Nancy Golubiewski is Research Ecologist at the New Zealand Centre for Ecological Economics, where she has researched the ecosystem services provided by natural and managed landscapes located within a Maori iwi’s ancestral land as well as contributed to projects on urban material/energy flow accounting and the development of a genuine progress indicator.
The Importance of Species
Perspectives on Expendability and Triage
Species diversity may affect ecosystem function, and different species may have disproportional influences upon ecosystem processes. To start off with, the null hypothesis for the relationship between biodiversity and ecosystem processes would state: Species diversity does not affect ecosystem function. Given the varied plant assemblages and ecosystems on Earth, however, diversity probably influences processes. Several alternative hypotheses have been proposed in order to elucidate the extent to which ecosystem function depends on diversity. Depending on the interpretation followed, the redundancy hypothesis implies that under existing conditions species richness is irrelevant or that a minimum level of diversity is necessary for proper system functioning, but most species overlap in their functional roles. A different view asserts that each and every species contributes to ecosystem functioning. Both the “Type 1” linear relationship and the “rivet” hypothesis are based on this idea of unique species contributions. The order of species deletions or additions does not matter in any of these hypotheses.
According to the Global Biodiversity Assessment (GBA), “There is no evidence that each and every species plays a unique role such that its absence would immediately result in a dramatic change in the functioning of ecological system”. Moreover, in most ecosystems, species diversity is higher than the level required for efficient biogeochemical and trophic function. Also, some researchers assert that richness is not important per se, but a higher level of richness increases the likelihood of including a productive species. Scientists acknowledge that the relationship between diversity and function most likely sits between the two extreme hypotheses listed above. Instead, a few species probably dominate processes and species richness does not matter past a certain threshold. This differs from the redundancy hypothesis in that the order of deletions or additions matters. The idiosyncratic response hypothesis summarizes the relationship: ecosystem function changes when diversity changes, but the magnitude and direction are unpredictable since the roles of individuals are complex and varied. The remainder of this article will consider the concepts invoked for the differential roles species may have in ecosystems. The focus will remain on autotrophs.
Species Traits
Species can individually affect ecosystem processes through unique traits. Examples of these unique traits include nitrogen (N) fixation, water redistribution, trace gas emission, high growth rates or unique secondary chemistry. A few species usually have values for traits quite different from the rest. These types of effects are often studied in the context of invasive species but can also occur in non-invasive situations. These functions may be predictable and may have profound effects if the species is added or deleted.
One framework for understanding species traits is that species modify available resources either through consumption or supply. The resource supply rate can also be affected by species. In particular, differences in tissue quality, which control litter decomposition, occur. Other environmental factors can be affected, including soil acidity and microenvironment (through evapotranspiration and insulation). These, of course, also will affect community processes. For example, in Asia Nepalese alder increases nitrogen (N) inputs while bamboo retains weathered potassium (K). Two processes affected by species traits- water redistribution and nutrient cycling- will be explored further below.
Hydraulic lift
Hydraulic lift denotes the phenomenon of water redistribution from deep, moist soil horizons to dry, shallow layers. Besides aiding the “hydraulic lifter”, water can also be provided to plants in the vicinity of the hydraulic lifter. The process involves passive movement of water from roots to soil water when the soil water potential is less than the xylem water potential (or, when one part of the soil has lower soil water potential than another part). One study found that groundwater lifted by sugar maple (Acer saccharum) was used by plants up to 2.5 meters (m) away from the tree, but no effect was seen further than 5 m from the tree. All neighbors used some fraction of the lifted water (3-60%), and differential plant effects were noted. Since plant water deficit limits production, the existence of a hydraulic lifter in a community may be a crucial presence. This also may affect biogeochemical conditions for helping mineral availability, nutrient acquisition, or microbial processes. To date, the phenomenon has been demonstrated in about 27 species of different life forms– grasses, shrubs, trees, and herbs– and scientists expect it to be more widespread.
Inverse hydraulic lift has also been demonstrated. Hydraulic lift may provide a significant amount of water needed for evapotranspiration and so contribute to this process at an ecosystem scale. On the other hand, demonstration of inverse lift may prevent neighbors/competitors from gaining access to water. These coarse-scale effects, in addition to the facilitation of neighboring plants, make hydraulic lift a community- and ecosystem-level process. It also may be a general process, as additional studies of different taxa and ecosystems indicate.
Nutrient cycling
In general, individual plant species can play an important role in soil fertility. Usually they create positive feedbacks to plant persistence. Plants from low-nutrient systems use nutrients efficiently and grow slowly, so the cycling of nutrients is slow; the opposite is true for high-nutrient sites. Sometimes the vegetation is more important than abiotic effects in soil process control. Species traits elucidate whether a species loss or addition will affect nutrient cycling. Plants affect the nitrogen cycle through litter quality and its subsequent influence on transformation rates. Competitive displacement may occur after a change in nitrogen (N) supply due to deposition from anthropogenic sources. Community composition in moist meadow tundra was shown to have a potential effect on the nutrient cycling of the community. Deschampsia caespitosa had a positive influence on the rates of net soil N transformations, whereas Acomastylis rossii did not respond to an increased N supply and had a negative influence on the cycle. Therefore, the replacement of Acomastylis rossii by Deschampsia caespitosa had positive feedbacks, and the litter could reinforce the patchiness of species composition and available N.
Particular species do not always affect processes, though. In semi-arid grassland, researchers found that plant presence, rather than plant species identification, probably had the most effect on ecosystem processes. They do acknowledge that specific plant characteristics, such as life span, biomass allocation, and tissue chemical allocation can affect nutrient dynamics and soil organic matter. The traits that best predict resource consumption are height (as related to an individual’s ability to capture light and exploit a large soil volume) or biomass per individual and relative growth rate (as related to individual ability to capture carbon and nutrients). These traits vary continuously among organisms.
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Main photo credit: National Biological Information Infrastructure