Diverse range of living things

Diversity of living things

Many small animals can be found in the schoolyard environment. Animals are capable of actively moving from place to place at some stage in their life cycle, and they feed by consuming other living things—or parts of them.

Most plants are fixed to one place and need to make their own food by a chemical process called photosynthesis. This chemical reaction usually occurs in the green leaves and uses energy from sunlight (carbon dioxide and water form glucose sugar and oxygen). This process of using carbon dioxide and producing oxygen only occurs in sunlight.

The schoolyard might have animals with a backbone (vertebrates) such as:

• birds (avians—feathers, lay eggs)
• lizards (reptiles—dry scales, lay eggs)
• frogs (amphibians—moist skin and lay eggs in water)
• and some mammals such as cats and dogs.

All mammals have fur and produce milk for their young. There are three groups of mammals:

• placentals (e.g. humans) that feed their unborn babies through a placenta
• marsupials (e.g. koalas and kangaroos) that feed their babies with a milk teat in a pouch
• monotremes (echidna and platypus) that lay eggs and feed their young by secreting milk onto the mother’s belly fur.

Many animals in the schoolyard are, however, likely to be very small and without a backbone (invertebrates) such as earthworms, snails, ants, ant lions, slaters, beetles and spiders. Their exoskeleton provides protection and a framework to which muscles are attached and which allows them to move.

The arthropods include:

• crustaceans, such as slaters and crabs
• arachnids have eight legs, a head and abdomen, such as spiders and ticks
• myriapods have many legs, such as centipedes and millipedes
• insects, the largest groups of arthropods, have a head, thorax and abdomen, six legs and antennae, such as ants and bees.

Scientists have developed different classification systems for animals based on the features and/or the origin of species. These are being revised constantly as new knowledge emerges.

Observing and understanding the diversity of all living things allows students to group living things so that they can identify patterns and change over different time scales. This allows function and relationships within systems to be identified and, models to be used to predict the consequences of change.

Life cycles

All living things have the potential to reproduce, and the offspring grow and develop through a series of stages. In some, the offspring look like small versions of their parents from the outset, but in others there are very different stages as the young develop. Many insects in particular go through a series of amazing changes.

Every flowering plant starts life as a seed. With the right amount of warmth, air and moisture, a seed starts to germinate by sending roots down into the soil and a shoot up towards the sunlight. If the plant receives enough light it grows to become a seedling, and eventually an adult plant. When it is time for the plant to reproduce, it produces flowers. After pollination and fertilisation have occurred, the flower develops into a fruit containing seeds. If the seeds experience suitable conditions for germination, the life cycle starts over again.

Every living thing goes through life stages. Although a developmental pattern is predictable for most animals, each living thing has a unique sequence of life stages that can make it difficult to make direct comparisons. The purpose of making these comparisons to gain an appreciation of the diversity among living things.

Living vs. non-living things

Looking at the world around us, we instinctively seek to identify living and non-living things. However, it is hard to absolutely define life.

Scientists agree that life on Earth generally has characteristics, including:

• Movement: All animals move, at least at some stage in their lives. Some plants can open and close their leaves, and sunflowers orient their flower to follow the Sun. However, for many plants their ‘movement’ is their ‘growth’. For example, roots explore the soil by growing into it.
• Respiration: Cellular respiration is a scientific term that describes the release of energy stored in organic compounds, for example, sugar. This is sometimes confused with our respiratory system (lungs) which is the area where the gas necessary for our cellular respiration (oxygen) and the by-product of the reaction is released (carbon dioxide). All living cells need to be able to release the chemical energy in sugars or other chemical molecules. Animals need to eat to gain these molecules, while plants use sunlight to produce their sugar molecules (photosynthesis). Both plant and animal cells use oxygen to break down the glucose sugar to produce energy (cellular respiration).
• Sensitivity: A living thing gathers information about its environment and reacts in consequence. For example, we avoid things that cause us pain. Plants react to their environment by growing towards the light or even by releasing alarm hormones when eaten by a predator.
• Growth: Living things have the ability to grow. Non-living things can also grow, such as stalactites, but it is an external process (the deposit of minerals on a spike) rather than an internal process (growing by means of absorbed energy and nutrients that are reorganised).
• Reproduction: Living things come from other living things and can often create new living things. A worker bee is sterile but is born from a fertile queen and is therefore alive. Plants have the ability both to reproduce sexually (creating seeds) and asexually, for example, runner plants.
• Excretion: Living things excrete things such as excess gases, salts and waste, in order to keep their internal composition constant.
• Nutrition: Living things need to acquire the necessary elements for growth and reproduction from the world around them. Animals need to eat other things to acquire energy to survive (heterotrophs). Plants need to absorb certain minerals, for example, phosphorous, in order to capture energy from the Sun (photosynthesise).

Adaptations

The first stage of understanding the diversity of living things is identifying the unique structural features of living things. The next step is to ask 'why' all living things are different. Other ways to ask this are 'how does this feature help the living thing survive' or 'what is the survival advantage of this feature'. For example, some animals that live in the desert have large ears. These features allow them to listen for predators so they can feed at night when it is cooler. Some plants have stick-like leaves that help minimise water loss. These features, which are important to the survival of an animal or plant in its native environment, are called ‘adaptations’. Adaptations can also be behaviours, such as the instinct to run from danger or the unfurling of leaves in sunlight. Adaptations evolve by means of natural selection.

Within populations there is variation among individuals, such as in the size of ears and the size of the leaves. If the conditions in the environment favour particular traits, such as larger ears that help to avoid predators at night and narrow thin leaves that help slow dehydration in water-poor environments, then the individuals who have them are more likely to survive and reproduce. If offspring inherit these traits then future generations are more likely to have larger eyes and waxier leaves.

Generally, scientific adaptations are identified at a population level. If a single individual has a mutation (change in the DNA that causes a change in its features), this changed feature may allow the individual to survive more easily to breed. If the individual’s offspring (children) inherit the same feature, they will also survive more easily in the environment. Gradually the new feature spreads through the population and can be considered an adaptation.

The ability to survive in an environment is only one thing that determines what features are common in a population. Other examples include the following possibilities:

• The necessity to reproduce to pass on genes. If individuals selectively choose their mating partners then traits might evolve due to preference. For example, if female birds prefer red chests then the population might evolve red chests. If there is competition to mate then males might develop special characteristics for fighting even if these make it harder to survive. For example, some species of stags fight each other with antlers and therefore have large antlers which can make it harder to walk around in a forest.
• If all individuals have the exact same genes for a particular feature then it will not change or disappear even if it can hinder survival. For example, even if it might be advantageous to have red fur, if all the individuals of the population have black fur then that feature cannot evolve unless the right mutation occurs.
• A mutant might have a structural feature that makes it more difficult to survive, but if the other individuals die from an unexpected event, for example, an avalanche, then the population might end up with that feature.
• Sometimes it doesn’t matter either way—structural features and instincts might not affect an individual’s ability to survive. In that case the population might end up sharing a structural feature or behaviour because of chance.

Species might have very different adaptations to the same environment, and not all these adaptations might be the ‘best’ adaptation. The adaptations that living things can evolve are constrained by traits they cannot change because they are integral to their development, such as plants not being able to develop muscles to walk to a water source and amphibians not being able to evolve fish scales. They are also constrained by traits that are not variable, for example, there might not be any individuals in the population with the ability to store fat in a hump, so even if it is a remarkable adaptation for camels it might not be an adaptation that other species will adopt.