Hills and Streams: An Ecology of Hong Kong

7. Foods and Feeding

7 Foods and Feeding Primary production Living organisms require energy for their activities and matter for their construction. Only green plants can make direct use of solar radiation as a source of energy and simple inorganic molecules as a source of matter. All other organisms, with the minor exception of some autotrophic bacteria, depend on green plants, the primary producers, for food. The key process in primary production is photosynthesis, which takes place only in the chloroplasts. In most algae, all cells have chloroplasts and can photosynthesize. In higher plants, in contrast, a varying proportion of the plant body consists of non-photosynthesizing living cells - functioning in support, storage and below-ground absorption - and dead cells, serving mainly for support. Largely as a result of this, the photosynthetic output per kilogram of plant biomass (the productivity to biomass ratio) decreases rapidly from aquatic communities, where most plant cells photosynthesize, through terrestrial grasslands and shrublands, with an increasing proportion of non-photosynthetic cells, to forests, where a large proportion of the biomass is dead bark and heartwood. When compared on the basis of area, however, primary productivity is higher in forest and closed shrubland than in grassland because the depth of the canopy means there are more photosynthetic cells per unit area. The productivity of grasslands on Hong Kong hillsides is also limited because of the death 105 Hills and Streams· An Ecology of Hong Kong of much of the foliage in winter (most trees and shrubs are evergreen) and the often considerable proportion of bare ground. There have been no measurements of the primary productivity of terrestrial communities in Hong Kong. Net primary productivity (i.e., photosynthetic production minus plant respiration, or the plant biomass that would be available for consumption by herbivores) is of particular interest. Comparisons with other parts of tropical and subtropical Asia suggest that the net primary productivity for mature forest in Hong Kong would be around 2 kg/m2/yr. This includes only aboveground production; below-ground production, which may be as much or more, is very difficult to measure accurately. The productivity of most existing plant communities in the Territory will be considerably less than this figure because few communities approach the maximum potential leaf cover (leaf area index) of 7-8 m2of leaf area per m2 of ground surface area that is typical of climax forest. Shrubland productivity is probably of the order of 1 kg!m2/yr - about half that of forest. Measurements of above-ground biomass for local vegetation are scarce also but data on similar vegetation types in Guangdong suggest a range from around 3 kg!m2 for shrubland to at least 20 kg! m2 for secondary forest. Shrubland production to biomass ratios are unlikely to exceed 0.3, while in secondary forest the figure would be less than O.l. Primary productivity in hill streams is very low. This is partly because of shortage of nutrients in the water. The phosphorus and nitrogen needed by algae (and, indeed, all organisms) to build proteins are present only in minute amounts unless the water has been polluted by sewage, agricultural fertilizers or livestock wastes. One nutrient present in excess is silica, which is used by diatoms (a type of algae) to build their cell walls or frustules. On its own, however, silica does not increase the amount of plant food available to stream animals. Production is limited further by shading. Streams provide a water source for hillside plants throughout the dry season and, even on an otherwise grass-covered hillside, riparian shrubs and trees may shade the stream bed completely. Algae are thus limited by light as well as by nutrients, and stream food chains depend largely upon plant litter derived from trees and shrubs lining the banks (see Chapter 6: Landwater interactions: streams). 106 Foods and Feeding Herbivores The greenness of Hong Kong hillsides suggests a superabundance of food for plant-eating animals. That this is not necessarily so, however, can be seen by imagining oneself having to rely on hillside plants for food. The vast majority of the available plant biomass is too tough, too poisonous or too low in nutritional value. Better-adapted mouth parts and a modified digestive system could overcome some of these problems but the basic fact remains: plant bodies are much poorer nutritionally than animal bodies. There are two major differences between the cells of plants and those of animals which influence...