Biological Invasions

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Biological Invasions

• Peter B. McEvoy
• Insect Ecology
• Ent 420/520

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Learning Objectives

• Highlight the importance of invasions
• Pose and answer basic questions about invasions
• Critique approaches based on expert opinion
• Critique statistical approaches
• Critique mathematical approaches
• Assess the current status and future prospects
  for a predictive theory of invasions

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Statistical Profile of an Invader(Sailer 1983,
Niemelä Mattson 1996)

• Taxonomic composition
• Geographic origins
• Mode of entry
• Economic status
• Effectiveness of quarantine procedures

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Who Are the Invaders?

• 66 of Invaders, 1383-2000 spp., Come From 3
  Orders
• Hymenoptera
• Coleoptera
• Homoptera

Sailer 1983
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Where Do Insect Invaders Come From? 66.2
Western Palearctic (i.e. Europe)
Sailer 1983
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How Many Invaders Become Pests?
Foreign species make up only 2 of fauna, but 40
of pests
43 of beneficial spp. are accidentally introduced
Over half of foreign species become pests
Sailer 1983
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How Can Invasions Be Prevented?Effectiveness of
Quarantine Procedures
Number of Species
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Asymmetry in the Insect Exchange Between
Continents
300 spp
Europe
North America
34-44 spp

• Asymmetry characterizes most if not all biotic
  interchanges between biogeographic regions

Niemela and Mattson 1996
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Hypotheses of Vermeij (1991) to Explain
Asymmetric Biotic Exchange.

• Transport by trade and human dispersal. Numbers
  of invaders going in any direction are
  proportional to the size of conduits for their
  passage
• Species pools as a source of colonists. Numbers
  of invaders reflect differences in the number of
  species available for dispersal from the donor
  environment
• Ecological opportunities for immigrants. Numbers
  of successful invaders are determined by the
  wealth of ecological opportunities for them on
  their arrival
• Intrinsic superiority of European species.
  Invaders from one donor environment are
  intrinsically competitively superior to those
  from other donor environment. Hostile takeover
  phenomenon.

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Forests of the World
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1. Transport Most Immigrants From NA to Europe
Follow Their Hosts

• Of the 44 NA insects invading Europe
• 17 Homoptera
• 15 Hymenoptera
• 6 Lepidoptera
• 4 Coleoptera
• 1 Thysanoptera
• 1 Diptera
• most have followed their introduced NA host
  plants, except for 5 broadly polyphagous species.

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2. Species Pools of Similar Size (100,000 spp)
Some Phytophagous Species in NA and Europe
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NA ? EU for Phytophagous taxaNiemela and Mattson
96
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Different Ecological Opportunities for Immigrants

• Potential hosts (area, species richness,
  similarity to native hosts, parasite-host
  synchrony)
• Enemy-free space (parasitoid species per host
  Breeding bird densities)
• Competition-free space (species packing)
• Bioclimatic conditions (high to low latitude
  insect transfers)

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Top-Down Biotic Resistance to Invaders
Carnivore
Parasitoid
Bird
Herbivore
Native
NIS
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Bottom-Up Resistance to Invaders
Consumer 1
Consumer 2
Resource
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Forest Cover NA 2x Europe
woodland cover 1910 woodland cover 1990s
woodland in conifers 1990s
Cover
Niemela and Mattson 1996
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Potential Hosts European Aliens Adopt Close
Relatives of European Hosts

• Most common hosts of exotics are genera common in
  Europe Prunus gt Malus gt Betula gt Populusgt Salix
  gt Pinus gt Quercus gt Pyrus gt Crataegus gt Acer gt
  Ulmus gt Alnus gt Picea)
• Least common hosts of exotics are genera not
  naturally represented in Europe Carya,
  Chamaecyparis, Robinia, Pseudotsuga, Thuja, and
  Tsuga)

Niemela and Mattson 1996
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Host specificity of invaders

• The majority of insects invading North American
  forests have, in fact, been rather diet
  specialized, contrary to expectation. For
  example, 68 of the European invaders are mono-
  or oligophagous.
• Matson et al. 1994

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Host Availability

• Similar hosts. Potential hosts abundant and
  closely related to ancestral hosts in both NA and
  Europe
• More hosts in NA. NA has 2x species and genera
  because many shared genera went extinct in Europe
  during Pleistocene
• More abundant and less fragmented in NA. NA tree
  abundance 2x Europe because Europe has higher
  human densities

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Intrinsic Superiority

• Strong selection for aggressive competitive and
  colonizing ability (Pleistocene glaciation and
  extinction, settlement and fragmentation)
• Adaptive traits
• Phenotypic plasticity
• Uniparental reproduction
• High reproductive potential
• Polyploidy
• High dispersal potential
• Protection from competitors and natural enemies
• Stress tolerance mechanisms such as dormancy

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High to Low Latitude Transfers

• Dormancy. Importance of entering and leaving
  dormancy at appropriate times
• Latitudes. Owing to vast differences in
  latitudes of the deciduous forests of Europe
  (43-60o N) and North America (30-48o N) and
  importance of photoperiod as a cue
• Asymmetry. Insects going Europe to NA better
  adapted for environmental synchrony than vice
  versa

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Necessary and sufficient conditions for invasion

• Ability to find potential hosts in new
  environment
• Ability to synchronize life cycles with
  conditions in new environment
• Ability to increase population size when rare
• Ability to colonize disturbed systems
• These conditions are more likely to be
  encountered by European insect traveling to NA
  than NA insect traveling to Europe based on
  ability of insect

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Biocontrol An Exception to the 10s Rule of
Williamson
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Predicting Risk to Native Plants in Weed
Biocontrol (Pemberton 2000)

• Field host use of 117 organisms established for
  biological control of weeds from 1902 - 1996
• Taxonomic groups 112 insects, 3 fungi, 1 mite,
  and 1 nematode
• Geographic areas Hawaii, the continental USA,
  and the Caribbean

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Biocontrol As an Invasion Process

• Target effects. How likely is control organism to
  establish, increase, spread, suppress target
  invader?
• Nontarget effects. How likely is control
  organism to increase and spread out of control to
  colonize and harm native hosts?
• Selected for success? BC agents presented with
  unlimited resources, enemy-free space, matching
  climatic conditions and still majority of them
  fail to control target or use predicted
  nontargets.

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Operational Definitions

• Use defined as introduced control organism
  completes its life cycle on non-target plant
  species.
• Closely related defined as congeneric species
  of plants and species in closely related genera
  previously classified as the same genus.
• Co-occurring defined as occurring together in
  the same state.

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Main Conclusions

• Risk is borne almost entirely by native plant
  species that are closely related (same genus or
  closely related genus) to target weeds
• Taxonomically related hosts. 15 spp bc insects
  use 41 native plant species
• 36 of 41 natives are congeneric with target weeds
• 4 others belong to two closely allied genera
• Taxonomically unrelated hosts. Only 1 of 117
  established biological control organisms uses
  native plants unrelated to the target weed.

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Elements of Safetyfor Protecting Native Plants

• Selecting the right environment select weed
  targets that have few or no native congeners in
  recipient environment
• Selecting the right organism introduce
  biological control organisms with suitably narrow
  diets

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Fisher-Skellam TheoryGrowth and Diffusion
Equation

• N N(x,y,t) local population density
  organisms/area at time t and spatial
  coordinates x, y
• D coefficient of diffusion
• f(N) a function describing net population change

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Asymptotic Rate of Spread

• For large time, the velocity (distance/time) VF
  for the advancing front approaches an asymptotic
  rate of spread, which depends on the intrinsic
  rate of increase ? and the coefficient of
  diffusion D.
• The radius of a species range should
  asymptotically increase linearly with time with
  slope ?(4?D)

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Data requirements

• Intrinsic rate of increase
• Diffusion coefficient

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Rice Water WeevilAccelerating spread, two scales
of movement
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Japanese Beetle Begins Slowly, Eventually
Constant Rate of Spread
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Gypsy MothNon-constant velocity related to
temporal variation in quarantine and spatial
variation in temperature
gt 7o C
Quarantine
lt7o C
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Small Cabbage White Butterfly
Increasing number of generations
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Predicted and Observed Rates of Spread
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Summary

• Retrospective studies of invasions yield a
  statistical profile of invader that may be useful
  for management (prediction and mitigation)
• Usefulness of diffusion models for understanding
  observed patterns of spread and predicting
  invasions

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General observations after our paper critique

• Clear, operational definitions
• Statistical basis for inference
• Control for opportunity for invasion
• Pitfalls of univariate approach to multivariate
  problem. Assumes other things are equal and they
  seldom are.
• Need for phylogenetically controlled comparisons.
  
• Anthropocentric bias to data. Aliens account for
  2 of our insects but 60 of our pests.
• Avoid Tautology. European insects are more
  invasive because of their competitive
  superiority.
• Argument by example leads to rebuttal by
  counter-example