|
¿How many butterflies arrive in Mexico ?
This is one of the most commonly asked questions about the Monarch butterfly. The short answer is, we don't know.
Despite multiple efforts to estimate how many millions of butterflies come to Mexico each year to hibernate, we still do not have a reliable number. Since the discovery of the hibernation sites more than 30 years ago, there have been various attempts to estimate the population size. Since 1985, the most commonly cited number is 10 million butterflies per hectare. If we calculate that the Monarchs use 9 hectares that puts their numbers at approximately 90 million butterflies. That said, the high mortality experienced in the winter of 2001-2002 suggested that this number could be a serious underestimation.
The problem of getting a reliable population estimate comes from the fact that it is very difficult to count the monarchs when they are gathered in such dense clusters. It is much easier to estimate population trends. It is also more important to know how the population size is changing from year to year or how the numbers from one site compare to another. Such information can be obtained by calculating relative abundance rather than absolute population size.
The main population index used for the Monarchs has been the forest area used by the colonies. This metric is imperfect because the relation between the area occupied and the number of butterflies is not necessarily a direct one. For example, the butterflies are sometimes very concentrated, but at other times quite dispersed. Moreover, the tree density in the different forests used by the butterflies can also vary.
Despite these limitations, the use of the area index has been consistent over the years and gives us a general indication of the population trends for the butterflies. The figure below shows these trends over the past 14 years:
Figure 1 shows that during the last 14 years, the area occupied by the Monarchs has varied substantially—from a minimum of 2 hectares in 2004 to 18 hectares in 1996, with an annual average of 8 hectares . The wide fluctuations are typical of insect populations, although it is important to have these estimates to know when the numbers go outside of their normal range.
The previous figure gives us a rough idea of population trends represented by the cumulative area occupied by a number of butterfly colonies. The monitoring efforts of WWF, in coordination with CONANP, started in 2004 include measurements from 11 sanctuaries that have historically been used by the butterflies (Table 1, Figure 2). During November, we survey areas to localize butterfly colonies and they are named according to the name of the site where they are found (ejido, indigenous community, private property, etc.) and the area occupied is measured every 15 days using a compass, topographic maps, and a Geographic Positioning System (GPS) (UTM projection, WGS 1984 datum).
 |
|
|
| |
|
|
| |
|
|
| |
Table 1. Hibernation sanctuaries in the Monarch Region. |
|
|
|
|
| |
|
|
| |
|
|
| |
Figure 2. Sanctuaries visited for population monitoring (click on the map above to see larger image). |
|
Monitoring reports on the Monarch butterfly colonies:
• Rendón-Salinas, E., S. Rodríguez-Mejía, M. Cruz-Piña y C. Galindo-Leal. Monitoreo de las colonias de hibernación de mariposa Monarca: superficie forestal de ocupación en diciembre de 2006 . Reporte. 6 pp. • Rendón-Salinas, E., A. Valera Bermejo, M. Cruz Piña, S. Rodríguez Mejía y C. Galindo-Leal. 2006. Monitoreo de las Colonias de Hibernación de Mariposa Monarca: Superficie Forestal de Ocupación en Diciembre de 2005. Reporte. 6 pp.
• Rendón, E. y C. Galindo Leal. 2005. Reporte Preliminar del Monitoreo de las Colonias de Hibernación de la Mariposa Monarca. WWF. México D. F. (inédito) 9 pp. |
