Faunal input at host plants: Can camel thorn trees use nutrients imported by resident sociable weavers?

Abstract

“Islands of fertility” result from the focussing of water and nutrients around many shrub or tree species due to plants foraging for resources. Plant–animal feedbacks may amplify the development of such islands through environmental modification due to, for example, faunal deposition of nutrients and seeds. Fauna residing within vegetation clumps are likely to exert stronger feedbacks on their hosts than itinerant species. We studied the interaction between camel thorn trees ( Vachellia erioloba ) and the colonial nests of sociable weavers ( Philetairus socius ) in the Kalahari. We hypothesized that the accumulation of biological material below the nests will alter the nutrient status of the soil beneath the nest trees, in relation to unoccupied trees and the surrounding grassland. We also suggested that this association will have both positive and negative effects on the camel thorn trees. We found that soil concentrations of N, P, and K were, respectively, 4, 4.6, and 1.2 times higher below trees with nests compared to control trees, indicating faunal concentration of nutrients. Soil δ 15 N values were higher below trees with nests than below control trees without nests. Foliar δ 15 N values were also higher in nest trees than in control trees, showing the trees accessed faunally derived N. Furthermore, foliar biomass per diameter of terminal branches was 27% higher in nest trees, suggesting that trees respond to nutrient input from the weavers with increased growth. Large barren areas in the subcanopy vegetation directly beneath the colonies were attributed to decreased water infiltration rates, as a result of accumulation of organic matter due to continuous deposition of feces, possibly limiting competitive species from establishing in the subcanopy. On the other hand, canopy volume was reduced in trees with nests due to nests occupying large volumes within the canopy, and nests frequently causing branch fall, indicating costs associated with hosting weaver colonies. Synthesis: We found nutritional benefits to camel thorn trees when hosting sociable weaver colonies. These benefits can potentially overcome important environmental constraints, but these are partially offset by the resulting costs to the host trees.

Ecological engineering across a spatial gradient: Sociable weaver colonies facilitate animal associations with increasing environmental harshness

“…Larger colonies may host hundreds of individual birds and nest chambers provide insulation, a crucial resource in arid environments, for its occupants (Lowney et al, 2020). Soils directly below colonies have particularly increased nutrient levels (Prayag et al, 2020) and this could result in direct effects on the local vegetation and animals. Weaver colonies have been shown to act as a resource to multiple species within the environment (Bolopo et al, 2019; Lowney & Charlton, 2017; Maclean, 1970; Rehn, 1965; Rymer et al, 2014) and maintain this impact throughout the year (Lowney & Thomson, 2021).…”

“…Most species that use weaver colonies are not obligate associates and can exhibit behavioural plasticity in their use of weaver colonies. We hypothesize that because invertebrate abundance is influenced by organic matter (Noy‐Meir, 1985) and this collects below colonies (Prayag et al, 2020), that a greater abundance of invertebrates will be observed at colony trees than the control trees. We also hypothesize that due to thermal insulation against hot and cold temperatures that colonies provide (Lowney et al, 2020) and increased resources in terms of invertebrate abundance, small‐ to medium‐sized birds and reptiles will associate with colony trees.…”

“…We also hypothesize that due to thermal insulation against hot and cold temperatures that colonies provide (Lowney et al, 2020) and increased resources in terms of invertebrate abundance, small‐ to medium‐sized birds and reptiles will associate with colony trees. We hypothesize that use of colonies for shade and territory marking (Lowney & Thomson, 2021) will increase the associations of large mammals, while herbivores are likely to forage more at colony trees due to the nutrient‐rich vegetation and increased foliar biomass (Prayag et al, 2020). Therefore, we expect increased number of large vertebrates at colony trees.…”

Ecological engineering across a temporal gradient: Sociable weaver colonies create year‐round animal biodiversity hotspots

“…The nesting chambers provide thermal benefits to occupants (Lowney, Bolopo, et al., 2020), a potentially crucial resource in arid environments. Food and nutrient resources may also concentrate at nest colonies, with soils below colonies massively enriched with nutrients (Prayag et al., 2020) that could have knock‐on effects on vegetation and animals in the local environment. Anecdotal data suggest a wide range of non‐obligate species also associate with these colonies (Maclean, 1973a).…”

“…We hypothesised that weaver colonies act as an important resource for multiple taxa due to the abiotic and biotic resources provided and we expected to observe more species, individuals and greater diversity at trees containing a colony and trees without a colony (control). We also hypothesised that colonies will provide different resources for different taxa; large mammals will use the structures for shade, whereas herbivores are likely to forage more at colony trees due to the nutrient‐rich vegetation and increased foliar biomass (Prayag et al., 2020). We also expected higher diversity of bird species associated with colony trees; large birds may use the top of colonies for nesting and perching, whereas small‐ to medium‐sized birds will likely use nesting chambers as a refuge from predators and to gain thermal isolation against hot and cold temperatures (Lowney, Bolopo, et al., 2020; Maclean, 1973a).…”