Transcript Persistence and Spatial heterogeneity

Allee effects and Spatial
Heterogeneity
By Nicholas Viveros and Jessica Oh
Persistence of invading gypsy moth
populations in the United States
By Whitmire et al.
• Spread of gypsy moth is
thought to be the result of
growth and coalescence of
isolated colonies in a
transition zone ahead of
infested area
• Isolated colonies are affected
by Allee effect and stochastic
events
Gypsy moth – basic information
• Introduced into the United States in 1868 by a
French scientist, Leopold Trouvelot, living in
Medford, Massachusetts.
• Overwintering eggs hatch in the spring and
larvae feed on both young and fully developed
foilage
• Dispersal – ballooning early instars and adult
male flight
Methods
• Placed over 50,000 pheromone-baited traps in
the transition zone.
• Traps placed at intertrap distance of 2 km and
placed using handheld GPS units within a 500
m radius from target coordinates
Results
• Isolated colonies in Wisconsin were 1.7x more
likely to persist than colonies in the
Appalachian region and the Midwest
• Isolated colonies were more likely to persist
when closer to the generally infested area
• The basal area of preferred host species and
land use did not explain differences in
persistence rates among regions
Gypsy Moth Management
• Entomophaga maimaiga – Japanese fungus
• natural enemy of gypsy moth larvae
• Specificity - the fungus affects only larvae of
Lepidoptera (order of butterflies and moths)
• Equally virulent in low and high gypsy moth
populations
Typical appearance of gypsy moth larvae
killed by Entomophaga maimaiga.
Questions
• What is the difference between persistence
and spread?
Questions
• What affects small isolated populations of
gypsy moths?
Questions
• Are there any other factors that can affect the
spread of gypsy moths that are not mentioned
in the paper?
Questions
• Should the paper have included a map to
orient the reader?
Source: US Forest Service
Questions
• All regions are political boundaries; how
would you define your regions?
Questions
• How can you make sure that you have the
correct resolution of data? What criteria
would you use to assess that the scale of the
datasets is biologically relevant to the species
in the experiment?
Questions
• Why do isolated colonies closer to the parent
populations persist at a greater rate?
Questions
• What could cause Allee effects to be
potentially different in different regions?
Spatial heterogeneity and rates of
spread in experimental streams
Simpson et al.
• Landscape models suggest that the spatial
scale and pattern of environmental
heterogeneity interacts with dispersal
scale/distance to determined spread rate
• Two possible processes:
- short-distance dispersal must spread
through each patch (end result = slower)
- long-distance events  can skip over less
suitable patches (end result= faster spread)
Species
• Freshwater diatom –
Nitzschia palea
• Disperse through water
flow
• Average movement rate of
90 cm/day
Methods and Materials
• artificial streams (unidirectional flow =
disperal bias towards downstream)
• control variable: phosphate (limiting resource)
availablity and distribution
Figure 1
Experimental design
• each environmental "patch" = agar plate
• stream bottoms: 21 agar plates - inoculation
site at central patch (upstream and
downstream)
• 3 possible resource levels/patches: low (20 µg
phosphate /liter),intermediate (55), high (100)
• heterogenous set-up = alternating high and
low vs. homogenous set-up = all intermediate
Results
• Spread rate slower in upstream direction than
downstream for heterogeneous stream - The
opposite phenomena found for homogenous
stream
Figure 3
Results cont.
• Differences in time to colonization (Tc) among
patches:
Tc(high) < Tc(average)< Tc(low)
i.e. phosphate variablity affects Tc
• positive linear relationship between growth
rate and phosphate level (Figure 4, part c)
Figure 4
Questions
• How were the nutrient levels varied in the
heterogeneous stream? Do you think that it
was effective in terms of the experiment?
Questions
• Why were the researchers unable to
disentangle dispersal from colonization in the
diatoms?
Questions
• Are there any factors that the researchers did
not control for?
Questions
• How did the researchers find the data seen in
this figure?
Questions
• What mechanism could account for the trend
that the rate of spread upstream was slower
in a heterogeneous stream than that of the
rate downstream?
Questions
• Why does environmental heterogeneity does
not influence downstream colonization rates?
Questions
• How would you test the hypothesis that the
stronger movement upstream is due to an
evolutionary bias?
Questions
• How does flow affect gliding motility?