Shrub encroachment is not always land degradation: Insights from ground‐dwelling beetle species niches along a shrub cover gradient in a semi‐arid Namibian savanna

Abstract

Shrub encroachment in semi‐arid savannas is induced by interacting effects of climate, fire suppression, and unsustainable livestock farming; it carries a severe risk of land degradation and strongly influences natural communities that provide key ecosystem functions. However, species‐specific effects of shrub cover on many animal groups that act as indicators of degradation remain largely unknown. We analysed the consequences of shrub encroachment for ground‐dwelling beetles in a semi‐arid Namibian savanna rangeland, where beetles and vegetation were recorded along a shrub cover gradient (<1% to >30%). Focusing on species niche breadths and optima, we identified two crucial shrub cover thresholds (2.9% and 10.0%), corresponding to major changes in the beetle communities with implications for savanna ecosystem functioning. Niche optima of most species were between the first and second thresholds; beyond the second threshold, saprophagous, coprophagous, and rare predatory beetles declined in numbers and diversity. This is problematic because beetles provide important ecosystem functions, such as decomposition and nutrient cycling. However, we also found that certain species were adapted to high shrub cover, thus providing examples of niche differentiation. Despite the predominantly negative effects of heavy shrub encroachment on beetle communities, shrubs in their early life stages apparently provide essential structures, which enhance habitat quality for ground‐dwelling beetles. Our results demonstrate that shrub encroachment can have mixed effects on ground‐dwelling beetle communities and hence on savanna ecosystem functioning. We, therefore, conclude that rangeland management and restoration should consider the complex trade‐offs between species‐specific effects and the level of encroachment for sustainable land use.

Interconnected effects of shrubs, invertebrate‐derived macropores and soil texture on water infiltration in a semi‐arid savanna rangeland

“…Apart from increasing infiltration, soil burrowing invertebrates have been found to improve ecosystem functioning, for example, by increasing soil turnover, decomposition and nutrient concentrations in the soil (Jouquet, Dauber, Lagerlöf, Lavelle, & Lepage, 2006). It is therefore of great importance to maintain a preferably heterogeneous environment with scattered shrubs and grass patches to create a suitable habitat for these organisms, as abundance and diversity of most arthropod groups peak at intermediate shrub cover (Blaum et al, 2009;Hering et al, 2018).…”

“…Shrub encroachment can be defined as the increase in density, cover and biomass of indigenous shrubs (Eldridge et al, 2011). There is strong evidence that shrub encroachment leads to changes in biodiversity of multiple taxonomic groups, including birds (Sirami, Seymour, Midgley, & Barnard, 2009), mammals (Blaum, Rossmanith, Popp, & Jeltsch, 2007) and arthropods (Blaum, Seymour, Rossmanith, Schwager, & Jeltsch, 2009;Hering et al, 2018), which may further feedback to the degradation process.…”

Partitioning of Water Between Differently Sized Shrubs and Potential Groundwater Recharge in a Semiarid Savanna in Namibia

“…This bush encroachment causes not only a significant reduction of the economic profitability and decrease in livelihoods but also leads to changes in vegetation structure and vegetation dynamics with major implications on different functions of the ecosystem (Reynolds et al, 2007; Eldridge et al, 2011; Soliveres and Eldridge, 2014). Ecological functions include biodiversity at many levels and the stable coexistence between bushes and perennial grasses (e.g., Blaum et al, 2009; Chown, 2010; Archer et al, 2017; Hering et al, 2019; Geissler et al, 2019). Water related ecohydrological functions that are affected by changes in vegetation include the modification of local climate, changed redistribution of water, and reductions of rainfall infiltration (Sharma, 1998; Bhark and Small, 2003; Maestre and Cortina, 2004; Wilcox and Thurow, 2006; Eldridge et al, 2011; Simonin et al, 2013).…”

Long‐term impacts of grazing management on land degradation in a rural community of Southern Italy: Depopulation matters

“…Abandonment of rural areas has induced a progressive renaturalization and expansion of forests showing contrasting effects and impacts. Greater absorption rate of CO 2 (Hering et al, 2019;Novara, Gristina, La Mantia, & Rühl, 2013;Robert & Saugier, 2003), biodiversity increase paralleling the expansion of natural landscapes (Laiolo, Dondero, Ciliento, & Rolando, 2004;Morán-Ordóñez, Suárez-Seoane, Calvo, & de Luis, 2011;Salvati, Perini, Sabbi, & Bajocco, 2012), soil erosion containment (Lana-Renault & Regüés, 2009;Lasanta, Arnáez, Errea, Ortigosa, & Ruiz-Flaño, 2009;Lenda et al, 2012;Moravec & Zemeckis, 2007;Navarro & Pereira, 2012;Salvati & Zitti, 2005;Zambon, Benedetti, Ferrara, & Salvati, 2018) increasing wood production (Ortuño Pérez & Martín Fernández, 2006) and, finally, a better regulation of hydrological cycle and water quality (García-Ruiz et al, 2008), are likely the most important outcomes of land abandonment, having positive environmental implications. At the same time, re-vegetation processes can induce negative effects on environmental sustainability because of pasture loss, decline of landscape quality, increase of fire risk and reduction of normal flooding (Bajocco, De Angelis, & Salvati, 2012;Bajocco, Salvati, & Ricotta, 2011;Sayadi, González-Roa, & Calatrava-Requena, 2009).…”