January 14, 2010

Irreplaceable natural services: A look at the plight of the Chihuahuan grasslands and the black-tailed prairie dog

ResearchBlogging.orgI've written in general about grasslands before, as a biome, making sure to note that these treeless plains have always been the stage of expansive growth and decline for both the animal world and the human world, a stage upon which our skill at mastering our environment and bending it to our will is most apparent. Our achievement in converting grasslands from complex ecosystem to agricultural workhorses is only matched by our negligence in understanding how these delicate systems work and the potential danger of reaching a point of no return in grasslands management.

This article from PLoS ONE,  provides a very clear, apt example of just how delicate this biome can be, and illustrates the services that native animals can provide in an ecosystem that would cost considerable sums to replace. Grasslands are rapidly being converted to shrubland and in some cases, bare ground. Agriculture has disturbed a major architect of the grasslands of northwestern Mexico: the black-tailed prairie dog (Cynomys ludovicianus).



These photos show the rapid loss of prairie dogs within the largest colony of the Janos grasslands, following two decades of intensive land use and drought. Note the sparse coverage of annual grasses and forbs and the lack of perennial grasses, which is characteristic of degraded grasslands in Janos. These plants are only available during the rainy season and most of the year the area is bare ground. doi:10.1371/journal.pone.0008562.g002


The 55,000 hectare prairie dog complex (an assemblage of eight colonies) in the grasslands of Janos Valley in Chihuahua was fragmented and reduced by 73 percent from 1988 – 2005. The researchers saw prairie dog densities drop from 25 per ha to a dismal 2 per ha and the average colony size from approximately 6,250 ha to 437 ha (though a couple of larger colonies do still exist). Almost across the board, vertebrates in the grasslands would suffer similar declines in the same time period.



The black tailed prairie dog is a keystone species of the Janos grasslands. It performs a critical role in shaping and maintaining the ecosystem to which it belongs in three ways:


  • They are a vital source of food for predators like raptors, owls and coyotes.
  • The burrows they create are used as homes and habitats by other life.
  • They prevent the encroachment of shrubland by eating the seeds and saplings of woody plants like ephedra and mesquite.


The last benefit is perhaps the most significant, at least to shrinking grasslands. The researchers were given unique opportunities to study the extent of this ecological boon, two cases: one where they could see the aftereffects of the removal of a colony and one where they could observe the expansion of one.

In 1988, the Los Ratones colony was poisoned, a colony comprising almost 1,600 ha. In the absence of the prairie dogs, 34 percent of the surrounding grassland was converted to shrubland by 1996. Just to be clear, that is 34 percent is not a percentage of the land that the colony covered, but a percentage of the surrounding grassland, which actually comprised more hectares than the Los Ratones colony itself. Why is that important to mention? The second case neatly dovetails.

The researchers watched the La Bascula colony expand 16 percent into the surrounding ephedra-dominated shrubland over a five year period, between 2000 and 2005. In the area around the colony, 81 percent of the ephedra had signs of 'prairie dog clipping' and, on average, the plants around the colony were 55 percent shorter than the plants on the colony outskirts, of which only 3 percent showed the hallmark of pruning. The prairie dogs had effectively cut a swath of 546 meters into the ephedra, converting a large area into grassland and maintaining it with their mere presence.

So when colonies of black-tailed prairie dogs are fragmented and reduced in size and number, the grassland is missing its most diligent protector, allowing the shrubland to expand. Some species have seemed to move and adapt to the shrubland when the ungrazed grassland habitat became more scarce:


...the bunchgrass lizard (Sceloporus scalaris), a species highly associated with perennial grassland habitat, ...was more abundant in the shrublands in our study. This species may have preferred the more structurally complex shrublands compared to the heavily grazed grasslands, as it has been found to be ten-times more abundant in ungrazed perennial grassland than grazed grassland.


It's not the standard, however. The endangered and threatened species that thrive in the grassland - particularly the carnivores and larger mammals - have seen startling declines.



Mammal and bird species in the Janos prairie dog grasslands showing dramatic declines in densities over time. (Note prairie dog densities are compared from 1994–2004.) Of the 33 bird species that were sampled, only those that exhibited a 2-fold or greater change over time are shown here. doi:10.1371/journal.pone.0008562.g009


I think it's important to mention that this article is not saying 'grassland good, shrubland bad'; it's highlighting how important one species can be to a delicate and anthropogenically overused ecosystem. In fact, the researchers make clear in the article that in general, species richness and diversity was actually higher in the shrublands. But the most threatened species depend on the grassland for their livelihood, which is why it's so important to try to preserve and manage it properly.

It's encouraging to see results. Often the rhetoric from NGOs and activists regarding the morality and practicality of protecting threatened areas starts to blend and lose impact, so it's vital to publicize studies that bring to light exactly the sort of "kick em in the wallet" attention grabbers that legislators need to see.

The researchers are working with ranchers in the area, trying to come up with management solutions instead of just kicking down the door and saying 'no', so to speak. They hope to use the cattle to clear spaces for the prairie dogs to recolonize, reduce the grazing pressure so that the fires can return, which in turn will clear more shrubland, making way for further colonization.

It's realistic, practical solutions like these that will not only protect the ecosystems that are in danger, but also forge new relationships with people a world away from the West. Can we really blame these people for being suspicious of our intentions, for assuming that this is just another mode of control or that we are disregarding their culture yet again, a culture certainly more intact and probably less confused than ours currently?

Ceballos, G., Davidson, A., List, R., Pacheco, J., Manzano-Fischer, P., Santos-Barrera, G., & Cruzado, J. (2010). Rapid Decline of a Grassland System and Its Ecological and Conservation Implications PLoS ONE, 5 (1) DOI: 10.1371/journal.pone.0008562

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