Investigation

The variety, or diversity, of life on Alaska beaches often is distributed in distinct bands, or zones, of biological communities that parallel the shoreline. Shoreline zonation is most distinct on rocky beaches, particularly on the faces of large boulders or bedrock outcrops where the plants and animals that are adapted to life on the rocky substrate also are adapted to different levels of exposure to the tides. Where the tidal range is large, the zones are spread out and can provide excellent opportunities for students to observe a diversity of seaweeds and animals in each zone. In places where the tidal range is smaller, the zones will be compressed. The presence of large boulders and the type of rock that forms tide pools provides other opportunities to observe diversity over small areas.

The communities on these rock habitats are very different from those adapted to live in soft-bottom substrates of sand or mud. Some beaches are mixtures of rock outcrops, boulders, cobbles, and areas where sand and mud are deposited. Beaches with this mixture of substrate types support the largest number of different species, and therefore the highest biodiversity. Although this activity was developed for rocky beaches where the zonation is distinct, the same approach can be used on other types of beaches with more than one type of substrate (e.g., boulders, large and small rocks, sand, and/or mud or to compare and contrast the diversity in different types of habitats.

In Alaska, the tidal cycle is semi-diurnal, with two cycles of high and low tides approximately every day. (Approximately, because the timing of the tides is a result of lunar cycle of the “tug” of the moon on the Earth, and the lunar day is slightly longer than 24 hours). In some locations in Alaska, tides have a large range between the highest high tides and the lowest low tides because the shape of inlets and channels “squeeze” the incoming water and cause it to pile up along the shoreline.

Who or What Lives Where, and Why
Alaskaʼs beaches are dynamic and often harsh environments for plants and animals. However, many species are uniquely adapted to live “between the tides.” This section of shoreline, which is the area uncovered by the lowest tides and covered by the highest tides, is known as the intertidal zone. The distribution of plants and animals in the intertidal zone appears to be dictated by a combination of biological adaptations to physical conditions and to inter- and intra-species competition and predation.

Upper limits for plants and sessile (fixed) animals are generally set by their tolerance to physical factors, while the lower limits are often set by biological interactions.

The intertidal zone is progressively more crowded at the lower tidal levels where complex dramas are played out in competition for space and food and the avoidance of becoming food. For mobile animals, behavior often provides important adaptations that influence where they can survive.

The most important factors that determine the suitability of the different tidal zones as habitat for each plant and animal species are tidal dynamics, exposure to waves, substrate type, and ecological interactions.

• Tidal Dynamics. The range and timing of tides are the primary controlling factors that dictate what kinds of organisms inhabit each zone. The action of the tides moves the salt waterʼs edge up and down the land in a predictable and regular sequence. When the tide is out, plants and animals in the higher part of the zone are exposed to drying (dessication) and extremes of heat and cold. But organisms that have adapted to survive these periods are again bathed in the rich “soup” of the ocean, or made buoyant when the tide returns.
  
  Lower down in the intertidal zone, exposure to drying and temperature extremes occurs less frequently and for shorter periods. A greater number of species is adapted to these less stressful conditions.
  
  
• Exposure to Waves. Wave action can extend the intertidal zone even higher up on the land, and strong wave action on open coasts can sweep away whatever is not clinging or firmly attached.
  
  
• Substrate Type: Rock, Sand, and Mud. Rocks of various sizes, from cobbles to boulders, provide plants and animals with a variety of niches (under rocks, in crevices and tide pools, on sides and tops of boulders) for potential attachment where the organisms can withstand the force of tides and waves and be sheltered from drying out and from temperature extremes. Animals that live on sandy beaches and mudflats must be adapted to burial by shifting sediments.
  
  
• Ecological Interactions. In addition to physical factors, relationships between and among organisms also affect and often control survival at any specific site. These relationships are especially important in the lower intertidal zone where competition, grazing, and predation control the composition of the biological community.

For more information about marine invertebrates commonly observed on Alaska beaches, see Meet the Invertebrates.

Splash Zone: The places on a beach exposed to occasional wave surges or spray of salt water often during storms.  Seaweeds and animals found here have to be hardy to survive long periods out of the water, exposure to sun, and fresh water from rain. Diversity is the lowest of all the zones.

Upper Intertidal/High Tide Zone: Wet during high tides. This is the part of the beach exposed to air most of the time, covered only during the higher tides in the cycle. Few types of plants, seaweeds, or animals can survive being dry most of the time when the tide is out, being submerged in salt water occasionally, the risk of being washed away by waves, and exposure to temperature extremes often on a daily basis. Diversity is low.

Middle Intertidal/Mid-Tide Zone: Wet and dry. These are the places on a beach closest to “land” that is usually covered and uncovered twice a day by the tides and where exposure to air lasts longer during periods of higher and lower tides and for a shorter amount of time during more moderate tides. Diversity is higher than further up the beach but only of organisms that can survive life both underwater and out-of-water. The diversity includes more animal predators and their abundance can limit whether or not their prey can survive in this zone.

Lower Intertidal/Low Tide Zone: Usually wet.These are the places on a beach exposed to air only during the lowest tides. The highest diversity of both seaweeds and animals are in this zone because they only have to tolerate short exposures to air that don’t occur every day. Some animals become stranded as the tide recedes but others can move up the beach to find food. Predator-prey relationships affect survival in this zone as do other relationships between species, especially competition.

In the primary grades, the NGSS emphasize observations of patterns and a foundational understanding of relationships between the external structures and their functions for plants and animals. The emphasis is not on classification for the sake of grouping and naming. The NGSS specifically exclude assessment of knowledge of animal species names in primary grades, however, learning Alaska Native names of culturally-important animals and animal groups receives emphasis in bilingual education in a number of Alaska school districts. Teachers should use their judgment in terms of the emphasis placed on assessing learning about the scientific or common names of different species or species groups.

Prior Student Knowledge: Students will have had prior experience observing plants and animals in different tidal zones on Sea Week field trips although they may not recognize the different zones. They will know how to group animals based on shared external structures (mollusks, crustaceans, echinoderms, bivalves, univalves) and have been introduced to variation in traits between parents and their offspring (plants and animals) and among individuals of the same type of plant or animal.

Students should also have foundational conceptual understandings in cross-cutting concepts of patterns, structure and function, and systems.

Possible learner preconceptions, misconceptions and instructional clarifications:

Learner Preconception/misconception: Plants and animals can live anywhere on the beach.

Instructional Clarification: Survival, and thus the pattern of distribution of plants and animals on a beach is related to many factors, as described in the Teacher Background section. The concept of adaptation (that in any given habitat, some organisms can survive well, some can survive less well, and some cannot survive at all) is a 4^th grade Life Science concept but students at this developmental stage can observe and collect data that will allow them to infer patterns that demonstrate that plants and animals cannot live anywhere on the beach.

Learner Preconception/misconception: Animals that live in tide pools or other places in the lower intertidal zone can survive out of the water.
Instructional Clarification: Many animals that live in tide pools spend their entire life in the same tide pool. Animals like tidepool sculpins that aren’t able to breathe air very long, so are somewhat restricted to pools. Other animals in the lower intertidal zone can spend a limited amount of time out of the water but need to stay wet to survive. If they move too far up the beach when the tide is out, they can dry out and die before the tide comes back in.