Essay on the Ecology of Different Animal Groups
Strong habitat segregations among ecologically similar and similar sized primate species have been reported in Southeast Asia (Chivers, 1973) and Africa. Possibly, competition may be an important factor in favouring the habitat-use patterns observed.
Tropical birds seem to share the following ecologically significant characteristics:
1. They have distinct breeding seasons which are the longest in species of humid forests and the shortest in the drier season.
2. Their clutch sizes are smaller than those of temperate birds. Commonly, two young are produced in wet tropical regions, and three prevail in more seasonal areas.
3. As compared to temperate areas, in humid tropical regions the nest predation rates are very high and there is very little nestling starvation. Small tropical birds have higher adult survival rates than temperate birds.
4. Most tropical forest birds are highly sedentary.
5. The incidence of communal breeding systems is much higher in tropical as compared to temperate birds (Fry, 1972).
6 Tropical forest birds seem to be restricted to narrower vertical zones of vegetation than in the case of temperate birds, and this seems to be associated with the greater species diversity in tropical forests.
7. Tropical habitats support more species of birds as compared to temperate habitats of similar vegetative structure and this does not seem to depend on the richness of plant species in the tropics (Orians, 1969). Another important difference is that the number of rare species of birds is much larger in the tropics.
Nilsson (1979) has found close correlations between community densities and avian species richness of different forest plots in Sweden. Another important observation was a close correlation between the standing deadwood and avian species density. In fact, Nilsson was able to successfully predict both bird community densities and species richness from the proportion of the basal tree and shrub area standing dead.
According to Nilsson 1979), vertical foliage structure is correlated with ecosystem productivity (not chiefly with bird species diversity). He implicated food availability as the ultimate determinant of variations in local bird species richness and their community density. Nilsson’s work has further shown that the overall bird community structure is the same in different types of forest.
The proportion of residents was higher and tropical migrants lower in coniferous than in nearby deciduous forests. Only some 10 per cent of the birds in the coniferous forest were tropical migrants in contrast to about 60 per cent in the eastern North America (Nilsson, 1979).
Some birds live in environmentally unstable areas. In these species, site fidelity is not an important factor in interspecific competition. If a part of a population of such a species evolves migratory behaviour, and if the productivity of this fraction of the population is greater than that of the remaining fraction, then migratory bahaviour will tend to be positively selected.
This often leads to the origin of completely migratory land bird species from unstable environments; in this case the migration is directed towards those areas where environmental stability and production during the breeding season are higher than in the non-breeding areas (Alerstam and Ecknell, 1979).
The migratory strategy is most fruitful for such simple-niche species as waders, swallows and terns. However, the success of this kind of strategy depends on two factors, viz., (1) that the birds escape competition during the breeding season by migrating to marginal areas, and (2) that their productivity in the breeding areas is high enough to permit some young ones to return to the non-breeding areas and to compete successfully with the remaining fraction of the population (Alerstam and Ecknell, 1979).
There are two main kinds of demographic strategy’ in birds. At one end of a series are species devoting much of the available energy to reproduction; these birds inhabit variable and rather unstable environments where their numbers are regulated by density-independent factors and where they tend to be well dispersed.
At the other end are those species which devote less energy to reproduction and more to surviving predation and competition; these mainly sedentary birds occur in fairly stable habitats and their numbers are regulated by density-dependent factors.
A rich and diverse community depends chiefly on an environment that is extensive and diverse and which supplies sufficient energy spread evenly over the year. Predation reduces competition and so favours species diversity. Most of these conditions prevail in the tropics where, for instance, guilds of frugivores occur. The development of such guilds is difficult or even impossible in temperate areas where fruit availability periods are too short.
Birds do not seem to play a major role either in the flow of energy or the cycling of nutrients in ecosystems. For instance, Holms and Sturges (1973, 1975) estimated that in some American forests the annual energy flow through the avian community did not exceed some 0.2% of the annual net primary production.
However, birds often exert important indirect effects on ecosystems by affecting the population levels of certain other organisms that are directly very important in nutrient cycling and energy flow. Insectivorous birds which eat some pest insect and control the pest population, are beneficial to man whereas grainivorous birds which eat seeds of crops or other plants useful to man, are often negatively important to man.
For instance, the house sparrow (Passer domesticus) and other species of Passer compete with man for crop or other seeds, producing a strong effect on agroecosystems. In Poland, P. domesticus causes such a serious damage to winter barley that farmers have now stopped growing this crop. In contrast, P. montanus is either neutral or even beneficial to farming in Poland (Pinowski, 1973).
Pianka (1973) has reviewed the status of knowledge about the ecology of tropical reptiles. Reptile communities display some species differences in habitat use and in food size. Separation of species by breeding season is quite rare in reptiles.
Microhabitat differences between species increase as the number of species living together increases. Separation with respect to food type seems, more important for those animals which feed on large foods (relative to their own body size) than for those feeding on small foods.
While much of modern ecological theory stems from the observation of birds, particularly from the studies of David Lack and Robert MacArthur, in recent years the study of lizard ecology has contributed substantially to our knowledge and understanding of ecology. Lizards, of course, are low- energy animals in contrast to the high energy birds (Huey et al., 1983).
Although much work has been done on the systematics of Asian fishes, very few ecological studies have been made though the diversity of fish biota in the tropical Asia has been well established. Critical studies are needed to establish how fish communities are organized, what they eat, what effects they produce on food resources, how adaptable they are, and what the effects of different seasons on their breeding are.
Some population studies of tropical insects have been undertaken in relation to medical, agricultural or sylvicultural aspects. In the case of butterflies and euglossine bees, there are indications of long adult insect life in the humid tropics, possibly because of year-round availability of resources and warm temperature.
During the growing season, the number of individuals and total biomass is much greater in secondary vegetation than in adjacent primary forest understorey. It is generally believed that greater habitat overlap is more common in groups which are largely made of generalized predatory and scavenger species.
The greatest species richness and biomass in forest understorey samples seems to occur at sites located within the altitudes 1000-2000 metres. There are some indications that the lower night temperatures result in greater net plant productivity. The abundance and species diversity of insects declines greatly above 3000 metres.