Is there a vacant lot, abandoned house, or field somewhere in your hometown that has been left undisturbed over a period of time? Think back to how it has changed over the last months or years. Perhaps it started with bare soil or some grass, and eventually you noticed taller grasses, bushes or even some saplings beginning to grow? Over time, you have witnessed the start of something ecologists call succession.
Succession is a series of predictable changes in an ecosystem over time. The changes are the result of a disturbance in the environment. Look at the diagram below that shows how succession would occur over time on the same piece of land:
You can see that succession proceeds in stages called seres, or seral communities that are based on dominant vegetation (the plant populations).
- The very first organisms to colonize after a disturbance are called pioneer species.
- Each sere has a different community of organisms that make it up. That community alters the environment by living there, leading to the next, newer stage.
- Without disturbance, a climax community may be reached which remains relatively stable.
- A new disturbance may disrupt succession at any point, resetting it.
Two types of succession exist, and are distinguished by what sort of environment you begin with after the disturbance:
|Type of Succession||Description||Starting Material||Possible Causes (Disturbances)|
|Primary Succession||Population of an area that was previously bare of life or soil. Begins with a pioneer species that can colonize bare rock, usually a lichen. Lichens are a mutualistic partnership between a fungus and an algae and are capable of photosynthesis and enzymatically breaking down rock or other dead organic matter. May end with a climax community.||barren land/rock, no soil or life
|Secondary Succession||Re-population of an area after its destruction due natural or man-made causes. Begins with some amount of soil. Pioneer species are plant species that can tolerate high light and low nutrients. May end with a climax community.||soil and/or other organisms
The volcanic eruptions and lava flows that built the islands of Hawaii are a good example of primary succession. Take a look at the pictures below and read more about this change over time.
Initially, a volcano erupts spreading lava. Over time, lava weathers and soil formation begins. Spores are blown into the area that develop into lichens. Eventually enough soil forms to hold windblown seeds and moisture so that grasses and weeds can grow. From this point changes continue similar to secondary succession.
As neat and tidy as succession may sound, the reality is that most communities and ecosystems don’t really stabilize. Disturbances are always occurring on large and small scales. Communities are typically near a climax community if they have high biodiversity. What is biodiversity? Let’s take a closer look!
Biodiversity is a measure of the variety of life in a community or ecosystem. For example, a desert has a lower variety of different species (or lower biodiversity) compared to the rainforest. Few species are specialized enough to survive in the harsh desert conditions.
The most biodiverse areas of the planet tend to be around the equator, where there are warmer temperatures and higher primary productivity. Primary productivity is a measure of how much organic matter (sugars) producers can put out to support the trophic levels of an ecosystem. These two factors (temperature, productivity) create more niches for organisms, which increases biodiversity.
One of the primary causes for the loss of biodiversity in recent years is habitat destruction by humans. Extinction, which is marked by the death of the last individual of a species, can be caused by loss of habitat, as well as other factors such as over-hunting, pollution and competition with invasive species. Invasive species are those organisms that are introduced to a non-native area and usually outcompete native species for resources.
Why is biodiversity important? It is important because each organism on Earth is part of links in a giant chain. This chain of millions of species provides us with food, air, clothing and more. Imagine a world without pollinators (to give us fruits/vegetables), producers (for oxygen) or bacteria (to fix nitrogen so that we can make proteins from our food chain). These are just a few examples of why biodiversity preserves the delicate balance on Earth.