Biotic & Abiotic Interactions in Ecosystems – Comprehensive Study Notes

Setting & Lesson Overview

Opening scene: narrator on Sandy Bay, Macquarie Island ➜ seals on sand, penguins near water, ocean, distant bluffs.
Purpose of lesson: answer “How do living and non-living factors in an environment interact?”
Three guiding goals
- Differentiate biotic vs. abiotic factors.
- Identify ecosystem levels of organization.
- Describe ways organisms compete for resources.

Any individual living thing.
- Examples given: sunflower plants, humans, reindeer, mushrooms (fungi), bacteria.
Activity: in a park photo, the duck, people, and plants qualify as organisms.

All organisms PLUS their remains or products.
- Visible: zebra, trees, grass, seals, penguins, wildebeests, flamingos.
- Invisible/less obvious: bacteria in soil, fungal spores on leaves.
“Once-living” clause: dead uprooted tree still counts as biotic because it was alive.

Physical & chemical components of habitat.
- Examples: water, snow, mountains, sunlight, oxygen, temperature, soil type, sand, rocks, bluffs.
Critical roles
- \text{Water}: solvent for life processes; animals drink, plants absorb.
- \text{Sunlight}: energy for photosynthesis → base of food web.
- \text{Oxygen}: terrestrial organisms draw from air; aquatic organisms from dissolved O$_2$.
- \text{Temperature}: sets tolerance limits, distribution ranges.
- \text{Soil}: nutrient profile & texture determine plant community; plants then influence animals.

Group of same species in same area, interbreeding.
- Example: the school of clownfish shown.

All populations of different species living close enough to interact.
- Example: clownfish + blue tangs + sea anemones.
- Interactions may include: predation, symbiosis, competition, shared shelter.

Community plus its abiotic environment.
- Example: coral-reef organisms + water, sand, sunlight, dissolved nutrients.

Occurs when \ge 2 organisms require the same limited resource.
Two scales
- Intra-specific: within a population (e.g., deer vs. deer for winter browse).
- Inter-specific: between different species (e.g., zebras & wildebeests at watering hole).

Energy/Food
- Deer in winter share shrinking plant supply; weaker individuals may starve.
- Rain-forest plants compete for sunlight ➜ tallest trees “win,” understory remains small.
Water
- African savannah: buffalo, elephants, giraffes, zebras converge on scarce watering holes; visible crowding illustrates pressure.
Space/Shelter
- Prairie dogs need large, flat, soft soil for tunnels; will kill ground squirrels entering colony—illustrates intense spatial competition.
Mates/Parental Protection
- Male hippos aggressively guard river stretches containing females; also defend calves from crocs, lions, hyenas, & rival bulls.
- Male fiddler crabs battle over burrows; burrow ownership directly tied to mating success.

Ecosystem management must consider both biotic interactions (e.g., overgrazing) and abiotic constraints (e.g., water scarcity under climate change).
Conservation plans often focus on keystone resources (waterholes, nesting sites) to ease competition and support biodiversity.
Understanding competition helps predict invasive-species impact: invaders that out-compete natives for limited resources can restructure entire communities.

Biotic = living/once-living; Abiotic = non-living/never-lived.
Organizational hierarchy: Organism → Population → Community → Ecosystem.
Interactions are multifaceted; competition arises whenever resources are finite.
Lesson objectives met: terminology clarified, hierarchy established, competition mechanisms illustrated through concrete examples.