Rapid anthropogenic modification of tropical forest habitats has led to unprecedented rates of population loss in terrestrial vertebrates (Newbold et al. 2015, Ceballos et al. 2017). Under this context, we integrate data from four independently surveyed landscapes using a standardized approach to document consistently negative mammal responses across a broad spectrum of structural change in novel anthropogenic habitats. The effects of complete forest habitat conversion were clearly more severe than those of habitat degradation. In fact, the number of species in either logged forest (LF) or secondary forest (SF) was only lower than that in adjacent primary forest (PF) when the number of primary forest species retained in degraded habitats was considered. Considering different forms of habitat degradation, mammal responses to logging were less severe than those to second growth (i.e., higher number of species and community integrity in LF). Considering commercial tree plantations, eucalyptus (EP) and oil palm (OP) monoculture performed similarly in terms of species retention. In relation to their respective baselines, EP harboured higher levels of overall abundance and community integrity, while OP retained higher species richness in terms of both the entire assemblage and primary forest species.
Undisturbed tropical forests typically exhibit complex vertical stratification, including large emergent trees (Kricher 2011). Human-induced disturbances often severely simplify forest structure, even in the case of low-intensity disturbance such as reduced-impact logging (Edwards et al. 2012, Gibson et al. 2011). This explains the consistently lower overall mammal species diversity observed, at least of forest specialists (Banks-Leite et al. 2014, Newbold et al. 2014). Notwithstanding, mammal responses were less severe within degraded habitat types (LF and SF) than in those converted into tree monoculture (EP and OP). Although habitat degradation drives a simplification in forest structure (e.g., low understorey tree density, absence of woody lianas, thinning of large canopy trees and heavy epiphytic loads; Melo et al. 2013), such changes are not as drastic as those induced by complete stand replacement with cropland and pasture. This drastically reduces the spectrum and availability of trophic and structural resources of old-growth forests (e.g., food and shelter; Benton et al. 2003), and induces elevated temperatures and lower humidity (Hardwick et al. 2015). Our results echo previous findings showing increasingly detrimental responses to gradually more intensive patterns of land-use change (Gardner et al. 2009), which has been shown for primates (Almeida-Rocha et al. 2017), herpetofauna (Palmeirim et al. 2017) and terrestrial biodiversity in general (Newbold et al. 2015).
Our results, however, partly contradict a global meta-analysis of the biotic effects of tropical forest disturbance which reported relatively mild changes in mammal assemblages between selectively logged and unlogged forests (Gibson et al. 2011). Although selective logging was the least detrimental form of habitat modification, our before-and-after study shows that over 20% of all mammal species found in adjacent PF sites had been apparently extirpated within 1 year after logging. In addition to the severity of habitat modification, mammal persistence in anthropogenic habitats further depends on species morphoecological traits (Whitworth et al. 2019). While some species may even benefit from habitat disturbance, particularly small-bodied habitat-generalists, forest specialists tend to be driven to local extinction (Banks-Leite et al. 2014, Newbold et al. 2014). This is particularly the case of large-bodied arboreal mammals (Cardillo et al. 2005, Dirzo et al. 2014), and matches our observation of multiple primate species declining in abundance particularly in oil palm plantations (see Fig. 4; Table S2). Indeed, strictly arboreal species are expected to be most severely affected by changes in forest structure (Estrada et al. 2017). Detection rates of arboreal species depend on the habitat structure and census technique (Johns and Skorupa 1987). In particular, the single use of terrestrial camera-trapping incurs a bias in detection rates against arboreal species, underestimating the negative responses to tropical forest disturbance (Whitworth et al. 2019). Studies based on terrestrial camera-trapping alone overlook important changes in the arboreal mammal fauna, partly explaining the weak mammal responses to forest disturbance observed elsewhere (e.g. Wearn et al. 2017). In this study, we considered only data collected on the basis of line-transect censuses on foot, which ensures the highly effective detectability of the most vulnerable arboreal fauna (Benchimol and Peres 2020). However, some other species, rare or less detectable by the sighting, may have been hidden in the census. Thus, the use of different types of methods to access the mammal fauna could demonstrate more contrasting data between anthropogenic habitats and their paired forests (Mendes-Oliveira et al. 2017).
Selective logged forests harboured the highest species richness and were the only modified habitat type sustaining a species composition similar to that of its PF baseline. Although treefall gaps generated by low-impact logging operations also alter the structure of the original forest, this is widely considered one of the most benign forms of extractive land-use for tropical forest biodiversity (Edwards et al. 2014, Gibson et al. 2011). In the Amazon, microclimatic conditions are known to recover relatively fast from selective logging (Mollinari et al. 2019). Short-term effects of selective logging on Amazonian forest mammals have rarely been detected (Azevedo-Ramos et al. 2006, Bicknell and Peres 2010, Laufer et al. 2015), even in hit-and-run illegal logging within protected areas (Carvalho Jr. et al. 2020). Nevertheless, while total species richness was higher in logged forest compared to adjacent unlogged forest, the number primary forest species was higher in the latter, suggesting that some forest specialists tend to decline whereas generalists and open-habitat species tend to increase. Moreover, we only considered the short-term effects of selective logging (i.e. 1 year post-logging). Long-term effects may include additional population declines (but see Bicknell et al. 2015) and eventually species losses as the extinction debt is paid (Kuussaari et al. 2009), which is further exacerbated by the compositional decay in tree assemblages over half a century (Richardson and Peres 2016).
Secondary forests experienced the second lowest difference in species richness and overall abundance in relation to adjacent old-growth. However, early successional forests diverged in their species composition and community integrity. Although second-growth habitats are critical to the persistence of forest species in human-modified landscapes (Chazdon et al. 2009), biophysical and compositional properties fail to converge with those of primary forests even after 25 years of regrowth and remain characterized by a hyper-abundance of pioneer trees (Chazdon 2014). This explains the similar overall species richness, but reduced number of primary forest species and community integrity recorded in the 14–19 years-old secondary forests examined here. Differences in species composition with adjacent PF are likely due to a delay in recovery of forest specialists (Acevedo-Charry and Aide 2019). While current evidence worldwide attributes a relatively high conservation value to tropical secondary forests (Chazdon et al. 2009, Sodhi et al. 2010), we emphasize the limited contribution of this habitat type in terms of composition profiles (Dunn 2004), which were comparatively more detrimental than those of selective logging (Gibson et al. 2011).
Both fiber (EP) and biofuel (OP) tree monoculture clearly retained the most species-poor mammal assemblages, accounting for between 47% and 55% fewer species than their adjacent baselines, and 71–92% in terms of overall abundance. Compared to old-growth forest, tree plantations amount to non-native homogeneous habitats in which microclimatic conditions are far more hostile. For example, daytime temperatures in oil palm plantations become 6.5°C hotter than in primary forest (Luskin and Potts 2011). The native plantation undergrowth in our study areas was also frequently cleared by herbicidal treatments to maximize crop yields. Given such drastic differences (Fitzherbert et al. 2008), mammal responses observed here were in agreement with the prevailing evidence across tropical forest landscapes (Newbold et al. 2015). Furthermore, while mammal community integrity in oil palm plantations was lower than that in eucalyptus stands, the number of primary species retained in former was higher than that in the latter. Therefore, we attribute a slightly higher conservation value to EP over OP due to the higher species similarity of the former compared to primary forest. Eucalyptus plantations generally support low (Martello et al. 2018) to moderate levels of biodiversity (da Rocha et al. 2012), and are primarily occupied by habitat generalists (Teixeira et al. 2019) particularly in young plantations (Martin et al. 2012). Biotic responses to eucalyptus monoculture were variable for different taxonomic groups but particularly mild for several invertebrate taxa (Barlow et al. 2007). Our results reinforce the notion that both types of tree plantations considered here amount to highly detrimental impacts on native biodiversity, particularly under a hostile landscape context where large areas of neighbouring primary forest are no longer available. In Southeast Asia, where oil palm is extensively planted in previously forested areas, only a few native vertebrate species were reported to use these plantations (Fitzherbert et al. 2008). Despite the presence of some epiphytes and their associated species (Fayle et al. 2010), oil palm plantations are extensively managed from clear-cuts lacking overstorey shade trees, severely limiting the capacity to even vaguely mimic a closed-canopy forest (Koh 2008). This is particularly alarming given that oil palm produces the world's most-consumed vegetable oil and has been predicted to vastly expand in lowland Amazonia (Martins and Junior 2016).
Reconciling economic development with biodiversity conservation in Amazonian forestlands implies prioritizing economic activities that induce the least amount of structural forest habitat change, which then will hopefully lead to the least detrimental effects of species assemblages (Gardner et al. 2009). Our findings indicate that reduced impact logging is a preferred option over silviculture of either eucalyptus or oil palm, but this ignores opportunity costs in terms of land-use revenues. In addition, given the relatively high conservation value of secondary forests in terms of the mammal species richness and abundance, this habitat type could be managed as a safety-net against the impacts of old-growth forest loss. An adequate assessment of mammal assemblage responses to tropical forest habitat modification should also consider large-bodied arboreal species, which tend to be highly vulnerable to forest canopy fracture but severely under-sampled by camera-trapping (Whitworth et al. 2019). Finally, we found that Amazonian mammal communities within any anthropogenic habitat type do not closely resemble those in adjacent areas of largely intact primary forests. We therefore recommend setting aside large blocks of primary forest as the best strategy to maintain the full complement of vertebrate species and integrity of ecosystem functions within any given working landscape (Peres 2005). This could be accomplished by creating small networks of protected areas that interconnect old-growth forests with the wider countryside landscape (Arroyo-Rodríguez et al. 2020). The efficient adoption of these measures by local to regional governments would contribute to minimise tropical forest biodiversity loss.