Unit Eight. The Living Environment


35. Populations and Communities


35.13. Ecological Succession


Competition, predation, and cooperation often produce dramatic changes in communities. This results in changes in ecosystems, by the orderly replacement of one community with another, from simple to complex in a process known as succession. This process is familiar to anyone who has seen a vacant lot or cleared woods slowly become occupied by an increasing number of plants, or a pond become dry land as it is filled with vegetation encroaching from the sides.


Secondary Succession

If a wooded area is cleared and left alone, plants slowly reclaim the area. Eventually, traces of the clearing disappear and the area is again woods. Similarly, intense flooding may clear a stream bed of many organisms, leaving mostly sand and rock; afterward, the bed is progressively reinhabited by protists, invertebrates, and other aquatic organisms. This kind of succession, which occurs in areas where an existing community has been disturbed, is called secondary succession.


Primary Succession

In contrast, primary succession occurs on bare, lifeless substrate, such as rocks. Primary succession occurs in lakes left behind after the retreat of glaciers, on volcanic islands that rise above the sea, and on land exposed by retreating glaciers. Primary succession on glacial moraines provides an example. The graph in figure 35.26 shows how the concentration of nitrogen in the soil changes as primary succession occurs. On bare, mineral-poor soil, lichens grow first, forming small pockets of soil. Acidic secretions from the lichens help to break down the substrate and add to the accumulation of soil. Mosses then colonize these pockets of soil (figure 35.26a), eventually building up enough nutrients in the soil for alder shrubs to take hold (figure 35.26b). These first plants to appear form a pioneering community. Over 100 years, the alders (figure 35.26c and inset photo) build up the soil nitrogen levels until spruce are able to thrive, eventually crowding out the alder and forming a dense spruce forest (figure 35.26d).



Figure 35.26. Plant succession produces progressive changes in the soil.

Initially the glacial moraine at Glacier Bay, Alaska, had little soil nitrogen, but nitrogen-fixing alders (photo above) led to a buildup of nitrogen in the soil, encouraging the subsequent growth of the conifer forest.


Primary successions end with a community called a climax community, whose populations remain relatively stable and are characteristic of the region as a whole. However, because local climate keeps changing, the process of succession is often very slow, and human activities have a major impact; many successions do not reach climax.


Why Succession Happens

Succession happens because species alter the habitat and the resources available in it, often in ways that favor other species. Three dynamic concepts are of critical importance in the process: tolerance, facilitation, and inhibition.

1. Tolerance. Early successional stages are characterized by weedy r-selected species that do not compete well in established communities but are tolerant of the harsh, abiotic conditions in barren areas.

2. Facilitation. The weedy early successional stages introduce local changes in the habitat that favor other, less weedy species. Thus, the mosses in the Glacier Bay succession of figure 35.26 fix nitrogen, which allows alders to invade. The alders in turn lower soil pH as their fallen leaves decompose, allowing spruce and hemlock, which require acidic soil, to invade.

3. Inhibition. Sometimes the changes in the habitat caused by one species, while favoring other species, inhibit the growth of the species that caused them. Alders, for example, do not grow as well in acidic soil as the spruce and hemlock that replace them.

As ecosystems mature, and more K-selected species replace r-selected ones, species richness and total biomass increase but net productivity decreases. Because earlier succes- sional stages are more productive than later ones, agricultural systems are intentionally maintained in early successional stages to keep net productivity high.


Key Learning Outcome 35.13. In succession, communities change through time, often in a predictable sequence.


Inquiry & Analysis

Are Island Populations of Song Sparrows Density Dependent?

When island populations are isolated, receiving no visitors from other populations, they provide an attractive opportunity to test the degree to which a population's growth rate is affected by its size. A population's size can influence the rate at which it grows because increased numbers of individuals within a population tend to deplete available resources, leading to an increased risk of death by deprivation. Also, predators tend to focus their attention on common prey, resulting in increasing rates of mortality as populations grow. However, simply knowing that a population is decreasing in numbers does not tell you that the decrease has been caused by the size of the population. Many factors, such as severe weather, volcanic eruption, and human disturbance, can influence island population sizes too.

The graph to the right displays data collected from 13 song sparrow populations on Mandarte Island (see map below). In an attempt to gauge the impact of population size on the evolutionary success of these populations, each population was censused, and its juvenile mortality rate estimated. On the graph, these juvenile mortality rates have been plotted against the number of breeding adults in each population. Although the data appear scattered, the "best-fit” regression line is statistically significant (statistically significant means that there is a less than 5% chance that there is, in fact, no correlation between dependent and independent variables).




1. Applying Concepts

a. Variable. In the graph, what is the dependent variable?

b. Analyzing Scattered Data. What is the size of the song sparrow population (based on breeding adults) with the least juvenile mortality? with the most?

2. Interpreting Data

a. What is the average juvenile mortality of all 13 populations, estimated from the 13 points on the graph?

b. How many populations were observed to have juvenile mortality rates below this average value? What is the average size of these populations?

c. How many populations were observed to have juvenile mortality rates above this average value? What is the average size of these populations?

3. Making Inferences. Are the populations with lower juvenile mortality bigger or smaller than the populations with higher juvenile mortality?

4. Drawing Conclusions. Do the population sizes of these song sparrows appear to exhibit density dependence?

5. Further Analysis

a. The fact that the song sparrow populations with lower juvenile mortality are a different size than those with higher juvenile mortality does not, in itself, establish that the difference is statistically significant. How would you go about testing these data to see if the relationship between juvenile mortality and population size is real?

b. What would you expect to happen if the researchers supplemented the food available to the birds? Explain.

c. What would you expect to happen if the researchers removed individuals from populations with more than 100 breeding adults, reducing each to 100?



Test Your Understanding

1. In the levels of ecological organization, the lowest level, composed of individuals of a single species who live near each other, share the same resources, and can potentially mate, is a(n)

a. population.      

b. community      

c. ecosystem.

d. biome.

2. When the number of organisms in a population remains more or less the same over time in the specific place where these organisms live, it is said that this population of organisms has reached its

a. dispersion.      

b. biotic potential.

c. carrying capacity.

d. population density.

3. Which of the following is a density-dependent effect on a population?

a. earthquake

b. increased competition for food

c. habitat destruction by humans

d. seasonal flooding

4. Which of the following traits is not a characteristic of an organism that has K-selected adaptations?

a. short life span

b. few offspring per breeding season

c. extensive parental care of offspring

d. low mortality rate

5. If the age structure of a population shows more older organisms than younger organisms, then the fecundity

a. will increase, and the mortality will decrease.

b. will decrease, and the mortality will increase.

c. and the mortality will be equal.

d. and the mortality will not change.

6. All the organisms that live in the same location make up a(n)

a. biome.

b. population.

c. ecosystem.

d. community.

7. For similar species to occupy the same space, their niches must be different in some way. One way for these species to both survive is through

a. competitive exclusion.

b. interspecific competition.

c. resource partitioning.

d. intraspecific competition.

8. A relationship between two species where one species benefits and the other is neither hurt nor helped is known as

a. parasitism.

b. commensalism.

c. mutualism.

d. competition.

9. The yellow-and-black-striped patterns of many kinds of stinging wasps are examples of

a. parasitism.

b. Mullerian mimicry.

c. commensalism

d. Batesian mimicry.

10. Succession that occurs on abandoned agricultural fields is best described as

a. coevolution.

b. primary succession.

c. secondary succession.

d. prairie succession.