The back of our truck was home on San Salvador in the Bahamas
If one were asked Ňwhat is the most widely distributed mammal in the worldÓ, most people would probably answer mice or rats. In this case however, most people would be wrong; the most widely distributed mammals are in fact bats. Bats can be found all over the world however, the greatest diversity of bats can be found in the neotropics where 75 bat genera can be found. Within the neotropics, the region richest in bat diversity is the region known as Middle America, the transition zone between the neotropics and the near arctic zones. This comparatively small region, which makes up only 1.8 percent of EarthŐs emergent land surface, contains 166 bat species, 28 of which are endemic to the region. Costa Rica specifically contains a total of 103 species (including 65 in La Selva and 78 in Guanacaste). Bats play a vital role in the ecology of tropical forests, serving as important pollinators, seed disperses and predators. However; they do not hold the same level of priority as other organisms of similar importance. This, in my opinion, is due to a lack of public understanding and the fact that there is relatively little profit to be made in their preservation. In this paper, I will attempt to rationalize the richness of the neotropical bat communities and the future of their preservation.
In reading A Neotropical Companion one quickly becomes aware that there are multiple theories on why rainforests are so diverse. One encounters the same problem when attempting to rationalize diversity in specific types of fauna, in my case bats. I am aware that there are many theories, I am also aware that no single one of them is completely correct, in this case however the one that I find to be the most likely is the one involving specialization due to intense competition. Overall I find this theory to be a likely explanation of neotropical bat diversity because, firstly there is a great deal of specialization among neotropical bats, and second there appears to be a great deal of evidence to support it.
In botany there is a rule called the law of limiting resources. In summary, this rule states that plants can coexist without severe interspecific competition as long as they are limited by different resources. In plants these resources are light, water and minerals (plants can be limited by various different minerals, much as how animals can be limited by a variety of different food sources). Although the needs of animals are some what different from plants, the theory should still hold up. Animals are limited by their need for food, water and shelter. The question now becomes, which one is the limiting factor for neotropical bats. Tropical rainforests are the wettest of terrestrial ecosystems, so one should be able to rule out water. This leaves food and shelter. Shelter in this case would refer to the habits or niches in which they reside. This however, as I will discuss at greater length later, may not be a limiting factor so much as an adaptation to avoid interspecific competition. Therefore the most likely limiting factor is food. Compared to their near arctic neighbors who are almost exclusively insectivores, the neotropical bats make use of a much wider range of food resources including carnivory, frugivory, herbivory, ominvory, piscivory, nectarivory, saguinivory, and of course insectivory. Based simply on this information, one can fairly easily conclude that neotropical bats have a high degree of specialization. This then leads to the idea that is the greatest focus of the paper; why is the bat community of the neotropics so diverse or rather how can it be so diverse. The answer to the question, at least in my opinion is simple. The neotropical region, is a habitat rich in resources, the only limiting factor for bats would competition for a common food resource. The way around this competition is clear; by specializing for different microhabitats and a diverse array of food resources bats are able to effectively reduce competition allowing many different species to coexist. Kalko illustrated this based on three factors: food, feeding method and habitat structure. By exploring the degree of variation within these three factors, one can gain an advanced understanding of how this theory works, and get a glimpse of the diversity of the bat community.
As mentioned before there are a total of six different food types exploited by neotropical bats (some would list herbivory, however there are no bats in the region that use plant material as their dominant source of food) this alone helps to prevent interspecific competition. If there is a great deal of competition for a food resource, natural selection would favor the bats that diverge and find novel new food sources, thus avoiding competition. Examples of this include the bulldog bat, who has adapted to catch fish, or the infamous Desmodus rotundus or vampire bat which survives primarily on the blood of mammals.
This works out well for the species that rely on more novel found sources, however, there is still a great deal of competition among the insectivores, who still make up 30-50 percent of all species in the neotropics; how do these bats manage to coexist and thrive when they are competing for the same food source? This can be explained as there is than one way to partition a food resource. Outside of finding a new source of food, bats can avoid competition by specializing for a specific type of food within a type. I will refer to this notion as food selection. The mechanism of selection in insectivores, for example is simple than one might think. Bats tend to select food in proportion to their own size. In insectivores this can be explained by differing echolocation frequencies. Smaller bats tend to have calls of higher frequencies, this allows them to detect smaller and larger insects, which in turn allows them to adopt more of a generalist strategy. Larger bats on the other hand have a lower frequency and therefore can only detect larger sized insects, so this prey selection is based on necessity.
Another major factor in resource partitioning is the species feeding method; bats can forage for the same food source in the same habitat, but if they have different method of foraging (they look in different places) than they can coexist with minimal competitive pressure. Once again, a prime example of this can be seen in insectivores. There are two basic strategies for insectivores, forage gleaners and aerial insectivores. While these two types are both insectivores, they are behaviorally and morphologically distinct. Foragers are adapted to snatch insects from substrates, and thus tend to have shorter rounded wings for greater maneuverability. Aerial insectivores on the other hand have longer, more pointed wings suited for hunting insects in mid flight. Another example would be differing foraging behaviors in frugivores. All frugivores make use of fruit as their major food source; however, some choose to consume the fruit on the spot while others prefer to take the fruit back to their roosts. This difference causes the bats to select certain fruits. For example, the larger carriers tend to select for larger fruits to cut down on return trips, where as smaller the smaller bats that prefer to eat on the spot can be less particular eating whatever is available.
The third and final factor is habitat structure. As was mentioned before, bats possess morphological adaptations to aid in their feeding methods, however adaptations are also required for their success in particular environments. Before discussing these adaptations however, it is important to discuss the environment to which they are adapting. The four major habitat use strategies are woodland-edge foraging, intermediate-clutter foraging, clutter foraging, and open foraging. The rainforest in particular is believed to be vertically stratified, with bats inhabiting differing levels within the three dimensional habitat. In these different levels, the bats have adapted for optimal foraging and maneuvering. For example, highly cluttered environments are favorable to smaller bats that are morphologically adapted for good maneuvering; where as more open areas are better suited for larger bats that are built for speed. Overall, the other two factors determine how the bats fit together in the ecosystem; this one determines where they fit in (physically).
Ecologically speaking, there is a great deal more to be said about bats than simply how their specialization allows them to coexist, they are also important parts of their ecosystems. Bats are very important as seed dispersers. 50-90 percent of all rainforest trees and shrubs depend on vertebrates as seed dispersers. Since bats make up 39 percent of the mammal population in the neotropics and a significant number of them are frugivores, one can reasonably assume that they play a fairly significant role. However, bats are not all that efficient at seed dispersal. A good number of the seeds consumed by the bats are deposited beneath the batsŐ roosts, or under the parent plant (these would be deposited by on the spot frugivores) dark shaded areas where germination is unlikely. The only seeds that would be likely to germinate would be those that are dropped in the transition between the food source and the roost. Some bats however, are vital as colonizing seed disperses. These would be the woodland-edge foragers, who tend to deposit their seeds outside of the forest, allowing it to expand or recolonize gaps.
Perhaps the most important ecological roles of bats, however are as predators and pollinators. Bats consume countless numbers of insects everyday in the rainforest ecosystems along with a number of other organisms. Nectarivory also plays an important role in the forest ecosystem. When flowers are in bloom, many different bats will visit the flowers to consume the nectar and pollen (even ones that arenŐt normally considered nectarivores), these bats serve as quite effective pollinators (although not the most efficient, since they arenŐt known for specializing for specific plants). For example, there are 52 pollinators in La Selva (vertebrate and invertebrate), four percent are bats.
It is fairly evident that bats are a vital part of the rainforest ecosystem, and like the rest of the ecosystem they are endangered and in need of conservation. Bats however, are not always given the attention that they deserve; convincing the general public that bat diversity should be a conservation priority can be a tough sell. The major reason for this is, unlike other parts of the forest bats do not offer a benefit to the human population that is obvious to the common person. Other organisms can be used for profit, like medicinal plants, bats however do not have a direct benefit to humans, serving to propagate the plants of the forest or reduce the insect populations. In fact, vampire bats are seen as a serious nuisance in countries were livestock is an important pat of the economy (countries like Costa Rica), a study from the 1970Ős reported that vampire bat vectored rabies cost more than 100 million dollars in lost livestock annually, and while this number very well could be lower today, the threat of such a problem would be a major concern to local farmers. In the past this lead to indiscriminant trapping and killing of bats as a counter measure, however it has become a priority to manage the problem without resorting to such extreme measures.
The present approach to conservation is based on identifying priority species, not unlike the World Wildlife FundŐs method of prioritizing ecosystems. Species are rated based on their importance to the ecology of the region and their susceptiblity to extinction. A species is classified as rare if it is locally rare, has a restricted distribution, or is a habitat specialist; if a species fits more than one of these criteria, they are considered to have an increased susceptibility to extinction. This method is deployed for similar reasons to others like it. With 166 species in the neotropics it would be difficult, if not impossible to preserve them all. By using the prioritization method, conservationist hope to protect the most endangered and most vital of species, in hopes that their protection will minimize the damage to the rainforest ecosystem as a whole.
Bats are a vital part of the rainforest ecosystem, and while their benefit to humans may not be obvious to the common person, it is important to realize that they play a vital role in the survival of the medicinal plants and the tremendous diversity of the neotropical region. In order to protect the region that is so valued by the world community and the resources there in we must not neglect to protect one of the regions most diverse organisms.
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