Agricultural activity continues to be an important source of economic income, along with other satisfactions, carried out mainly by people over 40 years of age. There is a sector of young people (between 20 and 39) who expressed their interest in agricultural activity. It develops in three annual agroecosystems: chinampa, ciénega, and slope. Campesinos identified agrestal weeds as components of the main crops and as useful species for different purposes. Differences were found in the weed diversity, composition, and abundance at the family and species level between the tree agroecosystems, with the highest values in slope. Alpha diversity values were similar, but dominance was higher in chinampa, and ciénega suggesting a decrease in agrobiodiversity. Differences between the three agroecosystems are explained by different factors: the type of agroecosystem, the similarity in the agricultural practices, and their location in the PERI and SUB areas.
Despite the high rate of migration that occurs in CDMX, most of the collaborators are native to the localities where they were interviewed, so this activity is linked to strong ancestral ties of land ownership, identity with their local roots, the permanence of certain traditions, and even to their ethnic roots -mainly Nahua roots-. There are different reasons that explain why they are campesinos in this megapolis: Some turned back to agricultural activity after they retired or became ill; others do not want to lose or abandon their lands, which were inherited from their parents, or grandparents. These reasons have been mentioned before (Torres-Lima and Burns 2002). With COVID 19 pandemic, many students and professionals retreated to their homes because of a lack of employment, or the impossibility of going to school. This isolation was the spearhead for promoting local organizations in different localities. An interesting case was observed in San Pedro Atocpan, where during the two years of pandemia, a collective was formed, Colectivo Rural Actopan, to preserve native maize seeds, promote their exchange, and even experiment with new varieties. The new generation, which consists of young people with at least bachelor's degrees, actively and methodically adopted this group after one of the collaborators who was over 60 years old created it (Daniel Vázquez, Pers. Comm.). Thus, agricultural activity has the potential to continue even when, in each generation, there is an increase in the percentage of people with higher education, which has been a trigger for the rapid change from agricultural activity to other productive activities. However, a strong local organization is necessary to avoid external factors that could affect land tenure (urbanization, criminal groups, invasions).
According to Hernández-Xolocotzi and Ramos (1977) slope, chinampa, and ciénega are agroecosystems because they provide natural resources for agricultural activities and have their own characteristics that differentiate them from others: the temporality, the spatial distribution, the forms of social organization, labor division, the tools and inputs used, the level of insertion in the market (v.g., local, regional) and the characteristics of the landscape where these are inserted (Martínez-Alfaro 2001; Casanova-Pérez et al. 2015). The historical component indicating its pre-Hispanic presence is also added (Table 8). However, we observed changes that arise from the campesinos’ own initiatives, given the changing demands of local, national, or even international crop markets, fluctuations in the sale price of different agricultural products, and pest and disease problems that reduce production and cause campesinos to look for new options. cultivation and, more recently, the decrease in the amount of rainwater and the increasingly erratic changes in the rainy season.
Changes in crop cultivation occurr concurrently with the introduction of new technologies, such as agrochemicals, or infrastructure, such as the excessive use of plastic bags or greenhouses. A negative pressure corresponds to the use of different agrochemicals, with the argument that, without their application, crop development is almost impossible. In the slope, nopal has the highest demand for pesticides, but maize requires more chemical fertilizer. Some authors suggest that the tendency to convert maize fields on nopal fields, which is related to "the best strategy that the collaborators have adopted to deal with the field, located at the urban-rural interface, the dependence on rain, the reduced use of machinery, and, therefore, the greater use of rudimentary procedures in its planting and harvesting, which allow them to take better advantage of the precarious production conditions" (Bonilla-Rodrguez 2014). Our findings suggest that this strategy has not been sufficiently regulated, and it is expanding with numerous management issues and serious market problems due to the expansion of the cultivation area of this species not only in CDMX but also in Morelos, which has created a very competitive market in recent years.
However, the results demonstrated that dependence to agrochemicals is more pronounced in chinampa and ciénega. This perception has already been reported (FAO 2015; González-Pozo et al. 2016; Dieleman 2017; Losada et al. 2017), as has the negative impact of its use detected in an increase in the concentration of nitrates in the waters near greenhouses (Méndez 2006) or the presence of ammonium, among others, in Lake Xochimilco-Chalco (Zambrano et al. 2009).
Almost uncontrollable amounts of trash are being generated. The level of contamination in the canals of Xochimilco, the inadequacy of water purification, and the precipitous decline in water levels are remarkable. During 2021 field trips, we observed the level of dirt in the water and the foul odors it emitted. The effects of the accumulation of significant amounts of organic matter are reducing the concentrations of dissolved oxygen in the water of channels and apantles (Mazari-Hiriart et al. 2008).
These changes in agricultural practices have resulted in substantial shifts in weeds diversity, abundance, and composition. Vibrans (1998) recorded 42 families and 256 ruderal species in the urban zone of the city, in areas below 2300 m a.s.l. A recent study to quantify knowledgeable and beneficial weed diversity and composition from the Cuajimalpa municipality (Rivera- Ramírez et al. 2021) reported 42 species and 19 botanical families. Additionally, Vieyra-Odilon y Vibrans (2001) and Sánchez-Reyes (2016) reported comparable data for maize fields in the State of Mexico.
In the case of flooded agricultural lands, such as ciénega AE, Sánchez-Blanco and Guevara-Ferrero (2013) reported a higher number of families (34) and species (133). This study examines the geographical phenomenon of the lake's desiccation and eutrophication, which has resulted in the exposure of salt-impregnated agricultural soils. Intermittent inundation and soil salinization were two of the issues that the collaborators mentioned. In our study, we found that ciénega presented the lowest species and family diversity, as well as the greatest number of Poaceae species with a dominance index (IV) greater than 0.1. This suggests that certain species are becoming dominant.
There are few studies that examine the effect of herbicides on weed diversity, composition, and abundance. Sánchez-Reyes (2016) compared these parameters in maize fields with and without herbicide application in Ixtlahuaca, State of Mexico. They recorded 43 species belonging to 17 botanical families. Although richness values were comparable whether herbicides were used or not, the composition did change, with a significant decrease in dicot dry weight and an increase in monocot dry weight in fields where herbicides were frequently applied. In the plots corresponding to ciénega and chinampa, a similar phenomenon was observed. In these AE, farmers cultivate the greatest diversity of annual crops dependent on various pesticides, such as ornamental and aromatic plants and vegetables. As a result, we discovered the highest proportion of Poaceae species with IV greater than 0.1, indicating this family's dominance.
In relation to the slope AE, the relatively high percentage of some native species and their low IV suggest that some traditional management practices still exist and are associated with Mesoamerican agriculture (Molina-Freaner et al. 2008). Despite the high use of chemical fertilizers, our findings indicate that only a small proportion of pesticides are used. According to Sánchez-Reyes (2016) and Rivera- Ramírez et al. (2021), this suggests that the fertilizer does not have any direct negative effects on weed diversity or composition. Molina-Freaner et al. (2008) discuss the role that certain agricultural practices (e.g., polyculture, rotation, intercropping, mechanized cultivation) can play in the weed richness and abundance, in the low values of dominance of some species, and in the presence of certain weeds that have adapted in some way to these agricultural practices.
Thus, the decrease or loss of traditional crops in chinampa and ciénega because of the substitution of commercial crops, the indiscriminate use of agrochemicals, and the possible effect of treated water used for irrigation has resulted in the loss of numerous native weed species and the increase of introduced species. Our findings reveal a trend in the number of native species versus alien species. According to Bye (1998) and Molina-Freaner et al. (2008), in cultivated fields that utilize traditional management techniques there will always be an increased presence of native weed species that have adapted to specific agricultural practices.
The comparison of weed diversity and composition between agroecosystems by zones (SUB, PERI) reinforces their differences, where slope plots of the PERI zone are clearly differentiated from chinampa, ciénega, and slope sections located in the SUB zone.
Our hypothesis was that, regardless of their suburban or peri-urban location, agroecosystems that preserve more traditional aspects of their agricultural practices will have a greater intra- and interspecific diversity of said species. This hypothesis is partially supported by the fact that the slope agroecosystem, located in the PERI zone, is the one with considerably more traditional agricultural practices and a greater proportion of traditional crops (corn, broad bean, and bean) that are primarily cultivated for subsistence. In the SUB zone, slope, ciénega, and chinampa plots will be used to cultivate commodities for sale, including nopal, squash, cempasuchil, broccoli, and tomato. Losada et al. (1996, 1998), Torres-Lima et al. (1994, 2008), and Torres-Lima and Burns (2002) have conducted a comprehensive analysis of the dynamics of agriculture from a historical perspective and the implications of being a campesino in a region with high urban growth in terms of the productive processes that occur between these zones. A duality of campesino-worker, campesino-employee, and educator-campesino.
Even though these changes have affected weed richness and composition between AE, traditional knowledge and use of weeds persist because: a) Campesinos classify this group of plants separately from the main crops and conceive of them as plants that grow spontaneously in cultivated fields, much like Espinosa-García and Sarukhán (1997), or Bye (1998) do. Consequently, the list includes perennial species such as tepozán, ahuehuete, and tabaquillo, which are species that sprout during the agricultural cycle and are then eliminated with weeding practices (manual or chemical); b) there is an important group of plants mentioned by almost all campesinos, which is similar to those reported in other studies, otherwise the study area and stand still through the years (Sánchez-Blanco and Guevara-Ferrer 2013; Linares-Mazari and Bye-Boettler 2015).The most significant number was associated with edible plants, followed by fodder and medicine. c) there is a group of species that maintains the náhuatl designation and reflects specialized knowledge based on their own experiences (Luna-José and Rendón-Aguilar 2012): xahuilisca, yolochichi, ocoxochil, pitzitlalcual, chilacaxtle, and chinantlaco. An ethnobotanical investigation aiming at collecting and documenting this knowledge in a systematic manner would be fascinating, given that many of them were not collected. It is important to note that, of the 216 registered species, we were only able to collect 111, so this information remains incomplete.