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Conservation Biology: Lecture Notes
9. Habitat Fragmentation
- Detachment or separation of expansive tracts into spatially segregated fragments
- Parks are becoming Islands, surrounded by developed areas
The Fragmentation Process:
Concerns about fragmentation
- Effects of reduction in area of remaining fragments.
- Effects of increasing isolation of patches.
- Effects of increasing disturbance from surrounding area (edge effects).
Theory of Island biogeography (MacArthur and Wilson 1967)
- Community-based model that
incorporates both island (patch) size and
- Hypothesizes that the number of species
on an island is a function of rate of
colonization and rate of extinction
- Mostly written about oceanic islands;
mostly applied to habitat fragmentation
- As isolation distance increases,
- As patch size decreases, extinction
- One or both changes have the same
effect: species loss.
- KEY MESSAGE: Diversity can decline
strictly by reduced dispersal
Conservation bio implications: This model easily represented population dynamics that
took into consideration the main factors that determine existence. It graphically made sense
and it could easily be applied to fragmented habits instead of only oceanic islands. It also
explains why extinction occurs in fragmented habitats (terrestrial as well)
- A group of spatially separated populations connected by dispersal (a population of
- Several distinct populations together with areas of suitable habitat which are
currently unoccupied (dispersal limitation).
- Each population cycles in relative independence of the other populations and
eventually goes extinct as a consequence of fluctuations in population size due to
random demographic events; the smaller the population, the more prone it is to
- The metapopulation as a whole is stable because immigrants from one population
re-colonizes habitat left open by extinction
- Pseudo sink :
Occurs when two habitats are very close together but one has a higher carrying
The poorer habitat becomes overpopulated
In a true sink, if you took away immigration factor, extinction would occur
In a “pseudo-sink” if you took away immigration, population levels would
stabilize and be normal
A risk is that you want to protect source habitats because they are what keeps
all the other populations in check. If you accidentally protect the sink, and not
the source, the entire metapopulation could be doomed for entire extinction.
Why has “metapopulation” theory replaced “biogeographical theory” in conservation bio?
- It is a more realistic representation of how ecosystem function that incorporates
local mechanisms of population dynamics.
- It is more realistic of what happens after fragmentation
- Re-colonization after extinction
- Heterogenous habitats
- Compared to the theory of island biogeography which assumes equilibrium takes
longer to reach
- It doesn’t take into account stochasticity, biotic interactions
Characteristics of Fragmented Ecosystems
-Altered microclimates within fragments (edge effects):
oIncreased range of temperatures
oImplications for: spp comp.; ecosystem processes
Wind: exposure to wind, desiccation
oChanges in soil moisture
oAltered hydrology; w/in & outside fragments