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General Terms Lectures 1-11

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Western University
Biology 2483A
Ron Podesta

Biology 2483A: Ecology Notes Lecture/Chapter One: Introduction Ecology is the study of interactions between organisms and environment and deals with the distribution and abundance of species within specific environments. (Not environmental science). There is not a balance in nature. Species don’t each have a specific role to play. When a species is removed they can always be replaced; although major influences can affect the environment greatly. The Ecological Maxims: 1. Organisms interact and are interconnected 2. Everything goes somewhere Ecology likes to monitor where: pollution, nutrients, energy, etc. 3. No population can increase in size forever Every species has a carrying capacity 4. Finite energy and resources result in tradeoffs A species will select where to ration their energy which they think will be most beneficial to survival: hunting, defense, camouflage, etc. 5. Organisms evolve: Evolution Adaptation: A characteristic that improves survival or reproduction. Natural selection: Individuals with certain adaptations tend to survive and reproduce at a higher rate than other individuals. If the adaptation is heritable, the frequency of the characteristic may increase in a population over time. 6. Communities and ecosystems change over time 7. Spatial scale matters Ecological Hierarchy: 1. Organism 2. Population: Group of individuals of a species that are living and interacting in a particular area 3. Community: Association of populations of different species in the same area 4. Ecosystem: Community of organisms plus the physical environment: biotic +abiotic 5. Landscapes: Areas with substantial differences, typically including multiple ecosystems 6. Biosphere: Entire planet plus all its organisms Ecosystem Processes: Producers: capture energy from an external source (e.g. the sun) and use it to produce food. Net primary productivity (NPP): Energy captured by producers, minus the amount lost as heat in cellular respiration. Consumers: get energy by eating other organisms or their remains. Ecosystem Flow of Energy and Nutrients: Energy: moves in a single direction only—it cannot be recycled and is lost as heat (convection, metabolism, conduction, latent heat exchange: respiration/sweating, radiation) Nutrients: are continuously recycled from the physical environment to organisms and back again Scientific Method: 1. Make observations and ask questions. 2. Use previous knowledge or intuition to develop hypotheses. 3. Evaluate hypotheses by experimentation, observational studies, or quantitative models. 4. Use the results to modify the hypotheses, pose new questions, or draw conclusions about the natural world. Amphibian Deformities and Mortalities: “Biological indicators” for environmental problems: due to permeable skin, eggs with no protective shells, double environment (land and water) means exposure in both areas Many became extinct and deformed. Parasite Ribeiroia causes deformities and mortality—cysts formed on amphibians Life Cycle of Ribeiroia: Eggs are in the water, and develop in snails (1 intermediate host), form free-swimming cercariae which then inhabit amphibian tadpoles (2 intermediate host), they form cysts here causing deformities, and when they are eaten by birds (definitive host), they mature to adulthood in the bird and release their eggs back into the water through bird feces. Factors Influencing Deformities: 1. Pollutants. Impair immune system making amphibians more susceptible to parasite 2. Fertilizer runoff. Increases algae growth and therefore snail population and then parasite population since it is the first intermediate host. 3. Habitat loss 4. UV exposure and Nitrate levels. High levels of both reduced mortality. 5. Pathogens. Chytrid fungus—causes a lethal skin disease. 6. Climate change Lecture/Chapter Two: The Physical Environment The physical environment determines where organisms can live, and the resources that are available. Thus, understanding the physical environment is key to understanding all ecological phenomena. Weather: Current conditions—temperature, precipitation, humidity, cloud cover. Climate: Long-term description of weather, based on averages and variation measured over decades. • Climatic variation includes daily and seasonal cycles, as well as yearly and decadal cycles. • Long-term climate change results from changes in the intensity and distribution of solar radiation. • Current climate change is due to increased CO2 and other gases in the atmosphere due to human activities. • Climate determines the geographic distribution of organisms. • Climate is characterized by average conditions; but extreme conditions are also important to organisms because they can contribute to mortality. Atmosphere contains greenhouse gases that absorb and reradiate infrared radiation emitted by Earth. These gases include: • Water vapor (H2O). • Carbon dioxide (CO2). • Methane (CH4). • Nitrous oxide (N2O). Without greenhouse gases, Earth’s climate would be about 33°C cooler. Solar radiation heats Earth’s surface, which emits infrared radiation to the atmosphere, warming the air above it. Warm air is less dense than cool air, and it rises —this is called uplift. Air pressure decreases with altitude, so the rising air expands and cools. Tropical regions receive the most solar radiation and the most precipitation. Uplift of air in the tropics results in a low atmospheric pressure zone—wet Downdraft of air around 30° creates a high atmospheric pressure zone—dry When air masses reach the troposphere– stratosphere boundary, air flows towards the poles. These three cells result in the three major climatic zones in each hemisphere—tropical, temperate, and polar zones. Areas of high and low pressure created by the circulation cells result in air movements called prevailing winds. The winds appear to be deflected due to the rotation of the Earth—the Coriolis effect. Water has a higher heat capacity than land— it can absorb and store more energy without changing temperature. Summer: Air over oceans is cooler and denser, so air subsides and high pressures develop over the oceans. Winter: Air over continents is cooler and denser; high pressure develops over continents. These are known as semipermanent high and low pressure cells. Major ocean surface currents are driven by surface winds, so patterns are similar. Speed of ocean currents is about 2%–3% of the wind speed. Ocean currents affect climate. Example: the warm Gulf Stream warms the climate of Great Britain and Scandinavia. Example: Labrador is much cooler because of the cold Labrador Current. Where warm tropical surface currents reach polar areas, the water cools, ice forms, water becomes more saline and denser and sinks (downwelling). Upwelling is where deep ocean water rises to the surface. Upwelling occurs where prevailing winds blow parallel to a coastline. Surface water flows away from the coast and deeper, colder ocean water rises up to replace it. Upwellings influence coastal climates. Upwellings bring nutrients from the deep sediments to the photic zone—where light penetrates and phytoplankton grow. This provides food for zooplankton and their consumers. These areas are the most productive in the open oceans. Coasts have a maritime climate: Little daily and seasonal variation in temperature and high humidity. Areas in the center of large continents have continental climates: Much greater variation in daily and seasonal temperatures. On mountain slopes, vegetation shifts reflect climate changes as temperature decreases, and precipitation and wind speed increase with elevation. When air masses meet mountain ranges, they are forced upwards, cooling and releasing precipitation. North–south trending mountain ranges create a rain shadow: The slope facing prevailing winds (windward) has high precipitation, while the leeward slope gets little precipitation. Vegetation can also influence climate. Albedo—capacity of a land surface to reflect solar radiation—is influenced by vegetation type, soils, and topography. For example, a coniferous forest has a darker color and lower albedo than bare soil or dormant grassland. Loss or change in vegetation can affect climate. Deforestation increases albedo of the land surface: Less absorption of solar radiation and less heating. Lower heat gain is offset by less cooling by evapotranspiration, due to loss of leaf area. Decreased evapotranspiration results in less moisture in the atmosphere and less precipitation. Deforestation in the tropics can lead to a warmer, dryer regional climate. Climatic variation over time: Earth is tilted at an angle of 23.5° relative to the sun’s direct rays. The angle and intensity of the sun’s rays striking any point on Earth vary as Earth orbits the sun, resulting in seasonal variation in climate. In temperate-zone lakes, stratification changes with the seasons.  Complete mixing (turnover) occurs in spring and fall when water temperature and density become uniform with depth. This pattern is due to the fact that Water has a higher heat capacity than land. Therefore at greater depths the water will retain the previous season’s temperature—in winter it gets warmer as you go down and in summer it gets colder. El Niño events, or the El Niño Southern Oscillation (ENSO), are longer-scale climate variations that occur every 3 to 8 years and last about 18 months. The positions of high- and low-pressure systems over equatorial Pacific switch, and the trade winds weaken. Upwelling of deep ocean water off the coast of South America ceases, resulting in much lower fish harvests. Greenhouse Gases Again: Over the past 500 million years, Earth’s climate has alternated between warm and cool cycles. Warmer periods are associated with higher concentrations of greenhouse gases. Earth has cycles of cool phases with formation of glaciers (glacial maxima), followed by warm periods with glacial melting (interglacial periods). These glacial–interglacial cycles occur at frequencies of about 100,000 years. We are currently in an interglacial period; these have lasted about 23,000 years in the past. The last glacial maximum was about 18,000 years ago. The glacial–interglacial cycles have been explained by regular changes in the shape of Earth’s orbit and the tilt of its axis—Milankovitch cycles. The intensity of solar radiation reaching Earth changes, resulting in climatic change. The shape of Earth’s orbit changes in 100,000-year cycles. (glacial-interglacial frequency) The angle of axis tilt changes in cycles of about 41,000 years. Earth’s orientation relative to other celestial objects changes in cycles of about 22,000 years. Lecture/Chapter Three: The Biosphere The biosphere is the zone of life on Earth. Biomes are large-scale biological communities shaped by the physical environment, particularly climate. Biomes are categorized by dominant plant forms, not taxonomic relationships. Plants occupy sites for a long time and are good indicators of the physical environment, reflecting climatic conditions and disturbances. Terrestrial biomes are characterized by growth forms of the dominant plants, such as leaf deciduousness or succulence. Plants have taken many forms in response to selection pressures such as aridity, extreme temperatures, intense solar radiation, grazing, and crowding. Similar growth forms can be found on different continents, even though the plants are not genetically related. Convergence: Evolution of similar growth forms among distantly related species in response to similar selection pressures. Temperature has direct physiological effects on plant growth form. Precipitation and temperature act together to influence water availability and water loss by plants. Water availability and soil temperature determine the supply of nutrients in the soil. Biomes Vary with Mean Annual Temperature and Precipitation: This does not account for seasonal variation Human activities influence the distribution of biomes.Land use change: Conversion of land to agriculture, logging, resource extraction, urban development. The potential and actual distributions of biomes are markedly different. There are nine major terrestrial biomes: 1. Tropical Rainforests High biomass, high diversity—about 50% of Earth’s species. Light is a key factor—plants must grow very tall above their neighbors or adjust to low light levels. Emergents rise above the canopy. Lianas (woody vines) and epiphytes use the trees for support. Understory trees grow in the shade of the canopy, and shrubs and forbs occupy the forest floor. Biome is disappearing due to logging and conversion to pasture and croplands. About half has been altered. Soils are nutrient- poor, and recovery of nutrient supplies may take a very long time. 2. Tropical Seasonal Forests and Savannas Yellow is drought period. Wet and dry seasons associated with movement of the ITCZ. Shorter trees, deciduous in dry seasons, more grasses and shrubs. Fires promote establishment of savannas; some are set by humans. In Africa, large herbivores—wildebeests, zebras, elephants, and antelopes—also influence the balance of grass and trees. On the Orinoco River floodplain, seasonal flooding promotes savannas. Less than half of seasonal tropical forests and savannas remain. Human population growth in this biome has had a major influence. Large tracts have been converted to cropland and pasture. 3. Hot Deserts High temperatures, low moisture. Sparse vegetation and animal populations. Low water availability constrains plant abundance and influences form. Many plants have succulent stems that store water. Convergence of this form is shown by cacti (Western Hemisphere) and euphorbs (Eastern Hemisphere). Both plants developed succulent stems separately in evolution. Humans use deserts for agriculture and livestock grazing. Agriculture depends on irrigation, and results in soil salinization. Long-term droughts and unsustainable grazing can result in desertification—loss of plant cover and soil erosion. 4. Temperate Grasslands Warm, moist summers and cold, dry winters. Grasses dominate; maintained by frequent fires and large herbivores such as bison. Grasses grow more roots than stems and leaves, to cope with dry conditions. This results in accumulation of organic matter and high soil fertility. Most fertile grasslands of central North America and Eurasia have been converted to agriculture. In arid grasslands, grazing by domesticated animals can exceed capacity for regrowth, leading to grassland degradation and desertification. Irrigation in some areas causes salinization. 5. Temperate Shrublands and Woodlands Evergreen leaves allow plants to be active during cooler, wetter periods. They also lower nutrient requirements—the plants don’t have to develop new leaves every year. Sclerophyllous leaves—tough and leathery—deter herbivores and prevent wilting. After fires, shrubs sprout from underground storage organs, or produce seeds that sprout and grow quickly. Without regular fires at 30–40-year intervals, shrublands may be replaced by forests. 6. Temperate Deciduous Forests Deciduous leaves in response to extended periods of freezing. Need fertile soils and enough water to support tree growth. Fertile soils and climate make this biome good for agriculture. Very little old-growth temperate forest remains. As agriculture has shifted to the tropics, temperate forests have regrown. Shifts in species composition are due to nutrient depletion by agriculture and invasive species, causing damage such as chestnut blight. 7. Temperate Evergreen Forests Includes temperate rainforests, but spans a wide range of environmental conditions. Commonly found on nutrient-poor soils. Evergreen trees are used for wood and paper pulp, and this biome has been logged extensively. Very little old-growth temperate evergreen forest remains. In some areas, trees have been replaced with non-native species in uniformly aged stands. Suppression of fires in western North America has increased the density of forest stands, which results in more intense fires when they do occur. It also increases the spread of insect pests and pathogens. Air pollution has damaged some temperate evergreen forests. 8. Boreal Forests Boreal Forests (Taiga): Long, severe winters. Permafrost (soil that remains frozen year-round) prevents drainage and results in saturated soils. Trees are conifers—pines, spruces, larches. Cold, wet conditions in boreal soils limit decomposition, so soils have high organic matter. In summer droughts, forest fires can be set by lightning, and can burn both trees and soil. In low-lying areas, extensive peat bogs form. Boreal forests have not been as affected by human activities. Logging, oil and gas development, occur in some regions. Impacts will increase as energy demands increase. Climate warming may increase soil decomposition rates, releasing stored carbon and creating a positive feedback to warming. This creates many forest fires. 9. Tundra Where growing season length and temperatures decrease, trees cease to be the dominant vegetation (the tree line marks the boreal to tundra transition). Characterized by sedges, grasses, forbs, and low growing shrubs. Human influence is increasing as exploration and development of energy resources increases. The Arctic has experienced significant climate change, with warming almost double the global average. Mountain Biological Zones: On mountains, temperature and precipitation change with elevation, resulting in zones similar to biomes. Smaller scale variations are associated with slope aspect, proximity to streams, and prevailing winds. Streams and rivers: Streams and rivers are lotic (flowing water) systems. Benthic organisms are bottom dwellers, and include many kinds of invertebrates. Some feed on detritus (dead organic matter), others are predators. Some live in the hyporheic zone—the substratum below and adjacent to the stream. Lakes and still waters (lentic) occur where depressions in the landscape fill with water. Littoral zone (near shore): Where the photic zone reaches the bottom. Macrophytes occur in this zone. Photic zone: Supports the highest densities of organisms, extends to about 200 m depth. Below the photic zone, energy is supplied by falling detritus. Pelagic zone: Open water beyond the continental shelves; dominated by plankton (small and microscopic organisms suspended in the water). Phytoplankton are photosynthetic, restricted to the upper layers through which light penetrates (photic zone). Zooplankton are non-photosynthetic protists and tiny animals (don’t need photic zone). The Deep Pelagic Zone: Below the photic zone, temperatures drop and pressure increases. Crustaceans such as copepods graze on the rain of falling detritus from the photic zone. Crustaceans, cephalopods, and fishes are the predators of the deep sea. Estuaries: Where rivers flow into oceans. Salt marshes: Shallow coastal wetlands dominated by grasses and rushes. Mangrove forests: Mudflats dominated by salt-tolerant trees. Sandy shores, Kelp forests, Coral reefs, Rocky intertidal are others. Lecture/Chapter Four: Coping with Environmental Variation: Temperature and Water The physical environment influences an organism’s ecological success in two ways: 1. Availability of energy and resources—impacts growth and reproduction. 2. Extreme conditions can exceed tolerance limits and impact survival. Energy supply can influence an organism’s ability to tolerate environmental extremes. The actual geographic distribution of a species is also related to other factors, such as disturbance and competition. • Because plants don’t move, they are good indicators of the physical environment. • A species’ climate envelope is the range of conditions over which it occurs. Physiological ecology is the study of interactions between organisms and the physical environments that influence their survival and persistence. • Physiological processes have optimal conditions for functioning. • Deviations from the optimum reduce the rate of the process. • Stress—environmental change results in decreased rates of physiological processes, lowering the potential for survival, growth, or reproduction. Acclimatization: Adjusting to stress through behavior or physiology. • It is usually a short-term, reversible process. • Acclimatization to high elevations involves higher breathing rates, greater production of red blood cells, and higher pulmonary blood pressure. Adaptation: Adjusting to envrionmental stress over many generations through natural selection • Individuals with traits that enable them to cope with stress are favored. Over time, these genetic traits become more frequent in the population. Acclimatization and adaptation require investments of energy and resources, representing possible trade-offs with other functions that can also affect survival and reproduction. Ecotypes: Populations with adaptations to unique environments. • Ecotypes can eventually become separate species as populations diverge and become reproductively isolated. Temperature: • Environmental temperatures vary greatly throughout the biosphere. • Survival and functioning of organisms is strongly tied to their internal temperature. • Some archaea and bacteria in hot springs can function at 90°C. • Lower limits are determined by temperature at which water freezes in cells (–2 to –5°C). • Metabolic reactions are catalyzed by enzymes, which have narrow temperature ranges for optimal function. High temperature destroys enzymes function (denatured). • Bacteria in hot springs - enzymes stable to 100°C; Antarctic fish and crustaceans -enzymes function at – 2°C; soil microbes - active at temperatures as low as –5°C. • Some species produce different forms of enzymes (isozymes) with different temperature optima that allow acclimatization to changing conditions. • Temperature also affects the properties of cell membranes, which are composed of two layers of lipid molecules. • At low temperatures, these lipids can solidify, embedded proteins can’t function, and the cells leak metabolites. • Plants that thrive at low temperatures have higher proportions of unsaturated lipids (with double bonds) in their cell membranes Ectotherms: Regulate body temperature through energy exchange with the external environment. • Ectotherm surface area-to-volume ratio of the body is an important factor in exchanging energy with the environment. • A larger surface area allows greater heat exchange, but makes it harder to maintain internal temperature. • Small aquatic ectotherms remain the same temperature as the water. • Some large ectotherms can maintain higher body temperature: • Skipjack tuna use muscle activity and heat exchange between blood vessels to maintain a body temperature 14°C warmer than the surrounding seawater. • Many terrestrial ectotherms can move around to adjust temperature. • Many insects and reptiles bask in the sun to warm up after a cold night, but this increases predation risk, increasing benefits of camouflage • Ectotherms in temperate and polar regions must avoid or tolerate freezing. Avoidance behavior includes seasonal migration to lower latitudes or to microsites that are above freezing (e.g., burrows in soil). • Tolerance to freezing involves minimizing damage associated with ice formation in cells. Some insects have high concentrations of glycerol, a chemical that lowers the freezing point of body fluids. • Vertebrates generally do not tolerate freezing temperatures. • Heat Stress: In hot environments can gain too much heat from the environment and body temperature can reach lethal levels. Endotherms: Rely primarily on internal heat generation—mostly birds and mammals. • Can maintain internal temperatures near optimum for metabolic functions. Can extend geographic range • Some other organisms that generate heat internally include bees, some fish, such as tuna, and even some plants. • Skunk cabbage warms its flowers using metabolically generated heat in early spring. • Endotherms can remain active at subfreezing temperatures. • The cost of being endothermic is a high demand for energy (food) to support metabolic heat production. • Metabolic rates are a function of the external temperature and rate of heat loss. • Rate of heat loss is related to body size and surface area-to-volume ratio. • Small endotherms with large surface area-to-volume ratio have higher metabolic rates, and require more energy and higher feeding rates than large endotherms. • Thermoneutral zone: The range of environmental temperatures over which a constant basal metabolic rate can be maintained. • Lower critical temperature: When heat loss is greater than metabolic production; body temperature drops and metabolic heat generation increases. • Metabolic Rates in Endotherms Vary with Environmental Temperatures: • Mammals in the Arctic have lower critical temperatures than mammals in tropical regions. • The rate of metabolic activity increases more rapidly below the lower critical temperature in tropical mammals as compared to Arctic mammals. Figure only graphing the lower critical temperatures and thermal neutral zone • Evolution of endothermy required insulation—feathers, fur, and fat. Insulation limits conductive and convective heat loss. • Fur and feathers provide a layer of still air adjacent to the skin. Some animals grow thicker fur for winter. • Some organisms can survive periods of extreme heat or cold by entering a state of dormancy, in which little or no metabolic activity occurs. • Small mammals have thin fur and not much fat for energy storage, but high demand for metabolic energy below the lower critical temperature. • They survive in cold climates by entering a dormant state called torpor. Body temperature and basal metabolic rates are low, which conserves energy. • Energy reserves are needed to come out of torpor. Small endotherms may undergo daily torpor to survive cold nights. • Longer periods of torpor, or hibernation, are possible for animals that can store enough energy. • Heat stress in animals: • Some organisms use behavioral changes to control exchange of energy with the environment. • Examples: Elephants swim and spray water onto their backs with their trunks to cool their bodies. • Moving into the shade reduces the amount of solar radiation received • Evaporative heat loss in animals includes sweating in humans, panting in dogs and other animals, and licking of the body by some marsupials. • Water stress in animals: • Arid conditions are a widespread challenge for organisms. • Some tolerate dry conditions by going into suspended animation. Many microorganisms do this, as do some multicellular organisms. • Desiccation-tolerant organisms can lose 80%–90% of their water. • Reptiles are very successful in dry environments. They have thick skin with layers of dead cells, fatty coatings, and plates or scales. • Mammals and birds have thick skin plus fur or feathers to minimize water loss. • Sweat glands in mammals are a trade-off between water loss resistance and evaporative cooling. Cryonics is the preservation of bodies by freezing, in hope that they can be brought back to life in the future. Two frog species live in the Arctic tundra and can survive winter in a semi-frozen state. They overwinter in shallow burrows, with no heartbeat, no blood circulation, and no breathing. • In most organisms, freezing results in tissue damage as ice crystals perforate cell membranes and organelles. • In animals that withstand freezing, the freezing water is limited to the space outside the cells. • Ice-nucleating proteins outside cells serve as sites of slow, controlled ice formation. • Additional solutes, such as glucose and glycerol are made inside the cells to lower the freezing point. Energy Exchange in Terrestrial Plants Conduction—transfer of energy from warmer to cooler molecules. Convection—heat energy is carried by moving water or air. Plants can adjust energy inputs and outputs. Transpiration rates can be controlled by specialized guard cells surrounding leaf openings called stomates. Variation in degree of opening and number of stomates control the rate of transpiration and thus leaf temperature. Transpiration - Stomatal Control of Leaf Temperature • If soil water is limited, transpirational cooling is not a good mechanism. • Some plants shed their leaves during dry seasons. • Other mechanisms include pubescence—hairs on leaf surfaces that reflect solar energy. But hairs also reduce conductive heat loss. • Pubescence was studied in three Encelia species (plants in the daisy family). • Desert species with high pubescence were compared with non-pubescent species from wetter, cooler habitats. • Plants of all three species were grown in both locations (common gardens). • In the cool, moist location, the three species showed few differences in leaf temperature and stomatal opening. • In the desert, species with no hairs maintained leaf temperature by transpiration; the pubescent species leaves reflected about twice as much solar radiation. • The desert species (E. farinosa) also has smaller, more pubescent leaves in summer than in winter, representing acclimatization to hot summer temperatures. • If air temperature is lower than leaf temperature, heat can be lost by convection. • Convective heat loss is related to speed of air moving across a leaf surface. • Boundary layer: A zone of turbulent flow due to friction, next to the leaf surface. • The boundary layer lowers convective heat loss. • Boundary layer thickness is related to leaf size and surface roughness. • Small, smooth leaves have thin boundary layers and lose more heat than large or rough leaves. • In cold, windy environments, convection is the main heat loss mechanism. • Most alpine plants hug the ground surface to avoid high wind velocities. • Some have a layer of insulating hair to lower convective heat loss. Lecture/Chapter Five: Coping with Environmental Variation: Energy Autotrophs: Assimilate radiant energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthesis). • The energy is converted into chemical energy stored in the bonds of organic molecules. • Sea slugs have functional chloroplasts that are taken up from the algae that the slug eats. • Photosynthesis - (most autotrophes): sunlight provides the energy to take up CO2and synthesize organic compounds. • Chemosynthesis (chemolithotrophy): Energy from inorganic compounds is used to produce carbohydrates. Chemosynthesis is important in nutrient cycling bacteria, and in some ecosystems such as hydrothermal vent communities. • Most of the biologically available energy on Earth is derived from photosynthesis. Photosynthetic organisms include some archaea, bacteria, and protists, and most algae and plants. Photosynthesis has two major steps: 1. Light reaction—light is harvested and used to split water and provide electrons to make ATP and NADPH. 2. Dark reaction—CO is 2ixed in the Calvin cycle, and carbohydrates are synthesized. • Photosynthetic rate determines the supply of energy, which in turn influences growth and reproduction. • Environmental controls on photosynthetic rate are an important topic in physiological ecology. Light response curves show the influence of light levels on photosynthetic rate. Light compensation point: Where CO2 uptake is balanced by CO2 loss by respiration. Saturation point: When photosynthesis no longer increases as light increases. Plants can acclimatize to changing light intensities with shifts in light response curves.  Shifts in light saturation point involve morphological and physiological changes. • Leaves at high light intensity may have thicker leaves and more chloroplasts. • Water availability influences CO 2upply in terrestrial plants. • Low water availability causes stomates to close, restricting CO2uptake. • This is a trade-off: Water conservation versus energy gain. • Closing stomates increases chance of light damage: If the Calvin cycle isn’t operating, energy accumulates in the light-harvesting arrays and can damage membranes. • Plants have various mechanisms to dissipate this energy, including carotenoids. • Plants from different climate zones have enzyme forms with different optimal temperatures that allow them to operate in that climate. • Nutrients can also affect photosynthesis: Most nitrogen in plants is associated with rubisco and other photosynthetic enzymes. • Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates. • But nitrogen supply is low, relative to demand for growth and metabolism. • Increasing nitrogen content of leaves increases the risk that herbivores will eat them, as plant-eating animals are also nitrogen-starved. Some metabolic processes decrease photosynthetic efficiency. Rubisco can catalyze two competing reactions: 1. Carboxylase reaction: Photosynthesis. 2. Oxygenase reaction: O is2taken up, carbon compounds are broken down, and CO is rele2sed (photorespiration). • Does photorespiration have any benefits? o Experiments with Arabidopsis thaliana plants with a mutation that knocks out photorespiration: o These plants die under normal light and CO co2ditions. o Hypothesis: Photorespiration may protect plants from damage at high light levels. • Used more in high O and high temperature 2 • However it is less efficient than photosynthesis so plants are at risk at high oxygen and temp • When atmosphere was like this (7 million years ago) they created a solution: C4 photosynthesis The C4 photosynthetic pathway reduces photorespiration, and evolved independently several times. • Many grass species use this pathway, including corn, sugarcane, and sorghum. It involves biochemical and morphological specialization. • CO 2ptake and the Calvin cycle occur in different parts of the leaf. • CO 2s taken up in the mesophyll by PEPcase, which has greater affinity for CO2, and does not take up O2. • CO concentration is increased in bundle sheath cells 2 where rubisco is operating in the Calvin cycle, which reduces O 2ptake by rubisco. • More ATP is required for the C p4thway, but higher photosynthetic efficiency gives these plants an advantage at high temperatures. • Transpiration losses are minimized because PEPcase can take up CO e2en when stomates are not fully open. • There is a close correlation between temperature and the proportion of C s4ecies in the community. o High temperatures = more C4 plants Crassulacean acid metabolism (CAM) minimizes water loss. • CO2 uptake and the Calvin cycle are separated temporally. • CAM plants open their stomates at night when it’s cooler and humidity is higher, and close them during the day. • CAM plants are often succulent, with thick, fleshy leaves or stems. • They are common in hot dry areas and some humid tropics with less access to water Heterotrophs: Obtain their energy by consuming energy-rich organic compounds from other organisms. • Some heterotrophs consume non-living organic matter. • Parasites and herbivores consume live hosts, but do not necessarily kill them. • Predators capture and consume live prey animals. • Some plants are holoparasites: They have no photosynthetic pigments and get energy from other plants (heterotrophs). Dodder is a holoparasite that is an agricultural pest and can significantly reduce biomass in the host plant. • Mistletoe is a hemiparasite—it is photosynthetic, but obtains nutrients, water, and some of its energy from the host plant. • The energy gain depends on the chemistry of the food, and how much effort is need to find and ingest the food o Soil microorganisms that feed on detritus invest little energy to find food, but the food has low energy content. o A cheetah hunting a gazelle invests a lot of energy to find, chase, and kill its prey, but it gets an energy-rich meal. • Multicellular animals have evolved specialized tissues and organs for absorption, digestion, transport, and excretion. • They have tremendous diversity in morphological and physiological feeding adaptations. • Humans view toolmaking as something that differentiates us from other animals o Chimps, crows, and dolphins do it too • Optimal foraging theory: Animals will maximize the amount of energy gained per unit time, energy, and risk involved in finding food. • Food availability can vary greatly over time and space. o If energy is in short supply, animals should invest in obtaining the highest-quality food that is the shortest distance away. • Profitability of a food item (P) depends on how much energy (E) the animal gets from the food relative to the amount of time (t) it spends finding and obtaining the food: E P = t • An animal’s success in acquiring food increases with the effort it invests; but at some point, more effort results in no more benefit, and the net energy obtained begins to decrease. • Marginal value theorem (Charnov 1976): o An animal should stay in a patch until the rate of energy gain has declined to match the average rate for the whole habitat (giving up time). o Giving up time is also influenced by distance between patches: The longer the travel time between food patches, the longer an animal should spend in a patch. o NOTE: this doesn’t just apply to distance but the giving up time between how long an organism should go after a certain energy source (ie. How long it should try to open a shell, etc.) o Optimal foraging theory does not apply as well to animals that feed on mobile prey. o The assumption that energy is in short supply, and this dictates foraging behavior, may not always hold. Lecture/Chapter Six:  Evolution and Ecology Case Study: Bighorn sheep hunting Trophy hunting removes the largest and strongest males—the ones that would sire many healthy offspring. In one population, 10% of males were removed by hunting each year, the average size of males and their horns decreased over 30 years of study. This is also being observed in other species: • African elephants are poached for ivory; the proportion of the population that have tusks is decreasing. • Rock shrimp are all born male, and become females when they are large enough to carry eggs. Commercial harvesting takes the largest individuals, which are all females. • Genes for switching sex at a smaller size became more common, resulting in more females, but smaller females lay fewer eggs. What Is Evolution? • Evolution can be viewed as genetic change over time or as a process of descent with modification. • Biological evolution is change in organisms over time. • Evolution can be defined more broadly as descent with modification. • As a population accumulates differences over time and a new species forms, it is different from its ancestors. • But the new species has many of the same characteristics as its ancestors, and resembles them
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