Species, Communities and Ecosystems

Understanding how life is organised starts with knowing the basic building blocks. A species is simply a group of organisms that can breed together and produce fertile offspring - think of all domestic dogs being one species despite looking completely different.

The organisation goes from smallest to largest: population (same species in one area) → community (different populations living together) → ecosystem $community plus all the non-living factors like temperature and soil pH$. Your local pond is a perfect example of an ecosystem.

Organisms get their food in three main ways. Autotrophs (producers) make their own food through photosynthesis, like plants using sunlight. Heterotrophs (consumers) eat other organisms to survive. Saprotrophs are the decomposers that break down dead stuff by releasing digestive enzymes externally - they're basically nature's recycling crew.

Quick Tip: Remember that abiotic factors are all the non-living things (pH, temperature, soil type) that affect where organisms can survive.

Food Chains and Community Interactions

Detritivores are different from saprotrophs because they actually eat dead organic matter and digest it internally - think of earthworms munching through leaf litter. The feeding order is straightforward: primary consumers eat autotrophs, secondary consumers eat primary consumers, and tertiary consumers sit at the top as apex predators.

Ecology studies how organisms interact with each other and their environment. Communities work brilliantly because they're self-sustaining through constant nutrient cycling - nothing gets wasted in nature.

The nutrient cycle keeps ecosystems running: plants absorb CO₂ and water to photosynthesise, consumers transfer nutrients by eating plants and each other, decomposers break everything down when organisms die, and nutrients return to the soil for plants to use again.

Remember: Communities are self-sustained because essential nutrients are constantly recycled - this is why ecosystems can function for millions of years.

Mesocosms and Chi-Squared Testing

A mesocosm is basically a mini-ecosystem in a jar that scientists use to study how communities work. Energy can enter and leave (through light), but matter stays trapped inside - making it perfect for controlled experiments on things like pH changes or temperature shifts.

Setting up a mesocosm needs three things: energy availability (clear glass for light), nutrient availability (decomposers for recycling), and a sealed container. You'd typically use pond mud, pond water with various organisms, and a large clear jar.

The chi-squared test determines whether your observed results happened by chance or if there's actually a relationship between variables. It tests the null hypothesis - basically asking "are these variables actually independent of each other?"

Exam Focus: If your calculated chi-squared value is greater than the critical value, reject the null hypothesis (variables are related). If it's smaller, accept it (variables are independent).

Chi-Squared Calculations

The chi-squared formula is: χ² = Σ $O - E$²/E, where O = observed frequencies and E = expected frequencies. To find expected frequency, multiply row total × column total, then divide by grand total.

Compare your calculated value to the critical value at 0.05 probability level. If calculated > critical, reject the null hypothesis. If calculated < critical, accept it and say "there is no significant difference between observed and expected results."

Chi-squared testing proves whether quadrant sampling gives you a reliable representation of a whole community. Random sampling ensures each element has an equal chance of selection - crucial for valid ecological studies.

Study Tip: Always use degrees of freedom when looking up critical values in chi-squared tables - this depends on your data structure.

Energy Flow in Ecosystems

Here's the reality about energy in ecosystems: reactions are never 100% efficient, and heat energy is always lost. Energy gets lost through movement, respiration, and all the metabolic processes that keep organisms alive, but it's constantly replenished by sunlight and feeding.

Ecosystems rely on solar energy for phototrophs, where chlorophyll traps light and converts it to chemical energy stored in carbon compounds. Plants then synthesise sugars and convert them into complex compounds like starch, cellulose, and proteins.

Biomass decreases along food chains because organisms lose CO₂, urea, and water. The heat produced from cell respiration helps keep warm-blooded animals at constant temperatures, but organisms can't convert that heat back into useable energy forms.

Key Point: Chemical energy gets converted into kinetic energy (movement), electrical energy (neurons), or energy for maintaining homeostasis - but heat energy is always lost to surroundings.

Trophic Levels and Energy Transfer

Food chains show how nutrients and energy pass from producers to consumers, while food webs show the interconnections between multiple food chains. Each organism's position is called its trophic level.

Energy transfer between trophic levels is incredibly inefficient - only 10-20% passes to the next level. The rest gets lost as heat, through incomplete consumption of prey, or in waste products. This is why there are fewer top predators than herbivores.

As you move up food chains, organisms need to eat more to compensate for energy loss. Higher trophic levels are less efficient because mobile prey requires more energy to hunt.

Pyramid diagrams represent energy flow and must be drawn to scale using kJ m⁻² yr⁻¹. Each level should have less than 10% of the energy from the level below it. Since biomass reflects energy content, biomass pyramids indirectly measure energy transfer.

Exam Essential: Energy flows linearly through ecosystems (sun → producers → consumers → heat), while nutrients cycle continuously through the system.

Biomass Pyramids and Population Limits

Biomass pyramids must be drawn to scale on graph paper, typically using 1 cm = 100 g. Each trophic level shows decreasing biomass, reflecting the energy lost between levels - you might see 2000 g of pondweed supporting only 200 g of insects.

Energy enters ecosystems from sunlight, flows through trophic levels with 10% efficiency, and is lost through heat, excretion, and incomplete consumption. Meanwhile, nutrients cycle continuously - gained from weathering rocks, recycled from dead organisms, and absorbed by producers.

Population growth gets limited by different factors. For animals: food availability, parasites and disease, predators, and nesting sites. For plants: light availability, temperature, CO₂ levels, and water supply.

Population Insight: When species spread into new areas, population growth follows an S-shaped curve - exponential growth, then transition phase, finally plateau at carrying capacity.

Population Growth Patterns

Sigmoidal $S-shaped$ growth curves show three distinct phases when species colonise new areas. The exponential phase occurs when natality exceeds mortality and resources are abundant - populations grow rapidly without constraints.

During the transitional phase, natality falls and mortality rises as resources become scarcer. Competition increases, and environmental resistance starts limiting growth.

The plateau phase represents carrying capacity - the maximum population the environment can support. Here, natality equals mortality, and populations stabilise due to food shortages, increased predation, and disease.

Real-World Application: Understanding carrying capacity helps predict how populations respond to environmental changes and resource availability - crucial for conservation and management decisions.

Carbon Compounds and Methane Production

Carbon dioxide dissolves in water to form hydrogen carbonate ions, lowering water pH. In terrestrial plants, CO₂ diffuses through stomata into spongy mesophyll and reaches chloroplasts for photosynthesis.

Methane production happens under anaerobic conditions when bacteria break down organic matter in swamps, lakes, and landfills. Farm animal manure and plant cellulose are major methane sources through a three-step bacterial process.

Peat formation occurs when organic matter doesn't fully decompose due to anaerobic, acidic conditions where saprotrophs can't survive. Over millions of years, compressed and heated peat becomes fossil fuels - coal, oil, and gas.

Calcium carbonate (CaCO₃) dissolves in acid and forms limestone rocks. Hard coral shells contain calcium carbonate and act as major carbon reservoirs in marine ecosystems.

Environmental Connection: Methane can either diffuse into the atmosphere (contributing to greenhouse effects) or accumulate underground - understanding this helps explain climate change mechanisms.

The Carbon Cycle

The carbon cycle involves reservoirs (carbon stores), fluxes (movement between stores), and carbon sinks (where carbon accumulates). Major reservoirs include the atmosphere, oceans, Earth's crust, and living organisms.

Nine key processes drive carbon cycling: photosynthesis removes CO₂ from atmosphere; respiration releases it back; decomposition by saprotrophs produces CO₂; methane production in anaerobic areas; peat oxidation; fossil fuel combustion; shell formation using calcium carbonate; limestone conversion to lime; and volcanic eruptions releasing molten rock and CO₂.

The cycle connects all Earth's systems - atmospheric CO₂ gets absorbed by plants, passes through food webs, returns via decomposition, and cycles between land, sea, and air through various chemical and biological processes.

Climate Impact: Human activities like fossil fuel combustion and deforestation are rapidly increasing atmospheric CO₂, disrupting the natural carbon balance that's maintained Earth's climate for millions of years.