“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”
(Hutchinson 1959). 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
(1956). 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 and Wilson 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 and Wilson 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 (1959) 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 [not immediately evident what this disparity is, though by the end of the paragrrph it becomes clearer – still, we seem to lack a crisp distinction between character displacement and limiting similarity here. Your text could cause one to think it is the difference between pattern and process, or the difference between phenomena that are the resultsof ecological assembly versus evolutionary divergence, and so on. ]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 (not a word seen frequently in ecology.
I did a full text search of the ecology and botany journals in the JSTOR archive (~1850 –
2001) and discovered the word has been used only 8 times, with 6 of those prior to 1910), 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. (of course those statistical methods were not so readily available in the 1950s…)
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’s 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 2003). 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 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.
Perhaps you could conclude with a statement of how we would craft hypotheses today, derived from Hutchinson’s insight, but tempered by out more sophisticated methodology and appreciation of ecological and evolutionary context.
Abrams, P. 1983. The Theory of Limiting Similarity. Annual Review of Ecology and
Systematics. 14: 359-376.
Brown, W.L. and Wilson? 1956. Character Displacement. Systematic Zoology. 5(2): 49-
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.
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.
Good job putting your paper’s idea in context – tracing both backwards and forwards.
Very well done!