Geologists know that throughout geologic time ecosystems change as the systems that affect ecosystem development change. Rises and falls in sea level, mountain building, erosion, climate change, and biotic evolution all impact ecosystem development.
However, ecosystems can also change over a much shorter period of time, from a few years to few hundred years for both natural reasons, and reasons brought on by human development. The first concept we will cover is natural succession, which describes natural change within ecosystems. Then, we will look at how human actions have impacted, changed, or degraded ecosystems.
Lesson
Learned: Biosphere 2. $200 million was spent in the
1990s to create a totally self contained and reliant ecosystem
in the Arizona desert. The idea was 8 scientists would live with
introduced plants and animals in a closed system, meaning, no energy,
nutrients or material could enter or leave the system--aside energy
from the sun. The project failed; most of the animals died off,
except for cockroaches and ants. The atmosphere inside the Biosphere
built up too much poisonous gasses, and the scientists ran out
of food (they were supposed to eat the plants and animals in Biosphere).
This experiment showed just how precarious the Earth's ecosystems
are. It now acts as a tourist attraction. It's
for sale if you are interested.
Natural Change
Natural Succession is the term to describe the gradual transition of an ecosystem from one biotic community to another. The transition is forced by changes to the plants/animals present in an area, or changes in the non-living environment in an area. Succession only occurs in an ecosystem when the variables affecting the ecosystem have changed. Succession does not go on forever, as eventually the ecosystem reaches a climax community (or ecosystem).
The climax community represents a final stage where the living and non-living components of an ecosystem remain stable and the biotic community continues on. However, over geologic time we know the variables effecting all ecosystems will eventually change, and the climax community will change into another type of ecosystem. Two graphics illustrating the concept of succession over time in a terrestrial (land) community and in an aquatic community are shown below.
Scientists hope an ecosystems changes for the better over time. How do we know if an ecosystem is improving over time? Common measures are:
- Species diversity (the more species, the more potential positive interactions and a decreased likelihood one species can dominate the environment).
- High productivity. The ecosystem produces a lot of nutrients (carbon, nitrogen, phosphorus) which helps the ecosystem grow, or helps adjoining ecosystems.
- Symbiosis. Species within the ecosystem work together in harmony to balance resource usage, so that one species is not "hogging" the resources.
- Synergy. The ecosystem works with adjoining ecosystems to share resources
A productive ecosystem (or a "good" ecosystem) maintains or increases these traits and acts as a good neighbor by not harming these traits in adjoining ecosystems. As we discuss change, scientists look for ecosystems that change or evolve to meet the four criteria above.
Succession from an agricultural field to a hardwood forest. The area changes ecosystems several times before reaching the hardwood forest ecosystem (Wright; Pearson/Prentice Hall).
To best describe how an ecosystem changes through time, imagine a vacant lot or vacant field, barren of any plant life. Over your life span, we can imagine that the area will soon be covered by grasses, and animal life will live in the grasses. The grasses will succumb to bushes and small trees. The trees will grow and cause the grasses and most bushes to die out. Over time, the trees will be replaced by hardwood trees (taller) trees.
Natural succession can be driven by changes to the biotic community. In the vacant lot example, succession is simply driven by changes in the plants and animals that fill that space. In the diagram at the bottom, we see a pond ecosystem slowly dissappears as it is filled in by the surrounding wetlands.
Not all natural succession improves an ecosystem. Natural processes: forest fires, landslides, floods, and volcanic eruptions can force an ecosystem into succession by destroying it or stressing it. How an ecosystem responds to a disturbance like these determines what will happen. If an ecosystem is fragile and close to being out of balance with its components and natural systems, it is more likely to permanently change.
Ponds and lakes are gradually filled in by the surrounding land ecosystem.