Losing our palms: The influence of landscape-scale deforestation on Arecaceae diversity in the Atlantic forest
Introduction
Anthropogenic activities have modified tropical forest landscapes over centuries, leading to a cascade of species extinctions mostly driven by habitat loss at the landscape scale (Wright, 2010, Arroyo-Rodríguez et al., 2013). Along the eastern Brazilian coast, the Atlantic Forest is one of 35 global biodiversity hotspots, harboring a large number of endemic species that are threatened by forest loss (Martini et al., 2007, Myers et al., 2000). The Atlantic Forest was colonized as long ago as the 1500s, and it has been drastically transformed by the impact of human colonization. Currently, only 12% of the original forest cover remains intact in a mosaic of old growth and secondary forest stands (Ribeiro et al., 2009).
Habitat loss and fragmentation has led to several changes in landscape structure and configuration, greatly affecting population and community dynamics (Fahrig, 2003). For instance, forest loss alone has proven to drive plant and animal species extinctions at the landscape scale in tropical forests (Banks-Leite et al., 2012, Lima and Mariano-Neto, 2014, Morante-Filho et al., 2015). Additionally, the remaining patches are prone to decrease over time, as they are more isolated and very close to the forest edge and therefore susceptible to forest fragmentation effects; indeed, recent analyses have shown that 70% of remaining forest worldwide is within 1 km of a forest boundary (Haddad et al., 2015). However, empirical studies have demonstrated that habitat loss leads to more pervasive effects on forest biodiversity than habitat fragmentation per se (see Fahrig, 2003). Forest cover at the landscape scale can thus be considered a proxy for habitat loss, with non-linear relationships usually observed between the amount of habitat and diversity metrics for different forest biota (Pardini et al., 2010, Estavillo et al., 2013, Ochoa-Quintero et al., 2015). Species richness and compositional similarity in several tropical plant families have been strongly and positively related to forest cover, with forest sites surrounded by a greater amount of forest cover exhibiting a higher number of species and greater similarity than severely deforested landscapes (Lima and Mariano-Neto, 2014, Andrade et al., 2015). Additionally, forest loss is expected to cause alterations to local forest structure, leading forest patches to retain early successional local attributes, including a reduction in the overall basal area and increasing canopy openness (Rocha-Santos et al., 2016).
Arecaceae is an essential botanical family for tropical frugivorous animals because it provides a high abundance of fleshy fruits and seeds (Zona and Henderson, 1989, Galetti et al., 2013). Additionally, palms are key components of the forest structure, and many species have been extensively harvested by humans in the tropics (Scariot, 1999). This family is widespread in both intact and fragmented tropical forests and includes species well-adapted to a wide range of environmental conditions (Uhl and Dransfiled, 1987). South America is center of endemism for Arecaceae (Pintaud et al., 2008), with studies revealing that environmental factors such as humidity, temperature and soil fertility strongly control species distributions and are therefore likely to affect family richness and composition (Bjorholm et al., 2005, Bjorholm et al., 2006, Salm et al., 2007). Most Arecaceae forest species cope well with moderate levels of anthropogenic disturbance (Scariot, 2001, Bjorholm et al., 2005, Bjorholm et al., 2006), yet alterations including reduced seedling recruitment, competition with alien species, and disruptions to mutualistic relationships with fauna can modify population dynamics and community composition (Scariot, 1999, Wright and Duber, 2001, Fleury and Galetti, 2004). Specifically, in the Brazilian Atlantic Forest, although some species can be found in disturbed areas (e.g., Bactris spp.; Silva and Tabarelli, 2001), others are extremely sensitive to forest disturbance (e.g., Geonoma spp.) and usually exhibit reduced rates of population growth in altered environments (Svenning, 2001). As a result, habitat loss, fragmentation and palm harvesting have been considered the main threats to palms (Scariot, 1999, Tabarelli et al., 2004, Galetti et al., 2006). However, no study has investigated the community and species-specific responses to habitat loss until recently.
Here, we investigated the responses of the Arecaceae family to habitat loss at the landscape scale by evaluating species turnover in nine 16 km2 landscapes ranging from 9% to 71% forest cover in the Brazilian Atlantic Forest. Additionally, we examined the influence of canopy openness on local diversity patterns. We specifically evaluated the overall species richness, composition and abundance of palms to both a forest cover gradient at the landscape scale and canopy openness at the local scale. We also classified palm species based on their habitat type and occurrence preferences to assess the responses of both groups to a reduction in forest cover. We hypothesized that palm assemblages will become heavily affected by reduced forest cover and canopy openness, with contrasting groups (“open-area” and “forest-interior” species) responding oppositely depending on the forest cover gradient. We predicted that (i) species richness and composition similarity will increase in landscapes surrounded by a higher amount of forest cover, following similar responses from other floristic groups (Rigueira et al., 2013, Lima and Mariano-Neto, 2014, Andrade et al., 2015); (ii) the overall abundance will not be affected by forest cover, given that certain ecological groups will be favored by disturbance and consequently compensate for the reduction in others (Scariot, 1999, Andreazzi et al., 2012); (iii) species richness will also increase in areas exhibiting lower canopy openness, as these areas exhibit a better-structured canopy (Hilário and Toledo, 2016, Rocha-Santos et al., 2016); and (iv) palm species will exhibit different levels of vulnerability to habitat loss, with forest-interior species and stems greatly increasing in more forested landscapes, whereas open-area species and stems will decline in landscapes containing higher amounts of forest cover.
Section snippets
Study area
This study is part of the REDE SISBIOTA, a research network designed to investigate how landscape-scale forest loss affects patterns of regional biodiversity and processes in anthropogenic landscapes. In this context, Landsat TM images of southern Bahia from 2011 (orbits 215/70 and 215/71) were first digitalized and used to select landscapes. The obtained map was then divided into cells of 4 × 4 km (16 km2) to estimate the percentage of forest cover based on the sum of old growth and secondary
Results
We sampled 1472 adult individuals belonging to 18 species and 7 genera within 4.5 ha of forest sampled across all nine plots (Table S1). Each plot contained between 5 and 16 species (mean ± SD = 9.6 ± 1.8). The most frequent species among all surveyed landscapes was Bactris hirta (N = 8 plots), whereas Euterpe edulis was the most abundant, with 323 individuals distributed in six areas. Some species of Bactris and all species of Geonoma were more frequently recorded in landscapes exhibiting more forest
Discussion
Our study clearly demonstrates the erosion of Arecaceae richness, composition and abundance associated to reduced forest cover within landscapes of the threatened Brazilian Atlantic Forest. To the best of our knowledge, this is the first study to document the influence of reduced forest cover on an entire palm assemblage, a key ecological group characterizing the structure and composition of tropical forest (Scariot, 1999). Habitat loss has strongly influenced the structure of palm assemblages
Conclusions
Our study demonstrates the sharp collapse in palm diversity in severely deforested landscapes in the Brazilian Atlantic Forest, a key structural and functional plant component of tropical forests (Aguirre and Dirzo, 2008). Approximately 45 palm species belonging to 10 genera occur in the remaining Atlantic Forest hotspot (Lorenzi et al., 2004), with 18 recorded at our studied site in southern Bahia, including the threatened Syagrus botryophora (Noblick, 1998) and one of the most economically
Acknowledgments
The present study represents publication number #22 of the REDE SISBIOTA, funded by the Brazilian National Council for Scientific and Technological Development (CNPq; 563216/2010-7) and the Universidade Estadual de Santa Cruz (Propp 00220.1100.1039 and 00220.1100.1095). We are grateful to all landowners who enabled us to work on their properties and to all field assistants, especially Rafael Chaves and Carol Cornélio. Funding for Tamiris L. S. Oliveira and Adrielle Leal during this study were
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