Individual shrubs, large scale grass cover and seasonal rainfall explain invertebrate-derived macropore density in a semi-arid Namibian savanna
Introduction
In drylands the risk of degradation is predicted to be greatest in semi-arid climate where both sensitivity to degradation and human population pressure are of intermediate value (Safriel et al., 2006). Degradation in semi-arid savannas is often caused by heavy livestock grazing which, in combination with other factors as precipitation and drought frequency, can lead to shrub encroachment (Jeltsch et al., 2000; Lohmann et al., 2012; Roques et al., 2001). The increase of shrub density, in combination with a decrease of perennial palatable grasses, often leads to a decrease in carrying capacity, impaired soil water budgets and nutrient cycling, landscape fragmentation, and habitat loss for local animal and plant species (e.g. Archer, 2009; Eldridge et al., 2011). There is a solid body of literature on the effects of shrub encroachment on vegetation structure (e.g. Lett and Knapp, 2005) and related abundance and diversity of various animal groups, as mammals (Blaum et al., 2007b, 2007a), birds (Sirami et al., 2009), and insects (Blaum et al., 2009; Hering et al., 2019; Wiezik et al., 2013). However, only few have focused on functional groups as macropore-building arthropods.
Especially in the light of shrub encroachment, macropore-building invertebrates might play a vital role in maintaining ecosystem services and counteracting degradation (Byers et al., 2006; Jouquet et al., 2006; Lavelle et al., 2006). Semi-arid savanna ecosystems, which are often affected by shrub thickening, are mostly nutrient limited and driven by low seasonal water availability (D'Odorico and Porporato, 2006; Sankaran et al., 2005). Thereby, it is crucial for plant survival that a relatively large fraction of the scarce rain water infiltrates into the soil and becomes available to plants and is not lost by surface run-off and evaporation (Popp et al., 2009). Furthermore, it is expected that in future precipitation events will become less frequent but more intense, due to climate change, leading to an increased risk of drought (Diffenbaugh and Field, 2013; Van der Esch et al., 2017). Therefore, we presume that the effect of macropore-building invertebrates on water infiltration dynamics and nutrient pools in soils will be of increasing relevance under future climatic conditions. The activity of macropore-building invertebrates is considered a potential buffer for climate change in savannas (Bonachela et al., 2015), and as these climate zones are most prone to degradation and extreme climate events, these organisms may play a crucial role in restoration management (Byers et al., 2006; Colloff et al., 2010; Kaiser et al., 2017). Due to their far reaching impacts on ecosystems, macropore-creating invertebrates can be regarded as ecosystem engineers.
Ecosystem engineers are defined as organisms that modulate the availability of resources for other species by causing physical state changes in biotic or abiotic materials (Jones et al., 1996, 1997). In soil, the relative importance of regulation imposed by ecosystem engineering is likely to be greater than trophic relationships (Lavelle, 2002). However, until today most research on macropore-creating organisms focused on earthworms in temperate ecosystems (Jouquet et al., 2006). In tropical and subtropical climate zones, including a large fraction of global drylands, the functional invertebrate species engineering soils are primarily termite and ant species (Jouquet et al., 2006).
In drylands, macropore-creating invertebrates can influence ecosystems on multiple scales inducing far reaching effects and feedbacks (Colloff et al., 2010; Jouquet et al., 2006; Lavelle et al., 2006). Termites, for example, have the capacity to build galleries and modify soil aggregates on different scales. On the scale of soil structure, termite activity impacts soil's clay content and nutrient cycling, thereby affecting inter alia the water holding capacity. On the soil profile scale, large amounts of soil are translocated and increased soil porosity is leading to higher water infiltration rates and preferential flow. On the landscape level, these combined effects increase heterogeneity by generating islands of fertility and are counteracting water runoff and erosion (Bottinelli et al., 2015; Jouquet et al., 2016). Thereby termite mounds shape the spatial structure of vegetation on large scales (e.g. Bonachela et al., 2015). Even though termites are the most prominent representatives of tropical soil ecosystem engineers, several subterranean ant and beetle species are also considered to impact ecosystems in a likewise manner. For example, Nkem et al. (2000) showed that ants influence soil structure, nutrient distribution, porosity, and infiltration beyond the parameter of the mound and into the surrounding ecosystem. Soil-burrowing dung beetles improve soil-hydrological properties, by increasing water infiltration and soil porosity, and reducing surface water runoff (Brown et al., 2010). However, most studies only analyzed the ecosystem function of a specific taxon of soil engineering invertebrates. In contrast, Colloff et al. (2010) demonstrated an increase in infiltration induced by the total number of invertebrate macropores. In a previous study, we also found a higher number of macropores under shrub canopies compared to open soil, increasing the sub-canopy soil's infiltration capacity (Marquart et al. in revision). Therefore, we argue that the number and sizes of macropores might give a good indication of the strength and effectivity exerted by macropore-building species on ecosystem functioning.
However, it remains unclear to which extent the activity of these mostly herbivore macropore-building invertebrates is influenced by the proximate presence of single plants or rather by broad scale vegetation cover. We would expect a higher number of invertebrate created macropores next to vegetation structures, especially under shrub canopies, compared to open soil. In particular, shrubs are known to provide favorable habitats for macro-arthropods (i.e. increased soil litter biomass and soil moisture, improved soil texture, and soil fertility) (Zhao and Liu, 2013). On the broader landscape scale, we hypothesize that macropores would be most abundant in areas with intermediate shrub cover levels, as arthropod richness is highest at medium encroached states, and arthropod abundance was found to be positively correlated with shrub cover (Blaum et al., 2009; Hering et al., 2019). Furthermore, vegetation cover in a grassland ecosystem was found to be correlated with the mound size of subterranean ant species (Blomqvist et al., 2000). Previous studies have found a higher soil macro-arthropod density during the rainy season compared to the dry season (Vikram Reddy and Venkataiah, 1990). Termites, as macropore-building species, display mixed results, but for semi-arid to arid environments their activity mostly peaked within the rainy season (e.g. Davies et al., 2015; Dawes-Gromadzki and Spain, 2003). Temporal dynamics of macropore-building arthropod activity within the rainy season have not been reported yet.
Here, we research if macropore dynamics are primarily controlled by vegetation structure on a small scale or by landscape wide vegetation cover in a semi-arid Namibian savanna rangeland. The largest fraction of Namibian land surface is used for extensive livestock farming as main agricultural activity, making shrub encroachment through degradation to one of the major social concerns (Namibia Ministry of Environment and Tourism, 2011). However, shrub encroachment in semi-arid savannas is a global phenomenon (Eldridge et al., 2011; Naito and Cairns, 2011) and therefore, its impact on soil faunal activity of general interest. In this study, we used two approaches at different scales of grass and shrub cover to test how vegetation affects invertebrate macropore numbers and size, as a proxy for soil-burrowing invertebrates’ activity.
Section snippets
Study site description
The study was conducted on the commercial livestock farm Ebenhazer in the western Kalahari, Omaheke region with a mean elevation of 1340 m above seas level (23°13′14.6″S 18°26′49.5″E; Fig. 1A). Livestock comprised mostly cattle and sheep with 0.04–0.08 livestock units (LSU)/ha. One LSU is the grazing equivalent of one adult dairy cow. Mean annual precipitation in this area is 267 mm, but precipitation is highly variable (annual precipitation coefficient of variance = 95 mm). Mean annual
Results
No significant negative correlation between shrub cover and cover of perennial grasses was not found (r = −0.43, p = 0.3). However, the highest perennial grass canopy cover of nearly 20% was found on the plot without shrubs. At the highest shrub cover, we found a rather high perennial grass cover of 16%. The canopy cover of annual grasses was over all low and did not exceed 2%.
Both, individual plants on a small scale and vegetation cover on landscape scale, as well as the recording time during
Discussion
In this study, we examined the interacting effects of small scale vegetation structures and vegetation cover at landscape scale on the amount and size of invertebrate derived surface macropores, as a proxy for soil-burrowing invertebrate activity, at three different times during a rainy season in a semi-arid Namibian rangeland. Our results indicated a positive effect of single shrubs on the small scale and grass canopy cover on the landscape scale on soil-burrowing invertebrate activity.
Conclusions
It is well documented that macropore-building invertebrates exert wide reaching effects on soil properties and are therefore considered as soil engineers which influence the availability of resources for other organisms, including microorganisms and plants (Jones et al., 1996, 1997). In this study we showed that the burrowing activity of invertebrates (as amount of actual found macropores) and therefore their beneficial impact on the ecosystem were controlled by vegetation on both the local and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge the financial support of the German Federal Ministry of Education and Research (BMBF) in the framework of the SPACES project OPTIMASS (FKZ: 01LL1302A). This research was carried out under research permit no. 2226/2016 of the Namibian Ministry of Environment and Tourism. We would like to thank Pieter Hugo for the possibility and infrastructure to conduct our research on the Ebenhazer farm.
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