2009 ecotones. Conservation policies regarding the Atlantic forest strongly emphasize the core formation Accepted 21 February 2009 (i.e. the rainforest stricto sensu) rather than the marginal habitats (e.g. restingas, swamps, and high alti- Available online 25 March 2009 tude campos) and species that are rare/endemic. To discuss this issue I revisit a hypothesis I have forwarded in 2002 that postulates that plant colonization of habitats marginal to the Atlantic rain forests Keywords: of the State of Rio de Janeiro was largely related to terrestrial nurse plants that originally, in the rainforest Conservation priorities habitat, were canopy plants such as epiphytes or hemi-epiphytes. Adaptations to water and nutrient Ecotone Marginal habitats restrictions, typical of life in the canopy, granted success to such plants upon migration to sandy, swampy Species commonness or rocky substrates in neighbouring areas. Many such species, then, behaved as nurse plants and favoured Species rarity colonization of these more extreme habitats by a number of other rainforest species. I now review recent evidence that corroborate this hypothesis, while examining the nature of such nurse plants. In all marginal habitats, nurse plants are often highly abundant locally and have high ecophysiological vigour, while both widespread and endemic species are found among them. Thus, nursing effect, local abundance, and ecophysiological performance are not related to species geographic distribution or to their spectrum of habitat preference. Paradoxically, several abundant nurse plant species have low Darwinian fitness. These studies provoke two reflections. First, the Atlantic forest sensu lato, i.e. the core formation plus the peripheral ones, should be treated collectively as a biodiversity hotspot, rather than the core rainforest formation alone. Second, widespread or common species play important functional roles in such marginal habitats and, despite their ubiquity, ecologically they might be less fit than rare/endemic ones at the local level due, for instance, to current constraints to sexual reproduction. Thus, they should also be targeted as conservation priorities. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Buckley and Kelly (2003), for instance, did not find bionomic differences while examining pairs of common-rare species within the Species rarity and/or endemism are often targeted by conserva- same phylogenetic group, within the same locality. Curiously, de- tion initiatives. These features are intuitively associated to vulner- spite its potential applied relevance, this topic remains short of ability and risk of extinction. At least from a probabilistic point of empirical evidences (Henle et al., 2004). Nevertheless, rarity review, a rare species is more likely to go extinct than a common one. mains more of a priority than commonness when it comes to bio- Moreover, fossil records indicate that extinct species often had diversity conservation. small geographic range or low local densities (McKinney, 1997). Meanwhile, global change remains a major issue and forecasts Thus, theory predicts (Henle et al., 2004) and it is also intuitive that about species extinctions and ecosystem change are increasingly a given rare species might be biologically more fragile than a com- pessimistic (e.g. Parmesan and Yohe, 2003; Root et al., 2003), albeit mon one. However, this is not necessarily true. Gaston and Kunin some controversy (Botkin et al., 2007; Scarano, 2007a). Following (1997) compared common and rare species, based on literature, the probabilistic rationale mentioned earlier, it is also to be ex- and did not find relevant bionomical differences between them. pected that rare and/or endemic species would be the main candi- dates for extinction. However, Holt (1990) argued that a given species will either change in abundance, evolve or become extinct * Address: Universidade Federal do Rio de Janeiro, CCS, IB, Depto. de Ecologia, in response to global change, and that we did not know enough Caixa Postal 68020, cep21941-970, Rio de Janeiro, Brazil. Tel.: +55 21 25626317; fax: +55 21 25626320. ecology, physiology and genetics to tell which species would meet E-mail address: [email protected] which of these outcomes. Holt’s argument still holds and there is 0006-3207/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2009.02.027 1202 F.R. Scarano / Biological Conservation 142 (2009) 1201–1208 apparently not enough empirical biological evidence to believe is clearly being made to only one of the various physiognomies that rare species might be less adaptable to environmental change. of a broader vegetation complex, namely the rain forest sensu stric- The uprising of the biodiversity–ecosystem function paradigm to. Although broader definitions exist (e.g. Morellato and Haddad, in Ecology also appeared as an indirect challenge to the notion that 2000; Oliveira-Filho and Fontes, 2000), Rizzini (1979) has offered species rarity should be more of a conservation priority than spe- the most comprehensive of all. He argued that the Atlantic forest cies ecological role. Do all species matter for the functioning of a of Brazil should be seen as a complex formed by several plant com- given ecosystem and its respective services? Are there expendable munities, including the rain forest at its core and peripheral vege- species? Is there a role for rare species? These types of questions, tation types such as forests (e.g. swamp forests and seasonally dry while provoking some controversy both among scientists and envi- forests) and also open vegetation types (e.g. open restingas, insel- ronmentalists (e.g. Srivastava and Vellend, 2005), have been ad- bergs and high altitude campos). Table 1 provides a brief overview dressed by an increasing number of researchers during the past of the main characteristics of the vegetation types comprised by two decades (for a review see Kareiva and Levin, 2003). the Atlantic forest complex in the State of Rio de Janeiro, where These aspects cast doubt as to which extent species rarity most of the research reviewed here has been conducted. Schematic should be treated as an undisputable indicator of conservation pri- representation of the spatial distribution of these vegetation types orities. Even if one fails to admit that there might be problems with can be found in Scarano (2002) and Lüttge (2006). this indicator, it seems at least that low levels of rarity in a given In the Atlantic forest complex there is a dualism between the community or a common species itself should not be a priori dis- core rain forest and its marginal habitats. While core rain forest carded as priorities based solely on their biogeographic features. presents elevated diversity, high levels of community endemism In this paper I discuss the risks of being biased against common and habitat destruction, the marginal vegetation types do not al- species or against vegetation types with low levels of endemism ways do so (although high altitude campos are a notable excep- or rarity when establishing conservation priorities. I use the Atlan- tion). Thus, marginal habitats are less of a conservation priority tic rain forest complex as a case study and, therefore, I revisit a and, for instance, it was long before Brazil had the creation in hypothesis forwarded in Scarano (2002) that plant colonization 1998 of the Restinga de Jurubatiba National Park, the first federal of habitats marginal to the rain forest sensu stricto (i.e. swamp for- conservation unit in the country to protect a restinga ecosystem ests, high altitude rock outcrops, and the open shrubby vegetation (Barbosa et al., 2004). of the coastal sandy plains locally called restinga) was largely re- Despite marked physiognomic and floristic differences, habitats lated to terrestrial nurse plants that before migration, in their ori- marginal to the Atlantic rain forest bear some striking structural ginal rainforest habitat, were canopy plants such as epiphytes or and functional resemblance to each other (Scarano, 2002). First, hemi-epiphytes. In this review I show new evidence reinforcing the floristic composition of the marginal vegetation is strongly the hypothesis, while focusing particularly on the nature of nurse influenced by the rain forest at the core of the complex. In the low- plants and rare plants at the marginal habitats. I sustain that the lands, the geologically young restingas and swamps have more ecological and evolutionary links between core and marginal hab- than 80% of their flora originated in the rainforest (Araujo, 2000). itats at the Atlantic forest complex

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