Empirical patterns in food webs the number of trophic levels

In the previous section, we examined very general aspects of food web structure - richness, complexity - and related them to the stability of food webs. In this section, we examine some more specific aspects of structure and ask, first, if there are detectable repeated patterns in nature, and second whether we can account for them. We deal first, at greatest length, with the number of trophic levels, and then turn to omnivory and the extent to which food webs are compartmentalized.

A fundamental feature of any food web is the number of trophic links in the pathways that run from basal species to top predators. Variations in the number of links have usually been investigated by examining food chains, defined as sequences of species running from a basal species to a species that feeds on it, to another species that feeds on the second, and so on up to a top predator (fed on by no other species). This does not imply a belief that communities are organized as linear chains (as opposed to more diffuse webs); rather, individual chains are identified purely as a means

s Scotch Broom

s Scotch Broom

Canton

Stony

Canton

Stony

x Chesapeake

J_I_il x Chesapeake

s St Martin

s St Martin

Mirror

Mirror

J_iJ

0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

Species removed / S

J_iJ

0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8

Species removed / S

O Most connected < Most connected, no basal deletions Random A Least connected

Figure 20.13 The effect of sequential species removal on the number of consequential ('secondary') species extinctions, as a proportion of the total number of species originally in the web, S, for each of 16 previously described food webs. The four different rules for species removal are described in the key. Robustness of the webs (the tendency not to suffer secondary extinctions) increased with the connectance of the webs, C (regression coefficients for the four rules: -0.62 (NS), 1.16 (P < 0.001), 1.01 (P < 0.001) and 0.47 (P < 0.005)). Overall, though, robustness was lowest when the most connected species were removed first and highest when the least connected were removed first. The origins of the webs are described in Dunne et al. (2002). (After Dunne et al., 2002.)

food chain length

Maximal food chains

4

11

12

2

4

11

12

3

i

12

4

11

13

2

4

11

13

3

i

13

4

12

2

4

12

3

B

12

4

13

2

4

13

3

B

13

S

11

12

2

S

11

12

3

g

12

S

11

13

2

S

11

13

3

g

13

S

12

2

S

12

3

10

13

S

13

2

S

13

B

12

2

B

12

Figure 20.14 Community matrix for an exposed intertidal rocky shore in Washington State, USA. The pathways of all possible maximal food chains are listed. 1, detritus; 2, plankton; 3, benthic algae; 4, acorn barnacles; 5, Mytilus edulis; 6, Pollicipes; 7, chitons; 8, limpets; 9, Tegula; 10, Littorina; 11, Thais; 12, Pisaster; 13, Leptasterias. (After Briand, 1983.)

of trying to quantify the number of links. Food chain length has been defined in various ways (Post, 2002), and in particular has sometimes been used to describe the number of species in the chain, and sometimes (as here) the number of links. For instance, starting with basal species 1 in Figure 20.14, we can trace four possible trophic pathways via species 4 to a top predator: 1-411-12, 1-4-11-13, 1-4-12 and 1-4-13. This provides four food chain lengths: 3, 3, 2 and 2. Figure 20.14 lists a total of 21 further chains, starting from basal species 1, 2 and 3. The average of all the possible food chain lengths is 2.32. Adding one to this gives us the number of trophic levels that can be assigned to the food web. Almost all communities described have consisted of between two and five trophic levels, and most of these have had three or four. What sets the limit on food chain length? And how can we account for variations in length?

In addressing these questions, we will conform to a bias that has pervaded investigations of food chain length - a bias in favor of predators and against parasites. Thus, when a food chain is described as having four trophic levels, these would typically be a plant, a herbivore, a predator that eats the herbivore, and a top predator that eats the intermediate predator. Assume the top predator is an eagle. Even without collecting the data, it is all but certain that the eagle is attacked by parasites (perhaps fleas), which are themselves attacked by pathogens. But the convention is to describe the chain as having four trophic levels. Indeed, descriptions of food webs generally have paid little attention to parasites. There is little doubt that this neglect will have to be rectified (Thompson et al., 2005).

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment