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[Latest update: October 31st, 2000]
Neotropical rainforests are legendary for
their vegetative diversity.
Up to 473 species of trees may grow in a single, one-hectare plot.
(Photograph by Robert Lücking)
Tropical rain forests are known for their high species richness at the large and small scale [WILSON, Biodiversity (1988)]. Current estimates indicate that 50-80 % of all organisms on earth live in the tropics [ROBINSON, Biotropica 24: 345-352 (1992)], most of them still unknown and hidden in the unexplored forest canopy [GOTTSBERGER et al., Ulmensien 10: 51-96 (1995)]. Factors that contribute to the high diversity at different scales are: (1) Geographical isolation; (2) habitat differenciation; (3) microhabitat differenciation; (4) niche relations; (5) the mass effect; (5) ecological equivalence [SHMIDA & WILSON, J. Biogeogr. 12: 1-20 (1985)]. These factors have been studied in foliicolous lichens at various levels [19, 21, 53, 54-57, 63], with the following results:
Geographical isolation: Of the 700 species accepted world-wide, 400-500 occur in each of the major tropical regions, 380 in a single country like Costa Rica, and 200-300 at a single site. Comparing the whole tropics (700 species) with the Neotropics (500 species), geographical isolation can explain 25 % of the world's diversity. Setting the Neotropics against Central America (400 species), only 20 % of its diversity is explained by geographical isolation [21, 53, 57, 63].
Habitat differentiation: Costa Rica's vegetation types can be roughly divided into five altitudinal zones (lowland, submontane, lower montane, upper montane, páramo) and three humidity provinces (rain forest, evergreen dry forest, semideciduous dry forest), which gives 15 possible combinations. Comparing then Costa Rica (380 species) with a single habitat type, the lowland rainforest (300 species), the habitat differentiation accounts for 23 % of Costa Rica's diversity [21, 57].
Microhabitat differentiation: The lowland rain forest can be divided into three principal microhabitats: shady understory, light gaps, and the canopy. Comparing a whole lowland rain forest site (280 species) with a single microhabitat, the shady understory (140 species), microhabitat differentiation accounts for 75 % of the habitat diversity [19, 54, 63].
Niche relations: Thus far, niche relations are not apparent in foliicolous lichens, since nutrient requirements seem to be similar, and there are no distinct phorophyte specifities or specific reproductive interactions with diaspore vectors. Niche relations are therefore close to zero and hardly explain microhabitat diversity [54-55, 63].
Mass effect: The mass effect means the enrichment of communities with individuals of species which do not belong to the particular community. Of the 140 species of foliicolous lichens found in the shady understory of the lowland rain forest, about 110 are genuine members of this community while 30 belong to other communities, in particular the light gap association. The mass effect in this case thus accounts for 20 % of the microhabitat diversity [56, 63].
Ecological equivalence: Niche relations, mass effect and ecological equivalence together summarize 100 % in explaining microhabitat diversity. If niche relations are close to zero and the mass effect accounts for 20 %, the degree of ecological equivalence is close to 80 %, which means that the 110 species of foliicolous lichens found in the shady understory of a lowland rain forest site principally have the same ecological requirements and occupy the same niche.
High small-scale diversity is particularly well-known in trees, the structural component of tropical rainforests. One hectar may include 100-200 tree species on an average, and up to 437 species where registered in a plot in Amazonian Ecuador [VALENCIA et al., Biodiv. Cons. 3: 21-28 (1994)]. These figures also hold for foliicolous lichens: A single site of a size of one hectare may shelter up to 280 species, and 82 species where reported from a single palm leaf. No less than 48 species where found on a small dicotyledon leaf of the size of a petri dish [21, 54, 63]. High diversity at the small scale is particularly striking when niche relations seem to be absent and the species are ecologically equivalent. Theoretically, a single niche should be occupied by a single species. However, high levels of sympatric ecological equivalence can be explained by intermediate disturbances and niche fragmentation [CONNELL, Science 199: 1302-1310 (1978); HUSTON, Am. Nat. 113: 81-101 (1979); 21, 57]. When several species are ecologically equivalent and sympatric, succesion starts with increasing diversity until one species eventually becomes dominant and outcompetes the others, finally reducing diversity to zero. Intermediate disturbances prevent succession from reaching this climax stage and in that way maintain a higher level of diversity. This effect is pronounced by division of the niche into spatially isolated fragments, where different successional histories may lead to partly different species assemblies which then increase overall diversity.
The two effects of intermediate disturbances and niche fragmentation can be observed in foliicolous lichens. For a community, leaf renewal is an intermediate disturbance which restarts succesion on the individual leaf, and the spatial isolation of leaves corresponds to niche fragmentation. The spatial and temporal dynamics are comparable to "gap" dynamics in rain forest trees which partly explains the high small-scale diversity found in these organisms. These mechanisms can also be demonstrated experimentally, making use of the fact that foliicolous lichens grow well on artificial surfaces like plastic and glass [SIPMAN, Lichenologist 26: 311-312 (1994); MONGE-NÁJERA & BLANCO, Trop. Bryol. 11: 5-9 (1995); 52]. An ongoing reserach project in "La Selva" investigates the effect of leaf demography and spatial isolation on overall diversity.
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The phenomenon that foliicolous lichens
grow well on any smooth substrate, including plastic and glass surfaces,
can be used to design demographic experiments with artificial leaves.
(Photographs by Robert Lücking)