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| Lecture28: Tropical Ecosytems Tropical biodiversity Introduction to tropics Definition—depends on who is defining, that is biologists often use specific plant assemblages, climatolgists may use specific atmos. conditions, geomorphologists may rely on specific physical processes.  KoeppenIn General, the tropics as falling within the 23.5N and 23. 5S. This latitudinal boundary defines the range of the subsolar point and this area therefore receives large amounts of solar radiation throughout the year, with little annual fluctuation.  brazilThis is reflected in climate graphs that show rainfall and T which are typical for the tropics. Compare with  SFClassification:  lushforest  paramo
PHOTO REMOVED PER REQUEST OF COPYRIGHT HOLDER sahel There are a variety of environments found in the tropics; humid, subhumid, dry, alpine, montane, etc. One way of looking at the different tropical climates is the Koeppen classification system. KoeppenYou can look at this more closely in the Christopherson textbook. To define a tropical climate, it uses a mean T of 18ºC for the coldest month, which excludes the cooler highlands. Like I said, we often think of the tropics as the humid lush environment of the tropical lowlands but around 25% of land surface within the tropics is considered highland, above 900m. As you can see, these classifications divide the tropics up into many subcatagories. There are many other classification systems and they very rarely fall out along the same boundaries. It is important to remember that whatever system of classification used, the categories at best describe transition zones both within the tropics and the tropics themselves.  rainfallPrecipitation: As a result of the stability in annual T, differences in precipitation are much more important than annual T as a climatic parameter. Most tropical rainfall is associated with convective processes rather than frontal systems. Remember that the ITCZ is basically composed of convective cells rising; convective uplift can occur away from the ITCZ simply because of the heat created by the high solar input. The annual migration of the ITCZ is an important factor in creating seasonality of rainfall that we see in the tropics. Areas closer to the equator experience less seasonality; those at higher latitudes tend to have more pronounced seasonality. Seasonality in the tropics refers to wet and dry as opposed to cold and warm. This is most obvious when looking at deciduousness in the tropics. Deciduous trees that we are familiar with in the temperate areas lose their leaves as an adaptation to cold winter months, deciduousness in the tropics is an adaptation to get through dryer times of the year (to not lose water through respiration). In general, rainfall varies dramatically throughout the tropics and has a strong influence on the ground in the form of species composition, diversity, and growth rates etc. A good example of this is the Yucatan peninsula, which has a pretty steep rainfall gradient over the latitudinal distance of 5º, with rainfall dropping from around 2500mm/year in the south to around 1500mm/year in the north. This is reflected on the ground: where forest still exists in the south, it is high, closed canopy, broad leaf luxuriant forest. In the north, the forest is comprised of much smaller trees, basically thorn scrub. Neo/Paleo: A quick point, that the tropics are often divided into two groups, along the lines of the old world/new world divide. That is the Americas being the new world, and the tropical areas of the Americas are referred to as the Neotropics Diversity in the tropics Measuring diversity and theories of tropical biodiversity.  butterfliesDiversity increases as one gets closer to the equator. Here is one image that shows, for example, that shows a common biogeographic pattern—the decrease away from the equator in number of swallowtail butterfly species. Measuring diversity by species richness: the total number of species found in a given area. Simpson’s Diversity Index: looks at the likelihood of two sampled individuals being the same species.  species/areaAnother important way of measuring at diversity in the tropics is the species-area curves. Species-Area curves show the number of species found as the unit area increases. Big difference between the tropics and temperate latitudes is that in the tropics, these curves tend to not level off because there are so many rare species. Thus, tropical diversity tends to be associated with a lot of rare types and a few common ones. For example, a 50ha plot in Panama with about 240,000 stems counted showed 309 species. 50% of the species had fewer than 110 individuals (17000). Again, any way it is measured, diversity increases toward the equator. Theories of biodiversity: Why is there so much diversity in the tropics?? This has been the topic of much debate, and many theories have been put forth. I. Interspecific Competition Hypothesis: Also known as niche partitioning. Basic idea is that 2 or more species seeking access to the same limited resource must subdivide the resource, or one will win over the other. The idea here is that if a resource is not subdivided, a species that is better adapted will eventually out-compete other species.  partitioningThis theory argues that high levels of competition in the tropics have led to increased specialization, which allows for greater packing of species into a smaller area.  Finchbills finchetreeThis high level of specialization definitely exists and is an interesting component of the tropics—one of the best examples of this is the finch population in the Galapagos. These finch poplulations were first described by Charles Darwin in the 1830’s and were important to his developing theory of natural selection. Basically on this island chain, there is a high diversity of finches with small differences in beak morphology. These differences in beak morphology reflect different feeding methods, which allow these species to co-exist without competing for food. This process that leads to this is known as adaptive radiation. Basically, a single species may, over time, branch out to form many species, each with a specialized adaptation for a specific resource.  fruitbat insect (Looks like another fruit bat to this web technician)Bats are another good example. There is generally an amazing array of bat diversity in the tropics. (fruit, insect)The high number of bat species falls out along food procurement strategies; in any given area you can find bats that eat meat, fruit, fish, blood, nectar, and insects. Insects can be eaten from all over the place as well. Problems: Plant Species don’t compete: There is very high plant diversity, but most trees compete for the same resources—space, light, moisture and nutrients. While there are some specialists, most trees are resource generalists and only differ along a small gradient of required resources. Not specific to the tropics There are people who are still looking at this theory. In the end, the argument is circular if one argues high level of competition results from and leads to many species that are after the same resource. II. Predation Hypothesis: Predators prevent prey species from competing with each other to the point of extinction. Predators will constantly switch their attention to the most abundant prey. Example of four caterpillar species—one wins out, but the birds begin preying more on the populous species, limiting it and allowing the others to survive. This is called density-dependence in plants and frequency-dependence in animals, meaning that the degree to which a prey will be selected is dependent on its density (if plant) or frequency (if animal). So in a mixture of species, as one species becomes more common, mortality increases, limiting that population and preventing it from out-competing other species. Note that this is essentially the opposite of the Interspecific Competition Hypothesis. Rather than specialization developing, predators would essentially prevent it by keeping competition levels low. Problems: Some studies have shown that large cats for example do not selectively choose the most populous food source, but eat everything they find. This would accelerate the extinction of those species that are already loosing out to competition Also not clear why this is limited to the tropics. So this is a strong theory, but evidence supports it shows an importance in structuring plant communities. However, it is a mechanism for diversity maintenance and does not address origin of diversity. III. Productivity/Resource Complexity Hypothesis: The tropics have more productivity (biomass) which allows more species to be accommodated. Abundance of resources can lead to specialization and increased diversity. Basically this is saying that rather than competition driving species to adapt, the availability of resources is so great that species will drift toward specialization and therefore increase diversity. lushforestTropical forests are structurally very complex, and this often does permit greater species packing. Resource complexity also includes disturbances such as storm damage, landslides, and tree fall gaps. Tree fall gaps provide a patch of bright sunlight on what would otherwise be a dark forest floor. People are beginning to think of tropical forests as essentially shifting mosaics of small disturbances in recovery. Specialization for light gaps, dark under-story etc is a response to the heterogeneous environment, which leads to high species diversity.  interdistIntermediate Disturbance Hypothesis, which falls under Resource Complexity. Intermediate refers to both the frequency and the scale of the disturbances. Disturbances occur at different time scales and intensity. Too frequent results in low diversity because only those species with an ability to quickly colonize will have time to do so. As frequency slows down, more species (those with lower dispersal/growth) are allowed time to colonize. If the frequency is too slow, there is time for competitive advantages to eliminate species and decrease diversity. Medium intensity disturbances also lead to higher diversity. Too small means only the nearest species will most likely colonize. Too large means that only species with high dispersal ranges can colonize. A medium size, with some species left alive, allows for the broadest range of potential colonizers. The tropics have a lot of rainforests of mixed age and composition, which supports this idea. Problem: Structural diversity does not correlate with species diversity. For example, there is no correlation between forest complexity and bird diversity in the tropics. It is not clear that the tropics are different than the temperate zone in regards to disturbance, seasonality. Also there is evidence that the patchy species distributions associated with forest gaps are transient and the resultant forest composition and diversity is similar to the rest of the forest. High productivity is probably essential to support the diversity of life forms, but not really a driving force This is a good hypothesis—but again, it does not stand alone. It does shed light on how tropical ecosystems work. IV. Stability-time Hypotheses. The tropics have been stable over a range of time scales. Seasonal variation is low; so is variation of solar insolation. Much of the tropics are geologically old, they are the cratons of old crustal material, heavily eroded because of the high amounts of precipitation. Glacial erosion is nonexistent in the tropical lowlands and for a long time people thought that there was little or no T change during glacial periods. This stability has provided the opportunity for high speciation and low extinction rates, leading to high diversity. This theory runs contrary to the Intermediate Disturbance Hypothesis. Problems: Would allow for development of diversity, but not drive it. Big problem—the tropics have not been climatically stable. The Pleistocene glaciations had an effect on tropical climates. We know from isotope and pollen studies that during glacial periods parts of central America and the Amazon basin were 4-6ºC cooler than today, CO2 was much lower, and there is strong evidence that much of Africa was drier. V. Refugia Hypothesis As the evidence for climatic instability in the tropics began to surface, a new theory developed based on unstable conditions, the Refugia Hypothesis: So this theory is a response to the evidence against the time/stability hypothesis.  refugiaThis theory states that during glacial periods, precipitation dropped so much in S America, that the Amazonian rainforest was broken up into smaller refugia, surrounded by vast areas of grassland. The shrinking and fragmenting of the forests resulted in geographic isolation, islands essentially, surrounded by seas of grassland. The populations on these rainforest islands would then speciate, creating new species. Once the glacial dryness ceased, the rainforest pockets would expand, the result being more overall diversity. This theory was put forth in the 1960’s based on the presence of centers of extremely high diversity, which tend to correlate today with areas of higher rainfall. The idea is that in the past these areas may have had higher rainfall, which could have supported the rainforests. This hypothesis has been the center of much debate, with much focus on the degree of environmental change in the tropics during the glacial period. One problem arises from the lack of evidence—lakes that hold good records are few and far between in the Amazon. Some records do exist however, and the evidence disputes the refugia hypothesis.  pollenPaul Colinvaux had produced a couple of records using proxy data (primarily pollen data) to show that Amazonian rainforests stayed more or less intact during glacial periods. These pollen diagrams are by no means conclusive however, and this remains one of the largest debates in tropical paleoenvironments. Conclusion: The Interspecific Competition, Predatory and Time Stability Hypothesis are all based on the idea of equilibrium in the tropics over long time periods. More recent understanding of both long term (glacial/interglacial) and short term (treefall gaps) instability have led to new ideas about the development of tropical biodiversity. Understanding the forces behind tropical diversity is important when one considers the large scale environmental changes that are being created by human activities such as logging and burning fossil fuels. |
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