CSEC Biology: Ecological Factors

Organisms are always interacting with their environments, either through exploiting the inorganic aspects of their surroundings, or with other living organisms. As a living thing yourself, you're always interacting with aspects of your environment, whether you're doing the bare minimum of breathing, or indirectly interacting with other organisms by eating food composed of organic matter.

Ecology can be defined as the study of living organisms in their environments and the relationships between these organisms and the abiotic and biotic factors of their environments.

As you may have already heard in the past (maybe in grade 6), living organisms exist in ecosystems, which are communities of living things sharing an environment and collectively interacting with each other and the abiotic factors of their environment. An organism's environment comprises the factors (abiotic and biotic, two terms we'll discuss more later) which are present in its environment and act upon it in some capacity.

The CSEC syllabus requires that you know how to distinguish between a few key terms related to ecology as well:

Abiotic vs Biotic Factors

Abiotic factors are the non-living, inorganic, chemical or physical aspects of the environment. For example, the air and soil would be considered abiotic aspects of the environment. Biotic factors, by comparison, are the other living organisms present in an ecosystem. These biotic factors can range from predators to prey to even parasites, which all interact with each other in an ecosystem.

Niche vs Habitat

If you're familiar with economics, you may understand the term 'niche' in relation to a particular market/consumer interest to which a business caters. In ecology, the concept of a niche is similar to the economic definition, as an ecological niche refers to an organism's particular specialized role within an ecosystem, just like how an economic niche is a specialized segment of the market. For example, certain fungi and bacteria in the soil serve the niche of decomposer, breaking down organic matter back to inorganic components which can be returned to the soil and reused by the food chain. On the other hand, a habitat is simply the place where an organism lives- such as how the habitat of phytoplankton would be in water bodies.

Population vs Community

A population comprises all the members of a particular species living together in a particular habitat. So, the group of all ackee trees in your community would be a population. However, a community is not limited to a particular species, but instead describes the group of all organisms living together in a particular habitat. For example, a lake has a community of organisms. Even your body has several bustling communities of microorganisms, from your large intestine to the surface of your skin. Since these are wide ranges of species, they are communities, but if we referred to only one species, it would be a population.

Species vs Population

A species describes a group of organisms sharing many close resemblances while being able to interbreed to produce viable, fertile offspring. For example, the Aedes aegypti mosquito is everyone's favourite species of mosquito. As mentioned before, a population is a more specific group, describing the group of organisms of a particular species living together in a particular habitat. So, the entire species of humans (Homo sapiens) would be far larger than the population of humans living in a single settlement.

As was stated above, an organism's environment is made up of both biotic and abiotic factors. The abiotic factors are the aspects of the environment which actually facilitate the survival of the organisms which inhabit it. As a result, they affect the organisms in a given environment, since organisms tend to require particular conditions to survive. Abiotic factors can be subdivided into three main groups: edaphic (soil-related) factors, climatic factors and aquatic factors.

These factors can be summarized in the following diagram:

Edaphic Factors

Although the term sounds fancy, edaphic factors are simply those abiotic factors relating to the soil and its composition:

Texture

Soil is formed from the weathering of rocks to form smaller rock particles. Different types of soil have different sizes of rock particles, which contributes to their texture. Gravel has the largest particle size (>2mm), followed by sand (0.02-2mm), then silt (0.002-0.02mm) and finally, with the smallest particle size, clay (<0.002mm).

Plants tend to need particular soil types to survive, but why? What is the result of the difference in particle size?

1. Smaller soil particles are able to hold more water due to capillarity and chemical forces, resulting in higher water content.

2. Larger soil particles have larger air spaces, resulting in higher air content in soil with larger soil particles.

3. Smaller soil particles make it harder for mineral ions to be leached (i.e. washed out of the soil), resulting in higher mineral ion content in soil with smaller particles.

4. Larger soil particles are easier to burrow through, thus some animals and plant roots are only able to penetrate soil with certain compositions.

Water

Water enters soil from rainfall and forms a film around the soil particles, which remains adhered due to capillarity and chemical forces.

1. Water is necessary for photosynthesis

2. Minerals in the soil are dissolved by this water, allowing them to be absorbed by the roots of plants

3. Water in the soil helps to prevent dessication (drying out) of animals in the soil which do not have waterproof coverings

Air

Air in the soil exists in the air spaces between soil particles.

1. Oxygen is necessary for aerobic respiration in the cells of plant roots and other organisms in the soil

2. Bacteria and fungi which act as decomposers in the soil require air to break down organic matter to form humus

3. Nitrogen gas in the air is needed by nitrogen-fixing bacteria to form nitrogen compounds like nitrates, which makes nitrogen available to plants (for the synthesis of useful compounds like proteins)

Mineral Nutrients

Mineral nutrients (such as phosphorous, nitrogen and magnesium) exist as dissolved ions in water in the soil. These mineral nutrients can come from decomposing organic matter as well as from surrounding rocks, from which some minerals can be dissolved.

1. Plants require mineral nutrients in order to grow healthy, as many minerals are needed to synthesize important compounds like proteins and chlorophyll

pH

Most plants prefer soil pH of around 6-7.2

1. pH can affect the availability of mineral ions in the soil. When the soil is too acidic or alkaline, some ions become less available while others can become overabundant and pose a risk to the plants. At alkaline pH values, greater than pH 7.5 for example, phosphate ions tend to react quickly with calcium (Ca) and magnesium (Mg) to form less soluble compounds. At acidic pH values, phosphate ions react with aluminium (Al) and iron (Fe) to again form less soluble compounds. Plants cannot absorb minerals through their roots if they are in insoluble compounds, as they rely on water to deliver dissolved minerals.

Humus

Humus is a sticky, brown material formed by bacteria and fungi after decomposing organic matter in the soil. Humus adheres to soil particles.

1. Humus can increase the air content of soil by allowing soil particles to bind to each other in clumps (called soil crumbs), thereby increasing the size of air spaces in the soil

2. Humus also adds mineral ions to the soil

3. Humus can also absorb and retain water and mineral ions, allowing the soil to have a higher and more consistent water content

Climatic Factors

As the name suggests, these are aspects of the atmosphere which affect terrestrial habitats (habitats on the earth rather than in water bodies). Terrestrial habitats include arboreal (tree) habitats and edaphic (soil) habitats.

Light

The duration and intensity of light in a particular area affect the living organisms in that habitat. Plants need light for photosynthesis, for which the rate is affected by the intensity of light. Changes in light duration and intensity comes with the seasons, and many plants and animals change their activities based on season, such as seasonal flowering, migration, hibernation and mating.

Temperature

Organisms tend to have a small temperature range within which they are able to survive. If temperatures are too low, ice crystals can form in cells and damage them, and many biological processes may be stopped due to insufficient activation energy. High temperatures can denature enzymes necessary for biological processes.

Temperature also affects the rate of photosynthesis and germination in plants, as well as the activity of some animals which hibernate in low temperatures.

Humidity

Humidity affects the rate of transpiration in plants as well as the rate at which water is evaporated from animals. Transpiration and evaporation tend to occur more slowly in high humidities, as the air is highly concentrated with water vapour, making it more difficult for more water vapour to enter the atmosphere.

Water Availability

Water is necessary for most chemical reactions in cells, and it is of course necessary for photosynthesis. Water enters terrestrial habitats through different forms of precipitation, such as rain and snow.

Wind

Wind is necessary for pollination of wind-pollinated flowers and seed dispersal of some plants. Wind affects some bird migration, the rate of transpiration in plants, evaporation of water from animals and sometimes the pattern of growth of vegetation.

Atmospheric Gases

Oxygen in the air is necessary for aerobic respiration, carbon dioxide is needed for photosynthesis and certain pollutant gases (eg sulphur dioxide and methane) can have negative effects on living organisms.

Aquatic Factors

These factors affect organisms in aquatic (water-based) habitats.

Salinity

The concentration of salt in the water of an aquatic habitat affects the organisms able to live within those habitats. Aquatic organisms tend to only be adapted to living specific saline conditions, so freshwater fish cannot live in the ocean, while saltwater fish cannot survive in freshwater. Similarly, organisms adapted to life in brackish water cannot exist in other salinities.

Water Movement

Organisms living in river are affected by constant water movement. Similarly, those in the ocean are affected by ocean currents, and those at shore lines are affected by changes in the tide.

Dissolved Oxygen

Aquatic organisms require oxygen dissolved in the water for use in respiration. Habitats with still water or with polluted water (such as those affected by non-biodegradable soaps) have low levels of dissolved oxygen, making it more difficult for organisms to survive than in areas with moving, cleaner water.