Fragmentation and Ecosystem Decay of a habitat island
Doug Bashford
� Area: Sci.Environm
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Msg#: 57 Date: 03-12-94 16:56
From: Alan Mcgowen Read: Yes Replied:
No
To: All Mark:
Subj: Ecocentral 14
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From: Alan McGowen <al...@igc.apc.org>
ECO CENTRAL 14
Fragmentation and Ecosystem Decay
The fourth root rule for equilibrium species diversity of a
habitat island discussed in ECO CENTRAL 13 says that diversity is
proportional to the fourth root of area. When habitat is
*fragmented* - cut into smaller habitat islands - the species
diversity of each fragment drops to its new, lower, equilibrium
value for the smaller area. Populations whose K (habitat area) is
no longer large enough to protect them against extinction (K less
than their Minimum Viable Population (MVP) size in the reduced
fragments) become extinct, mutualists and specialist feeders which
had depended upon then become extinct, etc. until a new stable
species composition has been achieved, at a lower species
diversity.
Models of this process based upon the equilibrium theory of
species composition of habitat islands predict that the new
equilibrium species diversity is approached inverse
exponentially, i.e. it is a sort of decay process. Because of
this, it has been called "ecosystem decay". [Lovejoy, 1984]
The mechanism behind this decay arises from the turnover of the
species composition of an ecosystem at equilibrium. Species whose
local populations become extinct are typically recolonized from
adjacent areas, or ecologically similar species take their place.
In a strongly coevolved climax community, species composition is
quite stable, and the turnover mainly reestablishes species that
had previously lived in the area.
This reestablishment of populations which have become extinct
occurs through dispersal mechanisms, and these mechanisms have a
probability of successful reestablishment which falls off with
intervening distance. Many dispersal mechanisms fall off inverse
exponentially with distance from the source. When distance is
increased by d, the probability of recolonization drops by a
factor C exp(-ad). In other words, the *immigration rate* of the
species into an area drops inverse exponentially with increasing
distance from the source, independently of the extinction rate in
the area. Since the species is expected to be at K in the area,
unless it is extinct, the immigration has no effect. When it
becomes extinct, the immigration reestablishes it, but only with
a probability inverse exponentially dependent upon the distance
which has to be traversed. Thus when a fragment is created from a
larger habitat area, not only is the K of every species within it
decreased (increasing the extinction rate due to stochasticity),
but the immigration rate and probability of reestablishment drops
because the distance to the nearest sources of the species for
recolonization has been increased.
The process of ecosystem decay has been studied on Barro Colorodo
Island. Called "the most studied piece of tropical forest on
Earth", Barro Colorodo was isolated during the creation of the
Panama Canal. Ecosystem decay is also being studied in detail in
forest fragments of varying sizes in the Amazon, north of Manaus
[Lovejoy, 1984]. Here a series of reserves, ranging in size from
1 to 1000 ha, are being set aside during deforestation, with each
size of fragment represented in several replicates. There is also
one 10,000 ha reserve set aside for comparison.
In estimating the rate at which biodiversity is lost due to
habitat reduction, decay models can be made with many different
levels of detail of assumptions about the extent of mutualist
interactions, specialist dependencies, and endemism (limited
geographical distribution of the species). However in many cases,
particularly in the tropics, our knowledge of the actual
biogeography is too limited to be able to choose among different
detailed models. A roughly inverse exponential pattern is common
among the models, however, and consistent with observation to
date, so the term "ecosystem decay" is well justified.
The study of ecosystem decay is likely to be one of the growth
industries of biogeography in the coming decades, since it is one
of the commonest phenomena on the earth today.
Ref.
Lovejoy, T. E. et al. 1984. Ecosystem Decay of Amazon Forest
Fragments. In _Extinctions_ ed. Matthew H. Nitecki, University of
Chicago Press, Chicago.
-!!!!!-
Alan McGowen
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