Chapter 54: Ecosystems

Ecosystem = all the organisms in a community plus the abiotic factors

can be seen as localized or global

Two new emergent characteristics:

1. chemical cycling (mostly recycled within the ecosystem)

2. energy flow (cannot be recycled)

Energy (sunlight) + plants = photosynthetic carbon fixation (chemical energy) used by heterotrophs, heat (energy) produced and lost to atmosphere

constantly need new source (sun, geothermal vents, chemical sources)

heterotrophic energy flow the basis for food chains, food webs (Fig. 54.1, 54.2)

Species in an ecosystem are divided into different trophic levels:

1. primary producers (autotrophs)--support all others; usually green plants; also algae, cyanobacteria (phytoplankton)

2. primary consumers (herbivores)--eat the producers; include insects, molluscs, grazing animals, birds, zooplankton

3. secondary consumers--eat the herbivores eg. spiders, frogs, carnivores (birds, fish mammals)

4. tertiary, quaternary consumers possible in richest, most species-diverse ecosystems

5. detritivores--derive energy from dead organic wastes; decomposers such as bacteria, fungi, earthworms, crayfish, millipedes, bald eagles!

I. Energy flow based on the photosynthetic equation:

6 CO2 + 6 H2O n C6H12O6 + 6O2

producers make organic molecules

heterotrophs, detritivores break these down to make ATP, with heat of respiration (Rs) as a byproduct

PSN activity sets the "spending limits" for an entire ecosystem

Intensity of solar energy varies with latitude, tropics receiving largest amount

varies locally and seasonally with cloud cover, dust, pollutants

only 1% of light reaching plants converted to PSN energy (Fig. 54.5)

this total Photosynthetic output = 170 billion tons of organic matter/year

primary productivity of an ecosytem = rate at which light E converted to chemical E (organics)

a. gross primary productivity (GPP) = total amount

b. net primary productivity (NPP) = GPP - Rs (respiratory needs for plant maintenance)= net amount available to consumers

50-90% of GPP left for NPP, depending on plant's efficiency

lower for large producers eg. trees w/ elaborate non-PSN structures

Productivity of different ecosystems( Fig. 54.3)

also affected by total area as % of total earth's surface

oceans = 25 % highest due to large surface area

tropical rainforests = 22% second highest due to intense solar radiation, rainfall

factors limiting productivity:

type of ecosystem

seasonal changes eg. temperature, rainfall, sunlight available (all available to larger extent in tropics)

supply inorganic nutrients

plants can remove these faster than replaced, yields slowing or stoppage of growth

limiting nutrient = one in shortest supply eg, N, P, K

only addition of this one can restart growth

productivity in seas:

highest near shore, shallow water, upper levels closest to light, heat

N, P usually more available at lower water levels (upwelling currents yield greater phytoplankton growth, Antarctic seas often more productive due to currents)

biannual turnover of water levels in lakes serves same purpose

Energy transfer and ecological pyramids (Fig. 54.5)

secondary productivity = rate at which consumers convert food eaten into own biomass

declines as move up the pyramid, due to energy loss as heat at each trophic level

also, not all consumed at next level (only 4% of grasslands consumed by insects)

of J consumed, app. 2/3 go to cellular respiration (maintenance); only 1/3 to producing new biomass--growth, producing offspring, available to next level of consumers

carnivores more efficient at energy to biomass conversion (meat more easily digested than vegetation); also consume more E in maintenance, esp. endotherms

ecological efficiency = net productivity level B/net productivity level A

10% a common value; 90% lost at each step

efficiencies very low, require broad base of pyramid to sustain any growth at upper levels

biomass pyramids = standing crop biomass (dry weight) of each trophic level--usually very bottom-heavy (Fig. 54.6)

may be inverted eg. aquatic phytoplankton vs. zooplankton (phyto- consumed so quickly, never reach large #'s ie. high turnover rate

multiplicative energy loss limits biomass of top levels carnivores , restricts food webs to 3-5 levels (Fig. 54.7) (1/1000 of PSN makes it to top consumers)

reason why lions, eagles, killer whales have no predators--too few of them to support another level

(pitch for vegetarian diet: more efficient to eat primary producers directly than their predators)

II. Chemical Cycling (Nutrient Cycling)--Fig. 54.8

chemical elements more likely to be limiting than sunlight

recycling of chemical elements from organic wastes and dead organisms by detritivores

biogeochemical cycles = biotic + abiotic factors of these cycles

A. global hydrologic cycle (Fig. 54.9)

evaporation over water, oceans > precipitation

net movement of water vapor over land via air currents

precipitation >evap. over land (90% due to plant transpiration), contributes to surface and ground water flow back to oceans

B. gaseous elements C, H, O, N, S take part in global cycles

C. less mobile elements P, K, Ca, Mg etc. are localized, 4 major reservoirs:

· living organisms--nutrients available

· fossilized deposits coal, oil, peat--unavailable to most

· soil, water, air--available

· minerals in rock--unavailable until weathering, erosion, lichens "unlock" these slowly

A. Carbon Cycle--Fig. 54.10

PSN + respiration = reciprocal processes, cycle CO₂ through atmosphere

CO₂ enters plants via stomata in leaves

[CO₂] atm fairly low app. 0.03%

recycled quickly due to high plant demand

at any one time, app. 1/7 of CO₂ in atmos. taken in by plants, and app. same amount returned via respiration

some C tied up longer in durable organics eg. wood

coal and oil result when decomposition slower than deposition; only burning or decomposition return C to atmosphere

***deposits (Carboniferous) that took app. 100 million years to form are being released by our industrialized civilization in app. 200 years; combustion of fossil fuels increases global CO_2, disrupts equilibrium

seasonal [CO₂] variations slight--in Northern hemisphere (more land mass)

lower in summer, due to greater PSN demand

higher in winter, when amount respired outstrips PSN

Aquatic environments, little free CO2 in solution:

a. . H2O + CO2 n H2CO3

b. H2CO3 + CaCO3 (from limestone abundant in oceans)n Ca2+ + 2 HCO3-

c. 2HCO3- n 2H+ + 2 CO32-

bicarbonate serves as CO2 reservoir, more shifts back to CO2 as used up in PSN

or PSN autotrophs may use HCO3- directly as a Carbon source

overall, 50X greater [CO2] in oceans than in atmosphere

may be able to buffer some human excesses in C release from fossil fuels

atmospheric [CO₂] steadily increasing since Industrial Revolution

burning of fossil fuels, deforestation and burning of wood

from 274 ppm in 1854 to 316 ppm in 1958 (14 % increase in 100 years)

to 351 ppm in 1993 (10% increase in 25 years!)

affects plants by raising PSN activity, more growth esp. in C₃ plants (more likely to be limited by scarcity of CO2 than C₄ plants)

causing greenhouse effect--reflection of heat (infrared radiation) back to earth by CO2 and water vapor in air, thereby retaining more solar heat

temperature of planet would be -18 C without it

B. Nitrogen Cycle--Fig. 54.11

earth's atmosphere is app. 80% nitrogen gas (N2)--inaccessible to plants, animals

only N2 fixing bacteria can reduce to NH3 (both free-living, symbiotic Rhizobium spp., cyanobacteria--aquatic)

also some small amount fixed by lightning in atmosphere

industrial fixation to manufacture nitrogenous fertilizers now may be tipping the balance, causing excess nitrogen levels, esp. in some aquatic ecosystems

plants can utilize NH3 directly - most NH3 undergoes nitrification by aerobic bacteria:

NH3 to NO2- to NO3-, then absorbed by roots, incorporated into amino acids, proteins

denitrification-- some bacteria obtain O2 from conversion of NO3- to N2

ammonification--organic nitrogen decomposed to NH3 by prokaryotes, fungi (detritivores) from their diet

most nitrogen cycled in natural systems from soil and water, not from atmosphere (NH₃ to NO3- to NH3 ) via plants, animals, detritivores

C. Phosphorus Cycle--Fig. 54.12

no atmospheric component

only one important form (PO43-) from weathered rock to soil

bound in soil by humus and soil particles, makes recycling difficult

erosion leads to run-off, leaching, deposition as ocean sediments, formation of new rock to complete the geologic cycle

nutrient cycling times

1. much faster in tropics--warmer, wetter, high demand leads to rapid decomposition and recycling, most tied up in living plants ( only 1-2% of total leaf litter, so soils appear impoverished

2. temperate forests leaf litter = 5-20% of organic matter due to slower decomp. so soils appear richer