Youth Science Lessons – Marine Ecology – Lesson 1 Beach & Dunes
Some basics on ecology
Let’s begin this unit by defining ecology. Ecology is the study of how organisms interact with their environment and with each other. For example, you can take the ecological community in your front yard. Your yard may be flat or sloped. This can impact the top of soil you may have, or how those soils may be “zoned” as you go from the top of the hill to the bottom. Your soil can be influenced by your geographic location as well. For example, are you near river or bay? Or are you more inland?
Soil is a mixture of inorganics (minerals), organics (nutrients), and water. As just mentioned, those mixtures are affected by your geographic location, elevation, and topography of the landscape. The components of these soils will impact which plants can grow there – hence the environment interacting with the organisms.
The pH, nutrients, and water content will all impact which plants can survive there. Each type of plant is considered a species. A species is defined as organisms who morphologically resemble each other, can breed and produce viable offspring. Each species of plant begins to grow into larger groups we call populations (a large group of the same species). You can see different populations of plants across your front yard, each competing with each other for the resources that the soil provides. Some win, some lose. As long as the soils provide, the population expands over the landscape to occupy new space.
But there are other populations of plants trying to do the same. Again, these populations compete against those of their own kind and those of different species for the available space and resources in your yard. This mass grouping of plant populations is called an ecological plant community (a large group of different populations).
Now here’s the kicker…
As these plants grow and expand their roots, they change the spacing between grains within the soil. This can provide more space for water and different gases. This changes the chemical composition of the soil. These changes COULD change the soil enough that the plants can no longer survive here. Thus, organisms interacting with the environment. Even more, the existing plant populations release waste products and consume the existing water and nutrients changing the environment even more. Again, the environment interacting with the organisms, and the organisms interacting with the environment.
As the original plant communities alter the environment they may, again, not be able to exist any longer. The new environment is more conducive to new plant species and the entire plant community in your yard may change. The original plant community is what we call the pioneer community and the process of community changing over time is called succession. This succession process continues until you reach what we would call the climax community. If you allowed your yard to interact with itself over time, you would eventually end up with a climax community. It would be interesting to see what kind of community this would be but, of course, we do not allow this. We have an idea of what we want our yard to look like and so spend a lot of time, energy, and money battling natural succession. We add commercial fertilizers to the soil to enhance plant populations we are interested in (lawns) undenounced that we are also providing a habitat that other plant populations we do not want (weeds) to thrive in. And so, we attack those weeds undenounced to us that in that act, we are yet again changing the ecology of the yard and so the cycle goes – us versus nature.
Let’s complicate things even more… animals.
Yes, as plant communities develop it attracts populations of animals. Some of these are herbivores who can/want to feed on the plant species growing there. The plants that survive these attacks either (a) develop defenses that retard herbivore attacks, or (b) reproduce at rates that can keep up with the attacks, or (c) both. Plants that cannot defend themselves from these herbivore predators either die or establish themselves in habitats that these herbivore predators cannot reach. Those plant species who can defend themselves increase in number. One would assume eventually that new herbivore predators who can overcome these plant defenses will eventually find their way to your yard and begin their attack. This will alter the animal communities which will in time alter the plant communities that survive which in time will alter the soil chemistry which in turn will alter the plant communities that can tolerate the chemical changes in the soil which in time will alter the animal communities who prey on them and so it goes. Ecology can be quite complicated but amazing to watch.
Of course, we can’t stop here… there are the carnivores, who feed on the herbivores. Those herbivore communities that make it are those who can feed on the plant communities that exist while at the same time defend themselves against the carnivore predators that are there. Their population numbers do well. But of course, there will eventually come carnivores who can eat them, and the herbivore population will change due to this. This will alter the plant communities and the soil chemistry and – and – and… you get it.
All of this time the animals (both herbivores and carnivores) expel waste as well – changing the chemistry of the soil and… here we go again. But there is another member of the community. The saprovore (the scavenger). These animals do not feed on the plants or animals per se, but rather on their decomposing bodies after death. They are actually the decomposers. As you throw them into the picture the process of change occurs again.
Lessons from all of this include
- Life changes – it always changes.
- Any alteration to the environment will trigger a myriad of both abiotic (non-living) and biotic (living) changes within the community.
Biology studies the mechanisms of living systems and how they function. Many biologists focus on one species, or a group with which specific species exists. Soil chemists study how the soil changes over time and which organic and inorganic compounds have the greatest impact on biological change over time. The ecologists’ study it all, understanding the “bigger picture” as they say. And keep in mind, this simple example we gave was of your front yard alone. There is your backyard, which could be COMPLETELY different. Then there is the park down the street, the landscape around the parking lot at the nearby grocery store, or the beach and dunes of our own barrier island. Let’s look closer to beach and dune communities.
Beach and Dunes
Let’s start with the environment of our own barrier island. Santa Rosa Island (near Pensacola FL) is a thin strip of sand about 44 miles long blocking the open Gulf of Mexico from the mainland. The sand is primarily quartz, a clear mineral that comes from granite and originated from granite rock in the Appalachian Mountains and appears white in the sunlight. There are numerous minerals within granite rock and they eventually erode or weather and are moved downstream by creeks and rivers. As the rivers slow, different minerals begin to settle out based on their densities, the denser settling first. So, beaches inland from the Gulf coast are a bit different from those we find on our barrier islands. Santa Rosa Island is almost pure quartz and gives the beaches the white sand for which they are famous for.
This white sand is very porous (holds very little water) and is nutrient poor (few organics). The waves and currents along the coast create what we call a longshore current which pushes the sand coming from the rivers into these long thin sand bars. There are numerous bars along the shores of the northern Gulf of Mexico, and few accumulate enough deposits that they emerge above sea level (barely) to form emergent sandy islands. The wind and waves are constantly shifting and moving these sands everywhere. These sand bars are very dynamic and look different each year you would visit them.
Seeds from mainland plants are brought to this new island by wind, water, and by nesting shorebirds or other visiting animals. The seeds may or may not germinate, depending on which plants. Those that can tolerate dry xeric conditions (little rainfall) and very little nutrients will make it and form the pioneer community. These pioneer plants will, as mentioned, alter the chemistry of the sand and add organics that will allow, in time, for other plants to exist here. And so, the process of plant succession begins.
Some of the plants will capture the drifting sand and form dunes. Some of the dunes will grow in size blocking the sea breezes and creating a different environment landward of them. Which will allow for the development of a completely different plant community. These dune plant communities will slow (not stop) the movement of the island – and things begin to settle.
The primary dune community are the dunes closest to the open water and are dominated by grasses and low growing vines. Grasses and ground vines do much better here because they can tolerate the high winds of the open Gulf. The most famous one is the sea oat (Uniola paniculata) but there are many others. These grasses have fibrous roots that form a “volleyball net” like structure to capture sand and form the primary dune.
Beyond the primary dune the wind is not as strong (blocked by the dune itself) and smaller shrubs can grow. All of these, like the grasses of the primary dune, have to be salt, drought, and poor nutrient tolerant. Seaside rosemary (Ceratiola ericoides) is one of the most abundant and fragrant. Many these shrubs are round in shape to help deal with what wind there is and look like little green sheep on a white field of sand, giving them a common name of “beach sheep”. In some spots there are low depression runnels in the dunes that can hold freshwater. These are known as swales and can form pockets of ephemeral freshwater ponds, which can support freshwater plants. The soils here are not dry white quartz but a combination of organic, inorganic compounds along with freshwater. Little oasis in a field of dry sandy dunes. This zone on the island is known as the secondary dune community.
Further inland on the island the wind slows even more allowing, of all things, trees to grow. Again, these trees must be drought, salt, and poor nutrient tolerant, and have had their seeds reach the island to have a chance to colonize here – and many, such live oak, pine, and magnolia, have done just that. However, with the harsher conditions on these islands, these trees do not grow as they do in town. Most are very short and exhibit what we call wind sculpting. This is the apparent “brushing back” of the tops of the trees towards the mainland. As the trees grow, they do eventually encounter the Gulf wind as they reach elevations above the secondary and primary dunes. This zone of the island has richer soils, less wind, and a greater variety of plants growing. It is known as the tertiary dune community or sometimes as the maritime forest.
The landward side of these islands where they meet the estuary do not experience the higher winds of the open Gulf (due to the blockage of the dunes themselves) and so, salt marshes can grow. These are communities of wetland grasses that we will highlight in our next lesson.
Animals will eventually arrive. Many, like sea turtles and shorebirds, are there is nest only and do not participate in the food chain by consuming the dune plants. But, if they can get there, other animals will. Beach mice, insects, and even deer have been found on these islands. Butterflies abound as they make their migrations and can fuel up on the variety of flowers found in these plant communities. There are numerous woodland birds that are attracted to the maritime forest communities and feed on seeds of many plants.
These herbivores attract carnivores like birds, snakes, lizards, and mammals. Skunk, raccoon, and armadillos are all common on the islands. Snakes like black racers, coachwhips and rattlesnakes are common as well. Armadillos frequently dig for insects and grubs, while opossums hunt a variety of wildlife including snakes. The mid-level carnivores attract the attention of top predators such as fox, coyote, and owls. In recent years there has been a resurgence of bald eagles and ospreys to the islands and alligators have been found as well. Keep in mind all animals found on barrier islands not only have to be able to reach them, but be able to find food, avoid predators, and tolerate the xeric conditions of this system. Ecologists love to study all of these interactions and understanding how barrier island systems function.
With the ecology of the island in balance, the island still moves – nothing will stop this – and it is particularly dramatic after tropical storms. However, the hardy survives, and the island continues on. Only with the arrival of humans has the ecology and dynamics of the island been altered. More on this in another lesson.
- Take an afternoon and explore the ecosystem we call your yard.
- Which plants are growing here?
- Which is the most abundant?
- Do you have any idea why these plants do so well?
- Are you managing the lawn, so they are chosen and competitors (weeds) are removed?
- Are the plant communities in different parts of your yard different?
- Why do you think that is?
- Is the soil in different parts of your yard different?
- What animals do you find in your yard?
- What do you think is attracting them?
- Can you tell who are the herbivores and who are the carnivores?
- If you can, take a dune walk. Begin at the shores of the Gulf of Mexico. Examine the sand and dig for animals. You will not find many plants here; can you explain why?
- As you move into the sandier portions of the beach walking towards the Sound what type of plants do you find?
- What animals do you see on the beach? What do you think each is feeding on?
- As you cross the primary dune be careful not to crush the grasses and notices which species are there. Which animals would you expect to find here?
- Notice the difference in the wind as you get behind the dune. Do you also notice the change in the plant communities?
- Continue crossing the island, noticing the environmental, plant, and animal changes as you do. Can you explain what triggered these changes and why those plants and animals made it?