Biodiversity Response to Climate Change in
the Middle Pleistocene:
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One of the most valuable contributions to ecology over the last 25 years has been long-term ecological research (LTER) in diverse ecosystems around the world. The U.S. National Science Foundation has supported such research at 26 sites in the United States and Antarctica since 1980. In 1993, the International Long Term Ecological Network formed to support interdisciplinary research and monitoring over the long term and large spatial scales; now 30 countries have formed LTER programs. Long-term ecological research has examined the dynamics of primary production, populations, nutrient cycling, and disturbances in terrestrial and aquatic systems. Such studies have been essential for documenting the dynamics of fundamental biophysical processes at different spatial scales, as well as the frequency and magnitude of disturbances and responses of populations and ecosystems.
But
many ecological processes occur on much longer timescales than 25 or even 100
years. These processes include (1) phenomena in which the driving factor
changes over longer time periods, (2) phenomena with long response times, and
(3) their interactions. The empirical record of these phenomena exceeds the
length of most long-term ecological studies. While these systems may be
amenable to modeling, the models need testing with data, ideally from many
examples. The historical record, stretching from human history to earth
history, is critically important for documenting and analyzing ecological
processes and interactions that occur on timescales of 102 to 106
years. This is the context for the book Biodiversity Response to Climate
Change in the Middle Pleistocene: The Porcupine Cave Fauna from Colorado,
edited by Anthony D. Barnosky. Porcupine Cave (Figure 1), located in the intermontane basin of South Park, central Colorado, contains vertebrate remains
from the late Pliocene to the present time, with a detailed record of mammalian
populations and communities from an interval spanning two mid-Pleistocene
glacial-interglacial transitions. In essence, Porcupine Cave is a LTER site.
The particular ecological relevance of Porcupine Cave is its record of faunal response to climatic changes during the transitions from glacial to interglacial conditions. According to Barnosky, the “faunal dynamics that characterize Porcupine Cave climatic transitions probably typify how ecosystems respond to climatic warming episodes at the high end of ‘natural’ warming rates” (p. 5). Such records establish a “baseline” in biodiversity and ecosystem processes; we need to understand the baseline in order to recognize when changes in diversity will lead to catastrophic shifts in ecosystem processes. Barnosky frames the relevance of the book in terms of the effects of current global warming on biodiversity and ecosystem processes.
The book has the nature of a monograph, with 26 chapters separately authored; but with over 20,000 fossils identified thus far and various systematic and paleoecological issues still in progress, it is clearly not the last word on the cave or its faunas. The book is divided into three sections—(1) the geology and environmental context of the cave and taphonomy of fossil assemblages, (2) systematic accounts of the vertebrate faunas, and (3) analysis of mammalian faunal change in relation to environmental change. Excavation started in 1985 in Porcupine Cave and the book synthesizes 15 years of fieldwork by crews from the Carnegie Museum of Natural History, the Denver Museum of Nature and Science, and the University of California Museum of Paleontology in Berkeley.
In Part 1 (The discovery and distribution of fossils, nine chapters), we learn that the cave sits in an Ordovician dolomite, which developed a karstic surface before burial by Ordovician sandstone. The ancient fissures predisposed the dolomite toward cave formation once these strata were uplifted and neared the surface. Inside the cave, both waterlain sediments and aggradation of rocky debris contain Plio-Pleistocene fossils. But the main agents of accumulation were mammals and birds. The most important contributors were woodrats (Neotoma), which collected myriad organic remains in their middens over a period of possibly two million years, based on dated materials in the cave. Among their retrievals were numerous raptor pellets and carnivore scats filled with bones of small vertebrates. Second in importance were various mammalian carnivores, including coyotes, bears, and badgers, that used the cave as a den. In addition, the cave may have been a natural trap from time to time, although relatively few skeletons of larger mammals are preserved.
The
cave contains 26 distinct fossil localities throughout its serpentine passages
(Figure 2); each locality represents hundreds to thousands of years of
accumulation. Most faunal information comes from seven localities, including
several that received detailed excavation. Dating has been accomplished mainly
by biostratigraphy. Among the cave localities, correlation is based on first and
last appearances of key mammalian taxa, on co-occurrences of taxa, and on
relative-abundance of several arvicolines (voles and lemmings). Arvicolines also
provide correlation to the absolute timescale and to global oxygen-isotope
stages. Magnetostratigraphy of the Pit, one of the better-studied localities
with stratified sediments (Figure 3), revealed three reversals within 1 meter of
sediment! These are interpreted as the Brunhes/Matuyama boundary and the
Jaramillo Event. One chapter describes the modern ecology of South Park in
terms of climate, bedrock geology and its relation to vegetation assemblages,
and fauna; this information is a point of reference for many of the paleoecological inferences for the middle Pleistocene fossil assemblages. Another chapter reviews the human history of the region from the Native
Americans to modern occupants.
A chapter about woodrat biology documents the
impressive taxonomic diversity of remains collected by the four species of
Neotoma living in Colorado today. The final chapter on taphonomic
modification and paleopathology of mammal bones illustrates (with high-quality
photos and drawings) a range of bone-damage patterns reflecting carnivore behaviors,
rodent gnawing, digestion, as well as disease in the living animals. This
section of the book would have benefited from a more comprehensive taphonomic analysis of the fossil assemblages, particularly with reference to
the extensive literature about woodrat middens.
Part 2 (Systematic accounts of taxa, 12 chapters) documents the faunal diversity that makes Porcupine Cave such an important middle Pleistocene site. Few chapters illustrate key specimens; this addition would have been valuable. One chapter mentions the presence of mollusks in some fossil assemblages, and Neotoma middens are usually loaded with plants but these are never mentioned. The vertebrate list is impressive—2 amphibians, 4 reptiles, 45 birds, and 73 mammals recognized from the 20,000 fossils identified thus far. Most fossils are middle Pleistocene in age (Irvingtonian North American Land Mammal Age), especially between 600 Ka to 1 Ma, but some are clearly lower Pleistocene or older (Blancan North American Land Mammal Age). Twenty-eight species (20 birds, 8 mammals) first occur in North America in these localities. At least seven mammals found in Porcupine Cave are now extinct. Chapter 10 provides faunal lists for all the fossil localities, as well as estimates of the number of specimens and individuals for the better-studied assemblages. The other chapters describe the status of taxonomic groups. Specimen numbers assigned to taxa are listed within the chapter (if the group is rare) or in an appendix. The cave records salamanders, toads, lizards, and several kinds of snakes—a more diverse herpetofauna than currently inhabits South Park. Even though birds are known from only about 200 identified specimens, 45 taxa are recognized. These range in size from various sparrows to a great horned owl; the most numerous species is the sage grouse. Mammalian carnivores are represented by 23 species. The most common species are coyotes, badgers, and spotted skunks. Rarer taxa include the now-extinct North American cheetah. Ochotonids (pikas) are moderately common and may represent the first documented pikas from the Blancan of North America. Leporids (rabbits and hares) are represented by as many as 10 species from a small set of localities in the Velvet Room of Porcupine Cave; as many as eight species were contemporaneous. No wonder there were so many coyotes.
Rodents are by far the most numerous mammalian fossils. The presence of geomyoids (kangaroo rats and pocket gophers), of murids (Peromyscus in particular), and porcupines is noted. Squirrels, Neotoma, and arvicolines each get an entire chapter. In the sciurid chapter, 11 taxa are described, mostly ground squirrels. Tamiasciurus hudsonicus is the only obligate tree squirrel. This chapter contains a biochronologic scheme based on species of Spermophilus and Cynomys; this scheme relies on the presence or absence of rare taxa for some levels, and no stratigraphic range diagram is given, so the utility of this scheme is difficult to assess. Variation over time in dental measurements signifies microevolution in the cheek teeth of two taxa. Many of the squirrel taxa live in the vicinity of Porcupine Cave today. Not so for species of Neotoma. This chapter's author, Repenning, recognizes five species, all extant taxa. Most of them do not live in South Park today but occur at lower elevations in Colorado. Porcupine Cave is the earliest record for all five species. Four occur in “great plains” habitats today; only N. cinerea, the bushy-tailed woodrat, lives in South Park. To explain the presence of great-plains taxa at such a high elevation in the middle Pleistocene, Repenning contrasts climatic change with tectonic uplift as explanatory hypotheses. No firm conclusion is reached but he provides plausible evidence for the tectonic hypothesis. Arvicolines are the most numerous mammals in the fauna. I did not find a tally of the total number of identified specimens, but the appendix for this chapter is 44 pages long! A minimum of 13 lineages is recognized, with as many as nine occurring in the same stratigraphic unit. Only four of these taxa are extant. Compared to other high-elevation fossil localities of Plio-Pleistocene age, Porcupine Cave shows significant differences in arvicolines—a pattern of geographic heterogeneity in montane faunas that also occurs today.
The last two faunal chapters, both well illustrated, cover ungulates. Equids are represented by 56 mostly fragmentary specimens. Nonetheless, four taxa are recognized—an ass, a hemionine, and one large and one small caballine. A similar number of fossils, most <20 cm long, are the basis for identifying 12 artiodactyls—a peccary, two camels, four deer, two pronghorns, one bighorn sheep, one mountain goat, and one muskox. The rare and fragmentary nature of the fossils make it unlikely that the artiodactyls were ever cave occupants; more likely, their isolated bones were brought in by other animals or washed into fissures. For two of the tables in this chapter, the anatomical abbreviations are undefined, making it hard to appreciate the significance of the measurements.
Part 3 (Effect of environmental change on the Porcupine Cave fauna, 5 chapters) documents faunal changes and their paleoenvironmental context. A detailed faunal analysis of the Badger Room locality includes construction of the climate space for temperature and precipitation for extant taxa considered ecologically equivalent to two rodents that are abundant in the fossil assemblage. Reconstructed temperature and precipitation patterns over the year indicate slightly longer, wetter, and possibly cooler winters than those of today. Also, the trophic and size structure of the 42 mammalian species of the Badger Room fauna is compared with the structure of the modern fauna. Faunal structure and richness are quite similar, despite about 40% turnover of taxa since the middle Pleistocene. Curiously, despite the detailed paleoenvironmental reconstructions, the authors never say whether the fauna represents a glacial or an interglacial interval.
The
chapter about faunal dynamics of the Pit documents quantitative changes in
rodent taxa in relation to the record of climate change inferred on the basis of
sediments. Sedimentary features of 14 strata indicate an alternation between
glacial and interglacial intervals spanning 780 to at least 900 ka. The faunal
analysis is based on almost 7000 identified fossils representing 1402
individuals. A rarefaction analysis indicates that samples from different
stratigraphic units have similar structure; samples with >500 specimens gain few
new species with increased sample size. The relative abundance of the original taxa in the
fossil assemblage is considered to be a good representation of the relative
abundance of populations because of the propensity of woodrats to collect
remains in proportion to their availability. The substantial sample sizes of
rodents and leporids are the basis for a series of small-mammal “pollen”
diagrams (e.g., Figure 4), in which changes in relative abundance over time are
used as the basis for paleoenvironmental reconstruction. The arvicoline pollen
diagram (Figure 4) is the basis for especially detailed inferences, largely
from variation in the frequencies of Mictomys sp., the bog lemming, and
Lemmiscus curtatus, the sagebrush vole—species whose modern ranges do not
overlap. This fascinating chapter needs a more explicit analysis of when
appearances and disappearances could have plausibly resulted from elevational shifts in
species ranges versus regional appearances and disappearances. Also, some
circular reasoning creeps in: changes in rodent frequencies are used to infer
changes in vegetation and climatic conditions, which are then viewed as the cause
of changes in rodent frequencies over time.
Marmot
tooth enamel was sampled for stable isotopes of oxygen and carbon form levels
1-7 of the Pit (spanning interglacial-glacial-interglacial sediments). Whereas
no significant changes in
18O
values occurred over time, the
13C
samples became more negative from older to younger levels. All
13C
values are within the C3 region of the carbon spectrum (<-8.0‰). The significance of this shift is not entirely clear, and it would be useful to
know how
13C
values in vegetation change along elevational gradients today.
The penultimate chapter is an analysis of dental variables in Marmota populations from the Pit. In light of the life-history of marmot populations living at high elevations, the authors generate expectations about dental traits that should be under selective pressure related to climatic change and other traits that should be selectively neutral in relation to climatic change. Analysis of changes over time in the mean and frequency distributions of dental variables shows no statistically significant changes in the traits hypothesized to change in relation to climate. The authors conclude that the climatic variations of this middle Pleistocene interval did not perturb marmot evolution; rather marmot populations have been stable and persistent to the present day.
The final chapter by Barnosky summarizes changes in lineages and faunas in light of the overarching question—how does climatic change affect terrestrial biodiversity? Barnosky summarizes changes in mammalian populations, in species richness, and in faunal structure over the middle Pleistocene sequence of glacial and interglacial intervals. Although notable changes did occur, particularly during the youngest transition from a glacial to an especially warm, dry interglacial, the climatic changes of these glacial cycles did not substantially disrupt mammalian lineages or faunal structure. The record at Porcupine Cave suggests that ecosystems are resilient to climatic changes of the magnitude of glacial to interglacial transitions, even as a moderate level of taxonomic turnover is ongoing. Species with specialized requirements (Mictomys, Lemmiscus in this sequence) showed greater fluctuations in abundance over time than species with general requirements. Greater turnover occurred among rare species than among common species. Lessons for the future involve generalization of the pattern from Porcupine Cave: climatic change of the rates and magnitudes of Quaternary glacial and interglacial cycles will affect population densities and geographic ranges, but will not alter ecosystem structure. Part of this resilience is maintained by genetic diversity among populations across the landscape level, open dispersal corridors, and the presence of many ecologically equivalent species at the regional level—in other words, metapopulations and metacommunities. The circumstances facing the 21st century are different, however. Global warming is predicted to exceed normal glacial-interglacial variation, many populations and species are already vulnerable to human activities, and dispersal is restricted by human modifications of the landscape. Barnosky concludes with an appeal to keep ecologically similar areas as connected as possible to allow natural migration and dispersal; to minimize anthropogenic global warming; and to maintain the functional integrity of ecosystems even if species composition changes.
The strengths of this book greatly outweigh its weaknesses. The strengths include the clear organization, the substance and quality of the contributions, and the scope of geological and paleontological documentation. Even though this book represents a progress report rather than a final summary, it convinces us that Porcupine Cave is “arguably the richest source of information in the world on Irvingtonian-age vertebrates” (p. 6). Linking the faunal analysis from the middle Pleistocene sequence of glacial cycles to effects of modern climatic change on biodiversity highlights the importance of the Quaternary record as a critical bridge between modern ecological research and paleoecology of Neogene and older records, where temporal and taxonomic resolution are usually lower. The main weaknesses of this volume are the absence of information about vegetation and the lack of illustrations for many of the systematic accounts. With such large sample sizes, the faunal data are ripe for a more quantitative treatment but that can come in later publications. Overall, this book is a fine achievement. I strongly recommend it for paleontologists and for neontologists concerned about the current mass extinction.