ABSTRACT
Pathogens affect plant communities in many ways, with widely different consequences (including negligible) and across vast ranges of spatial (cellular to landscape) and temporal (from the contemporary plant generation to evolutionary time) scales. Plant pathogenic fungi are ubiquitous in plant communities, and their impacts are diverse and often profound. They influence productivity, species composition and diversity, physical structure of communities and their genetic diversity, succession, and stability. Disease is a defining feature of some plant communities. At the same time, fungus-plant interactions are often so complex and subtle as to be nearly invisible. That any given plant community represents a dynamic interplay, usually over a long period, between plants, pathogens, and their environment is often not apparent unless the equilibrium is altered by a perturbation such as a novel pathogen, novel host plant, or an abrupt change in the environment. The effect of fungal pathogens on plant communities is obvious when it is catastrophic, for example, when a new pathogen such as
Cryphonectria parasitica
(Anagnostakis, 1987) or
Cronartium ribicola
(Maloy, 1997) is introduced to an ecosystem dominated by susceptible hosts, or when a susceptible host is grown in an environment more favorable to its pathogens, such as
Mycosphaerella pini
and
Cyclaneusma minus
on
Pinus radiata
in New Zealand (Gibson, 1972; Gadgil, 1984). But such examples are scant compared with the known diversity of plant-associated fungi and suggest that a more widespread and subtle dynamic exists between most plants and pathogens. In this chapter we (1) look to plant pathology for insights into the ecological implications of plant pathogenesis, (2) review the impacts
of invasive pathogens on plant communities as a source of hypotheses and tests of hypotheses on the roles of indigenous pathogens, and (3) illustrate the range and complexity of possible ecological interactions with observations from the forest ecosystems of western North America.
