Micromamíferos, cambio climático e impacto antrópico: ¿Cuánto han cambiado las comunidades del sur de América del Sur en los últimos 500 años?

Abstract

Diversas evidencias sugieren que la actual configuración de las comunidades -i.e., riqueza (número de especies) y diversidad (distribución de la abundancia de especies)- de micromamíferos en el sur de América del Sur se habría generado en el período posterior a la llegada de los europeos (1500 AD). En este trabajo se revisa el registro fósil para pequeños roedores y marsupiales en los últimos 500 años, con énfasis en aquellos de la región Pampeana y la Patagonia. Esta evidencia se comparó con los registros actuales correspondientes a más de 700 muestras de egagrópilas de estos mismos sectores geográficos. En una primera aproximación, se destaca que la riqueza y diversidad de las comunidades de micromamíferos fue mayor en la etapa previa al impacto antrópico más profundo, con una abrupta caída de ambos parámetros hacia el presente. Se sugiere que las transformaciones producidas por la ganadería y agricultura extensivas habrían provocado una uniformidad de hábitat beneficiosa para algunos taxones oportunistas, facilitando su dispersión y permitiendo el incremento, en algunos casos dramático, de sus poblaciones. En los últimos 500 años se registran variaciones significativas en la distribución de otros taxones, que en algunos casos implican extinciones locales involucrando cientos o miles de kilómetros. Sobre 46 especies de roedores consideradas como Preocupación Menor por la UICN y con registro fósil para este segmento temporal, 23 (50%) han experimentado retracciones en sus rangos de distribución o reducciones drásticas de su abundancia. Al menos 9 taxones se habrían extinguido completamente en América del Sur, incluyendo 1 marsupial, 1 quiróptero y 7 roedores. Fenómenos similares se registran en otras áreas del Hemisferio Sur, tanto en contextos continentales como insulares. Este panorama pone de manifiesto la necesidad de evaluar más cuidadosamente la situación de algunas especies con poblaciones consideradas como estables, pero susceptibles de verse afectadas frente a nuevos cambios extensivos en los ambientes (e.g., megaminería, expansión de la frontera agrícola), a juzgar por lo que indica el registro fósil.

Introduction: The last 500 years of the historical era, an interval that corresponds to the concerted dispersal of European explorers, traders, and colonists around the globe, has witnessed the global disappearance of ~90 mammal species. Besides the known cases of biological extinctions, this time period was also characterized by the regional extirpations of specialized taxa and by the expansion and population growth of some opportunistic species. Several lines of evidence suggest that the current configuration of small mammal communities -i. e. richness (number of species) and diversity (distribution of species abundance)- in southern South America would have been generated in the period after the arrival of Europeans ca. 1500 AD. In this study, we reviewed the fossil record for small rodents and marsupials during the last 500 years, with emphasis on the Pampean and Patagonian regions. Based on these findings, we offer some considerations concerning the biogeography and conservation of these species.

Methods: Micromammals from archaeological and paleontological sites have provided considerable information on environmental conditions during the Quaternary in South America. In this work, we reviewed several micromammal fossil samples, mostly generated by the trophic activity of owls. The use of this kind of data involves some extrinsic and intrinsic biases that must be considered at the time to study the diversity of past communities, such as the bird involved on the accumulations, its hunting techniques, the time of the year, the size and behavior of the prey species, etc. For this work, we compared the fossil samples with more than 700 owl pellet assemblages of the same geographical areas.

Results: Richness and diversity of small mammal communities was higher prior to the deepest human impact (> 0.5 ka), showing a pronounced drop in both parameters to the present. The regional extinction of some cricetid rodents and small marsupials was recorded, both in the Pampean (e. g. Bibimys torresi,Pseudoryzomys simplex) and in the Patagonian regions (e. g. Euneomys mordax, Lestodelphys halli, Tympanoctomys kirchnerorum), as well as the biological extinctions of the bat Desmodus cf. D. draculae and the cavy Galea tixiensis in the Pampas. At this same time, opportunistic species (e. g. Calomys spp.,Oligoryzomys longicaudatus) showed a dramatic increase of their populations, especially in the most extensively disturbed areas.

Discussion and conclusions: In the last 500 years, significant variations occurred in the micromammal assemblages of southern South America. These changes included the dispersion and increase (in some cases extensive) of some specialized species and the regional extinctions of others, involving hundreds to thousands of kilometers of distributional extent. We suggest that the changes produced by livestock and agriculture have caused extensive habitat uniformity which was beneficial for some opportunistic taxa (e. g. Calomys spp., Oligoryzomys longicaudatus), facilitating their dispersion and allowing the increase of their populations. Anthropic disturbances also include the extensive use of fire, urbanization, and introduction of exotic forbs and grasses. Of about 46 species rodents considered as Least Concern by the IUCN and with fossil record for the last 500 years, 23 (50 %) have experienced drastic reductions in their range or abundance during the Late Holocene. At least nine taxa are completely extinct, including one marsupial, one bat and seven rodents. Similar phenomena are recorded in other areas of the Southern Hemisphere, both in continental and island contexts. This evaluation of the fossil record highlights the need to evaluate more carefully the status of some species with supposedly stable populations, but which are likely to be affected by future extensive changes in their environments (e. g. mining, expansion of the agricultural frontier).