ReviewDynamics of avian species and functional diversity in secondary tropical forests
Introduction
Agricultural expansion in the tropics has led to large-scale deforestation (Gibbs et al., 2010), causing loss of forest species. Traditionally, protected areas have been seen as the best way to reduce deforestation and limit the resulting loss of biodiversity. These protected areas generally consist of natural or near-natural ecosystems, such as primary forest (Dudley, 2008). In the tropics such primary forests are generally considered to be irreplaceable for their biodiversity value (Gibson et al., 2011), as well as providing numerous ecosystem services. However, biodiversity declines continue in many tropical forest protected areas (Curran, 2004, Laurance et al., 2012). Additionally, it is not always feasible to designate sufficient land to adequately represent the range of communities found in specific biomes (Cox and Underwood, 2011) or support viable populations of all species (Struhsaker et al., 2005). Thus, it is clear that we cannot rely solely on protected areas of primary forest to conserve tropical forest biodiversity.
Forests that have been altered as a result of unsustainable use or natural disasters are considered degraded, and this includes secondary forests, which have undergone forest clearance (ITTO, 2002). While degraded tropical forests may be of lower biodiversity value than primary forests, given that over half of all tropical forests are now considered to be degraded (ITTO, 2002), they may provide a valuable opportunity for conservation. Wright and Muller-Landau (2006) suggested that expansion of secondary forests could play an important role in preventing extinctions by providing alternative habitat for forest species. Previous reviews suggest that secondary forests may provide habitat for forest specialists, but also that these forests differ in their conservation value depending on connectivity, disturbance history and, in particular, site age (Bowen et al., 2007, Chazdon et al., 2009, Gardner et al., 2007). Recently, the increasing recognition of the importance of degraded forests has led to ambitious restoration targets such as the Aichi Targets and the New York Declaration on Forests, which aim to restore more than 15% of degraded forests (Convention on Biological Diversity, 2010) and 200 million hectares of degraded forests (United Nations, 2014) worldwide, respectively. However, although there are numerous site and landscape level studies, there are a lack of syntheses on the benefits of secondary forests for biodiversity and ecosystem services, and those published are largely limited to impacts on plant communities and carbon storage (Derroire et al., 2016, Martin et al., 2013) or to a limited number of biodiversity metrics, such as species richness (Dent and Wright, 2009, Dunn, 2004a).
Measures of the conservation value of an ecosystem commonly use species-based metrics (Myers et al., 2000), with the value of an area measured by the community species richness or the presence of particular species of interest. A complementary approach to species-based metrics is to assess changes in functional diversity, which describes the range of functional roles played by species within a community (Petchey and Gaston, 2006). Ecosystem functioning in general tends to be correlated with both species richness and functional diversity, with indices based on traits (e.g. feeding behaviour) performing better than those based solely on species richness and abundance (Griffin et al., 2009, Petchey and Gaston, 2006). Both the identity and distribution of functional traits have been shown to be important in predicting function (Gagic et al., 2015).
In this study we focus on birds as they provide key functions, such as pollination, seed predation and dispersal, removal of carrion, and predation of other animals, in tropical forests, and as the roles of individual species can be characterized in terms of their feeding behaviour (Sekercioglu et al., 2004). We conducted a systematic review and analysis to assess: i) how avian species richness and species richness of forest specialists in secondary tropical forests compares with that of primary tropical forests; ii) the functional diversity of avian communities in secondary tropical forests compared with that of primary tropical forests; and iii) how both metrics change, and possibly recover, with secondary forest age.
Section snippets
Data collation
Using a standard methodology (Pullin and Stewart, 2006), a systematic review of the literature was conducted in May 2013 by searching Thomson Reuters Web of Knowledge with the terms bird* AND (secondary or disturb*) AND forest AND tropic*. Additional studies were found in the reviews by Barlow et al. (2007), Bowen et al. (2007), Gardner et al. (2007), Dent and Wright (2009) and Chazdon et al. (2009). Gilroy et al. (2014) and the PREDICTS database (Hudson et al., 2017), were searched for
Results
A total of 24 studies that aimed to sample the entire avian community with data on 44 paired secondary and primary forest sites were found through the systematic review (Table 2). These studies documented 29,023 observations of 1673 bird species. Sites were widely distributed across the tropics but most were found in the Americas or Asia (Table 2, Fig. 1). Secondary forest sites had regenerated for between one and 100 years but nearly half of the sites had been disturbed within 10 years prior to
Discussion
Our study represents the largest quantitative synthesis of avian responses to secondary tropical forest succession to date. Our results indicate that avian species richness is lower in secondary forests than in primary forests. Forest specialist species richness increases with secondary forest age and is likely to take over 100 years to recover. Regarding functional diversity, two metrics (FDis and FDiv) were similar in secondary and primary forests, whereas FEve was higher in secondary forests.
Conclusion
The conservation value of secondary tropical forests will vary depending upon the aims of conservation strategies. If the aims are to support overall or forest specialist species richness at primary forest levels then our results suggest that preservation of primary forests is vital, in agreement with previous reviews (Barlow et al., 2007, Bowen et al., 2007), although the richness of forest specialist species, and hence the conservation value of regrowth, does increase with secondary forest
Conflict of interest
The authors declare that they have no conflict of interest.
Funding
P.A. Martin was funded by a NERC PhD studentship and by NERC via the Biodiversity & Ecosystem Service Sustainability (BESS) programme (Project ref. NE/K01322X/1).
Acknowledgements
Thank you to the authors of all of the studies considered, in particular S.H. Borges, C. Banks-Leite, R.F. de Lima, D. Becker, R.K. Mulwa, E.L. Neuschulz, B. Maas, G. Ferraz, D. Edwards and J. Gilroy, to A. Symes and to BirdLife International for providing data.
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