- Research article
- Open Access
Global climate changes drive ecological specialization of mammal faunas: trends in rodent assemblages from the Iberian Plio-Pleistocene
- Ana R Gómez Cano†1,
- Juan L Cantalapiedra†1, 2,
- Aurora Mesa1,
- Ana Moreno Bofarull1 and
- Manuel Hernández Fernández†1, 3Email author
© Gómez Cano et al.; licensee BioMed Central Ltd. 2013
Received: 7 November 2012
Accepted: 24 April 2013
Published: 30 April 2013
Several macroevolutionary hypotheses propose a synchrony between climatic changes and variations in the structure of faunal communities. Some of them focus on the importance of the species ecological specialization because of its effects on evolutionary processes and the resultant patterns. Particularly, Vrba’s turnover pulse hypothesis and resource-use hypothesis revolve around the importance of biome inhabitation. In order to test these hypotheses, we used the Biomic Specialization Index, which is based on the number of biomes occupied by each species, and evaluated the changes in the relative importance of generalist and specialist rodents in more than forty fossil sites from the Iberian Plio-Pleistocene.
Our results indicate that there was a decrease in the specialization degree of rodent faunas during the Pliocene due to the global cooling that triggered the onset of the glacial events of the Cenozoic (around 2.75 Ma). The subsequent faunal transition after this critical paleoenvironmental event was characterized by an increase of specialization related to the adaptation to the new environmental conditions, which was mainly associated with the Pleistocene radiation of Arvicolinae (voles).
The pattern of faunal turnover is correlated with the development of the modern glaciations in the Northern Hemisphere around 2.75 Ma, and represents a reorganization of the rodent communities, as suggested by the turnover pulse hypothesis. Our data also support the resource-use hypothesis, which presumes the role of the degree of specialization in resources specifically related to particular biomes as a driver of differential speciation and extinction rates. These results stress the intimate connection between ecological and evolutionary changes.
Almost since the first publication of the seminal contribution about the tendency of species to form varieties , there has been a debate between models that consider the competition between species as the key for evolutionary changes [2, 3] and the ones that regard factors external to the species, usually biogeography or climatic changes, as the main drivers responsible of biotic evolution [4–7]. Nevertheless, an increasing consensus has appeared during the last decades on the critical importance of changes in the physical environment, rather than biotic interactions themselves, for the evolution of organisms and ecosystems at large spatial and time scales [8–11].
The habitat theory proposed by Elisabeth S. Vrba [12, 13] is one of the best known of such evolutionary models. This theory comprises a set of hypotheses showing the influence of global climatic shifts and the subsequent environmental changes on the turnover of species assemblages as a result of the drifting of geographic distributions, lineage originations and extinctions.
As a core part of the habitat theory, Vrba [7, 14] developed the resource-use hypothesis, which stresses the relationship between the ecological specialization of species and the processes that regulate their evolution. The different conditions imposed by the ecological characteristics of species, and especially the biomes that they inhabit, have an effect on the macroevolutionary patterns that are observed through time and space [7, 14–17]. In this way, the resource-use hypothesis gives a great value to the differences between the species that inhabit one biome exclusively (biome specialists or stenobiomic species) and others that obtain their resources from more than one biome (biome generalists or eurybiomic species). In order to clarify the concepts used for this work, it is important to indicate that biome specialization is not necessarily related to specialization in other ecological traits of the species. For example, Rhynchomys soricoides (Murinae, Rodentia), which has been described as vermivore , exhibits a high grade of specialization in its dietary habits but, since it inhabits three different biomes (equatorial rainforest, tropical deciduous woodland and temperate evergreen forest) [19–21], is a biome generalist. On the other hand, Stochomys longicaudatus (Murinae, Rodentia) has an omnivore diet , but it is restricted to the equatorial rainforest of Central Africa .
According to the resource-use hypothesis, during episodes of climatic triggering of habitat change, specialist species are more prone to suffer limitation of their resources and, consequently, they are more susceptible to habitat fragmentation, vicariance and directional selection. Since environmental changes have stronger effects on biome specialists than on generalist species, which can find their resources in different biomes, the former are predicted to have higher speciation and extinction rates than the latter [14, 17]. Such phenomenon results in an increase of the specialist species versus generalists in the global fauna, which has been observed in mammalian assemblages from both Africa [14, 15] and South America  as well as in the ruminants at the global scale .
Importantly, because of the constraints on the evolutionary history of species imposed by changes in the physical environment, most of the evolutionary changes in biotic lineages (including speciation, extinction or dispersion) should occur synchronically with global climatic changes. Such an evolutionary scenario is developed in the turnover-pulse hypothesis [12, 24, 25]. According to this hypothesis, most speciations and extinctions across diverse groups of organisms are not randomly distributed in time, but show statistically significant concentrations near times of major physical change. While most of these turnover-pulses affect few lineages and/or restricted geographic areas, some of them are massive and of global extent .
To test these predictions, here we focus on the rodent faunas from the Iberian Plio-Pleistocene. During this period, successive cooling pulses culminated with the establishment of continental northern-hemisphere glaciations and the modern ice age climate around 2.75 Ma [27–29]. This severe climatic event at the global scale is an ideal scenario for testing Vrba’s hypotheses. We used rodents as our study group because they are widespread, highly diverse and habitat-sensitive, which makes them one of the most environmental and climatically informative groups of mammals [30–34]. Besides, their Iberian fossil record is vast and extensively documented . Additionally, this group has been used to characterize intervals of great faunal change throughout the Cenozoic, usually associated with global climate fluctuations (e.g. [36, 37]). For these reasons they provide a suitable faunal set to test the predictions exposed above.
Fossil sites used in this work and values of average BSI
Age (Ma) *
Average BSI §
Cueva Millán 1a
Pinilla del Valle
Cueva del Agua
Cueva de los Zarpazos 4
Trinchera Dolina 10
Cúllar Baza 1
Trinchera Penal Tubo 2
Trinchera Penal 8
Trinchera Penal 7
Trinchera Dolina 6
Trinchera Dolina 5
Trinchera Dolina 4
Trinchera Dolina 3
Sima del Elefante
Barranco de Quebradas 1
Villalba Alta 1
La Gloria 4
The specialization degree of each species was measured using the Biomic Specialization Index (BSI) developed by Hernández Fernández and Vrba . This index indicates the number of biomes inhabited by the species, following the climatic classification of Walter , which recognizes ten biomes. Therefore, BSI equals 1 for most specialized species whereas generalist species could exhibit a BSI as high as 10. The data on the biome residence for all the rodent species were obtained from , who derived biome residences from identifying their living ecological analogues as estimated by ecomorphological studies of the dentition [41, 42].
Finally, following Hernández Fernández and Vrba , we calculated the relative frequency of specialist and generalist species in each fossil site in terms of the average value of the BSI of the species found in the site. Taking into account that some taxa on our lists were not identified to species level, we decided to conduct the analysis by two different ways. First, for the undetermined taxa, the BSI value was calculated as the average of all the species that belong to their upper taxonomic level. Second, to avoid the potential noise in the data due to unidentified taxa, we only analysed the taxa that were determined at species level in each fossil site. Since both analyses yielded very similar results, here we only discuss those corresponding to the latter version of the analysis (all taxa at the species level).
Following our predictions derived from the interaction between the resource-use and the turnover-pulse hypotheses, the beginning of the modern glaciations in the Northern Hemisphere (around 2.75 Ma) should be coincident with the rising of mammal faunas dominated by generalists (high average BSI values). After this critical event we should find a progressive increase in the specialization degree of the rodent faunas from the Iberian Pleistocene (decrease of average BSI). In order to identify significant changes in the average BSI of fossil assemblages through time, we carried out Multivariate Adaptive Regression Splines (MARS; ) on the data set. MARS is a method that identifies hinge points in a time series that automatically minimize the residual sum of squares (RSS). In this way time shifts in BSI trends do not need to be fixed a priori. Progressively adding hinge points to the model increases the number of parameters. Since overly complex models may result in stochastic error (inflated variance), we need to find an equilibrium between fit and complexity. MARS does this automatically using the Akaike Information Criterion (AIC) scores of each model, which measures the goodness of the fit of a statistical model while penalizing the number of parameters (the complexity).
Additionally, we expected to find different evolutionary responses to this event in different rodent clades according to their degree of specialization. More generalist groups are predicted to survive and proliferate after the environmental crisis at 2.75 Ma, becoming progressively more specialized. Meanwhile clades dominated by specialist species before the crisis are expected to decrease in importance, although the specialization degree may not change. These predictions were tested by means of t-Student comparisons between species pools before and after the inflection point yielded by the MARS analysis within different taxonomic groups. Following Wilson & Reeder , we studied the specialization trend of Sciuridae, Gliridae, Castoridae, Arvicolinae, Cricetinae, Gebillinae, Murinae and Hystricidae separately. Subfamilies within Cricetidae (arvicolines and cricetines) and Muridae (gerbillines and murines)  were studied independently due to their importance in Iberian Plio-Pleistocene rodent faunas, and because they have been traditionally taken as independent families in paleontological studies  and their monophyly has been demonstrated by molecular studies [46, 47].
The gradual decrease in the relative importance of specialist species during the Pliocene (increase in average BSI) is probably related to the progressive global cooling predominant during the Pliocene, which triggered modifications in the climate of southern Europe, from subtropical conditions with minor fluctuations of temperature to temperate conditions with noticeable annual thermal seasonality [49, 50].
Our results are consistent with a scenario where species dwelling in more than one biome (generalists) were more able to tolerate this change due to their ability to find resources in different environments. After the development of the first modern glacial events at the Northern Hemisphere the specialization degree of the faunas progressively increased (the value of average BSI decreases). This trend reflects a gradual “recovery” of the rodent communities after the ecological disturbance caused by the onset of the first glaciations. Finally, during the Holocene the ecological specialization (average BSI) of the faunas reached similar values to the ones found during the early Pliocene.
Correlation analyses between average BSI and the percentage of different rodent groups
Before 2.75 Ma
After 2.75 Ma
On the contrary, the other rodent groups of the Iberian Plio-Pleistocene were not able to take advantage of the new environments that were associated with the glacial-interglacial cyclicity of the Pleistocene climate. Particularly, in the case of Murinae there was a substantial decrease in the relative importance of this group in the rodent faunas from the Iberian Peninsula (Figure 4). From communities with representatives of eight genera during the Pliocene , the Pleistocene faunas only preserved species of Apodemus, Castillomys, Micromys and Stephanomys, the latter only surviving until 2.0 Ma [57, 58]. Most murine species were adapted to forested and warm environments [34, 42], which disappeared from Iberian latitudes with the onset of the modern glaciations. Today this family is predominantly distributed in tropical areas of the Old World, with only a few genera and species in temperate regions . Murine demise in the Iberian Peninsula would be due not only to the decrease in temperature, but probably also to changes in precipitation amount and seasonality, associate to the development of Mediterranean climate . Although such species loss was conspicuous in the Iberian Pleistocene, the specialization degree of this group did not show significant differences after 2.75 Ma (Figure 3).
Cricetinae was the other group showing a clear impoverishment over the analyzed interval. Whereas cricetid communities during the Pliocene held the genera Apocricetus, Blancomys, Ruscinomys, Celadensia or Trilophomys, Pleistocene cricetid faunas were only represented by the immigrant Allocricetus bursae. This taxon is closely related to the few genera that occupy today the arid environments present in the temperate regions of Eurasia since the onset of the glaciations. Apparently, the global cooling at the Plio-Pleistocene boundary imposed strong ecological limitations to the species adapted to arid and open environments, such as the Iberian cricetids [42, 61]. Similarly, the terrestrial squirrels, characteristic of many Pliocene faunas from the Iberian Peninsula (Atlantoxerus), belong to Xerini, a group that today is restricted to the African savannas and semideserts, and disappeared completely from the Iberian record due to the cooling of the climate. They were replaced in the Iberian Pleistocene by Sciurini (Sciurus) and Marmotini (Marmota); these groups are predominant respectively in forested and open temperate environments. In the case of glirids, there were no large faunal changes before and after the beginning of the Pleistocene northern glaciations. They maintained the same genera, which are still living, and there are no significant differences in the BSI of their species over the period studied here (Figure 3).
Finally, three minor Pliocene generalist groups, gerbils, beavers and porcupines, were unable to proliferate in the Iberian Pleistocene. In the first case this is probably due to their specific specialization to arid climates mostly developed in other regions of Eurasia. The other two families show extremely low species number, which may have hampered their diversification.
It seems that the Pliocene witnessed a process of disassembly within the Iberian rodent communities. The impoverishment of the Iberian rodent communities throughout the Pliocene stemmed from a loss of diversity that affected most of the rodent families. The recovery of species diversity during the Pleistocene was linked to the radiation of stenobiomic arvicolines that resulted in the establishment of a new rodent fauna (Figure 4). This global radiation is evidenced within the Iberian fossil record by local speciation (e.g. development of the endemic lineage Iberomys within Microtus[62–64]) as well as by immigration of new species from other Eurasian regions, being both processes spurred by global environmental change. Overall, such ecological reorganization of assemblages appears to be triggered by global climatic changes and modulated by the differences in ecological specialization of the implied species, similarly to what was observed in earlier periods of faunal replacement in Spain [36, 65].
Our results offer support for some hypotheses included in Vrba’s habitat theory, which predicts a proportional decrease of specialist species associated with severe global climatic changes, and a later recovery of this kind of species associated to a complete faunal turnover. The pattern of faunal turnover is correlated with the development of the modern glaciations in the Northern Hemisphere around 2.75 Ma, which triggered a reorganization of the rodent communities as predicted by the turnover pulse hypothesis. In the same way our data support the resource-use hypothesis, which stresses the role of the degree of specialization in resources specifically related to particular biomes as a driver of differential speciation and extinction rates. In this way, ecological and evolutionary changes are intimately connected.
Finally, this work shows that the exceptional quality of the fossil record in the rodent assemblages from the Iberian Peninsula makes this group a good case for the study of the relevance of ecological characteristics of species in the development of macroevolutionary processes.
This work is dedicated to Pierre Mein (Université Claude Bernard Lyon 1); without his research on small mammal faunas and his contributions to the biochronology of European mammal faunas this kind of studies would not be possible. The Editor Davide Pisani (The Natural History Museum, London), Cesar Laplana (Museo Arqueológico Regional, Alcalá de Henares), Raef Minwer-Barakat (Institut Català de Paleontologia, Sabadell), Pablo Peláez-Campomanes (Museo Nacional de Ciencias Naturales, Madrid), Jan A. van Dam (Universiteit Utrecht) and several anonymous referees are acknowledged for their comments on preliminary versions of the manuscript. This is a contribution of the Team of Paleoclimatology, Macroecology and Macroevolution of Vertebrates (http://www.pmmv.com.es) of the Complutensian University of Madrid as a part of the Research Group UCM 910607 on Evolution of Cenozoic Mammals and Continental Paleoenvironments. We acknowledge financial support from the Spanish Ministry of Science and Innovation, through the projects CGL2006-01773/BTE and CGL2010-19116/BOS.
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