Salim Ali School of Ecology and Environmental Sciences, Pondicherry University, Pondicherry 605 014, INDIA.
It has been postulated that species occupying similar ecological niches coexist because of their differential use of resources (Lack, 1971; MacArthur, 1972). By partitioning the available resources, these species are able to avoid interspecific competition. However, support for this theory from field studies is ambivalent (Wiens, 1989).
Woodpeckers may be considered as model organisms to test the competitive exclusion theory as several species, varying in size and level of specialization, typically co-exist in forest habitats both in temperate and tropical regions. Studies in the temperate regions have shown many ways that sympatric woodpeckers differ in foraging and nesting requirements (Williams, 1975; Alatalo, 1978; Conner, 1980; 1983; Bull et. al., 1986). These include differences in foraging behaviours, use of different micro- and macro-habitats and use of resources at different times. However, very few such studies have been conducted in the tropics (Short, 1978; Askins, 1983), where the diversity is higher and habitats more complex.
Woodpeckers are important components of forest communities and play a vital role in forest ecosystems. Being insectivorous, they help in controlling bark insects directly by feeding on them and indirectly by altering the micro-climatic conditions on tree barks (McCambridge and Knight 1972; Kroll and Fleet, 1979; Otvos, 1979; Kroll et al., 1980; Mendel et al., 1984).
As primary cavity nesters, capable of excavating tree holes, woodpeckers provide nesting opportunities to a host of secondary hole-nesters that depend on old nest-holes of woodpeckers or natural tree cavities (Short, 1979; 1982). Tree holes can be a scarce resource (Von Haartman, 1957; Collias and Collias, 1984), and often there is severe competition among both the primary excavators themselves or between the primary and secondary hole-nesters for occupation of ideal cavities (Short, 1979).
Forestry practices such as selective logging can be detrimental to woodpecker populations (Johns, 1989). Removal of dead trees or snags and large trees with decaying trunks or dead branches reduce nesting and foraging sites for woodpeckers (Conner et al., 1975, Scott, 1979; Martin, 1987). Loss of forest areas or their conversion into plantations, and habitat fragmentation, also affect woodpeckers adversely (Pettersson, 1985; Ligon et al., 1986; Conner and Rudolph, 1989; Mitra and Sheldon, 1993).
Their role as biocontrol agents has not been properly understood or appreciated in the monoculture plantations, especially in the tropics, where increased areas in the forestry sector are being brought under intensive cultivation of fast-growing tree species that are vulnerable to harmful insect pests (UNESCO, 1978; Speight, 1986).
Therefore, there is an urgent need to understand the ecology of tropical woodpeckers: their habitat needs in terms of foraging and nesting, their interactions with one another and other hole-nesters, and their population densities, in order to save them and their habitats.
This study was initiated with the following objectives at the Peechi-Vazhani Wildlife Sanctuary, Kerala, between 1991 and 1993:
1. To determine the resource utilization by sympatric woodpecker species to see if they differed in their foraging behaviour, foraging habitats, nest sites and nesting habitats.
2. To determine the densities of the various species in natural forests and manmade plantations.
3. To draw up strategies for conservation and management of woodpeckers and their habitats.