The multiple microhabitat pond microcosm is a simple model designed to recreate the diversity of life that might be found in a small freshwater pond. It is superior to a simple container or traditional aquarium because it more accurately recreates the natural subdivisions, physical diversity and plentiful resources of a real pond. This makes the total number of different species it houses far greater and prevents the gradual decline in diversity usually observable in more traditional systems. We plan to use this design as one of several different modules that will eventually be connected together to create a diverse enclosed environment containing hundreds of plant and animal species that will be used for teaching introductory biology classes. The project is scheduled for completion by the end of 2000.
In any pond, even a small one, the alert observer will notice that there are many different microhabitats where different species live:
The multiple microhabitat pond microcosm attempts to recreate the ecosystem of a natural pond by partially separating a 15-20 gallon aquarium into a number of nested compartments. Each compartment is designed to have slightly different physical conditions from the others. The compartments are generated using a set of plastic aquaria placed one inside the other and fixed together along one face using PVC cement. Limited movement of water, nutrients and small species between compartments is allowed by drilling two to four 5mm diameter holes in the walls of each plastic aquaria.
The act of manually harvesting live plant material from the outer-most compartment and returning it to the inner-most compartment for composting serves as a mild form of disturbance that helps to maintain a permanent gradient of environmental conditions and hence biological diversity.
The occasional addition of small samples of organic material or pond
water from different local sources at different times of year also helps
to boost diversity.
These miniature freshwater ecosystems are designed to mimic the ecology of a small, natural pond. If you get close to them and look carefully you will see that the sides of the tanks are covered with tiny living creatures. Many more are swimming or floating free in the water. Some of these are small animals, others are single-celled creatures called protsists and algae. On any given day, this system probably contains more than one hundred different species of plants, animals, protists and algae; many more than a traditional aquarium could hold. The organisms are organized into a complex food web that draws its energy from light and dissolved nutrients in the water. Most of the inhabitants are smaller than 5mm long. They are born, reproduce, die or get eaten within a few days or weeks, living their whole lives within this “little universe” generation after generation.
Biological diversity is the total number of different species that live together in a given environment. Each species occupies its own unique ecological niche. Many factors can affect biological diversity. This system has been specially designed to allow as many species as possible to live together in a relatively small volume of water. The nested arrangement of the tanks creates partially separated ecological zones or microhabitats. In nature, different microhabitats can occur naturally because of the physical size and structural diversity of the environment. Each microhabitat contains a different community of organisms that thrive there because they are all well adapted to the local conditions. Subdivision of an environment into smaller, partially separated microhabitats creates more niches, lowers competition between organisms and often allows more species to live in the same physical space. This is one factor that makes the biological diversity in this artificial system (and in nature) much higher than in an traditional aquarium, where there are very few separate and distinct niches. The diversity of this system is also affected by factors such as immigration, succession, predation, local extinction and by the availability of resources.
Immigration occurs when new species are introduced. They usually arrive when leaf-litter or organic material is brought in from a local pond and added to the inner-most tank. The organic material gradually breaks down and releases dissolved nutrients to the water. The inner-most tank contains a rich soup of nutrients and the others show a gradually declining concentration gradient of nutrients moving from the inner-most tank (or zone) to the outer-most.
Succession is the gradual change in the relative numbers of different species over time. These changes occur because of maturation, differential reproduction, predation and subtle changes in the physical conditions. In some ecosystems, such as oak forests, succession may occur until a climax state is reached. In other ecosystems, such as coral reefs, factors such as disturbance cause succession to continue indefinitely. Moderate levels of disturbance usually increase the biological diversity of an ecosystem. In this artificial system, the act of removing excess plant material and adding more decaying organic matter is a form of diversity-boosting disturbance that allows perpetual succession to occur. If you observe the community in this system over a period of months, you will see that it is always changing.
Local extinction of a species occurs when death rate exceeds reproduction. This can occur because of competition for resources within a given niche, excessive predation, disease or because the local physical conditions are no longer suitable for that species. Partial separation of the system generally makes extinction less frequent because organisms can sometimes recolonize one zone from another.
Another factor that affects diversity is the availability of resources. In freshwater ecosystems, the most important resources are light, stable surfaces to grow on and dissolved nutrients. These resources are plentiful in this artificial system because of the regular addition of organic material and because of the many flat, clear plastic walls, which provide a surface for organisms to live on, but still allow the penetration of light.
The exclusion of large predators such as fish also helps the diversity to stay high. Even though some types of predation can act as a disturbance and actually boost diversity in a natural pond, this artificial system is so small that even a single fish could quickly eat many of the small species to local extinction.
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