IB Environmental systems and societies 2.1B Population Notes

Population: The Unit Where Ecology Actually Happens

1. A population is a group of individuals of the same species living in the same area at the same time and able to interbreed.
2. Populations are the fundamental unit of ecology, the building blocks of communities and ecosystems.

What Makes a Group of Organisms a Population

1. Same species: the individuals all belong to one biological species.
2. Same place and time: they share a habitat and overlap in the same period.
3. Interbreeding: members can reproduce with each other, keeping genes flowing through the group.

Six Characteristics Ecologists Measure

1. Size (N): the total number of individuals in the population.
2. Density: the number of individuals per unit area or volume, such as trees per hectare or fish per cubic metre.
3. Distribution: the spatial arrangement of individuals, which can be random, uniform, or clumped.
   1. Clumped is the most common pattern in nature, driven by patchy resources or social grouping.
4. Age structure: the proportion of individuals in each age group, such as juveniles, adults, and post-reproductive adults.
5. Sex ratio: the ratio of males to females, which shapes reproductive potential.
6. Genetic diversity: the variation within the population that affects its resilience and ability to adapt.

Species vs. Population: Not the Same Thing

1. A species can have many separate populations living in different locations.
2. These populations may be isolated by geographical barriers or connected through migration.

Why Populations Matter Ecologically

1. Populations are the basic units of evolution, because genetic change accumulates within a population over generations.
2. They are the level at which we study species interactions such as competition and predation.
3. Understanding population dynamics underpins conservation, wildlife management, and resource planning.

Distribution Is Shaped by Biotic and Abiotic Factors

1. Where a population is found, its distribution, is set by both abiotic and biotic factors acting together.

1. Biotic factors are the living influences, such as predation, competition, food supply, and disease.

1. Abiotic factors are the physical and chemical conditions, such as temperature, light, water, pH, salinity, and oxygen.
2. Abiotic tolerance limits set the potential range, while biotic interactions often decide the smaller range a species actually occupies.
3. For example, freshwater below about pH 5.5 excludes many fish because low pH disrupts gill ion regulation, causing loss of sodium and chloride, and mobilises toxic aluminium.

Human Impact on Distribution

1. Humans reshape distributions by altering abiotic conditions, for example irrigation, urban heat islands, and pollution, which can expand or shrink a species' range.