1. Community Ecology BDC321 Mark J Gibbons, Room 4.102, BCB Department, UWC Tel: 021 959 2475. Email: mgibbons@uwc.ac.za Image acknowledgements – http://www.google.com

2. It takes hundreds of years for a field to develop into a mature woodland in moist temperate areas, during which time it is likely to be interrupted by disturbance and returned to an earlier successional stage. A community that appears to have reached a stable structure at the scale of km 2, will be a mosaic of miniature successions – every time a tree falls or is struck by lightening, a gap is created etc The space - time context

3. Patchiness Patch dynamic concept – views habitat as patchy, with patches being disturbed and recolonised by various species: disturbance events act as the reset mechanism. Extinction within a single patch without migration (closed system) = extinction for good Extinction within an open system is not necessarily permanent because of reinvasion from other patches Reinvasion can involve adults, but more usually, propagules. The order of arrival and subsequent recruitment levels will influence the nature and outcome of interactions

4. Successional Age # Species YoungOld GapPioneerClimax Dominance - Controlled Communities Some disturbances are synchronised, or phased, over large areas: the whole area then undergoes succession A climax mosaic produced by un - phased disturbances is much richer in species than one that is phased. Others are much smaller and produce a mosaic of patches at different stages of succession. Explain Figure

5. When looking at patch mosaics, two basic features of disturbance are important: 1) Frequency of disturbance 2) Disturbance, and hence patch, size PioneerMidClimax Time Diversity Pioneer Mid - Climax Competitive Exclusion Intensity (type) of disturbance.... When disturbance occurs Intermediate Disturbance Hypothesis Frequency of Disturbance

6. Diagrammatic representation of the time course of species richness in three gaps, and in the community as a whole, at three frequencies of un - phased disturbance. Dashed lines indicate the phase of competitive exclusion as the climax is approached. Highest diversity is maintained at intermediate levels of disturbance If disturbances occur very regularly, a pioneer community of low diversity is maintained; as communities mature so diversity increases as more species invade. At low levels of disturbance, community diversity reduced due to competition

7. Sample Time Boulder Size (N) % Bare Rock Species Richness MeanSERange Nov 1975 < 4978.01.70.181 – 4 50 – 29426.53.70.282 – 7 >29411.42.50.251 – 6 May 1976 < 4966.51.90.191 – 5 50 – 29435.94.30.342 – 6 >2944.73.50.261 – 6 Oct 1976 < 4967.71.90.141 – 4 50 – 29432.23.40.402 – 7 >29414.52.30.181 – 6 May 1977 < 4949.91.40.161 – 4 50 – 29434.23.60.202 – 5 >2946.13.20.211 – 5 Related to disturbance frequency NB: even some small boulders support high numbers of species Sousa (1979) Ecology 60: 1225-1239

8. Gap size influences community structure by influencing processes of recolonization GAP Recolonization in the centre of a large gap can only happen by distant dispersal Recolonisation in the centre of a small gap happens by local dispersal GAP Gap size

9. For Example: Mussel beds Recolonisation in the centre of a very small gap happens by lateral growth Tanaka & Magalhaes (2002) Marine Ecology Progress Series 237: 151-158

10. Area (cm 2 ) Perimeter (cm) P:A Ratio Patch Size Effects Square25200.8 Square100400.2 Square400800.2 Big Medium Small Brachidontes darwinianus Brachidontes solisianus Percent Cover Time Collisella subrugosa Chthamalus bisinuatus Density Time 1) Small gaps colonised first – laterally (mussels and limpets) 2) Large gaps – more barnacles. WHY? 3) Similar patterns for large and medium (same P:A ratio) implying that lateral recruitment more important than distant dispersal

11. Succession and Patchiness spreadsheet exercise

12. A founder-controlled community is one in which a large number of species are equally able to colonize a “gap” created by a disturbance, are equally able to survive the abiotic environment there and are equally able to persist until death: i.e. the result of disturbance is a competitive lottery, the winner being that species which happens to establish itself first. Obviously, the number of young that are available to occupy a gap should be no more for one species than another, otherwise the most productive species would come to dominate. Founder - Controlled Communities

14. Dominance Controlled Founder Controlled Two extremes of a continuum Whilst real communities may be closer to one extreme than another, component species and patches may be either dominance- or founder-controlled within the same community For Example: Half of the within-reef variation in fish assemblages attributable to habitat requirements of species, balance due to stochastic factors as emphasized by the lottery hypothesis.

15. THE END Image acknowledgements – http://www.google.com