Top Down and BottomUp

It is widely recognized that predator control (top-down) prevails at the higher levels of the food web, whereas resource control (bottom-up) is strongest at the lower trophic levels. Events at the topmost level have cascading effects down the food chain. One such cascading

Table 2 Typical densities and generation times of the major planktonic groups. The values can vary widely in time and space due to external environmental as well as internal biological factors

Density

Generation time

(no r1

(days)

Bacteria

108-1010

1-10

Picoalgae

108-109

1-5

Phytoplankton

104-108

1-30

Nanoflagellates

102-108

1-3

Ciliates

101-107

2-7

Rotifers

101-104

2-10

Copepods

101-103

2-20

Cladocerans

101-102

1-10

effect is the presence of planktivorous fish that may release the zooplankton predation on nanoflagellate populations that in turn can result in a decrease in bacterial abundances as nanoflagellates tend to control bacterial abundance. The relative strength of these regulation mechanisms varies among ecosystems of differing trophic status and with geographical ranges. Although trophic cascades have been detected in nutrient-poor lakes and in marine systems, the strongest cascading effects of top predators on phytoplankton have been observed in lakes with high nutrient concentrations (i.e., meso- and eutrophic lakes).

The relationship between nutrient status and the extent to which lower trophic levels were affected by alterations in fish population densities (i.e., the extent of the cascading effects) was shown not to be affected significantly by latitude or altitude in a study of 466 lakes from the temperate zone to the arctic. In eutrophic temperate lakes, top-down control or a mixture of the two regulation mechanisms have been shown to govern the microbial community. On the other hand, in oligotrophic temperate lakes a mixture of the two mechanisms or bottom-up influences alone prevail. The more oligo-trophic a system is, the more important resources appear to be in controlling the microbial food web. This general picture from temperate regions is supported by studies in arctic and alpine lakes as well as in marine ecosystems. However, experimental studies support strong top-down controls in most freshwater systems while the interactions in marine systems are more complicated. The reason for this is yet not clear but the fact that marine environments are often less rich in carbon, nitrogen, and phosphorus and have wide extensions ('boundlessness') than most lakes may play a role. An underlying aspect of this is that marine fish populations are much less abundant than freshwater ones, which implies that top-down controls are weaker and therefore strong cascading effects are less likely to occur.

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