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Case studies on the effect of climate change on the flora of Mexico

Volume 4 Number 2 - July 2007

O. Téllez, P. Dávila, M. Ayala, K. Gutiérrez & I. Melchor

Mexico belongs to a natural biological region that extends from southern USA to northern Central America. This region is called ¨MegaMexico 3¨ (Rzedowski, 1993). It is characterized by rich biological diversity, which is mainly restricted to the Mexican territory. Mexico is considered a megadiverse country, and is probably third in the world with 8-12% of the global biodiversity (Caldecott et al., 1996; Mittermeier & Mittermeier, 1992). With regard to reptiles, Mexico has the most species in the world, with 717 species (Flores, 1993), with regard to mammals, Mexico takes second place, with 456 species and fourth place with regard to amphibians, with 285 species. Finally, Mexico has approximately 25,000 plant species (Rzedowski, 1993; Villaseñor, 2003). In addition, the endemism rate in different plant groups is usually high. For example, of the 900 Cactaceae species found in Mexico, 687 are endemic. In the Compositae (>3,000 species), the endemism rate is above 50% (Turner & Nesom, 1997; Villaseñor et al., 1998) and in the Leguminosae (>2,200 species), the endemism rate is also more than 50% (Sousa & Delgado, 1993).

Unfortunately, this unique biodiversity is rapidly being lost. Official reports point out that more than 500,000 hectares of natural forest are being lost or severely degraded every year, especially in the S-SE of Mexico. Other assessments state that the annual deforestation rate in the country has reached 1.5 million ha (CCE, 2007) or even 75,000 to 2 million hectares per year (Lund et al., 2002). The FAO assessments, since the 80’s estimated the loss of 350,000 to 650,000 ha a year. A recent assessment of the deforestation in Mexico indicates that from 1976 to 1993, 29,765 km2 of forest was lost and from 1993 to 2000, about 54,306 km2 disappeared. Therefore, the annual deforestation rate has increased from 175,000 to 775,800 ha (Velásquez et al., 2002). The National Commission of Biodiversity of Mexico (CONABIO) has made important efforts to build a system of Priority Conservation Terrestrial Regions (RTPs) in the country (Arriaga et al., 2000), including 151 areas that cover about 500,000 km2. However, its biological and environmental richness representation is inadequate (Cantú et al., 2004).

So far, the efforts undertaken regarding biodiversity conservation, with a few exceptions, have not considered the effects of global climate change, especially regarding plants. It is only recently that research projects have tried to assess the effects of different environmental conditions caused by global climate change on the current distribution patterns of plant groups.

Since 2003, the Facultad de Estudios Superiores Iztacala (FES, Iztacala), Universidad Nacional Autónoma México (UNAM) have undertaken projects related to the conservation of the natural resources of the arid and semi-arid regions of Mexico. The efforts include in situ and ex situ conservation and incorporate the goal of assessing the effects of global climate change, as a long-term conservation criterion (Téllez-Valdés & Dávila-Aranda, 2003; Villaseñor & Téllez-Valdés, 2004; Téllez-Valdés et al., 2006). Accordingly, three main projects are currently being developed: 1) Sustainable management and human development; 2) Ex situ conservation of the natural resources of the arid and semi-arid regions of Mexico; 3) Conservation assessments.

These projects target different aspects related to the conservation of natural resources. In particular conservation assessments aim to assess different future climate scenarios, based on the use of ecological niche models in which the bioclimatic information is the main source of information. Thus, the idea is to evaluate the consequences of future climate change on the conservation of the flora and to analyze the role that the RTPs and the Natural Protected Areas of Mexico are playing to conserve its future biodiversity.

Due to the uncertain magnitude of climate change, a comprehensive study was undertaken to include various proposals that suggest different scenarios. In particular, the model includes information which emerged from the IPCC studies (Canziani & Diaz, 1998; Giorgi et al., 1998; Karl, 1998; Neilson, 1998) and the Canadian general models of circulation (HadCM2 and HadCM2a). A conservative scenario (HHGSDX50), assumes a CO2 increment of 0.5% per year, whereas the most drastic one (HHGGAX50) proposes a CO2 increment of 1% each year. With this information it is possible to generate different scenarios in the country between 2030 and 2100. For instance, 1) a conservative scenario that considers an increment of 0.5°C and annual precipitation reduction of 10%; (2) an intermediate scenario with a temperature increment of 1-1.5°C and an annual precipitation reduction of 10%; and 3) a drastic scenario with a temperature increment of 2-2.5°C and an annual precipitation reduction of 10%.

One of the most important in situ projects is the assessment of the conservation status of endangered or threatened Cactaceae of the Tehuacan-Cuicatlán Biosphere Reserve. In this project, the distribution patterns and reproductive capacity of these species are being evaluated and their habitat and the effects of land use are also being studied. Finally, the effects of climate change on their survival and future distribution patterns, especially those species that are more sensitive to climate conditions, are being assessed. This Reserve is unique as it represents the southernmost semi-arid region of North America and is one of the most diverse areas, with almost 3,000 vascular species in an area of 10,000 km2. As part of this project, a database of 5,000 records of 82 Cactaceae species growing in the reserve, including 21 that are endemic is being analyzed.

The possible future distribution of the Cactaceae species in response to global climate change has been evaluated. The distribution of the 82 species of Cactaceae show different responses to climate change, both increase and reduction (Figures 1 & 2). In general terms, only a few species extend their distribution area under climate change. In fact, almost 95% of species drastically reduce their areas and 50% seem to have no distribution possibly because they will be extinct by 2100. It is clear also, that most of the habitats associated with the latter species will also be lost. However, in other cases, the area appears to occupy a more protected region at higher altitudes, where the species can find a refuge in the Reserve. This effect might already have occurred in the Cuicatlán Canyon, where the distribution of many columnar Cactaceae and their pollinators coincides very well.

The differential response to global climate change shows interesting results. There are various species (Mammillaria crucigera, M. huitzilopochtli, M. kraehenbuellii, M. napina and M. pectinifera) with current restricted distribution patterns that seem to be less sensitive to climate change by showing lower reduction rates, than those with wider patterns such as Cephalocereus columna-trajani and Neobuxbaumia tetetzo, which in theory have a higher genetic diversity and can occupy various habitats with different environmental conditions (unpublished Thesis).

There is also information about the behaviour of other endemic cacti species with less than 10 currently known populations (Ferocactus haematacanthus, F. macrodiscus, Mammillaria huitzilopochtli, M. kraehenbuellii, M. napina, M. pectinifera and Stenocactus crispatus). In these cases the effects of climate change are also very interesting. For instance, Mammillaria kraehenbuellii, M. napina and Ferocactus haematacanthus, have high habitat specificity and very restricted distribution patterns; the two species of Mammillaria showed an increase in their distribution while F. haematacanthus, even in the drastic scenario, only seemed to have a slight reduction (see Figure 3).

In the future, it is hoped to include data relating to the dispersion rate of the species, as an indicator of their “escaping capacity” to find new and more adequate habitats, under the pressure of global climate change. In general, the Cactaceae are characterized by producing fleshy fruits that are dispersed by animals (especially insects, birds and bats), which could play an important role for the dispersion of the seeds to other areas, where they could probably establish and grow.

In addition, there is a current project which aims to regionalize the flora of the Baja California Peninsula, on the basis of the climate and conservation status of the species. This Peninsula is located in northwest Mexico and represents an arid region with 0-22 mm of precipitation. In this project priority areas for conservation using bioclimatic models are being selected that enable the incorporation of climate-change scenarios, as an important tool to assess its long-term conservation value. Within the framework of this project two databases have been analyzed. One includes 40,000 records with information on about 3,000 vascular plant species. The second has 6,000 records of 500 endemic species. Considering possible future scenarios in different timeframes based on an increase in temperature from 0.9-1.2ºC and 1.5-2ºC and decrease in precipitation of about 5-10%, the species show different responses to these changes. However, in general terms, most of the species seem to show a drastic reduction of their distribution (Figure 4).

For example, a group of species from the temperate areas of the Peninsula show an increase in distribution (Agave deserti, Astragalus orcuttianus, Eriogonum elongatum, Eriogonum hastatum, Garrya grisea, Ipomopsis effusa, Ipomopsis guttata, Nolina palmeri and Quercus peninsularis). However, others that currently have a wider distribution that partially enter the drier zones show a reduction and under a drastic scenario can even disappear.

The effects that climate change will have on the plant species of the drier areas of the Baja California Peninsula are diverse, due to the fact that most of the habitats in which they currently live will change. Some groups of species will have a reduced distribution or disappear, even in the less drastic future scenarios. However, those species inhabiting more temperate areas will probably be less affected. For instance, it seems that the wetter pine and oak forests, will contract more gradually than the dry habitats.

In summary, it is recognized that most species have only a few alternatives in the face of climate change. For instance, they can 1) migrate to appropriate environmental conditions; 2) adapt to the new environmental conditions; or 3) become extinct. Evidently, it is expected that the intrinsic capacity of each taxon or group of taxa to respond to these climate changes, will result in different behaviours that can probably be assessed by bioclimatic modeling. Consequently, the results obtained so far show that the effects of climate change on the flora of Mexico, although different, is in general terms negative to the survival of numerous taxa inhabiting different habitats.

It is evident that in most cases, the areas where plant species are currently living will change latitudinally and altitudinally or the species will even disappear. Consequently, these models can help to propose future in situ and ex situ conservation strategies that assess the long-term role of the protected areas.

Future projects for 2008

In 2008, the effect of global climate change on the areas and distribution patterns of wild relatives of cultivated plants will be assessed. Mexico is the centre of origin of several plant species that have been domesticated and played an important role as main food crops for humans. Wild species of some families such as the Cucurbitaceae and Gramineae will be studied in order to propose strategies that can mitigate the reduction of their distribution or avoid extinction by ex situ conservation.

In addition, conservation priority areas of the drylands of Mexico will be studied and selected. In this project, FES Iztacala, UNAM will collaborate with the Royal Botanic Gardens, Kew, UK through the Millennium Seed Bank. This will assess the effects that global climate change will have on the distribution of species that inhabit Mexican drylands and the role that natural protected areas can play for their long-term conservation. The role of the Seed Bank of Mexico located at the FES-Iztacala in the conservation of the plant species of the country will also be evaluated.

Finally, it is worth mentioning the increased concern of society and the academic world in relationship to environmental issues, including global climate change. In many ways they have been supportive and have financed research activities that are needed to provide the necessary information. For example, most of this work has been supported (2007-2008) by the FES, Iztacala, other UNAM institutions, the automotive company Volkswagen which through its programme ‘For love of the Planet’ has granted US$ 100,000 and the Royal Botanic Gardens, Kew contributed US$ 50,000.00. Evidently, due to the large size of Mexico, more funds are needed for research to understand and if it is possible, to protect the 25,000-30,000 Mexican plant species from global climate change.


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O. Téllez, P. Dávila, M. Ayala, K. Gutiérrez & I. Melchor
Laboratorio de Recursos Naturales UBIPRO
Facultad de Estudios Superiores, Iztacala (FES, Iztacala)
Universidad Nacional Autónoma México (UNAM),
Av. de los Barrios 1, Los Reyes, Tlalnepantla Estado de México, 54090. Ap. postal 314, Mexico