Ecology – Biology 3500
COMMUNITIES and ECOSYSTEMS UNIT Dr. Adams
Chapter 19: Nutrient Cycling and Retention
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
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.
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!)
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
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, third paragraph, first column, pg. 434. 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
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?
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
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?