Allee effects and Spatial Heterogeneity By Nicholas Viveros and Jessica Oh.

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 larvaekilled 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?