Movement of invasive aquatic plants through the horticultural trade: the example of Minnesota (US)
Invasive aquatic plants may be voluntarily introduced as a commodity itself (for aquaculture, aquaria, etc.) or involuntarily introduced as a contaminant with other aquatic plants. Studies were done in Minnesota (US) to evaluate the risks of introducing invasive aquatic plants both accidentally (as import contaminants) or deliberately (sales of aquatic plants, including prohibited species). Aquatic plants were ordered from vendors across the USA from May and September 2001 to determine the prevalence of movement of invasive plants into Minnesota via horticultural trade. 34 orders were made, which included orders for prohibited species in order to verify that regulations are being implemented. Ordered plants were placed in appropriate containers in a greenhouse and were then identified to verify whether the plant received was the plant listed on the invoice. Contaminants, such as plants (seeds), animals, algae, moss, or fungi, which were found associated with the ordered plants or their packaging, were also recorded and identified. A total of 681 individual plants (corresponding to 123 species) were received, and were composed of the following types: 66 emergent plants, 16 submersed plants, 34 floating leaved plants and 6 free-floating plants. The ordered plants had additional plants associated with them upon arrival in the following proportions:
|
Plant types
|
Number of ordered species
|
% of species with contaminants
|
|
Emergent
|
66
|
62
|
|
Submersed
|
16
|
100
|
|
Floating
|
6
|
100
|
|
Floating leaved
|
34
|
66
|
The following aquatic plants species were identified as contaminants of the 123 species originally ordered:
|
Contaminant species
|
Family
|
Origin
|
% of orders contaminated
|
|
Lemna minor
|
Lemnaceae
|
Cosm.
|
50
|
|
Azolla caroliniana
|
Azollaceae
|
N-Am.
|
30
|
|
Unknown
|
/
|
/
|
24
|
|
Spirodela punctata
|
Lemnaceae
|
N-Am.
|
20
|
|
Utricularia sp.
|
Lentibulariaceae
|
/
|
10
|
|
Spirodela polyrrhiza
|
Lemnaceae
|
Africa, Eur., Asia
|
10
|
|
Lemna trisulca
|
Lemnaceae
|
Eurasia
|
5
|
|
Wolffia sp.
|
Lemnaceae
|
/
|
2
|
|
Sphagnum sp.
|
Sphagnaceae
|
/
|
2
|
|
Myriophyllum sp.
|
Haloragaceae
|
/
|
2
|
|
Cabomba caroliniana (EPPO list of IAS)
|
Cabombaceae
|
S-Am.
|
2
|
|
Salvinia molesta
|
Salviniaceae
|
S-Am.
|
1
|
|
Egeria densa (EPPO list of IAS)
|
Hydrocharitaceae
|
N-Am.
|
1
|
|
Potamogeton sp.
|
Potamogetonaceae
|
/
|
1
|
|
Potamogeton crispus
|
Potamogetonaceae
|
Cosm.
|
1
|
|
Hydrilla verticillata
|
Hydrocharitaceae
|
Af.
|
1
|
|
Ceratophyllum demersum
|
Ceratophyllaceae
|
Cosm.
|
1
|
|
Marsilea sp.
|
Marsileaceae
|
/
|
1
|
|
Ricciocarpus natans
|
Ricciaceae
|
Trop.
|
1
|
|
Leersia oryzoides
|
Poaceae
|
N-hemisphere
|
1
|
Misidentified plants were found in 18% of the orders. The introduction of invasive plants as contaminating seeds was a minor pathway compared to the sale of prohibited plants and the unintentional inclusion of invasive plants (the whole plant being the contaminant). The inadvertent sending of extra species occurs more frequently than sending misidentified species. During this study, prohibited aquatic plants were easily purchased (Hygrophilla polysperma, Acanthaceae; Althernanthera sessilis, Amaranthaceae), indicating that present federal and state regulations are not adequate to stop the movement of plants into Minnesota. The following precautionary measures could be usefully applied by vendors to reduce the risk of introducing invasive species: fewer plant species in growing tanks, high pressure rinsing before packaging, and removal of soil.
Sources
Maki C, Galatowitsch (2004) Movement of invasive aquatic plants into Minnesota (USA) through horticultural trade. Biological conservation 118, 389-396