The tropical forests of the world are home to the most diverse assortment of insects that can be found anywhere on Earth. Costa Rica is home to more species of butterfly than can be found in all of the United States and Canada. (Kricher, 1999) The amazing species richness in the tropics, especially the new world tropics, is easy to see at first glance in a forest or reef, but it is not so easy to explain. Of the myriad of ideas concerning the origin of neo-tropical species diversity, many have been discredited. However, there are still many hypotheses that are hotly debated currently, and the actual truth may lie somewhere in-between the currently proposed hypotheses. In order to postulate a well informed hypothesis as researchers, scientists must study trends in previously observed phenomena. Thusly, the following examples of hypothetical models for plant, bird and reef diversity should parallel the models that would be used to explain insect diversity.
The tropics have been much more stable climatically than regions of higher latitude. This leads some ecologists to assume that tropical diversity can be attributed to stability of climate over a long duration of time. This possible explanation for tropical species richness is known as the Stability-Time hypothesis. This hypothesis is debated and largely considered incomplete because it does not incorporate many of the other factors that appear to foster species diversity. The ideas behind the hypothesis, however, do have merit, and in the case of insects, may be crucially important to the task of explaining species diversity. Insects are ectothermic organisms and, due to that fact, cannot be active year round in seasonal climates. However, in the tropics, insects are provided with adequate temperatures to be active year round. The temperature factor alone allows for nearly a 100% increase in time (compared to temperate forests) for reproduction, which should logically double the rate of genetic mutation and speciation. By the logic presented in the stability-time hypothesis, mountainous regions will probably have little insect diversity because the climate is often cold and insects living in such habitats would need to be specialized to tolerate the cold weather.
Species must compete with one another for resources; this occurs in all ecosystems and biomes. Individual organisms must also compete with members of the same species to gain access to needed resources. Therefore, competition for resources in the tropics, where species richness is high and organisms are very abundant, must be very fierce. The Interspecific Competition hypothesis states that high levels of diversity may have been attained by specialization and consequent speciation due to high levels of competition among species for limited resources. In other words, competition for resources facilitates species diversity because species will be forced to specialize to better compete for resources. There are some problems with this hypothesis though: few tropical insects predate only on one species. Many insects in the tropics, herbivorous and carnivorous alike predate on multiple species and many plants will be host to multiple species of insect. The low host specificity noticed in some studies may be reason to consider that the estimates for insect diversity are over-inflated and that, perhaps, there are many less species of insects in the world than previously estimated. (Novotny et.al., 2002)
Predation on these leaves is apparent because of the hole pattern. The culprit likely is sitting on the leaf, which conveniently serves as either a seat or a meal.
The Intermediate Disturbance hypothesis is one of the hotter ideas in modern tropical ecology, although there are many disagreeing factions. The main idea of this hypothesis is that disturbances, intermediate in both magnitude and frequency, allow more species to exist in the same area. Low rates of disturbance would lead to certain species being favored over others and, conversely, high rates of disturbance would favor opportunistic, pioneer species. This hypothesis is supported by many sources of evidence, but one of the most fundamental is the simple fact that the most common dispersion pattern in nature is clumped. Certain species will colonize an area when conditions are favorable, and the only way for a dynamic system to provide for many different species is for that system experience periodic disturbances. The intermediate disturbance hypothesis does not answer the question of how the tropics became species rich, but it does attempt to explain how species richness is maintained.
When old and large trees, such as this kashew tree, fall in the forest they creat large light gaps and, according to the intermediate disturbance hypothesis, allow for pioneer species to move in. This mechanism may serve to maintain tropical diversity.
In opposition to the Intermediate disturbance hypothesis is the Recruitment-Limitation hypothesis. (Tillman, 1999) This hypothesis deals with the limitations of sessile species from colonizing new areas due to seed dispersal problems and the fact that competition can only exist in species that are sufficiently close in proximity. While it may have some pertinence to insects, really does not serve well as a comparative model in insect diversity. The same problem may apply to the intermediate disturbance hypothesis in that the hypothesis is mainly a model for explaining plant and coral (both sessile) diversity. To use these hypotheses as a basis for predicting insect diversity, it is necessary to investigate insect herbivory patterns so that these hypotheses are applicable.
As stated earlier, few species of insect are 100% specialized to predate a single species of plant, in fact there are relatively few species of insect that are genus specific predators, although many genus specific herbivores and predators do indeed exist. Many insects will also share a host plant with other insects of different species, sometimes in the same vicinity on the plant. This fact make the interspecific competition hypothesis appear to be incorrect. This may not completely be the case, but it is certain that the interspecific competition hypothesis does not adequately explain insect diversity because species competition does not appear to be an issue when explaining herbivory patterns. Insect herbivory patterns also make it clear that the intermediate disturbance hypothesis cannot be used to explain all insect diversity. If insects predate multiple plant species, then there should be little need for disturbance to occur to keep insect diversity high. There are certainly exceptions to this statement such as Azteca ants which are dependent on cecropia trees (a pioneer species), and termites which feed mainly on dead wood and thusly require some level of disturbance to survive.
It is obvious that there must be more occurring in the tropics than just high plant diversity to account for the high insect diversity. So then, why do new orders of insect arise more frequently in the tropics then elsewhere on Earth? (Monastersky, 1992) The first place to look for this answer may be the most obvious, but possibly overlooked. It appears that there is a positive correlation between plant biomass and insect species diversity. (Haddad et.al. 2001) Perhaps the tropical environments, simply by having more plant material to feed insects, promote insect diversity by a very simple mechanism after all. There are, of course, many different situations to consider such as the effects of monoculture and differences between monoculture and polyculture. Diverse groups of plants tend to support more generalist insect species, but also are correlated with high insect diversity. (Haddad et.al. 2001) Plots of monoculture tend to have very high insect abundance, but relatively poor diversity since insects living in such areas may need to be specialized in order to survive.
Agricultural land use, such as this pineapple plantation in Costa Rica, seriously decreases the plant diversity in an area. Insect diversity appears to suffer as a concequence.
Armed with the knowledge that insect diversity is promoted by high temperatures and high plant biomass, we can now start to assemble a plausible hypothesis regarding the predictability of insect diversity in different ecosystems. Obviously, most insects require water, so that is a factor that must be considered, but it seems obvious that environments that are high in plant biomass and warm year round would predictably have the highest insect diversity as well. Conversely, environments with little plant biomass and/or low temperatures would predictably have low species diversity. By this logic, the final hypothesis that insects tend to thrive in benign environments was made to explain high insect diversity in tropical forests. Factors that may be limiting for insect diversity could be: broad temperature shifts, high elevation, lack of fresh water, low plant bio mass, soil acidity or basicity etc. As such, the lowland and mountain forests should have high level of insect diversity while rocky beaches or high level mountain terrain would predictably have low diversity.Quantitative data supporting this hypothesis could be obtained from any number of sources, but to avoid any seasonal or weather related effects, it would be best to take samples for a long period of time. Qualitative data is also useful in the validation of this hypothesis because the time for data collection was short and limited by weather phenomena. Because of weather and time related limitations the actual experiment was limited to only a few collections of quantitative data by ground collections at certain sites. In locations where limiting factors prevented sample collection, observations were made of the apparent species in the area.
Plant biomass is so abundant in the picture of a rainforest that is difficult to find anything that is not a plant. Experimental data suggests that high quantities of plant biomass foster insect diversity.
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