Fauna are defined as being the animal life of a region or geological period (8), and they are essential to the survival of the rainforest. Animals serve an integral role in the ecosystem of the rainforest, as they interact with all parts of the ecosystem, such as the flora, soil, air, and water systems.  They contribute to the various nutrient cycles, the energy cycle, and act as ambassadors for the jungle to humanity.  With approximately 500 species of mammals (1), 1600 species of birds (2), and 1 million species of insects (2) in a 2.5 million square mile area (1), the Amazon rainforest is considered one of the most biologically diverse places on the planet.  To ensure the health of the rainforest, we must preserve the health of the fauna.
        Because of the diversity of animal species and the constant discoveries of yet more species, it is impossible to characterize the Amazon's fauna by listing all the species.   However, it is possible to break the fauna of the Amazon into different categories and know that each category is necessary for the survival of others.  Doran and Safley define soil health as being "the continued capacity of soil to function as a vital living system... to sustain biological productivity, promote the quality of air and water environments, and maintain plant, animal and human health" (4).  This can also be applied to fauna; they are healthy if they are able to exist as a 'vital living system' and 'sustain biological productivity.'  This can also be generalized to the entire ecosystem. Costanza et al (3) proposed an "ecosystem health paradigm." Costanza discusses a combined effort of ecologists and economists to try to create a "unifying concept of environmental management that would meet the needs felt with regulatory agencies to adopt a broader set of management goals than used at the time." (Costanza).  Costanza found that an ecological system is healthy if it is "stable and sustainable."This is very difficult to measure directly; in fact it is nearly impossible.  Therefore, a proxy must be employed.  The proxy used by ecologists is bioindicators: "... a complex concept such as ecosystem health cannot be measured as such, but that it can be approached through a series of indicators, each of which will measure a certain aspect..." (van Straalen).  Thus, fauna can be very important to monitoring reliably the state of certain aspects of Amazon Rainforest health.
There are still many important questions to be resolved.  For instance, what is the relationship between species and ecosystem health?  Since one cannot investigate all species, w hich are the most important, the key species.  Ecological theorists have proposed answers to the former question.  Lawton (1994) tried to explain an interesting facet of the relationship between biodiversity, and ecological ability to function properly.  If all species are present and relationships unaffected, then one can be sure that ecological functions are constant.  However, the presence of all functions does not require the presence of all species.  He proposed 3 models to explain this relationship:

a) Redundant species hypothesis - With a decrease of biodiversity, ecosystem
functions are unaffected until the point where only a few key species remain.
If one of these species is lost, the system collapses.
b) Rivet hypothesis - With a decrease of biodiversity, ecosystem function will
decrease proportionally.  This represents a direct correlation between the two.
c) Idiosyncratic hypothesis - There is no relationship between biodiversity and
ecosystem functions.

There is some evidence for the redundant species hypothesis.  For example, Nordgren et al (1983) studied the effects of heavy metal contamination on soil respiration. Species of fungi were killed in a gradient surrounding the source of the metals.  However, respiration was only affected with an high level of metal (and therefore a high loss of species) near the source (Nordgren). In fact, there is a "general feeling...that functional redundancy indeed plays a role..." (van Straalen).  Nevertheless, despite great efforts arising from the Rio convention, there is very little empirical evidence to support any of Lawton's hypotheses (van Straalen).  Still, as Naeem and Li (1997) put it, biodiversity is "ecological insurance." (Naeem). Rather than looking at the number of species to show health, bioindicators can show continuation of attributes.
        Ideally, one would be able to monitor the health and population dynamics of every species in a particular ecosystem.Naturally, when discussing the Amazon rainforest, this is impossible.The biodiversity of the rainforest is such that not only are there vast numbers of species, many of them are very rare or endemic to the Amazon region.Therefore, in order to efficiently characterize the fauna of the rainforest, a different method mus a) t be used.
One such way is the use of bioindicators.  Bioindicators are species that are particularly sensitive to the environment, and provide information about ecosystem health.  Indicator species respond both to the presence or absence of other species as well as the presence of pollutants. By studying the population dynamics or by statistically sampling an indicator species, one can deduce much information about the rest of the ecosystem's health.
There are three kinds of bioindicators:
                compliance indicators: these verify that maintenance or restoration goals have been met
                diagnostic indicators: these help the investigation of observed disturbances
                early warning indicators: these reveal the first signs of a disturbance before most species are affected
      
 In an attempt to cover all three types of indicators, we selected two groups of animals to serve as indicator species: bats and amphibians.  Based off scientific papers, it was decided that bats would serve as good indicator species because they have an abundance of species, occupy almost every trophic level, contribute to ecological processes such as seed dispersal and pollination, and because they select specific habitats.(4)  The first three reasons make them good bioindicators, the last reason makes them easy to monitor.
        Amphibians were selected because they take in nutrients through their skin, so toxins in the environment build up faster in their bodies than in other species.   For instance, if someone is using a certain pesticide, we can monitor the frogs of the area and
test them to see conclusively if a decrease in population is resulting from exotoxicity.  This proves that the pesticide is having a negative effect and should be removed.  Similar experiments may be done to determine pollution by industries, such as mining or logging, or any other possible source of contamination.

There is a slight difference between bioindicators and key indicator species. While both of them are useful in deducing information about their environment, bioindicators tell us information about the environment through their population numbers or particular responses to the ecosystem, while key indicator species are those species that are essential to an ecosystem.That is, if this species were to disappear, a good part of the food web (indeed the whole ecosystem) could perish. A good example of key indicator species is the connection between otter, sea urchins, and kelp.If the otter were to disappear, there would be no species to eat the sea urchins, and their populations would grow as their food source, kelp, would disappear faster and faster.
As it applies to the rainforest, however, monitoring key indicator species to find out information on the ecosystem is not very efficient.That is, with such biodiversity, it is almost impossible to find a key indicator species.There are such vast numbers of species, and the Amazon food web is so complex that if one species were to disappear, the other species can quickly adapt. Every species in the Amazon consumes many different species, and is likewise consumed by many different species, therefore a key indicator is not readily apparent.

Bats as Indicator Species

With increasing human encroachment on the Amazon rainforest and its diverse faunal constituents, monitoring its impacts on the habitat and ecosystems becomes proportionally more imperative. Though satellite imagery, soil sampling, water analysis, and detailed air sensory may give researchers an idea about the general health of the habitat, the information provided by these techniques would not give a very clear picture of how human encroachment is impacting the animal life of the rainforest. Due to the sheer mass and diversity of the rainforest, it would be extremely difficult and probably completely unfeasible to attempt to monitor all of the animals that exist in the Amazon rainforest.Here is where indicator species come in: due to their inherent characteristics, preferred habitat and place in the food web, indicator species are extremely sensitive to the overall health of the rainforest's ecosystems. By monitoring the progress of these species over time, researchers are able to easily determine whether
or not an ecosystem is being affected by unusual or adverse conditions. One particular type of animal that has been selected to be a primary indicator species is the bat. Their commonplace occurrence in every trophic level in the canopy and their relative immobility by their maintenance of a permanent roosting place, bats are relatively easy to find in the rainforest. They are also significant contributors to the ecology of the rainforest, helping to maintain insect populations, pollinating flowers and dispersing seeds over broad areas.
        Bats are of the order Chiroptera, which is divided up into 18 families, in all totaling 986 known species in the world.  They inhabit most temperate and tropical regions of the globe and are one of the most numerous forms of mammals on the planet. Only rodents
have more species than bats.  The most obvious and unique distinctive feature that bats have is the capability of flight. They are the only mammals who have this capability, which is granted to them by skin membranes that extend out from the side of their bodies and their tails to connect their limbs with their main bodies. The forearms and fingers have been adapted to support these membranes, with long extended fingers and slender bones. The entire body of the bat is designed for flight, with flattened ribs, an extremely well supported shoulder girdle and clavicle, and a rigid sternum.
        Another highly unique characteristic of bats are their employment of echolocation for nocturnal orientation. Vocal sounds emitted through the nose or mouth by a bat in flight bounces off surrounding objects, effectively giving them a sensory system analogous to radar. This extra sense allows bats to avoid running into obstacles at night and to detect the position of flying insects or other potential food sources.
        Bats generally tend to roost in a permanent shelter, consistently returning to the same place to rest. Shelters can include cages, trees, crevices, and even buildings. These relatively secure areas are where bats hibernate when conditions are unfavorable, such as a climactic change or reduction in food supply. During hibernation their body temperatures drop significantly, reflecting a marked decrease in metabolism and oxygen consumption. Temperatures and metabolism return to their normal states immediately following the reawakening of the bats.
        A common method of characterizing bats is differentiating them by their distinct diets. Because of the overall species diversity of bats, these diets spread over a large range of food sources. Many can be characterized as follows:

        - Insectivorous:
                - most insect food obtained by flying
                - most will eat some fruit
                - largest and most diverse group of bats
        - Fruit-eating:
                - feed almost exclusively on fruit
                - will eat some green vegetation
                - sometime work together in groups
                - live in tropical environments where fruit is
constantly ripening
        - Flower-feeding:
                - diet consists mainly of pollen and nectar
                - will eat some insects found in flowers
                - mainly tropic and subtropical bats
        - Carnivorous:
                - prey on frogs, birds, lizards, small mammals, other bats
                - extremely varied diet
        - Fish-eating:
                - catch fish near or at the water surface

        All of the above types of bat can be found in the Amazon rainforest, making them an exceptionally good indicator species, since they are affected by multiple factors due to their reliance on a diverse amount of food resources. Due to the large biomass and
abundance of life in the rainforest, the Amazon is an especially ideal environment for large colonies and an assorted number of bat species that can be monitored at all different levels of the canopy.