Energy flow

Energy captured by photosynthesis flows through ecosystems through many pathways, whose variety is correlated with the species richness of the system. Species richness could influence the rates and quantities of energy flowing through the system in a number of ways.

Primary productivity and biomass accumulation Primary productivity of tropical forests is apparently affected by plant species richness only at levels far below those that characterize most mainland tropical forests (Vitousek and Hooper 1993; Wright 1996). Even highly fragmented and highly disturbed tropical forests have many more species than the minimum number needed to yield full primary productivity. Moreover, because nearly all tropical forest woody plants are Cj, loss of species is unlikely to affect the diversity of photosynthetic mechanisms except among herbaceous plants, which are minor components of undisturbed tropical moist forests. Therefore, to the best of our knowledge, biomass production in tropical forests under relatively constant conditions is insensitive to species richness.

However, the rate of biomass accumulation depends strongly on the nature and intensity of disturbance, and species differ in the speed with which they respond to disturbances. Therefore, although no data are available to lest the hypothesis, species richness may influence the rate at which biomass accumulates after disturbance (Denslow 1996), In addition, variability in rates of photosynthesis per unit area may be inversely related to species richness if, as seems likely, some species perform better in wet years and others perform better in dry years. That is, richness may result in buffering of production under conditions of environmental variability (Tilman and Downing 1994}. Variation in performance among tropical forest tree species is to be expected, but relevant data are yet to be gathered. Because tropical forest trees do not typically form annual growth rings, gathering data to measure the extent to which tree species richness buffers primary production under variable weather conditions may be difficult, but new methods to estimate the growth rates of tropical trees are being developed (Worbes and Junk 1989).

Forests that are naturally low in species richness grow on unusual tropical soils (Connell and Lowman 1989; Hart 1990). Examples include Mora excelsa stands adjacent to mangrove forests (Richards 1952) and Eperua forests of South America (Kiinge and Herrera 1983; Cuevas and Medina 1988; Herrera et al. 1988). However, these forests have not yet been studied sufficiently to determine their productive capacity, the degree to which that capacity is influenced by low species richness, or whether their interannual variation in total production is greater than in forests with greater species richness.

Within-planl carbon allocation and consumption Although secondary production has received less attention than primary production, its quantity, quality and temporal patterns are important components of ecosystem functioning. Secondary production, the summation of the growth of individuals and populations of all heterotrophic organisms, is completely dependent on primary production, nearly all of which in tropical forests results from photosynthesis by green plants. However, secondary production is not a simple function of primary production because plants have evolved a number of defensive structures and chemicals that deter consumption of their tissues by herbivores, parasites and pathogens. These defenses also lower the eflicicncy with which consumers are able to digest those tissues {McNaughton et al. 1989). Relationships between primary and secondary production are difficult to measure because many consumers are small or mobile, and because traces of their consumption of plant tissues may disappear rapidly.

The quantity of secondary production and its distribution among species are both potentially sensitive to species richness because different plants allocate their primary production in highly distinctive ways {Coley et ai. 1985). Plant species differ strikingly in the proportion of primary production they allocate to defenses, which defensive compounds they synthesize, the quantities and composition of their tissues that function to attract mutualists (Coley and Aide 1991; Davidson et al. 1991), and the physical and chemical composition of their wood. Because tropical climatic conditions allow heavy herbivore pressure throughout the year, tropical woody plants allocate relatively large amounts of energy to the production of chemical defenses (Levin 1978; Levin and York 1978; McKey 1979; Coley and Aide 1991) and resources that attract predators and parasites of herbivores (Simms 1992).

Plants influence animal biodiversity and productivity via two primary mechanisms. They provide the energy that supports animal populations, and they provide physical, temporal and biochemical heterogeneity. Wood, roots sap, extrafloral nectar, leaves, flowers, fruits and seeds are useful categories of tissue and fluid because they differ strikingly in their physical and chemical structure and because there appears to be relatively little overlap in the species of animals using those different tissues and fluids (Table 9.2). Thus, consumers of those tissues are usefully considered functional groups.

Consumers may also increase primary productivity by maintaining individual plants and plant populations in rapid growth phases by reducing the accumulation of living plant biomass, by reducing respiratory losses, and

TROPICAL FOREST ECOSYSTEM PROCESSES Tabic 9.2 Plant tissue and fluid types and their consumers

Tissue/fluid type

Wood Roots Sap

Flora! nectar Extrafloral nectar Leaves

Flowers Fruits

Seeds

Representative consumers

Termites, larvae of wood-boring beetles, girdling beetles Larvae of cicadas and beetles

Ants, aphids, membrascids, some hemiptcra and diptera. some marsupials

Lepidoptera, hymenoptera, birds, bats Ants

Larvae of many insects, otthopterans, adult beetles, leaf-eating monkeys, sloths, tree kangaroos, understory mammals, pathogenic fungi

Larvae of moths, flies, wasps, birds, monkeys

Sucking insects, frugivorous birds, fruit bats, frugivorous monkeys, other mammals

Ants, bruchid beetles, rodents, granivorous birds by recycling nutrients. These affects are important in alga! communities and grasslands, but in tropical forests, where the amount of standing biomass is high in relation to net primary production, the relatively small amount of new primary production typically consumed by herbivores probably has little effect on total net primary production (Huston and Gilbert 1996).

"Mobile link" species (Gilbert 1980), such as pollinators, seed dispersal agents and plant defense mutualists, have little impact on fluxes of energy and materials in ecological time, but they may be critical to the maintenance of the species richness of tropical forests. Many plants depend upon a small suite of frugivores for dispersing their seeds; loss of those species is expected to have major influences on the long-term population dynamics of many tree species (Howe and Smallwood 1982; Terborgh 1986a). Also, most frugivores that are effective seed dispersers are relatively large organisms, active throughout the year. Bccause few tropical plants have ripe fruit at all seasons, frugivores tend to be dietary generalists. Maintenance of the frugivore functional groups may depend upon the presence of a small number of tree species, e.g. Ficus spp., that ripen their fruits at times of year when most species are not fruiting (Terborgh 1986a). Because those frugivores may be the primary dispersers of the seeds of many other species of plants, secondary production and recruitment of plant species may depend strongly on the presence of a small subset of the total tree species in the forests.

Animal-animal interactions The animal species that eat the tissues of tropical forest plants support a complex array of commensals, predators, parasites and parasitoids. Many of these animals, such as blood parasites of vertebrates, predatory mites and parasitoid wasps, are tiny and inconspicuous, but they are thought to be the principal agents reducing herbivory in both polycuiture crop systems (Andow 1984) and natural vegetation (Gilbert 1977). Thus, these tiny organisms may act as rate regulators or "energy filters" (Hubbell 1973) by controlling herbivore populations, thereby reducing both the rate at which, and number of pathways by which, primary production becomes secondary production, for example, herbivorous insect larvae, which are major consumers of primary production, are attacked by both specialist and generalist invertebrate predators thai typically maintain their populations well below outbreak levels (Huston and Gilbert 1996). Social wasps and ants, which are important predators of foliage-eating insects, are, in turn, attacked by army ants. Similarly, the intensity of grazing and browsing of tropical understory plants by vertebrates such as agoutis, peccaries, deer, rhinoceroses, wild cattle and tapirs may be greatly reduced by predators such as tigers and jaguars. In forest fragments lacking these predators, browsing vertebrates can dramatically alter the structure and species composition of understory vegetation (Dirzo and Miranda 1987, 1991).

Detritus- detritivores Energy flow in ecosystems would quickly be reduced if the activities of detritivores were depressed or eliminated. The extremely high rates of decomposition of fine detritus on the floors of tropical moist forests, combined with the fact that in many, but not all (Jordan 1985: Brown and Lugo 1990), tropical forests most nutrients in the system are found in the bodies of plants, not in the soil, indicates that changes in rates of energy processing by soil detritivores have major consequences for energy flows in those forests.

Microorganisms and fungi dominate many detritivore communities, but on some tropical islands and on low-lying mainland areas adjacent to the ocean, crabs are remarkably abundant and have major effects on ecosystem processes (Cushman 1996). On Christmas Island, for example, the native red land crab, Gecarcoidea natalls, reaches densities as high as 2.6 and a biomass of more than It ha"1 (O'Dowd and Lake 1989). These crabs defoliate uncaged seedlings of tree and vine species within days, and they remove 39-86% of the annual leaf litter. The soil near burrow entrances has significantly higher concentrations of organic matter and mineral nutrients than soil elsewhere.

The taxonomy of tropical microbes is, unfortunately, extremely poorly known, and knowledge of the functional properties of microbial species is even poorer. Therefore, we do not know how many functional groups of tropica! forest microbes should be recognized, or how many species arc able to cleave particular chemical bonds in detritus. Consequently, we do not know how sensitive ecosystem processes may be to deletions of microbial species; nor do we know which functional processes are likely to have the least functional redundancy (Lodge et al. 1996).

Tropical trees differ markedly in tissue chemistry (Rodin and Basilevich 1967; Golley [983a,b), suggesting that they differ both in what they remove from the soil and in what they deposit on the soil surface. Soils under the legume Pentaclethra macroioba at La Selva, Costa Rica, have lower pH values than soils from areas away from individuals of this species, presumably because symbiotic microorganisms associated with P. macroioba trees fix nitrogen, which is then nitrified (Sollrns, unpublished data). Soils under female Tropins involĂșcrala individuals have higher phosphorus concentrations than soils under males (Cox 1981). The meager evidence so far available suggests that trees of different species may generate significant differences in the soils in the area affected by their roots and litterfall, but whether these differences are important for regeneration, growth and species richness in tropical forests remains to be determined (Parker 1994). Several studies have failed to detect significant differences in nutrient levels of soils between evergreen forests dominated by single species and mixed forests in India (Kadambi 1942), Zaire (Hart and Murphy 1989; Hart 1990) and Malaysia (Whitmore 1975).

9.6,2 Materials processing

The movement of materials in ecosystems is often strongly tied to movement of energy, but the two processes are often unconnected. Because uptake of mineral nutrients and their movement though plants is driven primarily by evaporation of water from surfaces of leaves, it is useful to consider materials processing separately from production for purposes of our analyses of the interfaces listed in Table 9.1.

Atmosphere-organism Plants, photosynthetic microorganisms and nitrogen-fixers are actively and massively involved with direct exchanges of materials with the atmosphere, but measurements of air quality above and within tropical rain forests are rare, and almost nothing is understood about atmosphere canopy exchange of aerosols in tropical forests. Plants intercept airborne particles, either as dry or wet deposition, and release in to the atmosphere carbon dioxide (especially at night), methane, a variety of volatile organic compounds and large quantities of water. Animals and microbes also release large quantities of carbon dioxide into the atmosphere. Exchanges of these materials by both groups appear to be directly proportional to the total biomass of organisms, irrespective of its distribution among species, except that exchange of materials is lower in systems dominated by woody plants. However, epiphytes depend primarily upon direct nutrient exchange with the atmosphere for their nutrients and water. The species richness of epiphytes is high in most tropical forests (Gentry and Benzing 1990; Gentry and Dodson 1987), but because they tend to grow on different parts of the trees, nutrient exchange rates may depend upon the richness of species in addition to their total biomass (Silver et al. 1996). The best evidence for the role of epiphytes in nutrient cycling comes from tropical cloud forests, where nutrient availability is often low due to low soil concentrations and waterlogging. In those forests nearly half of the foliage nutrient pool may be stored in epiphyte biomass (Nadkarni 1984). Nitrogen-fixing epiphytes, especially cyanobacteria, fix substantial amounts of nitrogen relative to other sources in tropical forests (Lodge et al. 1996). Also, because the volatile organic compounds produced by plants are highly species-specific, the composition of airborne volatiies may carry a signature of the spccies richness of the forest canopy. However, the significance, if any, of such a correlation for ecosystem processes is unknown.

Unlike most other nutrients, the major sources of nitrogen to ecosystems are precipitation and biological nitrogen-fixation by free-living bacteria and cyanobacteria, by bacteria having mutualistic associations with plants, by fungi, and by gut-dwelling symbionts of termites (Prestwich et al. 1980; Prestwich and Bentley 1981). In species-poor systems, such as those growing on young tropical lava flows, invasion of a single tree species and lichenized fungi with nitrogen-fixing bacteria may dramatically increase nitrogen input to the system, productivity and ecosystem development (Vitousek et al. 1987; Vitousek and Walker 1989). However, whether the quantities of nitrogen entering and cycling with tree-species-rich tropical forests are influenced by the number of species of free-living or symbiotic nitrogen-fixing microorganisms is unknown.

Bio tic interface In tropical forests, large quantities of nutrients are stored in live biomass (Jordan 1985). As we have already pointed out, the synthesis of defensive chemicals may result in conservation of nutrients by reducing losses to herbivores (McKey et al. 1978; Hobbie 1992), especially in forests where leaves arc long-lived (Jordan 1991). Within-plant nutrient transfer, measured as the difference between nutrients stored in live tissues and nutrients deposited in litterfall. may be significant for the conservation of some nutrients, particularly phosphorus (Vitousek 1984). Plant species differ strikingly in the defensive compounds they synthesize (Colcy and Aide 1991; Davidson et al. 1991) and the chemical composition of their wood. These differences, combined with differences in the degree to which plants recapture nutrients prior to discarding their leaves, could result in nutrient dynamics being influenced by tree species richness. Unfortunately, however, few studies have compared the composition of live and senesced tissues in tropical forests. (For a more extensive discussion of this interface, see Silver et al. 1996.)

Plant-soil Nutrients are taken up from the soil and forest floor through fine roots and eventually returned again through decomposition of litterfall and below-ground litter inputs (Went and Stark 1968; Start 1971; Stark and Jordan 1978; Cuevas and Medina 1988). The decomposition of litter is carried out primarily by microbes whose diversity and functioning are still poorly known. The role of microorganisms in the nitrogen cycle appears to be especially important for productivity and biomass accumulation in tropica! forests because different plant species require nitrogen in different inorganic forms (NH4, NO?, NOj), and nitrogen often limits rates of photosynthesis in tropical forests. An important functional group of organisms in tropica] forests are endomyeorrhizal fungi that have mutualistic relationships with at least 80% of tropical plants (Janos 1983). The growth of tropical moist forest trees may be especially sensitive to losses in microbial diversity because, unlike temperate forests, which have relatively few trees species but many fungal species, tropical forests have many tree species but relatively few species of endomyeorrhizal fungi (Malloch et al. 1980). Also, largc-scale, long-term conversion of forests to grasslands or cropland results in major changes in soil nutrient pools and the soil biota (Olson 1963; Hamilton and King 1983; Macedo et al. 1993; Henrot and Robertson 1994), which affects nutrient cycling on those agro-ecosystems and the potential for regenerating forests on those lands.

The consequences of forest disturbances for ecosystem productivity and nutrient cycling depend on the scale and frequency of those disturbances. In the Atlantic lowlands of Costa Rica, intermediate-scale experimental clear-cutting of forests on residual soils resulted in rapid, short-term increases in nutrient concentrations in soil solutions, increased percolation of water through the soil, and increased losses of soil nutrients (Parker 1994). With no additional disturbance, the large pulse of nutrients lost in percolating water was transient. Concentrations returned to predisturbance levels in less than 2 years. Small-scale disturbances, such as natural or artificial treefall gaps, do not result in increased soil nutrient availability (Vitousek and Denslow 1986) or solution losses (Parker 1994) compared with intact, forest. On the other hand, large-scalc, long-term conversion of forests to grasslands or cropland results in major changes in nutrient pools and the soil biota (Olson et al. 1968; Hamilton and King 1963; Mazdeva et al. 1992; Macedo and Anderson 1993; Henrot and Robertson 1994). In combination with extraction of nutrients in harvested biomass, these changes cause the productivity of transformed tropical agroecosystcms to decrease rapidly.

Atmosphere-soil Soil microorganisms, like the macroorganisms above ground, release to the atmosphere large quantities of carbon dioxide and methane (under anaerobic conditions), and the soil surface receives the atmospheric deposition that is not intercepted as well as throughfall and stem flow. Currently, tropical forests are a net source of atmospheric C02, but this is due to the reduction of the total area of forests and to extensive burning, not to loss of species per se (Hall and Uhlig 1991; Houghton 1991). Tropical forests release large quantities of methane to the atmosphere, much of it due to the activities of methanogenic bacteria in tropical wetlands (Bartlett and Harriss 1993). Gut symbionts of termites are also a significant source of methane (Wassmann et al. 1992; Martius et al. 1993). How emission rates of methane and other chemicals vary with biodiversity is unknown, and the lack of information on the identities and functional attributes of most soil microorganisms makes it impossible to identify the number of significant functional groups and the number of species found in most of those groups.

Soil-water table Most nutrients leave forested ecosystems through the soil. Because most tropical forests grow on deep, highly weathered soils with low nutrient-holding capacities (Sanchez 1976), the large volumes of water that move through the soil generate high potential losses of nutrients through leaching (Radulovjch and Sollins 1991). Rates of movement of water on and within the soil arc rcduccd by woody debris and fine litter, and by the extensive mats of fine roots that characterize tropical forests, especially those growing on nutrient-poor soils (Silver et al. 1996). Plant roots also recapture nutrients in the soil, and deep-rooted species may pump water from deep in the soil to the surfacc, where it may be transpired.

Empirical studies Only two studies that are useful for providing insights into the relationship between plant species richness and nutrient cycling in tropical forests have been carried out (see Silver et al. 1996, for more details). In Puerto Rico, Lugo (1992) compared nutrient-cycling processes between plantations with low species richness but of different ages with similar-aged secondary forests with higher species richness. The plantations had higher above-ground biomass and N and P pools than similar-aged secondary forests, but the secondary forests had greater root masses, deeper roots and higher root nutrient pools than the plantations. Plantations trees retranslocated more nutrients than secondary forest trees, so that the litterfall in secondary forests had greater nutrient concentrations, leading to faster rates of litter decomposition and nutrient mineralization. Both the greater root mass and depth of rooting, as well as the mix of litter, were functions of species richness, but the long-term implications of these results for nutrient cycling are unclear.

Ewell et al. (1991) studied the influence of species richness over a period of 5 years on experimentally manipulated early forest successional plots in Costa Rica. Species richness at the end of the study ranged from zero (bare ground plots) to about 125 species in an enriched succession. Soil nutrient pools were positively correlated with species richness, which was attributed to more effective nutrient retention and maintenance of soil properties favorable for plant production. Whether such effects would persist through later years of succession is unknown.

9.6.3 Functional properties over longer temporal scales

Thus far, we have concentrated on processes occurring at a specific location and over short time-frames, i.e. on scales of a few days to a few years. However, the functioning of tropical forest ecosystems depends on the formation and maintenance of the structure of the forest, which is, in turn, the result of photosynthesis and biomass accumulation and biogeochemical cycling over many decades or centuries. The ways in which the forests respond to perturbations such as fire, drought, strong winds, unusually heavy rains, invasions of exotic species and losses of species typically found in the system are also important. These responses may be influenced by the richness of plant species in the forests that are not expressed under constant conditions.

Provision and maintenance of structure The many plant species that live in tropical moist forests can be grouped into a small number of life forms. Most species of canopy trees are similar enough in their growth forms that loss of particular species would not change the vegetation structure very much (Ewell and Bigelow 1996). However, certain life forms, particularly palms, lianas and epiphytic bromeliads, are both structurally highly distinctive and represented by a few sympatric species. Thus, these plants may function as "structural keystone species" whose removal would influence the degree to which the forests can be changed by perturbations, and the rate of recovery of the forests after perturbations (Denslow 1996).

Resistance to invasion Many exotic species have been introduced into tropical regions, as they have into temperate regions (Drake 1989). However, existing data suggest that exotic species have seldom invaded undisturbed mainland tropical forests (Ramakrishnan 1991). Weedy plants and animals are confined primarily to highly disturbed habitats. Why tropical forests are resistant to invasions by alien species is not understood (Rejmanek 1996).

9.6.4 Functional properties over larger spatial scales

Although we have not devoted much attention to processes on regional scales, the approach outlined in this paper may help us to understand how species richness influences large-scale processes. To think creatively about linkages across space, we need to have categories of materials that move across landscapes, the agents that drive their movements, the distances over which they move, and how those transfers arc influenced by species richness (Table 9.3).

As indicated in Table 9.3, many linkages that connect local tropical forest ecosystems across space are probably strongly influenced by the total amount of vegetative cover, but they arc probably little influenced by species richness per se. The variety of species present in the canopy may influence the composition of the airborne volatile organic compounds, but the significance of that relationship, if any, has not been determined. Moving animals spread diseases and plant propagules, and concentrate nutrients around their

Table 9.3 Landscape-scale linkages in tropical forests

Linkage

Process

Distance moved

Distance Effect

Influence of biodiversity

Atmosphere

Release of CO;,

Local-»regional

Atmospheric

Probably none

organism

CH4, volatiles.

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