Paleoecology

Paleoecology is a branch of ecology and paleontology focused on studying the interrelationships between extinct organisms and the paleoenvironments in which they lived. The study of paleoecology developed when geologists and paleontologists began using fossils and geological data to examine environmental conditions in ancient ecosystems. Paleoecology has helped scientists understand the nature of environmental changes occurring on Earth and has enabled scientists to better predict how conditions might continue to change as climate change continues.

Development of Paleoecology

Ecology is a branch of biological and physical science that studies the relationships among individual organisms, populations, and the environments in which they live. Paleontologists developed a branch of research known as paleoecology, which focuses on the interrelationships between extinct organisms and the paleoenvironments in which they lived.

Early paleontologists often made inferences about ancient environments from the types of fossil organisms found in various sediment samples. Paleontologists uncovering marine fossils from sediment in a modern desert, for instance, would likely infer that the sediment in the area was once part of a submerged ocean environment. Similarly, an abundance of fossils representing herbivorous organisms might indicate that there was once abundant vegetation in a certain fossil site.

American geologist Kirk Bryan (1888-1950) is often credited as one of the scientists responsible for the emergence of paleoecology as a distinct field of paleontological research. Bryan's research in the 1920s and 1930s focused on understanding weather patterns in ancient environments by examining aspects of soil chemistry and the occurrence of fossilized pollen grains. Bryan went on to train several prominent students who later made significant contributions to paleoecology and other branches of geological research.

Paleoecology utilizes data derived from the study of paleozoology (the study of fossilized animal species) and paleobotany (the study of fossilized plant species) in an attempt to reconstruct assemblages of organisms that occupied ancient environments. In addition, paleoecologists utilize chemical analyses of soil and geological data regarding the movement of continents. As a whole, these data allow paleoecologists to create models of extinct ecosystems that help them understand how evolutionary drives and pressures create communities within specific environments. The study of these ancient ecosystems can provide insight for ecologists studying modern ecosystems and can help to create an understanding of how changes in environmental conditions alter the evolutionary path of an ecosystem.

Theoretical Concepts of Paleoecology

One of the most important theoretical underpinnings of paleoecology is the concept of substantive uniformitarianism, which is the theory that Earth's physical and chemical properties have remained unchanged throughout the planet's history. The concept of uniformitarianism was developed in the nineteenth century and popularized by scientists such as prominent Scottish geologist Charles Lyell and English scientist William Whewell, who believed Earth's properties and processes were universal constants that could be applied equally to ancient and modern environments.

Since the nineteenth century, scientists have come to understand that the Earth's physical and chemical properties are not constant and have changed considerably during the 3.4 billion years of life on the planet. Modern ecologists apply an altered version of uniformitarianism, which assumes that the Earth's properties have varied but remained relatively similar during most of life's history on the planet.

Applied to paleoecology, uniformitarianism holds that the materials that make up the Earth and the forces that drive chemical reactions and geological processes are similar today to those active in ancient ecosystems. This assumption allows paleoecologists to tentatively apply modern theories about ecological relationships to ancient ecosystems. For example, paleontologists might assume that ancient microorganisms were able to gain energy by utilizing sulfur in the same way as modern sulfur-metabolizing bacteria because the chemical reactions that drive the metabolism of sulfur functioned the same in the past as they do in the modern environment.

Similarly, the theory of actuopaleontology (also called analogy) holds that the features and behaviors of modern organisms are similar to the features and behaviors of ancient organisms. In general, actuopaleontology allows paleontologists to assume that extinct organisms behaved and functioned in similar ways to modern organisms. This applies not only to the structure and biological function of individual organisms but also to the formation of multispecies communities.

Therefore, when paleontologists observe a beak-like structure on a fossilized organism, they often assume that the beak was used like the beaks of modern animals living in similar environments. Similarly, paleontologists often assume that organisms in ancient ecosystems filled the same roles as similar organisms in modern ecosystems of the same type. Therefore, Paleontologists might expect organisms in an ancient marsh ecosystem to display behaviors and environmental roles similar to organisms in modern marshes.

Research occasionally invalidates hypotheses developed using the principle of actuopaleontology. An extinct organism might, for instance, evolve a certain feature that is similar to features of modern organisms, but it might have used this feature for a different purpose. Similarly, ancient ecosystems that are superficially similar to modern ecosystems might be found to contain vastly different types of organisms with different sets of relationships not seen in modern ecosystems.

Whether to assume hypotheses based on the theories of actuopaleontology or uniformitarianism can be determined with the help of parsimony, defined as the idea that, given an unexplained phenomenon, it is best to assume the simplest possible explanation of the available data. Parsimony is a core concept that can be applied to all scientific thought because it discourages scientists from positing unlikely and overly complicated explanations.

Most scientists, for example, believe that life originated on Earth about 3.4 billion years ago, but some scientists have postulated that life came to Earth from elsewhere in the galaxy and carried on meteors from a distant planet. Although either explanation could be correct, the former is the simplest explanation, as it does not propose the extra assumption that life evolved elsewhere in the universe.

Applied to paleoecology, parsimony dictates that paleontologists utilize theories that encourage the simplest and most direct explanation. The principles of actuopaleontology and uniformitarianism are parsimonious in that they assume that conditions in the past are similar to present conditions. While this may not always hold true, it is the simplest method for explaining similarities between extinct organisms and ecosystems and their modern counterparts.

One of the most important steps to understanding extinct ecosystems is to determine the climate of the ecosystem in question. Geological data provide a variety of evidence regarding the climate of paleoenvironments. For instance, large deposits of coal result from decomposing woody plants, whose existence indicates the presence of a lush, humid forest system. By contrast, some ancient soils display characteristic signs of extended ice or frost coverage. These ancient soils include glacial tills, which are pulverized or striated rock segments that bear the scars of contact with moving glaciers. Rocks that show striations left by standing water and cracks resulting from heating and drying were exposed to an ancient monsoon system in which hot, dry seasons alternated with intense rainfall and flooding.

Fossils also can be used as indicators of climate conditions. Ecologists have noted that the diversity of species is greatest near the equator and declines at higher latitudes; paleontologists have discovered the same trend in the diversity of fossil assemblages from different areas. This phenomenon can be partially explained by the warm and relatively stable climate near the equator, leading to the proliferation of plants and microorganisms that form the base of the food chain. Farther from the equator, the environment becomes more variable.

In the broad temperate zone, organisms are still plentiful, but diversity is lower overall than in equatorial zones. Here, organisms are more generalized, filling multiple niches because food is less plentiful than in zones closer to the equator. The number of species continues to drop in the higher latitudes, where arctic weather conditions dominate, and overall diversity is low.

By measuring the diversity of fossil species in a certain area, paleoecologists can begin to understand the relative climate of the region during the time that the fossils were deposited. By examining a diversity of fossils over time, paleoecologists also can begin to observe climate change in ancient environments.

An example of this approach is the research of paleontologist Warren O. Addicott, whose 1969 study of shallow-water mollusk fossils from the west coast of Mexico and the United States indicates that certain species of mollusk were gradually moving southward during the early portion of the Tertiary period (65 to 2.5 million years ago). Addicott believed that this movement indicates a pattern of global cooling that led to changing distributions of species requiring warm, tropical waters for feeding and breeding. Furthermore, Addicott's research also indicated that a pronounced period of global warming existed during the later part of the Tertiary, again demonstrated by the movement of mollusk species farther northward toward the upper edges of the United States.

Marine fossils are the most useful for determining the specifics of ancient climate conditions. Brachiopods, members of a phylum of shelled marine animals, first appear in the fossil record in the Cambrian period (542 to 488 million years ago) but are still represented by more than two hundred living species in modern marine environments. Brachiopods appear to be highly climate-specific. Paleontologists have found that characteristic assemblages of brachiopod species appear at each climate zone. Because extinct brachiopod species appear to have changed little in millions of years of evolution, paleoecologists can examine the distribution of modern brachiopods and, by applying the theories of analogy, parsimony, and uniformitarianism, can assume that ancient brachiopods tended to exhibit the same climatic preferences as their modern counterparts. Paleontologists can, therefore, use the distribution of these characteristic brachiopod faunas to track changes in climate distribution in periods as distant as the late Cambrian.

Organisms that form shells are especially useful for studying marine climates because the process of shell formation is closely tied to the chemical composition of the marine environment. Oxygen isotopes occur naturally in marine environments and in multiple varieties. In warm environments, certain types of oxygen isotopes tend to disappear from the environment as they engage in chemical reactions and dissolve in the surrounding water. Therefore, in warmer waters, only the more stable types of oxygen isotopes remain. By examining the fossilized shells of ancient organisms, paleoecologists can measure the proportion of stable to unstable oxygen isotopes and can attempt to determine whether the organism lived in a warmer or cooler environment.

Paleoecologists often attempt to determine the closest living relatives of species represented in the fossil record of the paleoenvironment to determine climate in ancient terrestrial environments. For instance, paleontologists have used the distribution of fossil rodent species as an indicator of alternating periods of warm, productive climate and periods of cooler climate with less abundant food sources during the Pleistocene period (1.8 million to 10,000 years ago). In other cases, paleontologists have theorized that the presence of fossilized reptiles indicates a mean temperature thatis in keeping with the temperature favored by similar-sized reptiles in the modern environment.

Paleontologists have also used the distribution of fossilized pollen grains to investigate climate change. Because plants produce pollen on an annual cycle, pollen grains may appear in layers in the sediment, with each layer representing a subsequent year in the history of the environment. Paleontologists can, therefore, retrieve a core sample of sediment from the environment of interest and then examine the distribution of pollen at each layer. In times of warmer weather and more rainfall, more pollen will appear in the sample, while cooler or drier temperatures will result in fewer pollen grains. By examining the distribution of pollen grains over longer periods, paleoecologists can begin to get an image of long-term trends in climate change.

Paleoecology of the Pleistocene and Holocene Epochs

Utilizing data from both geological sources and the distribution of fossil species, paleontologists have created a detailed model of the paleoecology of the Pleistocene period, when the Earth experienced its most recent period of global cooling. The Pleistocene is characterized by extended glacial periods when thick sheets of ice covered large portions of the temperate zone. These periods of intense cooling alternated with interglacial periods, when the climate grew warmer, and the glaciers melted across much of the landscape.

Evidence for climate change in the Pleistocene has been derived partially from the distribution of fossil species that appear to be characteristic of certain climate zones. Single-celled organisms called foraminifera are some of the most important fossil organisms of the period. These tiny-shelled microbes are associated with the growth of algae, and their distribution is tied closely to temperature and other climatic variables. The tiny shells of foraminiferans are abundant in many types of ocean sediment, and paleoecologists have used the proportion of oxygen isotopes remaining from the pulverized shells of foraminiferans to track shifting climatic patterns during the Pleistocene.

Paleontological knowledge of the Pleistocene ice age has also benefited from detailed investigations of fossilized pollen. The distribution of pollen grains shows distinct periods of increased productivity followed by periods of greatly reduced pollen production. Paleontologists have taken this as additional evidence that the Pleistocene experienced waves of glacial and interglacial conditions.

The Holocene epoch began approximately ten thousand years ago and continues into the twenty-first century. Paleoecologists believe the Holocene represents an intermediate period between the last ice age of the Pleistocene and the next ice age. The climate has continued to fluctuate, and the Earth experienced a period of cooling, beginning in the seventeenth century and ending at the beginning of the twentieth century. Sometimes called the Little Ice Age, this period had lowered temperatures across most of the temperate zone, but it did not affect the globe as a whole.

Evidence supports the assertion that human activity is accelerating the process of global warming across the planet. Paleoecological research has helped scientists understand how the environment will change if current trends in climate change continue and how this will impact plants, animals, and humans. The study of ancient ecosystems indicates that further global warming will force many species to migrate to stay within comfortable climatic zones. Past periods of global warming and cooling have also resulted in waves of extinction, during which thousands of species disappear from the Earth as they cannot adapt to changing environmental conditions. Paleoecological research can help scientists and governments prepare for and prevent extinctions or, where necessary, guide species reintroduction and replacement.

Principal Terms

actuopaleontology: the linguistic act of applying organismic features found on modern living animals to extinct organisms

brachiopod: any of a phylum of shelled marine animals that have formed valved shells superficially similar to such mollusk species as clams and oysters

foraminiferan: a single-celled aquatic animal with a shell composed of compacted minerals or grains of sand from surrounding sediment

paleoclimatology: the study of the climate of an extinct ecosystem as determined through geological evidence and the distribution of fossils

paleoecology: the study of the interrelationships between extinct animals and the paleoenvironments in which they lived

paleoenvironment: the extinct environment of a certain area during a chosen point in the history of the region

parsimony: the theory that, given unexplained phenomena, one should assume the simplest possible explanation for the observed data

substantive uniformitarianism: the theory that extinct animals would have been similar in structure and behavior to living organisms because the processes, rates of change, and terrestrial materials have been relatively stable throughout the Earth's history

temperate zone: those intermediate latitude zones of the Earth marked by seasonal changes in climate

Bibliography

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