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 Page - 1 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 Page - 2 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 Page - 3