John J. Ewel

EFFECTS OF BIODIVERSITY ON ECOSYSTEM FUNCTIONING: A CONSENSUS OF CURRENT KNOWLEDGE

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earths biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earths ecosystems and the diverse biota they contain.

Don't judge species on their origins

Conservationists should assess organisms on environmental impact rather than on whether they are natives, argue Mark Davis and 18 other ecologists.

Deliberate Introductions of Species: Research Needs Benefits can be reaped, but risks are high

The silent invasion of Hawaii by insects, disease organisms, snakes, weeds and other pests is the single greatest threat to Hawaii’s economy and natural environment.... Even one new pest-like the brown tree snake--could forever change the character of our islands. (Coordinating Group on Alien Pest Species 1996, P. 1). Reforestation in the tropics is so vastly behind deforestation that we cannot wait to fully appraise all the potential negative elements of domestication. Weediness is of consequence perhaps in Honolulu, but not in Addis or Delhi. (James Brewbaker, quoted by Hughes 1994, p. 244 ).

Tropical Soil Fertility Changes Under Monocultures and Successional Communities of Different Structure

For 5 yr we monitored the fertility of a volcanic-ash derived Inceptisol at a site in the humid tropics of Costa Rica. After forest felling and burning, we established four treatments in a randomized block design with six blocks: a sequence of monocultures (two crops of maize [Zea mays] followed by cassava [Manihot esculenta], then the tree species Cordia alliodora), successional vegetation, a mimic of successional vegetation that was physiognomically similar to the model but shared no species with it, and a species-enriched version of successional vegetation. In addition, one plot was maintained free of vegetation. Species-rich successional vegetation was effective at maintaining soil fertility, although we observed general trends of soil-nutrient decline beneath all treatments, presumably because of plant uptake. It proved possible to imitate the fertility-maintaining characteristics of successional vegetation by creating an equally species-rich community of different floristic composition, but the maintenance of fertility was not enhanced by further species enrichment. Successive peaks of nitrate-nitrogen in soil solution, extractable phosphorus, and extractable potassium occurred during the 1st yr, perhaps driven by an early increment of organic matter from postburn debris and roots. Organic matter, total nitrogen, and extractable sulfur were remarkably stable during the 5-yr period. Depletions of cations, decreases in effective cation exchange capacity (CECe ), and increases in acid saturation were related to treatment in the following order: bare soil > monocultures > the three diverse, successional communities. In the bare-soil plot, fertility decreased dramatically: there was a net loss of exchangeable cations and inorganic nitrogen, the phosphorus-fixation capacity increased, and acid saturation reached a potentially toxic 86%. At the start of the study, three of the blocks had soil with lower pH, lower CECe , and higher acid saturation. During the study this less fertile soil lost proportionally more cations and increased more in acid saturation and phosphorus-fixation capacity. The less fertile soil under monocultures proved exceptionally vulnerable to loss of fertility; after 5 yr under monocultures, for example, acid saturation reached 38% in the more fertile soil and 75% in the less fertile soil. In the species-rich communities, however, changes in soil fertility were far less marked.

A place for alien species in ecosystem restoration

Blanket condemnation of alien species in restoration efforts is counterproductive. Where their presence does not unduly threaten surrounding ecosystems, alien species can be tolerated or even used to good advantage, if they provide essential ecological or socioeconomic services. By speeding restoration or making it more effective, non-native species can provide economic and ecological payoffs. Risk is always an issue when alien species are involved, but greater risk taking is warranted where environmental conditions have been severely modified through human activity than where reassembly of a biological community is the sole goal of restoration.

Natural systems as models for the design of sustainable systems of land use

Natural ecosystems, whose components are the results of natural selection, are sustainable; most are productive, responsive to pests, and retentive of nutrients. Thus, they are appropriate models on which to base the design of new systems of land use. Abiotic and biotic stressors are related non-linearly; the nadir of total stress being mid-way along a gradient of environmental harshness. Superimposing the stress functions on Holdridges life zone chart yields four broad categories of environments for agriculture: climates where annual rainfall is similar to potential evapotranspiration, plus three other categories that are either too cold, too arid, or too wet. Extremely cold lands have no potential for agriculture. Lands that are arid or infertile can be used successfully, although the cost of compensating for environmental limitations increases exponentially with increasing abiotic stress. Grazing animals (which act as trophic buffers between people and environment) have proven successful in dry and infertile environments. The humid tropical lowlands epitomise environments of low abiotic stress but overwhelming biotic intricacy. Here it pays to imitate natural systems rather than struggle to impose simplicity on ecosystems that are inherently complex. The keys to success are to (i) channel productivity into outputs of nutritional and economic importance, (ii) maintain adequate diversity to compensate for losses in a system simple enough to be horticulturally manageable, (iii) manage plants and herbivores to facilitate associational resistance and not associational susceptibility, and (iv) use perennial plants to maintain soil fertility, guard against erosion, and make full use of resources.

Managing the whole landscape: historical, hybrid, and novel ecosystems

The reality confronting ecosystem managers today is one of heterogeneous, rapidly transforming landscapes, particularly in the areas more affected by urban and agricultural development. A landscape management framework that incorporates all systems, across the spectrum of degrees of alteration, provides a fuller set of options for how and when to intervene, uses limited resources more effectively, and increases the chances of achieving management goals. That many ecosystems have departed so substantially from their historical trajectory that they defy conventional restoration is not in dispute. Acknowledging novel ecosystems need not constitute a threat to existing policy and management approaches. Rather, the development of an integrated approach to management interventions can provide options that are in tune with the current reality of rapid ecosystem change.

Invasibility: Lessons from South Florida

South Florida contains more conspicuous introduced plants and animals than any other region in the continental United States. At the same time the region also encompasses one of the largest contiguous complexes of preserved ecosystems in the eastern U.S. (Fig. 13.1.). Everglades National Park, dedicated in 1947, covers about 2500 (terrestrial) km2; the Big Cypress National Preserve, established only a decade ago, occupies 2300 km2; and the Fakahatchee State Preserve, whose acquisition by the State of Florida began in 1974, contains about 200 km2. An additional 3600 km2 are included in the three diked basins with modified hydroperiods controlled since 1949 by the South Florida Water Management District. Most of the introduced species that cause concern in South Florida were present before government agencies gained control of these lands.

Seed dynamics during forest succession in Costa Rica

Soil seed banks and current seed inputs each play a role in tropical succession. We compared the abundance and floristic composition of seeds from these two sources at a Costa Rican site by germinating seeds from the soil, measuring seed inputs for 3 yr, and monitoring the earliest colonists in a forest clearing. There were an estimated 6800 viable seeds/m2 in the soil of 3.3-yr-old vegetation, 9500 seeds/m2 in 11-yr-old vegetation, and 7000 seeds/m2 in a 75-yr-old forest. An estimated 10100 seeds/m2 fell on the soil surface of the young successional vegetation during 3 yr and 3700 seeds/m2 fell during that same time in the forest. Locally produced seeds accounted for about 75% of the seed input to the soil surface early in succession. Seeds dispersed out of young successional vegetation increased the quantity and species richness of the seed input and storage in an adjacent forest. Much of the species richness of the young successional vegetation resulted from seeds dispersed there from other communities by animals. Deforestation stimulated germination of most seeds in the surface soil of the old forest, including seeds of the dominant canopy tree. The recruitment of seedlings from the soil seed bank numerically overwhelmed that from post-disturbance seed rain and sprouts. We evaluated patterns of soil seed storage during succession and predicted the ability of vegetation of differing ages to respond to disturbance. Immediately after disturbance the number of seeds in the soil plummeted due to mortality, low inputs, and germination. As the vegetation regrew, the soil seed bank increased to a peak after 4 to 7 yr, then gradually decreased to its pre-disturbance size. High-frequency pulses of disturbance should result in reduced species richness, dominance by species with long-lived seeds, and fast recovery by seedling recruitment from the soil seed bank.

Herbivory in Complex and Simple Tropical Successional Ecosystems

To investigate the relationship between herbivory and floristic complexity, we measured losses to herbivores in four 0.1- to 4-yr-old tropical ecosystems: (1) unman? ipulated successional vegetation, (2) successional vegetation with higher plant diversity than the unmanipulated succession, (3) an ecosystem of investigator-controlled species composition, designed to imitate the physiognomy and species richness of the successional vegetation, and (4) monocultures of maize, cassava, and Cordia alliodora. We measured herbivory rates (loss of leaf area per day) on dominant plant species in each system and aggregated these over species to estimate rates for plant communities. Although herbivory rates varied widely among species, losses to herbivores in terms of mass of leaf tissue lost per unit of ground area were approximately equal in the four systems, 71.5to78.5gm-2- yr~ *. Ecosystems with greater plant species richness lost a lower proportion of available leaf area and exhibited lower temporal variability in herbivory. Species-rich ecosystems had relatively constant, predictable rates of herbivory due to counterbalancing of low rates on some species with high rates on others. The rate of herbivory on any species was strongly influenced by the nature of the surrounding vegetation. Although surrounding vegetation often conferred protection upon potential pest targets, in some cases a plant species ex- perienced increased susceptibility to herbivores through association with other species.