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Ecology 19.docx

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Biology 2483A
Mark Moscicki

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Ecology-Lecture 19 Nov 19, 2013 Introduction  Lindeman studied energy relationships among the organisms and non living components in a lake ecosystem. Rather than grouping its plants, animals, and bacteria according to their taxonomic categories, they were grouped based on how they obtain their energy.  The term ecosystem was first used by Tansley (1935) to refer to all the components of an ecological system, biotic and abiotic, that influence the flow of energy and elements. The elements are primarily nutrients, but may also include pollutants.  The ecosystem concept integrates ecology with other disciplines such as geochemistry, hydrology, and atmospheric science. Primary Production  Primary production is the chemical energy generated by autotrophs during photosynthesis and chemosynthesis. (chemosynthesis generates a very small amount of energy)  Primary production represents an important energy transition (conversion of light energy from the sun into chemical energy that can be used by autotrophs and consumed by heterotrophs)  Primary productivity is the rate of primary production.  Energy assimilated by autotrophs is stored as carbon compounds in plant tissues, thus carbon is the currency used to measure primary production. Gross Primary Production  Gross primary production (GPP) is the total amount of carbon fixed by autotrophs in an ecosystem. GPP is equivalent to the total photosynthetic rate, thus it depends on the photosynthetic rate.  GPP is controlled by the rate of photosynthesis (influenced by climate) and leaf area index (LAI)— leaf area per unit of ground area (how much of the ground surface has leaf cover). The LAI is a unitless measure.  LAI varies among biomes  Less than 0.1 in Arctic tundra (less than 10% of the ground surface has leaf cover).  12 in boreal and tropical forests (12 layers of leaves between the canopy and the ground, on average).  Shading increases with the addition of each leaf layer. Because of shading, the incremental gain in photosynthesis for each added leaf layer decreases. Eventually, the respiratory costs associated with adding leaf layers outweigh the photosynthetic benefits.  Plants use about half of the carbon fixed in photosynthesis for cellular respiration to support biosynthesis and cellular maintenance. All plant tissues lose carbon via respiration, but not all tissues are photosynthetic (e.g., tree trunks). Therefore, trees tend to have higher respiratory losses than herbaceous plants. The respiration rate increases with temperature, so tropical forests have higher respiratory losses. Net Primary Production  Net primary production is the energy remaining after respiratory losses.  Net primary production (NPP): NPP = GPP – Respiration  NPP represents biomass gained by the plant. NPP is the energy left over for plant growth, and for consumption by detritivores and herbivores. NPP represents input of carbon in ecosystems (what remains in ecosystem, not just what it lost).  Carbon not used in cellular respiration can be allocated to growth, reproduction, storage, and defense against herbivory. Plants can respond to environmental conditions by allocating carbon to the growth of different tissues. The allocation of carbon is an investment in potential future NPP, but the demands for other resources determine whether it pays off. Plants tend to allocate NPP to tissues that acquire the resources that limit their growth.  Example: Grassland plants allocate more NPP to roots because soil nutrients and water are scarce. In contrast, plants growing in dense communities with neighbors that may shade them may allocate NPP preferentially to stems and leaves in order to capture more sunlight for photosynthesis.  Allocation of NPP to storage products (e.g., starch and carbs) provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events such as frost. This is stored in stems and roots.  Substantial amounts of NPP (up to 20%) may be allocated to defensive secondary compounds when herbivory is high. Measuring NPP  It is important to be able to measure NPP. NPP is the ultimate source of energy for all organisms in an ecosystem (determines amount of energy available to support that ecosystem). Variation in NPP is an indication of ecosystem health which varies tremendously over space and time. Year to year changes may be indicative of stresses to that ecosystem (drought, acid rain). Additionally, NPP is associated with the global carbon cycle. As a result, it is associated with global climate change. Terrestrial Ecosystems  In terrestrial ecosystems, NPP is estimated by measuring increase in plant biomass in experimental plots, and scaling up to the whole ecosystem (several methods).  The traditional technique is using harvesting methods.  To do this, you harvest aboveground biomass before and after growing season (at its maximum). The difference in plant biomass between the two is used as an estimate for NPP. This is a reasonable estimate of aboveground NPP if corrections are made for herbivory and mortality.  Measuring belowground NPP is more difficult. The proportion of carbon in roots exceeds that in aboveground tissues in many ecosystems (as seen before)Fine roots turn over more quickly than shoots meaning that they die and are replaced quickly. In addition, roots may exude carbon into the soil, or transfer it to mycorrhizal or bacterial symbionts. Therefore, harvests must be more frequent, and additional correction factors are needed.  Minirhizotrons are underground viewing tubes with video cameras. They allow direct observation of root growth and death, and have advanced the understanding of belowground production processes.  Harvest techniques are impractical for large or biologically diverse ecosystems (due to their labor intensive and destructive nature)  Several non destructive techniques have been created which allow for more frequent estimation, over much larger spatial scales. However, they are not as precise as harvest techniques.  Remote sensing is a method that relies on the reflection of solar radiation. The amount of light reflected depends on the wavelengths of the light. Different kinds of objects absorb or reflect some wavelengths more than others. Remote sensing takes advantage of light reflection and absorption to estimate the density and composition of objects on earths surface, in water, and in the atmosphere. Remote sensing can be done at large scales using satellites which transmit their measurements to receiving stations. It can estimate CO u2take and NPP, deforestation, desertification, and other phenomena.  Ecologists use remote sensing to estimate NPP by taking advantage of the unique reflective pattern of chlorophyll containing plants, algae and bacteria. Chlorophyll absorbs blue and red wavelengths and has a characteristic spectral signature. Ecologists measure the reflection of specific wavelengths and estimate NPP using several indices that have been developed. NDVI (normalized difference vegetation index) is an index that uses differences between visible light and near infrared reflectance to estimate the density of chlorophyll. (NIR red) NDVI  (NIR  red)  NIR = Near-infrared wavelengths (700-1000 nm)  red = red wavelengths (600 – 700 nm)  Vegetation has a high NDVI value; water and soil have low NDVI values. The above image shows that vegetation reflects more near infrared and red light (600-700nm and 700-1000nm) than bare soil or water. Also, due to the fact that vegetation absorbs red and blue, there are dips in the reflectance curve (unlike soil and water which absorb one colour)  Below image shows that terrestrial NPP is highest in the tropics, and it declines heading North and South  NPP can also be estimated from GPP and respiration measurements (directly). Change in CO 2 concentration is measured in a closed chamber. Sources of CO2 in a closed system are by plants and heterotrophs undergoing respiration.  Sometimes whole stands of plants are enclosed in a chamber or tent to study CO 2 exchange.  The net change in CO 2s GPP minus total respiration: Net ecosystem production or exchange (NEE).  Heterotrophic respiration must be subtracted from NEE to obtain NPP. Thus, NEE is a more refined estimate of ecosystem carbon storage than NPP (which only subtracts plant respiration).  We can also estimate NEE using frequent measurements of CO2 and microclimate at various heights of the plant canopy, and into the open air above the canopy. This is known as the Eddy correlation or eddy covariance. It takes advantage of the gradient in CO2 concentration between the plant canopy and the atmosphere that develops because of photosynthesis and respiration.  During the day (plants are photo synthetically active), CO2 is lower in the canopy than it is in the air above. At night, CO2 in the canopy is higher due to respiration.  Instruments are mounted on towers to take continuous CO mea2urements.  A network of eddy covariance sites has been established in North America to increase our understanding of carbon and climate. Aquatic Ecosystems  Phytoplankton do most of the photosynthesis in aquatic habitats (algae and bacteria).  Phytoplankton have short life spans, so biomass (plant material) at any given time is low compared with NPP; harvest techniques are not used.  Photosynthesis and respiration are measured in water samples collected and incubated onsite with light (for photosynthesis) and without light (for respiration).  The difference in the rates is equal to NPP (note that there are errors associated with the artificial environment of the bottles).  Remote sensing of chlorophyll concentrations in the ocean using satellites provides estimates of marine NPP. Indices are developed to indicate how much light is absorbed by chlorophyll, which is then related to NPP.  Below image shows that NPP is highest in zones of upwelling in coastal areas! Environmental Controls on NPP  NPP varies substantially over space and time. Much of the variation is correlated with climate (temperature and precipitation). NPP in Terrestrial Ecosystems  NPP increases as precipitation increases, up to a point (2,400mm per year). Once it reaches a certain point, NPP decreases in some ecosystems. At very high precipitation levels, there is usually heavy cloud cover, lowering the amount of available sunlight. High precipitation also leaches nutrients from the soil, and high soil water content results in hypoxic conditions.  NPP increases with increasing average annual temperature (does not mean carbon storage (NEE) is higher because respiration/carbon loss is also higher with warmer temperatures). Therefore, NEE may actually decrease.  We know that water availability and temperature has a direct effect on NPP. However, climate influence on NPP can also be indirect, mediated by factors such as nutrient availability, or the particular plant species found within an ecosystem.  An experiment looked at how NPP in a grassland ecosystem responded to year to year variation in precipitation. They also looked at the average annual NPP and precipitaton across several grassland ecosystems at different locations in the U.S. In grassland sites across the central United States, it was observed that NPP variati
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