Geoscience Reference
In-Depth Information
populations by introduced cats and rats has been a major cause of extinctions on islands such as Ascension Island,
whilst the escape and spread of alien 'garden' species ('invasive plants') can cause problems for endemic species.
Thus on Lundy Island in the Bristol Channel, UK, the endemic Lundy cabbage is threatened by the spread of
rhododendron, which not only outcompetes the Lundy cabbage physically but also secretes a chemical into the soil
which attacks the cabbage ('allelopathy'). Conservation bodies are trying to limit the damage to island wildlife. Physical
methods are erecting fences or clearing vegetation, herbicides can be sprayed, and biological control can be
attempted by introducing chemical contraceptives and viruses (e.g. myxomytosis in rabbits) against alien species.
Reintroducing lost species can be implemented; Scottish Natural Heritage has had a successful programme for
reintroducing sea eagles on the island of Rhum since 1975, and the Jersey Wildlife Trust has a breeding programme
for reintroducing the pink pigeon and the Mauritius kestrel on to the island of Mauritius. Such attempts are expensive,
and will not meet with guaranteed success unless the underlying cause is dealt with.
2.6
return to their equilibrium is the
resilience
. If a system is
unable to return to equilibrium it is
unstable
and therefore
has low resilience. A special case is where biological
populations do not return to equilibrium but cycle
indefinitely, like the lemming in the Arctic (
Plate 22.2
),
lynx in the subarctic, or red grouse in Britain (
Plate 22.3
).
Figure 22.6
shows the reaction of four ecosystems to a
perturbation. System A (a tropical rain forest) is stable; it
does not depart far from equilibrium, and returns rapidly
to it. System B is unstable; it passes beyond the stability
domain and collapses. System C (a boreal coniferous
forest) is stable, but less stable than A, owing to its larger
displacement from
K
and the longer time needed to return
to it (lower resilience). System D is the continual cycle
referred to previously (Arctic populations); it is dynami-
cally stable. All the above trends assume that the
perturbations are equally strong. In the real world the
behaviour of ecosystems will depend on the precise nature
of the perturbation and on its magnitude. Natural major
perturbations or disturbances which affect ecosystems
are drought, freezing, fire, insect pests and disease;
All Mediterranean
Asia Minor
2.4
Iberia-France-Italy
Balkans
North Africa
2.2
Sardinia
Corsica
Peloponnese
Sicily
2.0
Crete
Cyprus
Balearics
Rhodes
1.8
Cyrenaica
Formentera
1.6
Malta
1.4
Cabrera
Island
Other area
1.2
1.0
1
2
3
4
5
6
7
8
Area, km
2
(log A)
for Mediterranean islands and mainland regions.
Source: After Blondel and Aronson (2000)
Term
Definition
Units
Definitions
Stable
Returns to initial equilibrium
n.d.
after a perturbation
Stability of ecosystems is not an easy property to define.
Indeed, the ecological literature suffers from confusion;
in some cases the same term is used with different
meanings, and in others different terms are used to convey
the same meaning.
Table 22.5
presents the most acceptable
definitions of stability.
An ecosystem is
stable
if all variables return to the initial
equilibrium position (defined as
K
) after suffering a
perturbation or shock which has displaced the variables
from their equilibrium position. How fast the variables
Resilience
Speed of return to equilibrium
Time
after a perturbation
Persistence
Time before variable changed
Time
to new value
Resistance
Degree of change after a
n.d.
perturbation
Variability
Variance of population densities
s.d or c.v.
over time
Notes: n.d., non-dimensional; s.d., standard deviation; c.v., coefficient
of variation.
