BIOS 3010: ECOLOGY
Laboratory 11: Metapopulation Dynamics
Dr. Stephen Malcolm
Tetraopes tetrophthalmus on common
milkweed, Asclepias syriaca
Cerambycid beetle larva
http://woodypest.ifas.ufl.edu/images/bore2.jpg
Introduction:
As described in the last laboratory, spatial and temporal patterns of
distribution of individuals within populations or metapopulations is an important
consequence of their responses to the distributions of resources and conditions
as well as to different ecological processes, within the evolutionary constraints of
their life histories.
BIOS 3010: Ecology
Laboratory 11: Metapopulation Dynamics
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It is common for populations of species to be distributed among a
series of partially isolated subpopulations and for individuals to move
occasionally from one group to another. The size, number and degree of
isolation of the units of such a METAPOPULATION and the dispersal abilities of
individuals influence the regional persistence of a species. This makes the
dynamics of metapopulations an important determinant of species persistence
and it is a major concern for conservation biologists. This is especially true
because rarer species are more likely to show fragmented populations
distributions because they are more likely to be habitat specialists than common,
habitat generalists. Generalized resource use life histories are likely to result in
random or regular distributions of individuals in time and space. In contrast,
specialized life history patterns of resource use are more likely to result in
strongly aggregated distributions of individuals that communicate by dispersal
among the components of a metapopulation.
In this laboratory exercise we would like you to test this idea by
comparing the distributions in space of the herbivorous larvae of two beetle
species.
The two beetles are, Tetraopes tetrophthalmus, the red milkweed
beetle, and Popillia japonica, the Japanese beetle. T. tetrophthalmus is a
longhorn beetle in the family Cerambycidae and is a specialist herbivore of
milkweeds in the genus Asclepias, especially the common milkweed, A. syriaca.
P. japonica is an introduced pest species in the large family Scarabaeidae, that is
a broadly generalist herbivore of over 275 different plant species and the larvae
are serious pests of lawns, grasses and nursery stock.
Hypotheses:
In this exercise, please formulate null and alternative hypotheses
about the influence that such diet breadth differences are likely to have on
whether or not these two beetle species show a metapopulation distribution of
individuals in space.
The beetle life histories:
The Japanese beetle, P. japonica, was first reported in North America
in 1916 and now occurs in more than 20 states from southern Maine south to
Georgia and west to Kentucky, Illinois, Michigan and Missouri.
Adult beetles emerge from the soil from mid May to late July and feed
on many plant species and mate. Females lay batches of 40-60 eggs in small
batches 5-8 cm deep in the soil and larvae hatch and feed on roots, especially of
grasses. Three larval instars take approximately 140 days to complete and this
period is interrupted by winter. Larvae overwinter in cells approximately 13 cm
from the soil surface and in spring they move up to ground level and complete
feeding and pupate.
BIOS 3010: Ecology
Laboratory 11: Metapopulation Dynamics
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The milkweed beetle, T. tetrophthalmus, is a very unusual cerambycid
beetle because it is a leaf- and root-feeding herbivore of milkweeds rather than a
wood borer as is typical for most members of this large and diverse family of
beetles. Adult T. tetrophthalmus emerge from pupae in the soil in June and July
and crawl or fly to the tops of milkweed ramets where they feed on young leaves
and mate. Females lay long, thin eggs into the pith of adjacent grass stems and
the larvae emerge from these eggs and drop to the soil around the underground,
root-like stems of the common milkweed, A. syriaca. This milkweed species is
highly modular and a genet (the genetic individual) is commonly made of from
10s to 100s of ramets. Thus the probability that larvae will find root resources to
exploit is high. Larvae feed through late summer and early autumn and then
diapause in the soil near milkweed roots. Like the Japanese beetle larvae
complete their feeding in spring, pupate and adult emerge in mid summer to
complete the life cycle.
Both beetle species have a single generation per year.
Methods:
Organize yourselves into working groups of 3 or 4 and measure the
distribution of Japanese beetle and milkweed beetle larvae both inside and
outside milkweed genets. Randomly select samples and dig with a garden fork
to the depth of the fork tines and search carefully for larvae that look like those
illustrated above. Repeat your sample digs until you have a sample size of both
beetle species that will allow you to make statistical comparisons of their
distributions both inside and outside milkweed genets. Then replicate your
sample by repeating sampling at a further 2 milkweed ramets. This will give you
N = 3 and repeated samples at each location. Use the numbers of larvae of each
species inside and outside the milkweed genet to compare distributions.
BIOS 3010: Ecology
Laboratory 11: Metapopulation Dynamics
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