Monday, April 15, 2013

Diversity Inhibits Invasive Species

Bull Thistle
We know that diversity is important for the survival of many species which are threatened.  Now there is some evidence that plant diversity in an ecosystem can inhibit the spread of some invasive species.

A study reported at looked at the growth of bull thistle (Cirsium vulgare) and smooth brome (Bromus inermis) in 24 experimental prairie plots.  Half of the 3/4 acre squares were planted with 8 grass and 7 wildflower species.  The other half were planted with 100 species.  They found that the more diverse test plots contained less of these invading plants.

Bull thistle and other invasive thistle species are difficult to control due to the thousands of light feathery seeds the wind disperses over the field.  In many states like Kansas, the persistent presence of them in your field can lead to a daily fine.  Controlling them requires early spraying of the flat rosettes and later pulling or digging them out before they go to seed.  Anything that reduces their spread is welcome.

A separate pilot project studied these areas for the presence of poison hemlock.  Walking transects and counting the number of plants one meter on either side of the line, they came up with the rather impressive figures illustrated below.

Hemlock stands tend to be dense and choke out any other growth as they reach their 6 foot plus height.  They are poisonous to cattle (just ask Socrates) but for our purposes, they take over the landscape, eliminating other native species.

Poison hemlock stand
As I am married to a woman addicted to the removal of invasive species, I am "encouraged" to join her for a walk the fields with a sprayer (if I want dinner that night), looking for small hemlock and thistle before they get a head start on us.  The above is good news with a strong caveat.  It takes a lot of work and money to convert old fescue fields to native species.  Knowing what a typical mix of warm season grasses and forbs costs, I can only shudder to think what a field seeded with 100 species would cost.

The other obvious message is the importance of maintaining diverse species where they occur naturally.  While "nature abhors a vacuum", most invasive plants love one.