III. THE TROPICAL FORESTS AND THEIR FAUNA

The distribution of animal communities is limited by climatic conditions and by the large vegetational formations like lowland forests and montane forests, amongst others (BRACK, 1986). Temperature (which is associated with altitude) governs the distribution of these vegetational formations. The tropical forests of Peru - habitat of the primate communities - are found in the Amazonian region and on the northern Pacific coast (bosque tropical del Pacífico). Primates generally inhabit forests with warm temperatures, up to about 600 in a.s.l., with the exception of Lagothrix flavicauda, Aotus miconax and Callicebus oenanthe which only live in the cloud forests of the ceja de selva.

The tropical forest of the Pacific coast extends from northern Colombia to northern Peru. It is generally characterized by a humid climate with high temperatures and summer rains (December to March), but at Tumbes, on the northern coast of Peru, the climate is much drier. The bosque tropical del Pa o at Tumbes is characterized b trees of more than 30 m height and dense undergrowth. The vegetation is dominated by representatives of the genera Cedrela, Cecropia, Ficus and Centrolobium and by palms, lianas and epiphytes.

The fauna of this area corresponds to the Pacific province of the Amazonian zoogeographic subregion. This province reaches from,Central America (south of Mexico) along the Pacific coast through Colombia and Ecuador to the forests of El Caucho and Matapalo in Tumbes. Due to the isolating effect of the Andean chain and climatic and vegetational factors, the fauna is rich in species and includes a large number of endemic subspecies. Studies by BRACK (1986) present data on mammals, birds, reptiles, amphibians, fishes, and crustaceans. Nevertheless, further studies are required for a more complete knowledge of this biome, which covers an area of nearly 5260 kM2.

The evergreen forests of Peru are,found along the eastern slopes of the Andes, in the Amazonian plain, and in some parts of the western slopes of the Andes between 800 and 3800 m a.s.l. in northern Peru. These forests are interspersed by the grasslands of the Gran Pajonal in the central part of the selva and the Pampas de Heath in the extreme south-east of Peru.

Floral diversity and vegetation types vary with altitude and latitude. With increasing altitude, tree height decreases, trunk anatomy shows an increase in dead cells which make the wood more flexible and durable, leaves become more sclerophyllic and the proportion of epiphytes increases. According to TERBORGH (1971) and our own studies, the following types of forests can be distinguished in the ceja de selva and selva alta: a) dwarf forest (between 2500 and 3800 m a.s.l.) with trees lower than 15 m of height, microsclerophyllic leaves and humus-liking epiphytes; b) cloud forest (between 1300 and 2500 m a.s.l.) with common epiphytes like mosses, lichens, ferns, orchids and bromeliads, tangled undergrowth formed by climbing bamboo (Chusquea spp., Gramineae), and tree-like ferns; c) montane rain forest (between 600 and 1300 m a. s.l.) with trees up to 35 m high, dense undergrowth and a mayor abundance of tree-like ferns.

The high degree of endemism of the fauna in the ceja de selva and selva alta can be explained by the isolation effect of the complex orography. The endemic yellow-tailed woolly monkey, Lagothrix flavicauda, inhabits the cloud forests of the Departments of Amazonas and Cajamarca, and there are museum specimens of Aotus, Lagothrix, Ateles and Saguinus fuscicollis originating from altitudes of 1200 m a.s.l. in Chinchao in the Department of Huánuco.

The selva baja has a high genetic diversity of terrestrial as well as aquatic organisms. Conditions of temperature, relative humidity and precipitation are ideal for the existence of this diversity. Acid and humid soils (from alluvial deposits) together with high temperatures accelerate the decomposition of organic matter and allow for the enormous development of the vegetation. Although the floristic composition is very heterogeneous, the physiognomy of the vegetational formations has an homogenous aspect. More than 3000 plant species (trees, palms and species of the undergrowth) are thought to occur in this region. Trees may be taller than 45 m and offer extensive vertical and horizontal space for the arboreal fauna and for epiphytes. The homogeneity of the forest is interrupted by small areas of forest that grow on white sands, with heliophile and sclerophyllic species that form dense and entangled bushes. These are the so-called varillales and chamisales which are found in Yurimaguas and on the rivers Napo, Ucayali, Itaya, Nanay, amongst others. The forest is also interspersed by extensive palm formations like the aguajales (Mauritia flexuosa) and yarinales (Pythelephas sp.), which offer optimal habitats for ungulates.