Urban forests contain terrestrial and aquatic systems that support invertebrates and their complex and varied life cycle requirements (Wilbur
1980). Our focus is primarily on terrestrial and semi-aquatic invertebrates that occupy the different strata within urban forests, from belowground, to ground level, to understory, sub-canopy and canopy. Terrestrial invertebrates contribute to an array of ecosystem functions (Scudder
2009), which translate into a multitude of services for humans (Prather et al.
2013), but also disservices (Dunn
2010), collectively termed nature’s contribution to people (NCP) (Díaz et al.
2018). In this section, we explore the contributions of invertebrates to urban forests and how these forests support invertebrates performing diverse functional roles, recognising that some species may perform different and multiple functions depending on life cycle stage and that their functions in an ecosystem may change over the course of their life. For instance, both holometabolous (complete metamorphosis) and hemimetabolous (partial metamorphosis) insects can experience remarkable ecological niche shifts while transitioning between larval/nymph and adult life stages, e.g., from herbivorous caterpillars to pollinating butterfly and moth adults, or from predacious aquatic nymph to predacious aerial dragonflies.
Pollination
Pollination refers to the exchange of genetic material between plants via reproduction and is a critical process in the ongoing recruitment of new generations for many plant species. Urban forests are a significant habitat resource for pollinators, which primarily include bees (Anthophila), flies (Diptera), and butterflies and moths (Lepidoptera). For example, wild bee communities in remnant forests are stratified vertically in the forest canopy (Urban-Mead et al.
2021) and contain unique species unable to persist in surrounding built-up areas (Harrison et al.
2018; Landsman et al.
2019). Similarly, urban parks that contain patches of remnant forest host more butterfly species, including woodland specialist species, than parks that contain only planted vegetation (e.g., Kitahara and Fujii
1997). However, planted trees (including exotics) can also be important to pollinators (Buchholz and Kowarik
2019), confirmed by the barcoding of pollen sampled from four bee species in five different EU cities (Müller
2021). Additionally, pollinators supported by urban forests provide pollination services both within these forests and to surrounding urban and rural habitats.
Urban forests provide significant nesting resources for social and solitary wild bees. For instance, social bumblebees will forage in private or community gardens, but queens construct nests in the less-disturbed soils of urban parks and forest edges (McFrederick and LeBuhn
2006). Many cavity-nesting solitary bees nest in logs, snags and stumps, and some, for instance leaf-cutting bees, collect leaves from a variety of trees and shrubs to partition their brood cells in the nest (MacIvor
2016). Many bees, such as species of the genus
Xylocopa, depend on dead wood for nesting and are potentially limited by these resources in cities, which are found nearly exclusively in remnant urban forests. More generally, many pollinators rely on a variety of urban land covers to complete their complex life cycles, depending on remnant forest for nesting and flower-rich urban greenspaces for foraging. Consequently, ensuring adequate nesting resources in urban forests will improve pollination in nearby urban greenspaces where it is valued (e.g., in residential and community gardens).
In degraded urban forests, many weedy herbaceous species may be present, which often provide foraging resources for generalist pollinators, including non-native honeybees (Threlfall et al.
2015) that might interact with native bees of conservation concern (Colla and MacIvor
2017). However, weeds can have extended flowering periods, or flower at different times than native plants, expanding the foraging season for many groups of pollinators or potentially ‘filling the gap’ brought about by climate warming if flowering and fruiting phenologies shift and thereby create periods of low resource availability (Sherry et al.
2007). It is therefore important to value and appropriately manage a range of urban forest types, even those perceived as lower quality.
Urban forests also contain many tree and shrub species required by moths and butterflies for oviposition and subsequent offspring development, and the structure of the forest resource in the landscape is important for these taxa. Hardy and Dennis (
1999) showed that the proportion of forest in the urban matrix was positively correlated with butterfly diversity. Similarly, Kurylo et al. (
2020) found that butterfly species richness increased with tree cover across the urban matrix, and Lintott et al. (
2014) found that moth diversity in urban forests increased in larger, older, and less fragmented patches.
Predation
Predation is the mechanisms through which populations of more abundant species are regulated by complex top-down trophic interactions. Invertebrate predator–prey interactions are ubiquitous on the forest floor (epigaeic stratum), with the main taxa involved including carabid and rove beetles (Carabidae, Staphylinidae), ants (Formicidae) and spiders (Araneae). These predators exert top-down control on the epigaeic and edaphic (soil) invertebrate communities, including members of their own guild (i.e., intra-guild predation), thus contributing an important top-down ecological process (predation) that structures communities (Niemelä
1993; Vidal and Murphy
2018). Invertebrate community structure in urban landscapes is, however, different from that in rural landscapes, with a general trend of predacious groups shifting towards smaller-sized species (see Merckx et al.
2018), species capable of flight (Niemelä and Kotze
2009) and thermophilic species (Piano et al.
2017). These differences are in line with the general processes operating in urban landscapes, including habitat fragmentation and degradation and the urban heat-island effect. Furthermore, for the largely predacious carabid beetle taxon, Kotze et al. (
2012) argued that due to a long history of urban forest fragmentation, forest specialist species have all but disappeared from boreal cities, like Helsinki, although some remain in highly specialized habitats in the city, such as bogs (Noreika et al.
2015).
Research on the effects of the apparent decoupling of interactions between different trophic levels in urban forests (see Samways et al.
2010) is needed to evaluate the functional importance of this dominant epigaeic predatory guild. A non-urban example illustrates the complex effects of epigaeic predators on ecosystem processes: Lawrence and Wise (
2000) showed that the removal of spiders from the forest floor in a secondary oak-hickory-maple forest in Madison County, Kentucky, USA, resulted in increased densities of springtails (Collembola). Yet, rather than an increase in the rate of litter decomposition due to a greater number of springtails, the authors later reported lower decomposition rates in the absence of spiders due, in part, to the release of mesopredators of other potentially important decomposer groups, such as mites (Acari) or flies (Diptera) (Lawrence and Wise
2004).
Urban pest populations often flourish when resources such as food or habitat are increased or novel community structures result in decreases in competition and/or predation (Robinson
1996). Changes in the climate of urban areas – as well as a lack of natural enemies in the case of exotic species – can facilitate pest outbreaks (Meineke et al.
2013) and associated economic consequences (Kovacs et al.
2010), such as the northward expansion by the emerald ash borer (
Agrilus planipennis) into Canadian cities and towns (Herms and McCullough
2014) and the hemlock woolly adelgid (
Adelges tsugae) across the Northeastern USA (Paradis et al.
2008). Arthropod pests such as some species of mosquitoes (Culicidae), cockroaches and termites (Blattodea) and beetles (Coleoptera) require extensive management in cities because they threaten stored products, public health or building structures (Rust
2009). In urban parks and forests, arthropod pests can damage native vegetation (Ciceoi et al.
2017) through elevated levels of herbivory (Christie and Hochuli
2005), or negatively affect native animals through predation or competition. However, arthropod pests may be subject to top-down control in urban areas, as evidenced by decreased foliage loss in large cities across Europe as a result of elevated bird predation (Kozlov et al.
2017). Increases in urban forest pests are also of concern as they can spread to nearby, more natural landscapes, as was shown for the Asian long-horned beetle,
Anoplophora glabripennis (Dodds and Orwig
2011).
As indicated above, biological control has the potential to regulate arthropod pests in urban forests, thereby reducing the need for pesticides or other control agents and potentially lowering monetary costs in the long term (Olkowski et al.
1976; Kenis et al.
2017). The success of biological control in urban areas relies on diverse source populations of natural enemies, resource accessibility and the ability of these organisms to permeate through and persist in the urban matrix (Shrewsbury and Leather
2012; Frey et al.
2018). For example, urban vegetation fragments can be an important source for biological control agents such as spiders (Lowe et al.
2018) and parasitoids (Fenoglio et al.
2013), and can increase the diversity of predator communities in nearby urban gardens (Vergnes et al.
2012). Increasing supplementary resources for natural enemies within the urban matrix can also increase biological control services (Ellis et al.
2005; Egerer et al.
2018). However, biological control can be hard to achieve in urban areas as arthropod predator communities are often disrupted, limiting their ability to counter pest populations (Meineke et al.
2014; Gardiner and Harwood
2017).
Herbivory
Herbivory is the process through which the energy plants capture from the sun is transferred to the next level of organisms, and is therefore an essential process for life on Earth. Invertebrate herbivores are a taxonomically diverse and speciose ecological group, dominated by juvenile and adult stages of moths and butterflies (Lepidoptera), beetles (Coleoptera), bugs (Hemiptera), flies (Diptera) and grasshoppers and crickets (Orthoptera). Some are specialist feeders on certain host plants, while others have the capacity to feed on a wide array of hosts (Forister et al.
2019). The sheer diversity and abundance of insect herbivores in urban forests make the interactions between plants and insects a key driver in productivity and nutrient cycling (Hawlena et al.
2012).
Collectively, invertebrate herbivores in urban forests are not a homogenous functional group as they employ an extraordinary array of strategies to consume plant material (Strong et al.
1984). This variation in foraging strategy has equally varied impacts on plants. For example, herbivory can result in substantial reductions in photosynthetic area, the destruction of reproductive structures such as flowers or seeds and, in some instances, can promote disease if invertebrates themselves are disease vectors (e.g., Dutch Elm Disease,
Ophiostoma ulmi and
O. novo-ulmi), or if their herbivory creates entry points for pathogens. In an urban context, herbivory, when out of control (e.g., gypsy moth infestations in Eastern North America [Moeller et al.
1977; Schultz and Baldwin
1982]), defoliates trees and impacts recreation and the overall appreciation of urban forests (see also the “
Disservices” section). Therefore, overabundant invertebrate herbivores in urban forests are typically perceived as pests, particularly when the extent of defoliation is severe and the health of the urban forest is compromised (Raupp et al.
2010).
The engineering role of herbivorous insects is most apparent during population outbreaks that threaten the persistence of key plant species, especially when outbreaks interact with other disturbances such as fire (Parker et al.
2006; Halofsky et al.
2020). A range of factors may contribute to elevated levels of herbivorous insects and thus herbivory in urban forests, such as loss of key predators (Hochuli and Threlfall
2018) or parasitoids (Peralta et al.
2011; Nelson and Forbes
2014), changes in landscape structure and configuration (Fenoglio et al.
2012; Rossetti et al.
2017) and microclimate (Meineke et al.
2013; Dale and Frank
2017). Mechanisms driving the population ecology of insect herbivores remain a key frontier in identifying how their impacts in urban forests can be assessed (see “
Invertebrate responses to urban environments” section) and managed (see “
Managing urban forests for invertebrates” section).
Dispersal of seeds and microorganisms
As plants and microbes are sessile, their main mechanism for movement into new locations is through the dispersal of seeds, spores and other propagules. While some ant species are known for playing an important role in seed dispersal in urban forests (Thompson and Mclachlan
2007), there is emerging evidence that seeds are also dispersed by other insect taxa such as hornets (
Vespa spp.) (Chen et al.
2017), crickets (Grylloidea) (Suetsugu
2020) and dung beetles (Scarabaeoidea) (Milotić et al.
2019). Indeed, there are many examples where plants have co-evolved with invertebrates to such an extent that plants develop specialised structures that enable dispersal by specific taxa (e.g., the elaiosomes on seeds of
Acacia spp. that enable dispersal by ants). Yet, such ant-seed dispersal relationships can be disrupted in urban areas, as evidenced by elevated rates of seed dispersal after the restoration of ant communities via urban forest restoration efforts in Sydney, Australia (Lomov et al.
2009). Additionally, invertebrates assist with the movement of fungal spores, bacteria and other microorganisms through intentional (e.g., transporting fruiting bodies of fungi) or incidental means (e.g., through digestion and excretion or via surface adhesion) (Bray and Wickings
2019). For instance, some beetles act as transport for fungi, moving and injecting significant quantities and diversity of spores into dead wood and thus improving decomposition and accelerating the creation of hollows that provide habitat for other organisms (Seibold et al.
2019). The movement and foraging of invertebrate taxa such as earthworms (Grant
1983; Milcu et al.
2006), ants (Beattie and Culver
1982; Christian and Stanton
2004; Rowles and O’Dowd
2009) and dung beetles (deCastro-Arrazola et al.
2020) not only facilitate seed dispersal (and fungal dispersal, see next section) but may be important mediators of germination success and seedling recruitment by protecting seeds from predation and locating seeds in nutrient-rich microsites. Although seed dispersal in urban areas can be a significant driver of urban plant community composition, this interaction remains poorly understood (Cheptou et al.
2008; Johnson et al.
2018). Supporting urban forest invertebrate communities that provide seed and microorganism dispersal could be critical for the species and genetic diversity of urban organisms.
Organic matter decomposition and soil development
The decomposition of organic matter closes the nutrient cycle loop in urban forests by reducing the accumulation of dead material and returning nutrients back to the soil to become available to plants once again. There are many soil- and litter-dwelling invertebrates who perform these important functions. Macro-detritivores (e.g., earthworms, woodlice and millipedes) break down leaf litter into smaller pieces (comminution) making it accessible to micro-detritivores (e.g., springtails, oribatid mites) and bacteria and fungi (David and Handa
2010; Ossola et al.
2017). Estimates across various biomes and ecosystems (not including urban forests) show that the presence of complex decomposer communities, including macro-detritivores and their predators, can accelerate both carbon and nitrogen loss on average by 11% (Handa et al.
2014). Studies in urban habitats remain scarce and are much needed, but recent studies have confirmed the importance of soil faunal community complexity for litter decomposition in both urban gardens (Tresch et al.
2019a) and urban forests (Meyer et al.
2020).
Some invertebrates burrow into the soil but feed on the forest floor (e.g., anecic earthworms), which allows for the incorporation of organic detritus and nutrients from the surface deep into the soil profile, while promoting soil gas exchange and water infiltration (Ossola et al.
2015a). In fire-prone urban ecosystems, the removal of large quantities of plant litter from forests by detritivorous invertebrates can decrease fuel loads and fire risk for neighbouring communities (Buckingham et al.
2015). An increase in detritivore species richness significantly enhances the process of decomposition in urban greenspaces and urban forests, as shown in urban gardens in Switzerland (Tresch et al.
2019a,
b) and in urban forests in Melbourne, Australia (Ossola et al.
2016), despite the latter being dominated by exotic species from Europe. The dominance of exotic detritivore species, however, is not uncommon and numerous species are now ubiquitous in cities worldwide due to trade and the movement of soil and plant material (Tóth et al.
2020). For example, historic anthropogenic disturbance, over a century old, best explained the intensity of exotic earthworm invasion in a north-eastern North American peri-urban forest (Beauséjour et al.
2014). Exotic detritivorous earthworms in North American forests change plant species composition by favouring non-native plants and reducing the cover of native species (Craven et al.
2017) and by reducing the diversity and density of soil invertebrates (Ferlian et al.
2017).
Decomposing dead wood, including snags/stags (standing dead trees), old roots and fallen branches, is another important forest resource (e.g., Thorn et al.
2020), but not always assessed in urban forest management (Korhonen et al.
2020). Wood decomposition is a long process occurring in different parts of a tree and at different stages of its life, thus providing nursery and refuge resources (i.e., a habitat tree, see Bauerle and Nothdurft
2011) to many taxa and from different trophic levels. For example, dead wood can provide important habitat to springtail (Collembola) communities (Raymond-Leonard et al.
2020). Habitat trees and tree related microhabitats are also particularly important to saproxylic invertebrates, especially jewel beetles (Buprestidae), long-horned beetles (Cerambycidae) and bark beetles (Scolytinae) (Speight
1989; Grove
2002; Kraus et al.
2016) whose larval stage can last up to five years. A specific example is the European stag beetle (
Lucanus cervus), which often occurs in warm urban deciduous forests (Harvey et al.
2011). Saproxylic beetles are key actors in ecosystem processes such as wood decomposition and nutrient cycling (Dajoz
2000), and their richness, community composition and genetic diversity depend mainly on tree species identity, decay stage, wood size and volume (Schiegg
2000; Brin et al.
2011) and distribution (Horák
2011,
2018), as well as on the connectivity and management regime of old trees and woody debris (Vandekerkhove et al.
2013). Old trees and woody debris are a critical resource for this group of invertebrates, however these elements are often missing from urban forests due to public safety concerns and aesthetical preferences (Hauru et al.
2014; Le Roux et al.
2014), threatening the persistence of these animals and the functions they perform.
Many saproxylic invertebrates feed on nectar and pollen as adults, thus the distribution and configuration of floral feeding resources (meadows, flowering bushes and trees) outside urban forests are complementary (e.g., Colding
2007) to maintain viable populations within urban forests (Matteson and Langellotto
2010). Since saproxylic invertebrates are generally not highly mobile, such floral resources should be in close proximity to decaying wood in urban forests, or should be well connected through green corridors providing feeding resources and resting places (see also the “
Pollination” section).
Disservices
While biodiversity and nature offer many benefits to people, they can also give rise to negative interactions or consequences that can be considered “disservices”. Some examples include property damage by termites or other wood boring insects (e.g.,
Xylocopa), entomophobia (fear of insects) and major outbreaks of pests, both medical and economic. One of the disservices with the most direct consequences for humans occurs when invertebrates transmit diseases that pose a significant risk to public health (Lyytimäki et al.
2008). Arthropod-borne diseases are of significant concern in urban landscapes (LaDeau et al.
2015), with key groups being mosquitoes (Culicidae) (Lourenço-de-Oliviera et al.
2004; Rochlin et al.
2016; Murdock et al.
2017; Goodman et al.
2018) and ticks (Acari) (Maupin et al.
1991; Stafford and Magnarelli
1993; Frank et al.
1998; Uspensky
2017). The latter rely on vertebrate hosts also being present in forests; therefore, understanding how the interactions between host, tick, and pathogen are affected by characteristics of the urban environment is essential for reducing public health risk (Ostfeld and Keesing
2017). For example, Krystosik et al. (
2020) conducted a systematic review and found that solid waste associated with urban landscapes provided a breeding ground for zoonotic disease hosts (often mammals) and invertebrate transmission vectors. Given the potential of public health risks to shape perceptions and management of urban forests, it is vital that risks be assessed and compared against the benefits that these forests provide to nature and humans alike.