Insects as a Vital Sign in the Greater Yellowstone Ecosystem
by Diane M. Debinski
Insects far outnumber vertebrates in Yellowstone National Park (YNP), North America, and worldwide. In fact, 80% of all named species are invertebrates (Cardoso et al. 2011). Despite their abundance, ecological importance, and benefits to society, numerous opportunities for discovery and for elevating the understanding of insects’ contributions to health of ecosystems still remains. For example, even in well-studied places like YNP, studies of invertebrates often reveal previously undocumented species (Duffy 1999). Because of the growing need for clear and reliable indicators of ecosystem change, monitoring programs are increasingly relying on insects to provide biological evidence of ecosystem health. Insects are excellent indicators because they are relatively easily collected, have a short life span, a high reproductive rate, and great mobility in the environment. Because of these traits, insects react quickly to environmental changes. Insect development is affected by humidity, rainfall, and temperature (Kremen et al. 1993), making them sensitive to even the smallest changes in local habitat. Some insects may have short dispersal distances despite being winged. These types of species are some of the first to be affected under conditions of habitat loss or fragmentation (Sobrinho et al. 2003).
The Ecological Role of Insects
As a result of the co-evolution of plants and insects, the two are dependent on one another for survival. Insects provide pollination services to the majority of flowering plants (Waldbauer 2003), and many of these relationships are so specialized that in the absence of its pollinator a plant cannot reproduce. Conversely, in the absence of its food source an insect will not survive. Although bees (Hymenoptera) are the most common insect involved in pollination, flies (Diptera) are a close second (Larson et al. 2001). The associations between flies and flowers are commonly overlooked, but their role in pollination increases with increasing altitude, making flies important pollinators in sites like alpine meadows of the Greater Yellowstone Ecosystem (GYE). Seed dispersal is another example of the delicate symbiosis between plants and insects. In fact, 35% of flowering plants rely on ants for seed dispersal (Waldbauer 2003).
Insects provide a vital connection between plants and vertebrates. Without insects, many food chains would collapse. Herbivorous animals, most of which are insects, play a pivotal role as intermediaries in food chains by making the nutrients in plants available to animals that do not eat plants (Waldbauer 2003). During the dry summer months when grizzly bears in YNP are stressed for food, insects become an important part of their diet; Yellowstone grizzly bears are one of the only North American populations that consume insects in noteworthy amounts. Consumption of bees, wasps, and army cutworm moths (Euxoa auxiliaris) increases as higher-quality foods decrease in availability during August and September (Mattson 2002). Insects also regulate vertebrate populations through insect-borne diseases, parasitism, and herbivory competition. For example, ticks may cause significant blood loss, excessive scratching, and disrupt the eating patterns of their vertebrate hosts (Mooring and Samuel 1998).
Insects aid in the crucial process of nutrient cycling by moving soil, consuming carrion, and decomposing organic matter. Insect activity physically modifies the soil profile, improving the habitat for plant growth. Ants and other burrowing insects redistribute soil, bringing mineral-rich components from below and mixing it with organic matter, creating a fertile environment ideal for plant growth. In an area such as the GYE with large populations of large mammals, carrion decomposition is a significant issue. Carrion beetles are especially important in decomposition. Sikes (1994) found more than 50 species of carrion beetle present in the northern range of the GYE that are heavily dependent on ungulate carcasses.
Justification for Monitoring Insects As Indicators
Changes in insect populations are detectable in other levels of the food chain. For this reason, monitoring insect populations allows the prediction of effects on animals at higher levels in the food chain. Several groups of insects have been used to document long-term changes in habitats (Turin and den Boer 1988), and fossil records of insect communities have been used to construct climate histories (Atkinson et al. 1987). Because insect populations are responsive to habitat changes at both small- and large-scales (e.g., climate change, fire, or exotic species outbreaks), their use as biological indicators could offer a richer understanding of ecological change in YNP and across the GYE.
Climate Change: Insects are especially responsive to climate change because of their specialized habitat requirements. For instance, butterflies have shown rapid responses to climate change (e.g., Warren et al. 2001). The implications of changes in climate for butterflies are potentially serious, with particular concern expressed about montane butterfly communities where habitats are predicted to shrink. Many butterflies in the GYE are tightly correlated with specific meadow habitats and already show population changes associated with drought (Debinski et al. 2013). Drought-induced change may portend future climate-induced shifts to butterflies and other montane insects.
Fire: For species that predominantly live above ground, direct effects of fire can include incineration. In contrast, for species that live underground (ants, burrowing beetles, or other insects overwintering underground), there may be few, if any, direct effects of a fire. However, indirect effects of fire manifest themselves via the effects of fire on the vegetation insects use. For terrestrial insects that use dead wood, such as pine bark beetles, fire can produce a major boom in population growth (Sullivan et al. 2003). Given the predominance of lodgepole pine in YNP and across the GYE, these insects could be considered significant ecosystem engineers.
Exotic species: Exotic species, especially exotic plants, may be having large, yet undetected effects on terrestrial insects in the GYE. Roadways and hiking trails are primary areas for the introduction of exotic plant species because they are often transported via humans or horses. Some of the major exotic plant species in the GYE are dalmatian toadflax, spotted knapweed, Canada thistle, ox-eye daisy, houndstongue, and leafy spurge (“Invasive Plants as Indicators of Ecosystem Health,” this issue). These species have indirect effects on the insect community by changing the amount and relative abundance of plants and soil nutrients available to insects (Ehrenheld 2003). These changes may increase some insect species by providing additional nectar, food, or host plants; others may decrease because their preferred nectar, food, or host plant species are out-competed by the exotics (Levine et al. 2003).
Currently, there is no park- or region-wide monitoring program tracking changes in insect populations or communities across the GYE. Yet, this taxonomic group represents a wealth of relatively unexplored vital signs of ecosystem change. Impediments to the development of a monitoring plan for this species-rich group are significant (Cardoso et al. 2011). However, guidance and selection criteria for indicator ideas and broad-scale monitoring for large protected areas have already been developed (McGeoch 1998). The diversity and distributions of these charismatic microfauna should be considered a critical source of future insight for understanding habitat and ecosystem-level changes across YNP and the GYE. This valuable insight should compel us to explore future strategies to improve the awareness of insect diversity, formalize programs to monitor changes, and consider efforts to enhance insect conservation.
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Diane Debinski is the Department Head of Montana State University’s Ecology Department. She pursues research and teaching in the fields of conservation biology, landscape ecology, and restoration ecology. Some of the topics of her research include biodiversity preservation, effects of habitat fragmentation, and climate change. Dr. Debinski has been conducting both observational and experimental studies of plant and butterfly community responses to drought and environmental variation in montane meadows.