Invasive Plant Guide

horses in a pasture invaded by thistles

I’m in Argentina for six weeks, and it’s been interesting observing what species invasive here are well-known to me from North America.  One of the best ways of predicting species’ invasiveness is by knowing if they are invasive elsewhere after all! The following photos are from the southern part of Buenos Aires Province and from the Costanera Sur Ecological Reserve in the city of Buenos Aires.  Other plants not shown here have included white mulberry, Morus alba, tree-of-heaven, Ailanthus altissima, two other thistle species, Carduus spp., purple starthistle, Centaurea calcitrapa, Eucalyptus species, Acacia species, and castor bean plant, Ricinus communus. Monterey pine, Pinus radiata, and green ash, Fraxinus pensylvanica are two plants invasive here introduced from North America.

A good source of information on Argentine invasive plants has been Gekko, Grupo de Estudios en Conservacion y Manejo.

Centaurea solstitialis, yellow star thistle

Hedera helix, English ivy, under invasive tree species

Populus alba, white poplar

Cytisus monspessulanus, French broom

Dipsacus fullonum, common teasel (in Buenos Aires Costanera Sur Reserve)

Arundo donax, giant reed (Buenos Aires, Costanera Sur Reserve)

Finally a study specifically addresses why some invasive plant studies show a decrease in biodiversity, and some show an increase or no change in biodiversity.  Not surprisingly, scale matters.  If you walk through a forest heavily invaded by non-native shrubs, you often find no plants growing under the shrubs.  But in the context of the landscape, you can still find the native species that used to grow in that forest.  On the small scale, invasive plants decrease biodiversity, but at larger scales, biodiversity often remains unchanged.  The authors point out that some ecosystem services that occur at smaller scales such as water filtration, are still likely to be affected by plant invasions.

K. I. Powell, J. M. Chase, T. M. Knight. 2013. Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships. Science: 339 (6117): 316  DOI:10.1126/science.1226817

Maryland has posted its regulations for assessing plant invasiveness.  These regulations will be used to develop a two tier list of invasive plants.  Tier 1 plants will be banned from sale, and Tier 2 plants will be labeled as invasive.  Find out more at http://ipetrus.blogspot.com/2012/11/regulation-of-invasive-plants-in.html