Discussion
Although site-specific characteristics and seasonality dominated ant
assemblage structure, habitat type (open vs closed) had important
impacts on ant richness and composition. Open habitat had significantly
more species, but were less even than closed habitats; and, although
ants responded to gradients in soil characteristics, responses to
habitat structure changes were not gradual (Figure S4). This is
evidenced by the relatively larger role of categorial classification
(open versus closed) in explaining ant diversity. Responses to
thickening are, therefore, predicted to be abrupt. This threshold
response can partly be explained by the presence of the numerical
dominant A. custodiens in open habitats of some sites that were
mainly on clayey soils and Odontomachus traglodytes which is a
specialised as well as aggressive predator in sandy soils.
Andersen (2019) reasoned that the amount of vegetation cover is an
important driver of both flora and fauna communities. However in
contrast to our study, Nooten, Schultheiss, Rowe, Facey, and Cook (2019)
reported fewer ant species in open habitat. Notably their study aite was
in highly altered urban gardens of Greater Sydney Region of south-east
Australia. Pacheco and Vasconcelos (2012) also reported fewer species in
the least structurally complex habitats of the Brazilian Cerrado. In
contrast, Lassau and Hochuli (2004), Yusah and Foster (2016) and
Ahuatzin et al. (2019) reported a negative response of ant species
richness and diversity with habitat complexity. Similarly, Hethcoat et
al. (2019) reported more open and warmer pasturelands supporting more
ants than the compared primary or secondary forest.
Overall, habitat explained the third largest amount of conditional
variation in the ant assemblages of this study. Similarly, across the
western Soutpansberg mountains, vegetation types explained a significant
amount of variation in ant assemblages (Munyai & Foord, 2012) . This is
not surprising, given that habitat structure had been thought to
determine the movements of ants as they are usually impeded by grass
stems or woody twigs (Nooten et al., 2019). In the Amazon basin,
Guilherme et al. (2019) reported the effects of habitat complexity on
ant species foraging strategies and behaviour.
Not surprisingly, ant assemblages in the two habitats were significantly
different. It is, however, expected that natural characteristics in a
site, for example the amount of sunlight intensity, plant cover and
overall variation in habitat structure must have a clear effect on the
ant assemblage richness and composition (Campos, Soares, Martins, &
Ribeiro, 2006). This is largely because of the effect of ground surface
rugosity, which affects foraging activities, for example ant running
speed (Greve, Blaha, Teuber, Rothmaier, & Feldhaar, 2019). In this
study, closed habitat is largely structural more complex. In total, 24
% of the species were only recorded in the open habitat, compared to
the 8 % restricted to closed habitat. This, once again, emphasises the
important role of habitat openness, as detailed in Andersen (2019), and
references there in, as a crucial driver of ant assemblage composition.
However, this larger number of species in the open habitat could also be
explained by the species-area effect. Currently, open Savannas dominate
the Hluhluwe–iMfolozi Park is largely dominated by open savanna
vegetation type (Charles-Dominique, Staver, Midgley, & Bond, 2015),
covering >75% of the landscape. This in itself would
explain the larger number of species observed in the open habitat.
One site, Mansinya, had a distinct ant assemblage and therefore
highlighting on the importance of site-specific characteristics,
particularly that related to soil characteristics. The Mansinya site is
on sandy soil with specific species associated with this site (Figure
S2). Similarly, sites on their own were the most influential predictors
of ant assemblage structure, as suggested by both conditional and
marginal effects (Table 1). Whether these differences are related to
historical factors, spatial autocorrelation or variables that we did not
measure is difficult to say and beyond the scope of this study. In
general, Radnan, Gibb, and Eldridge (2018) noted that ants tend to be
more responsive to small-scale alteration on the soil surface than
overall changes in vegetation community composition. The influence of
site-specific characteristics, namely, vegetational complexity, litter,
and soil chemical properties were reported in Parui, Chatterjee, and
Basu (2015) as the driver of variation in ant assemblage composition in
the forested habitats of Eastern India.
The activities of the hyperabundant (72 % of the total abundance)Anoplolepis custodiens , increased in site with clay soils (Figure
3). In contrast, activities of Odontomachus troglodytes peaked in
sandy soils. Three other species (Myrmicaria natalensis ,Bothroponera soror and Monomorium junodi ), were more
active in loamy soils. Along an alluvial fan of the Tehuacán Valley of
central Mexico, Rios-Casanova, Valiente-Banuet, and Rico-Gray (2006)
reported high ant abundance in sandy soils. The latter study suggested
that sandy soils and habitat complexity could have favoured the
abundance of the dominant ants, which in turn could have also affected
overall ant diversity patterns in their study. Bownes, Moore, and Villet
(2014) also observed very large number of Anoplolepis custodiensin in citrus orchards of the Eastern Cape, South Africa while A.
custodiens has been observed to nest in open and well insulated soils
of the Karoo (Dean, 1992).
The high abundance, dominant behaviour, and impact of A.
custodiens on ant diversity have been documented in several southern
African studies (Mauda, Joseph, Seymour, Munyai, & Foord, 2018; Parr,
2008; Samways, 1983). Although this species is native to the region, it
is well known to prey on a variety of other invertebrates (Keiser,
Wright, & Pruitt, 2015), and also small livestock (Prins, Robertson, &
Prins, 1990), it is also a major pest in agriculture landscapes (Addison
& Samways, 2006; Addison & Samways, 2000). A. custodiens is
also well known for tending honeydew producing pest insect species, and
being nectar collectors, hence, regarded as one of the serious pests in
the agricultural systems (Dean, 1992).
A. custodiens respond
positively to disturbance (Addison & Samways, 2000; Mauda et al.,
2018). Coupled with the formation of supercolonies in open shallow soils
in particular (TC Munyai Personal observation), suggests that manual
removal of trees and shrubs should
not be done indiscriminately. The fact that this species can completely
dominate open habitat (Addison & Samways, 2006), clearing of sites
could facilitate invasion with
negative impacts not only on ant diversity, but also ecosystem services.
Particularly since A. custodiens is a polygynous (Samways, 1990),
generalist predator of other invertebrates (Dean, 1992), and small
vertebrates (Mauda et al., 2018; Prins et al., 1990), displacing other
ant species (Mitchell, 2000), and control ant assemblage structure in
the South African savanna ( Parr, 2008).
A. custodiens is particularly active in clay soils, and clearing
of woody species in the park need to be context specific.
Exploring the response of A.
custodiens to various woody
species control measures, be it manual removal of trees and shrubs,
using fire or browsing mammals as a management tool, should provide a
better understanding of possible impacts. Generally though, we consider
this a cautionary note around predicting local impacts based on
broad-scale global patterns.