The Origin of Predators
If it is not to be supposed that predators simply appear automatically alongside plant-grazing animals, some account needs to be offered for their appearance. In Idle Theory the explanation is that predators emerge when, for some grazers, predatory life is more idle than one feeding from plants.
The Predator Model
In line with the above reasoning, the simulated predators are no different in size or other attributes than grazers. They are, however, unable to graze on plants. They prefer to scavenge from dead grazers, but will attack live ones if none are at hand. They will also attack each other in extremis.
The effect of predators in the simulation model is, at minimum, to slow the multiplication of grazers. In these simulations grazer populations multiply and overgraze plants, and their population collapses. For the predators, this produces an excess of dead grazers on which to scavenge. But once these have either been consumed or disintegrated, a small population of grazers is confronted with a relatively large population of predators.
There are 3 possible results.
Thus, while this model will often exhibit relatively stable populations for long periods, the terminal event is regularly a grazer population explosion and crash, and a subsequent era in which either predators re-establish their hold on grazer populations, or kill off all grazers, or themselves die out. This is a picture of instability, not of predators regularly acting to hold down prey populations.
To some extent, this may simply reflect the small scale of the simulation model, in which there are hardly ever more than 10 predators, and very often only one. In these circumstances, the end result is almost the flip of a coin. In a larger model, with many more grazers and predators, perhaps some grazers and predators would regularly survive.
On the other hand, some studies of predation agree broadly with the results produced here by the Malthus simulation model.
It may the balance of predators and prey appears to be inherently unstable. This is because once a prey population begins to increase faster than its predators reduce it, either because of a sudden burst of reproduction, or increased mortality among predators, it becomes impossible for predators to restore the lost equilibrium. Furthermore, the predators most likely to survive are those which never decimate prey populations, and consequently that predators may play a minor role in restricting prey populations.
Stable populations - the arms race
In this simulation model, predator-prey population stability is seldom achieved for long. But in a quite different simulation model - the Tetra model - population stability is often maintained for long periods of time.
If population stability can be assumed, and predators usually restrain grazer populations, then grazers are likely to develop defensive strategies. Just as predators must select for faster-reproducing grazers, they are also likely to select for fast-escaping grazers (because they catch the slowest), or elusive camouflaged grazers (because they catch the most visible), or for powerfully armoured grazers (because they catch the least defended).
Thus an initial population of slow, undefended grazers will evolve into several sub-populations, each of which has its own defence system. This gradual specialization of grazers will force predators to in turn specialize - in speed, visual acuity, or power. Each specialist grazer population will acquire its own specialist predators. General purpose catch-all predators will be neither fast enough or powerful enough for the task.
This niche-specialization would apply not only to predators and grazers, but also to grazers and plants. For grazer-predated plants would also multiply more rapidly, and evolve thorns and spines and toxins. The result would be increasing specialization and re-specialization, until every creature was a specialist, adept at catching one form of prey, relatively inept at catching most others.
Such specialization requires prey (animal or plant) to work ever harder to maintain their defences, and for predators to work harder to overcome them. Thus long term stability must bring falling idleness for both predator and prey. With falling idleness must come falling population. At some point, the system must break down. And the likelihood is that the predators will be the first to go, leaving their specialist prey to multiply unchecked.
The present population of one modern natural predator (the cheetah) are almost genetically identical, indicating that they were the descendants of perhaps a single pair. Since cheetahs are the fastest mammals, this suggests that cheetahs got locked into an arms race with high-speed prey, and that they almost lost completely.
The scene near the end of such an arms race would be a dwindled population of relatively undefended fast-reproducing small plants and heavily defended shrubs, very fast or heavily armoured grazers, and extremely powerful predators. The system would break down in one of two ways. Either it would slowly break down, as a long succession of extinctions removed the least idle grazers one by one, taking with them their specialist predators. Or else overworked predators would die out, producing a population explosion would occur in their prey population, and a subsequent collapse in the plant population. This collapse in the already dwindled plant population would signal the end for many of the remaining armoured, overworked grazers and their specialist predators, in a catastrophic extinction event.
After this extinction event, one-time prey populations would gradually dispense with the speed and armour acquired during the arms race. With falling defence expenditures, idleness would increase, and numbers increase.
In The Blind WatchMaker, Richard Dawkins wrote:
I regard arms races a of the utmost importance because it is largely arms races which have injected such "progressiveness" as there is in evolution.
Certainly arms races are likely to produce all kinds of novel defence and attack strategies, and thus the appearance of progress. But the view here is that the increased energy expenditures required for all this inevitably reduce the idleness of the creatures, and push them towards extinction. Far from progress, arms races produce degeneration.
The arms race between predators and prey acts to decrease the idleness of both, as the accumulation of new weapons and defences increase their maintenance energy expenditures. Those that can stay out of these arms races are the most likely to survive, because they don't carry this extra baggage: it is the meek that inherit the earth.
Avoiding arms races is best for both predators and prey, because of the likelihood of a terminal extinction that neither survive. The best predator strategy seems to be to keep their populations low, and reproduce at something like replacement rates. This way they take only a small number of the prey population, and the consequent abundance of their grazer prey makes hunting easy. With only a few of their numbers being taken by predators, there is very weak selection pressure on grazers to increase reproduction rates, run faster, or construct defensive armour. Selection pressure acts instead to reduce reproduction rates, speed, and armour. This makes life even easier for predators.
The long-run outcome of this would be that both predators and grazer reproduction would tend towards replacement levels, with relatively low populations of both, and high plant densities. Both grazers and predators would live largely idle lives. The system would be stable.
In a restricted locality, like an island, this kind of stability would be reached much more rapidly than on a large continent. On an island, grazers would have either no predators, or very few. With low reproduction rates and small populations, few variants would appear: evolution would all but stop.
But if such an offshore island became connected to a continent, by the appearance, say, of a land bridge as sea levels fell, the idyll would be terminated. Fast-reproducing and efficient continental predators would migrate to the island. The likelihood is that these continental incomers would rapidly decimate its stable grazer populations before they had time to respond by increasing their reproduction rates. Grazer extinction, along with that of their small resident predator population, would be a likely result.
Author: Chris Davis
Last Edited: 8 april 1998