The nature of relationships between associated species is dynamic in both ecological and evolutionary time, and subject to change with changing conditions. The fluidity of transition between parasitism, commensalism, and mutualism is hinted at by fungi in the family Clavicipitaceae, several of which grow within the tissues of grasses. Some species are parasites, producing diseases such as ergot in rye. Other species are commensal, with little appreciable impact on the grass host, and others still are mutualistic, producing toxic alkaloids that protect their grass hosts from grazing. Similarly, among yucca moths, most species form obligate pollination mutualisms with their yucca hosts, but several species have become parasitic; laying their eggs in yucca fruits they have not pollinated and thereby exacting a heavy cost on the host.
Among parasitic associations in particular, changing ecological conditions can foster rapid changes in the demography and severity of infections, resulting in disease outbreaks. An alarming example is the outbreak of a chytrid fungus, parasitic in amphibians, in Central America since the late 1980s. The epidemic, which appears responsible for the extinction of over 70 species of frogs and toads in the region, has recently been linked to improved conditions for the fungus resulting from global warming.
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