Population growth What affects population size Starting activity

Population growth What affects population size?

Starting activity: Come up with a definition for the following • Population size • Population density

Lesson objectives: Pupils will be able to. . . • Explain the significance of limiting factors in determining the final size of a population; • Explain the meaning of the term carrying capacity; • Explain, with examples, the terms interspecific and intraspecific competition;

Population growth Algal cell capable of asexual reproduction • List as many reasons as you can as to why its population would stop growing? Very hard to investigate in a natural open environment like a pond. . so we don’t! In a lab, bacteria can be grown in a nutrient medium kept under constant conditions.

r/K selection theory of population growth Selective pressures drive evolution in one of two generalized directions: • Either r- or K-selection. • r refers to reproductive rate • K refers to a constant

Logistic growth curve (k-Strategists) Carrying capacity

Population size Boom and bust (r-Strategists) K Carrying capacity ‘Bust’ ‘Boom’ Time

r-strategists examples – Typically, r-selected species exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood. – Organisms with r-selected traits range from; – Bacteria. – Diatoms. – Micro fungi (Penicillium, Yeast). – Insects. – Weeds. – Various cephalopods e. g. Squid. – Some mammals, especially small rodents.

r- and K-strategists In ecological succession • r- strategists prosper at first, as the environment is unstable, however, as species settle the environment begins to become more stable and K- strategists take control.

r- and K-strategists In ecological succession • In areas of major ecological disruption or sterilisation (such as after a major volcanic eruption, as at Krakatoa or Mount Saint Helens), r - and K-strategists play distinct roles in the ecological succession that regenerates the ecosystem. • Because of their higher reproductive rates and ecological opportunism, primary colonisers typically are r-strategists and they are followed by a succession of increasingly competitive flora and fauna.

r- and K-strategists In ecological succession • Eventually a new equilibrium is approached (sometimes referred to as a climax community), with r-strategists gradually being replaced by Kstrategists which are more competitive and better adapted to the emerging micro-environmental characteristics of the landscape.

Human Population Growth Are humans r- or Kstrategists? Why? Where is the human population on its typical growth curve? Is there any reason to believe that we will not experience the “standard” pattern?

Limiting factors and carrying capacity • Limiting factors are collectively called Environmental resistance • Carrying capacity = maximum population size that can be maintained over a period of time in a particular habitat and resources available Density dependent (biotic) Limiting factors Density independent (abiotic)

Limiting factors affect population by influencing. . . . What factors? 1. 2. 3. 4. Birth rate (natality) Death rate (mortality) Immigration Emigration

What will influence the carrying capacity? Make a table and list as many factors as you can that would. . . • Increase the carrying capacity • Decrease the carrying capacity Discuss your ideas with your neighbour!

Checking understanding. . . • Explain the connection between carrying capacity and limiting factors Homework:

B C D A

Competition

What’s the difference. . • Intraspecific competition • Interspecific competition Give some examples of each.

Mallards fighting for mates

Inter-specific competition Example 1 - Paramecium P. caudatum P. aurelia

Interspecific competition & competitive exclusion principle 1934, Russian Biologist Carl Fredrick Gausse Competitive exclusion principle: no two species can occupy the same niche indefinitely when resources are limiting.

Growth curves of two Paramecium species

Inter-specific competition affects: 1. Size of two populations • When grown separately, populations of P. caudatum and P. bursaria are three times denser than when grown together

2. Distribution of two populations • P. caudatum survived towards top of tube - better adapted to feed on bacteria - more oxygen at top so more bacteria • P. bursaria survived towards bottom of tube - better adapted to feed on yeast - less oxygen = bacterial growth not favoured, more yeast survive

Shag - Phalacrocorax aristotelis Cormorant Phalacrocorax carbo • Mixed diet, sand eels and herring • Eats mostly flat fish, gobies and shrimps • Fishes in shallow estuaries • Fishes out to sea • Nests low on cliffs or shallow ledges • Nests on high cliffs or broad ledges

Shag Cormorant

Extinction of one population is not necessarily inevitable. . . • Interspecific competition could result in two populations, one being much smaller than the other, both remaining constant. • Other variables will take effect, environmental factors, for example.

Population distribution What can you explain from this diagram?

Acorn barnacle - Chthamalus • Able to live both low down and high up on shore • Smaller in size

Crenate barnacle - Balanus • Only survives lower down shore as not well adapted to tolerate exposure to air at low tide • Larger in size

Tasks: 1. Complete worksheet on barnacles. 2. Complete question sheet on competition 3. Complete competition exam question (Jan 05 Q 4) Finish all above tasks for homework