Conservation efforts, by definition, are aimed
at keeping things as they were. When this is
impossible,
ecologists and
policy makers try to
at least minimize
anthropogenic damage to the
Earth's species, without attempting so many
simultaneous tasks that each effort becomes
ineffectual. The species that receive this sort
of
triage tend to be
taxonomically unique, such as the
tuatara, or
charismatic
megavertebrates, such as the
giant panda. Such a strategy prioritizes species over
ecosystems, and can only be undertaken with the understanding that eventually all the tuataras and pandas on Earth may only be able to live in
zoos. However, triage remains a potentially useful tool if it
is applied based strictly on ecological
importance. If conservationists focus on
keystone species and
ecosystem engineers, they
will be most able to protect
diversity and
maintain
structure within threatened ecosystems.
Keystone species are
usually understood to act
trophically and to
exert an effect disproportionate to their
relative
biomass within an ecosystem. They
often occur at or near the highest trophic
levels of a
food web, as
carvinores with high
per capita effects and low collective biomass. However, keystones can act non-trophically as well,
pollinating or
dispersing other species. The
textbook case of a keystone species is the starfish
Pisaster, which
plays a major role in supporting diversity.
In one computer simulation, when it is removed, the
Simpson Diversity Score1
of its ecosystem fell by 4 points, as the
Pisaster’s prey were allowed to
competitively
exclude each other. Other species in the same simulation exerted far less of an effect on their
biotic environments (Power 1996).
A species can also be of particular importance
to the physical structure of an environment.
Ecosystem engineers change the quality,
quantity, and distribution of
resources other
than their own biomass. These organisms
allogenically construct or
autogenically compose
entire
habitats such as
coral reefs,
forests,
and
beaver ponds; in doing so they affect key
environmental attributes such as
nutrient
mixing,
temperature, and
drainage.
Excepting ecosystem engineers which are also
keystone species, such as
beavers,
leaf-cutter
ants, and
badgers, ecosystem engineers commonly
live at high densities and, like Antarctic
algae or
reef building coral,
may be near the lowest trophic level of a food
web (Lawton 1994). Their effects are drastic, but can be
taken for granted, potentially causing them to
be overlooked in conservation efforts.
There are, however, some ecosystem functions to
which all species contribute equally.
Low-diversity communities constructed
experimentally so as to resemble depauperate
descendants of high-diversity communities show
decreased rates of
carbon dioxide uptake and decreased
plant productivity, with a constant decline in
each function confirming Lawton’s “
ratchet”
hypothesis (1994). Another caveat lies in the
fact that many studies that appear to attribute
a major role to a given species in fact
attribute that role to a given
guild. A hypothetical experiment which labels an
ecosystem’s
decomposer as an ecosystem engineer
does not determine whether ecosystem function is
further dependent on a diverse guild of
decomposers, or whether one species is
sufficient. And even seemingly redundant
species can contribute to an ecosystem’s
resistance and
resilience in the face of
disturbances that may
never be experimentally observed (Walker 1992).
Conclusive decisions as to the allocation of
conservation resources can only be made for
ecosystems that are thoroughly studied and well
understood, and even then minor-interactors must
never be written off entirely.
Works Cited:
- Lawton, H. What do species do in ecosystems?
OIKOS 71(3), 1994.
- Power, M.E. and others. Challenges in the
Quest for Keystones. BioScience 46 (8):
609-620, 1996.
- Walker, B. H. Biodiversity and ecological
redundancy. Conservation Biology 6 (1): 18-23,
1992.
1This is a unitless ranking of an ecosystem's diversity that takes both its richness (the number of different species it contains) and eveness (the degree to which species are spatially interspersed with each other) into account.