PREDATOR / PREY RATIOS

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The large top predators in any system generally obtain most of their food intake from large herbivores (it is not usually worth the amount of effort required to hunt much smaller prey ). For warm-blooded predators , a large quantity of food is required (about 10 times as much as for an equal-sized cold blooded predator ), so that the larger the predator and the higher its metabolic rate , the rarer it will be in any ecosystem . Predator/prey ratios are calculated by working out the weight of the predator and its prey and counting the number of predators and the total number of prey.
predator/prey ratio = predator weight x number
prey weight x prey number
For modern examples such as the lion on the African savannah game parks, this ratio is about 1% or even less. For Permian cold-blooded predators such as Dimetrodon, the ratio is much higher, at 20%, equivalent to today's crocodiles and spiders. Prehistoric mammal predators such as sabre toothed tigers have a ratio of about 3 - 5%. Tyrannosaurus ratios are almost exactly the same as for sabre tooths, and large dinosaur predators average about 3.5%, with a range from 5% for good habitats such as late Cretaceous Alberta down to less than 1% for the much more difficult environment of late Cretaceous Mongolia.
Dinosaurs clearly fall into the same group as the unquestionably warm-blooded prehistoric mammals and are much lower than cold-blooded predators of today or the Permian period. There are several possible reasons for the lower values for the lion - the savannah is relatively open and provides little in the way of hunting cover, there is considerable interference from man etc, so that the lion is unable to operate at peak efficiency and so the herbivore population increases beyond its expected level.
The studies cited above are not considered flawless by all experts, and no studies have been done with bird predator/prey systems. Census counts based on incomplete fossil assemblages may be unrepresentative, and the assumption that predator density is always limited by prey density is largely untested.

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