Abstract
Land use change (LUC) is the leading cause of biodiversity loss worldwide. However, the global understanding of LUC's impact on biodiversity is mainly based on comparisons of land use endpoints (habitat vs non-habitat) in forest ecosystems. Hence, it may not generalise to savannas, which are ecologically distinct from forests, as they are inherently patchy, and disturbance adapted. Endpoint comparisons also cannot inform the management of intermediate mosaic landscapes. We aim to address these gaps by investigating species- and community-level responses of mammals and trees along a gradient of small scale agricultural expansion in the miombo woodlands of northern Mozambique. Thus, the case study represents the most common pathway of LUC and biodiversity change in the world's largest savanna. Tree abundance, mammal occupancy, and tree- and mammal-species richness showed a non-linear relationship with agricultural expansion (characterised by the Land Division Index, LDI). These occurrence and diversity metrics increased at intermediate LDI (0.3 to 0.7), started decreasing beyond LDI > 0.7, and underwent high levels of decline at extreme levels of agricultural expansion (LDI > 0.9). Despite similarities in species richness responses, the two taxonomic groups showed contrasting β-diversity patterns in response to increasing LDI: increased dissimilarity among tree communities (heterogenisation) and high similarity among mammals (homogenisation). Our analysis along a gradient of landscape-scale land use intensification allows a novel understanding of the impacts of different levels of land conversion, which can help guide land use and restoration policy. Biodiversity loss in this miombo landscape was lower than would be inferred from existing global syntheses of biodiversity-land use relations for Africa or the tropics, probably because such syntheses take a fully converted landscape as the endpoint. As, currently, most African savanna landscapes are a mosaic of savanna habitats and small scale agriculture, biodiversity loss is probably lower than in current global estimates, albeit with a trend towards further conversion. However, at extreme levels of land use change (LDI > 0.9 or < 15% habitat cover) miombo biodiversity appears to be more sensitive to LUC than inferred from the meta-analyses. To mitigate the worst effects of land use on biodiversity, our results suggest that miombo landscapes should retain > 25% habitat cover and avoid LDI > 0.75—after which species richness of both groups begin to decline. Our findings indicate that tree diversity may be easier to restore from natural restoration than mammal diversity, which became spatially homogeneous.
Highlights
Land conversion to agriculture is the major driver of global biodiversity loss, with consequences for ecosystem functioning and human wellbeing (Haddad et al 2015; Pfeifer et al 2017)
If the quadratic abundance model is preferred for tree abundance, it makes sense to use quadratic models for tree community attributes; likewise, the 3rd-degree model was selected for mammal community attributes as it was the best model for mammal occupancy
By showing that even after the positive effect of the intermediate disturbance, most mammal /tree species declined in occupancy/abundance at higher levels of land use and associated fragmentation, our results expand upon the existence of non-linear relationships and possible thresholds observed in forests (Andrén 1994; Hill and Caswell 1999; Mönkkönen and Reunanen 1999; Pardini et al 2010)
Summary
Land conversion to agriculture is the major driver of global biodiversity loss, with consequences for ecosystem functioning and human wellbeing (Haddad et al 2015; Pfeifer et al 2017). The expansion of agriculture, and the resulting loss and fragmentation of original habitats, leads to reduced habitat area, increased habitat isolation and novel ecological boundaries (Taubert et al 2018) These altered landscape characteristics amplify competition, reduce immigration and often increase predation, causing population declines, local species extinctions and changes in species compositions (Fahrig 2010; Pfeifer et al 2017). It overlooks the possibility that different land use-biodiversity relationships may exist in savannas and ignores the role of heterogeneous landscape mosaics at intermediate land use intensities with varying patch-matrix structures (Franklin and Lindenmayer 2009) While the former is essential for making accurate global biodiversity change projections, the latter is critical for informing local land and biodiversity management. In African savannas, patchy mosaic agricultural landscapes are widespread and will need to be managed carefully to meet both biodiversity and food security objectives
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