| Measures
of diversity depend on an eclectic taxonomy now being developed from comparisons
of morphology, developmental programs, carbohydrate chemistry, fatty acids,
and nucleotide sequences in a wide range of arbuscular fungal taxa obtained
from living culture collections. Developmental patterns in character origin
and transformation are providing clues of intrinsic causation in evolution
of diversity. Extrinsic causation is being identified from population level
dynamics as well as data on species numbers, abundance, composition, and
distribution. Detection of species is based solely on sporulation, so that
a combination of field sampling and various trap culture methods provide
a more comprehensive estimate of fungal community organization. Species
distributions rarely correlate with ecological gradients or hypothesized
phylogenetic relationships, suggesting that an important causal factor of
present-day distributions is dispersal over geologic time. Global distribution
of both derived and ancestral species and representation of all genera in
most plant root systems further indicate that local diversity has a strong
historical component, with ecological processes of subordinate consequence.
Ecological dynamics play a crucial role at the local level. They are governed
by multilevel diversity among and within organisms of a species assemblage,
such as differences in life history traits and heterogeneity of genetic
and physiological properties, respectively. |
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