BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall

963-5782

The joshua tree, a member of the lily family, is characteristic of the Mojave Desert

Lecture 18 Ecosystems

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Overview - Link to Course Objectives

Ecosystems

  • Include the abiotic factors affecting the total community living in a definable environment
    • Notice that here community refers to all of the species in the environment, not just a subset of similar species (like the bird community or the grazing community)
    • In practice, boundaries of ecosystems are often hard to define
        • Some edges are difficult to detect (we will deal with this in the next lecture)
        • Some species move between habitats and link ecosystems
  • Some ecosystem-level processes (= emergent properties of ecosystems)
    • Trophic structure (covered in community lecture)
    • Energy transfer between species and trophic levels
    • Material cycling (nutrient recycling) between species and trophic levels
  • Ecosystem measurements preview
    • Biomass
    • Total standing crop of any species or trophic level
      • standing crop is not the same as productivity, the new biomass added to a system in a given time, and large standing crops can be associated with either high or low productivity
    • Energy flow
      • Trophic level productivity (first level is Primary Productivity)
      • Does not always correlate with biomass, as small biomass can still produce large flows if individuals are productive
    • Nutrient flow
    • If level of a nutrient controls the productivity of a trophic level, it is termed a Limiting Nutrient
    • Flow rates of limiting nutrients important to both biomass and energy flow
    • Turnover rate important
      • Turnover is the ratio of total amount present to input/output rate
      • Low ratio means that nutrients do not reside in system or at the trophic level for long
      • High ratio means that nutrients reside in system or at the trophic level for long periods
      • The ratio gives the Residence Time
        • If the amount present is in some mass quantity (say, kilograms or tons or grams) and the rate is in mass per unit time (say grams per hour or tons per day) then a dimensional analysis of the ratio is, for example,
          • kilograms/(kilograms/day) - the kilograms cancel and the final dimension is days
          • 450 kilograms of Carbon/(0.5 kilograms of Carbon/day) = 900 days = Residence time for carbon in the system
        • this analysis will work if the amount present is in a density (e.g., grams/square meter) if the rate is in density units also (e.g., grams/square meter/day)

Some Physical Constraints on Flow (20.1)

  • First Law of Thermodynamics
    • Constrains the system to a complete accounting of all material and energy that enter or leave the system
      • If the system is in equilibrium, then net input = net output
      • Compounds that enter the system remain in the system until they or compounds made from them leave the system
        • This rule applies to those materials we intentionally or inadvertently add to systems, such as nutrients in our household waste or biocides used to control target organisms
  • Second Law of Thermodynamics
    • Constrains the system to a net loss (of heat energy) when energy is transferred between various components of the system
      • 10% Rule of thumb is that only 10% of the energy that leaves one trophic level is transferred to the next level and 90% is lost as heat
      • Constrains the system to a net increase in entropy unless sufficient energy enters the system to counter the tendency for entropy to increase

Primary Production (20.2)

  • Primary production of Terrestrial Ecosystems is often measured in dry weight (no water included) of producer tissue added to the system
    • This is often calculated as the Standing Crop of the system at the end of the growing season minus the Standing Crop of the system a the beginning of the growing season
      • Standing Crop is just what is seems to be, the total biomass present at some point in time
    • Problem - energy content of different tissues is not constant
    • Better expressed in terms of energy (calories or joules are most usual)
      • One can convert between biomass and energy through calorimetry
        • Calculates energy in a material by burning it and measuring the temperature change in the apparatus (usually one called a bomb calorimeter)
      • Of course, this does not measure total energy, but it does measure total potentially useful energy, since respiration will oxidize the tissue in any case
  • Gross Primary Production (GPP) is the amount of energy fixed by photosynthesis and chemosynthesis in an ecosystem
  • Net Primary Production (NPP) is GPP minus the energy lost to respiration by the producers
  • Net Change in Biomass (B) is NPP minus the energy lost to herbivory (G) and that lost to plant (algal) mortality (L)
    • B = NPP - G - L    or    NPP = B + G + L
  • Primary productivity in Aquatic Ecosystems often measured as O2 production in a pair of similar bottles, one transparent (Light Bottle) and the other opaque (Dark Bottle)
    • O2 levels are measured at the beginning and end of exposure of bottles to conditions (light, temperature) of interest and O2 production or consumption is the difference between the final and initial O2 concentrations
    • Light bottle represents NPP as the O2 produced by photosynthesis is reduced by the amount consumed by respiration
    • Dark bottle represents O2 consumed by respiration since no photosynthesis can take place
    • GPP = total O2 produced in light bottle - O2 consumed in dark bottle (remember, consumption will be a negative amount, so we are subtracting a negative here and GPP will be larger than NPP)
  • The relationship between productivity and standing crop
    • Primary Productivity and standing crop (total biomass) are not always positively correlated
      • Reefs - most productive but have a tiny standing crop because turnover is so high
    • Total continental standing crop versus total marine standing crop
      • Standing crop of marine producers is much smaller than terrestrial system producers with similar productivity, although the marine system supports lots of herbivores and carnivores
        • This means that turnover is much greater in marine systems
  • Production Efficiency is the proportion of energy fixed to that available (sunlight for photosynthesis)
    • GPP efficiency is never greater than 4%
    • NPP efficiency is usually 1% or less

Limits to Primary Production  (20.3, 20.4, 20.5, and 20.6)

  • Terrestrial ecosystems
    • In general, terrestrial ecosystem net primary production increases with increasing:
      • moisture availability as measured by evapotranspiration
      • length of growing season
      • temperature measured as mean annual temperature
      • limiting mineral nutrient availability (most often soluble organic nitrogen [NO3, NH4])
  • Aquatic Ecosystems
    • Aquatic ecosystem net primary production is affected by
      • Water Depth
        • photoinhibition occurs near surface
        • PAR absorption by the water and by solutes and suspended particles means that there is a depth at which PAR is reduced to the level which sustains just enough photosynthesis to offset the respiration needed for basal metabolism, so NPP is zero (this is the Compensation Depth)
        • Light Penetrates less in turbid waters
      • Temperature
        • Important to freshwater systems (turnover leads to recharging the epilimnion with nutrients)
        • Although deep ocean has constant 4°C temperature, surface temperatures (and photosynthetic zone) are much closer to air temperature and will vary with the season in temperate oceans
      • Nutrients
        • Nitrogen (NO3, NH4)
          • Very important in open ocean
        • Phosphorous
          • Important in freshwater systems
          • Eutrophication of lakes occurred by the addition of phosphate-detergents
              • Process of increasing the concentration of nutrients until unusual photosynthesizers (Cyanobacteria often) turn the water turbid and often decrease oxygen concentration
        • Iron is often limiting
          • Needed to make many of the electron-accepting members of electron-transport chains

Question: As CO2 builds up in the atmosphere, some predict that some dry terrestrial ecosystems will become more productive and that there will be little gains in aquatic systems. What is responsible for this? I'll give you a hint - it's not the CO2 concentration, but something affected by it.

Secondary Production (20.7, 20.8, 20.9, 20.10, 20.11)

  • Biomass produced by producers (primary production) can only move into two other components
    • Detritus feeders
      • Majority of plant/algal biomass dies and is eaten by detritivores
    • Herbivores (and algae feeders)
      • Important to plants and algae, as they eat living material
        • The importance of this pathway is that the size of the herbivore community may affect the size of the primary producer community
        • detritivores only get what dies and cannot affect the rate of dying, so they cannot have the same regulatory effect on the primary producer community as herbivores can
      • Terrestrial and Aquatic systems - the size of the herbivore community increases with primary productivity
  • Measuring secondary production
    • Gross assimilation (= Ingestion or the amount of plant/algal material eaten) is reduced by the amount of energy (and the compounds needed to supply it) used to maintain the herbivore/detritivore
      • Gross Assimilation = Assimilation + Waste (feces)
        • Gross Assimilation and Assimilation differ in that only the portion of ingested materials that crosses the digestive tract wall is Assimilation
      • Net Secondary Production = Assimilation - Maintenance Respiration
        • In endothermic vertebrates, maintenance is easiest to measure as the basal metabolic rate
        • Net production is measured as biomass increase, which can  be either the
            • Growth of individuals
            • Addition of new individuals
      • Assimilation Efficiency = Assimilation/Gross Assimilation
      • Secondary Production Efficiency is the ratio of Net Secondary Production/Assimilation
        • this is a measure of how much of the assimilated material or energy is used for production (and not lost to maintenance)
        • higher for ectotherms than for endotherms as the basal metabolic rate is lower in ectotherms and takes a smaller bite out of the assimilated materials
      • Consumption Efficiency is the ratio of Gross Assimilation of one trophic level/Net  Productivity of the next lower level
        • this efficiency measures the proportion of available productivity that gets eaten by the next trophic level
      • Lindeman Efficiency is different from the efficiencies discussed so far
        • the ratio of assimilation between trophic levels
        • Maintenance is included in both the denominator and numerator, and will cancel out if it is the same proportion for each trophic level
        • An alternative to Consumption Efficiency for comparing different levels of a trophic pyramid
        • Terrestrial ecosystems
          • Often near 10% for all but decomposers
          • Decomposers get what the herbivores and carnivores do not
          • Total energy flow not much affected by carnivores (who produce 10% of 10%, or only 1% of primary production)
        • Aquatic Ecosystems
          • Higher than for terrestrial systems, leading to rapid turnover in biomass
          • Often as high as 40 % for aquatic herbivores
          • Often as high as 80% for aquatic carnivores

Nutrient Cycling (21.1)

  • Remember that the input of material to a system will equal output
    • Immature, early succession systems may accumulate materials
    • Agricultural systems lose more that annual natural inputs can replace, and may lose materials (when no fertilization takes place)
  • Nutrients are cycled internally, so that they may be retained for a long time within a single trophic level or may move down the pyramid from consumers to producers
    • If you follow the nutrients, they will cycle within the system, a phenomenon called Nutrient Cycling, a form of recycling
  • Remember that ecosystems are always open
    • New materials arrive
    • System always leaks, so materials leave the system (no matter how slowly)
  • Many plants recycle internally (called retranslocation) by pulling essential nutrients (amino acids, lipids, minerals) out of leaves before they fall so that they can be re-used in next year's crop of leaves
    • store the nutrients in the stem (trunk) or underground (in roots or tubers)
    • smallest roots are also temporary and are regrown each year (called fine roots) and nutrients are retranslocated from them as well

Decomposition (21.2)

  • Decomposition is the complete breakdown of organic molecules into inorganic molecules
    • Decomposition is the role of the Detritivore Community
    • Most of the real chemical breakdown, in both aquatic and terrestrial systems, is done by bacteria and fungi, which secrete enzymes into the soil or water that can catalyze the breakdown
    • The overall process takes a large community of microbes, not just one species or even a few species
      • Hyphal fungi are well suited here
      • All fungi are heterotrophs and those with hyphal growth can penetrate into new habitat as they exhaust the present habitat's resources
  • Decomposition is an example of Succession, as one set of microbes finishes breaking down their substrates, another group that can break down the waste from the previous community replaces it
    • The first substrates to be consumed are proteins, nucleic acids, easily digestible lipids, and mono/oligosaccharides (glucose, disaccharides, etc.)
    • The second substrates to be consumed are the cellulose and hemicellulose components of plant cell walls as well as less digestible lipids
    • The last materials to be consumed are the Lignins
      • Lignins are a class of molecules with many variations but all are pholyphenolic compounds (a series of phenol rings bonded in various ways)
        • Lignins cross-link cellulose fibers to harden woody tissue
        • Breakdown of lignins yields little net energy and is done only by Basidiomycete fungi (no bacteria!)
  • Much of the above applies to algae (major primary producers in marine and most fresh water systems so they are the major producers of detritus) as well, except that:
    • There is no lignin
    • Some microalgae have mineral cell walls (silicon)
    • Cellulose is a major component of some algal walls but other fibrous carbohydrates (mannanes, xylanes, alginic acids) are common
    • One useful group of fibrous algal carbohydrates are the Sulfonated Polysaccharides from red algae
      • These are chains of galactose with suphonate groups attached and are a diverse set that includes
        • Carrageenan - a gelling agent in toothpaste, shoe polish, shampoo, and many foods (sauces, pates, and many ice creams - if your shake or ice cream does not melt into total soup - you've got carrageenan
        • Agar and Agarose - gels with the property of melting at much higher temperatures than the gelling temperature, agar is a mix of polysaccharides and agarose is purified to only one polysaccharide
  • Rate of decomposition (21.4)
    • Increases with quality of litter
      • High nitrogen/low lignin is high quality
      • Low nitrogen/high lignin is low quality
    • Temperature, moisture availability (terrestrial ecosystems) have their expected effects
    • Availability of oxygen increases rate of decomposition
      • Waterlogged soils are Anoxic and decomposition of cellulose and lignins is very slow
    • Net Primary Productivity and decomposition rates are usually positively related
      • High NPP rates result from high nutrient availability, which also means that the growth rate of the microbial decomposers is high
  • Decomposition and Mineral Nutrients (including Nitrogen) (21.5)
    • Mineralization is the process of converting organically bound N, P, and metal ions into inorganic (usually soluble) forms
      • Mineralization occurs as waste products are produced from microbial decomposition
      • Their solubility makes them vulnerable to Leaching from the soil
    • Immobilization is the binding of these nutrients into organic compounds after Uptake (absorption) by an organism
      • Plants immobilize nutrients when they absorb them and use them to synthesize new biomass
      • Microbial Decomposers will also immobilize nutrients when they produce new biomass
    • Because microbes involved in decomposition will both mineralize and immobilize nutrients, the portion of nutrients available for uptake by plants is Net Mineralization (= Microbial mineralization - Microbial immobilization)
  • Terrestrial Ecosystems (21.7)
    • Decomposition is a soil process - another reason to study soils and another reason why soils are so complex
    • Involves (for our purposes) two size-based animal groups:
      • Macrofauna - invertebrate detritivores (insects [springtails, larvae of many insect groups, termites, roaches and others], crustaceans [isopods and, in some situations, amphipods], mites, earthworms, snails, slugs, and millipedes
        • Macrofauna have an important role in the breaking down of detritus particles into smaller and smaller particles so that the detritus can be attacked by fungi and bacteria
        • Remember that the decomposition pathway supports more than one trophic level as decomposers have predators and parasites
      • Microfauna - bacteria, protists, small nematodes
  • Aquatic Ecosystems (21.6, 21.8, 21.9, 21.10, 21.11)
    • The macrofauna of freshwater system is similar to those found in soils except that there are no millipedes
      • In marine systems, there are no insects, the annelids are polychaetes, not oligochaetes (earthworms), and many more crustaceans are present
    • When particles size is small enough in aquatic systems, the particles can become suspended in the water column
      • These suspended particles are called POM (Particulate Organic Matter) and can be so small that they are essentially suspended indefinitely (like a solution)
    • In aquatic systems, the photic zone may be separated from the benthos (where most decomposition goes on)
      • In terrestrial systems this is not so, as plants have roots in the soil, the site of decomposition
      • Thus, in aquatic systems, the mineralization of nutrients may not get to the primary produces immediately
        • See the discussion of water column zonation (in both lakes and oceans) in Lecture 4
        • Nutrients are only mixed into the photic zone when thermocline (or halocline) breaks down and Turnover of the body of water can occur
      • Nutrient Spiraling - because running waters constantly move nutrients downstream, a nutrient molecule is not taken up at the same place where it was mineralized, but downstream from there
        • Nutrients in streams and rivers move downstream in a conceptual spiral of mineralization, then downstream, uptake, then downstream, mineralization again, then...
      • In oceans, mineralization occurs at great depths and comes to the surface (photic zone) in areas of Upwelling
        • Where winds push water away from continents, the water is replaced by water coming up from deeper layers
        • Upwelling results in increased primary productivity, which means increased secondary productivity (fish, in oceanic systems!)
          • El Niño (ENSO) happens when the winds fail off of the wester coast of South America, the surface water is not pushed west, and upwelling stalls.
          • Primary and secondary productivity crashes - a disaster for economies dependent on these fisheries

Biogeochemical Cycles

  • Nutrient cycles are the most thoroughly modeled ecological processes, also called  Biogeochemical cycles,  and are the paths of materials as they enter an ecosystem, pass through various components of the ecosystem, and finally exit the ecosystem
    • Local models - apply to one ecosystem
    • Global models - apply to the biosphere
    • Biosphere -- all earth is a single ecosystem
  • Nutrient pools - nutrients in a species, a trophic level, or in some abiotic portion of the system (such as the soil or the atmosphere)
    • Flux rates are the rates of flow from pool to pool
  • Sources - a pool that is outside of the system and is often considered to be inexhaustible, although the flow from the source may be very slow
    • Weathering of rock
    • Rainfall (local systems)
    • Groundwater flow (local systems)
    • Atmosphere (local systems)
  • Sinks - where materials go when they leave the system
    • Can often be sources too
    • Notice that atmosphere can be a pool in a global cycle, but a source in a local cycle
  • Inputs to Terrestrial Ecosystems
    • Absorption of gasses
    • Weathering
    • Dryfall - deposition of particles by wind
    • Wetfall - rain
  • Outputs from Terrestrial Ecosystems
    • Gas releases (CO2, H2S, N2)
    • Organic detritus lost from the system (into a lake or stream or windblown materials)
    • Leaching
    • Crop Harvests

Global Carbon Cycle

Sources, Outputs, and flows

Pools Flux pathways  
  Input Output
Producers photosynthesis respiration, herbivory, death
Consumers herbivory respiration, carnivory (between trophic levels), death
Decomposers death respiration
Detritus plants, animals consumption by decomposers
Fossil fuels anaerobic respiration human-caused combustion
Atmosphere/Water    
  • Ocean is generally considered to be in equilibrium with atmosphere, such that an alteration in one will inevitably lead to an alteration in the other
  • Since Carbon is the primary element fixed in biosynthesis, carbon cycles and energy flows are positively linked

Global Phosphorous Cycle

 

  • There is no atmospheric pool for P
  • P at the surface is always part of a phosphate ion
    • Pi, Po, Pp (Inorganic, Organic and Particulate Phosphorous)
    • The designation comes not from any change to the phosphate ion
      • Pi refers to phosphate in rocks or dissolved in water
      • Po refers to phosphate that is part of an organism or found in detritus
      • Pp is phosphate that is part of a particle that can be blown by the wind and can become part of dryfall
  • Input Sources -
    • weathering of some rocks (usually calcium containing rocks)
    • wetfall and dryfall deposition (terrestrial and aquatic)
    • River flow (into lake and marine systems)
  • Cycling goes on between uptake of phosphate by plants or algae, through the trophic pyramid, and it is released from dead tissue by decomposition
  • Output
    • River flow (terrestrial systems)
    • sediment deposition (aquatic systems)
    • wind
    • Crop harvesting

Global Nitrogen Cycle

 

note that the role of microorganisms is key

  • Fixation - some nitrogen is "fixed" (changed into a chemical form useful in living systems) by lightning and some is fixed by bacterial action (including the blue-green algae, the cyanobacteria)
  • nodulation a means of increasing rate of fixation in soils
  • Denitrification by other bacteria that use NH3 as an energy source and release N2 back into the atmosphere

Terms

Biomass, Energy flow, Nutrient flow, Turnover, Residence Time, Primary Production, dry weight, Standing Crop, calorimetry, bomb calorimeter, Gross Primary Production, Net Primary Production, Net Change in Biomass, Production Efficiency, Compensation Depth, Eutrophication, Secondary Production, Gross assimilation (= Ingestion), Assimilation, Net Secondary Production, basal metabolic rateAssimilation Efficiency, Secondary Production Efficiency, Consumption Efficiency, Lindeman Efficiency, Nutrient Cycling, Decomposition, Detritivore Community, Lignin, Anoxic, Mineralization, Immobilization, Net Mineralization, Macrofauna, Microfauna, POM (Particulate Organic Matter), Nutrient Spiraling, Upwelling, Biogeochemical cycle, Local model, Global model, Biosphere, Nutrient pool, Flux rate, Source, Sink, Fixation, Denitrification, Pi, Po, Pp (Inorganic, Organic and Particulate Phosphorous)

Last Updated March 15, 2007