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<< Chapter 6 | Chapter 7 | Chapter 8 >>
Chapter 7. Species Richness and Diversity
Species Diversity Introduction
Species Richness (s) is a relative term that refers to the number of species in a community, and is directly associated with measuring the diversity of species in a given area. A related term, evenness (E), is another dimension of diversity that defines the number of individuals from each species in the same area. Together, these terms have been used to describe species diversity patterns on Earth.
There are several hypotheses that have been proposed to explain species diversity patterns. Many of these hypotheses are based upon the idea that species are more diverse near the equator than near the poles. In other words, there is a recognized latitudinal gradient of species diversity on Earth. The hypotheses that support this latitudinal gradient can be divided into two groups: abiotic and biotic. The biotic hypotheses are those that explain species diversity patterns with relation to living organisms. The abiotic hypotheses, on the other hand, explain species diversity patterns with relation to non-living chemical and physical environmental factors.
Abiotic Species Diversity Hypotheses
Four commonly recognized abiotic hypotheses include:(1) The Time/Stability Hypothesis, (2) The Area Hypothesis, (3) The Productivity Hypothesis, and (4) The Metabolic Hypothesis.
The Time/Stability Hypothesis suggests that diversity is directly related to the length of time that an area of land has been around. Specifically, land that has been around for a long time, undisturbed, tends to have more diversity, and is therefore more stable when compared to land that has been around for a short period of time. For example, the land underneath a glacier that has just receded will be less diverse than land that never had a glacier.
The Area Hypothesis explains species diversity relative to land area. According to this hypothesis, the larger an area is, the more organisms it can support, which results in larger populations. Larger populations are characterized by fewer extinctions and more prevalent mutation events and are, consequently, more diverse.
The Productivity Hypothesis says that the more energy there is in a system, the more biomass there will be in that system. More biomass supports greater species diversity.
The Metabolic Hypothesis states that temperature has an impact on the metabolic rates of organisms. This expands on the idea that every population has a net energy flow that acts upon it. The main idea behind this hypothesis is that larger animals have relatively slower metabolic rates when compared to smaller animals . For example, a mouse must eat half of its body weight to keep from starving, while a human only needs to eat two percent of its body mass to live.
This phenomenon is explained by several ideas. One idea is that larger animals have proportionately less surface area and lose heat more slowly. So, pound for pound, they need less food. The other idea is that larger animals and plants must transport nutrients further and, as a result, use them up more slowly than their smaller counterparts.
The Metabolic Hypothesis, however, suggests that Temperature may also have an impact on an animal’s metabolic rate. Research shows that a 5˚C rise in body temperature is roughly equal to a 150% increase in metabolic rate. This is true for a large range of organisms, including aerobic microbes, plants, multicellular invertebrates, fish, amphibians, reptiles, birds, and mammals. Gillooly (2001) claims that the metabolic rates of these organisms are all derived from a general function using body size and temperature.
According to The Metabolic Hypothesis, organisms with faster metabolic rates have faster mutation rates. This is because they will change quicker and get through more generations more rapidly. Therefore, you would expect to see new species created in small organisms and warm environments.It has also been suggested that body size and temperature predict the density and growth rates of a population. This means that individuals in warmer environments consume resources more quickly, which results in fewer available resources and lower population densities.
The Metabolic Hypothesis has broad implications. According to Brown (2004), the hypothesis is said to be able to predict anything from life history attributes, development rates, mortality rates, and life spans, to rates of competition and predation, populaion interactions, and patterns of species diversity. In addition, this hypothesis offers an explanation for the peak of biodiversity near the equator.[]
Biotic Species Diversity Hypotheses
There are three commonly recognized biotic species diversity hypotheses. They include: (1) The Heterogeneity Hypothesis, (2) The Competition Hypothesis, and (3) The Predation Hypothesis.
The Heterogeneity Hypothesis suggests that the more spatially diverse the community is, the greater the species richness.
The Competition Hypothesis introduces the concept of r and k selection. r-selected organisms are described as rapid growing organisms that have broad resource requirements and k-selected organisms are described as slow growing organisms that have narrow resource requirements.
The Predation Hypothesis says that predation opens up niches and reduces prey. Fewer prey lead to liberated resources which, if used by other species, leads to an increase in species richness. The concept of a keystone species is important to this theory. The keystone species is the species that a community is dependent upon and is often a predator. The keystone species maintains stability of the community which will increase species richness.
In evolutionary ecology there are many questions regarding variation in species diversity among different areas. In addition to the preeceding abiotic a biotic hypotheses, Emerson (2005) predicts that species diversity can drive speciation and the prediction is supported strongly by the biogeographic theory. By controlling several physical features of the Canary and Hawaiian Islands, Emerson showed that the probability of extinction and speciation for each species on an island increased as the number of species increased. Based on this analysis, species number was positively related to diversification and species richness.
Hutchinson (1959) reports that there are so many animals on the planet because of three reasons: (1)The greater the length of food chain (greater species diversity), the more stable the environment is, (2) Smaller organisms undergo more evolutionary change due to large populations in which more mutations occur, (3) The greater number of niches available, the more species that can occupy them without competition. Communities of greater organism diversity and species richness will persist longer and outlast many communities that are less diverse, and have a shorter food chain.
Rapoport's Rule, named after Eduardo H. Rapoport, was proposed by George C. Stevens in 1989 as an additional explanation for the already widely accepted notion that species richness has an inverse relationship with latitude. Rapoport's rule introduced the idea that the length of the geographical range of a species, relative to the north and south axis of the earth, plays an important role in the already acknowledged latitudinal gradient of species richness. Specifically, species at higher latitudes have a wider geographical range than species at lower latitudes.
The geographical range of a species refers to the range in latitudes, from north to south, in which a species can be found. This range is largely dependent upon climatic stresses in a given region. Accordingly, species in high latitudes have evolved to tolerate extreme and highly variable climatic conditions (i.e. temperature and precipitation) and therefore have a wider geographical range. Species in low-latitudes do not gain a competitive advantage by having such broad climatic tolerances and therefore occupy narrower geographical ranges. This distinction forms the premise behind Rapoport's rule.
Assuming equal dispersal abilities of high and low latitude species, more low latitude species are randomly placed outside of their narrow geographic range than high latitude species. As a result, more low latitude species are placed in environments that they are not well suited for. These species, termed "accidentals", "artificially inflate species numbers and inhibit competitive exclusion." In other words, "accidentals" would normally be competitively excluded by the better adapted individuals in the same area, but due to the unique nature of narrow tropical geographic ranges, the success of the "accidentals" is only dependent upon their proximity to the areas in which they do well. Therefore, in lower latitudes, more species exist in habitats that they would normally not survive in. In accordance with Rapoport's rule,this phenomenon, called the "rescue effect", accounts for the higher species diversity in the lower latitudes as compared to the higher latitudes.
Rapoport's rule, although widely acknowledged, faces criticism from related studies. There is considerable doubt that Rapoport's rule is actually a causatory agent of the latitudinal species richness gradient but is, rather, an artifact of the phenomenon. The methods and logic used by Stevens are especially scrutinized, every possible error drawn out. For example, it has been suggested that Stevens over-emphasized species richness patterns in higher-latitudes in comparison to lower latitudes, resulting in conclusions that are only applicable to local, rather than regional, patterns. Such gaps in Stevens' methodology and evidence leave Rapoport's rule open to inquiry. Also, many exceptions to the rule have been demonstrated. However, whether or not Stevens' conclusions are completely accurate, Rapoport's rule plays a significant role in the study of species richness patterns and raises questions that promote further studies.
Diversity is measured for three main reasons: (1) to measure stability to determine if an environment is degrading, (2) to compare two or more environments, and (3) to eliminate the need for extensive lists. Diversity indices provide important information about the composition of a community. These indices not only measure species richness, but also take into account the relative abundance of species, or evenness. When measuring species diversity, species richness and evenness must always both be considered. In addition, indices provide important information about species rarity and commonness in a population. These are important and common tools used by biologists in order to understand community structure.
There are many indicies that can be used to calculate diversity. Some of the more valuable indicies that calculate diversity include the following: (1) Berger Index, (2) Simpson's Index, []which calculates the probability that two organisms sampled from a community of will belong to different species, (3) Shannon-Wiener Index, []which accounts for both abundance and evenness of the species present, and (4) Species Evenness Index. Each of the indicies uses different mathematical approaches/calculations to test/calculate diversity between two or more communities. These tools for measuring diversity are often used when determining community similarity, ethnic diversity, and biological diversity.
To learn more about the diversity indices, read Diversity index.
In addition to the previous indices, ecologists employ rank abundance curves, which are graphs ranking the most abundant species to the least abundant. They can be shown as a plot of number of species vs. number of individuals on a logarithmic scale that usually yields a normal distribution. This is because environments are usually undersampled, resulting in many singlets,especially in high diversity systems. Singlets make up the middle peak of the distribution, and the more sampling that is performed, the more the curve will shift to the right.
The number of unsampled species in cases of undersampling can be roughly estimated using the Chao estimator, in which Sestimate = Sobserved + F12 / 2F2 where F1 is the number of singletons sampled, and F2 is the number of doublets. There is also a method of estimating what percent of the total species is represented in a sample, called Good's coverage estimator, in which Coverage = 1 - (number of individuals in species / total number of individuals). These estimator equations allow researchers to have a good idea of how their limited sampling relates to the entire sampled population.
Measurements of species diversity are constantly being improved and/or critiqued by ecologists. Hurlbert (1971) argued that species diversity itself is a "nonconcept" and that theoretic results are not dependable. His view is that ecologists should abandon the "poorly defined" idea of diversity completely, as well as diversity indices, and rely more on direct observation. He came up with alternate equations called "species composition parameters" that are more direct than diversity indices. Although he thought these equations are more reliable than diversity indices, he still stated that they shouldn't be strongly depended upon in research. Soetaert and Heip (1990) also argued that species indices are not always accurate, but worked on finding a way to improve results. They approached the issue of sample-size dependence of diversity indices. They found that this dependence is highest for more diverse communities and those with more rare species. This is something that can be taken into account during sampling and utilization of the indices. Such endeavors to improve measurements of diversity have and will continue to improve ecological research methods as a whole.
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