Sunday, April 3, 2011

Beneficial Invasive Species

Having spent several afternoons with Mort Shurtz and Barb attacking dense stands of invasive Japanese Honeysuckle at Nathanael Greene/Close Memorial Park,  I was interested to find an article in suggesting that it had some ecological benefit.  Reading it shows that the devil is in the details.

Tomas Carlo and associates at Penn State University studied populations of robins and catbirds in urban and forested landscapes compared with areas of dense Japanese Honeysuckle.  There are already known to be "three to four times more fruit-eating birds such as robins and catbirds than there were just 30 years ago, especially in landscapes of high human presence."  Not surprisingly they found more of these birds in the honeysuckle where there were berries to eat.  Score one for the robins.

They then put pots of American nightshade, a locally common berry producing plant in these same areas.  They reported that fruit removal by birds from the nightshade pots was 30% greater in pots located in the honeysuckle areas.  They concluded that the dispersal of seeds was a "win-win-win for all three: the birds, the honeysuckle and the nightshades."

Here is where the problem begins.  Seed dispersal by birds occurs in the area where they eat or come to rest.  Dispersal does not equal propagation.  Birds eating in Japanese Honeysuckle thickets are both eating more of their berries, and dispersing honeysuckle and a smaller amount of other seeds in the same areas.  Aggressive honeysuckle will cover and choke out natives as well as spread laterally where roots and stems contact the ground.  The native berry producers have little chance to propagate and thrive in these areas.

Robins are hardly endangered and do not depend on the spread of Japanese Honeysuckle for their survival.  Catbirds numbers are declining, not from lack of food but from feral cats.  During their breeding season when berries are not available, robins eat primarily worms and catbirds eat insects.  Insects and larvae depend on the wide variety of native species that Japanese Honeysuckle chokes out.

Carlos suggests that "eliminating an invasive species could result in harm to the newly formed balance of the ecosystem."  This would be true if there was no other food source available, but there are many native plants that these birds have fed on for thousands of years.  If we are encouraging native plants and planting some in place of Japanese Honeysuckle we remove, there would be no threat to the birds.  Carlos also mentions that  "large-scale attempts to remove species could be a waste of time and tax dollars."*  The latter statement is true when applied to the country as a whole.  Local efforts in parks, conservation areas and restored habitat remains important to preserving what we can of our threatened ecology.

The "elephant in the living room" that isn't mentioned is the fact that many other bird (and other) species have special environmental or food needs which cannot be met by invasive species that are foreign to their habitat.  As Lisa Berger, Charley Burwick's wife puts it, "these are species that have not, or cannot nest in non-native or invasive species or glean fruit or insects that are not also dependent upon native species."

Ironically, this research became public during National Invasive Species Awareness Week.  "Eliminating an invasive species" is not longer an option so we will have to pick our fights carefully.  We have limited financial and human resources and need to focus on the most important areas to protect.  In the words of Kenny Rogers in The Gambler:
You got to know when to hold 'em, know when to fold 'em
Know when to walk away, know when to run.



1 comment:

  1. Tried to post earlier but doesn't seem to want my comment. The photo is a cedar waxwing and not a catbird.