Earth's ecosystems are diverse and complex, ranging from lush forests to arid deserts on land, and from freshwater lakes to vast oceans. These ecosystems support unique communities of organisms, each adapted to their specific environment and playing crucial roles in the flow of energy and nutrients.

Scientists use various methods to study ecosystems, including field observations, experiments, and modeling. These approaches help us understand how ecosystems function, how they respond to changes, and how we can better manage and protect them in the face of environmental challenges.

Ecosystem Types and Structure

Types of major ecosystems

• Terrestrial ecosystems support life on land
  • Forest ecosystems characterized by dense tree cover (tropical rainforests, temperate forests, boreal forests or taiga)
  • Grassland ecosystems dominated by grasses and herbaceous plants (savannas, prairies, steppes)
  • Desert ecosystems receive little precipitation (hot deserts like the Sahara, cold deserts like the Gobi)
  • Tundra ecosystems found in cold, treeless regions (Arctic tundra, alpine tundra)
• Aquatic ecosystems support life in water
  • Freshwater ecosystems contain water with low salt content
    • Lentic ecosystems are still water habitats (lakes, ponds)
    • Lotic ecosystems are flowing water habitats (rivers, streams)
  • Marine ecosystems contain saltwater
    • Coastal ecosystems are near shore (estuaries, coral reefs)
    • Open ocean ecosystems are offshore (pelagic zone is the water column, benthic zone is the seafloor)

Methods in ecosystem analysis

• Field observations and sampling collect data on ecosystem components
  • Transects and quadrats survey species composition and abundance
  • Mark and recapture estimates population sizes
  • Remote sensing uses satellite imagery to map ecosystem extent and change
• Experimental manipulations test hypotheses about ecosystem processes
  • Controlled experiments isolate specific variables (nutrient addition, predator exclusion)
  • Natural experiments take advantage of existing gradients or disturbances (elevation, fire)
• Ecosystem monitoring tracks changes over time
  • Long-term ecological research (LTER) sites are studied for decades (Hubbard Brook, Konza Prairie)
  • Sensor networks continuously measure environmental variables (temperature, soil moisture)
• Stable isotope analysis traces the flow of elements through ecosystems
  • Carbon isotopes ($^{12}C$ and $^{13}C$) indicate carbon sources and pathways
  • Nitrogen isotopes ($^{14}N$ and $^{15}N$) reveal trophic positions and nutrient cycling
• Ecosystem modeling integrates data and understanding
  • Conceptual models represent key components and interactions (food web diagrams)
  • Mathematical models quantify rates of change and fluxes (nutrient cycling equations)
  • Computer simulations predict ecosystem responses to perturbations (climate change scenarios)

Ecosystem Dynamics and Modeling

Ecosystem modeling techniques

• Types of ecosystem models represent different aspects of ecosystem structure and function
  • Conceptual models use flowcharts and diagrams to illustrate key components and interactions (energy flow, nutrient cycling)
  • Mathematical models use equations to describe ecosystem processes and quantify rates of change and fluxes (primary productivity, decomposition)
  • Computer simulations integrate multiple mathematical models to predict ecosystem responses to perturbations (land use change, invasive species)
• Applications of ecosystem models advance understanding and inform management
  • Understanding ecosystem functioning by testing hypotheses and exploring scenarios (carbon sequestration, nutrient retention)
  • Predicting responses to environmental changes such as climate change and pollution (species range shifts, eutrophication)
  • Informing management decisions about resource use and conservation (sustainable harvest, habitat restoration)
  • Identifying knowledge gaps and research priorities to guide future studies (data needs, process uncertainties)

Food chains vs ecosystem stability

• Food chains are linear sequences of energy transfer between trophic levels
  • Trophic levels are feeding positions in a food chain (primary producers, primary consumers, secondary consumers, tertiary consumers)
  • Energy loss occurs between trophic levels due to ecological efficiency and the 10% rule (only 10% of energy is passed to the next level)
• Food webs are complex networks of feeding relationships among species
  • Interconnectedness of species creates multiple pathways for energy flow and nutrient cycling (generalist predators, detritivores)
  • Omnivory (eating at multiple trophic levels) and intraguild predation (eating competitors) add complexity to food webs
• Ecosystem stability depends on resistance to and resilience from perturbations
  • Resistance is the ability to withstand disturbances without changing state (invasive species resistance)
  • Resilience is the ability to recover from disturbances and return to the original state (fire resilience)
  • Biodiversity enhances ecosystem functioning through redundancy (multiple species performing similar roles) and functional complementarity (species performing different roles)
    • Insurance hypothesis suggests that biodiversity provides a buffer against environmental variability
  • Trophic cascades are indirect effects that ripple through food webs
    • Top-down control occurs when predators regulate prey populations and indirectly affect lower trophic levels (wolf reintroduction in Yellowstone)
    • Bottom-up control occurs when resource availability limits higher trophic levels (nutrient limitation of primary productivity)
  • Keystone species have disproportionate effects on ecosystem structure and function relative to their abundance (sea otters in kelp forests)
  • Ecosystem engineers modify habitats and create opportunities for other species (beavers building dams)

Ecosystem processes and interactions

• Biogeochemical cycles describe the movement of elements through ecosystems (carbon cycle, nitrogen cycle, phosphorus cycle)
• Primary productivity is the rate at which producers create organic matter through photosynthesis or chemosynthesis
• Decomposition breaks down organic matter, releasing nutrients back into the ecosystem
• Ecological succession is the process of change in species composition over time
  • Primary succession occurs on newly exposed surfaces (volcanic islands, retreating glaciers)
  • Secondary succession follows disturbances to existing communities (forest fires, abandoned farmland)
• Carrying capacity is the maximum population size an ecosystem can support indefinitely
• Niche is the role and position a species occupies in its environment, including its interactions with other species and its use of resources