This question has taxed ecologists and foresters for centuries and has been the theme of thousands of publications. Despite this, we are still struggling to explain why one or just a few species do not come to dominate a forest by killing off the weaker species. Requirements for certain species are remarkably precise, as in the case of Abbot squirrels which have become so dependent on Ponderosa pine, whose seeds and shoot-tip bark they consume, that they are now found only in association with this tree (Attenborough, 2002). However, other more general considerations apply elsewhere. As has been noted previously, tropical forests tend to hold far higher numbers of species per unit area than any other forest. Tropical rain forests not uncommonly contain over 100 tree species in a hectare, and up to 283 per ha. Moreover, Pitman et al. (1999) found 825 tree species in 21 forest plots covering just 36 ha scattered through 400 km2 of the Amazon forest in Peru. This compares with 620 tree species in the whole of North America covering almost 25 million km2. The question of how species co-exist is thus even more pertinent in the tropics.
In general terms high diversity in the tropics is linked with 'productive habitats that have a benign, stable climate, a diverse physical environment, abundant biotic interactions, and long periods of time without major climatic or geologic disturbances' (Schemske, 2002, p. 167). Abundant biotic interactions, such as having specialized pollinators or prey to eat, reduce competition between species allowing them to co-exist (i.e. the ghosts of competition past), as exemplified in Section 6.2.2. If several species of tree, for example, were competing for the same pollinators, the best competitor would win driving the other extinct, but if they use different pollinators, that element of competition is removed. Such specialization is dependent upon a stable environment allowing a species to narrow its niche which, in turn, allows more species to be packed into the forest. So, a bird might eat the fruit of just one tree in the tropics while in temperate regions a similar bird would need a wider range of foods to cope with seasonal differences and year to year variations in fruit production. The main premise of this argument is that a reduction in competition between species allows more species to co-exist. However, it can also be argued that greater competition may help co-existence by suppressing the vigour of otherwise dominant species. Having stressed the value of stable conditions, non-equilibrium mechanisms (such as variation in habitat across an area, or variation in time by periodic disturbance or gentle changes in local climate) can contribute to species co-existence by giving spatial or temporary advantage to different sets of species, again preventing one set from driving others out. (Tropical forests, of course, show low stability in the face of large disturbances, so the emphasis above is on gentle or small-scale changes to enhance biodiversity.) The books Foundations of Tropical Forest Biology (Chazdon and Whitmore, 2002) and Species Coexistence (Tokeshi, 1999) are recommended for further development of these complex arguments.
A further solution has been borrowed from the evolution of genes. In the 1960s, when it seemed that the huge number of genes found in organisms was too many to explain just by natural selection, the neutral theory of molecular evolution came to the rescue. In this model, neutral, harmless mutations occur randomly, gradually increasing gene diversity beyond those selected for by natural selection. Stephen Hubbell (2001), who developed the Neutral Theory of Biodiversity, suggests a similar process may happen with species, in that diversity is increased by the accumulation of species by chance. If the environment is stable enough, for example, to allow a number of species to survive, then chance can lead to them accumulating into a rich forest. The mathematical models accompanying this theory do indeed often predict results very similar to those actually found in nature. Even this seems not to be the whole answer, since assemblages of species in the tropics build up more quickly than random accumulations could predict (see Nee, 2005).
The following sections explore some of the factors that affect biodiversity and co-existence in more detail.
6.5.1 Soil conditions and leaf litter as sources of diversity
Along with propagule availability, historical factors, light conditions and grazing by animals, soil conditions, notably soil pH and fertility, drainage and water availability, greatly influence biodiversity in the tree and shrub layers and understorey floras. Many of the 30 or so species of oak found in south-east USA are closely associated with particular soil conditions. White and black oaks (Quercus alba and Q. velutina) are found on well-drained upland clay soils, shingle oak Q. imbricaria on calcareous soils, water and willow oaks (Q. nigra and Q. phellos) in moist river basins, turkey and blackjack (Q. laevis and Q. marilandica) oaks on dry sand hills and live and laurel oaks (Q. virginiana and Q. laurifolia) in maritime forests. Terborgh (1992) compares these American forests with those of the primary lowland forests of south-east Asia, which are some of the most valuable and best stocked of any in the tropics. Their commercial value is largely due to the high proportion and many species of Dipterocarpaceae present. Many of these dipterocarps have very straight trunks, are up to 60 m tall, and have wood with excellent working qualities which is often sold under the misleading name of 'Philippine mahogany'. The two or three most abundant dipterocarp species present in a stand often characterize its floristic composition (particular groups of species growing under different dipterocarps) and are related to the substrate on which the forest grows, but the number of associations present is far greater than in the oak forests just mentioned.
Soil conditions and light intensity have a strong influence on bryophyte distributions and zonation; indeed mosses and liverworts are in general very good indicators of small-scale mosaic patterns involving soil pH, levels of mineral nutrients, humidity and illumination. Coppicing causes the relative humidity of the air to decrease, while the light and temperature range are increased. This has a differential effect on the growth of those bryophytes which are either able to invade or are present already. Gimingham and Birse (1957) found that the life form sequence - dendroid forms (e.g. Thamnium alopecurum and Mnium undulatum) and thalloid mats (e.g. Pellia epiphylla): rough mats (e.g. Eurhynchium striatum): smooth mats (e.g. Hypnum cupressi-forme): short turfs (e.g. Ceratodon purpureus) and small cushions (e.g. Orthotrichum anomalum) - occurred along a gradient in which light intensity increased and atmospheric relative humidity decreased. This sequence helps us to interpret the striking contrast in growth form distribution (see Section 3.1.1) which may be seen when tracing bryophyte communities along a stream which runs through both felled and unfelled regions of a wood.
The rugged bases of old ash stools in Hayley Wood in southern England bear species of moss and liverwort such as Lejeuna cavifolia, Porella platyphylla, Homalia trichomanoides and Neckera complanata, for which Cambridgeshire is otherwise too dry. Hookeria lucens, a moss of heavily shaded moist places, such as the deeply incised Seckley Ravine, Wyre Forest, English Midlands, is particularly susceptible to environmental change; exposure to direct sunlight kills it. Soil pH is also important. The liverwort Pellia epiphylla is favoured, like the moss Mnium hornum, by surface acidity, whereas Pellia endivifolia is a calcicole requiring alkaline conditions.
Some soil-dwelling organisms facilitate the presence of others. The nest mounds of ants (Fig. 6.8) are hotspots for litter-dwelling earthworms and 37 of the 369 ant-associated species of beetle recorded in Denmark and Fennoscandia are actually dependent on ants (i.e. are myrmecophilous).
Soil loss and deterioration can be very severe if forests are managed incorrectly; rates of erosion when tropical rain forests are felled in hilly country are very high. The influence of various trees on the degradation of forest soils is considered in Section 2.2.3, while recent declines in the floras of English ancient woodlands, often caused by nutrient enrichment (eutrophication), are discussed in Section 8.6.2.
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