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Ecology 21.docx

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Biology 2483A
Mark Moscicki

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Ecology-Lecture 21 Nov 26, 2013 Introduction  In addition to energy, all organisms require specific chemical elements to meet their biochemical requirements for metabolism and growth. Organisms get these elements by absorbing them from the environment or by consuming other organisms  Iron is required by all organisms for several metabolic functions. However, organisms obtain this iron from different sources in very different ways. However, the ultimate source of iron is solid minerals in earths crust which are subjected to chemical transformations as they move through the different physical and biological components of ecosystems.  Biogeochemistry is the study of the physical, chemical, and biological factors that influence the movement and transformation of elements (nutrients and non nutrients like tracers or pollutants).  Understanding biogeochemistry is important in determining the availability of nutrients which are chemical elements required for metabolism and growth. Nutrients must be in certain forms for uptake by organisms. The rate of physical and chemical transformations determines the supply of nutrients. Nutrient Requirements & Sources  All organisms share similar nutrient requirements. However, the ways in which these nutrients are obtained, the chemical forms of those nutrients that are taken up, and relative amounts of those nutrients that are required vary greatly. Nutrients enter ecosystems through the chemical breakdown of minerals in rocks of the earths crust, or through fixation of atmospheric gases  Amounts and specific nutrients required vary according to the organisms mode of energy acquisition (autotrophs vs heterotrophs), mobility, and thermal physiology (ectotherm vs. endotherm)  Differences in nutrient requirements are reflected in the chemical composition of organisms. Carbon (C) is the main component of structural compounds in plants; nitrogen (N) is largely tied up in enzymes. C:N ratios reflect relative concentrations of biochemical machinery in cells: Animals have lower C:N ratios (6 for humans); plants have C:N ratios of 10–40. As a result, herbivores must consume more food than carnivores to get enough nutrients such as N.  All plants require a set of essential nutrients. Some species have specific requirements. Some 4 and CAM plants require sodium. (All animals require it for maintaining pH and osmotic balance.) Some plants that host N-fixing bacteria require cobalt. Some plants in selenium-rich soil require it, but it is toxic to most plants.  Plants and microorganisms take up nutrients in simple, soluble forms from the environment. Using the simple forms, they synthesize the larger forms required for their metabolism and growth.  Animals mostly get nutrients in food as large, complex molecules. They take them up by consuming living organisms, or detritus. Some of these are broken down; others are absorbed intact, such as some amino acids. Ultimate Source of Nutrients  All nutrients are ultimately derived from two abiotic sources: Minerals in rocks and gases in the atmosphere. Over time, they accumulate in ecosystems in organic forms.  Nutrients may be cycled within an ecosystem, repeatedly passing through organisms and the soil or water. They may even be cycled internally within an organism (stored or mobilized for use as needs change)  The breakdown of minerals in rock supplies ecosystems with nutrients such as potassium, calcium, magnesium and phosphorous.  Minerals: solid substances with characteristic chemical properties, derived from several geologic processes  Rocks: collections of different minerals.  Elements are released from rock minerals by weathering, which is a two step process.  The first step is Mechanical weathering. This is the physical breakdown of rocks. Expansion and contraction processes (freeze–thaw and drying–rewetting cycles) break rocks into smaller pieces. Plant roots and gravity (e.g., landslides) also contribute.  Mechanical weathering exposes minerals to the processes of chemical weathering in which the minerals are subjected to chemical reactions that release soluble forms of the mineral elements (nutrients).  Weathering is one of the processes that result in soil formation.  Soil is a mix of mineral particles, organic matter (mostly decomposing plant matter), water, and organisms. The water contains dissolved organic matter, minerals, and gases (the soil solution).  Soil properties influence availability of nutrients to plants:  Texture: determined by particle size. The coarsest particles are sand. Intermediate sized particles are silt. Clays are the smallest particles (< 2 μm). Clays have a semicrystalline structure and negative charges on the surface that can hold onto cations and exchange them with the soil solution. As a result, clay 2+ + particles can serve as a reservoir for some nutrient ions such as Ca , K , and Mg . A soils ability to hold and exchange these cations is known as Cation exchange capacity. It is determined by the amount and types of clay particles present in the soil. Texture also influences soil water-holding capacity, and thus movement of nutrients in the soil solution. Soils with a high proportion of sand have large spaces between the particles, and do not hold water well. Water drains through quickly.  Parent material: The rock or mineral material that was broken down by weathering to form a soil. Parent material may be bedrock (common), or sediment deposited by glaciers (till), or by wind (loess), or by water. The chemistry and structure of the parent material determines the rate of weathering, and the amount and type of minerals released. Thus, it influences soil characteristics such as fertility.  Example: Soils derived from limestone have high levels of Ca , K , and + 2+ Mg . Soils derived from acidic granite have low concentrations of these nutrients.  The chemistry and pH of the parent material exerts an influence on abundance, growth, and diversity of plants in an ecosystem.  Gough et al. (2000) showed that variation in acidity of parent material pH was correlated with differences in plant species richness in Arctic ecosystems. They surveyed Arctic vegetation across gradients in soil acidity and found that the number of plant species increased as acidity decreased. This variation was attributed to the negative effects of soil acidity on nutrient availability, as well as its inhibitory effects on plant establishment.  Over time, soils undergo changes associated with weathering, accumulation/chemical alteration of organic matter, leaching (movement of dissolved organic matter and fine mineral particles from upper to lower layers) These processes form horizons, which are layers of soil distinguished by their color, texture, and permeability.  Climate influences rates of soil development (speeds weathering, leaching, etc). Soil development is fastest in warm, wet conditions. Tropical forest soils have experienced high rates of weathering and leaching for a long time, and are nutrient-poor. Most of the nutrients in these ecosystems are in the living tree biomass. In other terrestrial ecosystems, the proportion of nutrients in the soil is greater. When tropical forests are cleared and burned, the nutrients are lost in smoke and ash and soil erosion. These ecosystems can take centuries to return to their previous state.  Organisms, especially plants, bacteria, and fungi, contribute organic matter to soils. Organic matter is a reservoir of nutrients such as nitrogen and phosphorus. Organisms can also increase chemicalweathering rates through the release of CO a2d organic acids. Nutrient Transformations  Once nutrients have entered an ecosystem, they are subjected to further modification as a result of uptake by organisms and other chemical reactions. Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients.  The most important nutrient transformation is the decomposition of organic matter, which releases nutrients back into the ecosystem.  Detritus is an important source of nutrients, mainly nitrogen and phosphorous. Detritus includes dead plants, animals, and microorganisms, and egested waste products.  Nutrients in detritus are made available by decomposition, the process by which detritivores break down detritus to obtain energy and nutrients.  Decomposition releases nutrients as simple, soluble organic and inorganic compounds that can be taken up by other organisms.  Organic matter is derived primarily from plant matter (from above or below the surface). Fresh, undecomposed organic matter on the soil surface is known as litter (most abundant substrate for decomposition including leaves, stems, roots, dead animals). This litter is used by animals, protists, bacteria, and fungi. Animals such as earthworms, termites, and nematodes consume the litter, breaking it up into progressively finer particles. This fragmentation increases surface area, which facilitates chemical breakdown.  Mineralization: Chemical conversion of organic matter into inorganic nutrients (nutrients not associated with carbon). Heterotrophic microorganisms release enzymes into the soil that break down organic macromolecules.  Abiotic and biotic controls on decomposition and mineralization determine nutrient availability to autotrophs.  Rates of decomposition and mineralization are influenced by climate. Decomposition and mineralization rates are faster in warm, moist conditions. Soil moisture also influences the availability of water and O2to microorganisms. Dry soils may not provide enough water for detritovores to function. Wet soils have low O concentrations, which inhibits detritivores 2 (hypoxic conditions). We want warm temps with intermediate temperatures.  Not all of the nutrients released during mineralization become available for uptake by autotrophs. This is because some of the nutrients released are used by decomposers themselves. The amount of nutrients released during decomposition is dependent on the nutrient requirements of decomposers, and the amount of energy the organic matter contains.  The above factors can be approximated by the C:N ratio of detritus. This ratio represents energy content to nutrient content ratio. Organic matter/detritus with high C:N will have a low net release of nutrients because the heterotrophic microbes are limited more by nitrogen than by energy.  The properties of carbon compounds influence decomposition rates. Not all of the carbon in litter is equally available as an energy source for decomposers. The chemistry of that carbon determines how rapidly the material can be decomposed.  Lignin strengthens plant cell walls, and is difficult for soil microbes to degrade. It decomposes very slowly. The amount of lignin in cell walls varies with plant species. The rate of nutrient release from plant material containing high lignin (oak/pine) is lower than that of material with low lignin concentrations (maple/aspen)  Plant litter may co
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