Journal of Coastal Conservation

, Volume 22, Issue 4, pp 667–677 | Cite as

Complexities of shrub encroachment: are shrubs important for the maintenance of diversity in Themeda-dominated assemblages on coastal headlands in eastern Australia?

  • John T. HunterEmail author


Shrub encroachment is an issue worldwide with consequences which may have both positive and negative outcomes for landscape and community health. Themeda-dominated communities on headlands are listed as endangered within New South Wales, Australia with shrub encroachment listed as threat. Coastal headlands are considered harsh environments and positive effects on species diversity patterns may occur due to variables that ameliorate these conditions or through indirect mechanisms. Shrubs are a natural component of coastal headlands in eastern Australian and tall shrub encroachment may have both positive and negative effects on non-shrub communities. I test whether tall shrubs may have a functional role that positively affects species diversity within and between plots and the distribution of species within Themeda-dominated communities on headlands. 352 2 × 2 m plots were placed on 46 headlands along a 530 km stretch of coastline on the North Coast Bioregion, New South Wales, Australia. Within plots vascular plant species were scored on cover and frequency. Species density, diversity, evenness, turnover and gamma diversity were calculated. Variables tested include cumulative tall shrub height from circular transects at 2, 4 and 8 m from plots, slope, aspect, altitude, ground layer height, distance to closest seaward edge and macropod grazing intensity. Rarefaction was performed on a subset of sites that had no shrubs and sites within shrubs within 8 m of plots. The relative interaction intensity (RII) was calculated for all taxa with more than one observation against plots without shrubs and plots with shrubs within 8 m radius. Analyses were performed using Canonical Correspondence Analysis (CCA) and Generalised Additive Modelling (GAM) on species frequency data. A total of 303 native taxa were recorded within plots. The most significant positive effects on species density, evenness and species accumulation included increasing proximity to, and density of tall shrubs. Tall shrubs may allow direct facilitation of less stress tolerant plants to colonise but may have an indirect interaction by reduction in the dominance of Themeda triandra. Other minor significant variables included distance from headland seaward edge, southern and western facing slopes, higher altitudes and a decrease in ground layer height. 178 (80%) of species were found to have a positive RII score associated with the presence of nearby taller shrubs. Tall shrubs may directly facilitate greater species diversity by ameliorating local conditions and indirectly facilitate by reducing the over-dominance of some taxa. In spite of the positive effect of shrubs on diversity eventually tall shrub encroachment may cause replacement of the Themeda-dominated communities with shrubland. Encroaching taller shrubs are therefore shown to be both of benefit to floristic species diversity at all scales within these Themeda-dominated communities on headlands in eastern Australia but also a potential threat. These results highlight the complex nature of community interactions requiring nuanced and potentially novel approaches to management.


Evenness Facilitation Indirect effects RII Turnover Stress 



Thanks to Vanessa Hunter and Helen Morgan for assistance during field data collection and Mark Watt for general assistance with the project. The author declares no conflicts of interest.


  1. Adam P, Stricker P, Wiecek BM, Anderson DJ (1989) The vegetation of seacliffs and headlands in new South Wales. Aust J Ecol 14:515–545CrossRefGoogle Scholar
  2. Anthelme F, Michalet R (2009) Grass-to-tree facilitation in an arid grazed environment (air mountains, Sahara). Basic Appl Ecol 10:437–446CrossRefGoogle Scholar
  3. Armas C, Ordiales R, Pugnaire FI (2004) Measuring plant interactions: a new comparative index. Ecology 85:2682–2686CrossRefGoogle Scholar
  4. Ascheboug ET, Callaway RM (2015) Diversity increases indirect interactions, attenuates the intensity of competition, and promotes coexistence. Am Nat 186:452–459CrossRefGoogle Scholar
  5. Bertness MD, Callaway RM (1994) Positive interactions in communities. Trends Ecol Evol 9:191–193CrossRefGoogle Scholar
  6. Bertness MD, Hacker SD (1994) Physical stress and positive associations among marsh plants. Am Nat 144:363–372CrossRefGoogle Scholar
  7. Bertness MD, Shumway SW (1993) Competition and facilitation in marsh plants. Am Nat 142:718–724CrossRefGoogle Scholar
  8. Brooker RW, Maestre FT, Callaway RM, Lortie CL, Cavieres LA, Kunstler G, Liancourt P, Tiebörger K, Travis JMJ, Anthelme F, Armas C, Call L, Corcket E, Delzon S, Forey E, Kikvidze Z, Olafsson J, Pugnaire F, Quiroz CL, Saccone P, Schiffers K, Sifan M, Touzard B, Michalet R (2008) Facilitation in plant communities: the past, the present, and the future. J Ecol 96:18–34CrossRefGoogle Scholar
  9. Buss LW, Jackson JBC (1979) Competitive networks: nontransitive competitive relationships in cryptic coral reef environments. Am Nat 113:223–234CrossRefGoogle Scholar
  10. Callaway RM (1995) Positive interactions among plants. Bot Rev 61:306–349CrossRefGoogle Scholar
  11. Callaway RM (1998) Are positive interactions species-specific? Oikos 82:202–207CrossRefGoogle Scholar
  12. Callaway RM, Nadkarni NM, Mahall BE (1991) Facilitating and interfering effects of Quercus douglasii in central California. Ecology 72:1484–1499CrossRefGoogle Scholar
  13. Cole LJ, McCracken DI, Baker L, Parish D (2007) Grassland conservation headlands: their impact on invertebrate assemblages in intensively managed grassland. Agric Ecosyst Environ 122:252–258CrossRefGoogle Scholar
  14. Colwell RK (2013) EstimateS: Statistical estimation of species richness and shared species from samples. Version 9. Persistent URL <>
  15. Costello DA, Lunt ID, Williams JE (2000) Effects of invasion by the indigenous shrub Acacia sophorae on plant composition of coastal grasslands in south-eastern Australia. Biol Conserv 96:113–121CrossRefGoogle Scholar
  16. Croft P, Hunter JT, Reid N (2016) Forgotten fauna: habitat attributes of long-unburnt open forests and woodlands dictate a rethink of fire management theory and practice. For Ecol Manag 366:166–174CrossRefGoogle Scholar
  17. Cuesta B, Villar-Salvador P, Puertolas J, Rey Benayas JM, Michalet R (2010) Facilitation of Quercus ilex in Mediterranean shrubland is explained by both direct and indirect interactions mediated by herbs. J Ecol 98:687–696CrossRefGoogle Scholar
  18. Dexter T (2015) Reconciliation in the grasslands, re-introduction of burning to Themeda grass headland EECs. In ‘proceedings of the 10th biennial bushfire conference’. Nature Conservation Council of New South WalesGoogle Scholar
  19. Dexter T, Miles J, Lenson D (2015) Re-introducing burning to Themeda headland grassland EEC, Narooma, NSW Ecological Management and Restoration Project Summaries Posted 6 Oct 2015. Available at:
  20. Eldridge DJ, Soliveres S (2014) Are shrubs really a sign of declining ecosystem function? Disentangling the myths and truths of woody encroachment in Australia. Aust J Bot 62:594–608CrossRefGoogle Scholar
  21. Eldridge DJ, Bowker MA, Maestre FT, Roger E, Reynolds JF, Whitford WG (2011) Impacts of shrub encroachment on ecosystem structure and functioning: towards a global perspective. Ecol Lett 14:709–722CrossRefGoogle Scholar
  22. Eldridge DJ, Beecham G, Grace JB (2015) Do shrubs reduce the adverse effects of grazing on soil properties? Ecohydrology 8:1503–1513CrossRefGoogle Scholar
  23. Greenlee JT, Callaway RM (1996) Abiotic stress and the relative importance of interference and facilitation in montane bunchgrass communities in western Montana. Am Nat 148:386–396CrossRefGoogle Scholar
  24. Grellier S, Ward D, Janeau J-L, Podwojewski P, Lorentz S, Abbadie L, Valentin C, Barot S (2013) Positive versus negative environmental impacts of tree encroachment in South Africa. Acta Oecol 53:1–10CrossRefGoogle Scholar
  25. Griffith SJ (1992) Species recovery plan for Thesium australe. Endangered species program project 196. Unpublished report. Australian National Parks and wildlife service, CanberraGoogle Scholar
  26. Grime JP (1973) Competitive exclusion in herbaceous vegetation. Nature 242:344–347CrossRefGoogle Scholar
  27. Hammer O, Harper DAT, Ryan PD (2001) PAST paleontological statistics software package for education and data analysis. Palaeontol Electron 4:9Google Scholar
  28. Heegaard E (2004) Trends in aquatic macrophyte species turnover in Northern Ireland – which factors determine the spatial distribution of local species turnover? Glob Ecol Biogeogr 13:397–408CrossRefGoogle Scholar
  29. Hill MO (1973) Diversity and evenness: a unifying notation and its consequences. Ecology 54:427–431CrossRefGoogle Scholar
  30. Hillebrand H, Bennett DM, Cadotte MW (2008) Consequences of dominance: a review of evenness effects on local and regional ecosystem processes. Ecology 89:1510–1520CrossRefGoogle Scholar
  31. Hunter JT (2003) Factors affecting range size differences for plant species on rock outcrops in eastern Australia. Divers Distrib 9:211–220CrossRefGoogle Scholar
  32. Hunter JT (2005) Geographic variation in plant species richness patterns within temperate eucalypt woodlands of eastern Australia. Ecography 28:505–514CrossRefGoogle Scholar
  33. Hunter JT (2016) Monitoring of Pultenaea maritima. Report prepared for saving our species, Department of Environment and Heritage.
  34. Hunter JT, Hunter VH (2017a) Floristics, dominance and diversity within threatened grassy headlands of the north coast bioregion of new South Wales. Pac Conserv Biol 23:71–80CrossRefGoogle Scholar
  35. Hunter JT, Hunter VH (2017b) The complex nature of headland shrub encroachment: the case of Zieria prostrata. Ecol Manag Restor 18:115–119CrossRefGoogle Scholar
  36. Hupp N, Llambi LD, Ramírez L, Callaway RM (2017) Alpine cushion plants have species-specific effects on microhabitat and community structure in the tropical Andes. J Veg Sci 28:928–938CrossRefGoogle Scholar
  37. Johnson CN, Jarmin PJ (1987) Macropod studies at Wallaby Creek VI. A validation of the use of dung-pellet counts for measuring absolute densities of populations of Macropodids. Australian. Wildl Res 14:139–145CrossRefGoogle Scholar
  38. Kinney K (2012) Fingal headland maritime Themeda grassland restoration. Ecological Management and Restoration Project Summaries Posted 21 June 2012. Available at:
  39. Lembrechts JJ, Milbau A, Nijs I (2015) Trade-off between competition and facilitation defines gap colonization in mountains. AoB Plants.
  40. Levine JM (1976) Competitive interactions in ecosystems. Am Nat 110:903–910CrossRefGoogle Scholar
  41. Levine JM (1999) Indirect facilitation: evidence and predictions from a riparian community. Ecology 80:1762–1769CrossRefGoogle Scholar
  42. Lortie CJ, Brooker RW, Choler P, Kikvidze Z, Michalet R, Pugnaire FI, Callaway RM (2004) Rethinking plant community theory. Oikos 197:433–438CrossRefGoogle Scholar
  43. Lunt ID, Winsemius LM, McDonald SP, Morgan JW, Dehaan RL (2010) How widespread is woody plant encroachment in temperate Australia? Changes in woody vegetation cover in lowland woodland and coastal ecosystems in Victoria from 1989 to 2005. J Biogeogr 37:722–733CrossRefGoogle Scholar
  44. Maestre FT, Bowker MA, Puche MD, Blen Hinojosa M, Martinex I, Garcia Palacios P, Castillo AP, Soliveres S, Luzuriaga AL, Sanchez AM, Carriera JA, Gallardo A, Escudera A (2009) Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands. Ecol Lett 12:930–941CrossRefGoogle Scholar
  45. Malloch AJC (1971) Vegetation of the maritime clifftops of the lizard and Land’s end Peninulas, West Cornwall. New Phytol 70:115–119CrossRefGoogle Scholar
  46. Melo MC, Graly A, Brittain B, Walter GM, Oritz-Barrientos D (2014) Strong extrinsic reproductive isolation between parapatric populations of an Australian groundsel. New Phytol 203:323–334CrossRefGoogle Scholar
  47. Michalet R, Maalouf JP, Choler P, Clément B, Rosebery D, Royer JM, Schöb C, Lortie CJ (2015) Competition, facilitation and environmental severity shape the relationship between local and regional species richness in plant communities. Ecography 38:335–345CrossRefGoogle Scholar
  48. Morris EC, Skelton NJ, Durham SJ (1990) Vegetation of three headlands of the central coast of new South Wales – Norah, Wamberal and Wybung heads. Wetlands (Australia) 9:49–67Google Scholar
  49. Mureva A, Ward D (2016) Spatial patterns of encroaching shrub species under different grazing regimes in a semi-arid savanna, eastern Karoo, South Africa. Afr J Range Forage Sci 2016:1–13Google Scholar
  50. Nackley LL, West AG, Skowno AL, Bond WJ (2017) The nebulous ecology of native invasions. Trends Ecol Evol 32:814–824CrossRefGoogle Scholar
  51. NPWS (2015) Themeda grasslands on seacliffs and coastal headlands of the Coffs coast Regional Park and Moonee Beach nature reserve. Unpublished report. OEH: Coffs HarbourGoogle Scholar
  52. Parsons RF, Gill AM (1968) The effects of saltspray on coastal vegetation at Wilson’s promontory, Victoria, Australia. Proceedings of the Royal Society of. Victoria 81:1–10Google Scholar
  53. Rey PJ, Alcátara JM, Manzaneda AJ, Sánchez-Lafuente AM (2016) Facilitation contributes to Mediterranean woody plant diversity but does not shape the diversity-productivity relationship along aridity gradients. New Phytol 211:464–476CrossRefGoogle Scholar
  54. Saving Our Species (2016) Themeda grassland on coastal headlands in the NSW North Coast, Sydney Basin, and South East Corner Bioregions. Available at:
  55. Schultz N, Reid N, Lodge G, Hunter JT (2014) Seasonal and inter-annual variation in vegetation composition: implications for survey design and data interpretation. Austral Ecol 39:134–138Google Scholar
  56. Seastedt TR, Hobbs RJ, Sudling KN (2008) Management of novel ecosystems: are novel approaches required? Front Ecol Evol 6:547–553CrossRefGoogle Scholar
  57. Silva JLA, Souza AF, Jardim JG, Goto BT (2015) Community assembly in harsh environments: the prevalence of ecological shift in the heath vegetation of South America. Ecosphere 6:1–18CrossRefGoogle Scholar
  58. Söderström L (1981) Distribution of bryophytes in spruce forests on hill slopes in central Sweden. Wahlenbergia 7:141–153Google Scholar
  59. Soliveres S, Maestre F, Eldridge DJ, Delgado-Baquerizo M, Quero L, Bowker MA, Gallardo A (2014) Plant diversity and ecosystem multifunctionality peak at intermediate levels of woody cover in global drylands. Glob Ecol Biogeogr 23:1408–1416CrossRefGoogle Scholar
  60. Soliveres S, Smith C, Maestre FT (2015) Plant-plant interactions, environmental gradients and plant diversity: a global synthesis of community-level studies. Perspect Plant Ecol Evol Syst 16:154–163CrossRefGoogle Scholar
  61. ter Braak CJF, Šmilauer P (2012) CANOCO reference manual and users guide software for ordination (version 5.0). Microcomputer power, IthacaGoogle Scholar
  62. Tilman D (1982) Resource competition and community structure. Princeton University Press, PrincetonGoogle Scholar
  63. Tzanopoulos J, Mitchley J, Pantis J (2005) Modelling the effects of human activity on the vegetation of a northeast Mediterranean island. Appl Veg Sci 8:27–38CrossRefGoogle Scholar
  64. Vadigi S, Ward D (2012) Fire and nutrient gradient effects on the sapling ecology of four Acacia species in the presence of grass competition. Plant Ecol 213:1793–1802CrossRefGoogle Scholar
  65. Wang J, Meier S, Soininen J, Casamayor EO, Pan F, Tang X, Yang X, Zhang Y, Wu Q, Zhou J, Shen J (2016) Regional and global elevational patterns of microbial species richness and evenness. Ecography 40:393–402CrossRefGoogle Scholar
  66. Ward D (2010) A resource ratio model of the effects of changes in CO2 on woody plant invasion. Plant Ecol 209:147–152CrossRefGoogle Scholar
  67. Wilsey BJ, Potvin C (2000) Biodiversity and ecosystem function: importance of species evenness in an old field. Ecology 81:887–892CrossRefGoogle Scholar
  68. Wright A, Schnitzer SA, Reich PB (2014) Living close to your neighbours: the importance of both competition and facilitation in plant communities. Ecology 95:2213–2223CrossRefGoogle Scholar

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© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  1. 1.School of Environmental and Rural SciencesUniversity of New EnglandArmidaleAustralia

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