Biotic Integrity - Measures of Stream "Health"
Chances are you have probably heard about "stream health", environmental monitoring, the Water Action Volunteers (WAV) program, or an index of biotic integrity. In fact, today, many "citizen scientists" participate in the collection of aquatic macroinvertebrates in such projects like the Wisconsin SWIMS database (Surface Water Integrated Monitoring System). There are a number of multimetric indices that are ways that scientist determine "biotic integrity", a rather ambiguous concept as we will see. But in essence, biotic integrity is a measure of how close to "pristine" a stream reach is. Pristine or "pre-colonial settlement condition" is pretty difficult to quantify but these measures are generally a pretty reliable and informative piece of the picture.
Mathematically, indices of biotic integrity (IBI) are multimetric indexes meaning that "the number" is the sum of a number of metrics which each individually assess a different component of the biological community and together they present a measure of "stream health" - an imprecisely defined term, to be sure. The sum, typically scaled to where 100 is "the best", typically means the sites that are most similar to "pre-settlement" conditions. Often "100" is set as the best streams ("reference streams") in the area, not necessarily pre-settlement conditions as those are nearly impossible to determine in most cases. And, how do I put this nicely? IBI's are largely made for morons, you know, politicians. I kid - a bit - but it they are rather coarse and imperfect measures that are used to communicate to science in a rather "science-lite" sort of way. Small differences in numbers among streams or reaches or year to year may or may not mean a hell of a lot. Yet, none of this is to say they are useless. They do paint a pretty good picture of stream health - particularly if data are collected over a long time period where they can be used to detect improvements or degradation over long time periods. And often the individual metrics themselves tell us as much as does their sum.
IBIs are much easier to communicate to non-scientists than the much more complex statistics that are typically used by community ecologists. I have given a talk or two with a lot of multivariate statistics in it and even among those with some science background, their eyes glaze over a bit. Trying to communicate ordination and classification techniques to those not trained in statistics is nearly impossible. Giving them a map with a bunch of IBI scores is a much more useful way to communicate very similar information. This may be the greatest strength of IBIs - they are a way to communicate complex ideas in an understandable and useful way.
History of IBI's
The idea of measuring - or needing to measure - the health of a stream comes from the Clean Water Act (CWA) enacted in 1972. The CWA requires that we maintain the physical, chemical, and biological integrity of streams. The goals of the CWA were quite ambitious and they have not all been reached. The legislation states that all U.S. waters would be fishable and swimmable by 1983, that there would be zero water pollution discharge by 1985, and that the law would prohibit discharge of toxic amounts of toxic pollutants. Certainly we are some distance from these ambitious goals but between the CWA and the Clean Air Act that helped reduce acid precipitation, our waters are demonstrably less polluted then before these pieces of legislation. After all, the Cuyahoga River is no longer starting on fire.
Fish IBI's were first created by James Karr in 1981 in Illinois and used fish assemblages (communities) to assess the health of streams in a highly agricultural area. IBIs came about because of the CWA and the fact that physical and chemical measurements can be imperfect and do not necessarily correspond with biotic communities. The idea behind the IBI is that we can quantify community measurements that are indicative of healthy communities. Metrics are then created as a way to score how "integral" the site is based on the samples taken to represent that site. Metrics are typically based on the number of species in a particular category or the percent of all individuals with a particular characteristic. For example, a lack of or a low percentage of non-native species would score highly for a metric based on non-native species. And a site with a relatively high proportion of omnivores - which is an indicator of poor biotic integrity - would score low for a metric that measured % of the community that are omnivores. IBIs are created by determining which characteristics best represented the "ideal" conditions for streams in that region.
IBIs were first created to assess the biotic integrity of the agricultural streams of Illinois using fishes as bioindicators. Karr used metrics like the number of species (more species = higher score), species richness of particular taxa, the proportion Green Sunfish - an indicator of poor water quality, and a number of metrics such as how many fishes were captured, what proportion were omnivores, and the proportion of those with diseases, tumors, fin damage, and other anomalies. As you can see, some metrics indicate health whereas other metrics indicate environmental degradation and are scored inversely to their proportion (high incidents of physical anomalies return a low metric score). Since Karr's first IBI, biologists have developed IBI in other places and using other taxa like aquatic macroinvertebrates, algae, and aquatic plants. The idea has spilled over to terrestrial environments where insects, birds, and other taxa are used as biological indicators.
There are generally a number of categories that metrics are divided into to best assess the biotic integrity of a site. The idea behind multimetric indices is that the metrics are chosen to be diverse to best represent what ideal conditions for the area that the IBI was created for. IBIs may be developed for small regions, states, or larger geographic regions. And the metric chosen depend upon the system being assessed. For example, below is an example of how for coldwater streams, high species richness is an indicator of degradation. Trout streams that have the highest biotic integrity have Brook Trout (native) and sculpin but few - if any - other fish species. As streams warm, they tend to have more species. Or, as the figure below shows, as streams are improved and get colder, species richness declined. For more on how Driftless streams have improved over time, read The Re-Birth of the Driftless post.
Figure from Lyons et al. 1996.
What Fish and "Bugs" Tell Us?
IBIs came about largely because the CWA mandates that biotic integrity was maintained. Without a definition of what biotic integrity meant in the legislation, it needed to be defined. And it was important that it was defined because biotic communities can tell us something that habitat assessments and water quality sampling may miss. Because macroinvertebrates and particularly fishes are long-lived, they may tell us about events such as acidic snow melt, a spate of pesticides, or a drought or warm period that may affect the biota but may have been missed in our water quality samples. We talk about the biota as being integrative over time, meaning that their communities can inform us about relatively recent events. And with fishes being longer lived, they may inform us about events in the less recent past.
Source: Freund and Petty 2007
The figures above are the West Virginia stream condition index (SCI) based on macroinvertebrate assemblages and the Mid-Atlantic Highlands fish IBI plotted against an X-axis which is a measure of water quality variables that are indicative of acid mine drainage (AMD). PC1 factor scores increases as conductivity, total dissolved solids, acidity, and dissolved metal ions (aluminum and iron, in particular) that AMD contributes to streams increases. The large message from this figure is that "bugs" communities are better indicators of AMD impairment (they have a much better goodness of fit). One of the explanations for this are that bugs are less mobile and unable to avoid short-term impairments whereas fish can move to avoid short-term impairments. There is also some evidence that fishes are more susceptible to fragmentation because they are more mobile or their mobility is dependent upon streams where as insects may disperse through the air. And within the fish community, less mobiles species - like sculpin - are better indicators of local water quality and historic events. If sculpin are extirpated (locally extinct), they may not recolonize due to their less mobile nature. And more mobile fishes with larger home ranges - like trout and Smallmouth Bass - may more quickly repopulate a reach after a disturbance.
The figure above illustrates that fish and macroinvertebrate indices provide somewhat different information about the health of streams. Both indices were scaled so that the best site = 100 and all sites were scaled from there. It is noticeable that macroinvertebrate scores tended to be higher and show less impairment than the fish IBI shows. The best explanation for this is that many of our sites were unimpaired reaches that were upstream or tributaries of streams that were severely impacted by acid mine drainage. Many of these streams had depressed fish assemblages because some of the more mobile species had been extirpated. Another interesting finding was that almost all of the sites where fish scores were higher than the macroinvertebrate SCI score were streams that we sampled near their confluence with the Cheat River, a source of large river fish species. This is evidence of the importance of connectivity within watersheds.
Use of IBI's in Wisconsin
Wisconsin has a number of IBIs that are used. I used a figure above from a Wisconsin coldwater IBI publication that shows that increased species diversity in coldwater streams is an indicator of warmer streams. The coldest of streams have very low diversity as few fish species have evolved to live in the stressful conditions that exist in cold water. The state and region has a number of IBIs for fishes and macroinvertebrates - and other taxa - that are used to assess the health of ecosystems. One of the most interesting and exciting ideas is that stream monitoring is often done by citizen scientists. Water Action Volunteers (WAV) are often part of environmental groups, Trout Unlimited chapters, and other interested citizens. The IBIs used by citizen scientists are often simplifications of macroinvertebrate IBIs used by scientists that are trained in the taxonomy of the "bugs" they are sampling and whose samples are often more quantitative (a fixed amount of stream bottom is in each sample). The citizen science measures may be a little coarser in resolution but they have exponentially increased the amount of streams that can be sampled which had improved our knowledge of the biotic integrity of streams in Wisconsin.
The Take-Home Message
Indices of biotic integrity are a useful tool to understand both how communities respond to water quality and habitat conditions and they can provide insights that habitat and water quality may not provide. The longevity of macroinvertebrates and particularly of fishes means that they can be used to assess long-term conditions that water and habitat sampling may not capture. And fish and "bugs" inform us about different parts of the bigger picture. Fishes are indicators of regional conditions whereas macroinvertebrates are more informative about water and habitat quality (particularly sedimentation). IBIs also are useful as they are a way to communicate difficult ecological concepts to people that are not familiar with the community ecology measures and multivariate statistics typically used by scientists. IBIs give citizen scientists a great opportunity to get involved and make a meaning contribution to science and conservation.
(I think I did a pretty good job of not digging too deeply into the weeds...but please let me know if that was understandable enough.)