Emergent Properties

Emergent properties of ecosystems are a consequence of the synergistic effects of community composition on "ecosystem function" (flux of energy and materials through the ecosystem). The idea that the forest is more than the trees conveys the importance of the concept, but it can be applied broadly to every ecosystem. Species' impacts on turnover rates and productivity have profound effects. As a very simple example, Blair et al. (1990) found that while decomposition rates for mixed litter were similar to the decomposition rates for the litter of tree species incubated individually, there were significant alterations to N flux and abundance of decomposer organisms that could not be predicted based on the patterns detected in incubations of individual litter types. Indeed, one of the most hotly debated and difficult relationships to define is that of the importance of biodiversity to ecosystem function.

There have been a number of hypotheses developed to explain the inherent importance of biodiversity, but few studies have provided convincing support for these hypotheses. Questions such as whether biodiversity impacts characteristics including ecosystem productivity, nutrient retention, and stability have permeated the literature over the past several decades. In an era of decreasing global biodiversity and decreasing genetic diversity, these questions require immediate attention. Intellectually there is no denying that maintaining biodiversity is essential to maintaining the integrity of systems. There are two main problems with this challenge. The first is that measuring the complete biodiversity of a system and determining the degree to which it is in an "undisturbed state" is difficult. Second, biodiversity is not a term that is indicative of quality but quantity. Increasing diversity by increasing undesirable species such as nonnative invasive species will not maintain the integrity of a system, so biodiversity for biodiversity's sake is not the answer. So, what species or how many species should an ecosystem contain? The example above of Blair et al. (1990) suggests that species interactions impact ecosystem function, in this case N flux within a system, which will feed back to impact energy allocation to nutrient acquisition by the plant. Tilman (1996) demonstrated that ecosystem stability increased with aboveground species diversity. However, Klironomos et al. (2000) found that the presence or absence of arbuscular mycorrhizal (AM) fungi significantly changed the relationship of plant diversity to aboveground productivity. Without the fungi, productivity increased as plant species were added to a total of 15 species in a linear fashion. Productivity was maximized at 10 plant species in an asymptotic fashion with AM fungi present. The results suggest that considerations of the importance of diversity in ecosystem function must include an understanding of diversity of all participants, otherwise the importance of diversity among different components may be overlooked. In contrast, the island biogeography study by Wardle et al. (1997) described above found the islands with the greatest aboveground diversity had lower ecosystem process rates. Whether lower rates of nutrient cycling and decomposition are tied to lower belowground diversity remains to be seen.

One of the biodiversity hypotheses examines the degree to which there is functional redundancy in microbial communities. This hypothesis suggests that there are so many species that have the same function that loss of one will not alter the way that the system operates. There are no studies that can provide adequate support for or can eliminate this hypothesis. The amount of stress placed on ecosystems across the globe by anthropogenic influences has changed the way ecosystems operate. Whereas there may have been no impact of the loss of one species from a system before anthropogenic N deposition, this chronic disturbance may have changed the response of the system to the loss of one species. There are two main terms used to characterize the way that an ecosystem responds to a disturbance. While a system that does not change appreciably following a disturbance is said to be resistant, a system that changes but returns to its predisturbance state within a reasonable time frame is said to be resilient. The degree to which ecosystems are resistant or resilient may depend entirely on biodiversity and on the amount of stress currently on the system. Unfortunately, as much as this information is needed to protect ecosystems from degradation and protect species against loss, the stability of ecosystems is an emergent property that cannot easily be measured quantitatively as a parameter or by quantifying one or more of its component parts.

conclusion

Integrating across scales is a challenge that ecologists have faced for the past hundred years. Scientists that examine microorganisms or soil communities have always dealt with this particular issue. For example, how does one acquire soil samples, in a forest, that are representative of the organisms that live there, in a manner that will allow one to evaluate treatment differences or small-scale rates of change? Conversely, present interpretation of global-scale processes such as net primary productivity and C cycling require very little understanding of micro-bial community dynamics. Processes such as nutrient cycling impact site productivity, but productivity at a global scale can be predicted based on temperature and moisture patterns alone. In models, the microbially mediated steps of nutrient cycling are also predicted by patterns of temperature and moisture. The dilemma facing scientists is that if one considers the component parts of any elemental cycle, the rate that a nutrient becomes available for uptake is dependent on the life cycle of a soil bacteria or the reach of a mycorrhizal companion. In other cases, such as where there is a unique biological interaction (e.g., pathogenesis, mutualism, ecosystem engineering), prediction of population and community dynamics is critical. Failure to investigate the causes and impacts of microbial community structure over the long term will retard our ability to manage ecosystems for the greatest benefit to society and reduce our understanding of the impacts of species loss and global climate change. The role that each organism plays must be examined across spatial scales from molecules to ecosystems, and temporal scales from seconds to centuries, or our ability to predict problems or mitigate damage will be impaired. Integration across scales is a challenge that those that study below-ground systems can begin to facilitate.

The field of science known as ecology is more integrative than most other fields. Ecologists are dependent on specialists that reach across the breadth of the physical sciences. Scientists that study soil microbiology and biochemistry can contribute to and benefit from approaching the medium and organisms they examine as ecologists. Society has placed a great burden on scientists by damaging systems before understanding how they operate. Science is now charged with developing an understanding of these systems and finding ways to mitigate the damage. This can be accomplished only by integrating across scientific fields. As such, this brief introduction to ecology was prepared to stimulate an awareness of the contributions that studies of soils have already made to our understanding of the operation of the natural world and the need to continue to integrate scientific endeavors from molecules to the biosphere.

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