Globalisation and Invasive Species: Economic Insights from Theoretical Ecology

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Globalisation and Invasive Species Economic
Insights from Theoretical Ecology

• Charles Perrings
• University of York
• Washington DC
• August 2004

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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).

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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.

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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?

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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.

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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.

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The model
The model comprises equations of motion for
biomass and the limiting natural resource
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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

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Consumption

• Consumption rates are responsive to
• environmental fluctuations and
• species traits
• They give species responses to environmental
  fluctuations
• 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

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Ecosystem productivity

• Ecosystem productivity is production of new
  biomass per unit of time averaged across the
  metacommunity

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Environmental fluctuations

• Environmental values are assumed to exhibit
  autocorrelated sinusoidal fluctuations with
  period T

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Impact on local biodiversity

• Dispersal of species between communities has
  non-monotonic 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.

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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
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Impact of dispersion rate on diversity (a
0.001)
Source-sink effects begin to increase local
diversity. Regional diversity begins to fall.
Time
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Impact of dispersion rate on diversity (a 0.02)
Source-sink effects allow local (and
regional)coexistence of large numbers of species.
Time
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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
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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
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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.

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Impact of dispersion rate on productivity (a 0
to 0.2)
Time
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Impact of dispersion rate on productivity (a
0.2 to 0.4)
Time
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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.

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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.

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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
  ecological-economic communities?
• What are the implications for invasive species?

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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.

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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

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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

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What are the connections between dispersion and
exotic species?

• Trade involves the dispersion of species of many
  kinds
• 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

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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

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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.

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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.

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The world as a single community

• The advantages of moving from autarky to trade
  are well-understood.
• 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.

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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..

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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.

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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?

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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.

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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

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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.