Topic 2.6

Changes

http://www.youtube.com/watch?v=xMQ0Ryy01yE

Limiting Factors and Carrying Capacity

• Limiting Factors– Variables which constrain the growth of populations– These include temperature, light, water and nutrients– Wide range of environmental factors = stenoecious species– Narrow range of environmental factors = euryoecious species

• Carrying Capacity– The maximum number of organisms of a particular species that a

given area or ecosystem can sustainably supportpopulation size

time

carrying capacity

Limiting Factors and Carrying Capacity

oligotypic species mesotypic species polytypic species

Temperature

• Plants have different niches with regard to temperature

• Different temperatures may be limiting at different stages in a plant’s lifecycle

• e.g. The South African Protea flower only germinates after bush fires activate its seeds, kill off competitors and provide ash to enrich the soil with nutrients

Water• Too little water may cause water stress

in plants:– Germination may fail– Seedlings may die– Seed yield may be reduced

• Plants have developed adaptations to survive in different levels of water stress:– Hydrophytes – water tolerant plants– Mesophytes –plants that survive in moist

environments– Xerophytes – plants that survive in dry

environments

Population Growth

Population Growth

• When a species is introduced into a new environment its population grows in a characteristic way.

• The curve is generally called a sigmoid growth curve or S-curve

Population GrowthS-Curve

• The S-curve has three phases (1,2,3)– 1. Exponential growth phase

Resources are plentiful. Birth rate (natality) is greater than death rate (mortality)

– 2. Transitional phaseNatility falling, mortality rising. But natality is still greater than mortality

– 3. Plateau phasenatality = mortality. Something is now limiting the birth rate. Eg predation, competition for resources, food shortage, disease. In reality the population wobbles around k, with overshoot and die-back

k = carrying capacity

k

The s-curve is typical of long-lived species that are able to reach their carrying capacity – e.g. top predators

Population GrowthJ-Curve

• The J-curve only shows exponential growth

• The species does not reach the transitional or plateau phases, therefore growth does not slow down

The J-curve is typical of short-lived species that exploit a ‘boom and bust’ strategy – e.g. rodents, pest insects and microbes

Density-dependent Limiting Factors

• These factors increase mortality rate as population density increases– e.g. competition for resources, space, disease, predation,

parasitism– Predators are attracted to more dense populations, disease

spreads more easily• They act as negative feedback mechanisms and lead to

stability around the carrying capacity• They also act as natural selection factors which help to

increase the fitness of the population• They include internal factors such as density-dependent

fertility or size of available breeding territory and external factors such as predation or disease

Density-independent Limiting Factors

• These factors increase mortality rate but are not related to population density – they are generally abiotic factors. They may also reduce birth rate– e.g. extremes of weather (drought, hurricanes), long-

term climate change (natural and anthropogenic), geophysical events such as tsunamis and volcanic eruptions

• They may act in conjunction with density-dependent factors, but do not act to stabilise populations

Human Influences on Limiting Factors

• Human activities may cause population increases by:– Increasing availability of resources (e.g. fertiliser use)– Reducing competition (e.g. pesticide use)– Reducing predation (e.g. by over-hunting)– Introducing animals to new areas

• Human activities may cause population decreases (or even extinction) by:– Habitat destruction– Introduction of alien species– Over-hunting

r- and K-Strategists

• Slow growing, long-lived species which exploit S-curve growth are called K-strategists

• Fast growing, short-lived species which exploit J-curve (boom and bust) growth are called r-strategists

• Species which employ a strategy somewhere between these extremes are called C-strategists

• K-strategists tend to be large, have a small number of offspring and high level of parental care (e.g. mammals)

• r-strategists tend to be small, have large number of offspring and very little parental care (e.g. insects)

r and K Survivorship Curves1000

100

10

1

0 50 100

Percentage of lifespan

Num

ber o

f ind

ivid

uals

sur

vivi

ngK-strategists

C-strategists

r-strategists

r and K Survivorship Curves

• K-strategists tend to survive for their entire potential lifespan and then die over a short time period (e.g. humans, salmon)

• r-strategists tend to die at a young age, and those that don’t are likely to live a long time (e.g. turtles, oysters)

• Limiting factors that affect the shape of the curve include:– Competition for resources– Adverse environmental conditions– Predator-prey relationships

Ecological Succession

• The long-term change in a community after it starts to colonise bare substrate

• Pioneer community climax community• Bare rock lithosere

• Freshwater hydrosere

• Dry habitat xerosere

http://www.bbc.co.uk/programmes/p0038s03

Succession Model (Lithosere)Bare uncolonised rock exposed

Colonisation by lichens, weathering of rock and build up of dead organic material

Growth of moss, further weathering; soil begins to form

Growth of small plant such as grasses, leading to further improvement of soil

Larger plants can grow in the deeper and more nutrient rich soil

Climax Community of shrubs and trees

Succession

• Primary Succession– Occurs on a previously uncolonised substrate (e.g. a

newly formed bit of bare igneous rock, newly exposed surfaces such as landslips, newly quarried rock face, newly exposed sanbanks or sanddunes)

• Secondary Succesion– Occurs in places where a previously existing

community has been destroyed (e.g. by a forest fire, newly exposed sediment at the edge of a lake)

– This occurs faster since soil and seed already exist

Succession• Succession occurs in stages• The first colonisers are pioneer species

– Fast growing species such as lichens and mosses which don’t require thick soil

• Secondary species (especially grasses) then take over – These are slower growing and are able to outcompete the pioneer species

for light and nutrients• Tertiary species (especially herbaceous plants) then take over

– These take advantage of thicker soil appearing as litter breaks down• Quaternary species then take over

– Largely shrubs appear which are able to outcompete the herbaceous plants• Finally a climax community is produced (e.g. oak forest)

– Thick soil has been produced, trapping more moisture and nutrients– A community of soil organisms develops– An animal community develops to exploit the available niches

http://edroness.blogspot.mx/2014/02/ecological-succession.html

Private Life of Plants – episode 4

SuccessionFeature Start of Succession End of Succession

Amount of organic matter Small Large

Amount of Nutrients Small Large

Nutrient conservation Poor Good

Amount of detritus Small Large

Niches Few but wide Many but small

Size of colonisers Small Large

Life cycles Simple Complex

Growth patterns r-strategists dominate K-strategists dominate

Diversity Low High

Stability Low High

Productivity Low and unstable High and stable

pH High Low

Biodiversity Low High

Productivity

• In the early stages of succession, GPP is low due to the low density of producers. However, NPP is fairly high due to the low respiration rates in the whole community

• As succession proceeds, GPP increases as soils become more structured and the density of organisms increases. NPP stabalises as niches become occupied

• The ratio of GPP/NPP is called the P/R ratio. In the climax community, overall productivity is balanced by overall respiration – therefore as succession proceeds, the P/R ratio approaches a value of 1

Productivity1

time

P/R

Ratio

0

Climax Forest

Agricultural Crop

Plagioclimax

• An interruption to succession is termed plagioclimax• Humans are often the source of this• Sometimes it is accidental and sometimes on purpose (e.g.

farmers do not want forests to become established on their land)

• The greatest impact is through direct deforestation • Humans also affect succession by:

– Erosion of footpaths– Trampling of plants – Hunting– Land clearance– Introduction of grazing animals

Questions

1. Briefly describe the stages of primary succession

2. What is P/R ration and how does it change as succession proceeds

3. List the characteristics of a typical climax community

4. What is plagioclimax. Give examples of how humans may cause it