Imanishi Kinji ― What He Saw in the Kamo River In the spring of 1935, Imanishi Kinji kept returning to the Kamo River in Kyoto.[1]
He stepped into the water, turned over stones, and collected mayfly nymphs living beneath them. Then he moved to another place, came back on another day, and did it again.
Many species of mayfly lived in the river.
But they were not found everywhere in the same way. Some appeared more often where the current was fast. Others were found in gentler water. When the riverbed changed, the species found there changed as well.
Imanishi kept looking.
But a river is not a laboratory.[2]
The amount of water changes. The speed of the current changes. The temperature changes. So does the riverbed. The algae growing on the stones change, and as the seasons pass, the numbers of the creatures living there change too.
The same conditions never return in exactly the same form.
Still, Imanishi kept returning to the same river.
And within all that change, a certain way of appearing began to become visible.
One species was found more often in one part of the river, another species somewhere beside it. The actual distribution changed with place and time. There was no clear line drawn across the river separating one from another.
Yet when several species were observed together, they did not seem to be mixed at random.
The same pattern was not repeating itself.[3]
The river was different each time. And yet, within those differences, a similar kind of separation appeared again and again.
Imanishi would come to understand this way of appearing as sumiwake―a term often translated as “habitat segregation,” “habitat differentiation,” or “ecological partitioning.”[4]
But while studying mayflies, Imanishi was not looking only at mayflies.
The current differed from place to place. So did the riverbed. The algae growing there differed. And so did the species of mayfly that lived there.[5]
He examined these things one by one, while continuing to watch the Kamo River itself.
A single place, observed on a single day, could not reveal what became visible over time.
It emerged by returning, again and again, to a river that never stopped changing.
Notes
1. The Kamo River fieldwork
Imanishi’s fieldwork in the upper Kamo River from March to June 1935 survives in four field notebooks, later published as Imanishi Kinji Field Notes: Collecting Diary, Kamo River 1935 (Imanishi Kinji Fīrudo Nōto: Saishū Nikki Kamogawa 1935). The notebooks provide an important record of the fieldwork that preceded his later studies of mayfly distribution and sumiwake.
2. “A river is not a laboratory”
This sentence is the wording of this essay, not a quotation from Imanishi. It does not mean that Imanishi rejected analysis or controlled comparison. Rather, it draws attention to the changing conditions of the natural river in which his observations took place.
3. What is meant by “pattern”
“Pattern” here does not mean that Imanishi found the same spatial arrangement each time he returned to the river. Conditions changed with place and time, and so did the actual distribution of organisms. The word is used here in the broader sense of a recurring way of appearing, rather than the repetition of an identical configuration.
4. From distribution to sumiwake
In his 1938 paper on the life forms and life zones of mayfly nymphs, Imanishi explicitly shifted the focus of his earlier taxonomic work toward habitat and, particularly, distribution. His 1941 paper developed these observations further in terms of ecological structure revealed through the arrangement of life zones. Sumiwake (棲み分け) has been translated in several ways, including “habitat segregation” and “habitat differentiation.” The Japanese term is retained here because no single English expression fully carries its later conceptual range.
5. Looking beyond the mayflies
Imanishi’s field investigations did not treat mayfly distribution in isolation. His surviving field notes show him examining environmental conditions and considering connections among water conditions, attached algae, and the distribution of animals. The essay therefore describes him as studying individual conditions while continuing to observe the river in which they occurred.
6. Competition and Imanishi’s later thought
Later discussions of Imanishi connect sumiwake with competition, coexistence, species society, and his broader view of the biological world. This essay deliberately stops earlier. No direct evidence has been established here that his 1935 Kamo River observations began from a perceived absence of competition. The focus is therefore limited to the repeated observation of several mayfly species and the differentiated ways in which they appeared within a changing river.
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