by Scott Miller, BLM/ USU National Aquatic Monitoring Center
Historically, canaries accompanied coal miners deep underground. Their small lung capacity made them more vulnerable to low concentrations of carbon monoxide and methane than their human companions. As late as 1986, the acute sensitivity of these birds served as biological indicators of unsafe conditions in underground coal mines. Although human health concerns continue to drive development and application of bioindicators, the loss of ecosystem services, such as clean air, drinking water, and plant pollinators, has increasingly diverted our focus to the health of natural ecosystems. All species (or species assemblages) tolerate a limited range of chemical, physical and biological conditions from which we can infer environmental quality. Despite our many technologies, we find ourselves turning to the biota itself to tell us the story of our world.
Bioindicators are biological processes, species or communities used to assess the environment and how it changes through time. Changes in the environment are attributed to anthropogenic disturbances (e.g., pollution, land use changes) or natural stress (e.g., drought, late spring freeze), though anthropogenic stressors are the primary focus of bioindicators. Extensive development and application of bioindicators has occurred primarily since the 1960s. Over the years, we have expanded our repertoire of bioindicators to assist us in all types of environments (i.e., aquatic and terrestrial) using all major taxonomic groups.
Nowhere are bioindicators more broadly used than in freshwater ecosystems, where in the United States all 50 states are currently developing or implementing biomonitoring programs using aquatic macroinvertebrates, periphyton (i.e., algae and diatoms) and/or freshwater fishes. Although many state and federal programs use multiple organisms to assess the status and trend of freshwater resources, aquatic macroinvertebrates are ubiquitously used in all 50 states.
Aquatic macroinvertebrates are animals that do not have a backbone, can be seen with the naked eye and spend at least part of their lives in water. In general, members of the taxonomic class Insecta comprise the overwhelming diversity of aquatic macroinvertebrates in North America, which is conservatively estimated at approximately 11,000 unique species. For perspective, the biodiversity of freshwater fishes is estimated to be around 800 unique species in North America! In addition to their tremendous biodiversity, macroinvertebrates play critical roles in stream and river ecosystems. While many folks are likely aware of their contribution to fish production, they also serve as important conduits of energy between basal stream resources (i.e., algae, aquatic macrophytes, leaf litter) and riparian song birds and bats among other organisms.
Aquatic macroinvertebrates are preferred over other freshwater organisms as bioindicators because they possess many of the hallmark traits of good bioindicators. Specifically, they: 1. Are relatively long-lived and thus integrate conditions through time and space; 2. Are ubiquitously found in perennial stream systems; 3. Exhibit a variety of life history strategies (e.g., mode of respiration, reproductive output, feeding strategies) which can be used to discriminate among causes of impairment; and 4. Can be sampled and identified in an efficient and cost-effective manner. For example, an unimpaired Rocky Mountains stream or river segment can contain more than 60 unique taxa representing a range of habitat preferences and life history strategies. This taxonomic and functional diversity can capture the myriad of responses to different stressors and disturbances such as fine sediment, metals, nutrients and hydrologic alterations.
Whereas regulators traditionally conducted chemical assays and directly measured physical parameters of stream systems (e.g., thermal regime, salinity, nutrients), bioindication uses macroinvertebrates, periphyton or fishes to assess the cumulative impacts of both chemical pollutants and habitat alterations over time. Consequently, bioindication is fundamentally different than classic measures of environmental quality and offers numerous advantages. First, bioindicators add a temporal component corresponding to the life span or residence time of an organism in a particular system (~ 1 year for many macroinvertebrates), allowing the integration of current, past or future environmental conditions. In contrast, many chemical and physical measurements only characterize conditions at the time of sampling, increasing the probability of missing sporadic pulses of pollutants. Furthermore, bioindicators are able to capture indirect biotic effects of pollutants, while many physical or chemical measurements cannot. Clearly, a pipe dumping phosphorus-rich sewage into a lake will adversely impact the ecosystem. Intuitively, we may guess that excess phosphorus directly increases growth and reproduction of some species. Chemical measurements, however, may not accurately reflect a reduction in species diversity or how growth and reproduction of other species may decline due to competitive interactions.
The numerous benefits of using aquatic macroinvertebrate as bioindicators of freshwater biological integrity have spurred legislative mandates for their use in numerous states and the US Environmental Protection Agency (USEPA) is working to develop numeric criteria by which macroinvertebrates could be used to place streams on the USEPA’s 303(d) list of impaired waters. Yet, bioindicators are not without faults. Like the canaries in the coal mine, we rely upon the sensitivity of some bioindicators to function as early-warning signals. In some instances, we cannot discriminate natural variability from human impacts; thus, limiting bioindicators’ applicability in heterogeneous environments. Accordingly, populations of indicator species may be influenced by factors other than the disturbance or stress (e.g., disease, parasitism, competition, predation), complicating our picture of the causal mechanisms for change. Finally, the overall objective of bioindicators is to use a single species or group of species to assess the environment and how it changes over time; yet, this can be a gross oversimplification of a complex system. Consequently, practitioners should not view chemical, physical and biological monitoring as mutually exclusive, but as complementary and necessary for accurate stressor identification.
Like all management tools, we must be conscious of its flaws; however, the limitations of bioindication are clearly overshadowed by their benefits. From a heuristic perspective, bioindicators are a culmination of all ecological knowledge from cells to ecosystems. They bring together information from the biological, physical and chemical components of our world that manifest as changes in individual fitness, population density, community composition and ecosystem processes. From a management perspective, bioindicators inform our actions as to what is biologically sustainable and what is not. Without the moss in the tundra, the cutthroat in the mountain stream and the canary in the coal mine, we may not recognize the impact of our disturbances, before it is too late.