Populations
Ecology studies how organisms interact with each other and their environment. It starts with populations: groups of one species living in one place.
Levels of ecology
Ecology works at several levels: an individual organism; a population (all members of one species in an area); a community (all populations of different species living together); an ecosystem (the community plus its physical environment); and the biosphere, all life on Earth.
Measuring populations
Ecologists measure population size, density (individuals per area), and distribution (clumped, uniform, or random). Counting every individual is rarely possible, so they sample: counting plants in quadrats, or using mark-recapture for animals, where the proportion of marked animals in a second catch estimates the total.
Exponential growth
With unlimited resources, a population grows exponentially: it increases by a constant percentage each period, producing a J-shaped curve. Bacteria dividing every 20 minutes could, in theory, go from one cell to billions in a day. Exponential growth cannot last because resources run out.
Logistic growth and carrying capacity
In real environments, growth slows as resources become limited and levels off at the carrying capacity, the largest population the environment can sustain. This gives an S-shaped (logistic) curve. Populations may overshoot the carrying capacity and then crash.
Limiting factors
Density-dependent factors, such as competition, predation, and disease, have stronger effects in crowded populations. Density-independent factors, such as droughts, fires, and cold snaps, affect populations regardless of crowding. Both keep populations in check.
Life history strategies
Some species, such as mice and dandelions, reproduce early and produce many offspring with little care, suited to unstable environments. Others, such as elephants and humans, mature slowly and invest heavily in few offspring, suited to stable environments. Most species fall between these extremes.
Human population
The human population took until about 1800 to reach 1 billion, then grew rapidly with improved food production and medicine, passing 8 billion in 2022. Growth is now slowing as birth rates fall in many countries. Demographers study age structure to predict future growth.
Worked example
Ecologists mark 50 fish and release them. Later they catch 60 fish, of which 10 are marked. The estimate is total = 50 x 60 / 10 = 300 fish. The idea is that the marked fraction in the sample (1/6) matches the marked fraction in the whole population.
Key takeaways
Ecology spans individuals, populations, communities, ecosystems, and the biosphere. Populations are estimated by sampling. Unlimited resources give exponential growth; limited ones give logistic growth toward the carrying capacity. Density-dependent and density-independent factors limit populations. Human population growth is now slowing.
Flashcard
One species in one area
Check your understanding
A deer population grows rapidly, then levels off as food becomes scarce. The level it settles at is the: