Populations are dynamic entities. Populations consist all species living within a specific area and fluctuate based on a number of factors. Seasonal and yearly changes in the environment, natural disasters like forest fires and volcanic eruptions, and competition for resources between and within species all impact populations. The world’s human population is growing at an exponential rate. Humans have increased carrying capacity through migration, agriculture, medical advances, and communication. The age structure of a population allows us to predict population growth. Unchecked human population growth could have dire long-term effects on our environment.

Communities include all the different species living in a given area. The variety of these species is called species richness. Many organisms have developed defenses against predation as a result of interaction with other members of the community. Species may form symbiotic relationships such as commensalism or mutualism. Communities are described by their foundation and keystone species. 

Behaviors are responses to stimuli. They can be instinctual behaviors not influenced by the environment or learned behaviors, which are influenced by environmental changes. Instinctual behaviors include mating systems and methods of communication. Learned behaviors include imprinting and habituation, conditioning, and cognitive learning. Although the connection between behavior and genetics is well established, the explanation of human behavior as entirely genetic remains controversial.

The (a) Chinook salmon mates once and dies. The (b) pronghorn antelope mates during specific times of the year during its reproductive life. Primates, such as humans and (c) chimpanzees, may mate on any day, independent of ovulation. (credit a: modification of work by Roger Tabor, USFWS; credit b: modification of work by Mark Gocke, USDA; credit c: modification of work by “Shiny Things”/Flickr)

Spontaneous lipid formation and the negative charge of nucleotides could explain the development of structures required for prokaryotic life. Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. Bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Endosymbiosis proposes host cells and bacteria formed an endosymbiotic relationship when host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. These ingested bacteria became more specialized in their functions, with the aerobic bacteria descending to mitochondria and autotrophic bacteria descending to chloroplasts.

Most life forms on earth obtain their energy from the sun. Plants use photosynthesis to capture sunlight, and consumers eat those plants to obtain energy. Decomposers digest plant and animal matter.

Participants in the carbon cycle are roughly divided among producers, consumers, and decomposers of organic carbon compounds. Primary producers are land plants and photosynthetic bacteria. A related source of carbon compounds is the mixture of organic materials from dead plants and prokaryotes that have resisted decomposition. Consumers use the organic compounds generated by producers and release carbon dioxide to the atmosphere. Other bacteria and fungi, collectively called decomposers, carry out the breakdown (decomposition) of plants and animals and their organic compounds. Most carbon dioxide in the atmosphere is derived from the respiration of microbes that decompose dead plants and animals.