Introduction
Population ecology is a branch of biology that studies the dynamics of populations of organisms, especially the regulation of population size, life history traits such as reproductive strategies, and the interactions among species. This topic is crucial for understanding the broader ecological interactions and the sustainability of ecosystems. In this study note, we will delve into the key concepts of population ecology as outlined in the International Baccalaureate (IB) syllabus for Biology.
Populations
Definition and Characteristics
A population is a group of organisms of the same species living in the same area at the same time, which interbreed. Populations can be seen as interbreeding units of a species. One species may consist of any number of populations, from one to many.
Example
The Eastern Grey Kangaroo is a species of marsupial native to Australia. This species consists of multiple populations across various regions of Australia. There are populations of Eastern Grey Kangaroos in Queensland, New South Wales, Victoria, and other parts of the country. Although individuals from these different populations are capable of interbreeding, in reality, they very rarely do due to geographical isolation. Each population may have its own unique characteristics and adaptations based on local factors such as habitat, climate, and food availability.
Community
A community includes all of the different populations (of different species) living in the same area at the same time. It is a collection of interacting populations within an ecosystem. Each species within a community depends on other species for food, shelter, pollination, seed dispersal, etc.
Habitat
A habitat is the local environment in which an organism, species, population, or community normally lives. For example, badgers, deer, oak trees, and ants are all species that would live in a woodland habitat. A description of the habitat of a species can include both geographical and physical locations, as well as the type of ecosystem required to meet all environmental conditions needed for the survival of the organism, species, population, or community.
Population Growth
Carrying Capacity
The carrying capacity ($K$) of an environment is the maximum stable population size of a species that an ecosystem can support. It is determined by competition for limited resources. Every individual within a species population has the potential to reproduce and have offspring that will contribute to population growth. However, various abiotic and biotic factors prevent every individual in a population from making it to adulthood and reproducing, ensuring the population size of each species is limited at some point.
Example
In a forest ecosystem, as deer population density rises, competition for available food (grass, leaves, etc.) increases, placing limits on individual growth rates and overall population size.
Population Growth Curves
Population growth can either be exponential or limited by carrying capacity.
J-Curves
For some populations, when population growth is plotted against time, a J-curve is produced. A J-curve describes the growth pattern of a population in an environment with unlimited resources. The J-curve has three distinct phases:
- Lag Phase: Initial growth is slow due to the small population size.
- Exponential Growth Phase: Population grows rapidly as resources are abundant.
- Plateau Phase: Growth slows as resources become limited, eventually reaching a plateau.
S-Curves
An S-population curve describes the growth pattern of a population in a resource-limited environment. The S-population curve has four distinct phases:
- Lag Phase: Initial growth is slow when the population is small.
