BIOLOGY - Animal Species Richness

How many different species of birds, fish, aquatic macroinvertebrates and mussels have been found in each watershed?

Why is this important for biology?

The Animal Species Richness Index provides important information for two aspects of natural resource management. First, species richness, the number of species, provides an indicator of the health of an ecosystem and can be monitored for changes over time and, secondly, the index identifies areas of unusually high biological richness across multiple taxonomic groups, an important aspect used in conservation planning.

Aquatic and terrestrial systems have assemblages of organisms that developed in response to a range of abiotic to biotic conditions operating at a range of spatial and temporal scales (Ricklefs 1987, Gaston 2000, Jackson et al. 2001, Chase 2003).

Animal Species Richness Health Scores

Biology Index - Animal Species Richness (major scale)

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The number of species represents one characteristic of an assemblage that can reveal the presence of natural and human-induced changes, both episodic and chronic, and reflect the effect of such change on an assemblage over time (Gergel et al. 2002, Mason et. al. 2005, Hansen et al. 2005, Yagow et al. 2006, Flynn et al. 2009). A decline in species richness at a local scale is an indicator of changes in aquatic or terrestrial habitat (Rapport et al. 1985). Species richness is also influenced by ecosystem processes (Loreau et al. 2001, 2003) and is an important component of monitoring for ecosystem health (Pereira and Cooper 2006). The importance of richness and the continued decline in the number of species (Novacek and Cleland 2001, Pimm et al. 2006) as a result of the steady or increasing human-alterations of habitat (Millenium Ecosystem Assessment 2005) dictates that species richness be used as a biological indicator.

At large spatial scales (e.g., global) species richness patterns tend to coincide across various taxonomic groups (Gaston 2000); however, this pattern is not perfect and tends to weaken at smaller spatial scales (Prendergast and Eversham 1997, Maes et al. 2005, Schouten et al. 2009). Schouten et al. (2009) observed that plant richness was not similar to four other biological groups examined. As a result of these exceptions in the diversity patterns, multiple taxonomic groups representing aquatic and terrestrial environments (Rodrigues and Brooks 2007) should be examined.

Creating the Index

Input Data

Calculating the index

A Species Richness Index (SRI) is a summary of the richness of several taxonomic groups across watersheds. The SRI was created for: mussels, aquatic macroinvertebrates, fish, and birds. A count of live specimens of mussel species was recorded during Minnesota Department of Natural Resources (MN DNR) statewide mussel surveys. Aquatic macroinvertebrate species or genus counts and fish species counts were recorded during Minnesota Pollution Control Agency (MPCA) streams surveys between 1996 and 2006. Bird species counts are recorded from the annual breeding bird survey (BBS) from 1995 to 2008. The index was developed in three steps:

1. Calculate mean watershed value for each species group,
2. Score each species group from 0 (no species present) to 100 (highest mean watershed value), and
3. Combine the four ranked scores into one index score per watershed.

The range of values of the mean species count per watershed was:

20-50  Aquatic macroinvertebrates
0-12    Mussels
62-97  Birds
1-27    Fish

These values were scored 0-100 and combined into a final Animal Species Richness index value. The combined scores were also scaled 0-100 to give a final species richness score.

Index Results

Interpretation of results

After combining the species richness values from four different taxa into a single score for each watershed, the resulting range of values was quite narrow.  Different patterns of richness exist for each sub-index, so the extreme values are dampened when they are combined.  Reviewing each sub-index is important to any discussion of species richness in a specific watershed.

In general, the combined results show a greater species richness score in watersheds found in the upper reaches of the various river basins.  This includes the upper reaches of Red, Minnesota, and Upper Mississippi River basins, as well as the St. Croix Basin.  

The aquatic invertebrate results show a greater richness in the Rainy River Basin of northern Minnesota, while the bird richness results show a definite increase in species number on an west to east line through central Minnesota.

Animal species richness is often correlated with the availability of diverse habitats (Meffe, Carroll and contributors 1997). Most watersheds in Minnesota were ranked as moderately to lightly impacted based on the combined species richness scores with the highest scores in watersheds across the central portion of the state. Meanwhile, the scores for the individual taxonomic groups indicated much higher variability:

Aquatic invertebrates counts ranged from moderate to high with the highest numbers in the northern portion of the state and a mix of scores across the rest of the state with little apparent pattern.
• The number of fish species varied widely across the state with a relatively high number of species in many watersheds.
Birds demonstrated the lowest range of scores with the highest number of species in the central portion of the state.
• The number of mussel species was highly variable across the state. The highest numbers of mussels were found in watersheds along the St. Croix and Mississippi basins, which have some of the highest species richness in the United States outside of the Tennessee River basin. However, mussels may have always been rare in some areas in Minnesota, such as the northeast, thus there may be uncertainty whether mussels are declining, and may be classified as “rare”.

A reduction in the number of species may indicate a loss of access or connectivity in some part of the system (Meffe, Carroll and contributors 1997). Areas with structurally simple habitats, such as areas with extensive agriculture or urban areas, generally support fewer terrestrial or bird species than complex habitats, such as forested areas. For example, a forest has a number of structural layers including: grasses near the land surface, shrubs, and the canopy of trees, which together provide a number of diverse habitats for birds.

Relationship to other health components

Water quality

Degraded water quality directly influences the richness of aquatic species, often reducing the number of sensitive fish and invertebrate species able to survive in the system. Bird species that feed on aquatic plants and animals will likewise experience reductions in species richness as their preferred food sources become more limited.


Loss of connection between and within terrestrial and aquatic habitats, affects species richness and composition by limiting access to the full range of necessary habitat types.


The geomorphic setting plays a key role in determining the expected species richness of a watershed system. As watershed assessments are refined, regional target values for expected results will become even more important.


The richness of aquatic species in a watershed is closely related to the natural flow regime to which those species evolved. An altered system may have less diverse habitats, a more limited stream flow pattern and limited access to important refugia. The richness of bird species may also be reduced because of alterations to wetlands, floodplains and riparian habitats that may accompany hydrologic modifications.

Supporting Science

Scientific literature support

Biological diversity, i.e., the variety of living organisms and the ecological processes that support them, is an important concept in ecological study (Meffe, Carroll and contributors 1997). The conservation of biological diversity generated a movement and a new scientific discipline, Conservation Biology, in the early 1980s (Meffe, Carroll and contributors 1997). Diversity measures are often used to compare communities and to assess a change in richness related to a pollutant or conversion from one land use to another, such as draining a wetland to increase agricultural acreage.

Biological populations are best assessed using multiple taxonomic groups (Gore et al. 2001, Wang et al. 2006). Biological indicators developed for a specific taxonomic group (e.g. fish or macroinvertebrates) are sensitive to different human-induced changes and spatial patterns, thus at least two different taxonomic groups should be assessed when evaluating a watershed (Karr 1981, Brazner et al. 2007, Flinders et al. 2008). For example, macroinvertebrates (both aquatic invertebrates and mussels) are relatively immobile and live in close contact with both the sediments and the water column, thus reflecting impacts at a local scale (Weigel et al. 2003, Yagow et al. 2006, Brazner et al. 2007, Flinders et al. 2008, Stepnuck et al. 2008). Fish, on the other hand, tend to indicate watershed-scale impacts because they can move in response to changes into other areas of the watershed. Therefore, fish can tell us something about the stream and together with macroinvertebrates, provide more specific information on the causes and extent of stress (Flinders et al. 2008, Norton et al. 2000). Aquatic organisms can be used to assess changes in aquatic systems, whereas the number of bird species provides information about changes in terrestrial systems.

Confidence in index

This index is strongly supported, resting on a well-developed theoretical foundation that began in ecological research near the early years of the last century, with strong theoretical support in the technical processes of generating the base data and index and empirical tests of its application.

The advantages of the Species Richness Index are that it summarizes multiple taxonomic groups in a single number, and can be effective as a communication tool. When the same objectives and variables are used, the index can be used to convey relative differences in habitat quality between sites and over time. For example, care must be taken when comparing among sites because of natural variability in water or soil quality constituents. Data to update scores should be available in the future because the DNR mussel program has a monitoring component, the MPCA assesses fish and macroinvertebrates in watersheds every 10 years and the BBS is conducted annually, is well supported, and expected to continue.

Although fish, macroinvertebrates, and mussels have been assessed broadly across the state, some watersheds have fewer assessments than others and a small number of watersheds have not been assessed. As well, most fish and invertebrate assessments have been conducted in small streams within the 81 watersheds, because small streams represent 80-90% of stream miles within watersheds (Leopold et al. 1964) and most are “wadeable” by biologists. Thus, the condition of many larger rivers can only be inferred for fish and macroinvertebrates.

Species of fish, macroinvertebrates, and mussels are influenced by temperature. For example, fish are classified as “coldwater, “coolwater”, or “warmwater” based on the water temperature of the systems in which each group is found. The species richness found in coldwater is usually very low, with the most species found in warmwater, and an intermediate number in coolwater (Lyons 1996). A coldwater stream with no impairments in southeast Minnesota may only have five species (Lyons et al. 1996), whereas a warmwater stream in central Minnesota may have 25 species. Thus, species richness is related to the location of a watershed and is not consistent across the state.

Due to the wide variety of equipment and methods used for assessing fish populations in Minnesota lakes, comparable fish species data from lake surveys was not incorporated into this index. If comparable data for species richness in lakes becomes available, an additional metric could be added to this index.

The numbers of bird species has been collected systematically in Minnesota since 1968 by both agency biologists and volunteers, and compiled into the BBS. Thus, the BBS provides information about the trends in bird species for about 50 years. However, because breeding birds migrate to other areas, declines in the number of many species reflect a combination of changes in habitat in Minnesota and in the wintering grounds.

Next Steps

Future enhancements

The scoring for the Animal Species Richness index could be improved by having an expected species richness value for each taxonomic group for each watershed. There is a natural range in expected species richness across Minnesota that is not accounted for by the current index scoring, such as for fish species in coldwater streams.

Additionally, including vegetation or plant species richness would also enhance this index.  However, statewide data containing comparable plant species richness values is not available at this time.  

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