Introduction
The savanna biome dominates southern Africa, and is characterised by
continuous grassy understory, a discontinuous woody overstory (Archer et
al., 2017; Bond, 2019), and, therefore, considered a largely open
ecosystem (Pausas & Bond, 2020). However, various anthropogenic
activities (Osborne et al., 2018) pose a threat to this biome. One
pervasive threat is that of woody plant encroachment into savannas and
grasslands, linked to climate change in particular (Criado, Myers-Smith,
Bjorkman, Lehmann, & Stevens, 2020). Over the past two centuries
(Archer et al., 2017) encroachment rates have varied between -0.131 to
1.275% per year in South Africa (O’Connor, Puttick, & Hoffman, 2014),
and 0.25% per year across the rest of Africa (Stevens, Lehmann, Murphy,
& Durigan, 2017).
Broad-scale studies predict that woody plant encroachment threatens
biodiversity (Parr, Gray, & Bond, 2012), reducing the quality and
quantity of the habitat, and jeopardizing both ecological functioning
and services. In communal grazing lands and protected areas, bush
encroachment also reduce production forage and increase ectoparasites
(Archer et al., 2017). Overall, biodiversity is strongly affected by
woody plant encroachment, e.g. in Grampians National Park, southern
Australia, species richness of herbaceous growth forms was significantly
reduced (Price & Morgan, 2008) while bird communities in the Kingdom of
Eswatini showed significant shifts in structure (Sirami & Monadjem,
2012). This has real implications for conservation managers that aim to
maintain diversity.
Wingless arthropods and those with low dispersal ability will be the
worst affected by local habitat change (Blaum, Seymour, Rossmanith,
Schwager, & Jeltsch, 2009). However, gradients of shrub cover vary in
their impact, with grasshopper richness peaking at intermediate shrub
cover, but not affecting butterfly richness (Koch, Edwards,
Blanckenhorn, Walter, & Hofer, 2015). Hering et al. (2019) and Blaum et
al. (2009) reported similar mixed effects on beetles, with some families
increasing in abundance and richness at higher encroachment levels.
Different metrics might also respond differently, e.g. shrub species in
Australian woodlands were important drivers of arthropod community
composition, but did not affect their abundance (Kwok & Eldridge,
2016). However, general consensus is that woody plant encroachment will
have adverse effects on the ecological and ecosystem services (Eldridge
et al., 2011) provided by arthropods, for example pollination
(Kettenbach, Miller-Struttmann, Moffett, & Galen, 2017; Lara-Romero,
Garcia, Morente-Lopez, & Iriondo, 2016), and dung decomposition (Blaum
et al., 2009).
Ants (Hymenoptera: Formicidae), are among the most abundant terrestrial
arthropods (Hölldobler & Wilson, 1990), and perform a myriad of
ecological functions (Wills & Landis, 2018) and ecosystem services (Del
Toro, Ribbons, & Pelini, 2012). Their response to encroachment has
rarely been investigated and varies geographically from increased
abundance in the southern Kalahari (Blaum et al., 2009) and increased
richness in the south western USA (Bestelmeyer, 2005). However, Parr et
al. (2012) found no real differences between ant richness and activity
in open and closed habitats in Hluhluwe-Imfolozi Park, but observed that
more species were restricted to closed habitats and that these
restricted taxa were largely predatory (80% in the forest compared to
the 7% in open habitats). Their results mainly focus on these rare
taxa. Although they analyzed compositional differences, the
distance-based approach used leads to confounding location and
dispersion effects and has been shown to not reflect community
composition (Warton, Wright, & Wang, 2012). Here, we use generalized
linear models and their multivariate extensions to model ant species
abundance (Warton et al., 2012). Ant functional roles is largely the
result of dominant taxa in ant assemblages. Here, we complement their
study by focusing on the community as a whole, and ask (1) how
seasonality, soil and structural habitat complexity interact to affect
ant diversity
in
an African savanna biome experiencing extensive woody thickening, (2)
and identify species associated with these differences.