Globalisation and Invasive
Species: Economic Insights from
Theoretical Ecology
Charles Perrings
University of York
Washington DC
August 2004
Globalisation
• Globalisation is widely understood to mean the closer
integration of the world's economic systems, systems of
governance and cultures.
• This interpretation of the term is reflected in the 2003
World Development Report's reference to the world
‘coming to resemble a single community' (World Bank,
2002).
Globalisation and the dispersion of species
• Globalisation has increased both the speed with which new
species are dispersed, and the extent of their dispersion.
• The widening and deepening of trade links has
– connected previously unconnected ecosystems,
– created new pathways between those ecosystems and
– increased the rate of dispersion of species along those
pathways.
• The dispersion of new species includes both deliberate
introductions (new crops, livestock strains, biocontrol
agents and so on) and accidental introductions.
Biodiversity trends
• At the same time successive environmental assessments
(most recently the MEA) have shown a number of trends,
including:
– that the number of exotic species in many managed and
impacted systems has increased;
– that the diversity of native species within ecosystems
has fallen;
– that the diversity of all species across ecosystems has
fallen.
• How are these things connected?
Intermediate dispersion theory
• Loreau M., Mouquet N. and Gonzalez A. 2003.
Biodiversity as spatial insurance in heterogeneous
landscapes, PNAS 100 (22): 12765-12770.
• The paper explores the functional significance of
biodiversity change across space.
• Species that are functionally redundant in one place may
not be functionally redundant in other places if
environmental conditions vary across space, and
asynchronously over time.
• The dispersion rate determines local biodiversity.
The spatial insurance model
• If different communities experience different
environmental conditions, and if these conditions fluctuate
asynchronously over time, then different species will thrive
in each of the communities at different points in time.
• Differences in dispersion rates between communities will
affect the level of biodiversity in those communities.
The model
The model comprises equations of motion for biomass
and the limiting natural resource:
dN ij t 
dt
dR j t 
dt
a M
 eij t R j t   mij N ij t  
N ik t   aN ij t 

M 1 k  j
s
 I j  lR j t   R j t  cij t Nij t 
i
Variables and parameters
Nij(t) is biomass of species I in community j at time t
Rj(t) is amount of limiting resource in community j
M is the number of communities
S is number of species
cij(t) is rate at which species i consumes the resource in j
eij(t) is efficiency with which the limiting resource in j is
converted into biomass by species i
mij(t) is mortality of species I in community j
Ij is renewal rate of the limiting resource in j
lj is decay/loss rate of the limiting resource in j
a is dispersal rate of all species
Consumption
• Consumption rates are responsive to;
– environmental fluctuations and
– species traits
• They give species responses to environmental fluctuations
cij t   1.5  H i  E j t 
where:
– Hi is the ‘trait value’ of species i (i.e. a measure of its
best performance along an environmental gradient)
– Ej (t) is the environmental value of community j
Ecosystem productivity
• Ecosystem productivity is production of new biomass per
unit of time averaged across the metacommunity
S
 t  
M
 e c t R t N t 
i  j j 1
ij ij
j
M
ij
Environmental fluctuations
• Environmental values are assumed to exhibit
autocorrelated sinusoidal fluctuations with period T :
1 
2t  
E j t   sin  x j 
  1
2 
T  
Impact on local biodiversity
• Dispersal of species between communities has nonmonotonic effects.
– Low and high rates of dispersal are both associated with
low local biodiversity.
– Intermediate rates of dispersal are associated with high
levels of local biodiversity.
Impact of dispersion rate on diversity
(a = 0)
The species with initial
competitive advantage
excludes others.
Biomass varies with
environmental
conditions. Regional
diversity is high.
Time
Impact of dispersion rate on diversity
(a = 0.001)
Source-sink effects
begin to increase
local diversity.
Regional diversity
begins to fall.
Time
Impact of dispersion rate on diversity
(a = 0.02)
Source-sink effects
allow local (and
regional)coexistence
of large numbers of
species.
Time
Impact of dispersion rate on diversity
(a = 0.2)
The metacommunity
begins to behave as a
single community, and
species with average
traits begin to
dominate.
Time
Impact of dispersion rate on diversity
(a = 0.4)
The community behaves
like a single community.
The species with the
average trait
competititively excludes
others. Local and regional
diversity falls.
Time
Impact on productivity
• If there is a positive relation between local biodiversity and
either productivity or resilience, this implies that:
– very low and very high rates of dispersion may be
expected to be associated with low levels of
productivity/resilience and
– intermediate rates of dispersion with high levels of
productivity/resilience.
Impact of dispersion rate on productivity
(a = 0 to 0.2)
Time
Impact of dispersion rate on productivity
(a = 0.2 to 0.4)
Time
Ecological conclusions
• Biodiversity can affect ecosystem functioning in ways that
cannot be detected by small-scale experiments.
• Changes in ‘connectivity’ between communities at the
landscape level can alter both species diversity and
ecosystem processes.
• Beyond a certain point increasing landscape ‘connectivity’
will decrease species diversity, and will increase the
variability of productivity at the local level.
Ecological conclusions (cont)
• The intermediate-type species that dominate when
dispersal is high have lower variability than the species
that dominate local communities when dispersal is low.
• More connected metacommunities experience lower
variability in productivity than unconnected communities.
Economic implications
• How does this help us to think about the implications of
globalisation?
• What are the connections between trade and dispersion?
• What are the connections between dispersion and exotic
species?
• What does competitive exclusion mean in ecologicaleconomic communities?
• What are the implications for invasive species?
How does this help us to think about the
implications of globalisation?
• The spatial insurance model is a model of the behaviour of
more or less strongly linked members of an ecological
metacommunity.
• An economic analogue to the closed community within a
metacommunity is the autarkic state.
• An economic analogue to a metacommunity that behaves
as a single community is the fully integrated world
economy.
The economic implications
• The dispersion of new products, processes, technologies
and management strategies has changed both the nature
and management of local risks.
• As production systems have become more homogeneous,
the risks associated with those systems have become more
highly correlated, e.g:
– vulnerability to external stresses and shocks,
– vulnerability to pests and pathogens)
• The portfolio of natural capital stocks has been reduced
What are the connections between trade and
dispersion?
• The rate of dispersion in ecology is driven both by the
characteristics of the species and environmental conditions.
• In an ecological-economic system environmental
conditions include the pattern and volume of trade.
• The dispersion of species depends on the direction and
volume of trade:
– the direction of trade determines the distribution of
propagules
– the volume of trade determines propagule pressure
What are the connections between dispersion
and exotic species?
•
Trade involves the dispersion of species of many kinds
1.
2.
3.
4.
5.
6.
7.
•
•
Crops (flora providing foods, fibres and construction materials)
Ornamentals (flora providing decoration or amenity)
Livestock (fauna providing foods, fibres and draught power)
Pets (fauna providing amenity)
Biocontrol agents
Pests
Pathogens
1 - 5 are typically deliberately introduced through trade,
and may depend on repeated introduction, cultivation or
management.
6-7 are typically the unintended consequence of trade or
travel
What does competitive exclusion mean in
ecological-economic communities?
• Species introduced via trade may competitively exclude
others either ecologically or economically.
• Ecological competitive exclusion implies that the species
best adapted to local conditions out-competes others in
accessing resources.
• Economic competitive exclusion implies that dominant
products/firms out-compete others in accessing markets
and hence resources, e.g.:
– high yielding varieties of rice
– BT cotton
What are the implications for invasive
species?
• Invasive species are introduced species that spread (usually
following establishment and naturalisation), with either
more or less significant consequences for ecosystem
processes and functioning.
• The probability that any species will become invasive is
depends on its invasiveness (including propagule pressure)
and the invasibility of the host system (affected by
disturbance, fragmentation etc).
• Pests, pathogens and cultivated alien species can all have
similar effects, competitively excluding native species and
affecting the production of ecological goods and services.
What are the implications for invasive
species?
• By convention, alien species that persist only because of
domestication are not considered invasive. But they may
have equivalent ecological effects, and these should be
taken into account.
• The risks of successful introductions of alien invasive
species increase with the volume of trade in the species
itself, or in the goods and services on which it is a
passenger.
The world as a single community
• The advantages of moving from autarky to trade are wellunderstood.
• It is argued that trade should benefit the environment by:
– improving the efficiency of activities that use natural resources,
– reducing the environmental impact of activities,
– improving security of land tenure by assigning private property
which promotes investment in land conservation and
environmental stewardship,
– induce greater macroeconomic stability which encourages
investment in conservation activities,
– reducing poverty and hence pressure on open-access resources,
– lowering the costs of environmental protection, and
– helping diffusion of environmental protection technologies.
Implications of the intermediate dispersion
hypothesis
• The intermediate dispersion hypothesis suggests that
– the impact on local species diversity of the move from
autarky to trade should initially be positive,
but that
– the impact on local species diversity of the further
growth of trade might be expected to become negative
at some point.
• Both have consequences for productivity and resilience.
• There may also be economic analogues to this…..
Implications for the research problem
• What is the nature of the problem?
• Historically, research on invasive species has focused on
pest species - those whose spread is somewhat independent
of human behaviour, and whose impacts inflict appreciable
harm
• These are part of a spectrum of exotic species whose
introduction and spread has transformed local ecosystems
and the goods and services derived from those systems.
• The spectrum stretches from pests and pathogens, through
bio-control agents to cultivated crops.
Implications for the research problem (cont)
• In all cases the generic research questions are the same.
• At the micro level (of individual species) they are:
– What is the opportunity cost of the introduction and
spread of new species (in terms of forgone ecosystem
services)?
– Is this compensated by the net benefits offered by the
actions that lead to the introduction and spread of new
species, taking all other effects into account?
Implications for the research problem (cont)
• At the macro level they are:
– What are the consequences of the change in the mix of
species (induced by globalisation) for the functioning of
ecosystems?
– What effects does this have on the production of goods
and services and human well-being.
• These questions apply as much to species whose spread
depends wholly on human action as to those whose spread
is independent of human action.
Implications for the research problem (cont)
• We should be taking a fresh look at problems that have
previously been in the domain of health and epidemiology,
agriculture, forestry.
• We should be asking what the net consequences of changes
in the species mix have been in terms of the capacity of
local systems
– to support the production of valuable goods and
services
– to function over a range of environmental conditions
Implications for the research problem (final)
• The micro question has been posed by adding
environmental externalities to quite standard production
theory.
• The macro question has been dealt with far less
satisfactorily, although it is beginning to be posed in
questions about the optimal mix of production and
conservation activities in the landscape.
• We need to consider the impact of changes in species mix
at the same level.