National 5 Biology Unit 3 Notes

Ecosystem Basics and Food Relationships

Biodiversity refers to the range of species present in an ecosystem - the greater the variety of species, the higher the biodiversity. All organisms in an ecosystem are organized in levels: an ecosystem consists of all living organisms and non-living components in an area, a community includes all living organisms within a habitat, while a population refers to all members of a single species in a given area.

The sun provides the primary source of energy in ecosystems. Producers (green plants) convert light energy into chemical energy through photosynthesis, while consumers (animals) get energy by feeding on other organisms. This energy transfer is represented in food chains, which show the direction of energy flow from producers to primary, secondary, and tertiary consumers.

Food webs provide a more accurate picture of feeding relationships by showing interconnected food chains for all organisms in an ecosystem. These complex networks highlight how removing even one organism can have a catastrophic effect on the entire ecosystem, such as when overhunting disrupts natural balances.

💡 Think of food webs like a complex game of Jenga - remove one piece (species) and the whole structure can become unstable or collapse!

Ecological Niches and Competition

Every organism has a specific niche in its community - essentially its "job" or role within the ecosystem. A niche relates to the resources an organism needs (light, temperature, nutrients) and its interactions with other organisms (competition, predation, parasitism).

Competition occurs when organisms require the same limited resources for survival. Animals compete for water, food, space, shelter, nesting sites and mates, while plants compete for water, light, soil nutrients and space. Competition can be classified into two types:

Interspecific competition happens between different species. This competition is usually less intense because different species often need slightly different resources. Intraspecific competition occurs between members of the same species and is typically more intense because individuals need exactly the same resources.

The distribution of organisms in an environment is significantly affected by abiotic factors - non-living variables such as temperature, light intensity, soil pH, and soil moisture. Scientists measure these factors using specialized equipment like thermometers, light meters, and soil pH meters, being careful to avoid sources of error like body heat or improper probe placement.

🔍 When studying abiotic factors, remember that accuracy matters! Always clean probes between readings and take measurements at consistent times to avoid skewed results.

Sampling Techniques in Ecology

Ecologists use various techniques to study the distribution and abundance of organisms in their habitats. Quadrats are square frames used to sample plants or slow-moving organisms. They can be placed randomly or along a transect line, with the number of squares containing plants providing an estimate of abundance.

When using quadrats, it's important to establish clear counting rules (like counting plants that occupy more than half a square) and to take multiple samples to calculate averages. This minimizes errors and provides more reliable data about plant populations.

Pitfall traps are commonly used to sample small invertebrates living on the ground. To be effective, these traps must be checked regularly to prevent specimens from dying or eating each other, have drainage holes for rainwater, be level with the soil surface, and be covered to avoid detection by predators.

Both techniques have limitations. Quadrats might not accurately count mobile species, while pitfall traps will only collect ground-dwelling invertebrates, missing those that live higher in vegetation. Successful field studies require choosing the right sampling technique for the target species.

🌱 Good ecological sampling is like taking a poll - your results are only meaningful if your methods capture a truly representative sample of what's out there!

Organism Identification and Biotic Factors

Identification keys help scientists accurately identify different species. These keys come in two main forms: branched keys that use a flowchart-like system of yes/no questions, and paired statement keys (also called dichotomous keys) that offer two contrasting descriptions at each step.

While abiotic factors are non-living influences, biotic factors are living things that affect biodiversity. Important biotic factors include predation, grazing, and competition. Grazing impacts biodiversity in different ways depending on its intensity:

At low grazing levels, a few vigorous plant species typically dominate, reducing overall diversity. Moderate grazing can actually benefit biodiversity by preventing dominant species from taking over, allowing less competitive plants to thrive. However, overgrazing damages biodiversity by removing too many plants and preventing their recovery.

Predation creates a delicate balance between predator and prey populations that follows a cyclical pattern: prey populations increase when predator numbers are low, predator numbers then rise with abundant food, which causes prey populations to decrease, and finally predator populations decline due to food scarcity.

📊 The predator-prey relationship is a natural balancing act - neither population can grow unchecked because they directly influence each other's survival!

Indicator Species and Photosynthesis

Indicator species serve as biological barometers that reveal pollution levels through their presence or absence. For example, certain lichen species are highly sensitive to air pollution and disappear from areas with poor air quality. Water pollution indicators respond to contaminants like untreated sewage, agricultural fertilizers, and acid rain.

Photosynthesis is the fundamental process that powers ecosystems by converting light energy into chemical energy. The process requires carbon dioxide, water, and light energy to produce sugar and oxygen. This vital process occurs in two main stages:

The light reaction happens when chlorophyll in plant chloroplasts captures light energy and converts it to chemical energy in the form of ATP (adenosine triphosphate). During this process, water molecules are split, releasing oxygen as a byproduct.

In the carbon fixation stage (also called the dark reaction), hydrogen and ATP from the light reaction combine with carbon dioxide from the air. This energy-powered chemical reaction produces glucose, which plants use for growth and energy storage.

⚡ Without photosynthesis, life as we know it would be impossible! It's the ultimate energy converter, transforming sunlight into the chemical energy that powers nearly all life on Earth.

Photosynthesis Process and Limiting Factors

During photosynthesis, light energy absorbed by chlorophyll is converted into chemical energy in the form of ATP. Water molecules are split to produce hydrogen and oxygen, with oxygen diffusing out of the leaf. ATP and hydrogen then move to the carbon fixation stage where they combine with carbon dioxide to produce sugar.

Plants use this sugar in several ways: as an immediate energy source for respiration, as cellulose for building cell walls, or stored as starch for later use. This versatility allows plants to maintain growth and energy reserves even when conditions aren't ideal.

A limiting factor is any variable that slows or stops photosynthesis when it's in short supply. Three key limiting factors affect photosynthesis rates:

1. Light intensity: As light increases, so does photosynthesis - but only until another factor becomes limiting.
2. Carbon dioxide concentration: Higher CO2 levels increase photosynthesis rates until another factor limits the process.
3. Temperature: Warmer temperatures speed up photosynthesis (as enzymes work faster), but excessive heat (above 45°C) denatures enzymes, stopping the process entirely.

🔬 Understanding limiting factors is crucial for maximizing crop yields! Commercial greenhouses often increase CO2 levels and temperature to boost plant growth.

Energy Flow in Ecosystems

Energy flows through ecosystems following a fundamental principle: only the energy contained in an organism's body when it's eaten can be transferred to the consumer. This creates an unavoidable energy loss at each level of a food chain.

Scientists represent this energy transfer using pyramid diagrams. The simplest is the pyramid of numbers, which shows the relative number of organisms at each trophic level. However, this method doesn't account for organism size - a single tree might support thousands of insects, creating an inverted pyramid.

The most accurate representation is the pyramid of energy, which shows the total energy contained within organisms at each level of the food chain. This pyramid is always upright because approximately 90% of energy is lost at each transfer through heat, movement, and undigested material. Only about 10% of energy is passed to the next level.

This energy loss explains why food chains rarely extend beyond four or five links - there simply isn't enough energy remaining to support additional levels of consumers. It also illustrates why eating lower on the food chain (more plants, less meat) is more energy-efficient.

🔋 The 10% rule of energy transfer explains why apex predators are rare in nature - by the time energy reaches the top of the food chain, around 99.9% of the original energy has been lost!

Food Production and Fertilizers

Intensive farming has developed to meet the food demands of our growing human population. These practices aim to increase crop yields (the mass of food produced in an area of land) through techniques like increased fertilizer and pesticide use, but often with environmental consequences.

Fertilizers are essential in intensive farming because harvesting crops continually removes nutrients from soil without the natural replacement that would occur through death and decay. Nitrogen is the main component in most fertilizers because it's crucial for plants to produce amino acids and proteins.

In the natural nitrogen cycle, plants absorb nitrates from soil to make proteins. These nutrients pass through food chains, and when organisms die and decompose, nitrogen returns to the soil. Farming disrupts this cycle by removing plant material before decomposition occurs.

While fertilizers boost crop production, they can cause algal blooms when rain washes water-soluble fertilizers into lakes and rivers. The high nitrate levels cause explosive algae growth that depletes oxygen when the algae die and decompose, potentially killing fish and other aquatic organisms.

🌊 Fertilizer runoff is a classic example of unintended consequences - what helps crops grow on land can create "dead zones" in water bodies when the natural nutrient balance is disrupted!

Agricultural Technologies and Biological Controls

Genetically modified (GM) crops contain genes from other organisms to improve their characteristics. This technology may help reduce intensive farming's environmental impact. For example, some GM crops have received bacterial genes that make them toxic to insect pests, while others have enhanced nitrate absorption, reducing fertilizer needs.

Chemical pesticides are substances sprayed onto crops to kill organisms that reduce plant growth, such as weeds, insects, and fungi. While effective, these chemicals are often poisonous and can accumulate in organisms' bodies. Through bioaccumulation, pesticide concentrations increase at higher food chain levels, potentially harming top predators.

A historical example is DDT, a non-biodegradable insecticide used in the 1940s to kill mosquitoes. While effective against insects, DDT caused shell-thinning in birds of prey, demonstrating how pesticides can have unexpected ecological consequences.

Biological control offers an alternative to chemical pesticides by using one species to control another. For example, ladybirds effectively control aphid populations that damage plants. However, biological controls must be carefully selected - the infamous case of cane toads in Australia shows how introduced control species can become invasive pests themselves.

🐞 Unlike chemical pesticides that kill indiscriminately, biological controls like ladybirds target specific pests while leaving beneficial insects alone - nature's own pest management system!

Evolution Through Mutation and Natural Selection

Mutations are random changes to genetic material that create the raw material for evolution. These spontaneous changes can be negative (decreasing survival chances), neutral (having no effect), or advantageous (increasing survival chances). Importantly, mutations are the only source of new alleles (gene variants) in a population.

Mutagenic agents can increase mutation rates, including environmental factors like UV radiation, X-rays, gamma rays, and certain chemicals. These factors increase the likelihood of DNA changes occurring.

The resulting variation within a population makes evolution possible. While mutations create new alleles, sexual reproduction further increases variation by creating different combinations of existing alleles. This genetic diversity allows populations to adapt to changing environmental conditions over time.

Natural selection drives evolutionary change through several key steps. Organisms produce more offspring than the environment can support, creating competition. Individuals with advantageous inherited traits (like better camouflage, stronger immune systems, or more efficient feeding) are more likely to survive this competition and reproduce, passing their beneficial genes to the next generation. Over time, the population becomes better adapted to its environment as disadvantageous traits disappear and advantageous ones become more common.

🧬 Natural selection doesn't create new traits - it simply determines which existing variations survive and reproduce. Think of it as nature's quality control system!