Recent decades have been especially notable in the rapid accumulation of functional diversity studies (REF). Nevertheless, functional diversity still claims for a consensual definition (Petchey & Gaston 2006). A widely adopted definition is 'the value and the range of those species and organismal traits that influence ecosystem functioning' (Tilman 2001). Nevertheless, functional diversity studies may also focus on the importance of specific traits to individual fitness (Bradshaw 1987). Thus, the use of term 'function' may apply both to trophic levels and to evolutionary process (i.e., considering the function of adaptations). Regardless of the definition used, however, a consensual point is that functional diversity studies always considers organisms as 'dynamic entities that interact with their environment' (Calow 1987).
Following the definitions presented, historically functional diversity studies aimed to respond two distinct questions: how species influences ecosystem functioning (through functional effect groups), or how species respond to environmental changes (through functional response groups) (Hooper et al. 2000). Nowadays, the applicability of functional approaches were expanded to answer questions related to assembly rules (Cornwell & Ackerly 2009; D??az et al. 1998; Kraft et al. 2008), organism strategies facing severe abiotic conditions (Golodets et al. 2009; Grime 1974; Lavergne et al. 2003; Raunkiaer 1934; Westoby 1998), interspecific competition (Grime 1973), biodiversity estimates, and conservation questions (REF).
Here I reported the trajectory of functional diversity since its conception to present days, covering how concepts and applicability of the functional diversity changed along the time. I also provided an overview regarding indices used to measure functional diversity and discussed strategies to choose relevant functional traits and indices to answers different ecological question. Finally, I discuss the importance of functional diversity to economic and conservation purposes.
THE HISTORY OF FUNCTIONAL DIVERSITY
The perception that organisms could be categorized in functional groups is not recent. The first step towards the functional diversity idea was likely taken by the Greek Theophrastus, 300 B.C., in Enquiry into Plants. Theophrastus created the first botanic systematization by classifying plants according to height, and stem density (see Weiher et al. 1999).
New ideas emerged only on the 19th century, but now focusing in another functional goal: the influence of biodiversity over ecosystems. The emergence of this view was reported in On the Origin of Species (Darwin 1859) through observations of greater productivity in areas with more plant species.
In the early 20th century the functional view based on traits benefits to species was recaptured by Raunkiaer (1934) when he classified plants as life-forms, i.e., groups of organisms that respond similarly to biotic or abiotic conditions. With this classification, Raunkiaer aimed to understand plants strategies to face cold climates. At the end of the 50's Hutchinson introduced a new view in community ecology by assuming that communities are formed by groups of organisms sharing similarities regarding resource use (see Blondel 2003). It was further expanded when Root (YEAR) proposed the term guild to designate groups of animals exploiting similar resources (see Blondel 2003). However, it did not take long to a similar, but more widely applicable, term be proposed, 'functional groups' (Cummins 1974).
During 70's, ecological researchers were mainly interested in understanding species interactions with biotic and abiotic factors (Grime 1973, 1974), leading to development of new approaches to organisms classifications (Cummins 1974; Grime 1974). These new approaches aimed to classify species with respect to their roles in the ecosystem processes (Cummins 1974) and their interaction with other species (Grime 1974). For example, '.
Advances in functional ecology included the emergence of a specific journal, Functional Ecology, first published in 1987. At that time, researches topics were mainly focused on species strategies to survivorship and tolerance under distinct environmental conditions (Noble & Slatyer 1980). In the same decade, it was provided, for the first time, a clear definition of functional diversity, highlighting that 'function' is synonymous of 'adaptation', in the Darwinian sense (Bradshaw 1987; Calow 1987).
In the 90's a growing concern regarding how Earth would respond to human induced global changes motivated new ecological questions. The initial concern in explaining species distribution was gradually replaced by seek understand how species affect the functioning of ecosystems. Functional ecology had then its focus and applicability extended to address such questions. Species roles in ecosystem functioning began to be considered a key component of biodiversity (Chapin et al. 1997; Walker 1992) and the effects of different components of diversity in ecosystem functioning were accessed (Tilman et al. 1997). The need to estimate functional diversity in quick, easy and ecologically meaningful way led to new schemes of classification (Westoby 1998).
By 2000's classifications schemes such as LHS (Westoby 1998) began to be used to understand species response to several factors, such as disturbance (Golodets et al. 2009), or to predict species distribution along environmental gradients (De Frenne et al. 2010). The importance of functional diversity to ecosystem functioning (see Hooper et al. 2005) led to more specific questions such as how the order in which traits are lost affects functional diversity (Petchey & Gaston 2002b). Trait-based approaches, besides used earlier (e.g., Weiher & Keddy 1995), were expanded to understand how communities are assembled (Ackerly & Cornwell 2007; Kraft et al. 2008; Pakeman et al. 2011). At the same time, the wide use of functional diversity in ecological studies associated with a growing consensus regarding limitations of the functional group approach (Petchey & Gaston 2002b) fuelled the search for new measures of functional diversity (Botta-Duk??t 2005; Cianciaruso; Batalha; et al. 2009; Mason et al. 2005; Petchey & Gaston 2002a)
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