Effects of grassland plant species diversity, abundance, and composition on foliar fungal disease

  • Tilman D
  • Mitchell C
  • Groth J
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In an experiment that directly manipulated grassland plant species richness
and composition, decreased plant species richness (‘‘diversity’’) increased pathogen load
(the percentage of leaf area infected by species-specific foliar pathogens across the plant
community) in 1998. Pathogen load was almost three times greater in the average monoculture
than in the average plot planted with 24 grassland plant species, an approximately
natural diversity. Eleven individual diseases increased in severity (percentage of leaf area
infected by a single disease) at lower plant species richness, and severity of only one disease
was positively correlated with diversity. For 10 of the 11 diseases whose severity was
negatively related to diversity, disease severity was positively correlated with host abundance,
and in six of these cases, species diversity had no effect on disease severity after
controlling for the effects of host abundance. These results suggest that increased abundances
of individual host species at lower species diversity increased disease transmission
and severity. In 1996 and 1997, similar results for a smaller number of diseases sampled
were found in this experiment and another similar one. Although the effect of diversity on
disease was highly significant, considerable variance in pathogen load remained among
plots of a given diversity level. Much of this residual variance was explained by community
characteristics that were a function of the species composition of the communities (the
identity of species present vs. those lost). Specifically, communities that lost less diseaseprone
species had higher pathogen loads; this effect explained more variance in pathogen
load than did diversity. Also, communities that lost the species dominant at high diversity
had higher pathogen loads, presumably because relaxed competition allowed greater increases
in host abundances, but this effect was weak. Among plant species, disease proneness
appeared to be determined more by regional than local processes, because it was better
correlated with frequency of the plant species’ populations across the region than with local
abundance or frequency across the state. In total, our results support the hypothesis that
decreased species diversity will increase foliar pathogen load if this increases host abundance
and, therefore, disease transmission. Additionally, changes in community characteristics
determined by species composition will strongly influence pathogen load.

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  • David Tilman

  • Charles E. Mitchell

  • James V. Groth

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