Spetner repeats the claim later in his book (p. 199): 'Plants and animals hardly
ever increase their numbers so much that they exhaust their resources. They stop increasing well before they
deplete their habitat.'
Unfortunately, Spetner is not alone among the anti-evolutionists in his claims about presumed scarcity of
competition and predation having any role in natural selection. Richard Milton, in Shattering the Myths
of Darwinism, makes the following absurd claims:
Talk of survival immediately conjures up a lurid vision of competition between the various forms of animal
life in a hostile world: competition for scarce resources of food and living space, of ‘nature red in tooth
and claw’ as Tennyson pictured it for his enthralled Victorian readers. In reality such competition is very
rarely found in nature. One conservative estimate is that there are at least 22,000 common species of fish,
amphibians, reptiles, mammals, and birds. In addition there are said to be at least 1 million common insect
species. Some of the thousands of species -- notably humans -- do compete aggressively, killing competitors
for living space and food. But the species that do are very much in the minority. The overwhelming majority
of creatures do not fight, do not kill for food and do not compete aggressively for space in a way that results
in the ‘loser’ dying out.’ (p. 124-125) ‘The majority of carnivores do not feed on prey that they themselves
have just killed but rather are scavengers or carrion feeders. This includes legendary hunters such as lions
or sharks who frequently eat as a result of not their own direct efforts but those of another lion or shark.
There are two profound errors in Spetner’s and Milton’s claims, one of which was hinted at above. The first
error is that it is NOT necessary for organisms to exhaust their resources in order for them to compete.
Selection can act via competition as long as the organism actually compete; this can occur prior to resources
being completely exhausted. Of course, neither Spetner nor Milton clarify what he means by 'exhaust' or
'deplete'. Indeed, rarely in nature would every last bit of a resource be depleted by the consumer organisms;
most organisms have some minimal threshold of resource availability below which they cannot sustain themselves,
thus competition can readily occur at resource levels above absolute zero (Tilman 1981). These writers’ second
major error is the simple fact that many, many cases are known where organisms DO severely deplete their
resources -- either as competitors competing for some nonliving resource, or as predators driving their prey to
scarcity. Both of these examples demonstrate Spetner’s severe lack of knowledge about natural history. Below
we will expound on the second point.
Do organisms deplete their resources to levels that impose constraints on survival and reproduction (the main
components of fitness)? Spetner & Milton say 'no', but extensive basic and applied ecological research says
'yes'. Literally hundreds of studies have demonstrated the effects of depleted light levels on growth of
trees in dense stands (Mohler et al. 1978). Smaller individuals usually have stunted growth and often die for lack
of light. In fact, the widespread observation that plants die at a predictable rate in dense stands that are
growing, has become known as the ‘self-thinning law’ of plant ecology (White 1980, Weller 1990). This
commonly-observed mortality pattern is a result of competition among the neighboring plants, which have depleted
their resources to a level that the smallest individuals cannot survive. Numerous studies have also documented
similar patterns among animals. A classic example is the study by Branch (1975) of limpets on the coast of
South Africa. Branch studied limpets at naturally-occuring densities that spanned a tenfold range, from 125
to 1225 per square meter; as density increased, limpet size decreased, such that there was a asymptote in total
limpet biomass per square meter. Branch further demonstrated a strong correlation between limpet size and
reproductive output, showing that the competitive effect at high densities translated into fitness differences
among the limpets. Numerous other studies document the role of competition in dense populations, as a result
of reduced resource availability (Begon & Mortimer 1991). Spetner’s and Milton’s claim regarding the scarcity
of competition is flat wrong.
The claim that predators do not suppress prey populations is also wrong. Many studies that involve the removal
or addition of a species to an ecosystem document drastic changes in the abundance of their prey, whether that
prey be plants or other animals. For example, the recent reintroduction of wolves to Yellowstone National Park
has had a direct negative effect on the density and survival of elk (Ripple & Beschta 2004, Hebblewhite et al.
2005). The Yellowstone wolves have also caused decreases in abundance of smaller predators, probably through
a combination of predation and competition (Crabtree & Sheldon 1999). Nelson & Mech (2006) describe how wolves
completely extirpated deer from a 3000 km2 section of northeastern Minnosota by the mid-1970s, and have prevented
recolonization by deer for the ensuing 30 years. In Yellowstone, the areas where the elk no longer graze heavily
have experienced drastic increases in vegetation abundance, demonstrating that the elk were suppressing plant
abundance; this three-level chain of effect (predator-plant eater-plants) is called a trophic cascade in ecological
circles, and has been documented in a number of ecosystems (Pace et al. 1999, Schmitz et al. 2000). Such cascading
effects that occur two or three levels down the food chain from an apex predator can only appear if prey species
abundances are controlled by the consumer animals that are higher on the food chain. The conclusion is that
predators (including herbivores) often dramatically reduce their prey (plant or animal) abundance, in direct
refutation of Spetner’s and Milton’s claims to the contrary.
Finally, Milton’s claim that even charismatic, apex predators are actually predominantly scavengers is flat
wrong. As usual, Milton provides nothing to back up his baseless claim. A very cursory examination of the relevant
literature reveals the falsehood of his assertion. Funston et al. (1998) studied hunting and feeding behavior of
lions in Kruger National Park of South Africa, and reported that they 'acquire most of their food by hunting rather
than scavenging.' They documented that among three groups of lions, the percentage of carcasses scavenged rather
than captured ranged from 2 to 40%. Stander (1992) conducted a similar study in Etosha National Park in Namibia
and reported that 'lions scavenged rarely'. In the boreal forest of northern Scandinavia, the European lynx diet
was studied by Odden et al. (2006), and they found that 'most of the diet was obtained by predation, although we
did document several cases of scavenging.' Karanth (1993) documented predation and diet in India of three carnivores:
tigers, leopards, and dholes, and found that 'carrion was inconsequential in the diets' of all three predators.
Stahler et al. (2006) studied wolves in Yellowstone National Park and found that they scavenge only when they are
unsuccessful in capturing live prey for several days, which was rare because a pack on average killed a prey animal
every 2-3 days. In another study in northern Scandinavia, Linnell & Strand (2002), report that for the diminutive
arctic fox, 'there is little evidence for the importance' of scavenging. As far back as 1972, Kruuk documented
that even the spotted hyena, widely believed to be predominantly a scavenger, got only 1/3 of their diet by
scavenging in the Serengeti, and less elsewhere.
The examples cited above resoundingly refute both aspects of Spetner’s and Milton’s claims. Competition and
predation need not be as intense as these authors imply, and the existence of strong effects of competitors on
each other, and predators on prey abundances, decidely disproves such claims.
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