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.

Effects of mammalian herbivory and seasonal drought were studied for Virola surinamensis (Myristicaceae) juveniles on Barro Colorado Island, Panama. Seedlings were planted at three months of age and the juveniles were monitored for two years; Treatments included: intact plants protected from mammals by cages, defoliated plants similarly protected, and unprotected plants, each planted in treefall gaps, on gap edges, and in the shaded understorey.

Juveniles planted in treefall gaps survived seasonal drought far better than those planted on gap edges or in shaded understorey. Two years after establishment, juveniles protected from mammalian herbivory showed a 78% survival in gaps (mean 6.8% skylight), 50% survival on gap edges (mean 3.0% skylight), and 33% survival in shaded understorey (1.4% skylight). This advantage was due to accelerated growth in gaps. Juveniles in gaps increased 616% in height, 1075% in leaf number, and 1800% in total leaf area. Comparable numbers in edges were 247%, 378% and 690%; in understorey 33%, 222% and 289%. Accelerated growth in gaps permitted yearlings to survive drought that killed suppressed yearlings in understorey. Mean light differentials as small as 0.6% and 0.3% skylight significantly influenced survival on edges and in shaded understorey, respectively.

Mammalian herbivory killed juveniles directly, and defoliation by mammals strongly accentuated drought mortality by suppressing root development. Natural defoliation was not attributable to gap conditions. Demographic projections from experimental data suggest that mammalian herbivory kills at least 48% of the juveniles of this species over two years, and contributes to the death of 32% more that actually die of drought stress. These projections suggest that 14% of the juveniles of this species die of drought mortality, independent of herbivory, during the first two years. Herbivory most strongly affects plants < 0.5 m in height, and is a continuing source of mortality among suppressed juveniles in the understorey. Steep slopes and large seed size each enhanced juvenile growth and survival in the intermediate conditions of gap edges, but not under the extreme conditions of gaps or shaded understorey.

The context of establishment determines the ‘shade tolerance’ of this conspicuous canopy tree. Without serious mammalian herbivory or extreme dry seasons, V. surinamensis can easily recruit as a shade tolerant plant in the understorey. Under present conditions on Barro Colorado Island, it cannot. Persistence involves both the chances of arrival in different microhabitats, and survival therein. Projections that include both the forest area represented by gaps, gap edges, and understorey and the experimental results from this study indicate that juvenile V. surinamensis can survive for two years in gaps, edges, and understorey, but that the higher proportions of vigorous individuals survive in edges, gaps and understorey, respectively.

The abundance and distribution of reptiles and amphibians inhabiting tropical monsoonal wetlands in Kakadu National Park, northern Australia, were monitored between late 1979 and 1982. Seasonal trends in species richness, and the influence of various environmental attributes (including those caused by feral ungulates) on species distribution patterns were also examined.

These wetlands support a high proportion of the amphibian species known from the Park, but are of less importance to reptiles. Within the wetlands, the structurally more complex forested ‘margins’ support many more species than the treeless floodplains. Amphibian species distributions seem to be influenced primarily by elevation, through its effects on soil moisture and flooding levels, while reptile distribution patterns reflect a more complex set of environmental factors, with structural attributes (e.g. vegetation height structure and cover, refuge abundance, leaf litter cover and depth, extent of flooding) being of prime importance.

Feral buffalo and pigs, through their grazing, trampling and wallowing, may considerably influence such structural attributes and change the duration and extent of water lie, and thus indirectly affect species distribution patterns. Current buffalo culling programmes provide an opportunity to monitor vegetation and faunal changes, and quantify feral animal impact post facto.

One component of the high organismal diversity of tropical lowland rain forests is the existence of series of closely related, sympatric species. For example, the six distinct tree species of the monophyletic Polyalthia hypoleuca complex regularly grow sympatrically in various combinations throughout the rain forests of Malesia. Theoretical and empirical evidence has been presented indicating that the members of such series either (1) are well differentiated with respect to at least one major niche component; or (2) may lack differentiated niches. In this study, certain sympatric members of the complex are shown to have clearly diverged in autecological characteristics that affect their distribution including: (1) P. glauca, P. hypoleuca, and P. sumatrana differ in growth characteristics, height at maturity, seedling germination requirements, and are found on soils with differing degrees of hydration; (2) P. discolor grows to a different height and on different substrate types than does P. multinervis; and (3) P. glauca and P. hypoleuca seedlings differ in transpiration characteristics and response to flooding and drought.

Nitrogenase activity in the surface soil and litter layers from a tropical lowland rain forest and an adjacent fernland in Sri Lanka was measured by the acetylene reduction method. The total amount of acetylene reduced, a measure of biological nitrogen fixation, was greater in the fernland (38.4 µmol m−2 h−1) than in the forest (15.3–29.2 µmol m−2 h−1). In both ecosystems the nitrogenase activity was greater (59–85%) in the soil than in the litter layer. The rates of acetylene reduction in soil and litter showed an inverse relationship with the decay constants of mixed litter and a positive relationship was found between the rate of acetylene reduction and the C:extractable nitrogen ratio of the soil. The relatively greater rate of nitro genase activity coupled with the slower rate of litter decomposition in the fernland indicates that accumulation of nitrogen may result in the soil and litter layers of the deforested areas during their aggrading phases.

Fruit production by an understorey tree, Miconia centrodesma, was monitored in treefall gaps and under intact canopy in a Costa Rican lowland rain forest. Trees in gaps displayed much less seasonality in fruit production than trees of intact forest sites. For example, ripe fruits were common on gap trees for a six month period (January-June) when few or no trees under intact canopy were in fruit. The frequent and aseasonal fruiting of gap trees demonstrates that they are not constrained by phenological cueing mechanisms; the influence of such cues is overridden by habitat. Trees in gaps also produced larger crops, had more extended fruiting episodes, and fruited more frequently than shaded conspecifics. This level of intraspecific variation in fruiting behaviour suggests that treefall gaps play an important role in determining the reproductive success of M. centrodesma. A substantial proportion of an individual's lifetime seed output may be produced during the brief period it occupies a gap. In addition, the large and continuous supply of fruits produced in gaps by M. centrodesma and other understorey plants, may mean that gaps function as ‘keystone habitats’ by providing resident frugivores with fruit during periods of general fruit scarcity.

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 permanence and stability of 20 territories of the dikdik Rhynchotragus kirki, were studied at Arusha National Park Tanzania. Response criteria included the structure, size and vegetation attributes of territories, density of dikdik, and spatial distances between neighbouring territories. Fourteen of the territories studied were spatially fixed and stable while six were unstable. Two of the unstable territories were completely abandoned. Mean territory size was 0.11 and 0.13 ha during the rainy and dry seasons, respectively. 14.2% of circular territories and 42.9% of elliptical and kidney-shaped territories were stable. The figure of 55 dikdik km−2 sighted during morning censuses was lower than that for evening censuses by 13.5%. Spatial distance between territories ranged from 4.5 m to 100 m, an indication that population density might be a function of the size of suitable habitat. Although shrub canopy cover within stable territories decreased by 44.6% between 1982 and 1987, and some territories were realigned with changes in rainfall pattern, pair-bond was maintained by dikdik in 70% of the territories studied.

We studied the seed dispersal ecology of the Neotropical pioneer tree Cecropia peltata L. (Moraceae) in tropical dry forest by documenting its (1) fruiting phenology, (2) rates of visitation by vertebrate frugivores, and (3) rate of recruitment of juveniles in different habitats at Santa Rosa National Park in northwestern Costa Rica.

At Santa Rosa, C. peltata is a common but patchily-distributed plant. Fruiting in females is seasonal, and fruit availability peaks in June through August (the early wet season). Females bear a few ripe fruit per day over a 4–5 month, fruiting period. At least 28 vertebrate species (15 diurnal and 13 nocturnal species) eat Cecropia fruit; equal numbers of ripe fruit are removed during the night and day. Owing to their more gentle treatment of seeds in the gut, birds and bats probably are more effective dispersal agents than monkeys. The recruitment rate of juveniles into already established populations was about 0.8 ha−1 y−1 compared with a rate of 176 ha−1 y−1 during the colonization of a cleared roadside.

Termites, particularly the mound building, fungus growing Macrotermitinae, reach densities of up to 400 termites m−2 in soils of dry tropical Africa. The influence of Macrotermi tinae in increasing certain soil nutrients in mounds compared to adjacent soils has been documented, but the links between litter harvesting by termites, soil fertility, and global C cycling have not been explored. This study reviews the evidence from soil science, ecology and atmos pheric chemistry and generates hypotheses to explain the role of termites in dry tropical eco systems. It is suggested that termite activity exhaustively partitions litterfall among adjacent com peting colonies, where it is so thoroughly decomposed that little or no organic C is incorporated into the soils. Associated N, P, and cations build up in the mounds, but C apparently is emitted as CO2 and CH4 from the mounds. While not adequate to calculate nutrient fluxes through termites, the data available support the argument that termites contribute significantly to atmospheric fluxes of CO2 and CH4. Moreover, they suggest a coupling of regional soil forming processes and the global C budget.

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.

Seven species of rodent, Paraxerus ochraceus, Tatera robusta, Saccostomus meamsi, Praomys (Mastomys) natalensis, Acomys dimidiatus, Thamnomys (Grammomys) dolichurus and Hystrix cristata were recorded in the riverine forests of the Jubba Valley, southern Somalia, in the dry season of 1986. Acomys dimidiatus was the most common of the five species trapped. Overall trap success was low, but increased over successive days. Saccostomus mearnsi, Praomys natalensis, Acomys dimidiatus and Thamnomys dolichurus all showed signs of breeding, and pregnant females of A. dimidiatus and T. dolichurus were trapped until the end of the dry season. Analysis of stomach contents revealed that all five trapped species relied heavily on seeds and, where comparisons could be made, the degree of overlap in diets was high. The findings are discussed in relation to the environmental conditions in the forest and neighbouring habitats.

Bat communities in the understorey of primary lowland dipterocarp forest in peninsular Malaysia and Sabah were sampled with mist nets and harp traps to estimate their trophic structure. Overall, 4 of 26 species (15%) at one site, and 6 of 33 species (18%) at the other site were frugivores, while the remainder were insectivores. In terms of individuals, 7–12% of all captures were frugivores. However, the true proportion of frugivores at both sites was probably closer to 1%, because the sampling procedures were strongly biased towards frugivores. In contrast, previously published studies indicate that frugivores comprise 34–48% of species, and up to 80% of individuals in bat communities in the understorey of Neotropical forest. These results support the hypothesis, developed from studies of bird communities, that the under-storey of Malaysian dipterocarp forest supports relatively few obligate frugivores, at least in non-masting years. However, further data are required on bats in the canopy, and on the foraging ranges of bats to compare the absolute densities of bats in Malaysian and Neotropical forests.