Untitled Notes

Learning outcomes focus on latitudinal gradient, 3 hypotheses, and calculating the Shannon diversity index.
Key concepts:
- Biodiversity components: species richness, species evenness, genetic diversity.
- Latitudinal gradient: species richness typically higher toward the tropics; influenced by spatial/topographical heterogeneity, niche specialization, geological age, surface area, and climate/productivity.
- Hypotheses addressing richness patterns:
- Species-time: temperate regions are younger/ periodically glaciated; older communities often richer for some taxa; limited marine applicability.
- Species-area: larger areas harbor more species due to larger populations and habitat variety; limitations in explaining richness in some large areas (e.g., tundra, open oceans).
- Species-productivity: higher plant productivity supports more species; linked to evapotranspiration; caveats for broad continental comparisons.
- These factors are not mutually exclusive; evolutionary time, area, and productivity all influence richness.
Calculating species diversity:
- Needs both richness and relative abundances; example comparisons with same richness but different evenness.
Shannon Diversity Index:
- Definition: $H<em>s = -\sum p</em>i \ln p<em>i$ where $p</em>i$ is the proportion of individuals in species $i$ .
- Interpretation: higher $H_s$ indicates greater diversity.
- Example captures how different species abundances affect the index.

Elton’s Diversity–Stability Hypothesis:
- More diverse communities dampen the effects of disturbances.
- A community is stable when there is little to no change in species number or abundances over time.
- Example: Tilman 1996; evidence from grassland studies supports the idea that diversity contributes to stability.

Disturbance leads to non-equilibrium dynamics; succession sequences replace species over time.
Primary vs secondary succession:
- Primary: colonization of a lifeless area (e.g., after volcanic eruption).
- Secondary: recolonization of a disturbed area that retains life (e.g., abandoned farmland).
Mechanisms of succession:
- Facilitation: early species modify the environment to favor later species; climax as endpoint (Clements).
- Inhibition: early species hinder later arrivals (e.g., Ulva inhibiting Chondracanthus in marine intertidal zones).
- Tolerance: late-successional species tolerate competition; early species do not guarantee a particular endpoint.
Key point: succession outcomes are not guaranteed; multiple pathways exist (Connell & Slatyer 1977).

Equilibrium model (MacArthur & Wilson): source of species richness tends toward an equilibrium $(\hat{S})$ determined by immigration and extinction rates.
Predictions:
1) Species–area relationship: larger islands have more species.
2) Species–distance relationship: islands closer to the mainland have more species.
3) Turnover: species composition changes over time even if total richness remains relatively stable.
Data support (core ideas):
- Area: positive correlation between island size and species richness for multiple taxa.
- Distance: more distant islands harbor fewer species.
Concept: equilibrium theory explains colonization-extinction dynamics on islands (Log-scale representations often used).

Key concepts:
- Biosphere vs ecosystem: energy flow and biomass production within ecosystems.
- Producers vs consumers: autotrophs produce; heterotrophs consume.
- Trophic levels: primary producers, primary/secondary/tertiary consumers; apex predators.
- Types of consumers: herbivores, carnivores, omnivores; decomposers & detritivores.
Food chains vs food webs:
- Food chain: linear sequence of energy transfer.
- Food web: network of interconnected feeding relationships.
Energy transfer and chain length:
- Chain length = number of links between trophic levels.
- Only ~ $10\%$ of energy is transferred to the next level; the rest is lost as heat, maintenance, etc.
Ecological pyramids:
- Pyramid of numbers, pyramid of biomass, pyramid of energy.
- Pyramids can vary by ecosystem; decomposers may have large energy flow despite small biomass.
Producers and major marine producers:
- Phytoplankton are primary producers in marine systems; zooplankton and krill are key consumers.
Zones and productivity:
- Photic, littoral, profundal zones in aquatic systems; productivity generally declines with depth and distance from land.
Biomass and energy distribution:
- Most Earth biomass is on land; most animal biomass is in marine environments; detrital pathways dominate energy flow.
  -1990s–2018 notes:
- ~550 Gt C globally in biomass; humans have greatly reduced wild mammal biomass over time.

Primary production controls:
- Terrestrial: water, temperature, nutrients (N, P).
- Aquatic: light and nutrients; nitrogen and phosphorus are often limiting.
Aquatic production specifics:
- Photic zone, littoral zone, profundal zone describe light and depth-related variation.
- Phytoplankton dominate primary production; zooplankton, krill as key consumers.
- Upwellings bring nutrient-rich water to surface, enhancing productivity.
- Nutrient enrichment (e.g., runoff) can cause algal blooms and dead zones.
NPP and GPP:
- Gross primary production: $GPP$ = carbon fixed during photosynthesis.
- Net primary production: $NPP = GPP - R$ where $R$ is respiration; NPP is energy available to primary consumers.
Biomass distribution and turnover:
- Land biomass >> marine biomass in total, but marine producers often have rapid turnover.
- Biomass distribution varies by trophic level and environment; turnover rates differ between terrestrial and marine systems.
Practical takeaways:
- Primary production is limited by water, temperature, and nutrients; light limits aquatic production.
- Most primary production enters detrital pathways rather than supporting herbivores directly.
- Understanding NPP helps predict energy available to ecosystems and their structure.

Water, temperature, and nutrient availability are the main limiting factors for terrestrial and aquatic primary production.
In aquatic systems, light and nutrient availability interact to limit production; upwellings and nutrient inputs play critical roles.
Distribution of Earth’s biomass shows strong land dominance in total biomass, with notable marine biomass and rapid turnover in marine producers.
Equation recap: $NPP = GPP - R$ ; energy transfer between trophic levels is inefficient and drives pyramid shapes.