Community level systems Study Guide

Ecosystems are complex, dynamic systems made up of living organisms interacting with each other and their non-living environment. These interactions are influenced by various factors, which can be broadly categorised as abiotic (non-living) and biotic (living). Abiotic factors include physical and chemical elements like temperature, light intensity, soil pH, water availability, and oxygen levels. For instance, plants in a desert community are adapted to high temperatures and low water availability, while aquatic organisms require specific oxygen levels dissolved in water. Changes in these abiotic factors can significantly impact the survival, reproduction, and distribution of species within a community. For example, a severe drought (low water availability) can lead to widespread plant death, which in turn reduces food for herbivores, affecting their populations. Biotic factors are the biological influences on organisms within an ecosystem. These include interactions such as predation (e.g., foxes hunting rabbits), competition (e.g., two species of plants competing for light and nutrients), disease (e.g., a pathogen infecting a population), and symbiosis (e.g., mutualism where both organisms benefit). The presence or absence of a predator can dramatically change the population size of its prey, and in turn, affect the plant populations that the prey feeds on. Competition for resources like food, water, or mates can limit the population size of competing species. Diseases can decimate populations, creating opportunities for other species. Therefore, understanding both abiotic and biotic factors is crucial for accounting for the structure and dynamics of ecological communities.

Microorganisms, such as bacteria and fungi, are indispensable to the functioning of ecosystems, primarily through their role as decomposers. When organisms (plants and animals) die, their complex organic matter, including carbohydrates, proteins, and lipids, needs to be broken down. Decomposers actively break down these dead organic materials, recycling essential chemical elements back into the environment. For instance, they convert complex carbon compounds into simpler forms, like carbon dioxide, which is then released into the atmosphere and can be taken up by plants during photosynthesis. Similarly, nitrogen-fixing bacteria residing in soil or in the root nodules of certain plants convert atmospheric nitrogen gas into ammonia, which can then be converted into nitrates by nitrifying bacteria. These nitrates are a form of nitrogen that plants can readily absorb and use to build proteins and nucleic acids. This decomposition and conversion process is crucial because it ensures that nutrients are not locked away in dead biomass but are continually made available for new life, supporting the continuous growth and productivity of the ecosystem. Without microorganisms, vital nutrients would become scarce, and ecosystems would eventually cease to function effectively due to a lack of available resources for producers.