Predators mainly affect prey diversity by regulating the distribution of space utilizable by the various prey species. If a generalized predator feeds on various species of prey in proportion »o their abundance, no prey species can predominate. Janzen (1971) has observed that moth larvae which feed on the seedlings of Dioclea megacarpa tend to damage seedling vines more heavily close to the parent plants than those remote from it.
Based on his work on tiger-fish (Hydrocyon vittatus) in African lakes, Jackson (1961) suggested that the predation of this ferocious species militates against speciation since the tiger-fish roams freely in open waters and devours other species whole.
To avoid being eaten, the prey species must spend their lives in the cover provided by vegetation at least until they attain a size more than half of that of the predator. Before reaching this size, the prey species must be protected, and this tends to be achieved by breeding either in vegetation or in streams. Jackson’s view is, however, not shared by Fryer and lies (1972).
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According to Fryer and lies, predation by tiger-fish does not differ significantly from that by numerous predators in other groups present in a lake. Fryer and lies’ view is supported by the observation that in Lake Tanganyika which not only has a few species of Hydrocyon but also of another similar predator, Lates, and yet harbours a fairly large number of several prey species. Fryer and lies (1972) proposed that far from hindering, predation may even promote speciation.
In most lakes, small crustaceans constitute common prey. Of the predators, many invertebrate carnivores, e.g., the Odonata and the net-spinning larvae of Trichoptera, just wait for prey to come to them. But some predators actively search for their prey; for instance, leeches which swim well, or caddis larvae, which have efficient seizing organs.
Certain Planarians can neither move fast nor have efficient seizing organs, but lay trails of slime in which prey becomes entangled. Asellus seems to be the chief prey of Planaria, but with growth and increase in size of Asellus, its chances of being devoured by Planaria decrease. Planaria, therefore, seems to feed regularly only when prey is abundant. When it is scarce, they rest, and planarians are known to be able to tolerate starvation for long periods. Planaria in turn are preyed upon by Odonata, Newts, and Plecoptera.
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The following three relationships between predator and prey are known to be applicable in the case of Protozoa: (1) prey avoid predation in cover, (2) predators become inactive when prey becomes scarce; (3) prey occurs in isolated colonies which when found may be devoured by the predator.
In open water there is no hiding place for prey. In such a habitat, only small invertebrates can survive predation by fish. Some of these successful invertebrates tend to be transparent, and many also reproduce rapidly during favourable periods, entering a resting phase when conditions become adverse.
Recently, predation has been studied in small, temporary water bodies. Here, predation is mainly by invading individuals that were reared elsewhere. Some characteristic organisms are phyllopods and mosquito larvae; these organisms feed in the open and away from cover, an activity possible only where predation is slight.