Ecosystem Metabolism IISecondary ProductionChapter 26

Only 5% to 20% of energy passes between trophic levels.Energy reaching each trophic level depends on:net primary production (base of food chain)efficiencies of transfers between trophic levelsPlant use between 15% and 70% of light energy assimilated for maintenance.Herbivores and carnivores expend more energy on maintenance than do plants: production of each trophic level is only 5% to 20% that of the level below it.

Ecological pyramid of energy

Ecological EfficiencyEcological efficiency (food chain efficiency) is the percentage of energy transferred from one trophic level to the next:range of 5% to 20% is typical, as weve seento understand this more fully, we must study the utilization of energy within a trophic level

Intratrophic Energy TransfersIntratrophic transfers involve several components:ingestion (energy content of food ingested)egestion (energy content of indigestible materials regurgitated or defecated)assimilation (energy content of food digested and absorbed)excretion (energy content of organic wastes)respiration (energy consumed for maintenance)production (residual energy content for growth and reproduction)

Egested Energy in dung

Fundamental Energy RelationshipsComponents of an animals energy budget are related by:ingested energy - egested energy = assimilated energyassimilated energy - respiration - excretion = production

Assimilation EfficiencyAssimilation efficiency = assimilation/ingestion primarily a function of food quality:seeds: 80%young vegetation: 60-70%plant foods of grazers, browsers: 30-40%decaying wood: 15%animal foods: 60-90%

Net Production EfficiencyNet production efficiency = production/assimilationdepends largely on metabolic activity:birds:

Production Efficiency in PlantsThe concept of production efficiency is somewhat different for plants because plants do not digest and assimilate food:net production efficiency = net production/gross production, varies between 30% and 85%rapidly growing plants in temperate zone have net production efficiencies of 75-85%; their counterparts in the tropics are 40-60% efficient

Low production efficiencies due to high energy costs

Detritus Food ChainsEcosystems support two parallel food chains:herbivore-based (relatively large animals feed on leaves, fruits, seeds)detritus-based (microorganisms and small animals consume dead remains of plants and indigestible excreta of herbivores)herbivores consume:1.5-2.5% of net primary production in temperate forests12% in old-field habitats60-99% in plankton communities

Detritus Food Chains

Exploitation EfficiencyWhen production and consumption are not balanced, energy may accumulate in the ecosystem (as organic sediments).Exploitation efficiency = ingestion by one trophic level/production of the trophic level below it.To the extent that exploitation efficiency is

Energy moves through ecosystems at different rates.Other indices address how rapidly energy cycles through an ecosystem:residence time measures the average time a packet of energy resides in storage:residence time (yr) = energy stored in biomass/net productivitybiomass accumulation ratio is a similar index based on biomass rather than energy:biomass accumulation ratio (yr) = biomass/rate of biomass production

Aquatic systems-rapid cycling

Biomass Accumulation RatiosBiomass accumulation ratios become larger as amount of stored energy increases:humid tropical forests have net production of 1.8 kg/m2/yr and biomass of 43 kg/m2, yielding biomass accumulation ratio of 23yrratios for forested terrestrial communities are typically >20 yrratios for planktonic aquatic ecosystems are

Biomass Accumulation Ratios

Residence Time for LitterDecomposition of litter is dependent on conditions of temperature and moisture.Index is residence time = mass of litter accumulation/rate of litter fall:3 months in humid tropics1-2 yr in dry and montane tropics4-16 yr in southeastern US>100 yr in boreal ecosystems

Ecosystem EnergeticsComparative studies of ecosystem energetics now exist for various systems.Many systems are supported mainly by autochthonous materials (produced within system).Some ecosystems are subsidized by input of allochthonous materials (produced outside system).

Autochthonous versus Allochthonous ProductionRoot Spring study showed that assimilation of energy by herbivores (0.31 W per m2) exceeded net primary production (0.09 W per m2) - balance represents energy subsidy.autochthonous production dominates in large rivers, lakes, marine ecosystemsallochthonous production dominates in small streams, springs, and caves (100%)

Cedar Bog LakeLindemans study of a small lake in Minnesota uncovered surprisingly low exploitation efficiencies:herbivores: 20%carnivores: 33%residual production of plants and herbivores accumulates as bottom sediment

Cedar Bog Lake

An ecological pyramid of energy. The breadth of each bar represents the net productivity of a trophic level in the ecosystem. For this particular system, ecological efficiencies are 20%, 15%, and 10% between levels, but these values differ widely in different communities. Not all components of food can be assimilated. The undigested fibrous plant material in this elephant dung represents egested energy.High rates of energy metabolism usually result in low production efficiencies. Because this hummingbird expends so much energy in hovering flight, less than 1% of its assimilated energy is converted to growth and egg production over its lifetime. Plant litter forms the basis for detritus food chains in forest soils. (left) In Western Tennessee, decomposition of litter proceeds relatively slowly and soils have a proportion of organic matter. (right) In Panama, leaves and wood decompose rapidly under the tropical conditions, leaving exposed mineral soil on the forest floor. Copepods and other zooplankton cycle the energy and nutrients for the algal food very rapidly.Biomass accumulation ratios for primary producers vary among ecosystemsCedar Bog Lake in Minnesota has been the site of important studies on energy flow in aquatic ecosystems.