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Instituto de Recursos Naturales—Especialidad de Botánica, Colegio de Postgraduados en Ciencias Agrícolas, 56230 Montecillo, Estado de México, Mexico
Received for publication July 7, 1997. Accepted for publication October 15, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: agrestals • epianthropochory • epizoochory • Mexico • seed dispersal • weeds
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Bye, 1993
; Espinosa-García and Sarukhan, 1997
; Vibrans, 1998a
). The study of the ecology of this flora, which has developed under circumstances quite distinct from the flora of man-made habitats of Mediterranean or temperate climates, could help understand the evolution of the interesting group of plants which we usually call weeds. The dispersal strategy of a plant is one of the most important aspects of its biology and ecology. A successful species has to be able to deposit propagules (diaspores) in sites conductive to germination and survival. Short-distance dispersal permits establishment at a certain distance from the mother plant; long-distance dispersal permits the colonization of new habitats.
Weed seeds in arable fields move short distances passively during cultivation (Marshall and Hopkins, 1990
), sometimes helped by an explosive ejection mechanism or by ants. However, an additional mechanism for long-distance dispersal may be valuable, as fields tend to be unstable habitats. In general, long-distance dispersal in weeds is accomplished by either wind dispersal or direct or indirect dispersal by humans (Anderson, 1983
; Marshall and Hopkins, 1990
).
Contaminated crop seeds are a major anthropochorous long-distance dispersal pathway (Anderson, 1983
; Holt, 1988
). Some specialist weeds, known as crop mimics, have evolved seeds of similar diaspore mass and date of maturation as the crop, which are harvested and sown with it (Baker, 1974
; Barrett, 1983
). Seeds also disperse in the mud sticking to feet, hooves, wheels, and machinery (Baker, 1974
; Anderson, 1983
). These diaspores appear not to require special morphological features. Abundant circumstantial and limited experimental evidence show mud dispersal to be effective; the five most common temperate weeds are dispersed this way (Fenner, 1985
). Some authors consider it to be the most common dispersal mecanism for weeds (Holt, 1988
), while others think that wind dispersal is more important (Harper, Lovell, and Moore, 1970
; Anderson, 1983
).
Observations in Mexico point to another major dispersal pathway used by Mesoamerican weeds. Anyone who visits central Mexican rural areas in autumn will have to remove numerous prickly fruits from socks and trousers after a stroll in the countryside. If one enters maize fields for harvesting (or vegetation sampling) one emerges with every piece of clothing infested with scratchy diaspores. Indeed, the spine-studded fruits of the winding Sicyos deppei or Echinopepon milleflorum (both Cucurbitaceae) may even find their way into the inside of a shirt from above. The Tarahumara Indians frequently harvest only in their loincloths, in order to reduce this annoyance (R. Bye, personal communication, Jardín Botánico, Universidad Nacional Autónoma de México). In the central highlands, it is impossible to find a burr-free maize field, unless the field has been treated with herbicides.
A literature review shows that dispersal by appendages that cling to the exterior of animals (epizoochory by burrs) is relatively rare: general floras contain 0–6 % species with adhesive fruit (Sorensen, 1986
). This dispersal adaptation is known particularly from the herb layer in temperate forests (Harper, Lovell, and Moore, 1970
; Sorensen, 1986
), savanna-like grasslands in Australia (Willson, Rice, and Westoby, 1990
), the semiarid grasslands of Africa (Fenner, 1985
; Milton, Siegfried, and Dean, 1990
; Ernst, Veenendaal, and Kebakile,1992
), and disturbed habitats in general (Sorensen, 1986
; Guitián and Sánchez, 1992
).
Animal exteriors, particularly the fleeces of sheep and the tails of horses are frequently cited as long-distance vehicles for clinging diaspores (Fenner, 1985
). While there are some anecdotal data on transport of seeds on human clothing (Salisbury, 1942
; Healy, 1943
), the systematic clinging to human exteriors has not yet been reported.
In this paper, I address the following questions: (1) Do central Mexican maize fields contain an unusually high proportion of weeds with adhesive diaspores? (2) What is the principal long-distance dispersal vector for these plants?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The study area of 
90 x 120 km lies in the highlands of the Transverse Volcanic Belt of south-central Mexico (18°40'-19°30' N, 97°30'-98°40' W). It consists of a main valley (Puebla-Tlaxcala), several lateral valleys (Oriental, Tecamachalco-Tehuacán, and Atlixco), mountains (Sierra Nevada with Popocatépetl and Iztáccihuatl, Malinche) and hillchains. The physiographic variablity of the study area is wide in terms of altitude (1400-3000 m), soils (volcanic, calcaric, and saline), and climate (cold to temperate to seasonally warm-tropical). The valley is densely populated, with one large city (Puebla) and several large towns. Small to medium-sized farms dominate and agricultural practices are modified traditional (use of tractors, fertilizers, and some pesticides, but generally traditional varieties and planting procedures). The main crop is maize. Widespread agriculture has existed for at least 4000 yr, including irrigation for 3000 yr (Lauer, 1981
).
Only the 50 most important species were included in the analysis of dispersal adaptations. Species importance was defined by a combination of frequency and average cover value (data in Table 1). The selection was based on 378 Braun-Blanquet relevés (Reichelt and Wilmanns, 1973
; Dierssen, 1990
) placed systematically (every 5 km) along passable roads from 1988 to 1990. These relevés included a complete species list on an area of 40 m2 of maize field at least 2 m from the edge, along with a visual cover estimate according to the standard scale of the method. Of the 98 species that occurred at least 15 times in the relevés, the 50 taxa with the greatest average coverage were selected for inclusion in Table 1. This combination of criteria allowed the inclusion of the dominant plants from geographically less extended communities, for example of saline soils. Voucher specimens were deposited at the National Herbarium, Mexico City (MEXU).
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In order to support the conclusions arrived at through observation and dispersal adaptation analysis, 45 maize farmers were interviewed in the Toluca valley. This region is situated 120 km to the west of the Puebla-Tlaxcala high basin and has a very similar maize field vegetation (Rodriguez-Jimenez and Agundis-Mata, 1981
; Vibrans, 1998a
, pp. 102–103). The interviews were conducted by the author and students of the author among their relatives and neighbors in November and December of 1997. This form of interviewee selection was chosen in order to overcome the traditional distrust of farmers towards systematic questioning or questionaires. Municipios (counties) in the north, east, west, and center of the valley are represented (Ixtlahuaca, Zinancatepec, Toluca, Metepec, and Ocoyoacac).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In nine species (18% of the 50 most important taxa) the diaspores have adaptations to adhesive dispersal (awns or hooks), and another five species (10%) have dimorphic diaspores with and without clinging appendages. This means that over one-quarter of the species rely at least partially on this generally rare dispersal pathway. The table also shows that adhesive fruit appear mainly on tall and dominant species. The relationship between plant height and dispersal mode is highly significant in a 
2 test (P < 0.005).
Thirty-five species have no conspicuous adaptation. There is only one endozoochorous species with a berry, and none that is distributed by wind.
The questionaire applied to farmers in the Toluca valley shows that the majority of farmers have plants with clinging diaspores in all of their maize fields (39 of 45 interviewed farmers = 88%). Those that do not had applied herbicides (4 farmers = 9%) (two informants, 4%, did not answer this question). Indeed, comments of the farmers show that herbicides are applied mainly to make harvesting easier by eliminating the scratchy clinging fruit, rather than to reduce competition with maize.
The main agricultural activities where the clinging diaspores are a nuisance are harvesting/dehulling (45 interviewees = 100%), gathering feed for animals in the fields (19 = 42%), weeding (13 = 29%), and gathering potherbs (9 = 20%). During field preparation, sowing, and cultivation, clinging diaspores pose little or no problem.
Twenty-nine farmers (64%) said that clinging diaspores are usually felt on the whole body, 12 (27%) indicated the waist as the upper limit, two (4%) who use herbicides feel them up to the knees, and two said "it depends." Thirty-seven informants (82%) said the diaspores were very annoying, three (7%) said they were sometimes annoying or only some species, one (2%) said they were not annoying, and four (9%) did not answer the question.
Regarding countermeasures, 17 farmers (37%) said they cut, trample, or beat the plants down with a stick before they set seeds; 9 (20%) apply herbicides, 6 (13%) use both mechanical, and chemical methods of weed control, 11 (24%) said they do nothing, and two (4%) did not answer. Eleven (24%) of the farmers regularly and eight (18%) sometimes put on protective clothing (large plastic bags, overalls, old clothes) when they harvest.
Farmers were also asked where they pick the clinging fruit off their clothing. Twenty-five (56%) said it varies - in the field, after finishing work, or at home. Nine (20%) always pick them off in the field after work, four (9%) in the field when they start to annoy, and seven (16%) only at home.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The complete absence of wind dispersal among the important species of this study is, however, noteworthy. In contrast, the Asteraceae in the flora of the Valley of Mexico (Rzedowski and Rzedowski, 1985
) contain 53% wind-dispersed species (201 of 379). (A few wind-dispersed weeds do exist in Mexican maize fields, for example, several species of Conyza, Gnaphalium, Porophyllum, or Sonchus, but none of them reach high cover values.)
Central Mexican maize fields have an unusually high proportion of species with diaspores adapted to clinging by hooks and awns for dispersal (Table 2; 2 test for difference between floras in the proportion of adhesive/nonadhesive diaspores significant at P < 0.005). Similar percentages (>20% of the species) are only found in grasslands rich in herbivorous mammals (e.g., Willson, Rice, and Westoby [1990]
for Australia; Milton, Siegfried, and Dean [1990]
for Africa).
The adhesive fruit are usually presented in a similar way to the well-known Bidens species: the awns are turned to the outside, ready to be brushed off. The diaspores may remain attached to the receptacle for months, if there is no mechanical disturbance (which, of course, there will be in a cultivated field, sooner or later). Only Medicago polymorpha fruit have adhesive appendages on all surfaces, so they could be picked up by the soles of feet also.
As adhesive-fruited plants are mainly tall (see Table 1), the small mammals that live in maize fields are unlikely to function as principal long-distance dispersers. Large domestic draught animals are used only for plowing and cultivating, at times when the weed diaspores are either immature or lying on the ground. Birds are discounted because (a) the plants generally do not offer perches, (b) most of the fruits are large and presumably aversive to birds, and must therefore be preened off rapidly, (c) such fruit are rarely found on birds (M. Gurolla, personal communication, Instituto de Biología, Universidad Nacional Autónoma de México), and (d) burrs are generally thought to be dispersed by mammals rather than birds (van der Pijl, 1972
).
Harvesting and removal of maize cane, however, are generally done by hand. Also, maize differs from most other cereals in that people can walk around in the fields without damaging the crop, and in central Mexico they do so frequently. Farmers harvest weeds as forage for animals, spontaneous potherbs ("quelites"; Bye, 1981
) and other crops sown together with the maize, e.g., the traditional beans and squash, but also green tomatos (Physalis), Vicia beans, chilis, peas, amaranth, Tagetes flowers, and others. Also, maize fields are widely used for defecation. Therefore, humans appear to be the most likely long-distance vector for the species with adhesive fruit (epianthropochory).
This conclusion is supported by interviews with farmers, which show that clinging diaspores are nearly omnipresent towards the end of the growing season, unless herbicides are used. The clinging seeds are felt to be a major nuisance, and various measures are taken to alleviate the problem. It is also clear that farmers transport seeds over considerable distances, though most diaspores probably move within their field by the same mechanisms for short-distance dispersal used by the other seeds, principally cultivation. Long-distance dispersal is always a relatively rare event, but particularly important to weedy, colonizing species.
Possible explanations for the success of the "human" transport vehicle are various. It permits the transport of relatively large seeds (Harper, Lovell, and Moore, 1970
). Achenes of the dominant Bidens odorata (5–14 mm), Simsia amplexicaulis (3–5 mm), and Tithonia tubiformis (4–6 mm) are larger than the average wind-dispersed seeds of the Asteraceae, thus giving the seedlings advantages when competing with the fast-growing maize (see Fig. 1). The 201 wind-dispersed Asteraceae of the Valley of Mexico (Rzedowski and Rzedowski, 1985
) have an average achene length, without pappus, of 2.9 mm (median: 2.5 mm, range: 0.4–13 mm), whereas the 65 species (mostly weeds) with clinging appendages have an average achene length of 5.5 mm (median: 5.5 mm, range: 2–40 mm).
Transport on humans permits directed dispersal, as humans are likely to move within habitats favorable to weeds. Slash-and-burn systems were and continue to be widespread in Mexico (though not in the study area), with cultivated surfaces changing every few years. A mechanism for directed dispersal (Howe and Smallwood, 1982
) of just a few diaspores must be of considerable advantage to plants evolving in this habitat.
The dominance of adhesive-fruited species is perhaps also one reason why exotic weeds (see Table 1) are not very successful in central Mexico (Vibrans, 1998b
), in contrast to other regions in the Americas (Muenscher, 1955
; Oberdorfer, 1960
; Holzner and Numata, 1982
; Pimentel, 1986
).
These results open up several lines of inquiry on the origin and evolution of these adhesive-fruited weeds dispersed by humans. What habitat did they come from? Is there evidence that wild animals and adhesive-fruited Heliantheae (the tribe to which all of the dominant Asteraceae with awned achenes belong) have coevolved in some part of Mexico? Since some of the species appear to occur mainly in fields, has agriculture been practiced long enough in Mexico to allow obligatory weed species with their specialized adaptations to evolve (Scholz, 1996
), as we know them from the Near East and the Mediterranean?
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FOOTNOTES |
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2 E-mail for correspondence: heike@colpos.colpos.mx
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Anonymus. 1971 Common weeds of the United States. Agricultural Research Service, United States Department of Agriculture. Dover, New York, NY.
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———, and G. C. De Rzedowski. 1985 Flora fanerogámica del Valle de México. Vol. 2. Escuela Nacional de Ciencias Biológicas and Instituto de Ecología, México, D.F.
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Scholz, H. 1996 Ursprung und Evolution obligatorischer Unkräuter. In R. Fritsch and K. Hammer [eds.], Evolution und Taxonomie von pflanzengenetischen Ressourcen (Festschrift P. Hanelt). Schriften zu Genetischen Ressourcen: 4, 109–129. Zentralstelle für Agrardokumentation und -information, Bonn.
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———. 1998b Native maize field weed communities in south-central Mexico. Weed Research 38: 153–166.
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