Changing Biological Communities – Disturbance and Succession[1]

A biological community consists of all the types of organisms that live in a given area. The different types of organisms in a community interact and influence each other in multiple ways. Two important types of interaction are consumption (e.g. some birds eat insects) and competition (e.g. plants compete for sunlight).

1.Complete this table to give additional examples of consumption and competition for the organisms in a forest in the northeastern US (e.g. trees, bushes, deer, squirrels, spiders and insects).

Type of Interaction / Examples
Consumption
Competition

A biological community often appears to stay the same from one year to the next. However, over periods of decades or centuries, many biological communities show major changes.

A biological community may be disrupted by a disturbancesuch as a flood, volcanic eruption or human activity (e.g. removing trees and plowing to prepare a field for farming).After the disturbance ends, a biological community typically changes by natural processes; this type of change is called succession.

Often, succession gradually restores a biological community so it becomes similar to the community that existed before the disturbance. For example, when a farm field in the northeastern US is no longer used for farming, succession often gradually restores a forest community.

Annual and perennial plants and grasses are herbaceous plants (plants with soft stems). Shrubs and trees have woody stems. (

This graph shows the trends in the types of plants in fields that had not been farmed for 1-60 years.Herbaceous plants have soft stems (e.g. the grasses and goldenrod shown in the photo). Trees and shrubs have woody stems. It typically takes a decade or two for a tree to develop from seedling through the juvenile sapling stage.
/

(Data for fields in northwestern New Jersey from Ecological Monographs 22:195-215, 1952)

2. Use the information from thegraph to complete this table.

Year / Main type of plant you would see / Other types of plants in the field
Last year of farming / Farm crop (e.g. corn) / Some weeds (e.g. grasses, goldenrod)
5 years after farm field
abandoned
60 years after farm field abandoned

3.Explain why there are no or very few tall trees in an abandoned farm fieldin the first decade after the field is no longerfarmed.

During the first decade after these farm fields had been abandoned, most of the area was covered byherbaceous plants that grow and reproduce rapidly and grow well in full sun.This flowchart shows how these herbaceous plants became common in these abandoned farm fields.

4a.By 40-60 years after farm fields had been abandoned, the amount of herbaceous plants decreased. Give one reason why the amount of herbaceous plants decreased as the amount of trees increased.

4b.The types of herbaceous plants changed at different times during succession. For example, ragweed (which grows well in full sun and completes its life cycle in one year) was very common in the first year after a farm field was abandoned, but less common in later years. In contrast, wild strawberry (which can grow in partial shade and grows back each spring from its roots) was more common inyears 25-40. Explain the likely reasons for these changes in the types of plants at different times during succession.

Notice how the growth of trees changes the environment in ways that influence which other plants grow in a biological community. The growth of trees also influences which types of animals live in a biological community. For example, forests provide food for white-tailed deer, so these deer become more common in areas that have some forest.

5. The animals in a biological community can influence the growth and reproduction of different types of plants and this can influence plant succession. Use the information in this flowchart to explain how a large population of white-tailed deer can reduce the growth and reproduction of trees and thus can slow the rate of development of forests during succession. /



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6.An article in a New Jersey newspaper claimed that the damage caused by logging is permanent and the forest ecosystem never recovers. (An ecosystem includes the physical environment as well as the biological community in an area.) The reporter supported his claim with photos that showed (1) the forest ten years ago, just before all the trees were cut and (2) the same area today with mainly grass and flowers and no tall trees. How would you explain to the reporter thatthe evidence in these photos does not show that the forest ecosystem would never recover?

There are some cases where succession does not restore the original biological community after a disturbance. For example, disturbance can allow invasive non-native plants to become established and these plants can interfere with the expected pattern of succession. In one area in Connecticut, succession proceeded as expected for the first three decades after the end of farming, but by the end of the fourth decade an invasive non-native vine had spread over the western half of the area (shown in the left half of the drawing). This vine had originally sprouted on the western edge of the study area and hadn't yet reached the eastern half of the study area which had the type of forest expected as the result of succession (shown in the right half of the drawing).

(From Journal of Vegetation Science 10:483-492, 1999)

7.Explain why the trees in the western half of the study area were smaller than the trees in the eastern half.

8a.In this question you will create a large flowchart that shows multiple biological processes that play a role in succession in abandoned farm fields. In the upper part of the space, show the growth and reproduction of herbaceous plants. (Hint: Review the flowchart on page 2.) Also, show the effects of competition between trees and herbaceous plants.

8b.In the lower part of the space for this large flowchart, show the effects of deer on tree growth and reproduction.

8c.To link the processes in the flowchart to changes during succession in abandoned farm fields:

– Underline the processes that result in the gradual development of a forest.

– Circle the part of the flowchart that shows how competition contributes to a reduction in herbaceous plants during the later decades of succession.

– Use { } to indicate the part of the flowchart that shows how a large population of deer can slow the rate of development of a forest.

The physical environment affects the pattern of succession. For example, trees generally need more water than grass; therefore, in regions with limited precipitation succession may culminate in grasslands, rather than forests.

/ Major Biomes

To measure total precipitation, snow, sleet and hail were converted to the equivalent amount of centimeters of rainfall per year. / A biome is a large region with characteristic climate and plants adapted to that climate.

9.Use the information given above to explainwhy the eastern part of the US has a forest biome while much of the central US has a grassland biome.

A modelis a simplified representation of reality that highlights certain key features of a phenomenon. A good model helps us to better understand and visualize a phenomenon. Three different models of succession on abandoned farm fields in the northeastern US are shown in this handout: the figure on page 1, the graph on page 2, and the flowchartin question 8.

10a.Whatimportant points about succession are illustrated by the figure on page 1?

10b.What additional important points about succession are illustrated by the graph on page 2?

10c.Whatadditional important points about succession are illustrated by the flowchart in question 8?

10d.Mhat additional important points about succession are not included in any of these three models?

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[1]By Dr. Ingrid Waldron, Dept. Biology, University of Pennsylvania, © 2015. This Student Handout andTeacher Notes with learning goals, instructional suggestions and background information are available at .