ROCKY INTERTIDAL (TIDE POOLS) FIELD TRIP Pre-Lab Exercise

(modified by Keith Meldahl from an exercise developed by John Turbeville, Mira Costa College)

Use the information below to answer the pre-lab questions.

Introduction

The habitats of the rocky intertidal zone occur at the dynamic interface between land and sea.  Daily tidal fluctuations create ever-changing conditions where organisms must be highly adapted in order to survive.   

The rocky intertidal area can be divided into several zones based on time of exposure above sea level and on the presence of certain key species, called index species, which live within particular zones.  The figure below shows the tide zones and some of the index species that commonly occur in each zone.    

Terms

Index species: A species that occurs abundantly in a particular tide zone (splash zone, upper zone, middle zone, or lower zone), and whose presence therefore indicates that you are within that particular zone.

Keystone predator:  A predator whose activities prevent prey species from overrunning the environment.  In our local tidepools, sea stars act as keystone predators by preying on mussels and other shellfish that have few other natural predators. If the sea star is removed from the environment, the mussel population could increase uncontrollably, driving out other species.

An Ochre Sea Star feasting on a California Mussel (K. Meldahl)

DIVERSITY and DOMINANCE in the TIDE ZONES

Two concepts will help us understand what we observe when we visit the tide zones:

· Diversity:  The total number of species living in a particular tide zone. 

· Dominance:  The relative abundance of species in a particular tide zone.  High dominance means that a tide zone is dominated by one or two very abundant species.  Low dominance means that no species distinctly dominates in the tide zone.

The graph shows us that diversity and dominance generally relate to how stressful and variable a habitat is.

In general, organisms have an easier time living in low-stress, stable habitats than in highly stressful, variable habitats. Therefore, low-stress habitats tend to have high diversity (many species). However, in such habitats, competition among all of these species (for space, food, oxygen, or other resources) generally prevents any one species from dominating the habitat. Therefore, low-stress habitats tend to have high diversity but low dominance.

In contrast, highly stressful and variable habitats are harder places to live, and so they tend to have low diversity (few species). However, any species that are able to deal with the variable, stressful conditions in these habitats have little competition from other species. This often allows them to become very abundant and to dominate the habitat. Therefore, high-stress habitats tend to have low diversity but high dominance.

Think about these concepts as you read about the various tide zones below.

SPLASH ZONE and HIGH TIDE ZONE

The splash zone and high tide zone are exposed to the air most of the time.  Because of this, desiccation (drying out) of organisms is a near-constant threat.  Also, temperature and salinity fluctuations are high. The Sun bakes exposed organisms, and even isolated pools can become dangerously warm.  Evaporation of isolated pools can increase salinity. Conversely, when it rains, the same pools can fill up with freshwater and salinity will decrease. 

These factors result in a very high stress environment. This is a hard place to live!  Most organisms here have some type of adaptation to cope with the stress in these zones.  The most common is for creatures to clamp their shells together (or to a rock) during low tide to limit their exposure time to the air.  These factors mean that few species live here.  However, the species that are able to live here face little competition, allowing some of them to become very abundant and to dominate the environment.  In other words, the high tide zone is a zone of low diversity but high dominance.

Summary:

Desiccation threat: HIGH

Temperature/Salinity fluctuations: HIGH

Predation threat: LOW

Stress level: HIGH

Limiting factor: Ability to survive harsh physical conditions (desiccation, changing temperatures, changing salinity)

INDEX SPECIES: Periwinkle Snails, Limpets, Buckshot Barnacles Click the link to learn more about these organisms

Typical Splash Zone and High Tide Zone setting.  The green alga on the rocks is somewhat unusual; normally the green areas would be mostly bare rock.  (J. Turbeville)

MIDDLE TIDE ZONE

The middle tide zone is covered by water about half the time and exposed to the air about half the time.  This makes it a less difficult and stressful environment than the high tide zone, but still more difficult than the low tide zone with regard to physical stresses (desiccation, salinity fluctuations, temperature fluctuations).  You will find more species here than in the high tide zone, but generally not as many as in the low tide zone.  In other words, the middle tide zone is a zone of moderate diversity.  The middle tide zone is a zone of high dominance because two index species are very abundant here: California Mussels and Gooseneck Barnacles.  Part of the reason these two species are so abundant is the relative absence of sea stars compared to the lower tide zone. In the lower the zone, sea stars are keystone predators that keep the number of mussels and barnacles down.  Sea stars may move up into the middle tide zone when the tide is high, but they can’t stay there when the tide goes out because they are susceptible to desiccation. Mussels and barnacles are therefore able to become very abundant in this zone.

Summary:

Desiccation threat: MODERATE

Temperature/Salinity fluctuations: MODERATE  

Predation threat: LOW

Stress level: MODERATE 

Limiting factors: Ability to survive wave impact and desiccation; ability to compete for space

INDEX SPECIES: Gooseneck Barnacles, California Mussels, Aggregated Anemones Click the link to learn more about these organisms

Gooseneck barnacles and mussels in the Middle Tide Zone (J. Turbeville)  

LOW TIDE ZONE

The low tide zone is covered by water most of the time.  Because of this, temperature and salinity fluctuations are typically low, and the threat of desiccation is likewise low.  This results in a low stress environment in terms of physical factors. The main challenges that organisms face in the low tide zone are 1) competition for living space and 2) avoidance of predators.   Species diversity is typically higher here than in the other zones.  However, the presence of predators, and the intense competition for food and space, generally means that no single species dominates this zone.  In other words, the low tide zone is a zone of high diversity but low dominance.  You will often find many very cool, interesting creatures in the low tide zone.

Summary:

Desiccation threat: LOW

Temperature/Salinity fluctuations: LOW   

Predation threat: HIGH

Stress level: LOW to MODERATE 

Limiting factors: Ability to compete for space and avoid predators

INDEX SPECIES: Ochre Sea Stars, Solitary Anemones, and others Click the link to learn more about these organisms

Ochre Sea Stars in the low tide zone (K. Meldahl)