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Coexistence mechanisms - the role of spatial structure and competition-colonisation trade-offs

One of ecology's most fundamental conundrums is how multiple species coexist within a habitat, even though "survival of the fittest" suggests that a single species should competitively exclude all others. A generation of ecologists have developed theoretical models demonstrating that multiple species can coexist under certain conditions, but testing these theories in the field remains a challenge. The papers described in this section hark back to a time before the Forest Ecology and Conservation Group existed, when David Coomes sojourned each spring on the coastal sand dunes of Norfolk, as Dr Mark Rees' postdoc at Imperial College. What started out as a mission to demonstrate that competition-colonisation trade-offs promoted coexistence of annual plants turned into an altogether more interesting exploration of the influences of spatial structure on density dependence.

Our premise was that seed size mediated a competition-colonisation trade-off in annual plant communities: small seeds could be produced in large numbers, but they grew into initially tiny seedlings that were easily suppressed by those of larger-seeded species. Theoretical modelling had already demonstrated that such a trade-off could promote the coexistence of species; all we had to do was prove the theory correct. We developed an elaborate "neighbourhood modelling" method to quantify the intensity of competitive interactions among annual species, in my case within dune slack communities and in Lindsay Turnbull's case within limestone grasslands. Both Lindsay and I found evidence of competition-colonisation trade-offs1,2 but the competitive advantage of being large-seeded was not enough to strongly promote coexistence3.

We characterised the spatial structure of the annual plant communities - the degree to which species were aggregated in space and associated with other species3,4 using a novel analytical approach5. We then used 'virtual experiments' to randomly reorganise the community's spatial structure and quantify the effects on competitive interactions6. Our analysis, for the first time, quantified the magnitude and direction of the effects of spatial structure in a natural community, and supported the conclusions of theoretical models that spatial structure can have substantial impacts on component species and community dynamics6.

Species that are long-lived do not need to regenerate from seed very often in order to persist in communities. Multi-stemmed architecture may be an important trait for the persistence of trees in forest understoreys, but this idea has rarely been tested using long-term individual-level data. We used measurements of 8527 individual woody stems from 1985, 1996 and 2008 to model the growth, survival and recruitment of hazel (Corylus avellana) and hawthorn (Crataegus laevigata and C. monogyna) and found that multi-stemmed architecture is an advantageous trait for understorey trees in temperate woodlands and suggest that the low light levels of forest understoreys favour ‘persistence’, through multi-stemmed growth, rather than ‘regeneration’ niches (i.e. periodic recruitment through seed)8. Where regeneration does occur in forests, seedlings compete strongly for light and a slight height advantage can result in a much strong competitive advantage9.

  1. Turnbull, L.A., Coomes, D.A., Hector, A., & Rees, M. (2004) Seed mass and the competition/colonization trade-off: competitive interactions and spatial patterns in a guild of annual plants. Journal of Ecology, 92, 97-109. click
  2. Coomes, D.A., Rees, M., Turnbull, L., & Ratcliffe, S. (2002) On the mechanisms of coexistence among annual-plant species, using neighbourhood techniques and simulation models. Plant Ecology, 163, 23-38. click
  3. Coomes, D.A., & Grubb, P.J. (2003) Colonization, tolerance, competition and seed-size variation within functional groups. Trends in Ecology & Evolution, 18, 283-291. click
  4. Coomes, D.A., Rees, M., Grubb, P.J., & Turnbull, L. (2002) Are differences in seed mass among species important in structuring plant communities? Evidence from analyses of spatial and temporal variation in dune-annual populations. Oikos, 96, 421-432. click
  5. Coomes, D.A., Rees, M., & Turnbull, L. (1999) Identifying aggregation and association in fully mapped spatial data. Ecology, 80, 554-565. click
  6. Turnbull, L.A, Coomes, D.A., Purves, D.W., & Rees, M. (2007) How spatial structure alters population and community dynamics in a natural plant community. Journal of Ecology 95, 79–89. DOI link
  7. Bonis, A., Grubb, P.J., & Coomes, D.A. (1997) Requirements of gap-demanding species in chalk grassland: reduction of root competition versus nutrient-enrichment by animals. Journal of Ecology, 85, 625-633. click
  8. Tanentzap, A. J., Mountford, E. P., Cooke, A. S., & Coomes, D.A. (2012). The more stems the merrier: advantages of multi-stemmed architecture for the demography of understorey trees in a temperate broadleaf woodland. Journal of Ecology. DOI link
  9. Tanner, E.V.J., Teo, V.K., Coomes, D.A. & Midgley, J.J. (2005) Pair-wise competition-trials amongst seedlings of ten dipterocarp species; the role of initial height, growth rate and leaf attributes. Journal of Tropical Ecology, 21, 317-328. click