Biodiversity, Ecosystem Function, and Biogeochemical Cycles

Unit 5: Biodiversity, Ecosystem Function, and Biogeochemical Cycles

Overview

This unit examines the origins and measurement of biodiversity, the ecological and evolutionary factors shaping species distributions, and the flow of energy and cycling of matter within ecosystems. It also addresses the impact of human activities on ecosystem processes.

Biodiversity

Defining Biodiversity

• Biodiversity refers to the variety of life forms in a given area, encompassing diversity at the genetic, species, and ecosystem levels.

• Community biodiversity focuses on the diversity of species within a particular ecological community.

Measuring Biodiversity

• Species richness: The total number of different species present in a community.

• Species evenness: The relative abundance of each species in a community; high evenness means species are present in similar proportions.

• Relative abundance: The proportion of each species relative to the total number of individuals in the community.

Example: A forest with 10 tree species (richness) where each species has similar numbers of individuals (evenness) is more diverse than one dominated by a single species.

Patterns and Factors Affecting Biodiversity

• Extinction: The loss of species, which reduces biodiversity.

• Latitudinal effects: Biodiversity tends to be higher near the equator and decreases toward the poles.

• Elevation effects: Species diversity often changes with altitude, typically peaking at mid-elevations.

• Insular effects: Islands often have fewer species (lower richness) but high rates of endemism due to isolation.

• Time and stability: Older and more stable environments tend to support higher biodiversity.

• Habitat heterogeneity: More varied habitats support more species by providing diverse niches.

• Keystone species: Species with a disproportionately large effect on community structure; their removal can drastically alter ecosystem function.

Disturbance and Biodiversity

• Disturbance: Events (natural or human-caused) that change community structure by removing organisms or altering resource availability.

• Intermediate disturbance hypothesis: Moderate levels of disturbance can foster higher diversity than low or high disturbance levels.

Scales of Diversity

• Alpha diversity: Species diversity within a particular area or ecosystem (local scale).

• Beta diversity: The difference in species composition between ecosystems; measures species turnover across habitats.

• Gamma diversity: Total species diversity in a landscape or region (regional scale).

Ecosystem Structure and Function

Energy Flow in Ecosystems

• Energy: The capacity to do work; in ecosystems, energy flows from the sun to producers and then through various consumer levels.

• Food chain: A linear sequence of organisms through which energy passes (e.g., plant → herbivore → carnivore).

• Food web: A complex network of interconnected food chains in an ecosystem.

• Basal resource: The primary source of energy for a food web, typically photosynthetic organisms (plants, algae).

Primary Production

• Gross primary production (GPP): The total amount of energy captured by producers via photosynthesis in a given area and time.

• Net primary production (NPP): The energy remaining after producers use some for their own respiration; available to consumers.

Equation:

$\mathrm{NPP} = \mathrm{GPP} - \text{Respiration}$

Example: In a grassland, GPP is the total energy plants capture from sunlight, while NPP is what remains for herbivores after plants meet their own energy needs.

Biogeochemical Cycles

General Concepts

• Matter: Elements and compounds that cycle through ecosystems, unlike energy, which flows one way.

• Chemical cycles / Biogeochemical cycles: The movement of elements (e.g., carbon, nitrogen, phosphorus) through living (biotic) and nonliving (abiotic) components of ecosystems.

• Abiotic reservoir: Nonliving storage sites for elements (e.g., atmosphere, soil, water, rocks).

The Carbon Cycle

• Carbon moves between the atmosphere, organisms, and the earth via processes such as photosynthesis, respiration, decomposition, and combustion.

Equation (Photosynthesis):

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

Equation (Respiration):

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2 \rightarrow 6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O}$

The Nitrogen Cycle

• Nitrogen cycle: The movement of nitrogen among the atmosphere, soil, and living organisms.

• Nitrogen fixation: Conversion of atmospheric nitrogen ($\mathrm{N}_2$) into ammonia ($\mathrm{NH}_3$) by bacteria or lightning.

• Nitrogenase: The enzyme complex used by nitrogen-fixing bacteria to convert $\mathrm{N}_2$ to $\mathrm{NH}_3$.

• Ammonification: Decomposition of organic nitrogen into ammonia by decomposers.

• Nitrification: Conversion of ammonia to nitrate ($\mathrm{NO}_3^-$) by soil bacteria.

• Denitrification: Conversion of nitrate back to atmospheric nitrogen by bacteria, completing the cycle.

Example: Legume plants host nitrogen-fixing bacteria in root nodules, enriching soil nitrogen.

The Phosphorus Cycle

• Phosphorus cycle: The movement of phosphorus through rocks, water, soil, and living organisms; unlike carbon and nitrogen, phosphorus does not have a significant atmospheric component.

• Apatite: A mineral that is a major abiotic reservoir of phosphorus.

Human Impacts on Ecosystem Processes

• Cultural eutrophication: Excessive nutrient input (especially nitrogen and phosphorus) from human activities (e.g., agriculture, sewage) leading to overgrowth of algae in aquatic systems.

• Marine dead zones: Areas in oceans or lakes with low oxygen (hypoxia) caused by nutrient pollution, resulting in massive die-offs of aquatic life.

• Human activities can disrupt nutrient cycles, reduce biodiversity, and alter ecosystem function.

Key Terms Table

Term	Definition
Biodiversity	Variety of life at genetic, species, and ecosystem levels
Species richness	Number of species in a community
Species evenness	Relative abundance of species
Keystone species	Species with a large effect on ecosystem structure
Gross primary production (GPP)	Total energy captured by producers
Net primary production (NPP)	Energy available to consumers after producer respiration
Biogeochemical cycle	Movement of elements through living and nonliving systems
Nitrogen fixation	Conversion of atmospheric nitrogen to ammonia
Cultural eutrophication	Human-caused nutrient enrichment of water bodies
Marine dead zone	Low-oxygen aquatic area caused by nutrient pollution
Apatite	Phosphorus-containing mineral
Disturbance	Event that changes community structure
Intermediate disturbance hypothesis	Moderate disturbance promotes highest diversity

Additional info: Some definitions and explanations have been expanded for clarity and completeness based on standard biology curriculum.