Unit question: How do living things depend on each other and on their nonliving surroundings to survive?

Every organism on Earth is part of an ecosystem: a community of living things interacting with each other and with the physical environment around them. In this unit, we explore how energy moves through ecosystems and how organisms shape one another's survival.

Every ecosystem contains two categories of factors:

- Biotic factors are all the living components of an ecosystem: plants, animals, fungi, bacteria, and any other organism. - Abiotic factors are the nonliving physical and chemical components: sunlight, water, soil composition, temperature, air, and pH.

Biotic and abiotic factors interact constantly. The amount of sunlight (abiotic) determines which plants can grow (biotic). Those plants then determine which animals can survive in that ecosystem. Remove any key factor and the whole system shifts.

Energy enters most ecosystems from the Sun. Here is how it moves:

- Producers (also called autotrophs) capture sunlight and convert it to food through photosynthesis. Examples: grasses, trees, algae. - Primary consumers eat producers. Examples: grasshoppers, rabbits, deer. - Secondary consumers eat primary consumers. Examples: frogs, foxes. - Tertiary consumers eat secondary consumers. Examples: hawks, sharks. - Decomposers break down dead organisms and return nutrients to the soil. Examples: fungi, bacteria.

The 10% rule: Only about 10% of the energy at one trophic level passes to the next. The rest is lost as heat. This is why food chains rarely have more than four or five levels.

Organisms in an ecosystem interact in several ways. The key relationships to know:

| Relationship | Who benefits? | Example | |---|---|---| | Predation | Predator eats prey | Hawk eats a mouse | | Mutualism | Both organisms benefit | Bees pollinate flowers; flowers give bees nectar | | Competition | Neither benefits fully | Two deer competing for the same territory | | Parasitism | Parasite benefits, host is harmed | Tick feeding on a deer | | Commensalism | One benefits, other is unaffected | Barnacles on a whale |

These interactions shape population sizes, energy flow, and the structure of ecosystems over time.