The literature on tropical secondary forests, defined as those resulting from human disturbance (e.g. logged forests and forest fallows), is reviewed to address questions related to their extent, rates of formation, ecological characteristics, values and uses to humans, and potential for management. Secondary forests are extensive in the tropics, accounting for about 40% of the total forest area and their rates of formation are about 9 million ha yr−1. Geographical differences in the extent, rates of formation and types of forest being converted exist.

Secondary forests appear to accumulate woody plant species at a relatively rapid rate but the mechanisms involved are complex and no clear pattern emerged. Compared to mature forests, the structure of secondary forest vegetation is simple, although age, climate and soil type are modifying factors. Biomass accumulates rapidly in secondary forests, up to 100 t ha−1 during the first 15 yr or so, but history of disturbance may modify this trend. Like biomass, high rates of litter production are established relatively quickly, up to 12–13 t ha−1 yr−1 by age 12–15 yr. And, in younger secondary forests (< 20 yr), litter production is a higher fraction of the net primary productivity than stemwood biomass production. More organic matter is pro duced and transferred to the soil in younger secondary forests than is stored in above-ground vegetation. The impact of this on soil organic matter is significant and explains why the recovery of organic matter in the soil under secondary forests is relatively fast (50 yr or so). Nutrients are accumulated rapidly in secondary vegetation, and are returned quickly by litterfall and decomposition for uptake by roots.

We propose a model of the gains and losses, yields and costs, and benefits and tradeoffs to people from the current land-use changes occurring in the tropics. When the conversion of forest lands to secondary forests and agriculture is too fast or land-use stages are skipped, society loses goods and services. To avoid such a loss, we advocate management of tropical forest lands within a landscape perspective, a possibility in the tropics because land tenures and development projects are often large.

The Asian elephant's foraging strategy in its natural habitat and in cultivation was studied in southern India during 1981–83. Though elephants consumed at least 112 plant species in the study area, about 85% of their diet consisted of only 25 species from the order Malvales and the families Leguminosae, Palmae, Cyperaceae and Gramineae. Alteration between a predominantly browse diet during the dry season with a grass diet during the early wet season was related to the seasonally changing protein content of grasses.

Crop raiding, which was sporadic during the dry season, gradually increased with more area being cultivated with the onset of rains. Raiding frequency reached a peak during October-December, with some villages being raided almost every night, when finger millet (Eleusine coracana) was cultivated by most farmers. The monthly frequency of raiding was related to the seasonal movement of elephant herds and to the size of the enclave. Of their total annual food requirement, adult bull elephants derived an estimated 9.3% and family herds 1.7% in quantity from cultivated land. Cultivated cereal and millet crops provided significantly more protein, calcium and sodium than the wild grasses. Ultimately, crop raiding can be thought of as an extension of the elephant's optimal foraging strategy.

The 71 ha Bukit Timah Nature Reserve contains the largest surviving area of primary rain forest in Singapore. The recorded vascular plant flora of the forest consists of at least 854 species: 787 angiosperms in 109 families, 65 pteridophytes and three gymnosperms. 58.6% of the species are trees or shrubs, 18.7% climbers, 12.3% terrestrial herbs, 8.8% epiphytes and hemi-epiphytes, 1.0% saprophytes and 0.6% parasites. The four families with most species -Rubiaceae, Euphorbiaceae, Orchidaceae and Moraceae - account for a quarter of the angio-sperm flora but contribute few large trees. Community reproductive phenology is charac-terized by supra-annual bursts of general flowering and fruiting, contrasting with a low back-ground level. After an exceptionally dry and sunny February, 1987, more than 150 species in 42 families flowered between late March and late May, followed by an equally well-defined fruiting peak 13 weeks later, between late June and late August.

Length-mass relationships within an assemblage of tropical snakes are used to describe morphological groups. We report patterns of body size based on length and mass measurements of 603 individual snakes of 27 species captured at La Selva, Costa Rica from March 1982 through August 1984. This assemblage of snakes is composed of at least four morphological groups each of which consists of species with similar habitat preferences. These groups are heavy-bodied terrestrial forms, light-bodied arboreal forms, long-tailed leaf-litter forms, and forms of unextreme relative mass and tail length. This tropical snake assemblage is more diverse in species richness and morphological diversity than a temperate assemblage.

Flooded savannas in south-west Venezuela consist of seasonal, marshy and flooded grassland communities, the distribution of which depends on soil level. Seasonal grasslands are dominated by tuft-forming, rhizomatous C4-grasses (exemplified by Paspalum chaffanjonii in this study), while flooded grasslands are dominated by stoloniferous grasses which develop rooted, floating culms during the rainy season (Leersia hexandra and Hymenachne amplexi-caulis). The interface between these two communities is a marshy grassland dominated by stoloniferous grass species which tolerate flooding to a depth of 10 to 25 cm (Panicum laxum and Leersia hexandra). All perennial grass species in flooded grasslands behave as typical C3 plants, while marshy grasslands are dominated by Panicum laxum, a species with reduced photo-respiration, low RuBP/PEP-carboxylase ratios, and low absolute RuBP-carboxylase activity compared to C3 grasses. It also has lower photosynthetic rates than the other grass species. Hymenachne amplexicaulis appears to be the least drought tolerant from the species selected, in accordance with its distribution in the wettest side of the flooding gradient studied. The other species showed marked reduction in relative water content and pronounced increase in leaf proline content in drought experiments lasting 6 to 30 days. Alcohol dehydrogenase increased markedly in response to anaerobiosis in the root environment in the tuft forming grasses, while the stoloniferous species with ascending culms were least affected by this treatment, probably as a result of better aeration inside the culms and also to the production of adventitious roots in upper nodes. Nitrate reductase activity increased as a result of anaerobiosis in the roots but not in the leaves, of all species except Leersia hexandra. No difference among the species was found in malate accumulation, or the activity of malic enzyme.

Necrophagous Diptera were sampled in a seasonal tropical mixed forest in Hong Kong from February 1985 to May 1986 using carrion-baited traps. Six families of flies were caught but only Calliphoridae, Muscidae and Sarcophagidae were considered. They made up a total of 14,669 flies of 21 species in the sample. Total species abundance followed Fisher's log-series distribution. The commonest five species were calliphorids, constituting 79.44% of the total combined catch. Temperature was apparently the main factor affecting seasonality of necrophagous Diptera in Hong Kong. Fluctuations in total abundance, species richness and H' followed seasonal changes in weekly mean air temperatures.

The growth response of Paspalum vaginatum S.W. and P. orbiculare Forst to different environmental conditions and the interactions between the two species were deter-mined experimentally in order to find out some of those factors which may be responsible for the species distribution. There was no difference in response (as measured by growth) of P. vaginatum to any of the soils tested but this was not so for P. orbiculare. P. vaginatum grew signifi-cantly better in waterlogged soil than in either moist or dry soil, while the soil moisture condi-tions had no significant effect on the growth of P. orbiculare. Salinity in the range of 0-50% that of sea water had no significant effect on the growth of P. vaginatum, however higher salinity significantly suppressed the growth. In P. orbiculare, salinity above 10% caused a signi-ficant decrease in growth to the extent that the species did not survive at 100% sea water concentration. In competition, P. vaginatum was the stronger competitor in red earth, the soil from its natural habitat, under saline conditions and in waterlogged conditions. The results are discussed in relation to the natural habitats of the species and used to explain, in part, their distribution.