Weak and Strong Top Down Effects in Similar Food Webs

Insects on plants provide good examples of the variability of top-down strength. While some food chains lack identifiable top-down effects, others with similar species are the opposite, and natural enemies greatly suppress the numbers and biomass of their food populations. A simple chain apparently without top-down effects is that on the roses in the author's yard and close by on the campus of the University of California, Davis. The buds of these roses support a modest number of aphids in spring, upon which a few ladybird beetles and syrphid fly larvae feed. The larvae of the syrphids and both larvae and adult ladybirds are predators of aphids. Also, one or two species of parasitoid wasps attack a few of the aphids in some but not all years. While in principle any or combinations of these species of natural enemies could suppress the aphid populations, in practice the author could find no evidence that they do. The predators eat only a few of the aphids. The predators are not numerous enough, they eat aphids too slowly, and they arrive too late to provide much, if any, top-down effect in this small system. The roses do not suffer much at all as a result of the low aphid densities that appear each spring. This is evident as the buds from which aphids were removed produced roses as lovely as those that bore the densest aphid colonies. It can be inferred that the ladybirds and syrphid fly larvae did not affect aphid numbers because the buds from which they were excluded ended up with no denser aphid colonies than those with the most ladybirds and syrphid larvae. An additional important piece of evidence of weak or absent top-down effects is the fact that very few of the immature aphids are killed by any of these three natural enemies. More numerous, earlier, and more voracious species of natural enemies could suppress the aphids. And, higher densities or different species of aphids could harm the roses. However, factors other than natural enemies come into play to limit aphids, syrphids, and ladybirds to the virtually innocuous densities in the author's garden and on campus. For example, the roses have traits that make them both resistant to and tolerant of the aphids. These resistance traits contribute to slowing the growth of herbivorous insect populations, and plant tolerance traits lead the plant to be able to withstand the feeding of the herbivore.

In contrast, another simple insect plant food chain in the author's yard and on campus with a single species of ladybird beetle, a parasitic wasp, and a herbivorous insect species related to aphids is an exemplar of top-down control. The Vedalia beetle, Icerya purchas, was introduced into California from its native Australia in 1888 for biological control of the cottony cushion scale in the citrus industry. Populations of this nonnative homopteran herbivore exploded in California and within a few years grew so dense over such wide areas as to threaten the citrus farmers with ruin. The threat was reversed by top-down forces brought by the Vedalia beetles, which were introduced to California from Australia. The population growth rate of Vedalia beetles in California and their success in suppressing numbers of cottony cushion scale was phenomenal. By fall of 1899, it was difficult for researchers to find a living specimen of the scale in some areas of southern California, while

Figure 2 A trophic cascade produced by strong top-down effects in a simplified food web for mid-latitude lakes of intermediate (mesotrophic) productivity. The left column illustrates a lake without effective piscivorous fish in which planktivorous fish suppress zooplankton densities and cause the indirect effect of burgeoning phytoplankton populations in mid-summer. On the right, an effective piscivore population reverses the cascade below. The planktivorous fish population is suppressed, zooplankton become dense and suppress the summer phytoplankton. Heavy sport fishing can reduce populations of piscivores and shift a lake from the right to the left side of the figure. Reproduced from Srong DR (1995) Population and community ecology. In: McGraw-Hill Yearbook of Science and Technology, p. 319. New York: McGraw-Hill.

Two links

Piscivores sparse or absent

Planktivores abundant

Planktivores abundant

Herbivores sparse

Herbivores sparse

Phytoplankton abundant

Three links Piscivores abundant

"VL

Planktivores sparse

Herbivores abundant

Phytoplankton sparse #

these pests had literally coated citrus branches only 18 months earlier. The tandem populations of this predator and prey were tightly linked in the long growing season of southern California. The Vedalia beetle adult lays large numbers of eggs. Both larvae and adults feed on both immature and mature scales, and the larvae feed upon the eggs of the scale insect. Vedalia beetle has as many as 20 generations per year and can build up very dense local populations that drive scale densities to very low levels. The Vedalia beetle has been similarly successful in biological control of cottony cushion scale in other parts of the world. The parasitoid wasp Cryptochaetum iceryae was also introduced to California for control of the cottony cushion scale. The author could find very few live cotton cushion scale on citrus or other host plants that support them in his yard and on campus. The lion's share have been killed by either the Vedalia beetle or C. iceryae.

Perhaps the best-known example of weak top-down effects in aphid food webs is that of the tree-dwelling aphids in the United Kingdom. The sycamore aphid, the lime aphid, and the Turkey oak aphid have been closely examined by Tony Dixon and his students at the University of East Anglia over a 40-year period. They found that these arboreal aphid populations are regulated by intraspecific competition as mediated by nutrition provided by the tree. The local and particular characteristics of trees (size, foliage, phenology, and their spatial arrangement in forests) also have a large influence on the performance of the aphid generation and the size of succeeding generations. Abiotic conditions as influenced by the weather are another important influence. While the weather cannot regulate populations about an equilibrium density because there is no negative feedback from populations to the causes of weather, these aphid populations are nonetheless greatly affected by the weather. This makes any regulation by predators more difficult to detect. None of Dixon's many studies indicated that top-down forces from the natural enemies of the aphids contributed much, if anything, to the numbers of these aphid populations in space or through time. For the sycamore aphid, the food web effects of the natural enemies - parasitoids and predators - were particularly well studied. The aphid parasitoids are Hymenoptera that lay eggs and develop inside aphids. Four of the parasitoids are host specific and attack no other aphids; the fifth species has other insect hosts. The insect predators include eight species of various kinds.

The reasons for weak top-down effects in these tree-dwelling aphids are the weak linkage between aphids and the population dynamics of the natural enemies. The parasitoids and predator insects of these arboreal aphids specialize upon and cause most mortality to young aphids in early instars. The young aphids are particularly abundant. It is reasonable that evolution has driven the predators and parasitoids to specialize upon young aphids, which provide much greater resources for them than old aphids. The older instars of these aphids, which live later in the growing season, have a much larger effect upon the dynamics of subsequent aphid generations than do early instars upon which the natural enemies specialize. Dixon and his colleagues learned that the natural enemies have finished their annual influence upon the aphid populations before this crucial later part of the growing season. The physiological condition of the host tree has the largest influence upon performance of later instars and this condition determines the survivorship, growth rate, and fecundity of the older aphids. The older aphids do better in places and in years with more vigorous trees. In addition, the natural enemies have longer generation times than the host/prey aphids. Because successive generations of the aphids move around, the longer generation times of the enemies mean that top-down forces are not located where and when aphids are more abundant; the enemies cannot track the population densities of aphids well. Finally, the parasitoids of these aphids suffer heavy hyperparasitism and also suffer heavy predation in the mummy stage, which also diminishes the ability of the parasitoid populations to track the aphids. The conclusion is that these tree-dwelling aphids are not subjected to effective top-down forces.

Finally, consider a well-researched food web made up of two food chains in which strong top-down effects in one are affected by the other (Figure 3).

The arthropod community on papaya in Hawaii studied by Jay Rosenheim and his students illustrates reticulate predatory interactions among food chains. There are strong top-down influences when the predatory beetle, Stethorus, suppresses populations of a herbivorous mite to low levels. However, the presence of a spider that entangles beetles in its web prevents the top-down effects and allows the herbivorous mite to increase and harm the papaya and its fruit. This interruption of top-down effects does not extend to the predacious

Nesticodes rufipes (tangle-web spider)

Stethorus siphonulus

(predatory beetle) (predatory beetle)

Tetranychus cinnabarinus (herbivorous mite)

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