APES test 3

Ecological Principles, Evolution, and Biogeochemical Cycles: A Briefing

Ecosystem Dynamics: Energy Flow and Productivity

I. Foundational Principles of Ecosystems

The functioning of all ecosystems is governed by a set of fundamental principles that dictate the movement of energy and matter.

• Energy Flow vs. Nutrient Cycling: Energy flows unidirectionally through an ecosystem and is not recycled. It is lost as heat with each conversion between trophic levels. In contrast, nutrients exist in a fixed supply and must be continuously recycled for life to persist.

• Interdependence of Components: Life relies on a delicate balance of biotic (living) and abiotic (non-living) components. Any disruption to these components will have a corresponding impact on the entire ecosystem.

• Niche and Interconnectivity: Each organism occupies a specific role, or niche. Due to this interconnectedness, any factor that affects one organism will inevitably impact the rest of the ecosystem in some way.

II. Trophic Structure and Food Webs

The organization of an ecosystem is defined by the feeding relationships between its organisms, known as the trophic structure.

Trophic Levels and Organism Roles

A trophic, or feeding, level is determined by an organism's food source. Energy and nutrients are transferred through these levels.

• Producers (Autotrophs): These organisms create their own food. On land, the primary producers are plants; in open water, they are phytoplankton. Most use photosynthesis, but some, particularly near hydrothermal vents, use chemosynthesis.

• Consumers (Heterotrophs): These organisms obtain energy by feeding on other organisms.

  • Primary Consumers: Herbivores that feed on producers.

  • Secondary Consumers: Carnivores that feed on herbivores (e.g., spiders, frogs).

  • Tertiary (or higher) Consumers: Carnivores that feed on other carnivores.

  • Other Consumer Types:

    • Omnivores: Feed on both plants and animals.

    • Decomposers & Detritivores: Break down dead organic matter (detritus), returning nutrients to the soil. Examples include fungi, bacteria, and various insects like bark beetles and termites.

    • Scavengers: Consume dead animals.

Food Webs

A food web illustrates the complex network of feeding relationships within an ecosystem.

• Structure: Arrows in a food web always point from the organism being eaten to the organism that eats it. The structure is hierarchical, with producers at the bottom, followed by successive levels of consumers, and top predators at the apex.

• Analysis: A food web allows for the identification of an organism's trophic level(s), the extraction of individual food chains, and the identification of organisms at each trophic level, including the top predators.

III. Energy Transfer and Ecological Efficiency

The movement of energy from one trophic level to the next is inherently inefficient, which has profound implications for ecosystem structure.

The 10% Rule

Only a fraction of the energy consumed at one trophic level is converted into biomass at the next.

• Ecological Efficiency: This measures the percentage of energy transferred between trophic levels. It typically ranges from 2% to 40%.

• The 10% Approximation: For simplified mathematical calculations, ecological efficiency is commonly expressed as 10%. This means approximately 90% of energy is lost at each transfer.

• Energy Loss: The majority of energy is not transferred because it is used for metabolic processes or lost from the system. For example, of the energy a caterpillar consumes, approximately 50% may be lost as waste, 35% used for respiration, and only 15% converted into caterpillar biomass.

IV. Ecological Pyramids

Ecological pyramids are graphical representations used to visualize the distribution of energy, biomass, or number of organisms across trophic levels.

• Energy Pyramid: This pyramid illustrates the amount of energy available at each trophic level. It is always pyramid-shaped due to the progressive loss of energy at each successive level. Energy is typically measured in calories (cals), kilocalories (kcals), Joules (J), or kilojoules (kJ).

• Biomass Pyramid: This shows the total mass of organisms (biomass) stored at each trophic level at a given time. It is usually pyramid-shaped. However, in certain ecosystems like the open ocean, the pyramid can appear inverted because producers (phytoplankton) are consumed so rapidly that their biomass at any given moment is very low compared to the primary consumers (zooplankton). Biomass is measured in units like kg/m² or g/m² (often as dry weight).

• Pyramid of Numbers: This pyramid depicts the total number of individual organisms at each trophic level.

V. Ecosystem Productivity

Productivity is the rate at which biomass is generated in an ecosystem, forming the basis of its energy budget. It is distinct from biomass, which is the total stored mass at a given time.

Gross vs. Net Primary Productivity

• Gross Primary Productivity (GPP): The total rate at which producers capture and store chemical energy as biomass. It represents the total amount of photosynthesis in an ecosystem.

• Net Primary Productivity (NPP): The rate at which producers create biomass minus the rate at which they use some of that energy for their own respiration. NPP represents the energy available to consumers. The relationship is defined by the equation: NPP = GPP - R (where R is Respiration).

The efficiency of photosynthesis is generally low, with only about 1% of the solar energy striking producers being captured and converted into GPP. Of this GPP, a significant portion (e.g., 60%) is lost to respiration, with the remainder (e.g., 40%) supporting the producer's growth and reproduction as NPP.

Productivity Dynamics

• High Biomass, Low NPP: A slow-growing forest can have a very low NPP but store a massive amount of biomass in its wood and root structures accumulated over many years.

• Low Biomass, High NPP: Algae in the ocean can have a very high NPP due to rapid growth and reproduction, but their stored biomass is low because they are consumed just as quickly.

VI. Core Biological Processes

Several fundamental biological processes drive the flow of energy and matter in ecosystems.

• Photosynthesis: The process used by producers to convert light energy into chemical energy (glucose), using carbon dioxide and water.

• Aerobic Respiration: A cellular process that releases energy from glucose using oxygen. Its products are carbon dioxide (CO₂) and water (H₂O).

• Anaerobic Respiration (Fermentation): A process used by some decomposers to break down glucose without oxygen. Its products can include methane, ethanol, acetic acid, and hydrogen sulfide (H₂S).

• Chemosynthesis: A process used by some organisms, particularly in environments without sunlight like deep-sea volcanic vents, to produce food using chemical energy from inorganic compounds instead of light energy.

VII. Ecosystem Vulnerabilities and Disruptions

The interconnected nature of ecosystems makes them vulnerable to disruptions at any level.

• Loss of Key Components:

  • Loss of Top Predators: Can lead to overpopulation of herbivores, causing overgrazing and ecosystem degradation.

  • Loss of Primary Producers: Causes the collapse of the entire food web, as the energy base is removed.

  • Loss of Decomposers: Would halt the recycling of nutrients, making them unavailable for producers and eventually starving the ecosystem.

• Case Study: Colony Collapse Disorder: The phenomenon of disappearing bee colonies raises critical questions about its causes (e.g., pesticides, disease), its potential impacts on agriculture and natural ecosystems that rely on bee pollination, and the viability of alternatives to bee pollination.

VIII. Quantitative Analysis and Key Formulas

The study of ecosystem dynamics often involves mathematical calculations to quantify energy flow and productivity.

Essential Equations

Application Example

Eco-math problems demonstrate how these principles apply to real-world scenarios. For instance, by using the 10% rule and an animal's caloric requirements, one can calculate the area of plant material required to support a top predator like a fox. Such calculations reveal why top predators require large territories to obtain sufficient energy from the producer base.

Ecological Niches and Species Adaptation

Defining the Ecological Niche

An organism's ecological niche describes its role and position within an ecosystem. This includes where it lives, its patterns of activity, how it obtains energy, and how it interacts with other organisms and the habitat.

• Example: Dung Beetle: The dung beetle lives in leaf litter and undergrowth. It consumes fecal matter for energy, classifying it as a detritivore. It also rolls dung into spheres, a unique behavior that illustrates its interaction with its habitat.

Fundamental vs. Realized Niche

The concept of a niche is further refined into two specific categories:

• Fundamental Niche: This is the idealized set of conditions—including habitat, resources, and climate—where a species could survive, grow, and reproduce without interference from predators or competitors. It represents the full potential range of an organism.

• Realized Niche: This is the actual set of conditions in which a species exists. The realized niche is a subset of the fundamental niche, narrowed by limiting factors such as competition for resources and the presence of predators.

  • Example: A mouse's fundamental niche might include an entire house. However, if cats (predators) are present downstairs, the mouse's realized niche will be restricted to the attic, where it can avoid them.

Ecological Range of Tolerance

Every species has an ecological range of tolerance for various environmental factors, such as temperature.

• Within this range, there is an optimal zone where the species thrives, grows, and reproduces. This corresponds to its fundamental niche.

• At the extremes of this range (e.g., very cold or very hot temperatures), the organism may only be able to survive. It will not have sufficient energy to grow or reproduce. The species is merely tolerating the conditions.

Case Studies: Polar Bears and Mangroves

• Polar Bears: The fundamental niche for polar bears involves hunting seals on frozen sea ice, which is their ideal situation. Due to climate change and melting ice, their realized niche is increasingly shifting to swimming and hunting in open water, a less efficient and more stressful environment that falls within their range of tolerance but is not optimal for thriving.

• Mangroves: Mangrove trees exhibit a broad ecological range of tolerance, as they can grow in conditions ranging from fresh water to salt water. However, their realized niche is often in areas with a salinity of around 35 ppm, which is where they are typically found.

Niche Generalists vs. Specialists

Organisms can be classified based on the breadth of their fundamental niche.

Humans are considered niche generalists due to their ability to inhabit virtually every environment on Earth. The distinction between a generalist and a specialist can sometimes be a matter of interpretation, but extreme cases are clear.

Environmental Change and Niche Dynamics

Environmental shifts, such as climate change, directly impact species' niches and distribution. An analysis of tree distribution in North America since the last ice age illustrates this:

• Spruce Tree (Specialist): As the climate warmed, the spruce tree's range shifted progressively north. It could not tolerate the warmer temperatures of the lower latitudes, demonstrating a narrow ecological niche.

• Pine Tree (Generalist): The pine tree demonstrates a very broad niche, with a range extending from Florida to Alaska, indicating its ability to thrive in vastly different climates.

Evolution and Natural Selection

The Mechanism of Natural Selection

Natural selection is a primary mechanism driving evolution, which is defined as the change in a population's genetic makeup over time. The process relies on several key factors:

1. Variation: Individuals within a species exhibit a variety of traits (e.g., different colorations).

2. Environmental Pressure: The environment presents challenges, such as predators.

3. Differential Survival & Reproduction: Individuals with traits better suited to the environment (adaptations) are more likely to survive and reproduce. Biological fitness is measured not by lifespan, but by the number of offspring an organism produces.

4. Inheritance: Advantageous traits are passed down to offspring, increasing their frequency in the population over subsequent generations.

• Example: Green Frogs: In a habitat with dark logs, lighter green frogs are more visible to predators and are selected against. Darker green frogs camouflage better, survive longer, and produce more offspring. Over a long period, the frequency of genes for darker coloration will increase in the frog population.

The Role of Random Variation and Mutation

It is critical to understand that the variations upon which natural selection acts are random.

• Organisms cannot will themselves to develop beneficial traits.

• Mutations and genetic recombination create new variations by chance.

• The effect of a new trait can be negative (reducing fitness), neutral (no effect on fitness), or positive (increasing fitness). A positive trait that helps an organism survive and reproduce is called an adaptation.

Case Study: Antibiotic Resistance in Bacteria

The development of antibiotic resistance in bacteria is a powerful, observable example of natural selection.

1. Variation: Within a bacterial population, random mutations mean some bacteria may possess traits that offer resistance, such as an enhanced cell wall or enzymes that break down the antibiotic.

2. Environmental Change: The introduction of an antibiotic dramatically alters the bacteria's environment.

3. Selection: The antibiotic kills susceptible bacteria, while the resistant bacteria survive. These survivors have higher fitness.

4. Reproduction & Inheritance: With their competition eliminated, the resistant bacteria reproduce easily, passing the resistance gene to all their offspring. They can also transfer these genes to other bacteria.

5. Result: The antibiotic becomes less effective for future treatments. This is a significant challenge in hospitals, necessitating strict hygiene and the ongoing development of new antibiotics. It is also why antibiotics should only be taken for bacterial (not viral) infections.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism, including all its alleles (e.g., carrying a recessive allele for a trait).

• Phenotype: The observable, expressed physical traits of an organism.

• Example: Tongue Rolling: The ability to roll one's tongue is a vestigial structure from primate ancestors that likely helped strip leaves from branches. It serves no purpose for modern humans. The loss of this ability represents an evolutionary change.

Speciation, Extinction, and Genetic Modification

The Process of Speciation

Speciation is the evolutionary process by which new biological species arise. A critical requirement for speciation is reproductive isolation, where two groups can no longer interbreed to produce viable, fertile offspring.

There are two primary modes of speciation:

1. Allopatric Speciation: This occurs when populations become separated by a geographic barrier.

   • Mechanism: A physical barrier like a river or mountain range prevents gene flow between populations. Over time, the isolated populations evolve independently, accumulating genetic differences until they can no longer interbreed.

   • Examples: The unique marsupials of Australia, which evolved in isolation after the continent separated. Darwin's finches, where different populations evolved unique traits on the various Galápagos Islands.

2. Sympatric Speciation: This occurs when new species evolve from a single ancestral species while inhabiting the same geographic area.

   • Mechanism: Reproductive isolation arises from factors other than geography, such as behavioral changes or genetic mutations.

   • Examples: The apple maggot fly, which is diverging into two groups based on a preference for mating on different types of fruit (apples vs. hawthorns). Polyploidy in plants, where a mistake in cell division leads to an extra set of chromosomes, making the new plant reproductively incompatible with its ancestors.

Factors Influencing Adaptation and Survival

A population's ability to adapt to changing environmental conditions is influenced by four key factors:

Other important considerations include mobility, which allows species to move to more suitable habitats, and symbiotic relationships (e.g., bats and cacti), which require both species to co-evolve to survive changes.

Extinction Events

• Past Extinctions: The fossil record shows several mass extinction events, including a major oceanic extinction ~250 million years ago and the KT event that eliminated the dinosaurs ~65 million years ago.

• The Sixth Mass Extinction: We are currently in a sixth mass extinction event, which is being caused by human activities. The primary drivers are often summarized as "HIPPCO":

  • Habitat loss

  • Invasive species

  • Pollution

  • People (Population growth)

  • Climate change

  • Overharvesting

• Recovery: Based on the fossil record, recovery of biodiversity after a mass extinction event takes millions of years. Life will likely survive, but the planet's ecosystems will be fundamentally and dramatically different.

Genetically Modified Organisms (GMOs)

GMO technology represents a rapid form of artificial selection, where desired genes are directly inserted into an organism's genome. Applications include:

• Agriculture: Creating crops that grow faster or are resistant to drought and pests.

• Pollution Cleanup: Engineering bacteria that can consume oil spills.

• Biofuels: Using modified bacteria to convert algae into biodiesel.

Biogeochemical Cycles

Biogeochemical cycles describe the pathways of essential elements and nutrients as they move between biotic (living or once-living) and abiotic (non-living) components of an ecosystem.

Foundational Concepts

• Limiting Factors: Nutrients or conditions that can limit the growth of a population. Nitrogen and phosphorus are the two most common limiting nutrients in terrestrial and aquatic ecosystems, which is why they are primary components of fertilizers.

• Movement Through Ecosystems: Producers absorb abiotic nutrients and convert them into biomolecules. Consumers obtain these by eating producers. Decomposers (bacteria and fungi) are uniquely capable of breaking down dead organic matter and waste, returning nutrients to their abiotic forms. Detritivores cycle nutrients within the biotic system but do not return them to the abiotic system.

The Water (Hydrologic) Cycle

Key processes in the water cycle include:

• Runoff: Water flowing across the Earth's surface into bodies of water.

• Infiltration: Water seeping from the surface into the soil.

• Percolation: The downward movement of water through soil due to gravity.

• Transpiration (Evapotranspiration): The evaporation of water from the leaves of plants.

The Carbon Cycle

The carbon cycle has two main components: a fast cycle involving living things and a slow cycle involving long-term storage.

• Carbon Sink: A reservoir that stores carbon for a long period. Major sinks include:

  • Oceans

  • Fossil fuels

  • Soil

  • Old-growth forests

• Carbon Source: A reservoir where carbon is stored for a short period before being moved elsewhere (e.g., living plants and animals).

• Human Disruption: Human activities are moving massive amounts of carbon from slow-cycle sinks into the atmosphere, disrupting the global climate. The two primary disruptions are:

  1. Burning of fossil fuels.

  2. Deforestation, which reduces the capacity of forests to absorb atmospheric CO2.

The Phosphorus Cycle

• Key Feature: The phosphorus cycle has no atmospheric component.

• Mechanism: It is primarily driven by the rock cycle. Weathering and erosion of rocks release phosphorus into the soil and water. It is then taken up by plants and cycled through the food web. Its slow release and lack of an atmospheric phase make it a major limiting nutrient.

The Sulfur Cycle

• Key Feature: A major source of atmospheric sulfur is volcanic activity.

The Nitrogen Cycle

The nitrogen cycle is essential for life, as nitrogen is a key component of proteins and nucleic acids. Although the atmosphere is ~78% nitrogen (N2), the strong triple bond in N2 gas makes it unusable by most organisms, rendering nitrogen a limiting factor. The atmosphere is the largest nitrogen sink.

The cycle involves five main steps, largely driven by bacteria:

1. Nitrogen Fixation: The conversion of atmospheric N2 into usable forms.

   • Biotic Fixation: Bacteria on the roots of legumes convert N2 into ammonium (NH4+).

   • Abiotic Fixation: High-energy events like lightning convert N2 into nitrates (NO3-).

2. Assimilation: Producers (plants) absorb ammonium or nitrates from the soil and incorporate the nitrogen into biomolecules.

3. Ammonification: Decomposers break down nitrogen-containing biomolecules in dead organic matter and waste products, releasing the nitrogen as ammonium (NH4+).

4. Nitrification: Nitrifying bacteria in the soil convert ammonium (NH4+) into nitrates (NO3-), the form most preferred by plants.

5. Denitrification: In low-oxygen soil conditions, denitrifying bacteria convert nitrates (NO3-) back into atmospheric N2 gas, completing the cycle.