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Determining the most harmful Non-indigenous Aquatic Species and each species’ “Harm Score”
To determine the Most Harmful Nonindigenous Aquatic Species (NAS) within states West of the 100th Meridian with significant Bureau of Land Management (BLM) acreage, Wildlife Forever asked 12 professional biologists to rank their Top 20 most harmful NAS out of a list of 104 NAS provided by the U.S. Geological Survey.
The western “BLM States” analyzed are 11 contiguous states that have large-scale BLM land holdings: Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming.
Each identified species was assigned points based upon the biologists’ ranking responses. If a biologist ranked a species as the most harmful, that species was awarded 20 points, if ranked second most harmful, it was awarded 19 points, and so on.
After tallying the points from all the ranking surveys, a list of the 25 most harmful NAS was created for what Wildlife Forever calls the “Battle of the Bads.” To determine the final ranking order, we asked the biologists to re-rank the NAS list, but this time choosing from the 25 species selected by the first round of surveys.
The final “Battle of the Bads” NAS list of the Top 25 most harmful species, was then assigned a “Harm Score” (HS) per species based on their rank within the Top 25. The most harmful NAS has an HS of 25, the second most harmful an HS of 24, and so on. On the next page is the final Battle of the Bads list as a result of the second round of NAS ranking responses.
|25 Most Harmful NAS|
|1||Quagga / Zebra Mussels||25|
|4||New Zealand Mudsnail||22|
|21||Armored Sailfin Catfish||5|
|25||Red Swamp Crayfish||1|
Determining the Prevalence of each species
From maps obtained through the U.S. Geological Survey, Wildlife Forever was able to determine the locations of each NAS. The maps were generated from the Nonindigenous Aquatic Species Database and authored by Justin Procopio under the supervision of Wesley Daniel of the USGS.
The maps contain the geographical area of each NAS overlayed onto a map of the U.S. Congressional Districts within states West of the 100th Meridian.
Urban congressional districts, like Nevada’s District 1, home to Las Vegas, were not included in this analysis. In total, 56 predominantly rural congressional districts were analyzed.
For each district and each species, it was loosely estimated whether the species was prevalent to a ‘major’ or ‘minor’ extent within a given Congressional District. A species’ full Harm Score was assigned in cases of ‘major’ prevalence and half a species’ Harm Score in cases of ‘minor’ prevalent.
Yellow represents areas where the Common Carp has been introduced
Determining each Congressional District’s “Infestation Score”
A district’s “Infestation Score” is the sum of all the species prevalent in the district, weighted by each species’ “Harm Score.”
Again, during each district’s calculation, if a species was judged to be a major infestation, its full HS was assigned. If it was estimated to be a minor infestation, only half of its HS was assigned, rounded up to the nearest whole number.
For example, Nevada’s District 3 has an Infestation Score of 74. The NAS present in the district are:
- Quagga/Zebra Mussels (HS = 25)
- Eurasian Milfoil* (HS = 24/2 = 12)
- Asian Clam* (HS = 16/2 = 8)
- Bullfrog (HS = 11)
- Common Carp (HS = 10)
- Armored Sailfin Catfish* (HS = 5/2 = 3)
- Western Mosquitofish (HS = 4)
- Red Swamp Crayfish (HS = 1)
* indicates a minor infestation
Nevada District 3’s Infestation Score = 25 + 12 + 8 + 11 + 10 + 3 + 4 + 1 = 74
A high Infestation Score means NAS are a major problem in that district. A low Infestation Score shows a relatively minor infestation status in the district.
The report authors recognize that the above scoring formula is inexact, however, it is hoped that the report provides a jumping off point for dialogue on NAS prevalence on BLM lands in the West.
Graphing each state’s Congressional Districts by Infestation Score
To illustrate which districts in each state have the worst NAS problems, a bar graph was made for each state. The graphs visualize the Infestation Score of each state’s Congressional Districts. See page 7.
Ranking all of the analyzed Congressional Districts
To determine which Congressional Districts are facing the worst NAS problems, we ranked them by Infestation Score. See page 11.
Ranking the NAS by Total Infestation Score
To determine which NAS are most prevalent and problematic, we calculated each species’ Total Infestation Score.
The Total Infestation Score is the sum of each species’ Harm Score multiplied by its prevalence.
For example, the Rusty Crayfish, which has an HS of 23, is prevalent in 5 districts, and somewhat prevalent in one district. Its Total Infestation Score = 23 x 5 + 12 x 1 = 127
Alternatively, the New Zealand Mudsnail, which has an HS of 22, is prevalent in 35 districts and somewhat prevalent in one district. Its Total Infestation Score = 22 x 35 + 11 x 1 = 781
Even though the biologists consider the Rusty Crayfish more harmful than the NZ Mudsnail, it is much less prevalent than the NZ Mudsnail, giving it a lower Total Infestation Score.
While the Total Infestation Score is not rigorously scientific, it does indicate which NAS natural resource agencies may want to focus on.
One drawback of the Total Infestation Score, is that the districts vary greatly with size. For example, both Montana and Wyoming only have one congressional district, despite their very large geographic area. Whereas California has many more districts, some of which are quite small. Thus, our Battle of the Bads rankings skew a bit towards species that are problems to a greater or lesser extent in states with more districts.