“Homage to Santa Rosalia or Why are there so many Kinds of Animals?”
A Tribute to a Classic Paper Fostering Classic Debate
by Jennifer Gruhn
Close to 50 years ago, G. E. Hutchinson used the theory of character displacement
to explain the Earth’s “enormous number of animal species” and published his findings in
a 1959 article entitled “Homage to Santa Rosalia or Why are there so many Kinds of
Animals?” Hutchinson recognized that character displacement creates a measurable and
consistent size ratio among competing species. Since the publishing of Hutchinson’s
paper, the claim that species could express this critical difference of size sparked much
research and debate. Ecologists and other biologists alike began measuring species, and
statisticians considered whether Hutchinson’s findings could be valid statistically.
Hutchinson’s basis for his theory was also scrutinized; was it the displacement of
character driving a consistent and minimum ratio, or was it a limit to similarity?
Hutchinson’s use of character displacement to describe a size ratio among competing
species and the quantitative data he gathered to justify this ratio not only fulfilled his
purpose by helping to explain “Why there are so many Kinds of Animals” but also
opened and currently opens new doors in ecological research.
Hutchinson’s paper is initially about a visit to Sicily, where he came upon an
artificial pond near the sanctuary of Santa Rosalia. After observing the diversity of the
Corixidae genus brooding in the pond, he was inspired to consider what was limiting
additional noticeable genera from inhabiting the pond, and in that case, what causes our
current number of species in general? He ponders over the magnitude of diversity that a
food chain harbors and what limits additional levels from being added to the food chain.
He mentions how natural selection limits the efficiency of predator and prey, so that one
does not eliminate the other and refers to the great diversity of plants, and how their
diversity correlates with the number of animal species inhabiting the plants.
Most notably though, Hutchinson provides some insight into two species that
occupy the same level on a food web; he considered how different in size these two
species must be in order for one to not completely fulfill the other’s niche. Hutchinson
uses mammal and bird case studies to justify a consistent size ratio found between the
two species’ sizes. He compares the skull sizes of mammal species and the culmen sizes
of bird species. His data shows that co-occurring species express a size ratio to each
other of 1.1 to 1.4, with a mean of 1.3. Hutchinson calls this ratio a result of character
displacement and considers the size comparison a “kind of difference necessary to permit
two species to co-occur in different niches but at the same level in a food web.” He
further says that the species’ specific size differences are critical enough to prevent a
third species from co-occurring, if it does not assume the ratio of about 1.3 when
compared to the other species. Hutchinson briefly elaborates on yet other factors
affecting diversity, such as the general size of a species and the mosaic nature of the
species’ habitat.
The term “character displacement” used by Hutchinson as the source driving the
critical 1.3 ratio was coined, but not first described, by W. L. Brown, Jr. and E. O.
Wilson. Brown and Wilson wrote a paper entitled “Character Displacement” three years
before Hutchinson wrote about the 1.3 ratio. To foreshadow and give light to
Hutchinson’s findings, it helps to understand the meaning of character displacement.
Brown et al. explained character displacement using an example of two different species
populations that occur both together and separately in space. Where the two species
occur individually, they have similar character; yet where they occur together, they have
distinct character (Brown et al. 1956). This phenomenon arises from the “genetic
interaction of two (or more) newly-evolved cognate species” (Brown et al. 1956). These
species show evidence that competition causes their disjoint character once the species
are proximally close. As explained before, Hutchinson took this idea further by creating
the 1.3 ratio for the two species’ sizes. He therefore elaborated on the term character
displacement, assuming it could be further defined by a maximum similarity ratio that the
species must express in order for their coexistence.
This disparity between the two definitions of character displacement (that used by
Brown et al. and that used by Hutchinson) is important. The disparity is significant
because Hutchinson’s 3.1 ratio is actually an example of another ecological phenomenon,
later called limiting similarity.
That Hutchinson “misdiagnosed” the cause for the consistent ratios found in his
research is exactly the argument made by Peter Abrams in his 1983 paper entitled “The
Theory of Limiting Similarity.” Abrams explains the common occurrence of mistaking
limiting similarity for character displacement:
“Although I am not aware of any published arguments that support
this belief, there does seem to be a widespread view that character
displacement results in species having utilization patterns close to the
limiting similarity. This view may be the result of a failure to distinguish
between limiting similarity and character displacement, which dates back
at least as far as Hutchinson’s famous ‘Homage to Santa Rosalia’ paper…”
Limiting similarity is loosely the idea that two or more different species have a limited
amount of similarity they can share in order to coexist, before one or more species is
excluded from the system (Abrams 1983). This notion falls right in line with
Hutchinson’s research, supporting a limiting similarity of size between the mammal and
bird species he measured.
In 1981, Hutchinson’s paper was confuted, this time for its statistical significance
and reasoning behind the limiting similarity ratios. Simberloff and Boecklen’s Meta
analysis, “Santa Rosalia Reconsidered: Size Ratios and Competition,” was a stab at
Hutchinson’s methods of data collection/calculation/and display as well as Hutchinson’s
confirmation that competition itself drove the size disparities of the mammals and birds
of his research. “Santa Rosalia Reconsidered” expresses the concern that textbooks and
the lay press use Hutchinson’s size differences to support new ecological theory without
statistically testing the research’s data.
Simberloff et al. tested Hutchinson’s and others researchers’ data for constancy
(randomness and independence of data) and minima (whether a minimum ratio is larger
than would happen by chance), but the data was largely incomplete to allow for such a
thorough analysis (Simberloff et al. 1981). There is also the conjecture that there were
multiple sets of data in Hutchinson’s study, as well as other studies; however, Hutchinson
chose to show only those sets of data that supported the 3.1 ratio trend (Simberloff et al.
1981). Lastly, Hutchinson easy exploitation of the null hypothesis (the hypothesis that
competition drove the size disparities among the species in his research) was an easy
target for disapproval in “Santa Rosalia Reconsidered” because this null hypothesis is
generally considered unfalsifiable.
It is valid that multiple studies have refuted the famous 1.3 ratio for a ratio
describing species size differences. In fact, the “1. 3 rule” should rather be called the 1. x
rule, if it should be named after any number at all (Simberloff et al. 1981). Beak size and
prey size of bird guilds produced a ratio of 1.14 in past research (Root 1967) and body
size and prey size of Anolis genus lizards produced a ratio of 1.4 (Schoener 1968).
Simberloff put it best when he said “it is simplistic to think morphological sizes are the
predominant index of feeding differences” (Simberloff et al. 1981).
Nevertheless, current research continues the quest to find species to species size
ratios and make yet other assumptions from them. Jason Knouft wrote a paper in 2003
entitled “Convergence, Divergence and the Effect of Congeners on Body Size Ratios in
Stream Fishes.” Knouft writes that “The use of size ratios as indicators of niche
differentiation in similar coexisting species has been a major point of interest for
ecologists since Hutchinson’s (1959) classic paper…” Knouft no doubt was inspired by
Hutchinson to measure species ratios in order to determine whether Etheostoma spp.
populations were converging of diverging.
Body size ratios have even permeated the field of geology; Case Hermoyian wrote
a paper entitled “Testing the Role of Competition in Fossil Communities using Limiting
Similarity.” Hermoyian confesses to using a “Hutchinsonian” ratio of mean size to
measure morphological traits from the fossil record and make assumptions about
competing species (Hermoyian 2002).
Hutchinson’s calculations of a species-species size ratio perhaps produced more
controversy than he had ever imagined. From Abram’s critique that limiting similarity
was the reason for a consistent ratio, to Simberloff’s concern for the statistical
significance and application of the 3.1 ratio to predator/prey dynamics, Hutchinson’s
“Santa Rosalia” paper was a success because it fostered such deliberation. There is
current research inspired by Hutchinson, but the question remains- for what future
ecological research is the ratio of species’ sizes a valid tool? Simberloff effectively
negated its benefit to predator/prey research. However, the ratio is potentially applicable
to specific situation-dependent circumstances, such as its use in fossil records, or to
determine whether populations are diverging or converging. Future consideration must
be made, however, on the statistical significance of these ratios once they are computed.
There is no doubt though that the species’ size ratio will live long in ecological research.
Jennifer Gruhn
Bibliography
Abrams, P. 1983. The Theory of Limiting Similarity. Annual Review of Ecology and
Systematics. 14: 359-376.
Brown, W.L. 1956. Character Displacement. Systematic Zoology. 5(2): 49-64.
Hermoyian, C.S. 2002. Testing the Role of Competition in Fossil Communities Using
Limiting Similarity. Geology. 30(1): 15-18.
Hutchinson, G.E. 1959. Homage to Santa Rosalia or Why Are There So Many Kinds of
Animals? The American Naturalist. 93(870): 145-159.
Knouft, J.H. 2003. Convergence, divergence, and the effect of congeners on body size
ratios in stream fishes. Evolution. 57(10):2374-2382.
Root, R. 1967. The niche exploitation pattern of the blue-gray gnatcatcher. Ecology.
37:317-350.
Schoener, T.W. 1968. Some Niche Differences in Three Lesser Antillean Lizards of the
Genus Anolis. Ecology. 49(5):819-830.
Simberloff, W.B. 1981. Santa-Rosalia Reconsidered Size Ratios and Competition.
Evolution. 35(6): 1206-1228.