Please Note: The title and description of this exercise are listed incorrectly at the end of Chapter 20 in the first printing of the textbook. The correct description is as follows:
“This web exercise (www.sinauer.com/ecology) explores the effects of extinctions and introductions in marine communities on food web structure. You will interpret data from a recent paper that demonstrates that species extinctions and introductions are occurring preferentially at different trophic levels.”
(This exercise is based on Byrnes, J. E., P. L. Reynolds, and J. J Stachowicz. 2007. Invasions and Extinctions Reshape Coastal Marine Food Webs. PLoS ONE 2(3): e295.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
In ecosystems worldwide, human activity has resulted in an increase in both native species extinctions and biological invasions. On the local and regional scale, rates of gains of species may be comparable or even exceed that of extinctions. Thus, species diversity may remain relatively constant at least in the short run.
Even if the diversity of species in an ecosystem remains roughly constant, it is possible that the composition of those species may change due to species introductions and extinctions. The processes that lead to species extinctions are different than those involved in species introductions. Thus, it is likely that the species gained from introductions will be different from those that are lost from extinction. For instance, larger animals are likely to have smaller population sizes and thus be more prone to extinction. On the other hand, smaller animals and plants that tend to have short generation times and a large number of possible offspring per individual parent may be more able to invade a community.
The differences in the attributes of the species likely to go extinct and those likely to become introduced suggest that the extinctions and introductions may occur at different trophic levels. Jarrett Byrnes and John Stachowitz at the University of California at Davis and Pamela Reynolds at the Bodega Marine Laboratory in California examined records of introductions and extinctions to address how such changes in species composition affect food webs.
Figure 1 Food web showing the connections between all trophic functional groups. Arrows represent one group consuming the group to which the arrow points. Shading indicates trophic level (none = 1, light = 2, moderate = 3, dark = 4). (Click to enlarge.)
Question 1
Figure 1 shows a generalized food web. The researchers assigned all species of their studies to one of the categories shown here. To which trophic level are deposit feeders assigned?
Question 2
According to the food web organization presented in Figure 1, how many trophic levels separate a predator from an herbivore?
Question 3
Based on the discussion in the Introduction about population size and the likelihood of extinction, do you think it more likely that extinctions would occur with more frequency at trophic level 2, or at trophic levels 3 and 4? Explain.
Figure 2
Question 4
The researchers looked at marine species that had gone extinct either globally or regionally historically. The distribution of these extinctions across the different categories and trophic levels is shown in Figure 2. The percentage of species in each category is shown, and the trophic levels are color-coded (white= 1, light grey = 2, moderate grey = 3, dark grey = 4). Which trophic level suffered the largest percentage of extinctions?
Question 5
About what percent of extinctions were in trophic levels 3 and 4?
Question 6
Do the actual results of extinction patterns in the food web match what you had hypothesized in Question 3?
Figure 3
Question 7
The researchers also looked at introductions of marine species into the San Francisco Bay, the Gulf of the Farallones, and Australia. The distribution of these introductions across the different categories and trophic levels is shown above. The percentage of species in each category is shown, and the trophic levels are color coded as before. From which trophic level did the most species introductions arise?
Question 8
What is/are the major difference(s) between the introductions in Australia as compared with those from the San Francisco Bay and the Gulf of the Farallones?
Figure 4 (Click to enlarge.)
Question 9
The researchers took a closer look at patterns of extinctions and introductions as they relate to trophic levels in the Wadden Sea in the Netherlands. In Figure 3, the top panel shows the distribution of species in the Wadden Sea before human-associated changes. To which trophic level did the most Wadden species belong before the changes?
Question 10
The middle and bottom panels of Figure 3 show the distribution of extinctions and introductions in the Wadden Sea, respectively. In which two trophic levels did most of the extinctions occur? What percentage of species extinctions belonged to these trophic levels?
Question 11
In which trophic level did most of the introductions take place? What percentage of species extinctions belonged to these trophic levels?
Question 12
Based on the patterns of extinctions and introductions, what would you expect to happen to the percentages of species in each of the four trophic levels in the Wadden sea.
