Oecologia

, Volume 153, Issue 4, pp 985–995 | Cite as

Positive and negative effects of grass, cattle, and wild herbivores on Acacia saplings in an East African savanna

Community Ecology

Abstract

Plant–plant interactions can be a complex mixture of positive and negative interactions, with the net outcome depending on abiotic and community contexts. In savanna systems, the effects of large herbivores on tree–grass interactions have rarely been studied experimentally, though these herbivores are major players in these systems. In African savannas, trees often become more abundant under heavy cattle grazing but less abundant in wildlife preserves. Woody encroachment where cattle have replaced wild herbivores may be caused by a shift in the competitive balance between trees and grasses. Here we report the results of an experiment designed to quantify the positive, negative, and net effects of grasses, wild herbivores, and cattle on Acacia saplings in a Kenyan savanna. Acacia drepanolobium saplings under four long-term herbivore regimes (wild herbivores, cattle, cattle + wild herbivores, and no large herbivores) were cleared of surrounding grass or left with the surrounding grass intact. After two years, grass-removal saplings exhibited 86% more browse damage than control saplings, suggesting that grass benefited saplings by protecting them from herbivory. However, the negative effect of grass on saplings was far greater; grass-removal trees accrued more than twice the total stem length of control trees. Where wild herbivores were present, saplings were browsed more and produced more new stem growth. Thus, the net effect of wild herbivores was positive, possibly due to the indirect effects of lower competitor tree density in areas accessible to elephants. Additionally, colonization of saplings by symbiotic ants tracked growth patterns, and colonized saplings experienced lower rates of browse damage. These results suggest that savanna tree growth and woody encroachment cannot be predicted by grass cover or herbivore type alone. Rather, tree growth appears to depend on a variety of factors that may be acting together or antagonistically at different stages of the tree’s life cycle.

Keywords

Acacia drepanolobium Woody encroachment Competition Positive interactions Tree–grass interactions 

References

  1. Angassa A (2005) The ecological impact of bush encroachment on the yield of grasses in Borana rangeland ecosystem. Afr J Ecol 43:14–20CrossRefGoogle Scholar
  2. Archer S (1995) Tree–grass dynamics in a Prosopis-thornscrub savanna parkland—reconstructing the past and predicting the future. Ecoscience 2:83–99Google Scholar
  3. Archer S, Schimel DS, Holland EA (1995) Mechanisms of shrubland expansion—land-use, climate, or CO2. Clim Change 29:91–99CrossRefGoogle Scholar
  4. Augustine DJ, McNaughton SJ (2004) Regulation of shrub dynamics by native browsing ungulates on East African rangeland. J Appl Ecol 41:45–58CrossRefGoogle Scholar
  5. Belsky AJ (1994) Influences of trees on savanna productivity: tests of shade, nutrients, and tree–grass competition. Ecology 75:922–932Google Scholar
  6. Belsky AJ, Mwonga SM, Amundson RG, Duxbury JM, Ali AR (1993) Comparative effects of isolated trees on their undercanopy environments in high- and low-rainfall savannas. J Appl Ecol 30:143–155CrossRefGoogle Scholar
  7. Bertness MD, Callaway RM (1994) Positive interactions in communities. Trends Ecol Evol 9:191–193CrossRefGoogle Scholar
  8. Bertness MD, Ewanchuk PJ (2002) Latitudinal and climate-driven variation in the strength and nature of biological interactions in New England salt marshes. Oecologia 132:392–401CrossRefGoogle Scholar
  9. Bond WJ, Midgley GF, Woodward FI (2003) The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Glob Change Biol 9:973–982CrossRefGoogle Scholar
  10. Callaway RM, Nadkarni NM, Mahill BE (1991) Facilitation and interference of Quercus douglasii on understory productivity in central California. Ecology 72:1484–1499CrossRefGoogle Scholar
  11. Callaway RM, Brooker RW, Choler P, et al. (2002) Positive interactions among alpine plants increase with stress. Nature 417:844–848Google Scholar
  12. Coe M, Beentje H (1991) A field guide to the acacias of Kenya. Oxford University Press, OxfordGoogle Scholar
  13. Dean WRJ, Milton SJ, Jeltsch F (1999) Large trees, fertile islands, and birds in arid savanna. J Arid Environ 41:61–78CrossRefGoogle Scholar
  14. Dickie IA, Schnitzer SA, Reich PB, Hobbie SE (2005) Spatially disjunct effects of co-occuring competition and facilitation. Ecol Lett 8:1191–1200CrossRefGoogle Scholar
  15. Dublin HT, Sinclair ARE, McGlade J (1990) Elephants and fire as causes of multiple stable states in the Serengeti-Mara woodlands. J Anim Ecol 59:1147–1164CrossRefGoogle Scholar
  16. Gadd ME, Young TP, Palmer TM (2001) Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanlobium. Oikos 92:515–521CrossRefGoogle Scholar
  17. Goheen JR, Keesing F, Allan BF, Ogada D, Ostfeld RS (2004) Net effects of large mammals on Acacia seedling survival in an African savanna. Ecology 85:1555–1561CrossRefGoogle Scholar
  18. Hocking B (1970) Insect associations with the swollen thorn acacias. Trans R Entomol Soc Lond 122:211–255Google Scholar
  19. Holmgren M, Scheffer M, Huston MA (1997) The interplay of facilitation and competition in plant communities. Ecology 78:1966–1975CrossRefGoogle Scholar
  20. Knoop WT, Walker BH (1985) Interactions of woody and herbaceous vegetation in a southern African savanna. J Ecol 73:235–253CrossRefGoogle Scholar
  21. Lauenroth WK, Aguilera MO (1998) Plant-plant interactions in grasses and grasslands. In: Cheplick GP (ed) Population biology of grasses. Cambridge University Press, Cambridge, pp 209–230Google Scholar
  22. Lortie CJ, Brooker RW, Choler P, et al. (2004) Rethinking plant community theory. Oikos 107:433–438Google Scholar
  23. Ludwig F, Dawson TE, Kroon H, Berendse F, Prins HHT (2003) Hydraulic lift in Acacia tortilis trees on an East African savanna. Oecologia 134:293–300PubMedGoogle Scholar
  24. Madden D, Young TP (1992) Symbiotic ants as an alternative defense against giraffe herbivory in spinescent Acacia drepanolobium. Oecologia 91:235–238CrossRefGoogle Scholar
  25. Maestre FT, Valladares F, Reynolds JF (2005) Is the change of plant–plant interactions with abiotic stress predictable? A meta-analysis of field results in arid environments. J Ecol 93:748–757CrossRefGoogle Scholar
  26. Midgley JJ, Bond WJ (2001) A synthesis of the demography of African acacias. J Trop Ecol 17:871–886CrossRefGoogle Scholar
  27. Misurelli DL (2002) Large ungulate impacts on bird populations in an East African savanna. State University of New York, Albany, NY, p 79Google Scholar
  28. O’Connor TG (1995) Acacia karroo invasion of grassland: environmental and biotic effects influencing seedling emergence and establishment. Oecologia 103:214–223CrossRefGoogle Scholar
  29. Odadi W, Young TP, Okeyo-Owuor JB (2007) Effects of wildlife on cattle diets in Laikipia rangeland, Kenya. Rangeland Ecol Manage 60:179–185CrossRefGoogle Scholar
  30. Okello BD, O’Connor TG, Young TP (2001) Growth, biomass estimates, and charcoal production of Acacia drepanolobium in Laikipia, Kenya. For Ecol Manage 142:143–153CrossRefGoogle Scholar
  31. Otieno SG (2004) Effects of domestic and wild herbivore utilization on herbaceous layer aboveground primary production in a central Kenya grassland. University of Nairobi, NairobiGoogle Scholar
  32. Palmer TM, Brody AK (2007) Mutualism as reciprocal exploitation: African plant-ants defend foliar but not reproductive structures. Ecology (in press)Google Scholar
  33. Palmer TM, Young TP, Stanton ML, Wenk E (2000) Short-term dynamics of an acacia ant community in Laikipia, Kenya. Oecologia 123:425–435CrossRefGoogle Scholar
  34. Pratt DJ, Gwynne MD (1977) Rangeland management and ecology in East Africa. Hodder and Stoughton, LondonGoogle Scholar
  35. Pringle RM, Young TP, Rubenstein DI, McCauley DJ (2007) Herbivore-initiated interaction cascades and their modulation by productivity in an African savanna. Proc Natl Acad Sci USA 104:193–197PubMedCrossRefGoogle Scholar
  36. Rebollo S, Milchunas DG, Noy-Meir I, Chapman PL (2002) The role of a spiny plant refuge in structuring grazed shortgrass steppe plant communities. Oikos 98:53–64CrossRefGoogle Scholar
  37. Riginos C, Milton SJ, Wiegand T (2005) Context-dependent interactions between adult shrubs and seedlings in a semi-arid shrubland. J Veg Sci 16:331–340CrossRefGoogle Scholar
  38. Roques KG, O’Connor TG, Watkinson AR (2001) Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence. J Appl Ecol 38:268–280CrossRefGoogle Scholar
  39. Rousset O, Lepart J (2000) Positive and negative interactions at different life stages of a colonizing species (Quercus humilis). J Ecol 88:401–412CrossRefGoogle Scholar
  40. Scholes RJ, Archer SR (1997) Tree–grass interactions in savannas. Annu Rev Ecol Syst 28:517–544Google Scholar
  41. Skarpe C (1992) Dynamics of savanna ecosystems. J Veg Sci 3:293–300CrossRefGoogle Scholar
  42. Stapley L (1998) The interaction of thorns and symbiotic ants as an effective defence mechanism of swollen-thorn acacias. Oecologia 115:401–405CrossRefGoogle Scholar
  43. Tewksbury JJ, Lloyd JD (2001) Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size. Oecologia 127:425–434CrossRefGoogle Scholar
  44. Tobler MW, Cochard R, Edwards PJ (2003) The impact of cattle ranching on large-scale vegetation patterns in a coastal savanna in Tanzania. J Appl Ecol 40:430–444CrossRefGoogle Scholar
  45. van Auken OW (2000) Shrub invasions of North American semiarid grasslands. Annu Rev Ecol Syst 31:197–215CrossRefGoogle Scholar
  46. van Vegten JA (1984) Thornbush invasion in a savanna ecosystem in eastern Botswana. Vegetation 56:3–7CrossRefGoogle Scholar
  47. Vetaas OR (1992) Micro-site effects of trees and shrubs in dry savannas. J Veg Sci 3:337–344CrossRefGoogle Scholar
  48. Western D, Maitumo D (2004) Woodland loss and restoration in a savanna park: a 20-year experiment. Afr J Ecol 42:111–121CrossRefGoogle Scholar
  49. Young TP, Stubblefield CH, Isbell LA (1997) Ants on swollen-thorn acacias: species coexistence in a simple system. Oecologia 109:98–107 CrossRefGoogle Scholar
  50. Young TP, Okello BD, Kinyua D, Palmer TM (1998) KLEE: a long-term multi-species herbivore exclusion experiment in Laikipia, Kenya. Afr J Range Forage Sci 14:94–102Google Scholar
  51. Young TP, Stanton ML, Christian CE (2003) Effects of natural and simluated herbivory on spine lengths of Acacia drepanolobium in Kenya. Oikos 101:171–179CrossRefGoogle Scholar
  52. Young TP, Palmer TM, Gadd ME (2005) Competition and compensation among cattle, zebras, and elephants in a semi-arid savanna in Laikipia, Kenya. Biol Conserv 122:351–359CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Department of Plant SciencesUniversity of CaliforniaDavisUSA
  2. 2.Mpala Research CentreNanyukiKenya
  3. 3.Center for Population BiologyUniversity of CaliforniaDavisUSA

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