Functional traits in agroecology: Advancing description and prediction in agroecosystems
2017; Wiley; Volume: 55; Issue: 1 Linguagem: Inglês
10.1111/1365-2664.13039
ISSN1365-2664
AutoresAdam R. Martin, Marney E. Isaac,
Tópico(s)Agricultural Innovations and Practices
ResumoJournal of Applied EcologyVolume 55, Issue 1 p. 5-11 EDITORIALFree Access Functional traits in agroecology: Advancing description and prediction in agroecosystems Adam R. Martin, Adam R. Martin Department of Physical and Environmental Sciences, and the Centre for Critical Development of Toronto Scarborough, Toronto, Ontario, CanadaSearch for more papers by this authorMarney E. Isaac, Corresponding Author Marney E. Isaac marney.isaac@utoronto.ca Department of Physical and Environmental Sciences, and the Centre for Critical Development of Toronto Scarborough, Toronto, Ontario, Canada Department of Geography, University of Toronto, Toronto, Ontario, Canada Correspondence Marney E. Isaac Email: marney.isaac@utoronto.caSearch for more papers by this author Adam R. Martin, Adam R. Martin Department of Physical and Environmental Sciences, and the Centre for Critical Development of Toronto Scarborough, Toronto, Ontario, CanadaSearch for more papers by this authorMarney E. Isaac, Corresponding Author Marney E. Isaac marney.isaac@utoronto.ca Department of Physical and Environmental Sciences, and the Centre for Critical Development of Toronto Scarborough, Toronto, Ontario, Canada Department of Geography, University of Toronto, Toronto, Ontario, Canada Correspondence Marney E. Isaac Email: marney.isaac@utoronto.caSearch for more papers by this author First published: 11 December 2017 https://doi.org/10.1111/1365-2664.13039Citations: 39AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat 1 THE IMPACTS AND PARADOXES OF MODERN AGRICULTURE Agricultural expansion and intensification are contributing to the world's most pressing environmental issues including global climatic change, widespread depletion and contamination of soil and water resources, major disruptions in the Earth's biogeochemical cycles and rates of species extinctions that are conspicuous on geological time-scales (Burney, Davis, & Lobell, 2010; Foley et al., 2005, 2011; Vitousek et al., 1997). Additionally, dominant models of agricultural production—largely envisioned as intensively managed monocultures—have resulted in widespread financial indebtedness of farmers, declining farmer autonomy in management decision-making, and the loss of crop genetic resources (Olson, Morris, & Mendez, 2012; Tomich et al., 2011). Paradoxically as well, while intensive industrial agriculture has increased food production rates to levels that are unprecedented in human history, they have ultimately resulted in pervasive reductions in global food security and sovereignty (Altieri & Toledo, 2011) to the extent that issues of food production, distribution and nutrition occupy a central role in the United Nation's Sustainable Development Goals (SDGs; Pérez-Escamilla, 2017). 2 FRAMING THE FIELD OF AGROECOLOGY Agroecology as a scientific discipline, an on-farm practice, and a social or political movement is gaining momentum as a contemporary lens to critique, evaluate and manage the myriad of global socio-economic and environmental issues surrounding food production (Mendez, Bacon, & Cohen, 2013). As a biophysical science, agroecology merges ecology, global change science, plant biology and soil science in order to address many of the most pressing issues of our time including food security, biodiversity loss and climate change (Altieri, 1999; Altieri, Nicholls, Henao, & Lana, 2015; Lin, 2011; Tomich et al., 2011; Tscharntke et al., 2012). But while agroecology is well positioned to address many of these pressing global issues, the science of agroecology lacks a theoretical framework that allows for the development and testing of generalizable hypotheses; especially those that are broadly relevant for farm-scale agricultural management, regional-level land-use planning or international environmental policy. Agroecological research based on a plant functional trait approach may provide such a generalizable framework. 3 TRAIT-BASED ECOLOGY FROM THEORY TO APPLICATION In the late 1970s through the 1990s, ecologists began to argue that differences in ecological strategies among plants represent the basis for explaining and predicting individual-, species- and ecosystem-level responses to environmental change (Bazzaz, 1979; Bazzaz & Carlson, 1982; Givnish, 1988; Grime, 1977, 1979, 1988; Lambers & Poorter, 1992; Reich, Walters, & Ellsworth, 1992; Westoby, 1998; Westoby, Falster, Moles, Vesk, & Wright, 2002). While such studies took a number of different approaches—e.g. the "competition, stress, disturbance" (CSR) theory (Grime, 1977, 1988), or "early-, mid- and late-successional species" classification schemes (e.g. Bazzaz, 1979)—such studies were broadly similar in that they focused on identifying a small number of functional types that could be used to categorize the ecological differences among a large number of plant species. In the 2000s, plant ecologists began to refine these ideas, by evaluating the ecologically meaningful characteristics of plants and plant parts—or the functional traits—which both (1) quantitatively differentiate species from one another along axes of ecological differences and/or functional biology (Reich et al., 1999; Westoby, 1998; Westoby et al., 2002; Wright et al., 2004, 2005), and (2) are most important in mechanistically predicting plant responses to, and impacts on, surrounding environments (Lavorel & Garnier, 2002). Thanks in part to the immense amount of species' trait data that emerged in the early 2000s (e.g. Wright et al., 2004), which has since been consolidated into massive trait databases (e.g. Kattge et al., 2011), studies differentiating trait-based ecological strategies have now come to encapsulate thousands of plant species (Diaz et al., 2016), and a global understanding of relationships between ecosystem processes and plant functional biology is being defined and refined (Diaz et al., 2004). On the basis of this rich history of theoretical and empirical research, over the past 30 years, trait-based ecology has emerged as one of the dominant paradigms in terrestrial ecology, and the theory and principles of trait-based ecology are now widely used to evaluate a remarkable range of questions and hypotheses in global change biology (e.g. Garnier, Navas, & Grigulis, 2016). From an applied perspective, trait-based ecology has more recently been embraced as a critical means by which scientists can test hypotheses on, and recommend management of, managed terrestrial ecosystems (e.g. Cadotte, 2011 and references therein). The transition of trait-based ecology into resource management fields was facilitated by clear empirical evidence that plant functional traits, and functional trait diversity are strong mechanistic predictors of ecosystem functioning (Cadotte, Cavender-Bares, Tilman, & Oakley, 2009; reviewed by Garnier et al., 2016). Scientists are now adopting trait-based approaches to research in many applied fields including restoration ecology (e.g. Funk, Cleland, Suding, & Zavaleta, 2008; Laughlin, 2014), urban ecology (Duncan et al., 2011) and more recently, agroecology (Garnier & Navas, 2012; Martin & Isaac, 2015; Milla, Osborne, Turcotte, & Violle, 2015; Wood et al., 2015). 4 TRAIT-BASED AGROECOLOGY Opinions that trait-based ecology might be efficacious in addressing major questions in agricultural systems first emerged over 5 years ago in a review by Garnier and Navas (2012) that summarized the concepts, tools and applications of functional ecology within agroecology. Then in 2015, three review papers published near-simultaneously (Martin & Isaac, 2015; Milla et al., 2015; Wood et al., 2015) summarized in greater detail the types of broad agroecological questions, hypotheses and management decisions that might be informed by trait-based research. Specifically, these papers reviewed how trait-based ecology could address questions and hypotheses surrounding the ecological consequences of crop breeding (Milla, Morente-Lopez, Alonso-Rodrigo, Martin-Robles, & Chapin, 2014; Milla et al., 2015); how functional traits underpin crop yield and yield models (Gagliardi, Martin, Virginio, Rapidel, & Isaac, 2015); how functional traits influence agricultural nutrient cycling (Garcia-Palacios et al., 2013); and how functional diversity inform an understanding of agricultural contributions to global net primary productivity (Monfreda, Ramankutty, & Foley, 2008). But while these and other examples were drawn from the literature, these review papers (Martin & Isaac, 2015; Milla et al., 2015; Wood et al., 2015) also highlighted the lack of research employing trait-based approaches to test hypotheses on linkages between on-farm diversity and agroecosystem functioning. This lack of research requires some qualification. Plant breeders, crop biologists, agronomists, ecophysiologists and farmers have certainly allocated considerable attention to understanding and selecting for crops that express certain reproductive, leaf, root, phenological and chemical traits (e.g. Meister, Rajani, Ruzicka, & Schachtman, 2014; Meyer, DuVal, & Jensen, 2012). However, this vast amount of scientific and informal on-farm research tends to focus on understanding and managing linkages between only certain functional traits—those forming part of a crop domestication syndrome (Meyer et al., 2012)—and yield. In comparison, trait-based agroecology as envisioned by applied ecologists (Garnier & Navas, 2012; Martin & Isaac, 2015; Milla et al., 2015; Wood et al., 2015), and expanded upon by the research in this Special Feature, seeks to understand how knowledge of the variation in the functional traits and trait diversity of both crop and non-crop species can be used to predict, manage and enhance multiple critical ecosystems functions other than yield alone. Discussing the value and potential application of trait-based agroecology must deal with issues of yield to some degree, as it is the key ecosystem service upon which many livelihoods are predicated. But this Special Feature has a broader goal, in seeking to significantly advance a trait-based understanding of how a wider range of above- and below-ground agroecosystem functions are linked to functional trait diversity among and within both crop and non-crop species. This Special Feature includes seven studies that employ functional trait-based approaches, to frame, test and interpret management-relevant hypotheses in agroecosystems. In doing so, cumulatively these studies provide arguably the single largest empirical and conceptual contribution to a trait-based understanding of agroecological structure and function. Specifically, these studies provide novel insights and evaluations into a wide range of agroecological questions, including how functional trait diversity is driving agricultural nutrient cycles and below-ground processes (Blesh, 2017; Pommier et al., 2017); how knowledge of functional traits can inform the restoration of degraded agricultural land (Lohbeck, Winowiecki, Aynekulu, Okia, & Vågen, 2017); how functional traits can be used to infer the ecological consequences of crop domestication (Roucou et al., 2017); how functional diversity-based approaches can be used to assess nutritional diversity in agricultural systems (Wood, 2017); and lastly, how farmers value, and make management decisions based on, functional traits of crop and non-crop species (Damour, Navas, & Garnier, 2017; Isaac et al., 2017). 5 WHAT DO WE KNOW ABOUT FUNCTIONAL TRAITS IN CROPS AND AGROECOSYSTEMS? In this Special Feature, Damour et al. (2017) make a major conceptual contribution to a broader understanding of the potential applications of trait-based agroecology, though a revised "response/effect" framework that aids researchers in hypothesizing linkages and feedbacks between plant functional diversity and ecosystem processes (Lavorel & Garnier, 2002). In their revised trait-based response and effect framework, Damour et al. (2017) outline hypotheses on how, and at which stages of management, knowledge of plant functional traits enters into agroecological management prescriptions. Damour et al. (2017) outline a key differentiation that functional diversity in agroecosystems (and in turn rates of agroecosystem service provisioning) is comprised of both planned and spontaneous plant diversity, drawn from a pool of available crop cultivars, and non-crop species (such as weeds) drawn from regional species pool respectively. The processes that result in these different functional diversity components are governed by a set of longer term strategic management decisions, shorter term on-farm tactical decisions, as well as multiple interacting socio-economic and biophysical constraints operating in different farms, regions or landscapes. Damour et al. (2017) then discuss how all of these factors lead to prospective differences in the "functional profiles" of the targeted agroecological plant communities (i.e. plant assemblages envisioned by farmers when making management decisions) vs. the realized plant communities (i.e. plant communities actually emerging following interactions between management decisions and the biophysical environment). In hypothesizing a sequence under which these different process occur, as well as the factors governing each stage, Damour et al. (2017) contribute a theoretical framework that can be employed to evaluate multiple linkages between different stages of management, on-farm environmental change, functional trait diversity, and the structure and functioning of agroecosystems. 6 FUNCTIONAL TRAIT ECOLOGY AND CROP DOMESTICATION Prior reviews and research in the field of trait-based agroecology have highlighted how intense artificial selection has resulted in major shifts in functional trait syndromes of crops (Martin et al., 2017; Milla et al., 2014, 2015), and how this variation then governs rates of agroecosystem functioning (e.g. Garcia-Palacios et al., 2013). Of all themes addressed in the trait-based agroecology literature to date, this area has arguably the strongest empirical support from studies comparing functional trait syndromes in crops vs. wild ancestors (e.g. Milla et al., 2014). In this Special Feature, Roucou et al. (2017) make a major contribution to the theme of understanding the traits other than those strictly related to yield that have also been selected for during the process of crop domestication. Through a comparative evaluation of above- and below-ground functional trait variation among 40 different accessions of wheat (Triticum turgidum), Roucou et al. (2017) provide strong empirical support for the hypothesis that due to evolutionary constraints in functional trait trade-offs, crop breeding has (likely inadvertently) also selected for suites of traits that favour high rates of resource acquisition. Their contribution is essential in defining a more comprehensive understanding of the ecological consequences of an expanded crop domestication syndrome (cf. Meyer et al., 2012), and presents lines of evidence that support the concept of functional trait-based screening for future crops (Milla et al., 2015). More practically, their research presents novel ideas surrounding how complementary functional trait syndromes can contribute to the ecologically based selection of genotypes for intercropped agroecosystems. 7 FUNCTIONAL TRAIT ECOLOGY FOR MULTI-FUNCTIONALITY IN AGROECOSYSTEMS In the applied ecology literature, studies that evaluate the complex linkages between functional traits, functional diversity and ecosystem functioning have recently begun to explore concepts surrounding "multi-functionality": the idea that multiple ecosystem functions should be considered as a management goal, vs. one or a narrow set of functions (Gamfeldt, Hillebrand, & Jonsson, 2008). The concept of multi-functionality has clear implications for trait-based agroecology, where managers may manipulate on-farm diversity to enhance certain ecosystem services beyond yield alone, or alternatively, minimize the net-negative trade-offs that might occur among ecosystem functions (Finney & Kaye, 2017; Schipanski et al., 2014). Research by Blesh (2017) addresses this theme by evaluating hypotheses on how functional diversity in temperate North American organic polyculture systems influences multiple agroecosystem functions. Blesh (2017) contributes novel empirical evidence on how the on-farm selection of crops with complementary functional traits, across a gradient of management-relevant environmental conditions, enhances the multi-functionality of agroecosystems including biological nitrogen (N) fixation (BNF), soil N retention and weed suppression. Blesh (2017) also contributes an important conceptual advancement to applied trait-based ecology, through a more detailed discussion on BNF in functional ecology. On the one hand, trait-based studies commonly view the ability to fix atmospheric N as a binary functional trait, most widely observed in plants in the Fabaceae family (Perez-Harguindeguy et al., 2013); however, Blesh (2017) points to the fact that in agroecosystems, quantitative rates of BNF should be viewed as a key ecosystem function. The work of Blesh (2017) therefore indicates that certain conventions in trait-based ecology surrounding how plant functional traits are defined need to be revisited when applied in agroecological contexts. From a management perspective, Blesh (2017) makes an important contribution to understanding how inter- and intraspecific trait variation across environmental gradients and management systems plays a key role in mediating relationships between functional diversity on agroecosystem functioning. Of all the potential applications of trait-based agroecology, understanding how traits mediate multiple ecological interactions that impact different aspects of nutrient cycles is arguably of the highest importance to farmers and land managers. At the same time, a key theme at the leading edge of trait-based ecology is better understanding how to characterize the diversity and role of functional traits in microbial species (Green, Bohannan, & Whitaker, 2008). Pommier et al. (2017) address both of these themes, by offering a robust example of how a functional trait-based approach provides new insights into our understanding and prediction of linkages between microbial diversity and nutrient cycles. Pommier et al. (2017) distil trait-mediated interactions among plant and microbial species that affect nitrogen (N) dynamics in European-managed grasslands agroecosystems, showing that variation in nitrate and ammonium leaching and soil organic matter content is largely explained by microbial traits including the maximum rate of nitrification and fungi: bacteria ratios. To date, this contribution by Pommier et al. (2017) is among very few studies that incorporate diversity of microbial traits with soil and plant metrics, representing an important advancement in the understanding of microbial-controlled ecosystem services in agroecosystems. 8 FUNCTIONAL TRAIT ECOLOGY FOR LAND MANAGEMENT AFTER THE CROPS HAVE GONE Agricultural abandonment has also emerged as a major global socio-economic and land-use trend, with major implications for human well-being, global economic systems and the Earth's biophysical environment. In turn, restoration of agricultural lands is now emerging as both a challenge and opportunity for conservation biology (e.g. Queiroz, Beilin, Folke, & Lindborg, 2014), climate change mitigation (e.g. Pan et al., 2011) and the revitalization of other ecological processes such as nutrient cycling (e.g. Knops & Tilman, 2000). Trait-based approaches to the planning and managing of biodiversity and ecosystem functioning in abandoned agricultural lands represent a major emerging vanguard for policymakers and practitioners. In this Special Feature, Lohbeck et al. (2017) provide empirical evidence on how plant functional traits and trait diversity is linked with the restoration of ecosystem functions in degraded agricultural lands. Lohbeck et al. (2017) show that enhancement of functional diversity in degraded East African agroecosystems is positively associated with key parameters of soil remediation including increased organic carbon stocks and decreased soil erosion, while the abundance of invasive species is negatively associated with these parameters. Lohbeck et al. (2017) then suggest that strategies for restoring degraded agricultural land should be informed by principles of functional ecology, with concrete policy- and practice-relevant suggestions that include farmer-lead tree selection during restoration processes. 9 FUNCTIONAL TRAIT ECOLOGY FOR AGRICULTURAL LIVELIHOODS AND POLICY While the conceptual (Damour et al., 2017) and empirical (Blesh, 2017; Lohbeck et al., 2017; Pommier et al., 2017; Roucou et al., 2017) studies in this Special Feature address major gaps in the scientific literature of agroecology, there remains an major "elephant in the trait-based agroecology room": do farmers and policymakers base their decisions on the same traits that ecologists consider vital to ecosystem functioning? Earlier papers have summarized the scant evidence that farmers make decisions based on functional ecology prioritized traits (Martin & Isaac, 2015). But here, Isaac et al. (2017) tackle this challenging theme directly by analysing farmer perceptions on how variation in the leaf economics traits of coffee (Coffea arabica) influence the management and understanding of agroecosystem functions. Through in-depth interviews and surveys, coupled with a visual elicitation tool dubbed "the leaf book," Isaac et al. (2017) show that farmers diagnose major management concerns based on, and adjust their practices at least in part according to, intraspecific variation in leaf economics traits. This research represents a major contribution to the literature on how farmers employ an understanding of plant functional traits other than yield alone to manage agroecological systems. From a wider ecological perspective, this research stands as the first to extend a major theme in the functional ecology literature—the concept of trait trade-offs and spectra (Wright et al., 2004)—into the sphere of local ecological knowledge. In theorizing the presence of a "Farmer Economics Spectrum," Isaac et al. (2017) contribute a conceptual advancement through their hypothesis that land managers acknowledge and make decisions based on the trait trade-offs that underpin inter- and intraspecific variation in plant functional biology. Wood (2017) also addresses the policy relevance of functional trait-based agroecology, through a novel empirical analysis that extends methods in functional ecology—namely functional diversity metrics—into the realm of human nutrition. Broadly, Wood (2017) presents arguments that trait-based agroecology must focus beyond the traits that mediate biophysical interactions, and instead also consider traits that are empirically linked with human well-being or other social aspects of agroecosystems. Wood (2017) then illustrates the value of this concept, by employing principles of functional trait diversity to derive a new nutritional diversity metric: potential nutritional adequacy (PNA). This metric represents a major advance in how plant functional trait diversity can be empirically linked with the nutritional outcomes associated with enhancing on-farm diversity. In his application of this new metric using data from Senegal, Wood (2017) shows that neither intensification nor diversification represent a singular approach to increasing PNA in small-holder agroecosystems; this represents a more measured and policy-realistic approach to addressing questions of food security, as compared to arguments that fall on one side of the "intensification vs. extensification" debate (Foley et al., 2011). Wood (2017) then proposes that PNA could be paired with other indicators of environmental outcomes as a means to assess the joint benefits of, or prospective trade-offs among, the provisioning of food security, food sovereignty and other ecosystem services in agroecosystems. 10 FUNCTIONAL TRAIT ECOLOGY FOR THE FUTURE OF SUSTAINABLE AGRICULTURE The research in this Special Feature demonstrates how a functional trait-based approach represents meaningful advances in integrating leading edge ecological sciences into an understanding of the linkages between agricultural diversity and agroecosystem functioning. The papers here address themes in agriculture that span from genetic to regional scales of investigation. Taken as a whole, we hope that they represent at least a small step in having trait-based agroecology contribute to the sustainable management of agricultural systems, the restoration of degraded agricultural lands, the understanding of the crop nutrition-food security nexus and the overall enhancement of agroecosystem functioning. Based on the empirical and conceptual advances put forth in this Special Feature, clearly there is a role for a trait-based approach to agroecology in guiding the composition and function of sustainable agroecosystems. Contemporary sustainable agriculture is more relevant than ever, particularly under the United Nations SDGs launched in 2015: 17 global goals with 169 total targets, designed to frame a global sustainable development agenda through the year 2030. Generally, the SDGs fall into the two primary realms: socio-economic development such as human health, education, social justice and inequality, and environmental sustainability including clean water provisioning, climate change mitigation and adaptation, and terrestrial and aquatic resource conservation. Many of the SDGs straddle this broad divide, perhaps no more than SDG 2 that calls for "End hunger, achieve food security and improved nutrition, and promote sustainable agriculture." This SDG, which entails targets surrounding agricultural production and sustainability, provides a clear opportunity for trait-based agroecology to inform a better understanding of the types of farming systems that can move towards meeting these targets. For instance, SDG 2 Target 2.4 calls for the implementation of "…resilient agricultural practices that increase productivity and production, that help maintain ecosystems, that strengthen capacity for adaptation to climate change, extreme weather, drought, flooding and other disasters, and that progressively improve land and soil quality." Sustainable Development Goal 2 Target 2.5 calls for the maintenance of a "…genetic diversity of seeds, cultivated plants, farmed and domesticated animals and their related wild species, including through soundly managed and diversified seed and plant banks." If not explicitly, the promise of trait-based agroecology is well-encapsulated by language within the SDGs that indicates a strong role for agroecology (and applied ecology more broadly) in helping to address some of the world's most pressing socio-economic and environmental concerns. In agriculture broadly, the realized and potential impacts of climate change, depletion of soil resources, as well as the loss of crop and non-crop diversity, require new approaches to frame, test and prescribe management for resilient agroecosystems. Trait-based ecology has advanced our understanding of the causes and consequences of shifts in ecological functioning of natural ecosystems, and based on the research in this Special Feature, represents a novel means of advancing our understanding of agroecosystem structure, function and performance. Most importantly, as this Special Feature demonstrates, trait-based approaches can play a key role in diagnosing crop–environment interactions, predicting crop and non-crop interactions, understanding future impacts of crop domestication and, ultimately, prescribing ecologically sound and sustainable management practices. ACKNOWLEDGEMENTS The authors thank Marc W. Cadotte, Jos Barlow, Alice Plane and Kirsty Lucas for guidance, advice and technical assistance in organizing this Special Feature. The authors also thank all authors for their contributions (and patience), and also extend their gratitude to the participants of the special session on "Functional traits in tropical agroecology" at the 2016 Annual Meeting of the Association of Tropical Biology and Conservation, where earlier discussions on trait-based agroecology were held. M.E.I. is supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, as well the Canada Research Chairs program. DATA ACCESSIBILITY Data have not been archived because this article does not contain data. REFERENCES Altieri, M. A. (1999). The ecological role of biodiversity in agroecosystems. Agriculture Ecosystems & Environment, 74, 19– 31. Altieri, M. A., Nicholls, C. I., Henao, A., & Lana, M. A. (2015). Agroecology and the design of climate change-resilient farming systems. Agronomy for Sustainable Development, 35, 869– 890. Altieri, M. A., & Toledo, V. M. (2011). The agroecological revolution in Latin America: Rescuing nature, ensuring food sovereignty and empowering peasants. Journal of Peasant Studies, 38, 587– 612. Bazzaz, F. A. (1979). Physiological ecology of plant succession. Annual Review of Ecology and Systematics, 10, 351– 371. Bazzaz, F. A., & Carlson, R. W. (1982). Photosynthetic acclimation to variability in the light envir
Referência(s)