{'fraction_non_alphanumeric': 0.02996772706316275, 'fraction_numerical': 0.01913324112494237, 'mean_word_length': 5.665130568356375, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Ecosystem to regional impacts of introduced pests and pathogens: historical context, questions and issues

Infestation by exotic pathogens and pests, and the resulting decline in native species is an important ecological, economic and evolutionary process that may alter ecosystem structure and function, and exert devastating impacts on natural resources and aesthetic conditions (Castello et al., 1995; Enserink, 1999; Everett, 2000). Introduced pests may dramatically change forest composition, structure and microenvironment; alter critical ecosystem processes such as nutrient cycling and retention; and increase ecosystem susceptibility to further disturbances, including invasion by exotic plants and animals (Vitousek, 1986; Ramakrishnan & Vitousek, 1989; Mack & D’Antonio, 1998; Mack et al., 2000). In the north-eastern USA, pathogen outbreaks have occurred historically and increasingly in the last century (Table 1; Castello et al., 1995; Liebhold et al., 1995; O’Keefe & Foster, 1998; Hall et al., 2002), and have initiated precipitous declines in such dominant species as chestnut, elm and beech (Liebhold et al., 1995). Importantly, outbreaks often lead to shifts in harvesting strategies of host trees, including increases in both the amount and rate of pre-salvage and salvage logging (Frothingham, 1924; Irland et al., 1988; Radeloff et al., 2000), potentially generating more profound ecosystem disruption than the pest or pathogen itself. Despite historical and ongoing introduction of pests and pathogens into forest ecosystems, there remains a critical lack of information regarding the factors controlling their impact across a range of spatial scales as well as the response of forest ecosystems and associated wildlife species to the selective mortality of dominant tree species. There has also been precious little attempt to place the understanding of these responses into a broad framework of ecosystem perturbation and recovery. Broad questions include: What regional, landscape, site and stand factors control the pattern of forest decline and mortality? What is the magnitude and trajectory of vegetation and microenvironment responses initiated by the selective removal of a dominant tree species and how do these affect ecosystem processes? How do pre-emptive logging and forest species decline because of the exotic pests differ in their impacts on forest structure, composition and ecosystem function? and finally, how do wildlife species respond to these species-selective processes? Because of the global movement of pests and pathogens, answers to these fundamental scientific questions are essential for understanding the function, dynamics and management of forests world-wide and for the development of appropriate management responses in forest ecosystems. One of the best-known examples of overstory loss in North America occurred following the introduction and subsequent spread of chestnut blight (Cryphonectria parasitica). This fungal disease effectively eliminated overstory chestnut (Castanea dentata) from forests extending from Maine to Georgia in the early twentieth century and dramatically altered the structure and composition of eastern deciduous forests (Liebhold et al., 1995). Unfortunately, despite a good understanding of the compositional and structural changes generated by this blight, we know very little concerning ecosystem response to the loss of chestnut. Similarly, impacts of chestnut loss on wildlife species are also largely unknown, outside the anecdotal information that squirrel populations crashed with the loss of chestnut mast, woodpeckers apparently increased because of the food and habitat in dead trees (cf. Smith et al., 2000), and seven moth species that fed exclusively on chestnut are now extinct (Opler, 1978). The recent unimpeded infestation of the hemlock woolly adelgid (Adelges tsugae; HWA) across the north-eastern USA provides an unusual opportunity and critical imperative to examine the ecological consequences of the removal of a core tree species, eastern hemlock, on forest composition, structure and function, as it occurs. The HWA, an introduced aphid-like insect from Japan that attacks and kills eastern hemlock, is generating widespread mortality and initiating intensive logging of hemlock from Journal of Biogeography, 29, 1471–1474