Ecology Biology 3500
COMMUNITIES and ECOSYSTEMS UNIT                                               Dr. Adams

Chapter 19:  Nutrient Cycling and Retention
  Nutrient Cycles
   
Energy makes one pass (flows) through the ecosystem; nutrients, however, are recycled
   
Cycles exist for ANY atom/mineral necessary for survival by any organism, but three are
        especially prominent in their importance/effects:  P, N, and C.
    For each -- need to know basic uses in organisms, basic components of the cycle (see Figs.
        19.1 - 19.4), basic sources/pools (and sinks), basic entry/exit points into/from cycle.
   
Other important aspects:
        P cycle:  no major atmospheric pool, largest pools in rocks (available through weathering) and
            dissolved phosphate ions in water (more readily available); although lots in soil, mycorrhizae
            play major role in uptake into organisms. In oceans, organisms, make use of dissolved ions
            until some settle in sediments.  To return to usable pool, must be uplift and weathering again.
            Residence time in the biosphere is likely in 1000's of years.
        N cycle:  although large atmospheric pool, only a few organisms (cyanobacteria, free living soil
            bacteria, and certain mutualistic root-associated bacteria) can break molecular N2 bonds,
            and fix nitrogen.  Lightning, interestingly enough, can also break N2 bonds and fix nitrogen.
            Upon death of organisms, their nitrogen is often released during decomposition (of proteins/
            amino acids particularly) as ammonia (a process called ammonification).  Ammonia can be
            converted by other bacteria to nitrates (nitrification).  Nitrogen can exit the organic pool by
            bacterial denitrification, releasing N2 back to atmosphere.  Residence time -- 625 yrs.
        C cycle:  large atmospheric pool, continuously replenished by cellular respiration and refixed by
            photosynthesis; dissolved carbonates may end up out of reach in sediment/rocks (until
            weathered).   Atmospheric C also influences climatic conditions.
        Don't forget the H20 cycle!

  Rates of Decomposition:  Decomposition -- breakdown of organic matter (with CO2 release).
    Mineralization -- conversion of organic forms of nutrients into inorganic (during decomposition),
        which makes these nutrients available to be absorbed by the producers.
    Influenced directly by temp., moisture, and surrounding chemical environment.
    Examples:
      Terrestrial
        Mediterranean woodlands in SW Spain:  wetter-- more decomposition (Fig. 19.5).
            differences between species of tree leaves:
                 best predictor of loss was the toughness/%N ratio (see equation page 415 & Fig. 19.6)
        Temperate woodlands:  compared New Hampshire to North Carolina:
            best predictor was % lignin:% N ratio, with higher decomposition in general for NC,
            probably because of higher temps (but also possibly higher N availability).
        And, in general, where productivity was measured to be higher (with higher AET), no surprise
            that decomposition higher in same areas (warm and moist) (see Fig. 19.8).
              So, do tropical rain forests have higher decomposition rates than temperate??
        So, does soil composition within climatic areas influence decomposition rates?  Indirectly, yes.
            Since richer soil typically means higher productivity, it also means higher rates of litter fall and,
            in turn, higher decomposition rates.  I would call this faster turnover, or simply cycling, rate.
      Aquatic -- influenced by temperature and chemical surroundings
        In aquatic systems, higher lignin content slows, higher nitrate content increases, and, to a point,
            higher phosphorus concentration increases decomposition rates.

  Nutrient cycling/retention and organisms
      Aquatic
-- Streams/rivers
          In moving water, nutrient spiraling occurs (very little cycling in place).
            Spiraling length and retentiveness of stream ecosystems -- effects of organisms:
                Example:  stream invertebrates in Arizona's Sycamore Creek; mayflies and chironomids.
                    Increasing percent available nitrogen consumed increases retentiveness.
      Terrestrial  -- gophers, prairie dogs, ground squirrels (burrowing mammals)
          Remember increase in plant diversity?  These organisms: 
            Alter the N cycle -- bring nitrogen poor soil to surface
            Increase light penetration to the surface of the ground   
            At colonies, reduce overall plant biomass, but increase nitrogen content of younger leaves
                (may explain why bison like feeding close to prairie dog colonies)
          In general, grazing speeds the rate of nutrient cycling in these ecosystems (DUH!)
        Plants
          Introduced Acacia and alterations of the S. African Fynbos (high diversity, low soil fertility)
            Acacia litter has 10X the nitrogen content than some of the native litter, likely because it is
                leguminaceous with mutualist bacterial nitrogen-fixers
          In Hawaii, Myrica trees (from islands near the Iberian peninsula) have similarly increased
             nitrogen availability in invaded Hawaiian ecosystems.

  Disturbance and Nutrients
      Not surprisingly, clear cutting of forested plots increases nutrient loss from the ecosystem
        For example, loss of nitrates may increase as much as 10X (see Fig 19.23) upon clear-cutting
        Nitrogen losses are greatest from altered forests where warm temps and high precipitation
            promote faster decomposition.  *On the flip side, in these ecosystems, rapid regrowth of
            plants may help to reestablish control of nitrogen loss following the original disturbance.
      From stream ecosystems, significant nutrient loss is typically episodic, associated with periodic
        flooding.  For example, in years of high stream flow (in Bear Brook, NH), streams lose more
        phosphorus than erodes into the stream; in years of low stream flow, more of the phosphorus
        "stays put".  Inputs are in the form of dissolved phosphorus, and phosphorus in fine/coarse
        particulate matter (including decaying organics) that "move" into the stream in relatively equal
        amounts; exports are any phosphorus that gets removed with the flow, and seems to be
        dominated by fine particulates, suggesting that physical and biological processes increase P in
        fine particulate form.  When the details were analyzed more carefully, major losses were
        associated with specific events (strong storms/spring snow melt), and major inputs were during
        autumn leaf fall.

Chapter 20 -- Succession and Stability
    Know definitions of:  Succession, including primary/secondary succession, pioneer and
        climax communities
  Community Change during Succession
-- Species/guild changes through succession
      Examples:  Glacier Bay, Alaska (subarctic zone; see Figs. 20.2 and 20.3); requires millenia
        Subarctic Glacier Bay is a good natural study area due to retreating glaciers.  Species diversity
            rose rapidly for first couple hundred years, then begins to level off.  Taller shrubs don't reach
            maturity until after about 3 decades, and tree species don't appear as a significant component
            for nearly 100 years.  Even then, low shrub/herb diversity continues increasing through the
            first millenium, with just a few tree/tall shrub species remaining, though in large numbers
          Note that the rate of change is a lot slower than in tropical ecosystems, and that number of tree
            species would be MUCH higher in the tropics.
        Temperate Forests  -- SE U.S.  (a couple of centuries)
          Virtually all of the deciduous forest in the piedmont/montane areas of the SE were cleared at
            some point, so ideal area for studying secondary succession.  Please READ the description
            of piedmont succession, first paragraph, second column, pg. 444.  General trends are weedy
            species initially, followed by rapid growing pine seedlings and more trees later, with woody
            species nearly levelling off at a century and a half into the succession.  Bird species follow a
            similar trend.  (see Figs. 20.4 and 20.5).
        Intertidal communities -- example is 1 to 1.5 years
         
Remember the Sousa disturbance studies from Chap 16?  Succession on rocks stripped of
            attached organism cover;  reaches maximum species diversity (of just 6 or 7) quickly.
        Stream communities -- algal/diatom succession taking a couple of months
          Sycamore Creek, NE of Phoenix.  Flash floods can remove algal/diatom community and
            initiate new round of succession.  Both diatoms and other algal organisms reach max
            diversity after about 20 days, and decreased after 50 days -- WHY??     
                Macroinvertebrate species diversity was dominated by one crane fly species' aquatic 
            larva, though most species present before the flood remained present throughout the study.

  Ecosystem Change during Succession --
    Increases in overall biomass, production, nutrient retention; and important soil changes
      Examples: 
        Soil changes at Glacier Bay (see Fig. 20.10).  Organic content, nitrogen content, moisture all
            increased, especially at the climactic "Spruce" stage, while pH and phosphorus content
            decreased.  Note that organic and nitrogen content apparently tied together (not surprising)
        The Hawaiian Island chain represents a natural experimental example of 4 million years of
            succession.  Organic carbon and nitrogen in the soils changes from young to old sites (and,
            as above, tied together; see Fig. 20.12).  Total phosphorus remains unchanged, but
            available (weatherable) phosphorus is largely depleted after about 20,000 years, and
            remains a limiting factor to primary production at sites 20,000 + yrs. old.
        As suggested above*, when allowed to recover after "clearing", ecosystem nutrient retention
            increases quickly and dramatically (see Fig. 20.15).
        Model of Ecosystem recovery (hence, succession):  reorganization phase, aggradation phase,
            transition phase (with peak biomass followed by slight decline) and "steady" state phase.
            In the Sycamore Ck. example from above, overall algal biomass was beginning to level off
            after the first month, as was primary production and invertebrate biomass.  Nitrogen
            retention reaches a peak at 30 days, and then drops off significantly, possibly associated 
            with the transition phase mentioned.  By 90 days post-flood, biomass loss may be evident.

  Possible Mechanisms of Succession -- note the word "possible;" see Fig. 20.20
    Facilitation -- early successional species in turn modify the ecosystem conditions and in turn
        facilitate their own replacement, making the conditions better for other successional species.
        These species in turn are replaced by yet other species, until a point is reached where the
        current residents no longer facilitate colonization by others.  This is the climax community.
    Tolerance -- this model suggests that the entire suite of colonizing species is largely there at the 
        start, and the species that dominate later in succession are simply those that tolerate the early
        conditions as well as whatever changes take place along the way.
    Inhibition -- like tolerance model, early on there are a large number of species; in this case all
        make establishment by other less likely (inhibit recruitment).  In turn, those that last are long-
        lived, light disturbance-resistant species. 
    All models partly to completely "reset" by major disturbance
                        EVIDENCE for particular mechanisms?
    Aquatic:  Intertidal
        Sousa -- with isolation and removal experiments, evidence suggests that the inhibition mech.
            is at work with successional species of algae in the intertidal.  This also clarified what
            ultimately causes early species to fail -- vulnerability to physical and biological factors causing
            mortality.  The dominant early Ulva gives way as the occasional exposure to drying wind/
            intense sunlight, and herbivory allows more resistant species to succeed.
        Turner -- her studies suggest facilitation for some species; the later successional surfgrass
            Phyllospadix produces hooked seeds which require mid-successional algae to latch onto and
            ultimately germinate.  Remove the alga and the Phyllospadix doesn't grow.
    Terrestrial:  Old Field/Forests
        Keever -- Again, both inhibition and facilitation seem to be involved, even with the same species.
            Horseweed (Erigeron) inhibits growth of Aster, but Aster stimulates growth of Andropogon.
        Chapin -- following deglaciation.  Both inhibition and facilitation appears to take place through
            much of the successional stages seen, with a lot of inhibition taking place at the climax stage
            (see Fig. 20.24). 
    Should NOT be surprising that different mechanisms are functioning for different species.
        Important question:  What keep grasslands/prairies as a "climax" community?

  Community and Ecosystem Stability
   
Definitions:  Stability, resistance, resilience
        Stability can occur because of lack of disturbance; however, some communities may remain
            relatively stable in the face of some significant "disturbance", due to resistance or resilience.  
       
Are some communities more resistant, and, if so, why?  more resilient?
    The Park Grass experiment:  a meadow at Rothamsted Exper. Station, in England, that has been
        constantly monitored since 1862.  Changes that have been seen have been almost completely
        involving changes in abundance of species already there; virtually no new colonizers have been
        recorded.  When plots were modified with fertilizers of different composition, proportions of
        legumes/other species increased, the % increase depending on the fertilizer used.  Although
        biomass remained quite consistent throughout the study periods and percentages of groups
        overall, individual populations of particular species changed substantially.  So, are communities
        stable in nature?  Probably depends on how you define "stable" -- what you measure in the
        community.  In forest community, studies require a great amount of time and are difficult to
        replicate (!). 
    Sycamore Creek, AZ (the return to) -- replicate studies following disturbance easier.
        Algal recovery higher in upwelling zones, where nitrogen is made more available from the
            underlying sediment.  The algal "community" is, therefore, more resilient in upwelling areas.
            The regions of upwelling/stationary/downwelling zones also remain quite stable even after
            many flood events; the stability lies in the underlying geomorphology of the streambed --
            upwelling occurs where bedrock is near the "surface".  Why does this make sense?