Sampling Records
Occurrence records were collected from nine sources (Table 1). The U.S. Geological Survey’s datasets provided the majority of records (74.7%), followed by Early Detection & Distribution Mapping System (18.6%), and the Great Lakes Invasive Network (3.8%). After the removal of duplicates, the final database had 120,511 records for 143 non-native species from eight taxonomic groups (Table 2). The earliest record in the database is the sedge Carex vesicaria, first recorded in 1842. The full dataset then runs to 2019 and includes 38 additional years relative to Jacobs & Keller (2016). Of the eight broad taxonomic groups (Table 2), non-native plant species occurred in highest numbers (n = 59 species), followed by fishes (n = 46 species), and mollusks (n = 16 species). Common carp (Cyprinus carpio) was the most often recorded species in Illinois (n = 71,536 records). Four carp species (C. carpio, Hypophthalmichthys molitrix, Ctenopharyngodon idella, and Hypophthalmichthys nobilis) account for 78.96% of all AIS records. Although carp species are widely distributed in Illinois, their high record count is partly a result of intensive sampling efforts of the USGS Long Term Resource Monitoring – Fish (Table 1) program, which primarily sampled in the southern portion of the Mississippi River and targets fishes. This program also reports each fish as a record even when multiple fish were captured during a single effort. Other data sources may include multiple individuals in each record.
Table 1: Data sources and the corresponding number of records used for the creation of the AIS database. Sources: Early Detection & Distribution Mapping System (EDDMapS), Great Lakes Invasive Network (GLIN), Illinois Natural History Survey (INHS), United States Geological Survey (USGS) Long Term Resource Monitoring (LTRM) and Nonindigenous Aquatic Species (NAS). Note: INHS and USGS LTRM possessed multiple databases with different management teams – the name of each sub-database is denoted next to the parent entity name (Vegsrs = vegetation stratified random sample; Vegtransect = vegetation transect sample).
Database
|
Number of Records
|
EDDMapS
|
25,776
|
GLIN
|
5,231
|
INHS - Crustaceans
|
252
|
INHS - Fish
|
1,486
|
INHS - Mollusks
|
2,264
|
USGS LTRM - Fish
|
83,609
|
USGS LTRM - Vegsrs
|
186
|
USGS LTRM - Vegtransect
|
716
|
USGS NAS
|
19,026
|
Total Records
|
138,546
|
Number of Duplicates
|
18,035
|
Final Total (Duplicates Removed)
|
120,511
|
Table 2: Taxonomic groups and the number of species in each that has been introduced and established in Illinois.
Group
|
# Introduced
|
# Established
|
Plants
|
59
|
41
|
Fishes
|
46
|
28
|
Mollusks
|
16
|
10
|
Crustaceans
|
15
|
10
|
Coelenterates
|
2
|
0
|
Diatoms
|
3
|
1
|
Parasitic Worms
|
1
|
0
|
Reptiles-Crocodilians
|
1
|
0
|
Total Species
|
143
|
92
|
Introduction and Establishment
Of the 143 non-native aquatic species that have been recorded in Illinois, 92 are established (119,769 records) and 51 have failed to establish (742 records). Species that failed to establish have generally been recorded infrequently and are native to regions that are climatically different to Illinois. For example, water lettuce (Pistia stratiotes), a floating aquatic plant native to Florida, has been recorded in Illinois XX times but has not persisted through winter to become established (USGS 2020). Likewise, the American alligator (Alligator mississippiensis), native to the southern US, has been recorded on four occasions but has not become established.
Our methods were slightly different to those of Jacobs and Keller (2016). First, we did not include records of hybrids because of difficulties with identification and the lack of genetic confirmation for these records. The Jacobs and Keller (2016) database included nine hybrids as introduced and established. Second, we removed seven plant species that were included in Jacobs and Keller (2016) because they are not considered to be obligate aquatic. If Jacobs and Keller (2016) had rejected hybrids and only included obligate aquatic species, they would have found 83 introduced and 44 established species, as opposed to the 99 introduced and 60 established species they reported. A final difference in methods is that we included species that have been recorded in the Illinois portion of Lake Michigan but not elsewhere in Illinois. This accounted for eight introduced species in our database, three of which have only been recorded after the Jacobs and Keller (2016) study, and two established species.
The number of newly introduced species per decade increased between 1842 and 2019. (Figure 1; linear regression, n = 18 decades, coefficient = 0.098, r² = 0.606, p = 0.0001). Over the same period, the number of newly established species recorded per decade also increased (Figure 1; linear regression, n = 18 decades, coefficient = 0.064, r² = 0. 0.571, p = 0.0002). A second-order polynomial regression is a good fit to the cumulative number of introductions (y = 0.004x² – 15.81x + 14,600, r² = 0. 977, p = <2.2e-16) and establishments (y = 0.003x² – 11.22x + 10,400, r² = 0. 981, p = <2.2e-16) per year (Figure 2). The average rate of new introductions was 0.803 per year (1 every 14.9 months) for the entire timespan (1842 to 2019) and 1.9 per year (1 every 6.3 months) for the most recent 30 years (1989-2019). For new establishments, the average rate was 0.517 per year (1 every 23.2 months) for the entire timespean and 1.2 per year (1 every 10 months) for the most recent 30 years (1989-2019).
The number of established species reported per year has increased over time (Figure 3). The largest number of established species reported in a single year was in 2014, when 57 species were recorded. We plotted the annual number of sampling records (a proxy for sampling efforts) against the annual number of established species recorded and observed a logarithmic relationship (Figure 3). The curve becomes approximately horizonal at around 34 established species per year, which indicates that current sampling methods and level of effort are sufficient to record approximately 34 of the 92 established species in Illinois each year.