Functional mechanisms of biodiversity

Ecological insights into system function, especially in terrestrial systems, arc widely recognized as difficult to obtain because of the connected nature of most ecosystems. Relationships involving only a small number of system components can usually be described, while the bulk of the system's functional attributes remain undetermined. Improving techniques in observation, experimentation and analysis arc, however, affording better and better opportunities for expanding quantitative knowledge of system function in relation to compositional and structural diversity.

Nevertheless, Springett (1976) provided some early evidence for the identifiable role of species diversity in ecosystem function in Western Australia. In that study, the diversity and abundance of soil microarthropods and litter decomposition were compared between natural woodlands and plantations of Pinus pinaster. No clear relationship was found between arthropod abundance and decomposition, but there was a significant correlation between species diversity and decomposition rates. This relationship indicated a large effect at low diversity values, but a tailing off of the response at higher levels. This supports the theoretical model of an asymptotic relationship between diversity and function, as suggested by Vitousek and Hooper (1993), and the notion that a certain minimum numbers of species (or types of species which may be referred to as functional groups) may be required for full ecosystem function. While additional species may add little to the ability of the system to support the essential processes, they may still provide an important insurance against disturbance and change (Hobbs et al. 1995b).

Based on observations at a broader scale, chaparral vegetation in southern California has provided the opportunity to gain some insights into the temporal role of diversity in system function during the post-fire period of that fire-prone vegetation. Fire is essential for the release and recycling of nutrients tied up in mature vegetation. However, released nutrients are vulnerable to loss from the system by volatilization and with post-fire runoff. As much as 66% of nitrogen in the soil and litter layer can be lost from a chaparral system during an intense fire (DeBano et al. 1979), and natural input levels are so low that full replacement of soil nitrogen could take more than 60 years for pre-fire levels to be reached where industrial pollution makes no contribution (Schiesinger and Gray 1982). Rundel (1983) has pointed out that on many chaparral sites symbiotic nitrogen-fixers such as annual Luptnua species, or the subshrub Lotus scoparius are post-fire pioneers, while less efficient asymbiotic micro-biota still play an important role in rebuilding nitrogen pools (Dunn and Poth 1979). Also assisting in maintaining the nutrient balance on recently burned sites are members of the post-fire annual flora, which through rapid growth are able to capture nutrients which might otherwise be lost with runoir. These organisms represent a highly diverse group of fire-specialists that occur only on burned sites, and then disappear after one or two years. Generalists persist for much longer after fire. Swift (1991) provides evidence that fire-specialists and generalists have very different nitrogen utilization strategies. Fire-specialist plant species such as Phacelia brachyloba and P. minor appear to have a preference for ammonium nitrogen over nitrate nitrogen, thereby being able to take advantage of high levels of the former found in the post-fire environment. On the other hand, their nitrogen-use efficiency is much lower than more persistent generalist species such as Crypt ant ha intermedia, Phacelia cicutaria and Brassica nigra. The exact role of these broad groups of species in nutrient relations of chaparral systems is not clear, but the fine-scale pattern of soil nutrient distribution after fires (Rice 1993) may require a great deal of structural and functional diversity for all of the key nutrient cycling processes to occur. Without the full complement of post-fire nutrient cycling functions, a chaparral system may degrade into one of a less diverse type, such as that dominated by Adeno-s torn a fasciculatum.

Control 50 100 150 Ch [email protected]

Treatment

Figure 7.4 Germination responses of Acacia species in a laboratory experiment following different heat (50, 100 and 150°C) and charcoal (Ch) treatments (Atkins and Hobbs, 1995)

Control 50 100 150 Ch [email protected]

Treatment

Figure 7.4 Germination responses of Acacia species in a laboratory experiment following different heat (50, 100 and 150°C) and charcoal (Ch) treatments (Atkins and Hobbs, 1995)

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