Biodiversity in an Intertidal Marine Ecosystem

Objectives for Today's Lab:

Describe the diversity of living organisms in a coastal marine ecosystem .
Determine how species richness varies along a gradient from high intertidal to low intertidal zones.
Explain the importance of intertidal habitats and some of the human activities that impact them.

Pre-Lab Notebook Preparation:

Write the Lab Title on a new page on the right-hand side of your notebook. *Remember to include the Lab Date!*
Write the Purpose paragraph and your Hypothesis for the lab.
Draw the required Data Table (see Procedures, step 3).
Add an entry for this lab in your Table of Contents.

Experimental Design

Model Organisms: Intertidal flora and fauna
Experimental Question: How does species richness vary along a habitat gradient from the high intertidal to the low intertidal?
Independent Variable: Intertidal zone
Dependent Variable: Species richness

Hypothesis

I hypothesize that species richness in the low intertidal will be (lower/higher) than in the high intertidal OR I hypothesize that species richness is not related to intertidal zone.

Background Concepts:

Hypothesis Formulation: ‘Null’ vs. ‘Alternate’ Hypotheses

Scientists set 'null' (no difference or relationship) hypotheses that they attempt to falsify. For example, if you wanted to test whether species richness decreases as you move farther away from the water, your null hypothesis would predict that species richness would not differ significantly with distance. Why? Because you are seeking to falsify this null hypothesis. The hypothesis you are testing to be supported is called the 'alternate' hypothesis, and in this case predicts that species richness will be higher in the low intertidal vs. the high intertidal. You would then assess species richness and if it was indeed higher in the low intertidal, you would reject the null hypothesis and find support for the alternate hypothesis.

Ecosystems

The root "eco" derives from 'oikos,' the Greek word for 'home.' Ecology is a branch of biology where scientists study the living organisms (biota) in a particular area (an ecosystem) and how they interact with each other and with the physical world (abiota) around them.

The Intertidal Marine Ecosystem

Zonation pattern on sandy beaches of the Atlantic coast of North America.

Castro, Peter, and Michael E. Huber. 2013. Marine Biology. 10th ed. Boston: McGraw Hill Education, New York, NY.

The intertidal zone is the area between the mean low tide and mean high tide, and represents the transition from fully terrestrial to fully marine conditions. By contrast, the subtidal zone is the area that is always submerged. The intertidal zone will be exposed during low tide.

Intertidal zone substrates can be rocky or soft bottom. Some organisms (epifauna) live on the surface, like mud snails on soft bottoms, or barnacles on hard rock substrates. Other organisms (infauna) live in the substrate, such as clams and worms that burrow in soft sediment.

Challenges of Living in the Intertidal

Due to the exposure to air, organisms face a variety of challenges, including:

Desiccation (water loss)
Temperature changes (can be extreme)
Salinity changes (can be extreme)
Interrupted feeding
Wave action and tides
Oxygen availability and build-up of CO₂at low tide
Limited space

Organisms like this kelp withstand waves by being flexible. In calm water the plant stands upright, but when a wave hits, the plant bends over and the fronds fold up allowing the seaweed to reduce water resistance.

Castro & Huber (2013)

Intertidal Zones

The distribution of dominant plants and animals within each zone in the (a) Pacific and (b) Atlantic coasts of North America.

Castro & Huber (2013)

Zonation in the intertidal is due to varying degrees of exposure.

The upper (or high) intertidal is the most exposed. The lower intertidal is least exposed.

There is typically more competition in the lower intertidal because it is the least extreme of the zones.

The lower intertidal tends to have more species for the same reason.

Species Richness

Species richness is the number of different species (or higher-order taxa) that are documented at a particular site. It is important to consider species richness when monitoring ecosystems, because healthy ecosystems generally support more species compared with those that are disrupted (often by human activities).

In this lab, you will calculate species richness simply by counting the number of species in the sampling area.

Procedures:

1. Establish the Sampling Area

Image source: Salish Sea Sciences, Friday Harbor, WA

Quadrats are square frames with an internal grid that are used for sampling the distribution of plants and animals in a particular area. Salt marshes are often sampled using quadrats that are placed at intervals along a transect.

Because you are interested in evaluating species richness across multiple zones (upper, middle and lower) you are going to run a transect tape (meter tape) perpendicular to the water line, from the water's edge up through the extent of the upper intertidal zone.

2. Sketch the Sampling Area

In your lab notebook, you will sketch and label a simple image of the sampling and surrounding area. Include the following information:

Approximate scale (in meters)
Direction of the coastline
Surrounding land uses (e.g. buildings; roads)
Your sampling area

3. Evaluate Species Richness

Your main goals are to determine how many different species are present in each quadrat you place along your transect, and to identify each species using iNaturalist.

It is also possible to estimate percent cover of each species, by estimating how many of the 25 units in the quadrat are occupied by each species (each unit equals 4% of a square meter). This would give you an estimate of species evenness in your sampling area.

Collecting Data:

Place your quadrats on one side of the transect tape at five meter intervals. Walk on the opposite side so that you do not disturb the areas you wish to sample. Record the distance from the water's edge in your field data table.
Identify every plant/animal that falls within the quadrat frame. You should use the iNaturalist app in the field but it is also good practice to photograph your quadrat in case you need to complete your species identifications back in the lab.
Record the scientific name (genus and species) and common name of each species in the field data table.
Optional: Determine the percent cover of each species and record in your data table (you will need to add an additional column).

*Note: In the sandy habitat most organisms will be submerged within the sediment matrix, so you will have to dig. Look for tell-tale siphon holes and worm tubes projecting from the sediment surface.

Data Analysis & Writing:

Questions to Consider

Do you notice changes in the number and types of species present as you sample down the transect? What are they?
Is there consistency across the class replicates with respect to species richness vs. intertidal zone?
What are some of the limitations or biases that may impact your results?
Are there any introduced species in your survey? What is the significance of this?
The presence of a balanced composition of taxonomic groups tends to be an indicator for ecosystem health. Look at the animal groups that you identified. What are their major roles in the coastal food web (e.g. are they predators, herbivores, detritivores, filter feeders?) Does there appear to be a balanced composition of groups relative to these trophic (feeding) categories?

Thinking Ahead to your Capstone project!

You may choose to perform a diversity survey for your capstone project, building on the techniques you used today. In lab 5 (meiofaunal diversity) you learned how to measure diversity using Simpson's Diversity Index. This equation uses both species richness and species evenness to estimate overall biodiversity, and may be used to perform a more robust assessment of intertidal diversity. You may choose to evaluate plant diversity, animal diversity, or both. If you are interested in assessing either animal diversity or algal diversity, you should consider transects that run parallel to the water, rather than perpendicular.

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