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Lecture 20

Lecture 20 - Energy Flow and Food Webs

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Biology 2483A
Hugh Henry

LECTURE 20: ENERGY FLOW AND FOOD WEBS Introduction  The ecological roles of organism are determined by their trophic interactions – what they eat and what eats them  This determines the influence of an organism on the movement of energy and nutrients through an ecosystem  Trophic levels describe the feeding positions of groups of organisms in ecosystems  All organisms are either consumed by other organisms or enter the pool of dead organic matter (detritus). In terrestrial ecosystems, only a small portion of the biomass is consumed, and most of the energy flow passes through the detritus.  Dead organisms and feces are consumed by organisms called detritivores (primarily bacteria and fungi) in a process called decomposition  Detritus is considered part of the first trophic level, and detritivores are part of the second level  Much of the detritus in streams, lakes, and estuaries is derived from terrestrial organic matter  These external energy inputs are called allochthonous inputs  Energy produced by autotrophs within the system is autochthonous energy  Allochthonous inputs can be very important in stream ecosystems (up to 99.8%)  The importance of autochthonous energy inputs increases from the headwaters toward the lower reaches of a river  Water velocity decreases, and nutrient concentrations increase as you go downstream Energy Flow among Trophic Levels  The amount of energy transferred from one trophic level to the next depends on food quality and consumer abundance and physiology  The second law of thermodynamics states that during any transfer of energy, some is dispersed and becomes unuseable: energy will decrease with each trophic level  A trophic pyramid portrays the relative amounts of energy or biomass of each trophic level  Some of the biomass at each level is not consumed, and some of the energy is dispersed in the transfer to the next level  In terrestrial ecosystems, energy and biomass pyramids are similar because biomass is closely associated with energy production (A)  In aquatic ecosystems, the biomass pyramid may be inverted. The primary producers are phytoplankton with short life spans and high turnover (B)  Inverted biomass pyramids are more common where productivity is lowest, such as nutrient-poor regions of the open ocean  Phytoplankton turnover is high, associated with high growth rate and short life span compared with phytoplankton of nutrient-rich waters  Herbivores on land consume a much lower proportion of autotroph biomass than herbivores in most aquatic ecosystems  On average, about 13% of terrestrial NPP is consumed; in aquatic ecosystems, an average of 35% NPP is consumed  There is a positive relationship between net primary production and amount of biomass consumed by herbivores. This suggests that herbivore production is limited by the amount of food available  Why don’t terrestrial herbivores consume more of the available biomass? o Several hypotheses have been proposed: 1. Herbivores are constrained by predators, and never reach carrying capacity Predator removal experiments support this hypothesis in some ecosystems. 2. Autotrophs have defenses against herbivory, such as secondary compounds, spines, etc. Plants of resource-poor environments tend to have stronger defenses than plants from resource-rich environments 3. Phytoplankton are more nutritious for herbivores than terrestrial plants Terrestrial plants have structural components such as wood, which have few nutrients Freshwater phytoplankton have carbon:nutrient ratios closer to those of herbivores than to those of terrestrial plants  Trophic efficiency – amount of energy at one trophic level divided by the amount of energy at the trophic level immediately below it  Trophic efficiency incorporates three types of efficiency: o Proportion of available energy that is consumed (consumption efficiency) o Proportion of ingested food that is assimilated (assimilation efficiency) o Proportion of assimilated food that goes into new consumer biomass (production efficiency)  Consumption efficiency is higher in aquatic ecosystems than in terrestrial ecosystems  Consumption efficiencies also tend to be higher for carnivores than for herbivores  Assimilation efficiency is determined by food quality and the physiology of the consumer  Food quality of plants and detritus is low because of complex compounds such as cellulose, lignins, and humic acids that are not easily digested, and low concentrations of nutrients such as nitrogen and phosphorus  Animals have carbon:nutrient ratios similar to the animals consuming them  Assimilation efficiencies of herbivores and detritivores are 20-50%; carnivores are about 80%  Endotherms digest food more completely than ectotherms and thus have higher assimilation efficiencies  Some herbivores have mutualistic symbionts that help them digest cellulose  Ruminants (cattle, deer, camels) have a modified foregut with bacteria and protists that break down cellulose. They have higher assimilation efficiencies than other herbivores  Production efficiency is strongly related to the thermal physiology and size of the consumer  Endotherms allocate more energy to heat production, and have less for growth and reproduction than ectotherms  Body size affects heat loss in endotherms. As body size increases, the surface area-to-volume ratio decreases  A small endotherm (e.g., a shrew), loses a greater proportion of its heat across its body surface than a large endotherm, such as a grizzly bear, and will have lower production efficiency  Change in food quantity or quality impacts trophic efficiency and can determine consumer population size  Steller sea lion populations in Alaska declined by about 80% over 25 years  Smaller body size and decreased birth rates suggested food quantity or quality might be a problem  Prey quantity was not declining but the sea lions had shifted from a diet of mostly herring (high in fats) to more cod and pollock (half as much fat)  The change in diet reflected a shift in the fish community  Decline in Steller sea lion populations may also be related to trophic interact
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