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Ecologically Effective Populations
For Highly Interactive Species
(Adapted from Rewilding North
America)
In an important recent paper in Conservation Biology, Michael
Soulè, Jim Estes, Joel Berger, and Carlos Martinez del Rio show that
top carnivores and other “highly interactive species” need to be
present in an ecosystem in “ecologically effective densities.” They
propose two goals for the “laws and policies that apply to the
conservation of biodiversity”:
The first is the goal of geographic representation of interactions,
which calls for extensive geographic persistence of highly
interactive species. Conservation plans and objectives (design,
management, and recovery) should provide for the maintenance,
recovery, or restoration of species interactions in as many places
as feasible, both within the historic range of highly interactive
species or in other sites where the consideration of climate change
and other factors is appropriate.
The second goal concerns ecological effectiveness within ecosystems,
communities, or landscapes: Conservation plans should contain a
requirement for ecologically effective population densities; these
are densities that maintain critical interactions and help ensure
against ecosystem degradation. This goal replaces the de facto
nonecological practice of requiring only the attainment of minimum
viable populations.
The point at which highly interactive species fall below the
densities needed to regulate their ecosystem is called the
“breakpoint.” For example, there is a breakpoint in sea otter
density after which otters can no longer control the sea urchins
overgrazing the kelp community, or, as Soulè and company write,
“Abrupt phase shifts between kelp-dominated and deforested states
thus occur with changing abundance of sea otters.” The breakpoint
may vary for the same species depending on the presence of other
predators or different ecological conditions, or “ecological
effective densities will depend on context: they are not the same
everywhere and under all circumstances.”
Based on this, they argue that U.S.
Fish and Wildlife Service Endangered Species Act “recovery
objectives are becoming indefensible in light of increasing
knowledge from community ecology.” They recommend that “conservation
plans should call for recovery or repatriation of such interactive
species at ecologically effective densities in as many places as are
currently realistic.” A highly interactive species is one whose
“virtual or effective absence leads to significant changes in some
feature of its ecosystem(s).” Surveying those endangered and
threatened species with published recovery plans, they find that “a
substantial proportion of [threatened and endangered] species are
strong interactors” and that “ecological function is simply unknown
for the majority of listed species.” This is why the whole approach
to endangered and threatened species recovery needs to be changed
from only preserving a minimum viable population that will stave off
extinction of the species to restoring highly interactive species in
all available habitat in their former ranges and at population
densities where they can prevent or repair ecological wounds.
Conservationists should embrace Soulè and company's call for a new
approach to species recovery as detailed in their ecologically
effective populations paper. We need to popularize it until its
wisdom is widely accepted among conservationists, biologists, and
land managers, just as the need for connectivity and the role of
top-down regulation are widely accepted in the conservation
community today.
(Adapted and condensed
from Rewilding North America by Dave Foreman [Chapters 8 and
14]. Copyright © 2004 by the author. Reproduced by permission of
Island Press, Washington, D.C. Quotes are from the books and papers
below.)
Books
Rewilding North
America by Dave Foreman (Island Press 2004).
Order
from The Rewilding Project.
Articles
PDFs not yet
available:
Michael E. Soulè,
James A. Estes, Joel Berger, and Carlos Martinez del Rio,
“Ecological Effectiveness: Conservation Goals for Interactive
Species,” Conservation Biology 17, no. 5 (October 2003):
1238-1250. The landmark paper on ecologically effective populations.
Michael E. Soulè,
James A. Estes, Brian Miller, and Douglas L. Honnold, “Strongly
Interacting Species: Conservation Policy, Management, and Ethics,”
BioScience 55, no. 2 (February 2005) 168-176. Specific
proposals for changes in the laws and approaches for endangered
species recovery based on ecologically effective populations.
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