8172

• UCREFRP Catalog Number: 8172
• Author: Jacobi, G. Z., J. E. Sublette, S. J. Herrmann, D. E. Cowley and M. D. Hatch.
• Title: Final Report
• USFW Year: 1998.
• USFW - Doc Type: Investigations of an Index of Biotic Integrity in New Mexico.
• Copyright Material: NO

Preliminary IBI Statistics
As part of the analyses for this report, the
potential utility of several summary
statistics for development of an IBI was
investigated. These statistics include: (1)
number of unique chironomid species, (2)
frequency distributions for number of
chironomid species and number of fish
species, (3) additional investigations into
number of fish species versus elevation, and
(4) the potential utility of an additional
environmental variable, distance from
headwaters.
The number of unique chironomid species
per site was regressed onto the total mmaber
of chironomid species per site (Figure 23).
The significant linear regression (R2=0.70)
indicated that about one-fourth of the
chironomid species at a site can be expected
to be found nowhere else. This level of
biotic site-specificity is probably unequaled
among any other group of animals. It
underscores the potential utility of
chironomids as indicators of habitat quality.
The strong linear relationship suggests that
the total number of chironomid species, the
number of unique chironomid species, or
both, could provide useful summary
information for inclusion in an IBI. It also
suggests that for a given number of species,
sites that lie above the regression line may
prove to have higher integrity than sites that
fall below the line. In anticipation of
inclusion of chironomid species in an IBI, a
manuscript handbook of keys and
illustrations of the pupae of New Mexican
chironomids is being prepared (Appendix
4).
A frequency distribution for number of
chironomid species was produced where
numbers of species were grouped into
ranges of 10 (Figure 24). Assuming that
the number of chironomid species is
correlated positively with environmental
integrity, the histogram indicates that
integrity at most of the upper Rio Grande
sites is moderately to highly degraded, and
that only a few sites have a high diversity of
chironomid species. The disjunct nature of
the distribution suggests that if the number
of chironomids at a site were to be included
in an IBI, a threshold indicator function
might be appropriate. For example, given
the break in the distribution above .30
species at a site, one might give a site a
higher integrity score if it has 30 or more
chironomid species. The distribution of
chironomid subfamilies with respect to
elevation (Box 3) suggests that a metric
based on subfamily might have utility in an
IBI.
The frequency distribution for benthic
macroinvertebrate taxa (Figure 25) suggests
bimodality, i.e., one grouping of sites with
10 to 20 benthic macroinvertebrate taxa and
a second grouping of sites with more than
20 benthic macroinvertebrate taxa. The
break in the distribution suggests that sites
having more than 20 benthic
macroinvertebrate taxa may have better
integrity than sites having fewer taxa.
Figure 26 provides a histogram for the
frequency distribution of number of fish
species per site. Two headwater stream
sites in the upper Rio Grande drainage had
only one fish species, a condition consistent
with the generally depauperate fish fauna in
headwater systems (e.g., Stewart and Loar,
1994). The remainder of the distribution
displayed bimodality. The modes in the
distribution are consistent with the
observation from cluster analysis of a
mid-elevation group of sites and a lower
elevation group of sites. The middle
3?0
19