title: Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation.
abstract: In savannas, the tree-grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.
authors.affiliation: Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands. corwaal@gmail.com
authors.forename: Cornelis
authors.initials: C
authors.lastname: van der Waal
authors.forename: Ada
authors.initials: A
authors.lastname: Kool
authors.forename: Seline S
authors.initials: SS
authors.lastname: Meijer
authors.forename: Edward
authors.initials: E
authors.lastname: Kohi
authors.forename: Ignas M A
authors.initials: IM
authors.lastname: Heitkönig
authors.forename: Willem F
authors.initials: WF
authors.lastname: de Boer
authors.forename: Frank
authors.initials: F
authors.lastname: van Langevelde
authors.forename: Rina C
authors.initials: RC
authors.lastname: Grant
authors.forename: Mike J S
authors.initials: MJ
authors.lastname: Peel
authors.forename: Rob
authors.initials: R
authors.lastname: Slotow
authors.forename: Henrik J
authors.initials: HJ
authors.lastname: de Knegt
authors.forename: Herbert H T
authors.initials: HH
authors.lastname: Prins
authors.forename: Hans
authors.initials: H
authors.lastname: de Kroon
mesh_terms: D000818:Animals; D002118:Calcium / Q000032:analysis; D017753:Ecosystem; D058751:Livestock / Q000254:growth & development / Q000378:metabolism*; D008274:Magnesium / Q000032:analysis; D009584:Nitrogen / Q000032:analysis; D009751:Nutritional Requirements; D010758:Phosphorus / Q000032:analysis; D006109:Poaceae / Q000254:growth & development / Q000378:metabolism*; D011157:Population Dynamics; D011188:Potassium / Q000032:analysis; D036226:Seedlings / Q000254:growth & development / Q000378:metabolism; D012987:Soil / Q000032:analysis* / Q000737:chemistry; D013019:South Africa; D013997:Time Factors; D014197:Trees / Q000254:growth & development / Q000378:metabolism*
chemical_list: D012987:Soil; D010758:Phosphorus; D008274:Magnesium; D009584:Nitrogen; D011188:Potassium; D002118:Calcium