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1969
Journal of the Lepidopterists' Society
177
ON THE ECOLOGY OF NATURAL DISPERSAL: DIONE MONETA POEYII IN TEXAS (NYMPHALIDAE)
Lawrence E. Gilbert1 Department of Zoology, Parks Road, Oxford, England
On December 31, 1964, the neotropical butterfly Dione moneta poeyii (Butler) was taken by my companion as we collected in the brush country of south Texas near Catarina (Dimmit County). That area was experiencing most unusual weather, complete with warm temperatures, abundant flowers, and thousands of butterflies. Had my companion not continued to collect randomly, we would not have seen Dione moneta, for after taking a large series of the similar Agraulis vanillae (L.) (see fig. 1), I had shifted my attention to other species. Doubtless I would have ignored the Dione as just another A. vanillae had it entered my field of vision.
No similar species to A. vanillae are listed in U. S. field guides or checklists. An attempt to identify the specimen soon expanded into an inquiry concerning its possible origin and the reasons for its occurrence in Texas. This investigation stimulated some thoughts about the nature and general importance of "stragglers" and about the criteria for deciding whether a given record represents natural dispersal.
The possibility that this Texas record of D. moneta represents natural dispersal from Mexico was examined, and the results reported below. In addition, the ecological facts gathered in this study lend some evidence towards possible clarification of the term "subtropical" as applied to the south Texas region, an area well known for its records of "stray" tropical butterflies. Thus, the purpose of this paper exceeds simply recording another tropical butterfly for Texas.
Import vs. Straggler
The term "straggler" is familiar to most American lepidopterists. It refers to an individual butterfly which undergoes long range geographical displacement by natural means (i.e. by its own powers of flight, with or without the help of such factors as wind). That we term such an individual butterfly "straggler" indicates a lack of certain information about that individual. A straggler may be only one of a vast migrating group, or it may be a lone stray.
I will avoid trying to distinguish migration from other forms of natural movement, since it seems more important to distinguish the naturally dis-
1 Present address: Department of Biological Science, Stanford University, Stanford, Calif.
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Gilbert: Natural dispersal by Dione Vol. 23, no. 3
persed individuals from the imported ones. Biological information gained from an unusual record depends largely upon which of these two categories is applicable to that record. On the other hand, the biological significance of both categories depends upon how successfully the new region is invaded. In either case, if populations of a species establish themselves in a new region, at best they provide unique opportunities for microevolutionary studies (Burns, 1966), and at worst, they seriously threaten the future of the native fauna, flora and agriculture (Elton, 1958). Is it worthwhile to attempt to distinguish naturally dispersed individuals from imported individuals of species which, because they rarely or never established permanent populations in a new region, seem to be of little biological consequence? The answer is yes, if we consider that a single fertilized female reaching a new region is theoretically capable of starting a new butterfly population. For most species, the chance of such successful invasion (by natural means) occurring during a human life is near zero. Considered in the context of geological time however, the occasional long distance dispersal by a species becomes more significant. Certainly the earth's islands were not all populated by man's importation of new species, or by large scale migrations. The "stragglers" have played their part.
The Investigation of Natural Dispersal
Because of the extreme difficulty in tracking even large migrations of insects over long distances, the investigation of a single stray (if there are such things) seems impossible except in retrospect. In one instance, a widely observed climatic event, wind, has been correlated with observed dispersal in the Lepidoptera. In this case, when two species of African moths appeared in Britain, their probable flight path was compared with synoptic weather conditions. It was concluded that this event did represent natural dispersal, and that wind was an important factor in promoting the long flight (French, 1967).
Such an approach seems to be the only reasonable way to study most records of geographically displaced insects. Even if the original collector takes no data other than locality and date, later workers might possibly make conclusions about the origin of a specimen by carefully studying past climatic and other ecological records for the region in question.
Most of the important points to consider when attempting to give a record natural dispersal status fall into two categories. First, one should consider available evidence concerning the butterfly's flight characteristics (migratory or not, etc.), and habitat preference (altitudinal limits, etc.) within the species distribution (Ford, 1945). Second, the region in which the displacement occurred should be checked with respect to
1969 Journal of the Lepidopterists' Society 179
Fig. 1. A, Dione moneta poeyii (Butler) 9, dorsal view. B, The same specimen, ventral view. (Note: For further details refer to Emsley (1963), and for a color plate see Seitz (1924) pi. 84e (labelled Dione hutleri). C, Agraulis vanillae (L.) $, ventral view.
overall weather patterns and other ecological factors such as condition of vegetation over the projected path, topography, and other butterflies on the wing at the same point in time.
Dione moneta: Flight Characteristics; Distribution Like A. vanillae, D. moneta is a strong, fast flier. Though A. vanillae is a known migrant, no records could be found of migrations of Dione moneta. Emsley (1963) provides the most recent indication that D.
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moneta had previously been taken in Texas: ". . . on occasions it has been recorded in Texas. . . /' However, no record of these Texas specimens could be found by Emsley upon a review of the museum notes taken for his 1963 paper. Any authors, including Emsley (1963) and Michener (1942), who extend the genus Dione Hiibner to the U. S. must refer (at least indirectly) to the distribution of Dione moneta poeyii (Butler) quoted by Stichel (1907): "Honduras, Mexico, Texas." Strangely enough, Stichel provides no supporting data for such a claim, nor does he refer to literature which does. Even so it would still seem strange that Dione moneta is not mentioned—even as a "vague" or "doubtful" record—by Klots (1951), Ehrlich (1961), or dos Passos (1964). According to Hoffmann (1940) D. moneta poeyii ranges through most of Mexico. Emsley (1963) records that Dione moneta seems to be confined to montane habitats above 1000 meters throughout its range. However, Emmel (1961) provides interesting evidence that at least in the more northerly, marginal part of its range, D. moneta (= D. butleri Stich.) flies at lower elevations during the winter dry season. He reports taking this butterfly near Gomez Farias during January at an elevation of about 900 feet. My own field work in that region during July-August 1965, turned up no Dione moneta below 1100 meters.
Generally speaking, the Sierra Madre Oriental is poorly known biologically in northern Mexico (Martin, 1958), and this generality extends to our knowledge of butterfly distribution in the temperate and tropical forests of Mexico (Monroe, 1963; Hovanitz, 1958). Therefore, it is possible that the normal range of this species extends much further north than is now known.
Dione moneta: Ecology of Occurrence in Texas
Since this butterfly occurs at elevations up to 7000 feet in northern Mexico, there would appear to be no montane barriers to it between its normal range and Catarina, Texas, some 350 miles due north (see fig. 2). Many plants in D, moneta s montane chaparral habitat near Carabanchel are found in the low country of northern Mexico and southern Texas (Personal observation). A northward emigrating or straying individual of this species should not be stopped by the vegetational changes if the weather remains favorable.
The climatic conditions in November and December 1964 were unusual for south Texas. The temperatures were warm throughout the period, with readings above 90° F several days in the last weeks of December. According to the U. S. Weather Bureau's National Summaiy of Climatological Data (1964), every state southeast of a line from West Texas to Maine experienced their monthly maximum temperatures for
1969 Journal of the Lepidopterists' Society 181
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Gilbert: Natural dispersal by Dione Vol. 23, no. 3
December between the 22nd and 27th of that month. These temperatures ranged from 56° (Michigan) to 92° (Texas) and were generally above 70°. This is evidence of the widespread nature of the late December conditions. Unusually heavy rainfall was recorded in the late summer of 1964 in southern Texas, and no freeze had occurred before December (see fig. 3), with the result that most plants were still green and many were in flower (e.g. Lantana). Steady south winds, estimated at 10-15 m.p.h., prevailed during the whole of the last two weeks of December, which indicates, at least roughly, conditions existing in Mexico.
The butterfly species found together with a displaced butterfly might provide important evidence concerning its possible origin. Listed below are some of the species on the wing with Dione moneta in Catarina, Texas, December 31, 1964.
Agraulis vanillae, Anaea andria, Asterocampa leilia, Brephidium exilis, Colias eurytheme, Colias cesonia, Danaus gilippus, Danaus plexippus, Euptoieta claudia, Eurema daira, Eurema lisa, Eurema mexicana, Eurema nicippe, Hemiargus isola, Kricogonia castalia, Lihythea sp., Mestra amymone, Papilio polyxenes, Phoebis sennae, Pieris protodice, Precis lavinia, Strtjmon melinus, Vanessa atalanta, Vanessa virginiensis.
Of these species, 30% are known migrants, 17% have been previously recorded together in mixed migratory flights (this includes A. vanillae) and 17% have been known to migrate northward in the spring (Williams, 1930,1958).
The Subtropics of Southern Texas and Northern Mexico A substantial part of the transition from the wet-dry seasonality of the northeastern Mexican tropics to> the hot-cold seasonality of the southeastern U. S. occurs between southern Tamaulipas and central Texas. Localities near the center of the region (i.e., in southern Texas) may vary greatly in early winter environment from one year to the next. The direction and extent of this variation depends upon at least three factors: 1.) The amount and distribution of late summer-early fall precipitation. 2.) The time of occurrence and intensity of Canadian cold air masses moving south. 3.) The time of occurrence and intensity of warm tropical air masses moving north.
Many plants on the semi-arid fringes of the tropical zones possess the ability of shedding their leaves in response to the drought of the tropical dry season. It follows then, that in the semi-arid region of Texas and Mexico being discussed, summer drought results in a leafless and bleak winter much sooner than usual. Likewise, early killing frosts may leave the countryside gray and bare before mid-November (irrespective of summer rainfall).
1969
Journal of the Lepidopterists' Society
183
100
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Fig. 3. Climatic Data. A, Graph of daily maximum temperatures recorded at Catarina weather station December 1, 1964 to January 8, 1965. B, Monthly rainfall totals, Catarina, Texas, May 1964 to December 1965.
(Note: the rainfall total for August equals or exceeds the yearly total for that area in drouth years, and is roughly one-half of the normal yearly average.)
On rare occasions, such as the winter of 1951, extremely cold air masses reach southern Tamaulipas (Martin, 1958). On the other hand, northward bound tropical warm air masses often bring southern Texas relief
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Gilbert: Natural dispersal by Dione Vol. 23, no. 3
from winter cold. Such mild weather, however, will not bring back leaves and flowers previously lost by summer-fall drought, by early frost, or by a combination of these factors.
Because of the various climatic factors which must interact in proper sequence and in proper intensity, the "tropical" winter of 1964 was indeed unusual for the Catarina area of South Texas. Essentially this phenomenon can be looked upon as a temporary northward shifting of the tropical temperature and rainfall regimes. On the other hand, the winter of 1951 represents the reverse situation. It is thus clear that the nebulous concept, "subtropical," as applied to southern Texas and northern Mexico, would be better understood if the unpredictable nature of the climate in this region could somehow be conveyed by the prefix of the term.
Conclusion and Summary
The occurrence of Dione moneta in Texas was but one in a series of unusual ecological events during the tropical winter weather of 1964-65. Numerous facts indicate that this record represents natural dispersal on the part of this butterfly species. It will now be useful to summarize the more significant of them:
1. Dione moneta is a strong flier, and is closely related to a known migrant, Agraulis vanillae.
2. Dione moneta is on the wing during December and January only 350 miles south of Catarina, Texas.
3. Though normally restricted to montane habitats above 1000 meters, D. moneta has been known to shift to lower elevations in northern Mexico during the winter dry season.
4. Many suitable adult food plants of this species remained green and in flower over a wide area of northern Mexico and southern Texas during the winter of 1964-1965.
5. Widespread, striking climatic conditions, including steady south winds and warm temperatures were correlated with the occurrence of D. moneta in Texas.
6. Among twenty-four other butterfly species in company with this specimen of Dione moneta, 30% were known migratory species, several being northward spring migrants.
To conclude, I hope that collectors will keep on the watch for Dione moneta, especially during exceptionally warm winter weather in "subtropical" Texas and Mexico. In addition, similar investigations of other tropical butterfly records for Texas would be of great interest since many questions about the distribution and dispersal of tropical insects are yet to be answered.
1969
Journal of the Lepidopterists' Society
185
Acknowledgments I greatly appreciate the help of the following persons: Thomas Gilbert, Mrs. Ira T. Burns, David Lees, Professor E. B. Ford, F.R.S., and Dr. J. A. Powell.
Literature Cited
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dos Passos, C. F., 1964. A synonymic list of the Nearctic Rhopalocera. Lepid. Soc, Mem. 1, 145 pp.
Ehrlich, P. R., & A. H. Ehrlich, 1961. How to Know the Butterflies. Wm. C. Brown Co., Dubuque, Iowa, 262 pp.
Elton, C. S., 1958. The ecology of invasions by animals and plants. Methuen, London.
Emmel, T. C, 1961. Spring Collecting in Mexico: The Gomez Farias Region of Southwestern Tamaulipas. J. Lepid. Soc, 15(3): 197-199.
Emsley, M. G., 1963. A morphological study of imagine Heliconiiae (Lep. Nymphal-idae) with consideration of the evolutionary relationships within the group. Zoologica, N.Y., 48: 85-130.
Ford, E. B., 1945. Butterflies. Collins, London, 368 pp.
French, R. A., 1967. Long distance movement of two migrant Lepidoptera in relation to synoptic weather conditions. Biometeorology, 2( I): 153.
Hoffmann, C. C, 1940. Catalgo sistematico zoogeografico de los Lepidopteros Mexicanos. Anales Inst. Biol. (Univ. Nac. Mex.) 11(2): 639-739.
Hovanitz, W., 1958. Distribution of butterflies in the New World. In: Zoogeography, American Association for the Advancement of Science, Pub. No. 51, Washington, D.C.: 321-368.
Klots, A. B., 1951. A field guide to the butterflies of North America, east of the Great Plains. Houghton-Mifflin Co., Boston, Mass., 349 pp.
Martin, P. S., 1958. A Biogeography of reptiles and amphibians in the Gomez Farias Region, Tamaulipas, Mexico. Misc. Publ. Mus. Zool. Univ. Mich., 101, 102 pp.
Michener, Charles D., 1942. A Generic Revision of the Heliconiinae (Lepidoptera, Nymphalidae). Amer. Mus. Novit., No. 1197: 1-8, 17 fig.
Munroe, E. G., 1963. Characteristics and history of the North American fauna; Lepidoptera. Proceedings of the XVI International Congress of Zoology, Washington, D.C., 4:21-27.
Seitz, A., 1924. Macrolepidoptera of The World. Alfred Kerhen, Stuttgart, Germany, Vol. 5, 1139 pp.
Stichel, H., 1907. Lepid. Rhop., Nymphalidae, Dioninae. Gener. Ins. Fasc. 63 pp., 1-38, Fig. 1, Pis. I-III.
Williams, C. B., 1930. The Migration of Buttterflies. Oliver and Boyd, London, 473 pp. 1958. Insect Migration. Collins, London, 235 pp.