Biodiversity in small-scale tropical agroforests: a review of species richness and abundance shifts and the factors influencing them

Abstract

SUMMARYAlthough small-scale agroforestry systems (swiddens, complex and single-crop-dominated agroforests, and homegardens) form a diverse and important tropical land use, there has been no attempt to collate information on their value for biodiversity. This paper reviews 52 published studies that compared species richness and/or abundance between agroforests and primary forest, and 27 studies that compared biodiversity parameters across agroforests. The former covered a broad range of taxa and geographical areas, but few focused on homegardens, while those comparing across agroforestry systems were biased towards studies of plants (21 studies) and homegardens (13 of 27). Of 43 studies comparing species richness or diversity across habitats, 34 reported lower richness in agroforests than in adjacent forest. There was also high β diversity between primary forests and agroforests. Patterns of abundance shifts were less straightforward, with many species traits (for example diets) being generally poor indicators of response to agricultural disturbance. Among the few trends identified, restricted-range or rare species, and terrestrial and some understorey vertebrates tended to decline most, and open country species, granivores and generalists increased most in agroforests. Variability in biodiversity retention across systems has been linked most strongly to economic function, management intensity and extent of remnant forest within the landscape, as well as more subtle cultural influences. Species richness and abundance generally decrease with increasing prevalence of crop species, more intensive management, decreasing stratum richness and shortening of cultivation cycles. Increasing holding size did not necessarily reduce α diversity. Knowledge of the general effects of small-scale agroforestry on biodiversity is substantial, but the great diversity of systems and species responses mean that it is difficult to accurately predict biodiversity losses and gains at a local level. Further work is required on the influence of spatial and temporal structure of agricultural holdings on biodiversity retention across agriculture/succession/forest mosaics, how β diversity across individual holdings influences biodiversity across landscapes, and ultimately on how agricultural intensification can be best managed to minimize future losses of biodiversity from tropical landscapes.

Ecosystem Services in Biologically Diversified versus Conventional Farming Systems: Benefits, Externalities, and Trade-Offs

“…While such agroforestry systems are typically less species rich than native forests (Scales andMarsden 2008, Jackson et al 2009), agroforestry systems support significant components of tropical biodiversity (e.g., 25 to 65% of forestdwelling plants and animals across 9 taxa and 14 countries (Bhagwhat et al 2008: 36 studies, 69 comparisons)), as do landscape mosaics comprising farmlands and natural or seminatural habitats (Daily et al 2001, Mayfield andDaily 2005). Management intensification in tropical agroforestry systems causes declines in species richness.…”

“…Management intensification in tropical agroforestry systems causes declines in species richness. Scales and Marsden (2008) reviewed studies of slash-and-burn fields, home gardens, and complex agroforestry systems (not including coffee), concluding that increased disturbance and reduced rates of forest regeneration decreased species richness in 83% of studies (N = 24). For coffee systems in Latin America, Philpott et al (2008a), using meta-analysis, found that management intensification caused declines in bird (N = 12) but not ant (N = 4) species.…”

Spatial and temporal variation in crop diversity in agroforestry homegardens of southern Ethiopia

“…Similar trends in homegarden dynamics have also been reported in other regions. Several studies (Nair 2006;Peyre et al 2006;Wiersum 2006;Scales and Marsden 2008) found that there is a tendency towards a gradual decrease in species diversity and structural simplification as a result of intensification of crop production. Such intensification is often related to the advent of commercialization.…”

“…However, as indicated by our data, also the need to intensify staple food production may also result in such a biodiversity loss, as well as that crop intensification may only involve a specific portion of the garden. Moreover, some studies found that dualpurpose homegardens may have higher diversity than subsistence-only homegardens, and that remote homegardens can have lower biodiversity than those close to urban centers (Scales and Marsden 2008). This indicates that the advent of cash cropping does not always result in a loss of biodiversity.…”

Sistemas agroflorestais como alternativa para as reservas legais

“…There are AFS that basically consist of simple clusters, where the paradigm is the same as that of a monoculture, of competition, where you have the combination of certain species employed to make better use of production factors, inputs and manpower; whereas other more complex AFS are based on another paradigm, which, with their ecological principles, look to the forest for their essence (PENEIREIRO, 2003). These systems can therefore be classified on a scale of sustainability and conservation potential (BHAGWAT et al, 2008;PENEIREIRO, 2003;SCALES;MARSDEN, 2008).…”