Dear Katie and Others,
I can understand (and partially agree with) some of the negative
reaction that many have had to the Davis et al. article in Nature. I
do not share the authors' desire to extol the virtues of non-native
species (except perhaps for agricultural and medicinal species).
However, after reading the article, I came away with a very different
impression of what the authors were trying to say than have many of
the posters.
The main points I took away from the article were 1) there have been
unsubstantiated claims of harm by non-native species 2) the costs of
dealing with successful non-native species may in some cases outweigh
the benefits, and 3) there should be no double-standard with regard
to how we view native and non-native species that have negative
impacts on ecosystems.
I agree with all three points.
The Wilcove et al. 1998 study claiming that the invasive species were
the second leading cause of extinctions needed to be addressed. There
was not much empirical data to back up this claim (at the time at
least), and yet it and similar reviews get cited often. Empirical
support is necessary for scientific credibility. Contrary to what one
poster said, I do not agree that empirical evidence is not needed
when there is observational evidence. Looks are deceiving, and most
of the examples of observational harm he cited are in fact also
supported by empirical studies. Purple loosetrife, on the other hand,
is notorious for decimating native plant diversity, but the empirical
support for this claim is at best conflicting. Furthermore, some
species (e.g., Microstegium vimineum) may have greater *per capita*
impacts on relatively species-rich ecosystems in which they are not
the most productive or abundant (Brewer 2011, Biological Invasions).
This would not have been revealed by simple observation. I think the
numbers of studies showing negative effects of non-native species
since the Wilcove et al. 1998 study have been increasing (judging
from what I've seen in issues of Biological Invasions). But this may
be in large part because an increasing number of ecologists have
recognized the lack of data (or conflicting data) on harm for some
notorious invaders and have taken it upon themselves to investigate
the issue. I was partly inspired to investigate effects of cogongrass
(Imperata cylindrica) on longleaf pine vegetation after reading
Farnsworth's study of purple loosestrife, which surprisingly showed
little effect of this species on native plant diversity. I don't
think these efforts to obtain empirical data are a waste of time.
The limited supply of conservation funding requires that we take a
cost-benefit approach to control of non-native species perceived to
be invasive. In north Mississippi, a funding priority of the National
Forest is the control of kudzu. While it almost certainly retards
succession (although I'm not aware of scientific studies
demonstrating this), I can find no evidence that it has displaced
native species. It was largely planted in eroding, agricultural
wastelands that were devoid of diversity in the first place. Yes,
it's expanding into adjacent forests, but very slowly, as it is the
least shade tolerant vine in the southeast US. It creates its
preferred light environment by killing a tree at a time, but this is
not a rapid process. I'm not saying it shouldn't be controlled, but
should its control be given priority over, say, reversing the effects
of fire suppression in upland forests in north Mississippi (as is the
case now)? The latter management activity would be much more
effective at restoring native biodiversity and perhaps a better use
of limited biodiversity conservation dollars.
I demonstrated that cogongrass dramatically reduced resident plant
species diversity AND the abundance of plant species indicative of
longleaf pine ecosystems (Brewer 2008, Biol Inv. 2008). I think this
non-native species is certainly deserving of its bad reputation and
control efforts. But what about native eastern red cedar (Juniperus
virginiana)? In the black belt prairie of north Mississippi, fire
suppression has led to a dramatic increase in its abundance, which in
turn has resulted in catastrophic reductions in native plant and
animal diversity and the displacement of numerous endemic
state-listed plant and animal species. It has converted a diverse
grassland ecosystem into a species-poor cedar thicket. Indeed, I
would argue that it has had a greater impact on diversity and biotic
homogenization in Mississippi than has cogongrass (certainly much
more than kudzu). Yes, there is concern about this native species,
and there are some efforts to control/remove it, but there is no
Eastern Red Cedar Task Force in Mississippi. There is no Cooperative
Weed Management Area formed around its control in Mississippi (as
there is for cogongrass and kudzu). Is this a double standard? I'm
not sure. I will say, however, that when I tell the cedar story to
members of the general public, I'm often asked whether the fires that
maintained the prairies were natural or set by humans and if
restoration would require controlled burning. The fires were mostly
set by humans in the past, and prescribed burning would be necessary
today. Why is that important? Is it okay for this native species to
decimate native diversity and endemic species if this is just Nature
taking its course?
There is good reason to be concerned about non-native species. Some
do become invasive and have catastrophic effects on the ecosystems to
which they have been introduced. I think we're all aware of this
(including the authors, who said as much and provided a couple of
examples). Some non-native species that become invasive and injurious
have done so probably because they have escaped their natural enemies
or have brought with them novel weapons or traits, which in turn
gives them a competitive advantage, an advantage that can (but does
not necessarily) lead to the displacement of native species. So, our
fear of alien species is not just xenophobia or nativism. There are
evolutionary and ecological reasons for it. At the same time,
however, mismanagement of ecosystems (e.g., fire suppression) and
climate change have given rise to negative effects of some native
species on others. Although I would have written the article
differently, I agree with the authors' conclusions that we should 1)
try to prevent invasive alien species from being introduced to new
areas and 2) prioritize management activities so as to minimize
negative effects of some species on others (regardless of their
origin). We need to do a better job of both.
Steve Brewer
At 2:15 PM -0400 6/10/11, Katie Kline wrote:
An essay published in the June 8 issue of Nature is causing
something of a stir. Eighteen ecologists who signed the essay,
titled "Don't judge species on their origins," "argue that
conservationists should assess organisms based on their impact on
the local environment, rather than simply whether they're native,"
as described in a recent Scientific American podcast.
In the essay, Mark Davis from Macalester College, St. Paul,
Minnesota and colleagues argue that adherence to the idea of
non-natives as "the enemy" is more a reflection of "prejudice rather
than solid science," wrote Brandon Keim in a Wired Science article.
As the authors wrote, the "preoccupation with the native-alien
dichotomy" among scientists, land managers and policy-makers is
prohibitive to dynamic and pragmatic conservation and species
management in a 21st century planet that is forever altered by
climate change, land-use changes and other anthropogenic influences.
As a result of this misguided preoccupation, claim the authors, time
and resources are unnecessarily spent attempting to eradicate
introduced species that actually turn out to be a boon to the
environment; the authors cite the non-native tamarisk tree in the
western U.S. as an example of this...
Read more and comment at
http://www.esa.org/esablog/ecologist-2/speaking-of-species-and-their-origins/
--
J. Stephen Brewer
Professor
Department of Biology
PO Box 1848
University of Mississippi
University, Mississippi 38677-1848
Brewer web page - http://home.olemiss.edu/~jbrewer/
FAX - 662-915-5144
Phone - 662-915-1077
