There have been some concerns raised in the land trust community that one of the impacts of climate change will be the displacement of some native species by others that are expanding their ranges. A recent article by Attorney James L Olmsted entitled: "The Butterfly Effect: Conservation Easement, Climate Change and Invasive Species"suggests a number of changes that land Trusts can make to their easement language to anticipate this problem, but the underlying premise that in-migrating North American species "will in many cases be invasive" is on questionable scientific ground.
It is wrong to think
of species and natural communities as static and restricted to where they are
today, or were at the time of European contact.
The term “Invasive” is
both relative in space and time and too
broadly applied to North American species that are expanding their
natural ranges in response to environmental factors and opportunities. Birds have been doing this for a very long
time. The black vultures now present in
large numbers in Connecticut were not found north of Maryland in the first part
of the 19th century (all those dead horses at Gettysburg gave them a
beachhead). Cardinals were not part of
my mother’s Massachusetts girlhood. Coyotes
are filling an available large predator niche after the extirpation of wolf and
cougar populations.
The term “Invasive” has more validity when it is restricted to introduced species, and then only to those which have such characteristics as spreading across spatial gaps, establishing virtual monocultures and multiple dispersal methods. Having these attributes, species should be demonstrated to displace and outcompete native species to be considered invasive. Under this definition, House Sparrows are invasive, but Cattle Egrets which, bless their hearts, got here by crossing the Atlantic Ocean and the Caribbean all on their own, are not.
When I was part of the Massachusetts Invasive Plant Advisory Group that developed criteria to determine which species should be considered invasive or potentially invasive in the Commonwealth, we had a very hard debate about Black Locust (Robinia pseudoacacia), a central Appalachian species which can be problematic in pine barren systems in the Northeast. Had the glaciers receded a few thousand years earlier, Robinia would likely have expanded its natural range a few hundred miles further north, resulting in a different kind of natural community where it overlapped with pitch pine and scrub oak. Humans helped it make the jump, planting Black Locust for fence poles (which sometimes resprouted!).
My advice for anyone drafting a conservation easement or management plan is to start by answering the question; “What are we trying to conserve and managing for?” The question of invasiveness relates directly to whether a species impacts the viability of conservation targets. The best example I can remember from my TNC days concerned a fen in NJ that was also a bog turtle site. The fen had a large and expanding incursion of purple loosestrife (an exotic species non-native to North America). There were two possible conservation targets to manage for at this site: the rare natural community represented by the fen, and the federally threatened bog turtle species. The condition of the fen was severely degraded and attempting to eradicate the loosestrife threatened worse disturbance as well as the bog turtles that still were using it, so it was determined not to try to manage the fen as fen, but as bog turtle habitat. The bog turtle basking areas were being shaded out by the loosestrife, so the management prescription was to cut the loosestrife stalks by hand each year before they set seed. This took several days of cutting by hand, but was the best response available to conserve the primary conservation target.
So, if we are managing for rare and restricted habitat types, some of which will not be viable in their current configuration, or indeed in any form with climate change, we are making a choice to prioritize them against the prevailing forces of change. That may indeed be the right thing to do, but even then the calcareous fens of Connecticut will not have the same species composition and structure as those in Maryland even when our climate changes to that of Maryland today. There are special gaps that are unlikely to be crossed by native fen species present today in Maryland but not in Connecticut. That is the beauty of natural variation. Diversity matters, but it plays out in many different ways from site to site.
Especially with large, “functional” landscapes, the idea is not to manage them to maintain exactly the species types and forest composition of today, but so that they are robust and resilient enough to maintain biodiversity, in whatever forms may be viable in the future. Invasive plants may well be a factor that needs to be accounted for, but it does not begin or end with a list of species that are “meant to be here” and others that are not.
