Populations and communities — IB Diploma Biology SL

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Populations and random sampling

   Practical

   A population is an interacting group of organisms of the same species living in an area. Members normally breed with each other, and reproductive isolation separates one population from another. Population size is usually estimated by random sampling, for example with randomly placed quadrats for sessile organisms, because counting every individual is rarely possible.

2. Capture–mark–release–recapture and the Lincoln index

   Capture–mark–release–recapture estimates the size of a population of motile animals. A sample is caught, marked and released; later a second sample is caught and the marked individuals in it are counted. The Lincoln index gives the estimate: population size = (M × N) ÷ R. It assumes marked animals mix randomly and are as likely to be recaught.

3. Carrying capacity and density-dependent factors

   Carrying capacity is the maximum population size that an environment can support sustainably. It is set by limited resources such as food, water, space, light or nesting sites. Density-dependent factors such as competition, predation and the spread of pathogens push a population back towards the carrying capacity by negative feedback, while density-independent factors cause fluctuations.

4. Population growth curves

   Practical

   Population growth curves model how numbers change over time. When resources are plentiful and limiting factors are absent, a population grows exponentially, giving a J-shaped curve. As density-dependent factors take effect, growth slows and numbers level off at the carrying capacity, giving a sigmoid (S-shaped) curve. Exponential growth plots as a straight line on a logarithmic scale.

5. Intraspecific and interspecific relationships

   Intraspecific relationships are between members of the same species. They compete for limited resources such as food, territory and mates, but may also cooperate, for example by hunting in packs. A community is all the interacting populations in an area. Interspecific relationships include herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity.

6. Mutualism

   Mutualism is an interspecific relationship that benefits both species. In root nodules of legumes (Fabaceae), Rhizobium bacteria fix nitrogen for the plant and receive sugars. Orchid mycorrhizal fungi supply sugars and minerals to the orchid and receive carbon compounds once it photosynthesises. Zooxanthellae in hard corals supply photosynthetic products; the coral provides carbon dioxide, nutrients and a sheltered, sunlit position.

7. Invasive species and testing for competition

   An invasive species is an introduced species that spreads and outcompetes endemic species because it acquires resources more effectively. Interspecific competition is indicated, but not proven, if one species is more successful when the other is absent. It is tested by laboratory experiments, field observations and removal experiments. A chi-squared test checks whether two species are associated.

8. Predator–prey relationships and control of populations

   Predator–prey relationships are an example of density-dependent control of animal populations. When prey are abundant, predators thrive and increase; heavier predation then reduces prey, so predators decline and prey recover. Populations can be controlled top-down, by predators at higher trophic levels, or bottom-up, by the supply of resources such as nutrients, but one usually dominates.

9. Allelopathy and secretion of antibiotics

   Allelopathy is the release of chemicals by a plant that inhibit the germination or growth of other plants nearby, such as juglone from black walnut. Secretion of antibiotics is the release by microorganisms, such as Penicillium fungi, of chemicals that kill or inhibit bacteria. Both release a substance into the environment to deter potential competitors.