Ecosystem Function

Nutrients & Nutrient Cycling

Ecosystem function and nutrients

• Elements that are required for the growth, maintenance and reproduction of organisms

• Unlike energy, which eventually leaves the system as heat, nutrients are recycled over and over again

• Phosphorous (P), carbon (C) and nitrogen (N) are particularly important for life• cycling of these three elements is one of the

major functions of ecosystems

Phosphorous

• Major constituent of nucleic acids, cell membranes, energy transfer systems, bones and teeth

• Limits plant production in many aquatic habitats

• Influx of P into rivers and lakes from sewage and agricultural runoff artificially stimulates production in these systems, disrupts ecosystem balance

Phosphorous

• Phosphorous cycle is chemically uncomplicated, contains fewer steps than other cycles• P does not generally undergo oxidation-

reduction reactions• No significant atmospheric pool – most P

occurs in mineral deposits and marine sediments

Phosphorous

• Phosphorous is slowly released to terrestrial and aquatic ecosystems through the weathering of rock

• Plants assimilate P from soil, often with help from mycorrhizae, and the P is recycled through the ecosystem

• Unassimilated P is washed into oceans, where it remains in dissolved form until it is finally deposited in ocean sediments

Phosphorous Cycle

Nitrogen

• Ultimate source of nitrogen for life is molecular nitrogen (N2) in the atmosphere

• Atmospheric nitrogen represents largest pool of nitrogen on earth

• A tiny fraction of this N2 dissolves into water or is converted by lighting into readily assimilated forms

• Most nitrogen enters the biological pathways of the nitrogen cycle through fixation by microorganisms

Nitrogen

• Although nitrogen fixation / denitrification account for only a small fraction of the earth’s total nitrogen flux, nearly all the nitrogen that is cycled through biological systems enters the cycle through nitrogen fixation by microorganisms

• Nitrogen cycle is more complex because nitrogen atoms can take on a greater variety of oxidized and reduced forms

Nitrogen Cycle – Fixation & Assimilation

• Specialized bacteria reduce atmospheric nitrogen to biologically useful forms with the enzyme nitrogenase (only works under low oxygen concentrations)

• Fixed nitrogen is available to plants as either ammonium (NH4

+) or as nitrate (NO3-)

• Plants further reduce these compounds into an organic form

• This organic nitrogen is the most reduced form, with the highest potential chemical energy

Nitrogen Cycle - Ammonification

• Organic nitrogen compounds are used by plants (and their consumers) to construct proteins

• Proteins are eventually metabolized, and excess nitrogen is excreted into the environment as waste – this step in the nitrogen cycle is called ammonification (carbon is oxidized in this step, but nitrogen is not)

Nitrogen Cycle – Nitrification & Denitrification

• The ammonia excreted as waste can be further metabolized by microorganisms in the soil…• Nitrification is the oxidation of ammonia to

nitrite, and then from nitrite to nitrate:NH3 -> NO2

- -> NO3-

• Under anaerobic conditions, reduction is thermodynamically favored, and nitrate may be reduced to nitric oxide This process, called denitrification, results in the loss of nitrogen from soils (as a gas):NO3

- -> NO2- -> NO …and… NO -> N2O -> N2

Nitrogen Cycle

Carbon

• C is the foundation of all organic molecules• Atmospheric C compounds such as carbon

dioxide and methane significantly influence global climate

• Three classes of processes cause carbon to cycle through ecosystems:• photosynthesis/respiration• exchange of CO2 between atmosphere and

oceans• precipitation of carbonate sediments in

oceans

Carbon Cycle – Photosynthesis & Respiration

• about 85 GT of carbon is assimilated by photosynthesis each year

• approx. 2,650 GT carbon present in total organic matter present in biosphere (in living organisms as well as organic detritus and sediments)

• average residence time of carbon in organic matter (i.e., time from photosynthetic assimilation to release as CO2 by respiration) is:

2,650 GT / 85 GT per year = 31 years

Carbon Cycle – Ocean-Atmosphere Exchange

• CO2 dissolves readily in water; ocean contains about 50x as much CO2 as the atmosphere

• CO2 continuously exchanged across the ocean/atmosphere boundary – total amount in ocean remains constant until new CO2 enters system from a source outside the atmosphere-ocean system (e.g., burning of fossil fuels)

• Ocean provides an important sink for excess CO2 produced by human activities

Carbon Cycle – Precipitation of Carbonates

• Once it dissolves in water, carbon dioxide forms carbonic acid, which readily dissociates into hydrogen, bicarbonate, and carbonate ions

• When present, calcium equilibrates with the carbonate ions to form calcium carbonate

• Calcium carbonate has a low solubility, so it readily precipitates out of the water column to form sediments

Carbon Cycle

Carbon Cycle & Ancient Atmosphere

• Geologists and paleoecologists can estimate the levels of atmospheric carbon dioxide throughout the earth’s history• burial of organic matter• precipitation of carbonates in marine

sediments

Carbon Cycle & Ancient Atmosphere

Cycle of nutrients through terrestrial ecosystems

weathering & atmosphericnutrient input

Inorganic soil nutrients

plant biomass

plant detritus

groundwater & stream runoff

Nutrient Inputs

• new inorganic nutrients are added to terrestrial ecosystems through:• weathering of the bedrock underlying the

soil • atmosphere - as particulates, ions dissolved

in precipitation, or molecular elements (N2) assimilated by bacteria

Nutrient regeneration in terrestrial ecosystems

Inorganic soil nutrients

plant biomass

plant detritus

mineralization via decomposition

• mineralization = conversion of organic molecules to inorganic nutrients

• decomposition usually the slowest (rate limiting) step in nutrient cycling

Nutrient regeneration in terrestrial ecosystems

Inorganic soil nutrients

plant biomass

plant detritus

mineralization via decomposition

• Uptake of inorganic nutrients by plants and the decomposition of detritus by microorganisms are both biochemical processes influenced by temperature, moisture, and chemical composition of the environment

Nutrient regeneration in terrestrial ecosystems

• Chemical characteristics of leaf litter that influence decomposition rate include phosphorous concentration, nitrogen concentration, carbon:nitrogen ratio, and lignin content

• Increased moisture and temperature also tend to increase the rate of decomposition

Nutrient regeneration in terrestrial ecosystems

• decomposition, assimilation rates very high in tropical forests (very little organic matter / nutrients in soil)

• decomposition very slow in boreal forests – dead plant matter builds up on ground and forms thick deposits

Nutrient cycling in aquatic ecosystems

• in rivers, lakes & oceans, organic matter sinks to the bottom and accumulates in deep layers of sediment deposits

• nutrients are regenerated and returned to zones of productivity (near the surface where there is light for photosynthesis)

• nutrient cycling is generally slower in aquatic systems, because nutrients must travel great distances to reach photic zone for assimilation

• anaerobic nature of many sediments also slows decomposition and changes the biochemical pathways by which nutrients are regenerated

Nutrient cycling in aquatic ecosystems

• mixing of bottom and surface waters is necessary for aquatic ecosystems’ productivity

• stratification hinders nutrient cycling in aquatic ecosystems by preventing the mixing of nutrient-rich deep water with surface waters where phytoplankton live

How does ecosystem function respond to natural and human

perturbations?

Effects of disturbance on nutrient cycling

• disturbance increases nutrient losses from ecosystems

Effects of disturbance on nutrient cycling

• nutrient loss post-disturbance is less severe in cold and/or dry climates

• nutrient loss after disturbance is most rapid in warm, moist conditions that promote rapid decomposition• assimilation of nutrients by plants is critical for

nutrient retention in these ecosystems• luckily, these are also environmental

conditions that would promote more rapid plant growth, so effect should be mitigated in nature

Ecosystem nutrient changes with succession

• as vegetation develops, soil nitrogen content tends to increase

• organic carbon content will also increase• fraction of phosphorus that is biologically

available tends to decrease (assimilated and retained by plants)

Ecosystem nutrient changes with succession

Effects of excess nutrients

• ecosystem function and structure can be disrupted by excess nutrient input

• in terrestrial ecosystems, excess nutrients generally lead to reduced species richness, simplified community structure• favor a few competitively dominant species

Effects of excess nutrients

• in aquatic environments, excess nutrients can come from natural or human sources• fertilizers, runoff, sewage• excessive upwelling

Effects of excess nutrients

• in aquatic ecosystems, population booms and subsequent death of primary producers (e.g., algae, phytoplankton) leads to huge detritus inputs…decomposition of dead organisms then strips waters of their oxygen

• at extreme – waters can become hypoxic (dead zones where oxygen content too low to support life)

Effects of excess nutrients – hypoxic dead zones

Effects of excess nutrients – hypoxic dead zones

Effects of excess nutrients – hypoxic dead zones

links between nutrient cycles

• link between N and C cycles is highly dependent on the global distribution of land cover:• photosynthesis requires N• historically, N fairly limiting in most

ecosystems• fixation and storage of C has been closely

linked to the N cycle

finally…

• Competition• Consumer/resource interactions• Facilitation• Community diversity & succession• Community structure• Ecosystem function

(…lectures 9-14)

Competition

• Niche concept, fundamental vs. realized niche• Relationship between intraspecific competition

and K• Evolutionary effects of intraspecific competition

on populations• Competitive exclusion principle• Predictions of Lotka-Volterra model for

interspecific competition• Interspecific competition and species

distributions

Consumer-Resource (exploitative) Interactions

• Major types of consumer-resource interactions and their common features

• predation, parasitism, herbivory…• Consumers can control distribution and abundance of

their resource species• Predator-prey cycles: why they occur, what they

represent• Lotka-Volterra model for cyclic predator-prey

interactions: general features, simplifying assumptions• Stabilizing factors that allow predator-prey cycles to

persist• Alternative stable states for predator-prey cycles

Facilitation

• differences between mutualism (obligate or facultative) & commensalism; what is meant by symbiosis (examples of each)

• different ways in which facilitation can benefit one or both species (release from predation, competition or stress, energy or nutrient access, transport, etc.) and examples

Community Diversity & Succession

• typical patterns of species abundance in communities (lognormal)

• measures of community diversity – species richness and abundance, Shannon-Wiener index, rank abundance curves

• factors controlling community diversity: environmental heterogeneity, disturbance

• intermediate disturbance hypothesis

Community Diversity & Succession

• types of succession (primary, secondary)• community stages (pioneer, climax)• different succession models, their predictions,

examples of empirical evidence (if available)

Community Structure

• what community structure is, how it is assessed• trophic levels, feeding interactions, food webs• effects of species richness on food web

complexity• effects of food web/trophic interactions on

species richness• keystone species, examples• trophic cascades• effects of introduced species on trophic structure• energy flow and trophic structure / energy

pyramids

Ecosystem Function

• differences / connections between nutrients, energy & productivity

• basics of P, N, & C cycles• cycling of nutrients through aquatic and

terrestrial ecosystems, nutrient inputs, regeneration, and outputs

• effects of disturbance on nutrient cycles• ecosystem effects of excess nutrients

further suggestions

• time management – budget time for entire exam prior to beginning

• be precise (avoid the shotgun approach)• be accurate

Remember the big picture

• water• energy• resources• reproduction• time & equilibrium• economics

Remember the big picture

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