10.2 Terrestrial environments

Terrestrial environments have undergone major changes throughout Earth's history. From ancient forests to vast grasslands, these habitats have been shaped by climate, atmospheric composition, and tectonic activity. Understanding past terrestrial environments provides insights into life's evolution on land.

Studying ancient terrestrial ecosystems reveals complex food webs and interactions between plants, animals, and microorganisms. Fossils preserved in these settings offer glimpses into past life forms and their adaptations to changing conditions. This knowledge helps us grasp how terrestrial life has evolved and diversified over time.

Terrestrial environments of the past

• Terrestrial environments have undergone significant changes throughout Earth's history, shaped by a complex interplay of biotic and abiotic factors
• Studying past terrestrial environments provides valuable insights into the evolution of life on land, climate change, and the dynamics of ecosystems over geological timescales
• Paleontological evidence, such as fossils and trace fossils, along with geochemical proxies and sedimentological indicators, allows researchers to reconstruct the characteristics and conditions of ancient terrestrial habitats

Factors shaping terrestrial environments

Climate and weather patterns

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• Climate, determined by factors such as latitude, atmospheric circulation, and ocean currents, exerts a strong influence on the distribution and characteristics of terrestrial environments
• Precipitation patterns, including the amount, seasonality, and form (rain, snow, etc.), play a crucial role in shaping vegetation and ecosystem dynamics
• Temperature variations, both spatial and temporal, affect the growth and survival of terrestrial organisms, as well as weathering and erosion processes
• Extreme weather events, such as droughts, floods, and storms, can have significant impacts on terrestrial habitats and their inhabitants

Atmospheric composition

• The composition of Earth's atmosphere has varied significantly over geological time, with fluctuations in the levels of oxygen, carbon dioxide, and other gases
• Atmospheric oxygen levels have influenced the evolution of terrestrial life, particularly the emergence and diversification of land plants and animals
• Changes in atmospheric CO2 concentrations have been linked to global climate shifts, such as the alternation between greenhouse and icehouse conditions
• Other atmospheric components, such as nitrogen and sulfur compounds, can affect soil chemistry, plant growth, and ecosystem processes

Tectonic activity and landforms

• Tectonic processes, including plate movements, mountain building, and volcanism, shape the topography and geography of terrestrial environments
• Uplift and subsidence can alter the elevation and exposure of land surfaces, influencing climate, weathering, and erosion patterns
• Volcanic eruptions can have both immediate and long-term effects on terrestrial habitats, through the deposition of ash, lava flows, and the release of gases
• Landforms, such as mountains, plateaus, and basins, create diverse microhabitats and environmental gradients that support a wide range of terrestrial ecosystems

Soil formation and properties

• Soil formation is a complex process involving the weathering of parent material, organic matter accumulation, and the activity of soil organisms
• Soil properties, such as texture, structure, pH, and nutrient content, are influenced by factors such as climate, vegetation, and geological substrate
• Well-developed soils, with distinct horizons and a rich organic component, are essential for supporting diverse terrestrial ecosystems and facilitating nutrient cycling
• Paleosols, or fossilized soils, provide valuable information about past terrestrial environments, including climate, vegetation, and landscape stability

Major terrestrial biomes

Forests and woodlands

• Forests and woodlands are characterized by the dominance of trees and a complex vertical structure, including canopy, understory, and forest floor layers
• Tropical rainforests, such as those found in the Amazon and Congo basins, are known for their high biodiversity and year-round warm, wet conditions
• Temperate forests, including deciduous and coniferous types, experience distinct seasonal changes and support a diverse array of plant and animal species
• Woodland habitats, such as savannas and shrublands, have a more open canopy and a significant presence of grasses and herbaceous vegetation

Grasslands and savannas

• Grasslands are dominated by herbaceous vegetation, particularly grasses, and are found in regions with moderate to low precipitation and seasonal drought
• Savannas are characterized by a mix of grasses and scattered trees or shrubs, often maintained by periodic fires and grazing by large herbivores
• Temperate grasslands, such as the North American prairies and Eurasian steppes, experience cold winters and hot summers, with rainfall concentrated in the growing season
• Tropical savannas, like the African Serengeti and Brazilian Cerrado, have a more pronounced dry season and support a diverse array of grazing and browsing mammals

Deserts and arid regions

• Deserts are defined by their low precipitation levels, typically receiving less than 250 mm of rainfall per year, and often experience extreme temperature fluctuations
• Hot deserts, such as the Sahara and Australian Outback, are characterized by high daytime temperatures, sparse vegetation, and specialized adaptations of flora and fauna
• Cold deserts, like the Gobi and Antarctic Dry Valleys, have lower temperatures and may experience freezing conditions, with limited plant growth and unique microbial communities
• Arid regions also include semi-arid habitats, such as shrublands and steppes, which have slightly higher precipitation levels and more extensive vegetation cover

Tundra and cold environments

• Tundra environments are found in high latitudes and at high elevations, characterized by low temperatures, short growing seasons, and a lack of trees due to the harsh conditions
• Arctic tundra, occurring in the northern hemisphere, is dominated by low-growing plants, such as mosses, lichens, and dwarf shrubs, and supports a variety of adapted animals
• Alpine tundra is found at high elevations above the treeline in mountain ranges worldwide, with similar vegetation and ecological characteristics to Arctic tundra
• Permafrost, or permanently frozen ground, is a key feature of tundra environments, influencing soil properties, hydrology, and the distribution of plant and animal communities

Terrestrial food webs and ecosystems

Primary producers and vegetation

• Primary producers, mainly plants and some microorganisms, form the foundation of terrestrial food webs by converting solar energy into organic compounds through photosynthesis
• Vegetation structure and composition vary across different terrestrial biomes, depending on factors such as climate, soil, and disturbance regimes
• Plant functional types, such as trees, shrubs, grasses, and forbs, have distinct ecological roles and contribute to the diversity and complexity of terrestrial ecosystems
• Plant-soil feedbacks, involving interactions between vegetation and soil microorganisms, play a crucial role in nutrient cycling and ecosystem dynamics

Herbivores and plant-animal interactions

• Herbivores, including both vertebrates (mammals, birds, reptiles) and invertebrates (insects, mollusks), consume plant material and play a key role in shaping vegetation structure and composition
• Grazing and browsing by large herbivores, such as ungulates and elephants, can influence plant community dynamics, seed dispersal, and nutrient cycling
• Insect herbivory, including leaf-chewing, sap-sucking, and gall-forming, can have significant impacts on plant growth, defense mechanisms, and ecosystem processes
• Plant-animal mutualisms, such as pollination and seed dispersal, are essential for the reproduction and dispersal of many plant species and the maintenance of biodiversity

Carnivores and predator-prey dynamics

• Carnivores, including both vertebrates (mammals, birds, reptiles) and invertebrates (insects, arachnids), consume other animals and play a crucial role in regulating prey populations and shaping community structure
• Predator-prey interactions, such as those between lions and wildebeest or between spiders and insects, involve complex behavioral and evolutionary adaptations
• Trophic cascades, where changes in top predator populations affect multiple levels of the food web, can have far-reaching consequences for ecosystem structure and function
• Scavenging and decomposition by carnivorous species, such as vultures and carrion beetles, contribute to nutrient cycling and the breakdown of organic matter

Decomposers and nutrient cycling

• Decomposers, primarily bacteria and fungi, break down dead organic matter and recycle nutrients back into the soil and atmosphere
• Soil microorganisms, including bacteria, fungi, and protozoa, play a vital role in decomposition, nutrient mineralization, and the maintenance of soil fertility
• Mycorrhizal fungi form symbiotic relationships with plant roots, facilitating nutrient and water uptake in exchange for photosynthetically derived carbon compounds
• Nutrient cycling, involving the transfer of essential elements such as carbon, nitrogen, and phosphorus between biotic and abiotic components of the ecosystem, is critical for sustaining terrestrial productivity and biodiversity

Terrestrial fossil preservation

Fossilization processes in terrestrial settings

• Fossilization in terrestrial environments is generally less common than in aquatic settings due to the greater exposure to weathering, erosion, and decomposition processes
• Rapid burial, either by sediments or volcanic ash, is often necessary for the preservation of terrestrial organisms as fossils
• Permineralization, where minerals precipitate within the pores of organic material, is a common fossilization process for terrestrial plants and some animals
• Carbonization, involving the compression and alteration of organic matter under anoxic conditions, can preserve the detailed morphology of leaves, seeds, and other plant structures

Taphonomy and biases in preservation

• Taphonomy, the study of the processes affecting an organism from death to fossilization, is crucial for understanding the biases and limitations of the terrestrial fossil record
• Differential preservation of hard parts, such as bones, teeth, and shells, relative to soft tissues, leads to an overrepresentation of certain taxa and an incomplete picture of past ecosystems
• Transport and reworking of fossils, particularly in fluvial and alluvial settings, can result in the mixing of remains from different time periods and environments
• Temporal and spatial biases, such as the preferential preservation of fossils in particular sedimentary environments or during specific geological intervals, affect the interpretation of terrestrial paleocommunities

Exceptional terrestrial fossil deposits

• Konservat-Lagerstätten are exceptional fossil deposits that preserve soft tissues, traces of behavior, and other rare features, providing unparalleled insights into past terrestrial life
• Amber, fossilized tree resin, can trap and preserve small organisms, such as insects, spiders, and plant fragments, in exquisite detail
• Volcanic ash deposits, such as those at Pompeii and Herculaneum, can preserve the remains of terrestrial organisms and their habitats through rapid burial and casting
• Permafrost and ice, in regions like Siberia and Alaska, have yielded well-preserved remains of Pleistocene megafauna, including mammoths, woolly rhinoceroses, and cave lions

Trace fossils and ichnology

• Trace fossils, also known as ichnofossils, are the preserved evidence of biological activity, such as tracks, burrows, and feeding marks, rather than the remains of the organisms themselves
• Vertebrate tracks and trackways, including those of dinosaurs, mammals, and birds, provide information on locomotion, behavior, and paleoecology
• Invertebrate traces, such as insect burrows and leaf mines, offer insights into the interactions between plants and animals in ancient terrestrial ecosystems
• Ichnology, the study of trace fossils, contributes to the reconstruction of past environments, ecological relationships, and evolutionary trends in terrestrial settings

Reconstructing past terrestrial environments

Paleosols and soil indicators

• Paleosols, or fossil soils, preserve evidence of past climate, vegetation, and landscape conditions through their physical, chemical, and biological properties
• Soil horizons, formed by the vertical differentiation of soil properties, can indicate the intensity and duration of weathering, as well as the influence of past biotic factors
• Clay mineralogy, particularly the presence of minerals like kaolinite, smectite, and illite, can provide information on past climate conditions, such as rainfall and temperature
• Soil carbonate accumulations, including calcrete and caliche, are indicative of arid to semi-arid environments and can be used to reconstruct past precipitation patterns

Paleobotany and plant fossils

• Paleobotany, the study of fossil plants, provides crucial information on the composition, structure, and evolution of past terrestrial vegetation
• Macrofossils, such as leaves, seeds, and wood, offer detailed morphological and anatomical information that can be used to identify plant taxa and reconstruct paleoenvironments
• Microfossils, including pollen and spores, are abundant and widely dispersed, allowing for the reconstruction of regional vegetation patterns and climate conditions
• Plant biomarkers, such as leaf wax lipids and terpenoids, can provide insights into past temperature, precipitation, and atmospheric CO2 levels

Vertebrate and invertebrate fossils

• Vertebrate fossils, including bones, teeth, and other hard parts, provide information on the diversity, ecology, and evolution of past terrestrial animals
• Mammalian tooth morphology and wear patterns can indicate dietary preferences and habitat use, allowing for the reconstruction of ancient food webs and environmental conditions
• Invertebrate fossils, such as insects, mollusks, and arachnids, offer insights into the structure and function of past terrestrial ecosystems, as well as plant-animal interactions
• The presence and abundance of certain indicator taxa, such as crocodilians, primates, and coral snakes, can be used to infer past climate conditions and habitat types

Geochemical proxies and isotopic evidence

• Stable isotope ratios of carbon, oxygen, and nitrogen in fossil materials can provide information on past climate, vegetation, and ecological relationships
• Carbon isotopes in fossil plant tissues and soil organic matter can distinguish between different photosynthetic pathways (C3 vs. C4) and indicate the relative abundance of grasses and woody plants
• Oxygen isotopes in fossil teeth and bone can be used to reconstruct past temperature, precipitation, and humidity conditions
• Clumped isotope thermometry, based on the ordering of heavy isotopes in carbonate minerals, can provide precise estimates of past terrestrial temperatures

Evolution of terrestrial life

Colonization of land by plants and animals

• The earliest evidence of terrestrial life dates back to the Ordovician Period, with the appearance of simple, non-vascular plants (bryophytes) and arthropod traces
• The Silurian Period saw the diversification of vascular plants, including the first appearance of lignified tissues (wood) and the development of root systems
• The Devonian Period, known as the "Age of Fishes," witnessed the radiation of early tetrapods, which later gave rise to the first truly terrestrial vertebrates (amphibians)
• The Carboniferous Period was characterized by the extensive development of coal forests, dominated by giant lycophytes, horsetails, and tree ferns

Diversification of terrestrial organisms

• The Permian Period saw the radiation of early synapsids (mammal-like reptiles) and the diversification of seed plants, including conifers and cycads
• The Triassic Period marked the emergence and diversification of dinosaurs, as well as the appearance of the first mammals and modern insect groups
• The Jurassic Period witnessed the dominance of dinosaurs, the radiation of birds, and the diversification of flowering plants (angiosperms)
• The Cretaceous Period saw the continued dominance of dinosaurs, the radiation of modern mammal groups, and the widespread expansion of angiosperm-dominated forests

Adaptations to terrestrial habitats

• The evolution of vascular tissues, including xylem and phloem, allowed plants to transport water and nutrients efficiently and colonize a wide range of terrestrial environments
• The development of seeds and pollen in plants facilitated reproductive success and dispersal in terrestrial settings, reducing dependence on water for fertilization
• The amniotic egg, which provides a protected and self-contained environment for embryonic development, was a key adaptation that allowed vertebrates to fully transition to life on land
• The evolution of endothermy (warm-bloodedness) in birds and mammals enabled active lifestyles and the colonization of diverse terrestrial habitats, including cold and arid regions

Mass extinctions and their impact

• The Late Ordovician mass extinction, caused by global cooling and glaciation, had a significant impact on early terrestrial communities, particularly invertebrates
• The End-Permian mass extinction, the most severe in Earth's history, resulted in the loss of an estimated 95% of marine species and 70% of terrestrial vertebrate species
• The End-Triassic mass extinction, linked to massive volcanic eruptions and climate change, paved the way for the dominance of dinosaurs in the Jurassic Period
• The End-Cretaceous mass extinction, caused by a combination of volcanic activity and an asteroid impact, marked the demise of non-avian dinosaurs and the rise of mammals as the dominant terrestrial vertebrates

Terrestrial environments and climate change

Greenhouse vs icehouse Earth

• Earth's climate has alternated between greenhouse and icehouse states throughout its history, with significant implications for terrestrial environments and biota
• Greenhouse periods, such as the Eocene, are characterized by high atmospheric CO2 levels, warm global temperatures, and reduced temperature gradients between the equator and poles
• Icehouse periods, such as the Pleistocene, are marked by lower atmospheric CO2 levels, cooler global temperatures, and the presence of continental ice sheets at high latitudes
• Transitions between greenhouse and icehouse states are driven by a complex interplay of factors, including plate tectonics, volcanic activity, and variations in Earth's orbit and axial tilt

Carbon cycle and atmospheric CO2

• The carbon cycle, involving the exchange of carbon between the atmosphere, oceans, and biosphere, plays a critical role in regulating Earth's climate and the distribution of terrestrial ecosystems
• Atmospheric CO2 levels have varied significantly throughout Earth's history, with higher concentrations during greenhouse periods and lower levels during icehouse periods
• The weathering of silicate rocks, which consumes atmospheric CO2 and releases calcium and magnesium ions, acts as a long-term negative feedback mechanism on global temperature
• The burial of organic carbon in sediments, such as coal and oil, represents a long-term sink for atmospheric CO2, while the oxidation of this buried carbon can