1. ESS Topic 2.6 - Changes
2.6.1 Explain the concepts of limiting factors and carrying capacity in
the context of population growth.
A population is a species of organisms living in the same place at the same time. Organisms
within a population interbreed and interact with one another and their physical environment
throughout their lives.
There are 4 main factors controlling population sizes:
1. natality - births increase the population
2. mortality - deaths decrease the population
3. immigration - movement of individuals into an area increases the population
4. emigration - movement of individuals out of an area decreases the population
Populations can theoretically grow to an infinite size, but available resources are finite, so
individuals must compete for resources (remember intraspecific and interspecific
competition!).
Carrying capacity (represented by a capital 'K') is defined as, "The maximum number of
organisms, in a given species, that can use a given area of habitat without degrading the habitat
and without causing stresses that result in the population being reduced."
(Source:www.desertmuseum.org/invaders/invaders_glossary.php)
Limiting factors are those factors (biotic and/or abiotic) which keep populations in check - they
prevent population numbers from growing too large and overrunning an ecosystem.
Examples of limiting factors:
• disease
• parasites
• accidents
• disaster
• hunting & predation
• starvation
• competition for resources
• available oxygen and/or nutrients
Resources for exploring limiting factors and population dynamics:
• Limiting Factors case study from Lake Winnipeg, Canada (PDF)
• Limiting Factors in Wildlife Conservation from the International Hunters Education
Association
• Fundamentals of Populations and Population Growth

2. ESS Topic 2.6 - Changes
• What is a Limiting Factor? from WiseGeek.com
2.6.2 Describe and explain S and J population curves.
Many populations are capable of growing at an exponential rate (exponential growth), such
that a population of only 2 individuals doubles at a predictable rate
(2,4,8,16,32,64,128,256,512,1024,2048...).
Once the population's use of resources begins to outstrip the ecosystem's ability to replenish
the resource, competition for those limited resources increases, and population growth slows
due to environmental resistance - any factor that limits increases in the population.
Population growth is generally represented by 2 different curves: S-curves and J-curves.
S-curves
• population grows exponentially at first, then growth slows as population #'s approach
carrying capacity
• results in a stable population
• consistent with density-dependent limiting factors, described below
Here is the classic s-curve (Source:
http://www.saburchill.com/IBbiology/chapters02/049.html)
J-curves
• rapid exponential growth quickly surpasses carrying capacity (called 'overshoot')
• sudden collapse ('diebacks') brings population #'s below carrying capacity
• population fluctuates over time
• consistent with density-independent limiting factors, described below
Here's an example of a j-shaped population curve. What do you think this graph indicates about
human population growth? What factors might have contributed to the relatively slow growth
in human population prior to the 19th century? (Source:
http://www.tutorvista.com/content/biology/biology-iv/population/malthus-population-
theory.php)
2.6.3 Describe the role of density-dependent and density-independent
factors, and internal and external factors, in the regulation of
populations.

3. ESS Topic 2.6 - Changes
Density-dependent limiting factors
• depend on the size of the population
• effects of the factors increase as the population grows
• act as negative feedback
• tend to be biotic
• two categories of density-dependent factors:
◦ internal factors - within a single species
■ limited food, space, or territory
■ reduced fertility rates
◦ external factors - between species
■ populations of predators or prey
■ diseases spread more easily in densely-populated areas
Density-independent limiting factors
• do NOT depend on the size of the population
• tend to be abiotic
• not part of a feedback system
• effect the population regardless of its size
2.6.4 Describe the principles associated with survivorship curves
including, K- and r-strategists.
K-strategists
• populations stabilize at or near the carrying capacity of an ecosystem
• s-shaped curves
• good competitors
• adapted to stable ecosystems
• longer lives
• slower growth
• more 'parental' care of offspring gives each individual a better chance of surviving and
reproducing
• higher proportion of offspring survive
r-strategists
• populations overshoot carrying capacity, then collapse
• j-shaped curves
• adapted to disturbed sites and ecosystems

4. ESS Topic 2.6 - Changes
• shorter lives
• rapid growth to reproductive maturity
• reproduce rapidly
• produce many offspring with relatively high mortality rates so that at least some of the
offspring survive to reproduce
• little or no 'parental' care of offspring because there are too many offspring to care for
2.6.5 Describe the concept and processes of succession in a named
habitat.
Succession - change in a community over time resulting in a stable climax community with
complex interactions
Primary succession - also known as a prisere; succession that develops from bare rock.
Primary succession is rare because almost all of Earth's surface has already undergone several
stages of succession over the past 4.5 billion years
Sere - one in a series of communities at a particular location; they change over time in seral
stages
• Hydrosere - primary succession in water
• xerosere - primary succession on dry land
• psammosere - primary succession on sand dunes
Seral stage - one step or phase in succession. There are several seral stages in primary
succession:
1. bare, inorganic surface -
◦ 'soil' is almost exclusively mineral content with few microbes and/or organic
matter
◦ nutrient-poor substrate
◦ limited water-holding capacity due to low OM content
◦ extremes of sun, temperature, and wind
2. Colonization -
◦ pioneer species adapted to extreme conditions
◦ almost exclusively r-strategists
◦ simple soils develop from windblown dust and rock decomposition from lichens
◦ still harsh sun, wind, and temperatures
3. Establishment -
◦ diversity increases as the number of producers able to live in the area increase
◦ soil microbes and invertebrates add OM and water-holding capacity
◦ weathering adds nutrients to soil
4. Competition -
◦ larger plants create new microclimates

5. ESS Topic 2.6 - Changes
◦ diveristy increases due to new microclimates
◦ k-strategists begin to establish, outcompeting r-strategists for light, space, and
nutrients
◦ microclimates make sun, temperatures, and wind less extreme
5. Stabilization -
◦ fewer new species colonize because niches are already occupied
◦ food webs become more complex with more specialized niches
◦ specialized niches favor k-strategists
6. Seral climax -
◦ stable and self-perpetuating community
◦ dynamic equilibrium of nutrient cycles
◦ patchwork of new/old growth maintains high levels of species diversit
Secondary succession -
• succession on established soils
• early community was destroyed by natural causes (fire, flood) or human activity (farming)
• established soils support wider variety of seeds
• dormant seeds remain in soils from earlier community
• fewer seral stages due to already established soils
Pioneer community - first organisms to colonize bare rock; often lichens
Subclimax community -
• community present when movement through seral stages is interrupted by an abiotic or
biotic factor
• succession will not continue until the limiting factor is removed
Climax community -
• community of plants and animals which produces conditions favoring their own
perpetuation, and which will not undergo transition unless disturbed by external forces.
(www.cabq.gov/aes/glossary.html)
• stable, in dynamic equilibrium
Plagioclimax -

6. ESS Topic 2.6 - Changes
• Plagiosere - a series of plant communities that are interrupted or disturbed by an outside
agent (www.en.wiktionary.org/plagiosere)
• the combination of plant species in an area brought about by human interference. Much
of the European landscape is plagioclimax. (www.tuition.com.hk/geography/p.htm)
• an area or habitat in which the influences of the human race, have prevented the
ecosystem from developing further. (en.wikipedia.org/wiki/Plagioclimax)

7. ESS Topic 2.6 - Changes
Zonation -
• different species occupying different spaces within a community or location based on a
gradient of some biotic or abiotic factor
◦ different species of mangroves based on tidal movement
◦ different species of plants and animals where mature trees fall in a forest
◦ montane meadows surrounded by forest due to combinations of temperature,
wind, precipitation, and soil leaching
2.6.6 Explain the changes in energy flow, gross and net productivity,
diversity and mineral cycling in different stages of succession.
Gross productivity -
• low during early stages of succession
◦ few producers
◦ limited or no soil to support rapid growth of new/more producers
• relatively high in later stages & climax communities
◦ more producers capable of growing once soil has developed
Net productivity -
• relatively high during early stages
◦ the system is adding biomass
◦ energy lost to respiration is limited because population numbers are low
◦ P:R ratio is high
• falls during later stages
◦ established producer/consumer cycle means productivity & respiration are more
balanced
◦ P:R ratio approaches 1:1
Biodiversity -
• low at early stages
◦ few producers adapted to bare rock
◦ few producers = no food source for consumers
• increases through later stages of succession
◦ food webs become more complex as number of producers increase
◦ specialized niches developing with high competition

8. ESS Topic 2.6 - Changes
◦ some individuals of pioneer species maintain their intitial foothold and don't
disappear from the community
◦ new species immigrate from other locations
• falls slightly (but still relatively high) in climax communities
◦ specialized niches established
◦ weaker organisms out-competed for resources (competitive exclusion)
Mineral cycling -
• low during early stages of succession
◦ not many producers to break down bare rock
◦ limited time for erosion to build soil matrix
◦ limited soil = few soil microbes to cycle minerals and nutrients
• increases rapidly throughout succession
• remains relatively high during climax community
◦ especially high in tropical zones due to year-round growth
2.6.7 Describe factors affecting the nature of climax communities.