Habitat degradation proceeds at an ever-escalating pace. Land is cleared, pollutants are released into the air, land, and water, and roads are cut with increasing frequency as the human population continues growing exponentially. The effects of habitat degradation compromise the quality not only of the land area directly around the degradation, but also typically affect additional land surrounding the site.

A frequent consequence of habitat degradation is a simplification of the environment. As land is compromised, conditions essential for the persistence of indigenous species vanish and those species go extinct. As a consequence, the only species that are left after the degradation are those that can cope with the radical environmental changes. These species are only a subset of the original community.

Many scientists have argued that as biodiversity is reduced, the stability of the ecosystem is also reduced, in that the ecosystem is less able to respond to environmental perturbations than it was originally. SAY WHY Therefore, a possible long-term outcome of habitat degradation is that the ecosystem will not be able to recover quickly.

Habitat degradation is usually thought of in terms of human activities that clearly and directly damage the environment, such as when land is cleared for farming or development or when pollution is dumped into the ocean. Such activities directly and immediately decrease the biological utility of the environment in their immediate area.

However, there are many human activities that may unexpectedly degrade habitat, including the release of chemicals that have led to the ozone hole and global warming. An equally important but less obvious degrader of habitat is the introduction of exotic species. The most successful introduced species, such as the gypsy moth in the Northeastern U.S., have directly contributed to the extinction of indigenous species. Approximately 68% of all the extinction events where the main causative factors are known involve the introduction of an exotic species. The introduction of the domestic cat alone into Hawaii has been estimated to be responsible for the extinction of more than twenty species of birds found only there.

The majority of introduced species are successful. Successful introduced species are successful because they are introduced. As a consequence of being taken from their original location, they are isolated from the predators, parasites, and pathogens that evolved to interact with them. Therefore, in their new habitat the introduced species experience "predatory release" and are often able to grow without most of their normal ecological restraints. Consequently, population sizes of these species tend to explode and decimate their host species and competitors.

A similar situation is currently happening in the Northeastern U.S. involving the Eastern Hemlock tree (Tsugae canadensis) and an introduced pest feeding on them called the Hemlock Woolly Adelgid (HWA, Adelges tsugae). The HWA, an aphid-like insect that feeds on Hemlock phloem, was introduced into the Pacific Northwest in 1924 from Japan with a timber delivery and had made its way to the east coast by the 1950’s.

Since its introduction into the eastern U.S., HWA has expanded its range primarily to the Northeast and now is found as far north as Massachusetts, covering approximately 25% of the entire Hemlock range. In many areas infested with HWA, the health of Hemlock has declined precipitously, with heavily infested trees yellowing and eventually becoming entirely defoliated shortly before the trees die. There are no natural predators that can effectively control the population of the HWA, and it seems that the northern spread is slowed only by cold temperatures.

Hemlock is a significant and valuable forestry and ornamental tree, and also provides many valuable unique benefits to forests that have the tree growing in them. As such, Hemlock is a tree that is very much worth preserving from this artificially introduced species that threatens to extensively degrade habitat in the Northeastern U.S. and greatly reduce the biodiversity therein.

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I. (2 hours) Estimate the number of acres in Black Rock Forest covered by Hemlock since 1937. Compare aerial photos to determine areas of maximum Hemlock stand degradation and calculate changes in total area of hemlock stands.

Methodology:

1. Locate Cascade Brook and Black Rock Hollow, the two main hemlock stands in Black Rock Forest, on digitized versions of aerial photographs from 1937, 1970, 1977, 1980, and 1998.
2. Trace the boundaries of each hemlock stand represented in the five time slices.
3. Using the 3x views of the aerial photos, count the number of complete sectors that are covered in each time slice by hemlock.
4. Estimate the number of partially covered sectors.
5. Calculate the total area covered by hemlock, using the grid size to hectare conversion provided.
6. Isolate those areas in the hemlock stands that are changing most rapidly.
7. Do an ANOVA test comparing the area covered by the hemlock in each of the time slices to determine whether the changes in distribution over time are significant.
8. Compare your results with your classmates.

II. (7 hours) With your lab partner, design, conduct, and analyze a field-based experiment that examines some community ecology aspect of the Hemlock-adelgid relationship. Several possible questions follow. Be sure to include questions of your own design.

To improve the results of the experiment, multiple groups should collect data for the same study. Talk to the other groups to find partners interested in addressing similar questions.

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You must finish both components of Assignment 18 before the end of the day. Tomorrow morning, before the lecture, all lab groups will present 10 minute oral reports. Each of you will present a part of your group’s results. You will be graded not only on the outcomes of your experiments, but also on how well you work together and on the final oral presentation.

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