Using a 4-year field experiment, Wilson et al. [19] found that the hemlock woolly adelgid (HWA; Adelges tsugae Annand) and elongated hemlock scale (Fiorinia externa Ferris) can significantly alter the foliar chemistry of eastern hemlock (Tsuga canadensis (L.) Carrière), although the effects from the two pest species are also very different. High tree mortality caused by insect herbivory can open up the canopy and increase light and temperature on the forest floor [20]. Massive pest infestations can greatly affect overall forest health, leading to reduced ecosystem primary and net productivity, carbon sequestration, and aboveground carbon storage (biomass), but increased decomposition rate (Table 1, Figure 1) [2,21].

Figure 1. Examples of how exotic pests may initiate short-medium-long-term effects on community- and ecosystem-level patterns and processes. Impacts of exotic pests on ecosystems can both be direct and indirect (e.g., pest-induced changes in trees at the individual, population, and community levels), which could eventually lead to landscape-level changes (Lázaro-Lobo and Ervin 2021). Feedback effects could be found at all organization levels, although varying time legs may also exist.

Mounting evidence shows that nonnative pest invasion can cause profound cascading and cross-trophic effects on food webs and many other ecosystem processes [31,38]. For example, modified hemlock foliar chemistry by the hemlock woolly adelgid [19] will affect other component plant species and associated herbivory activities and the entire ecosystem’s chemical profile. Additionally, avian community composition could be altered by HWA infestation because it causes high mortality of the hemlock trees that birds rely on [40]. Numerous specialist arthropod species dependent on ash (Fraxinus) may be extirpated because of the decimation of this host species by the invasive emerald ash borer (EAB, Agrilus planipennis Fairmaire) [4,41,42].

Existing evidence shows that, in general, planted forests appear to be more vulnerable to pest invasions than natural forests, possibly due to their lower biodiversity [43,44,45]. For example, planted poplar trees (Populus spp.) in China were seriously damaged by the star beetle and Chinese red pine (Pinus massoniana Lamb.) was seriously affected by pine wood nematode (Bursaphelenchus xylophilus), as evidenced by extensive tree death in these plantations e.g., [46].

In addition to the direct effects on host trees, exotic pests can also pose direct (and indirect) effects on native insects, especially pollinators, through various and sometimes complex interactions, including competition and predation. For example, a recent meta-analysis by Debnam et al. [47] shows that exotic pollinators can displace native insect and bird pollinators in certain cases, but their direct effects on native pollinators can be context-dependent, ranging from mutualism to antagonism.

A problematic indirect effect of insect and disease infestation is an “invasional meltdown,” during which the mortality of native tree hosts facilitates the invasion of non-native plants [4]. For example, forests experiencing high levels of ash mortality because of EAB infestations in Michigan and Ohio experienced increased growth of invasive woody shrubs, such as multiflora rose (Rosa multiflora Thunb.), Amur honeysuckle (Lonicera maackii (Rupr.) Herder), and autumn olive (Elaeagnus umbellata Thunb.) [42,48]. In Hawaii, rapid ʻŌhiʻa death (ROD), a recently discovered wilt disease of the widespread endemic Metrosideros polymorpha Gaudich. caused by the fungal pathogens Ceratocystis lukuohia and C. huliohia [49,50], may cause dramatic increases in non-native tree dominance in Hawaiian forests that are intensified by feral ungulate disturbance and competition with non-native plants in the understory [51]. In a reversal of these dynamics, the introduction of exotic plants could lead to the arrival of associated exotic pests that can affect ecosystems in different ways, such as altering forest succession and leading to species replacement [52].

There is abundant evidence that the loss of trees can directly and negatively affect water availability [53,54]. Some exotic pests cause direct disturbances on the soil surface through their movements and migration. Some build nests (large and small) or drill holes in the ground that affect soil structure and nutrients, as well as forest carbon dynamics. For example, soil nesting near tree roots by invasive ants was found to reduce carbon fixation and storage of Acacia drepanolobium Harms ex Sjöstedt saplings in Kenya, suggesting that direct interactions between invasive ants and plant roots in other ecosystems may strongly influence carbon fixation and storage [37]. Additionally, Warren et al. [55] found evidence that Brachyponera chinensis Emery, an invasive ant species in eastern North American forests, does not provide the seed dispersal services of the native ant that it replaces, potentially shifting ecological dynamics in these forests. Meanwhile, the feeding and burrowing behaviors of invasive earthworms in eastern North American forests, including at least three pheretimoid “jumping worm” species, reduce carbon storage in the forest floor, redistribute soil nutrients, and change basic soil properties, such as bulk density and soil pH, all causing substantial impacts on ecosystem functions and cascading effects on forest organisms [56].

Indirect geochemical and geophysical impacts from exotic pests on forest ecosystems include altering water and energy cycling, such as interception, runoff, storage, and recharge that subsequently influence surface albedo, evaporation, and transpiration [26]. Altered vegetation structure, including canopy height and density, will affect light penetration and wind speed [29]. A good example of indirect effects from exotic pests is the effects of losing eastern hemlock (due to HWA), a keystone species in the Southern Appalachian Mountains of the United States, on nitrogen (N) dynamics (mostly declining N retention except where N availability is high). Furthermore, pest and forest management through chemical use (i.e., in pest control) can definitely affect water, soil, and overall habitat quality [57].

The loss of trees caused by exotic pests can negatively and indirectly affect water quality [53,54]. For example, high tree mortality may increase nitrogen mineralization and nitrification and nitrate leaching to groundwater and/or surface waters [20,26,58]. In riparian ecosystems, the loss of riparian species can affect nutrient subsidies to rivers and streams. For example, ash leaves rapidly decompose, and therefore likely release nutrients relatively quickly when they fall into or near streams [33]. The loss of ash to EAB may, in some cases, shift to greater proportions of leaf litter from species, such as oaks (Quercus), that take longer to decompose, and therefore alter the timing of nutrient inputs into aquatic systems.

The effects of exotic pests on trees and water can negatively and indirectly affect soil conditions. For example, exotic pests may increase soil C and N levels but reduce C:N ratio e.g., [33]. An altered soil moisture regime can then affect the diversity and activity of soil microorganisms [14,15].

Canopy herbivory and frass deposition from native insects can affect soil nutrient dynamics [59] and nutrient cycling [20], but invasions from nonnative pests could substantially enhance such effects, causing much greater damage to the extent that the hosting ecosystem may not be sustainable over the long term [2].

Meanwhile, the loss of ash to EAB may indirectly affect forest soil chemistry, given that decomposed ash litter can contribute significantly to nutrient availability [60].

3. Short- vs. Long-Term Effects

4. Climate Change May Enhance the Impacts of Pests on Forest Ecosystems