Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 336

hyphae, 10 7 yeasts, 10 6 protozoa, 10 5 nematodes and thousands of assorted
worms, mites, collembola and other fauna. In addition, embedded in this
numerically huge community are tens of thousands of genetically distinct
bacteria and archea, thousands of fungal species and hundreds of proto-
zoan, nematode, worm, insect and arachnid species. The possible origins of
such diversity are discussed lucidly by Tiedje et al . (Chapter 6); here we
explore some of the consequences in relation to soil organic matter dynamics.
Concepts
The functional consequences of biodiversity, and especially of such
diversity as prevails in soils, are at present poorly understood from both a
theoretical and experimental standpoint. It can be hypothesized that diverse
systems are more productive, sustainable and resilient, and that a reduction
in biodiversity may result in an impoverishment of ecosystem functions
(Giller et al ., 1997; Wolters, 1997). The reasoning is based mainly on three
mechanisms:
1. Repertoire . At the most basic level, for a biologically mediated process
to occur, organisms which can carry out that process must be present.
A highly diverse system will have a wider repertoire of abilities - or a
more extensive 'toolkit' - that will permit a greater range of functions to
be carried out. This is the most basic form of 'functional' diversity and
emphasizes the point that if the tool is not available the job will not get
done. The diversity of the repertoire in a system is not necessarily related to
the taxonomic diversity since one organism may carry out many functions
from both a biochemical and physical (e.g. 'ecosystem engineering')
perspective.
2. Interactions . Organisms influence other organisms to varying degrees,
in a positive or negative manner; the more diversity that prevails the more
possibilities there are for such interactions to develop. In terms of produc-
tivity and sustainability, positive interactions may accelerate nutrient
cycling, negative interactions may regulate pathogens. Thus the behaviour
of individuals and populations can depend strongly on their biotic
context, and community-level controls have been clearly demonstrated
in experimental systems (e.g. Janzen et al ., 1995; Toyota et al ., 1996).
3. Redundancy . The more organisms there are that can carry out a particu-
lar process, the more likely it is that if some are incapacitated or removed,
the process will not be affected; those that remain simply fill the gap. This is
also vaunted as the 'spare wheel' hypothesis (Andrén et al ., 1995).
Four main hypotheses linking biodiversity and function currently are
postulated; these are summarized in Table 5.1.