Environmental Engineering Reference
In-Depth Information
portant enterprise in ecological science. But reintroducing once-lost species
back to their native habitats can be a formidable task because it presumes
that the we can draw individuals from somewhere else on the landscape (i.e.,
the species have not yet gone extinct) and that suitable habitat is available
to support the reintroduction.
Species also do not live in splendid isolation of each other.There are di-
rect and indirect lines of dependency that make up the natural economy
(chapters 7 and 8).These dependencies can have an important bearing on
the success of reintroductions, especially if we have limited insight into how
they arose over the eons in the first place.This uncertainty leaves us with
the sobering prospect of what Pimm (1991) has termed “Humpty Dumpty
effects.”
The concept has its roots in the familiar nursery rhyme in which all the
parts of a broken Humpty Dumpty are present, but he cannot be put back
together because it is unclear how the pieces fit together originally.To elab-
orate this point, let's consider a hypothetical restoration scenario and use the
chaparral food web (figure 6.2) for illustrative purposes. In chapter 6,I ex-
plained how habitat fragmentation led to local extinctions of bird species
that comprised the chaparral food ecosystem because fragmentation dis-
rupted important lines of dependency among top and middle (meso) pred-
ators whose effects cascaded down to bird species causing local bird species
extinctions. Suppose that a conservation agency decided that it would re-
store the chaparral ecosystem in southern California by buying parcels of
land to reverse development and reconnect that habitat fragments. Suppose
also that the conservation agency could find individuals of the different bird
species elsewhere on the landscape and that it decided to reintroduce them
en masse in order to hasten ecosystem recovery.This strategy might be suc-
cessful, but it might not because of an ecological phenomenon known as
priority effects. Priority effects mean that species in an ecosystem do not
assemble haphazardly but instead follow a strict sequence.
For example, let's suppose a primordial chaparral habitat was comprised
only of the roadrunner and its prey: insect species and lizards. Suppose that
from time to time the other bird species (the wrens, quail, and gnatcatcher)
attempted to become established in the chaparral but they were unsuccess-
ful because roadrunners, which also prey on bird eggs, prevented the other
bird species from growing in abundance. Suppose, however, that over time,
the roadrunner population built to a sufficient size to support a population
of carnivores such as foxes that preyed upon roadrunners.The limitation of
