Community level systems Revision Notes

What is Interdependence?

Interdependence describes how organisms in a community rely on each other for survival. This reliance can involve various aspects such as food, shelter, pollination, seed dispersal, and nutrient cycling. For example, producers (like plants) create their own food through photosynthesis, providing the base of most food webs. Herbivores eat plants, and carnivores eat herbivores, showing a direct food chain interdependence. Decomposers break down dead organisms, returning nutrients to the soil, which plants then absorb, completing a vital nutrient cycle. Without these connections, the flow of energy and matter through an ecosystem would stop, affecting all populations.

Examples of Interdependence

A key example is the relationship between plants and animals. Plants provide oxygen for animals to breathe and food as primary producers. Animals, in turn, help plants by pollinating flowers, allowing them to reproduce, and by dispersing seeds, helping new plants grow in different areas. Another example is the role of microorganisms like bacteria and fungi as decomposers. They break down dead plants and animals, releasing essential minerals and nutrients back into the soil, which are then absorbed by living plants. These complex webs of interaction ensure the stability and balance of an ecosystem.

What is Competition?

Competition occurs when organisms require the same limited resources to survive, grow, and reproduce. These resources can include food, water, light (for plants), space, and mates. Competition can be intraspecific (between individuals of the same species) or interspecific (between individuals of different species). For example, a group of deer might compete for a limited patch of grass, or foxes and badgers might compete for the same type of prey. Competition is a fundamental ecological interaction that strongly influences population sizes, distribution, and the evolution of species through natural selection.

Consequences of Competition

The outcome of competition is that successful competitors obtain more resources, leading to better survival rates and reproduction, while less successful competitors may decline in population, be forced to move to different areas, or even face extinction. Intense competition can drive evolutionary adaptations, as species develop traits that make them more efficient at resource acquisition or allow them to utilise different resources, reducing direct competition. Understanding competition helps scientists explain why certain species are found in specific habitats and how changes in resource availability can impact community structure.

Decomposition: The Foundation of Nutrient Cycling

Microorganisms, primarily bacteria and fungi, act as decomposers in an ecosystem. Their essential role is to break down dead organic matter, which includes dead plants, dead animals, and waste products. This process of decomposition converts complex organic compounds into simpler inorganic substances. Without decomposers, dead organisms would accumulate, and essential nutrients would remain locked within their tissues, making them unavailable for new growth and life in the ecosystem. This process is a continuous cycle that ensures the recycling of materials.

Carbon Cycling and Microorganisms

In the carbon cycle, microorganisms play a crucial role by releasing carbon dioxide through respiration. As decomposers break down dead organic material, they respire, producing carbon dioxide as a byproduct. This carbon dioxide is then released into the atmosphere, where it can be taken up by plants during photosynthesis. This completes the loop, making carbon available for the synthesis of new organic compounds in living organisms. Microorganisms also play a part in forming fossil fuels over geological timescales, although their immediate impact is on decomposition.

Nitrogen Cycling and Microorganisms

Microorganisms are vital for the nitrogen cycle. Atmospheric nitrogen gas is unusable by most organisms. Nitrogen-fixing bacteria, which can live freely in the soil or in the root nodules of legumes, convert atmospheric nitrogen into ammonia. Other bacteria then convert ammonia to nitrites, and then nitrites to nitrates (nitrification). Nitrates are a form of nitrogen that plants can readily absorb from the soil to produce proteins and other critical biomolecules. In addition, denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere, completing the nitrogen cycle.

Recycling Essential Nutrients

Beyond carbon and nitrogen, microorganisms are also responsible for recycling other key nutrients like phosphorus and sulfur. By breaking down organic compounds containing these elements, they release them into the soil and water in forms that can be reabsorbed by plants. This continuous recycling ensures that ecosystems do not run out of essential minerals, enabling sustained life and productivity. Without these microbial processes, ecosystems would eventually run out of usable nutrients, leading to their collapse.

Understanding Abiotic Factors

Abiotic factors are non-living physical and chemical elements of an ecosystem that influence living organisms. Key examples include temperature, which affects metabolic rates; light intensity, critical for photosynthesis; water availability, essential for all life processes; soil pH, influencing nutrient uptake by plants; and oxygen levels, vital for respiration in aerobic organisms. For instance, in an aquatic environment, low dissolved oxygen due to pollution can cause fish populations to decline. In terrestrial environments, extreme temperatures can limit the distribution of plants and animals. Changes in any of these factors will directly impact the types and numbers of species that can survive in a given community.

Understanding Biotic Factors

Biotic factors are the living components of an ecosystem that affect other organisms. These interactions can be categorised into several types: predation, where one organism hunts and kills another; competition, when organisms vie for the same limited resources like food, water, or space; disease, caused by pathogens that can spread through populations; and symbiosis, which includes mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed). For example, a sudden increase in a prey population can lead to an increase in its predator population, and vice-versa. The arrival of an invasive species can introduce new competition, negatively impacting native species.

Impact on Communities (Foundation Tier)

Both abiotic and biotic factors work together to shape communities. If light intensity decreases, plants may grow slower, reducing food for herbivores. If a new predator is introduced, prey numbers might fall, which could then lead to an increase in the vegetation that the prey ate. A practical way to investigate these factors is through transects and quadrats, used to measure the distribution and abundance of organisms in different areas. For example, you could measure soil pH or light intensity along a transect and correlate it with the number of a particular plant species found.

Investigating Factors Using Sampling

To study how abiotic and biotic factors affect communities (PAG B2), scientists often use sampling techniques. Quadrat sampling involves placing square frames randomly or systematically in an area to count or estimate the abundance of organisms. Transect lines are used to study how species distribution changes across an environmental gradient, such as from a footpath into a field. Alongside organism counts, abiotic factors like temperature, light intensity, soil moisture, and pH are measured at intervals along the transect or within quadrats. This allows for correlation between factor variations and species distribution or abundance. Safety precautions include assessing for uneven ground, wearing appropriate footwear, and handling soil samples safely.

Levels of Organisation in an Ecosystem

Ecosystems are organised in a hierarchical manner, starting from the individual and expanding to the entire planet. Understanding these levels helps in studying and managing life on Earth. The key levels are individual organism, population, community, ecosystem, and the biosphere. Each level has unique characteristics and interactions. An individual is a single living thing. A population is a group of individuals of the same species living in the same area. A community consists of all the different populations that live and interact in a specific area.

Defining an Ecosystem

An ecosystem is a more comprehensive level that includes both the living (biotic) components and the non-living (abiotic) components of an environment, interacting together. Biotic factors include plants, animals, fungi, and bacteria. Abiotic factors are physical and chemical elements like temperature, light intensity, water availability, pH of the soil, and mineral content. The interactions between these biotic and abiotic components are what define an ecosystem, such as how plants absorb sunlight and water for photosynthesis, or how decomposers recycle nutrients back into the soil.

Examples of Ecosystem Scales

Ecosystems can vary significantly in scale. A small pond can be considered an ecosystem, with its specific populations of fish, insects, and aquatic plants all interacting with the water, light, and dissolved oxygen. A large forest, with its numerous communities of trees, mammals, birds, and insects, all interacting with the climate, soil, and topography, is also an ecosystem. The entire Earth, encompassing all its varied environments and living organisms, is referred to as the biosphere, representing the largest possible level of ecological organisation, where all ecosystems collectively operate.