Long-term dynamics of an epiphyte community - pattern and process
Final Report Abstract
The thorough understanding of the structure and dynamics of a local community along with the underlying mechanisms remain a central issue in community ecology, particularly in hyperdiverse tropical rain forests, but virtually all empirical evidence is derived from trees. This study focused on epiphytes in the only permanent plot world-wide dedicated to this plant group. Ten years after an initial census of an area of 0.4 ha in 2002, we recensused this and an even larger adjacent area (totaling 0.9 ha) for all occurring epiphytes with the help of a canopy crane in lowland Panama. We discovered a substantial increase in abundance (+50%) and a slight increase in species numbers (+5%) in the original area. Changes in the composition of epiphyte assemblages on individual trees in the 10-yr period was directional - there was a significant tendency towards ecological convergence with a consistently increased resemblance of assemblages on individual trees overtime. Low population densities make it unlikely that processes like competition and facilitation can explain this pattern, while colonization and differential mortality/extinction processes are more promising candidates. Vertical segregation, however, was consistent over time. Ex situ and in situ experiments explored whether the regeneration niche is the crucial ontogenetic stage, when these spatial patterns are determined. The results are mixed, in some cases clearly demonstrating strong ecological filtering of the early life stages, while in others begging for alternative explanations. Yet another potentially deterministic factor for community structure is host tree identity. The long-standing notion of host preference was tested for consistency in time by identifying host tree-epiphyte associations in the 2002 and the 2012 census. The fact that about one third of previously found associations could not be validated indicates that the usual statistical methods to identify such associations result in large type 1 errors. The remaining associations may be considered robust enough to warrant experiment scrutiny of the underlying mechanisms of these associations in the future. We also used the data to examine the notion of a predictive value of population structure for future population growth. This long-standing notion, was has already been recently refuted for trees, can now also be considered disproven for epiphytes. However, we consistently found a rather tight correlation of current population structure and population size, which is probably a consequence of the unsaturated nature of the community and a high dynamic of individual populations not dominated by persistence but by recruitment. Permanent plots in tropical forests are a true success story, because long-term comparisons allowed many insight that were previously impossible about tropical biodiversity. We will try hard to continue censussing the epiphytes at San Lorenzo in the future and would very much welcome similar efforts at other field sites to allow comparisons and generalizations.
Publications
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Do current population structures really predict community dynamics of vascular epiphytes? Jahrestagung der GTÖ 2011, Frankfurt
Katrin Wagner, Mendieta G., Zotz G.
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Long term dynamics of a vascular epiphyte assemblage in a lowland rainforest. Jahrestagung der GfÖ 2011, Oldenburg
Mendieta G, Katrin Wagner, Zotz G
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Vertical segregation of vascular epiphytes and the regeneration niche. Jahrestagung der GfÖ 2011, Oldenburg
Wagner K, Mendieta G, Bogusch W, Zotz G
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Host preferences in vascular epiphytes: how stable are patterns in time? 6. Canopy Conference, Oaxaca, Mexico, 2012
Katrin Wagner, Mendieta G, Zotz G
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Long-term dynamics of vascular epiphytes, a 10 years observational study in a lowland rainforest. 6. Canopy Conference. Oaxaca, Mexico, 2012
Mendieta G, Katrin Wagner, Zotz G
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2013. The role of the regeneration niche for the vertical stratification of vascular epiphytes. J. Trop. Ecol. 29: 277-290
Wagner, K., W. Bogusch, and G. Zotz
