Biology (university) · Part 10
Ecology and the environment
How populations grow, how species interact, how energy and nutrients move, and how we protect biodiversity.
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- Population growthWith no limits, a population grows exponentially, faster and faster. In reality, limits such as food and space slow it down, and it levels off at the carrying capacity (K), the largest population the environment can support. This S-shape is logistic growth.
- Ecological nicheThe role of a species in its environment: where it lives, what it eats, when it is active and how it interacts with others. Two species cannot occupy exactly the same niche for long (competitive exclusion).
- SymbiosisA close, long-term relationship between two species. Mutualism helps both. Parasitism helps one and harms the other. Commensalism helps one and does not affect the other.
- Trophic levelA feeding level in a food chain: producers (plants and algae), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and so on. Decomposers break down dead matter.
- Energy transferOnly part of the energy at one trophic level reaches the next. Much is lost as heat through respiration or stays in uneaten parts. A rough rule is about 10 percent passes on, so food chains rarely have more than four or five levels.
- Carbon cycleCarbon moves between the air, living things, soil, oceans and rocks. Photosynthesis takes carbon dioxide from the air into plants. Respiration, decay and burning fuels return it. Burning fossil fuels adds carbon that had been stored for millions of years.
- Nitrogen cycleNitrogen gas in the air cannot be used by most organisms. Nitrogen-fixing bacteria convert it to ammonia, nitrifying bacteria convert ammonia to nitrate that plants absorb, and denitrifying bacteria return nitrogen gas to the air.
- Ecological successionThe gradual change in species over time in an area. Primary succession starts on bare ground, such as new volcanic rock. Secondary succession follows a disturbance, such as a fire, on soil that already exists, and is faster.
- BiodiversityThe variety of life: the number of species, the genetic variety within them and the variety of ecosystems. Higher diversity tends to make ecosystems more stable. Main threats are habitat loss, invasive species, pollution, over-harvesting and climate change.