Spatial variability and abiotic determinants of termite mounds throughout a savanna catchment

Davies, Andrew B.; Levick, Shaun R.; Asner, Gregory P.; Robertson, Mark P.; Rensburg, Berndt J. van; Parr, Catherine L.

doi:10.1111/ecog.00532

Cited by 94 publications

(138 citation statements)

References 41 publications

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“…Even though Picker et al (2012) and Juergens (2013) have convincingly shown strong correlation between fairy circles and ants or termites, respectively, they do not provide an explanation for the observed lack of any larger-scale clustering at distances  60 m found here (see also Davies et al 2014). Even though Picker et al (2012) and Juergens (2013) have convincingly shown strong correlation between fairy circles and ants or termites, respectively, they do not provide an explanation for the observed lack of any larger-scale clustering at distances  60 m found here (see also Davies et al 2014).…”

Section: Hypothesis On Self-organization By Plantscontrasting

confidence: 71%

Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

et al. 2014

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The mysterious 'fairy circles' are vegetation-free discs that cover vast areas along the pro-Namib Desert. Despite 30 yr of research their origin remains unknown. Here we adopt a novel approach that focuses on analysis of the spatial patterns of fairy circles obtained from representative 25-ha aerial images of north-west Namibia. We use spatial point pattern analysis to quantify different features of their spatial structures and then critically inspect existing hypotheses with respect to their ability to generate the observed circle patterns. Our working hypothesis is that fairy circles are a self-organized vegetation pattern. Finally, we test if an existing partial-differential-equation model, that was designed to describe vegetation pattern formation, is able to reproduce the characteristic features of the observed fairy circle patterns. The model is based on key-processes in arid areas such as plant competition for water and local resource-biomass feedbacks.The fairy circles showed at all three study areas the same regular spatial distribution patterns, characterized by Voronoi cells with mostly six corners, negative correlations in their size up to a distance of 13 m, and remarkable homogeneity over large spatial scales. These results cast doubts on abiotic gas-leakage along geological lines or social insects as causal agents of their origin. However, our mathematical model was able to generate spatial patterns that agreed quantitatively in all of these features with the observed patterns. This supports the hypothesis that fairy circles are selforganized vegetation patterns that emerge from positive biomass-water feedbacks involving water transport by extended root systems and soil-water diffusion. Future research should search for mechanisms that explain how the different hypotheses can generate the patterns observed here and test the ability of self-organization to match the birth-and death dynamics of fairy circles and their regional patterns in the density and size with respect to environmental gradients.

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Section: Hypothesis On Self-organization By Plantscontrasting

confidence: 71%

Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

et al. 2014

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“…1 mound·ha −1 , see Davies et al. ) Macrotermes mound densities were selected using Light Detection and Ranging (LiDAR) data from the Carnegie Airborne Observatory Alpha sensor package (CAO‐Alpha; Asner et al. ).…”

Section: Methodsmentioning

confidence: 99%

Variable effects of termite mounds on African savanna grass communities across a rainfall gradient

Davies¹,

Robertson

Levick

et al. 2014

J Vegetation Science

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Questions: Termite mounds of the genus Macrotermes are prominent features in African savannas, forming nutrient hotspots that support greater plant diversity, which is of higher nutritional value than the surrounding savanna matrix. However, little is known about grass communities on and around mounds or how the functional importance of mounds varies across sites. As mean annual rainfall increases, savannas in southern Africa become increasingly dystrophic through increased denitrification (including pyrodenitrification) and 1 the leaching of soil nutrients. The functional importance of mounds is concomitantly expected to increase as the difference in foliar nutrient levels between mounds and the savanna matrix increases. We tested this prediction on grass communities across a rainfall gradient i) to determine the degree to which grass assemblages differ between termite mounds and the savanna matrix, ii) to determine the spatial extent to which mounds influence grass communities, and iii) to investigate whether these patterns differ across savanna types. Location: Kruger National Park, South AfricaMethods: Grass communities were surveyed at three savanna sites differing in mean annual rainfall (550 -750 mm.yr -1 ). Grass diversity and tissue nitrogen concentrations were measured on and off termite mounds and along transects away from mounds in order to calculate the spatial influence of termite mounds on savanna grass communities. Using termite mound densities estimated from airborne LiDAR, we upscaled field-based results to determine the percentage of the landscape influenced by Macrotermes termite activity.Results: Although species richness of grasses was lower on mounds than in the savanna matrix, the assemblage composition varied significantly, with higher nutrient concentrations in grasses located on mounds. This pattern became more distinct with increasing rainfall. The spatial extent of these nutrient-rich grasses also differed across the rainfall gradient, with a larger sphere of influence around mounds in wetter areas. Mounds distinctly altered grass communities over ~2% of the entire landscape. Conclusions:Our results show that Macrotermes mounds are important components of savanna heterogeneity, and reveal that the functional importance of mounds increases with increasing rainfall.

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“…The four in each set of plots form part of a burning experiment, but are representative of 6 each savanna type and its fire regimes. Fire effects were not expected to influence sampling because both termite diversity (Davies et al, 2012) and mound distributions (Davies et al, 2014) display remarkable resistance to fire. Furthermore, the same four burning regimes were sampled in each habitat, thus controlling for any potential fire effects.…”

Section: Study Sites and Termite Samplingmentioning

confidence: 99%

Seasonal activity patterns of African savanna termites vary across a rainfall gradient

et al. 2015

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Seasonal variations in temperature and moisture are strong drivers of biological activity and diversity. Termites are an important insect group previously shown to respond to seasonal variation, but results are mixed with unclear patterns across habitat types. We investigated seasonal variation in termite species density, activity levels and assemblage composition across three seasons (wet, transitional and dry) and four savanna types across a rainfall gradient (450 mm.yr -1 -900 mm.yr -1 ) in South Africa using cellulose baits. Termites responded to seasonality in all savannas investigated, with lower species density and activity levels during the dry season compared to the wet and transitional seasons. In the more arid sites (≤ 550mm rainfall per year) activity levels were highest in the wet season, while at wetter sites (≥750mm rainfall per year) the highest activity was recorded in the transitional season. Assemblage composition did not differ much between seasons across all sites, but differences in both composition and activity levels across seasons were more pronounced in wetter sites compared to drier ones. Our results demonstrate that seasonal patterns in termite diversity vary with mean annual rainfall, with larger variation in wetter habitats where climatic variation between seasons is greater.

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Spatial variability and abiotic determinants of termite mounds throughout a savanna catchment

Cited by 94 publications

References 41 publications

Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

Adopting a spatially explicit perspective to study the mysterious fairy circles of Namibia

Variable effects of termite mounds on African savanna grass communities across a rainfall gradient

Seasonal activity patterns of African savanna termites vary across a rainfall gradient

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