Summary

In this chapter, we shift the focus to systems that usually have at least three trophic levels and with 'many' species.

We describe 'unexpected' effects in food webs, where, for example, the removal of a predator may lead to a decrease in prey abundance.

The indirect effect within food webs that has received most attention is the trophic cascade. We discuss cascades in systems with three and four trophic levels, and address the question of whether cascades are equally common in all types of habitat, requiring a distinction to be made between community- and species-level cascades. We ask whether food webs, or particular types of food web, are dominated by either top-down (trophic cascade) or bottom-up control. We then define and discuss the importance of keystone species.

answers are uncertain - but it is important that we discover them

Figure 20.19 Pictorial representation of the results of an analysis of a food web from Chesapeake Bay (see also Figure 20.13) in which interactions between the 45 taxa were quantified and the taxa assigned to compartments (the number of which was not predetermined) in such a way as to maximize the differential between the connectance within compartments (in this case 0.0099) and that between compartments (in this case 0.000087, more than two orders of magnitude lower). Food webs may be considered compartmentalized if that differential is sufficiently large. Arrows represent interactions and point from predator to prey: solid color, within compartments; dashed lines, between compartments. (After Krause et al., 2002.)

1 Phytoplankton

2

Benthic producers

3

Bacteria <1 |m (small)

4

Bacteria >1 <2 |m (medium)

5

Bacteria >2 |m (large)

6

Acartia tonsa (copepod)

7

Microciliates

8

Macrociliates

9

Predaceous ciliates

10

Chrysaora quinquecirrha

(sea nettle)

11

Mnemiopsis leidyi

(comb jelly)

12

Nemopsis bachei (jellyfish)

13

Cladocera

14

Other zooplankton

15

Anchoa mitchilli larvae

(anchovy)

16

Anchoa mitchilli eggs

17

Fish larvae

18

Marenzelleria viridis

(polychaete)

19

Nereis succinea (polychaete)

20

Hetermastus filiformis

(oligochaete)

21

Other polychaetes

22

Corophium lacustre

(amphipod)

23

Leptocheirus plumulosus

(amphipod)

24

Other meiofauna

25

Macoma baithica

(Baltic clam)

26

Macoma mitchelli

(rosy clam)

27

Rangia cuneata

(wedge clam)

28

Mulinia lateralis (coot clam)

29

Mya arenaria

(soft-shelled clam)

30

Crassostrea virginica (oyster)

31

Callinectes sapidus

(blue crab)

32

Anchoa mitchilli

(bay anchovy)

33

Micropogon undulatus

(croaker)

34

Trinectes maculatus

(hogchoaker)

35

Leiostomus xanthurus (spot)

36

Cynoscion regalis (weakfish)

37

Alosa sapidissima

(American shad)

38

Alosa pseudoharengus

(alewife)

39

Alosa aestivalis

(blue-back herring)

40

Brevoctia tyranus

(menhaden)

41

Morone americana

(white perch)

42

Morone saxatilis

(striped bass)

43

Pomatomas saltatrix

(bluefish)

44

Paralichthys dentatus

(flounder)

45

Arius felis (catfish)

Figure 20.19 Pictorial representation of the results of an analysis of a food web from Chesapeake Bay (see also Figure 20.13) in which interactions between the 45 taxa were quantified and the taxa assigned to compartments (the number of which was not predetermined) in such a way as to maximize the differential between the connectance within compartments (in this case 0.0099) and that between compartments (in this case 0.000087, more than two orders of magnitude lower). Food webs may be considered compartmentalized if that differential is sufficiently large. Arrows represent interactions and point from predator to prey: solid color, within compartments; dashed lines, between compartments. (After Krause et al., 2002.)

Any ecological community can be characterized by its structure, its productivity and its temporal stability. The variety of meanings of 'stability' is outlined, distinguishing resilience and resistance, local and global stability, and dynamic fragility and robustness.

For many years, the 'conventional wisdom' was that more complex communities were more stable. We describe the simple mathematical models that first undermined this view. We show how, in general, the effects of food web complexity on population stability in model systems has been equivocal, whereas for aggregate properties of whole model communities, such as their biomass or productivity, complexity (especially species richness) tends consistently to enhance stability.

In real communities, too, evidence is equivocal at the population level, including both studies that have examined the relationships between species richness and connectance and those that have manipulated richness experimentally. Again, turning to the aggregate, whole community level, evidence is largely consistent in supporting the prediction that increased richness increases stability (decreases variability). We stress, though, the importance of the nature, not just the richness, of a community in these regards, returning to the importance of keystone species.

Limitations and patterns in food chain length are discussed. We examine the evidence that food chain length is limited by productivity, by 'productive space' (productivity compounded by the extent of the community) or simply by 'space' - but that evidence is inconclusive. We examine, too, the arguments that food chain length is limited by dynamic fragility (ultimately unconvincing) or by constraints on predator design and behavior. There is a clear need for rigorous studies of many more food webs before acceptable generalizations can be reached.

We examine work linking the prevalence of omnivory and its effect on food web stability, noting that earlier work found omnivory to be rare and destabilizing, whereas later work found it common and with no consistent effect on stability.

Finally, we ask whether food webs tend to be more compartmentalized than would be expected by chance. As long as habitat divisions are subtle, the evidence for compartments is typically poor, and there are even greater difficulties in demonstrating compartments (or the lack of them) within habitats. There is, though, a clear consensus from theoretical studies that communities will have increased stability if they are compartmentalized.

Chapter 21

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