Tropical rainforests and the need for cross-continental comparisons

Ecologists have tended to overemphasize the common features of tropical rainforests on different continents. In reality, the five major tropical rainforest regions (tropical America, Africa, Southeast Asia, Madagascar and New Guinea) are distinct ecological and biogeographical entities. Although it is easy to find examples of at least superficial convergence between unrelated organisms in these different regions, there are many other cases where convergence is incomplete or there are no obvious ecological equivalents. Pantropical comparisons with standardized methods are needed for the insights that they can provide into rainforest ecology and the help that they can offer in identifying conservation strategies that are appropriate to regional conditions. Here, we suggest ways in which the practical difficulties of such pantropical comparisons can be minimized.     

The archaeology of Australia's tropical rainforests

Archaeological research in the Australia's northeast Queensland rainforest and margins has revealed a human antiquity of at least 8000 cal year BP within the rainforest and at least 30,000 years on the western edge. Rainforest occupation before 2000 cal year BP was at generally very low levels, after which time settlement of this environment became intensive and probably permanent. Exploitation of toxic varieties of nuts began about 2500 cal year BP, peaking after 1500 cal year BP. This economic development appears crucial to successful human adaptation to rainforests in the area and was pivotal in facilitating the long-term permanent human settlement of the wet tropics. The role of fire, El Niño Southern Oscillation (ENSO) activity and shifting vegetation regimes were important catalysts in providing opportunities for permanent Australian rainforest Aboriginal occupation. The results have implications for global understandings of rainforest occupation by modern people. It demonstrates the wide temporal and spatial variability of human rainforest colonization processes worldwide.     

Tropical grain legumes as important human foods

Grain legumes continue to occupy an important place in human nutrition as sources of protein, vitamins and minerals. From a nutritional point of view chickpea, pigeonpea, mung bean, urd bean, cowpea, lentil, soybean and peanut are the important grain legumes for the millions of people in semi-arid and tropical regions of many Asian and African countries. These legumes are used in various food forms after suitable processing depending on the regions of their production and consumption. Such aspects as production and consumption, processing and food uses, chemical composition, and effects of processing on the nutritive value of these legumes are the important topics of this paper. To enhance their utilization, new potential and diversified food uses have been highlighted. Future research needs and priority areas are listed to improve their utilization and nutritional quality.     

Bushmeat hunting changes regeneration of African rainforests

To assess ecological consequences of bushmeat hunting in African lowland rainforests, we compared paired sites, with high and low hunting pressure, in three areas of southeastern Nigeria. In hunted sites, populations of important seed dispersers—both small and large primates (including the Cross River gorilla, Gorilla gorilla diehli)—were drastically reduced. Large rodents were more abundant in hunted sites, even though they are hunted. Hunted and protected sites had similar mature tree communities dominated by primate-dispersed species. In protected sites, seedling communities were similar in composition to the mature trees, but in hunted sites species with other dispersal modes dominated among seedlings. Seedlings emerging 1 year after clearing of all vegetation in experimental plots showed a similar pattern to the standing seedlings. This study thus verifies the transforming effects of bushmeat hunting on plant communities of tropical forests and is one of the first studies to do so for the African continent.     

Impacts of warming on tropical lowland rainforests

Before the end of this century, tropical rainforests will be subject to climatic conditions that have not existed anywhere on Earth for millions of years. These forests are the most species-rich ecosystems in the world and play a crucial role in regulating carbon and water feedbacks in the global climate system; therefore, it is important that the probable impacts of anthropogenic climate change are understood. However, the recent literature shows a striking range of views on the vulnerability of tropical rainforests, from least to most concern among major ecosystems. This review, which focuses on the impact of rising temperatures, examines the evidence for and against high vulnerability, identifies key research needs for resolving current differences and suggests ways of mitigating or adapting to potential impacts.     

African rainforests: past,present and future

The rainforests are the great green heart of Africa, and present a unique combination of ecological, climatic and human interactions. In this synthesis paper, we review the past and present state processes of change in African rainforests, and explore the challenges and opportunities for maintaining a viable future for these biomes. We draw in particular on the insights and new analyses emerging from the Theme Issue on ‘African rainforests: past, present and future’ of Philosophical Transactions of the Royal Society B. A combination of features characterize the African rainforest biome, including a history of climate variation; forest expansion and retreat; a long history of human interaction with the biome; a relatively low plant species diversity but large tree biomass; a historically exceptionally high animal biomass that is now being severely hunted down; the dominance of selective logging; small-scale farming and bushmeat hunting as the major forms of direct human pressure; and, in Central Africa, the particular context of mineral- and oil-driven economies that have resulted in unusually low rates of deforestation and agricultural activity. We conclude by discussing how this combination of factors influences the prospects for African forests in the twenty-first century.     

Ecological sensitivity of Australian rainforests to selective logging

To assess the ecological consequences of selective logging in the major rainforest areas in Australia, past studies are reviewed with respect to the following: recovery of stand structure; regeneration capacity; individual tree growth and species composition of stands; hydrologie impact; floristics; wildlife; soil nutrient levels; fire susceptibility; and incursion of weeds and diseases. Although rainforest logging has become a sensitive issue in the community and its effects are of concern to ecologists, relatively few scientific studies of selective logging have been published. Thus, the full nature and extent of ecological changes to Australian rainforests due to this form of disturbance cannot yet be determined. However, some cautious interpretation of the studies that have been completed is possible. Following a single selective logging, the changes indicated by individual studies often appeared to be relatively minor. It is suggested that many of these effects are not extensive or irreversible and might not persist beyond structural recovery of the rainforest. However, two changes were identified as likely to persist beyond structural recovery. These are: a post-logging difference in the proportional representation of major overstorey tree species and a reduction in the numbers of large-diameter trees. Further, results indicate that more extensive and longer-lasting changes may result from multiple selective loggings, especially if the time between successive loggings is short. Even with light logging intensities, a conservative interval of at least 60 years between selective loggings, to allow canopy and below-canopy conditions to be restored, is indicated. Apart from disturbance frequency, the studies reviewed raise questions as to whether long-term natural disturbance effects per se are distinguishable from long-term selective logging effects. A need for further work is highlighted. Overall, there appears to be a distinction between the recovery capability of the more northerly mainland rainforests and those of Tasmania. Slower growth in the more southerly rainforests, compounded by a geographical susceptibility to summer drought, increases the possibility of fire damage following selective logging.     

Attribution of changes in precipitation patterns in African rainforests

Tropical rainforests in Africa are one of the most under-researched regions in the world, but research in the Amazonian rainforest suggests potential vulnerability to climate change. Using the large ensemble of Atmosphere-only general circulation model (AGCM) simulations within the weather@home project, statistics of precipitation in the dry season of the Congo Basin rainforest are analysed. By validating the model simulation against observations, we could identify a good model performance for the June, July, August (JJA) dry season, but this result does need to be taken with caution as observed data are of poor quality. Additional validation methods have been used to investigate the applicability of probabilistic event attribution analysis from large model ensembles to a tropical region, in this case the Congo Basin. These methods corroborate the confidence in the model, leading us to believe the attribution result to be robust. That is, that there are no significant changes in the risk of low precipitation extremes during this dry season (JJA) precipitation in the Congo Basin. Results for the December, January, February dry season are less clear. The study highlights that attribution analysis has the potential to provide valuable scientific evidence of recent or anticipated climatological changes, especially in regions with sparse observational data and unclear projections of future changes. However, the strong influence of sea surface temperature teleconnection patterns on tropical precipitation provides more challenges in the set up of attribution studies than midlatitude rainfall.     

South American palaeobotany and the origins of neotropical rainforests

Extant neotropical rainforest biomes are characterized by a high diversity and abundance of angiosperm trees and vines, high proportions of entire-margined leaves, high proportions of large leaves (larger than 4500 mm2), high abundance of drip tips and a suite of characteristic dominant families: Sapotaceae, Lauraceae, Leguminosae (Fabaceae), Melastomataceae and Palmae (Arecaceae). Our aim is to define parameters of extant rainforests that will allow their recognition in the fossil record of South America and to evaluate all known South American plant fossil assemblages for first evidence and continued presence of those parameters. We ask when did these critical rainforest characters arise? When did vegetative parameters reach the level of abundance that we see in neotropical forests? Also, when do specific lineages become common in neotropical forests? Our review indicates that evidence of neotropical rainforest is exceedingly rare and equivocal before the Palaeocene. Even in the Palaeocene, the only evidence for tropical rainforest in South America is the appearance of moderately high pollen diversity. By contrast, North American sites provide evidence that rainforest leaf physiognomy was established early in the Palaeocene. By the Eocene in South America, several lines of evidence suggest that neotropical rainforests were diverse, physiognomically recognizable as rainforest and taxonomically allied to modern neotropical rainforests. A mismatch of evidence regarding the age of origin between sites of palaeobotanical high diversity and sites of predicted tropical climates should be reconciled with intensified collecting efforts in South America. We identify several lines of promising research that will help to coalesce previously disparate approaches to the origin, longevity and maintenance of high diversity floras of South America.     

The ecology of water-filled treeholes in Australian rainforests: Food web reassembly as a measure of community recovery after disturbance

Ten water-filled tree holes in a subtropical rainforest were emptied out thereby imposing a major perturbation on the community of organisms which occupied these habitat units. During the process of food web reassembly following this disturbance, the average number of predator species in the web increased as did the average number of prey species. Predator-prey ratios increased in magnitude with the number of days after the disturbance. Furthermore, the characteristic mean predator-prey ratio of the original community was gradually restored as the food web reassembled. The number of prey species was found to be a better predictor of the number of predator species during community recovery than vice versa. The increasing number of trophic links in the food web over the period of reassembly, due mainly to the increase in the number of predacious species in the web, provides a good overall measure of community recovery. The paper discusses the use of relationships among the food web statistics described within a model of food web reassembly. Such a model can be used to assess the rate of recovery of a community after a disturbance of the food web at the regional level of resolution. Further, the question of whether or not the rate of community recovery can be used to estimate ecosystem recovery with the aid of food web statistics is examined.