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[Latest update: October 31st, 2000]
Deforestation and land use change are the
most important factors which put into danger the structural integrity
of tropical rainforests and other tropical habitats.
(Photograph by Robert Lücking)
The delicate symbiotic nature of lichens is one of the reasons for their sensibility and aptitude as bioindicators of environmental parameters and air pollution [HAWKSWORTH & ROSE, Lichens as Pollution Monitors (1976); RICHARDSON, Biological Indicators of Pollution (1987); ARNDT et al., Bioindikatoren (1987); WIRTH in ELLENBERG, Zeigerwerte von Pflanzen in Mitteleuropa (1992); SEAWARD, Environ. Rev. 1: 73-91 (1993)]. In tropical regions, lichens have been used as bioindicators only in a few instances [e.g. WOLSELEY & AGUIRRE-HUDSON, Glob. Ecol. Biogeogr. Lett. 1: 170-175 (1991); GALLOWAY, Glob. Ecol. Biogeogr. Lett. 2: 37-45 (1992)]. Except for urban areas, pollution is a less important problem in the tropics, and instead, attention must be drawn to the destruction of tropical forests and land use change [WILSON, Biodiversity (1988)].
Ideal properties of a bioindicator include prompt and accurate response to environmental parameters, easy identification, possibilities of standardization and application by non-specialists, commonness and wide geographical distribution, and genetic homogeneity. Tropical communities are more diverse and their underlying factors more complex than in temperate regions. Data sampling needs to be more extensive and accompanied by ex-situ evaluation, and bioindicators should therefore permit collection of large data sets and their quantitative evaluation with low expense. Foliicolous lichens are particularly promising as bioindicators in the tropics. Their relationship to the substrate, the living leaf, is reduced, and their dependence on atmospheric factors particularly pronounced. Foliicolous lichens exhibit an accelerated life cycle and immediately react upon environmental changes. They grow essentially two-dimensional and are of similar size orders, making quantitative approaches more feasable. Their taxonomic background is excellent and they are particularly useful for ex-situ evaluation, since they are easily collected in large quantities and quickly processed for investigation and documentation.
Foliicolous lichens are suitable for indication of the following environmental parameters [44]: (1) Altitudinal zonation, reflecting the so-called "Massenerhebungseffekt" [GRUBB & WHITMORE, J. Ecol. 54: 303-333 (1966); 27, 51]; (2) seasonality, i.e. the amount of precipitation and their annual distribution; (3) anthropogenic disturbance; (4) microsite, differenciating between different light intensities in the shady understory, natural light gaps and the outer canopy. Based on our studies on habitat and microhabitat preferences [3, 4, 6, 19, 21, 27, 40, 53-58, 61, 63-64], indices were defined for 300 foliicolous lichen species [44], and a List of environmental index values for foliicolous lichens was compiled. This list, which can also be used as a reference of the ecology of each species, represents a working base which should not be uncritically used but is thought to be gradually improved and enlarged by more detailed studies, especially in the Paleotropics.
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The species of Badimia, with their
large, colourful apothecia, are restricted to undisturbed primary forests,
while Asterothyrium is an indicator of anthropogenic influence.
(Photographs by Robert Lücking)