Do Camelthorn Trees Use Sociable Weavers to Forage for Nutrients?

Kervin D. Prayag PRYKER001 Do camelthorn trees use sociable weavers to forage for nutrients? BIO4000W (Hons) Department of Biological Sciences University of Cape Town Supervised by: Michael D. Cramer (Department of Biological Sciences, UCT) Robert L. Thomson (Percy Fitzpatrick Institute of African Ornithology, UCT) November 2016 Plagiarism declaration I know that plagiarism is wrong. Plagiarism is to use another’s work and pretend that it is one’s own. I have used the Harvard convention for citation and referencing. Each contribution to, and quotation in, this thesis from the work(s) of other people has been attributed, and has been cited and referenced. This thesis is my own work. I have not allowed, and will not allow, anyone to copy my work with the intention of passing it off as his or her own work. Signature Kervin Deveshwar Prayag ………………………………………. Date ………………………………………. i Abstract “Islands of fertility” result from focussing of water and nutrients around many shrub- or tree- savanna species due to plant 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 resident within vegetation clumps are likely to exert stronger feedbacks on their hosts than itinerant species. We studied the relationship between camelthorn trees (Vachellia erioloba) and the spectacular colonial nests of sociable weavers (Philetairus socius) in the Tswalu Kalahari reserve in the Northern Cape, South Africa. We hypothesized that nutrient inputs from the sociable weaver nests have a positive effect on tree growth and nutrient status. We also hypothesized that nests have negative consequences for the trees through reduced foliar canopies and increased branch fall. We measured tree leafiness, foliar and nutrient contents and δ15N values across pairs of trees with and without nests. We also measured sub-canopy vegetation cover in the islands of fertility below the trees. There were no significant differences in foliar N, P and K between nest and control trees, although δ15N differed strongly. Trees with nests, however, had 27% higher leafiness of terminal branches. Trees thus utilise the nutrients made available through bird faeces for growth, although they do not accumulate more foliar nutrient. Canopy volume was, however, reduced in trees with nests due to these occupying large volumes and to branch fall. Sub-canopy vegetation cover was surprisingly decreased. We conclude that the benefits to the camelthorn trees through faunal deposition of nutrients does not come without costs to the trees, and that this plant-animal interaction has measurable effects on the associated sub-canopy vegetation. Keywords: sociable weaver, camelthorn, semi-arid savanna, plant-animal interactions, positive feedbacks, faunal nutrient input ii TABLE OF CONTENTS Plagiarism declaration i Abstract ii Introduction 1 Materials and methods 4 Study system 4 Sampling procedure 4 Loss of biomass 4 Foliar canopies 5 Sub-canopy vegetation 5 Biomass accumulation, foliar nutrient concentrations and foliar N ratios 5 Biological nitrogen fixation 6 Data analyses 7 Results 8 Discussion 9 Acknowledgements 13 References 14 List of figure captions 19 Figures and tables 21 Appendix 31 Introduction In arid and semi-arid environments where resources are scarce, conditions can be limiting in terms of growth and reproduction for plants. In these environments, plants are known to use diverse strategies to gather resources. Such strategies include deep roots, biological nitrogen fixation, and inverse hydraulic lift (Schulze et al. 1991; Jackson et al. 1996; Schulze et al. 1998; McCulley et al. 2004). The use of these strategies have been documented for camelthorn trees (Vachellia erioloba) in arid and semi-arid regions (Schulze et al. 1991; Canadell et al. 1996; Seymour 2008). Camelthorns are an iconic and dominant tree species in arid and semi-arid regions of Southern Africa, and are critical to animal and plant communities in the area (Coates Palgrave 2005). In its Kalahari range, camelthorns are known to host large nest colonies of the gregarious bird species Philetairus socius, commonly called sociable weavers. In arid and semi-arid savannas, in addition to its other resource acquisition strategies, the camelthorn tree may be able to forage for nutrients from the soils enriched by the deposition of faeces below its canopy when hosting a sociable weaver colony. Interactions between plants and animals are abundant in natural systems with important impacts on structure, function and diversity of ecosystems through processes such as pollination, seed dispersal, habitat provisioning or nutrient cycling (Ehrlich and Raven 1964; Quek et al. 2004; Tiffney 2004; Bascompte and Jordano 2007). Interactions can range from being mutually beneficial for both partners, to being one-sided and benefit only the one partner (Andersen and Braithwaite 1996). For example, plant-pollinator interactions are often mutually beneficial: in return for pollinating the flowers, the pollinator is rewarded with nectar (Ramírez et al. 2011). Positive interactions can involve cost and benefits for either one, or both of the partners (Addicott 1986). As an example, grazing pressure by ruminants leads to a loss of biomass in plants, but deposition of faeces by the animals (faunal nutrient input) can increase the nutrients available to the plants (Whitehead 2000). The plants can therefore outweigh the costs of grazing pressure by investing in this increase in resource availability to increase productivity, nutrient content and reproductive output (van der Wal et al. 2004). Other potentially mutualistic plant-animal interactions are the ones between birds and plants. For example, a tree hosting an insectivorous bird species can experience decreased pressure from insect herbivory (Mäntylä et al. 2011). Costs and benefits to camelthorn trees hosting sociable weavers have however not yet been documented. 1 In the interaction between sociable weavers and camelthorns, the benefit to the birds through habitat provisioning by camelthorns has previously been documented: as one of the few very large tree species in arid and semi-arid savannas, the camelthorn is one of the limited structures on which sociable weavers can build their colonies (Dean et al. 1999; Spottiswoode 2009). Sociable weaver colonies can be very large in size and one nest can host up to 500 individuals (Maclean 1973; Covas et al. 2004). Faeces and nest material continuously falling from the large nest colonies therefore result in very high levels of localized faunal nutrient input enrich soils in nutrients beneath the camelthorn trees (Dean et al. 1999). Studies have shown that in addition to enriching soils in nutrients in terrestrial systems, seabird guano can also alter the soil nitrogen isotopic ratio (δ15N) (Wainright et al. 1998; Szpak et al. 2012). Using the 15N natural abundance technique, it was shown that plants make use of nutrients from these soils enriched by guano deposition (Wainright et al. 1998). In a tandem study investigating the soil chemistry below camelthorns trees with nests and without nests, it was found that soil nutrients and 15N abundance increased in the presence of a nest (du Toit 2016). It remains unknown whether the δ15N in camelthorns is similarly altered when the tree hosts a sociable weaver nest, or whether the tree makes use of the abundant nutrients for growth. In Kalahari ecosystems, camelthorn trees are known as ecosystem engineers as they create islands of fertility below their canopies which provides favourable conditions for sub-canopy plant species to establish and grow (Dean et al. 1999; Seymour 2006). Faunal nutrient input through the deposition of sociable weaver faeces below the tree canopies can lead to an enhancement of these islands of fertility. There is also increasing evidence that sociable weaver colonies can benefit other animal species through food and habitat provisioning (Rymer et al. 2014). Examining the costs and benefits to camelthorns when hosting sociable weaver nest colonies is therefore of importance, as camelthorns, and potentially sociable weavers, play an important role in these Kalahari ecosystems. In the present study, we examine the costs and benefits to camelthorns of hosting sociable weaver nests. We also examine potential effects of this plant-animal interaction on the sub-canopy vegetation in the islands of fertility below the trees. We hypothesize that nutrient inputs through the deposition of sociable weaver faeces have a positive effect on tree growth and foliar nutrient status, and an effect on foliar δ15N. We also hypothesize that nests have negative consequences 2 for the trees through reduced foliar canopies and increased branch fall. Lastly, we hypothesize that nutrient inputs from the sociable weaver nests will increase sub-canopy vegetation cover below the trees. To test these hypotheses, camelthorn tree growth, foliar nutrient concentrations, foliar nitrogen isotopic ratios, change in biomass and sub-canopy vegetation cover of camelthorn trees with and without nests were measured, analyzed, and compared. 3 Materials and methods Study system The study was undertaken in the semi-arid Tswalu Kalahari reserve in the Northern Cape, South Africa (27°13′30″S and 22°28′40″E, altitude 1020–1586masl) (Figure 1). This savanna system is characterized by reddish-brown sandy soils with an open canopy of trees and shrubs that includes species such as Vachellia

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