Understanding Biogeochemical Cycles, Population Ecology, and Biodiversity

Posted on Aug 27, 2024 in Biology

Biogeochemical Cycles

Life is linked to the availability of about 25 chemical elements. H, C, O, N, P, and S make up 99% of all living matter and can act as biolimiting elements. These elements relate to:

  • Living things
  • The geological environment (atmosphere, hydrosphere, lithosphere)

This interconnectedness is known as Biogeochemical Cycles. There are two main groups:

Gaseous Nutrient Cycles

  • Main reservoir: the atmosphere
  • The most important: C, N, and O

Sedimentary Nutrient Cycles

  • Main reservoir: the lithosphere
  • Nutrients are released slowly through weathering processes
  • The most important: P and S
  • Circulation is much slower, and elements tend to settle
  • These elements are often limiting factors

Carbon Cycle

The pathway of carbon in the biosphere:

  • CO2 is incorporated into living matter through photosynthesis
  • CO2 is incorporated into shells and skeletons through biochemical processes
  • CO2 is re-released through cellular respiration and the transformation of organic compounds

Part of the carbon is removed from the main cycle through a very slow process that stores huge amounts in sedimentary rocks. This carbon returns to the main cycle through the burning of fossil fuels and organic matter, and through the dissolution of carbonated water on carbonate rocks.

Nitrogen Cycle

Nitrogen cannot be used directly by the vast majority of organisms. Atmospheric nitrogen (N2) must be transformed into the nitrate anion (NO3) before being assimilated. Organisms capable of fixing atmospheric nitrogen include:

  • Symbiotic nitrogen fixers: Bacteria and some fungi
  • Free-living nitrogen fixers: Aerobic bacteria, anaerobic bacteria, and cyanobacteria

Nitrogen fixation (N2 to NO3) is enhanced by the cultivation of legumes and the industrial production of fertilizer. Annually, nitrogen fixation exceeds the natural rate by 10%. Improper handling of fertilizers and nitrogenous wastes can lead to eutrophication and rapid ammonification.

After the incorporation of the nitrate anion, waste products of metabolism and organic wastes are transformed into ammonia (NH3) by decomposing microorganisms. This process is known as nitrogen mineralization (Protein to NH3), and is carried out by bacteria of the genus Clostridium.

Nitrification

The transformation of ammonia to nitrate is carried out by chemosynthetic bacteria in two phases:

  • Genus Nitrosomonas: Converts ammonia into nitrite anion (NO2)
  • Genus Nitrobacter: Converts nitrite to nitrate anion (NO3)

This process is known as nitrification (NH3 to NO3).

Denitrification

The conversion of nitrate anion (NO3) to atmospheric nitrogen (N2) is done by some species of fungi and denitrifying bacteria in anaerobic conditions. This process is known as denitrification (NO3 to N2).

Phosphorus Cycle

Phosphorus is one of the most important nutrients for living organisms. It is found in nucleic acids, ATP, and skeletons and shells. It is of great importance as a limiting nutrient.

Producers require inorganic phosphate (PO4), which is transferred in the form of organic phosphate as part of organic molecules that contain ATP, nucleic acids, and phospholipids. Decomposers transform organic phosphate back into inorganic phosphate. Much of the phosphate is lost from the food web due to physical processes such as sedimentation. Some biological processes (phosphate deposition in skeletons and shells, excretion, etc.) also cause loss of phosphorus from ecosystems.

POPULATION ECOLOGY

Population

A population is a group of organisms of the same species capable of reproducing with each other and occupying a specific area. Some important factors influencing population dynamics include:

  • Birth rate (tn): Number of individuals born per unit time. This can be further divided into:
    • Birth potential: The birth rate when the population is not subject to adverse conditions.
    • Real birth: The birth rate when the population is subject to environmental constraints.
  • Mortality rate (tm): Number of deaths that occur in a population per unit time. This can be further divided into:
    • Potential mortality: The minimum number of estimated deaths in a population.
    • Actual mortality: The actual number of deaths.
  • Population size (N): The total number of individuals within a population.
    • Ecological density: Number of individuals per unit area or volume occupied or habitat.

Population density should be kept in balance for the survival of the species. A very small population density can hinder sexual reproduction due to difficulty in encounters between individuals of different sexes.

  • Immigration rate (i): The number of individuals entering the population from other places.
  • Emigration rate (e): The number of individuals of the original population who leave and go elsewhere.
  • Biotic potential (r): The theoretical growth in the number of individuals when birth potential coincides with the birth rate and death rate coincides with potential mortality.

Growth Rates of Population (TC)

TC = tn – tm + i – e

A positive TC indicates population growth, while a negative TC indicates population decline.

Growth Curves

Plotting the number of individuals in a population over time results in growth curves. There are two main types:

J or Exponential Growth Curve

  • The population grows exponentially until it exhausts a resource.
  • There is an abrupt stop and rapid decline in the number of individuals.
  • Unstable over time.

S or Sigmoid (Logistic) Growth Curve

  • Several phases:
    • Lag phase: The population is growing slowly.
    • Exponential phase: Growth is rapid.
    • Stationary phase: The rate of growth slows.
  • K (Carrying capacity): The maximum theoretical density of individuals that an area can support.
  • Populations are more stable.
  • May present oscillations due to variations in K:
    • Cyclical oscillations
    • Acyclic oscillations

Coping Strategies

There are two main strategies for survival and population growth: r-strategy and K-strategy.

R-Strategy

  • Microscopic or small species
  • Exponential growth (J-shaped curve)
  • Typical of ephemeral or unstable environments
  • Opportunistic and invasive
  • Produce many offspring with little parental investment

K-Strategy

  • Sigmoid growth (S-shaped curve)
  • Population fluctuates but remains close to the carrying capacity (K)
  • Lower biotic potential
  • Greater ability to compete
  • Greater longevity
  • Reduced number of offspring with significant parental investment
  • Present in stable environments

Survival Curves

Survival curves are graphic representations of the variation in the number of individuals of a population in relation to age. There are three main types:

Curve A

  • Low initial mortality rate
  • Sharp decline in survival after a certain age
  • Typical of humans in developed countries

Curve B

  • Constant death rate
  • Typical of species that constantly renew their tissues

Curve C

  • The most common type
  • High initial mortality rate
  • Decreasing mortality as age increases

Tolerance Curves

Tolerance curves describe the range of environmental conditions that a species can tolerate.

  • Stenoic Species:
    • Require very specific environmental conditions to survive.
    • May have a small geographic range.
    • Narrow tolerance limits.
    • Can have high population densities under optimal conditions.
    • Typically K-strategists and specialists.
  • Euryoic Species:
    • Tolerate a wider range of environmental conditions.
    • Maximum population densities are usually not very high.
    • Tend to be r-strategists and generalists.

Ecological Succession

Ecosystems change over time through a process called ecological succession. This is a continuous process that leads to the formation of a climax community, which is in equilibrium with the physical environment.

Based on the dominant species, a series of stages can be defined:

  • Lichens and mosses
  • Annual grasses
  • Shrubs (bush)
  • Trees (forest)

Types of Succession

  • Primary Succession: Occurs in an area lacking a pre-existing community, such as dunes or new islands.
  • Secondary Succession: Occurs in an area where an existing community has been disturbed, such as after a fire or flood.

Changes in Ecological Succession

Comparing the initial and final stages of ecological succession reveals several changes:

Structural Changes

  • Changes in species composition:
    • Early stages: Dominated by r-strategists
    • Final stages: Dominated by K-strategists
  • Increased species diversity:
    • Final stages:
      • Increased complexity in ecosystem structure
      • Increased number of trophic levels and interactions
      • Complex food webs

Functional Changes

  • Absence of untapped niches, leading to ecosystem stability
  • Progressive increase in biomass due to the increasing number of species
  • Variation in the metabolism of the community:
    • Early stages: P/R > 1 (autotrophic ecosystem)
    • Final stages: P/R = 1
    • Ecosystems tend to evolve towards a stable metabolism (P/R = 1)

Biodiversity

Biodiversity refers to the variety of organisms living on our planet. It includes multiple levels of organization:

  • Genetic diversity: Variability of genes and chromosomes within a species.
  • Population diversity: Diversity of populations within a species.
  • Species diversity: Encompasses all living species.
  • Ecosystem diversity: Integrates all previous levels, including the physical environment and interactions.

A major threat to biodiversity is ecological regression, which can be caused by natural or anthropogenic factors. An example of ecological regression is the degradation of the Tablas de Daimiel National Park in Spain.