Speciation in the tropical rain forest and the concept of the niche

Two statements about the tropical rain forest are constantly reiterated:(1) it is the richest in species of all plant communities, (2) it is unusual among species-rich communities in the frequency of series of closely related (or at least congeneric) apparently sympatric species (cf. Fedorov, 1966). The comparison of species diversity in plant communities of very different physiognomy, e.g. forest and grassland, presents difficulties. On a species/area basis it is possible that some non-forest communities, e.g. the fynbosch (sclerophyll scrub) of the Cape and sclerophyll communities in Australia and New Caledonia, may be richer than the tropical rain forest, but the latter is certainly much richer than any other forest community. The total species-richness of the rain-forest flora depends in part on the large number of synusiae present, e.g. of lianes, epiphytes, ground herbs etc., but only trees will be discussed here. The species diversity of rain-forest trees varies regionally (e.g. richest in south-east Asia, poorer than elsewhere in most parts of tropical Africa) and according to site conditions (e.g. poorer on podsols and on sites with impeded drainage than on well drained latosols in the same area). The frequency of groups of closely related species varies in different regions and in different storeys in the same region. The very large number of Shorea spp. and other nearly related groups of Dipterocarps in the A storey in south-east Asia is very striking, but in Africa and America, the numerous congeneric species in the B and C storeys is equally remarkable, e.g. Diospyros and Rinorea in Africa, Miconia in tropical America. Some families tend to be represented in all storeys, but not by many species in any one, others in only one or two storeys, but by many species in the same storey. The common occurrence of groups of morphologically similar and nearly related tree species in the rain forest seems in contradiction to Gause's competitive exclusion principle. These questions suggest certain reflections on the concept of the niche as applied to plants. If it applies in any exact sense to rain forest trees it implies that species which occupy niches must stand in a different relation to the resources (light, water, mineral nutrients) of the environment and to other organisms of the ecosystem. It is well known that even unrelated rain-forest trees of the same storey tend to be very similar in such features as crown form, size and shape of leaves etc., and are often hard to distinguish in the sterile condition. Characters of possible ecological importance in which they differ include the following:usual mature height (determines the place of the tree in the stratification), growth rate, life span, shade tolerance, type of dispersal mechanism, reproductive strategy, flowering season etc. These tend to be correlated together and not combined at random, e.g. shade-intolerant species tend to be fast-growing and to have efficient (wind or bird) dispersal mechanisms, shade-tolerant to be slow-growing and to be gravity-dispersed. But even if allowance is made for all these known differences of possible ecological importance it is difficult to believe that they are enough to fit as many species to occupy different niches as seems to be required by the competitive exclusion principle. Other differences between species probably exist:possibilities are in the way different species exploit the mineral resources of the soil (root activity, differences in recycling of nutrients depending on different length of life of leaves) and defence mechanisms against herbivores and pathogens. The latter may be biochemical or biological as in myrmecophytes. We can only conclude that we know very little about the autecology of rain-forest trees. Comparative studies of selected species from different storeys in the same stand may be very valuable.