6.4 Biomes and Biodiversity
Key Takeaways
- Terrestrial biomes are defined primarily by temperature and precipitation; tropical rainforests have the highest NPP and species richness, deserts and tundra have the lowest
- Aquatic biomes split into freshwater (lakes, rivers, wetlands) and marine (estuaries, intertidal, neritic, pelagic, abyssal, coral reefs), with estuaries and coral reefs among the most productive
- Biodiversity has three levels - genetic, species, and ecosystem - and species diversity combines richness (number of species) and evenness (distribution of individuals)
- The latitudinal diversity gradient shows species richness peaks at the equator across nearly all taxa, driven by productivity, climate stability, and evolutionary time
- The species-area relationship (S = cA^z) explains why larger habitats hold more species and why habitat fragmentation reduces biodiversity even when total area is preserved
Why Biomes and Biodiversity Matter for the Praxis Biology Exam
A biome is a large geographic region characterized by a distinct climate, set of dominant plants, and characteristic animals. The Praxis Biology exam expects you to match biomes to their climatic signatures (temperature and precipitation), identify dominant vegetation, and reason about why species richness is highest near the equator. Biodiversity is tested at three levels: genetic, species, and ecosystem.
Major Terrestrial Biomes
The map of terrestrial biomes is shaped by latitude, elevation, and moisture. Use temperature and rainfall to discriminate biomes on the exam.
| Biome | Climate signature | Dominant vegetation | Notable feature |
|---|---|---|---|
| Tundra | Cold year-round, < 250 mm precipitation; permafrost | Mosses, lichens, dwarf shrubs (no trees) | Short growing season; carbon-rich permafrost |
| Boreal forest (taiga) | Long cold winters, short summers; moderate precipitation | Coniferous evergreens (spruce, fir, pine) | Largest terrestrial biome by area |
| Temperate deciduous forest | Four distinct seasons, 750-1,500 mm precipitation | Broadleaf trees that drop leaves seasonally (oak, maple, beech) | Eastern U.S., Europe, East Asia |
| Temperate grassland | Hot summers, cold winters; 250-750 mm precipitation | Grasses, few trees | Prairies, steppes; deep fertile soils |
| Desert | < 250 mm precipitation; extreme temperature range | Cacti, succulents, drought-deciduous shrubs | CAM photosynthesis; low NPP |
| Tropical rainforest | Warm year-round, > 2,000 mm precipitation evenly distributed | Tall broadleaf evergreens with layered canopy | Highest biodiversity on land; nutrient-poor soils |
| Tropical savanna | Warm, with distinct wet and dry seasons | Grasses with scattered drought-tolerant trees (acacia) | Periodic fires; large grazer migrations |
| Chaparral | Mediterranean climate: wet winters, hot dry summers | Drought-resistant shrubs, small trees | Fire-adapted; coastal California, Mediterranean |
| Temperate rainforest | Cool, very wet (> 2,000 mm); mild temperatures | Massive conifers (redwood, Sitka spruce) | Pacific Northwest; high biomass |
A reliable shortcut: tropical rainforests have constant warmth and high rainfall, while deserts have extreme daily temperature swings and little rainfall. Tropical savannas are distinguished from grasslands by temperature (warm year-round) and from rainforests by seasonal drought.
Aquatic Biomes
Aquatic biomes are classified by salinity (freshwater vs marine), depth, and flow.
Freshwater
- Lakes and ponds - standing water with layered zones (littoral, limnetic, profundal, benthic).
- Rivers and streams - flowing water; species adapted to current and oxygen gradients.
- Wetlands - saturated soils part of the year (marshes, swamps, bogs); among the most productive ecosystems on Earth and critical for water filtration.
Marine
| Zone | Location | Key feature |
|---|---|---|
| Estuary | Where rivers meet the sea | Mix of fresh and salt water; nursery for many fish |
| Intertidal | Coastline between high and low tide | Extreme physical stress; barnacles, mussels, starfish |
| Neritic | Continental shelf, above ~200 m | Sunlit, productive; most commercial fisheries |
| Pelagic (open ocean) | Off the shelf, surface waters | Low nutrient, vast volume; phytoplankton-driven |
| Abyssal | Deep-sea floor, > 2,000 m | Cold, dark, high pressure; chemosynthetic communities at vents |
| Coral reefs | Warm, shallow tropical waters | Among the most biodiverse ecosystems; threatened by warming and acidification |
Levels of Biodiversity
The U.S. Geological Survey defines biodiversity at three levels:
- Genetic diversity - variation in alleles within a population. High genetic diversity buffers populations against disease and environmental change.
- Species diversity - the number of species (richness) and their relative abundance (evenness) in a community.
- Ecosystem diversity - variety of biomes, communities, and ecological processes across a region.
A common Praxis distractor confuses species richness (count of species) with species evenness (how equally distributed individuals are among those species). A community with 10 species can be more diverse if all are present in equal numbers than if 90% of individuals belong to one species.
The Latitudinal Diversity Gradient
Species richness is highest near the equator and declines toward the poles. The pattern is observed across nearly every taxonomic group - mammals, birds, insects, trees, and marine invertebrates. Major hypotheses include:
- Higher productivity at the equator supports more species.
- Climate stability over evolutionary time allows specialization.
- More time since the last glaciation allowed for speciation.
- Greater area of tropical zones over geological history.
Biodiversity Hotspots
A biodiversity hotspot is a region with exceptional concentrations of endemic species (species found nowhere else) facing exceptional loss of habitat. Conservation International defines a hotspot as a region that has lost at least 70% of its primary vegetation and contains at least 1,500 endemic vascular plant species. There are roughly 36 recognized hotspots, including Madagascar, the California Floristic Province, the Caribbean Islands, and the Tropical Andes.
The Species-Area Relationship
The species-area relationship is the empirical observation that larger areas contain more species. It is often modeled as S = cA^z, where S is species number, A is area, c is a constant, and z is a slope typically between 0.2 and 0.35 for terrestrial systems. The relationship underlies island biogeography theory (MacArthur and Wilson) and explains why fragmenting habitat into smaller patches reduces biodiversity even if total area is conserved.
A biome receives less than 250 mm of precipitation per year and has dwarf shrubs, mosses, lichens, and permafrost as its dominant features. Which biome is described?
Forest A contains 20 species but 95% of individuals belong to a single dominant species. Forest B contains 20 species with individuals distributed roughly evenly among them. Which statement best describes the biodiversity of the two forests?