Topdown or bottomup control of food webs Why is the world green
We have seen that trophic cascades are normally viewed 'from the top', starting at the highest trophic level. So, in a three-level trophic community, we think of the predators controlling the abundance of the grazers and say that the grazers are subject to 'top-down control'. Reciprocally, the predators are subject to bottom-up control [abundance determined by their resources]: a standard predator-prey interaction. In turn, the plants are also subject to bottom-up control, having been released from top-down control by the effects of the predators on the grazers. Thus, in a trophic cascade, top-down and bottom-up control alternate as we move from one trophic level to the next.
But suppose instead that we start at the other end of the food chain, and assume that the plants are controlled bottom-up by competition for their resources. It is still possible for the herbivores to be limited by competition for plants - their resources - and for the predators to be limited by competition for herbivores. In this scenario, all trophic levels are subject to bottom-up control [also called 'donor control'], because the resource controls the abundance of the consumer but the consumer does not control the abundance of the resource. The question has therefore arisen: 'Are food webs - or are particular types of food web - dominated by either top-down or bottom-up control?' [Note again, though, that even when top-down control 'dominates', top-down and bottom-up control are expected to alternate from trophic level to trophic level.]
Clearly, this is linked to the issues we have just been dealing with. Top-down control should dominate in systems with powerful community-level trophic cascades. But in systems where trophic cascades, if they exist at all, are limited to the species level, the community as a whole could be dominated by top-down or bottom-up control. Also, there are some communities that tend, inevitably, to be dominated by bottom-up control, because consumers have little or no influence on the supply of their food resource. The most obvious group of organisms to which this applies is the detritivores [see Chapter 11], but consumers of are trophic cascades all wet?
top-down, bottom-up and cascades nectar and seeds are also likely to come into this category [Odum & Biever, 1984] and few of the multitude of rare phytophagous insects are likely to have any impact upon the abundance of their host plants [Lawton, 1989].
The widespread importance of top-down control, foreshadowing the idea of the trophic cascade, was first advocated in a famous paper by Hairston et al. [1960], which asked 'Why is the world green?' They answered, in effect, that the world is green because top-down control predominates: green plant biomass accumulates because predators keep herbivores in check. The argument was later extended to systems with fewer or more than three trophic levels [Fretwell, 1977; Oksanen et al., 1981].
Murdoch [1966], in particular, challenged these ideas. His view, described by Pimm [1991] as 'the world is prickly and tastes bad', emphasized that even if the world is green [assuming it is], it does not necessarily follow that the herbivores are failing to capitalize on this because they are limited, top-down, by their predators. Many plants have evolved physical and chemical defenses that make life difficult for herbivores [see Chapter 3]. The herbivores may therefore be competing fiercely for a limited amount of palatable and unprotected plant material; and their predators may, in turn, compete for scarce herbivores. A world controlled from the bottom-up may still be green.
Oksanen [1988], moreover, has argued that the world is not always green - particularly if the observer is standing in the middle of a desert or on the northern coast of Greenland. Oksanen's contention [see also Oksanen et al., 1981] is that: [i] in extremely unproductive or 'white' ecosystems, grazing will be light because there is not enough food to support effective populations of herbivores: both the plants and the herbivores will be limited bottom-up; [ii] at the highest levels of plant productivity, in 'green' ecosystems, there will also be light grazing because of top-down limitation by predators [as argued by Hairston et al., 1960]; but [iii] between these extremes, ecosystems may be 'yellow', where plants are top-down limited by grazers because there are insufficient herbivores to support effective populations of predators. The suggestion, then, is that productivity shifts the balance between top-down and bottom-up control by altering the lengths of food chains. This still remains to be critically tested.
There are also suggestions that the level of primary productivity may be influential in other ways in determining whether top-down or bottom-up control is predominant. Chase [2003] examined the effect of nutrient concentrations on a freshwater web comprising an insect predator, Belostoma flumineum, feeding on two species of herbivorous snails, Physella girina and Helisoma trivolvis, in turn feeding on macro-phytes and algae within a larger food web including zooplankton and phytoplankton. At the lowest nutrient concentrations, the snails were dominated by the smaller P. gyrina, vulnerable to predation, and the predator gave rise to a trophic cascade extending to the primary producers. But at the highest concentrations, the snails were dominated by the larger H. trivolvis, relatively invulnerable to predation, and no trophic cascade was apparent [Figure 20.6]. This study, therefore, also lends support to Murdoch's proposition that the 'world tastes bad', in that invulnerable herbivores gave rise to a web with a relative dominance of bottom-up control. Overall, though, we see again that the elucidation of clear patterns in the predominance of top-down or bottom-up control remains a challenge for the future.
Continue reading here: Strong interactors and keystone species
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