Building Science Concepts: Tidal communities

Explore the science concepts that underpin knowledge and understanding about the interrelationships and food webs in tidal communities.

The New Zealand Ministry of Education’s Building Science Concepts (BSC) series presents sets of interlinking concepts that build stage by stage towards overarching science concepts or big ideas. A big idea shows how a fully developed understanding of the concept might look, but recognises that such an understanding might not be achieved until New Zealand Curriculum level 7 or 8.

This resource is a partial replication of Building Science Concepts Book 22 Tidal Communities: Interdependence and the Effects of Change. The background information on this page, combined with the information in the interactive, covers the science notes provided in the original BSC book. The overarching science concepts (big ideas) and how they may be scaffolded in sequence are illustrated in the interactive below.

Introduction to tidal communities

This resource builds on information from Life between the tides – unpacking more of the details of the interrelationships amongst the beach inhabitants and the potential impacts of changes both natural and human induced. Studying this environment gives students opportunities to explore concepts of:

• the variety and interdependence of living things
• the effects of change, both short term and long term, on an environment
• the relationship between living and non-living elements in an ecosystem.

Te ao Māori connections

The topic is also a context for incorporating an awareness of values associated with kaimoana and kaitiakitanga. Within te ao Māori, seashores occur at the border of the realms of Tangaroa (god of the sea) and Tāne (god of the forest). Many pūrākau are set at this boundary. Similarly, there are many tikanga that are observed there and support the survival and management of this area of environmental richness.

Beaches are places of great importance to Māori as sources of food. Each iwi has its own particular protocols to observe when visiting coastal sites or gathering any sea-based resources. It is good to arrange for someone from your local iwi to visit to discuss these protocols with students. Introduction of kupu Māori will encourage development of dual perspectives on understanding this environment. Glossary of kupu Māori mō te ara o Hinekirikiri (Māori words associated with the intertidal zone) is available in downloadable PDF format.

Feeding relationships in tidal communities

Feeding relationships in any community of living things are complex. Coastal beaches provide a wide variety of habitats for living things. These organisms have developed many ways of exploiting their habitats. Each type of beach has its own particular pattern of feeding relationships. Models such as food webs are representations scientists use to illustrate these relationships clearly.

A food web will always start with:

• a producer – plants such as rimurimu (seaweed) or pūkohu wai (algae), which get eaten by
• a kaiota (herbivore) – such as tungangi (cockles), tuatua or mud lugworms, which get eaten by
• a kaikiko (carnivore) – such as whai (stingrays) and tōrea tai (oystercatchers).

In the tidal community, the most common producers are algae. Seaweeds are one type of algae, but there are many others. Most of these are microscopic and make up meroiti tipu (phytoplankton).

Food webs can also start off with dead plant and algal material and the wastes and dead bodies of animals. Detrital food webs are often overlooked, but without them, all the detritus would accumulate. Nutrients that are locked up inside the detritus are recycled back into the seawater, mud and sand for plants and algae to consume. Detrital food webs follow the pattern of:

• detritus – dead and waste organic matter is eaten by
• decomposers – fungi, bacteria and animals such as mud worms and mōwhiti (sandhoppers), which specialise in eating detritus and then go on to be eaten by something else.

Usually any one type of animal can eat a range of foods so there will be multiple pathways showing on the food web. Making a food web in a range of contexts enables students to develop understanding of the concept of food webs as well as mutual dependence that exists in all ecosystems.

Changing environments

Water and wind are two of the most powerful agents of change in nature. Their actions can be slow and gradual or fast and dramatic. The effects of their actions are readily seen on all beaches. Wave action can move great quantities of sand and shingle and erode away cliffs and rocks. Wind can create or flatten tāhuahua (sand dunes), erode cliffs and rocks and uproot plants.

Storms combine wind and wave action to create extreme turbulence that can scour out rock pools and destroy complex ecosystems. However, these habitats are recolonised within a short time because even the species that live in one place on the rocks usually have free-swimming young among the drifting plankton. The seaweed spores arrive and attach to the rock, the mobile young of limpets and barnacles arrive to feed on the seaweeds as they begin to grow, browsing snails follow, shrimps, crabs and the cockabullies arrive and so on until the balance is restored.

Storms inland can also create dramatic changes for the beach. Huge amounts of water can flow down rivers to the sea, carrying vast quantities of silt as well as larger organic debris, such as branches and trees. In 2019, an old landfill site was breached by a river in flood resulting in tonnes of material being washed up along kilometres of the West Coast of the South Island.

In river estuaries, debris can build up and change the course of the river to the sea, leaving what were once tidal mudflats high and dry. The seeds of dry-ground species, carried there by water and wind, may then have the right conditions to germinate and grow. In New Zealand, storms can result in the debris from sawmilling operations inland being washed into streams and rivers and thundering downstream, smothering beaches with bark and offcuts.

Changes from people’s activities

Human activities can make big changes to beaches. In urban areas, buildings may encroach on beaches, reducing the range of beach habitats. Mudflats and tidal areas may be drained for subdivision. Run-off from streets may be washed out through drains to coastal waters, and sometimes untreated effluent from homes and industry is drained into coastal waters.

In rural areas, nutrients from fertilisers and animal wastes can intrude into waterways and from there to the sea. In estuaries, these nutrients encourage the growth of certain plants that can remove the oxygen from the water and kill the animal life. Deforestation of hillsides inland allows rain to erode the soil, clog the rivers and silt up estuaries and mudflats. People may overharvest shellfish, leaving too few to sustain significant populations. Vehicles driven on beaches can crush shellfish that live in the wet sand and damage sand dune habitats.

Introduction of competing species, whether accidental or deliberate, may endanger native beach species. The introduction of marram grass to stabilise dunes has endangered pīngao, the native sand sedge, which is highly valued by Māori for fine weaving. The South African spider Steatoda capensis (accidentally introduced to New Zealand) has supplanted the native katipō spider (both are poisonous) on many beaches.

Minimising impacts

People also devise ways of minimising or rectifying the effects that they have on beaches and the tidal communities living there. They exercise their responsibilities of guardianship (kaitiakitanga) by:

• making laws prohibiting vehicles on beaches
• putting in place rāhui for fishing and collection of kaimoana
• setting catch limits for fish and shellfish
• fencing off sand dunes
• prohibiting the removal of driftwood
• designating sensitive areas as parks and reserves to protect habitats and their populations of living things – for example, planting pīngao grass to stabilise sand dunes.

Alternative conceptions

Students may have alternative conceptions when interpreting the arrows in food web diagrams. They may think the arrow should point from the eater to the eaten – for example, from the bird to its food, the crab. The direction of the arrow in fact represents a transfer of energy. The arrows point from the eaten towards the eater because that is the direction in which the energy provided by the food is flowing.

Students may ignore the role of small species within the tidal community. The tiny creatures that feed on detritus (which may seem to be just rubbish) or the algae and the small invertebrate animals that feed on them may seem less important. They may focus instead on larger animals such as birds of fish. This may lead students to think that it is the larger and more attractive animals that are essential and that it doesn’t matter if these smaller organisms die out.