Popular Science Monthly/Volume 66/February 1905/On the Relations of the Land and Fresh-Water Mollusk-Fauna of Alaska and Eastern Siberia
|ON THE RELATIONS OF THE LAND AND FRESH-WATER MOLLUSK-FAUNA OF ALASKA AND EASTERN SIBERIA.|
By WILLIAM HEALEY DALL,
HAVING recently completed a census of the land and fresh-water mollusk-fauna of Eastern Siberia and Alaska, together with a discussion of the relations of its elements, it seemed that a summary of the results in their bearing on geographical distribution of animal life in the boreal regions might have some interest for the readers of the Popular Science Monthly.
In the region north of latitude 49° the molluscan population of North America is rather scanty. For distributional purposes it must be divided into two series, one containing the aquatic forms and the other the land shells. The distribution of water animals is carried on by different means from those influential in the dispersal of terrestrial forms, and any discussion which combined the two without distinction would be liable to contain errors of fact and deduction.
The vast territories under consideration have a number of drainage systems which in the tabulation of species should be distinguished. The chief of these are:
1. The Labradorian.—This comprises the area of drainage into Ungava Bay and the Atlantic north of the straits of Belleisle and the 'Height of Land' including the Labrador coast and the northeastern part of the Ungava District of the Dominion of Canada.
2. The Canadian.—This system comprises the drainage of the St. Lawrence and the Great Lakes south and east from the 'Height of Land,' including the island of Anticosti.
3. The Hudsonian.—This, the largest system of all, includes the entire area draining into Hudson Bay, including Keewatin, the southeastern corner of the Mackenzie District, eastern Athabaska, the whole of Saskatchewan, the southeastern two-thirds of Alberta, Assiniboia and Manitoba, the drainage area of the Red river of the north in the Dakotas, and northeastern Minnesota, all of Ontario, Quebec, and Ungava north and west of the 'Height of Land.'
4. The Mackenzian.—This vast system includes the basin drained by the Mackenzie river and its tributaries, covering northwestern Alberta, northeastern British Columbia, and the northwestern two thirds of Athabaska and the Mackenzie District.
5. The Columbian.—This includes the coast drainage of British Columbia and Southeastern Alaska, the basins of the Fraser and Columbia rivers, the coastal part of the State of Washington, and the northern part of Idaho and Montana, west of the Selkirk Range and its more southern equivalents in the Eocky Mountain region. The northwestern extension of this system in Alaska, between the Coast Ranges and the sea, including the Alaska Peninsula and the Aleutian Islands, being really an extension northwestward of the Columbian system, might perhaps for convenience be called the Alaskan sub-region.
6. The Yukonian.—This system includes the entire drainage of the Yukon river, the tundra north of it and the basin of the Kuskokwim; or all of Alaska north, northwest and west of the Alaskan range, as well as that of the basin east of the Coast ranges drained by the Yukon and its tributaries.
We know through the researches of General G. K. Warren, IT. S. Engineers, that a considerable portion of the original Mississippi drainage near its former headwaters has, through changes of level of the earth near the 49 th parallel, been captured by the Hudsonian drainage system, and it is not unreasonable to suppose, that a certain proportion of the Mississippi fresh-water fauna was captured with the streams in which it lived. Some such supposition seems necessary to explain the far northern extension of certain Mississippi Valley species in the Hudsonian region, while other drainages equally suited to their inhabitation are destitute of them.
Another factor in the distribution of land as well as fresh water shells is the former existence of a continental ice-cap by which the entire mollusk fauna of the region occupied by ice would be exterminated; though, in the interglacial periods, the external fauna might advance as the ice retreated, only to be driven out again on the return of the ice. Such fluctuations have been well shown, by the researches of Dr. Coleman, of Toronto, to have occurred in the valley of the Don, near that city.
The vast territories included in these drainage systems are, it is true, only partially and imperfectly explored for mollusks. Yet certain portions of them are tolerably well known and the uniformity imposed on the fauna by its high northern position and unvaried conditions, leads to the belief that, while much is yet to be known in tracing out the details of distribution, little is to be expected in the way of absolutely new species, even from this immense territory yet to be explored.
It would be rash to conclude that nothing new remains to be found, but our expectations should certainly be moderate.
There are a few characteristic fresh-water forms in Greenland, Labrador and the Hudsonian region, but in the main, the molluscan population of the waters of the region described may be regarded as a northward extension of the Northeast American and Mississippi Valley faunas, diminished by the elimination of all the less hardy species and reinforced by the addition of a small number of circumboreal forms. The total fresh-water fauna comprises 138 species, of which 90 occur in the Hudsonian system, but only 23 extend to the Mackenzie basin, though the Yukon system may boast of 29. Some eccentricities of distribution will be referred to later.
The distribution of land shells is not affected to any serious extent by rivers or streams, but is doubtless, in the main, the result of the slow movement of individuals.
The pulmonate fauna of Alaska is composed of four elements; contributions from the faunas of Asia, of the Pacific Coast region of America, of the Hudsonian or Canadian region, and of the special circumboreal or common subarctic fauna of the whole northern hemisphere.
Differences of latitude mean for the snail not so much differences of temperature corresponding to the latitude, as differences of the annual active period, which diminishes as one proceeds northward. Snails at Point Barrow (and there are such) must remain in a state of hibernation at least nine months in the year, and I suspect that this more probably brings a limiting strain on the vitality of the organism, than would the mere occurrence at times of a specially low temperature.
The Alaskan land shells number 86 species, of which 16 are circumboreal, and the same number (though not all the same identical species) are common to the northeastern part of Siberia. Fifty species are held in common with the Pacific fauna, while 34 are part of the Yukon fauna which is so intimately allied to that of the Hudsonian region. These latter two groups are limited by topographic features. Thus the fauna of the Hudsonian region, in constantly diminishing number of species, is extended to the northwest through the Yukonian region north of the Coast range and the Alaskan range, to Bering Sea on the west and the Arctic coast on the north.
In like manner the fauna of the Columbian region of the Pacific Coast, is extended along the Alaskan coast and islands south and west of the two ranges mentioned, and between them and the sea. Some of the most characteristic and larger species are cut off in their northwestern extension by the area near Mount St. Elias, where along one hundred miles of coast immense glacier fields extend to the very border of the sea. The last representatives of this fauna disappear among the eastern Aleutian islands. In British Columbia a few species belong to the Valley region between the Rocky and the Cascade Mountains, and do not reach the sea coast, but these are too few to modify essentially the general rule, and like the valleys themselves, these species disappear a short distance north of the 49th parallel. I was able to show, some thirty-five years ago, that in a broad way the distribution of the birds and plants presents analogous features.
Fauna of Northeastern Asia.
The land shell fauna of the northeast extremity of Asia has little individuality, but represents a mingling of the depauperated extremes of the faunas of northeast China and of Europe with that series of species which is sometimes called the circumpolar or circumboreal fauna.
Much of the apparent poverty of the fauna may be due to insufficient collecting, but even when the most generous allowance for this factor is made, it still remains certain that the molluscan population is far less in variety than might reasonably be expected.
The palaearctic fauna of Europe appears to extend clear across Northern Asia, losing a large proportion of its species on the way, until (if the circumboreal species be excluded) only about thirty species reach the headwaters of the Lena and the barrier of the Stanovoi range. A very remarkable local fauna exists in the great 'Relictensee' of Siberia, Lake Baikal, but it does not appear to have tinctured the East Siberian fresh-water fauna outside of that lake, to any appreciable extent. It is possible that the comparatively recent emergence of a large part of Eastern Siberia from the sea, and the presence of the vast desert region to the south and west, may enter into the explanation of this sparse shell-fauna, as well as of some of the peculiarities of the Baikal faunula.
Southeast of the Stanovoi range we find between the mountains and the sea, the valley of the Amur and several smaller valleys, such as the drainage-basins of the Ud and the Tugar. To the southwest the sources of the Amur emerge from the deserts of Gobi and Dauria, and along the line of these water courses has crept a certain number of molluscan forms intimately related to or identical with those of Mongolia, China and the Orient. This forms the second element of the fauna of Northeast Siberia.
The number of purely endemic species is remarkably small and a portion of those claimed to be of this character are probably mere local mutations of widespread palearctic forms already known. Yet it would seem as if a more thorough exploration must add largely to the species now known, and it is almost incredible that the luxuriant fertile valleys of Kamchatka and the innumerable streams and lakes of that country should not be well populated with mollusks.
There are a few species which seem to be common to the shores of Bering Sea, both Asiatic and American, such as Succinea chrysis, Pyramidula pauper, Punctum conspectum, and Anodonta beringiana. There is one local species, Eulota weyrichi, known only from Sakhalin Island; and another, Helicigona subpersonata from the valley of the Ud. Three forms of Vivipara (of which two are probably variants of Chinese forms) are the only exclusively local species of the vast Amur Valley, or drainage, not known from other regions. Nine specially Kamchatkan species have been described, but about half of them are doubtfully distinct.
The total number of land and fresh-water mollusks known from the Amurland, Sakhalin, Kamchatka, the Chukchi peninsula, and the Asiatic coast north of the Amur and east of the Stanovoi range, is only eighty-one. Of these thirteen are circumboreal species, and twelve are supposed to be locally peculiar. Of the remainder
|Europe and West Siberia contribute||55||per cent.|
|Northeast China contributes||22||"|
|In common with America there is||13||"|
Of these erratic species a few may be especially mentioned. Margaritana margaritifera, as is well known, is absent from the whole of the great northern central region of North America, though it appears in the Lower Saskatchewan, the sources of the Missouri and in eastern Canada, while on the Pacific it ascends at least to latitude 56° N. In eastern Asia it is known from Kamchatka, Sakhalin Island, the upper portion of the Amur basin, and southern Mongolia—but I find no authoritative record of it thence westward to Northern and Middle Europe. Schrenck did not find it on the lower Amur.
Phisa fontinalis is reported from the upper Amur and (in a duck's crop) the desert of Dauria, but is not known from Siberia proper, though common in Europe. There is an entire absence of typical Physa throughout East Siberia, so far as reported; and only one species of Ancylus or Unio is known from east of the Yenisei river of Siberia.
Aplexa hypnorum is known from Northern Europe, Western Siberia and the Chukchi peninsula, but has not been reported from Eastern Siberia, or the Amur, though abundant in Alaska, and reaching on the Taimyr peninsula to 73° 30' North Latitude.
Zoögenites harpa is known from Northeastern Scandinavia in Europe; from Northeastern America, the Hudson Bay territory and Southeastern Alaska, in America; but in Siberia it is recorded only from the easternmost margin; the Chukchi peninsula, Bering Island, Kamchatka and the lower Amur. These singularities of distribution must await much more extended knowledge before they can be adequately discussed, but it is believed that to some extent they are due to the transgression of the sea, or of glacial ice, over part of the area in which a species might naturally be expected to occur, thus delaying the occupation of the entire region by the species concerned.