Alpha Diversity, Beeta Diversity, Gamma Diversity
Measuring Biodiversity
Alpha diversity, Beeta Diversity, Gamma Diversity
Ecologists have developed ways to characterize species diversity in a given area:
Within-habitat diversity or alpha-diversity: refers to a group of organisms interacting and competing for the same resources or sharing the same environment. Measured as number of species within a given area.
Between-habitat diversity or beta-diversity: refers to the response of organisms to spatial heterogeneity. High beta-diversity implies low similarity between species composition of different habitats. It is usually expressed in terms of similarity index between communities (or species turnover rate) between different habitats in same geographical area (often expressed as some kind of gradient).
Geographical diversity or gamma-diversity
Neutral processes that regulate species diversity
Neutral processes are those that occur independently of any differences among species, as though the species were genetically identical. They will affect diversity regardless of the ecological characteristics of a region. For example, there is a continual rain of seeds and spores onto the soil, and which species happen to land in a site suitable for growth is largely a matter of chance.
Immigration
Immigration provides a continual source of new diversity for a region. How important it is depends on the balance between the number of propagules that come from outside and the number produced by resident individuals. If the area is large (a few square kilometers), most young individuals will be recruited from the resident population, but in small areas (a few square meters), reproduction by residents may be overwhelmed by immigration. Thus the importance of immigration increases as the size of the area decreases.
Some organisms are dispersed much more broadly than others. The very small spores of ferns, for example, may be carried by wind for hundreds of kilometers from their parental site. The seeds of plants such as dandelions and poplars are much larger but have special devices to facilitate wind transport. The effectiveness of immigration in providing new recruits to an area is seen dramatically after a natural disaster destroys all life in an area. After the devastation of a volcanic eruption, for example, plants and animals quickly return: first the groups with effective long-distance dispersal, and later, those who disperse more slowly but are better competitors once they arrive.
Extinction
Extinction of a species or a population will occur for one of two reasons: as a result of accidents (environmental fluctuations) or because of population interactions.
a) accidents: events that trigger extinctions for no predictable reason - volcanos, rising sea level, an ice storm, any environmental circumstance that wipes out an ecological niche.
b) population interactions that are not neutral processes: predation and competition can result in negative growth rate and ultimately, extinction. However, on their own, predation and competition rarely cause extinctions directly; they cause population densities to become very low and then a random accident may drive the vulnerable population to extinction.
The probability that enviornmental or population fluctuations will cause an extinction depend on how abundant the organism is and how large its range is.
a) abundance: if the chance that any given individual will die in a given period of time is p, then the chance that all individuals in a population of size N will die within that same period of time is pN. If the population is large, the probability that this will happen is very small: for example, if p = 0.5 in a given year, then the probability that all individuals in a population of N = 1000 will die at the same time is so small that it is unlikely to occur in a billion years. If, on the other hand, N = 10, the population is likely to become extinct within a thousand years, a relatively short period, and certainly much shorter than the time necessary to produce a new species. Small populations are thus at high risk of chance extinction.
b) range: disturbances that kill all the individuals in a given area happen all the time. Smaller and more localized disturbances are more frequent than large and widespread disturbances - treefalls are more frequent than forest fires, landslips more common than earthquakes. A species that is restricted to a few small sites is therefore at higher risk of being extinguished by an environmental fluctuation than one that occurs at many sites over a large area.
Biological Surveys
Biodiversity surveys are undertaken to find out what organisms exist in a given area. The data that is gathered from these surveys is used for numerous purposes such as:
- monitoring endangered populations;
- evaluating conservation priorities of an area;
- bio-prospecting
Museum and herbarium specimens provide a valuable record of the location of organisms but such data are rarely systematic and often subjective. This is why field surveys are so important.
Despite their importance in biodiversity research, there are no well-defined rules as to how to perform such surveys. This is due to the vast differences between surveys in terms of the goals of the survey, of available resources and time, the area to be surveyed, the organisms to be found, and a myriad of other factors that change from survey to survey.
Still, there are some basic questions that must be addressed before any survey is begun and these questions are explained below.
What are the objectives of the survey?
The survey's objective must be determined:
Do we want
- to inventory the species in a given area?
- monitor the populations already known to exist there?
- model the processes driving diversity in the system?
These are all valid scientific objectives for carrying out a biodiversity survey. Since it is likely that only a sample of the actual diversity of an area can be surveyed, the goals of the survey are extremely important to ensure that the results are useable.
The objective will guide the answers to the next most important questions: what kind of diversity will we look at and how will we measure it?
What kind of diversity are we measuring?
Genetic diversity
Some argue that the fundamental unit of biodiversity is the gene. Genetic diversity is the degree of variability of the genetic material of an organism. Species are defined by the differences in their genes. High genetic diversity indicates populations that can more easily adapt to changing situations and environments, and also a greater assortment of materials that can be found, increasing the chances of finding a useful compound.
However, exact assessment of genetic diversity is both time-consuming and prohibitively expensive, requiring modern laboratories and expensive chemicals. We have so far been able to account for all the genes in just one species of bacteria! Realistically, investigators could only examine a minute fraction of the genetic diversity to be found using this approach, and time is often a constraint.
Species diversity
Fortunately, genetic diversity can be estimated by species diversity, and this has become the standard unit of measurement in most biodiversity surveys.
Species have the advantage of being natural biological divisions and easily identifiable; their diverging appearances were the basis by which they were classified in the 18th century, and modern phylogenetic techniques more often than not produce species divisions similar to those of classical taxonomic divisions. For many groups of organisms, such as birds and flowers, public interest means that identification of many species is already known by large numbers of people.
The degree of genetic variability at the species level, and indeed at any taxonomic level, can be maximized by taking species that differ by one another by as many characters as possible. If these characters represent different genetic elements, then the divergent species should represent greater genetic diversity.
Higher taxon diversity
In the case of some groups of organisms, such as insects, the numbers of species is so large that it is not practical to identify them all. Fully half of the 1.5 million identified organisms are insects. Furthermore, species of many microorganisms have not yet been identified or named. Only identification to higher taxonomic groups, such as to the genus or family level, may be necessary or even possible. This method works well up to families, if the species observed are more or less similar. If the species are quite different this method is less useful, since diversity would be underestimated.
If the intent of the survey is to generate a list of which particular species are to be found in an area, then this method is unsuitable; conversely, if simply estimating the number of species to be found is the aim, this method is acceptable.
Indicator groups
Sometimes, indicator species may be used to select areas as priorities for conservation and protection because it is assumed that if a given indicator organism is protected, then a number of other organisms may be protected as well.
Using the presence of indicator species as representative of many species can make a survey less costly and time-consuming. However, the complexity of most ecosystems makes it unlikely that any one (group of) organism(s) can serve as an indicator of community structure and function. Indicator groups must be used cautiously at any time, especially when the relationship between the indicator group and the target group is negative; it may be that the two simply live in different locations that are not usually found together at the scale of previous surveys. If the goal of the survey is to establish some cause and effect, then the indicators may not provide the same relationship between the cause and effect as the target groups. Furthermore, setting conservation goals based on indicator organisms may result in the inadvertent loss of species that were not adequately protected.
Surrogate environmental measurements
To estimate the number of species in an area, surrogate measurements, such as net primary production for plants, may be used, which, although crude, are readily available. The major problem with this method is that specific species are not identifiable by this method, so that estimates of total diversity may be generated, but not the particulars. This method works best for larger areas, where the effects of local differences and chance are minimized.
Methodology: How will the survey be carried out?
The actual methodology of the survey depends the goal of the project, the unit of biodiversity being measured, and how the data will be analyzed. Data collected can be either qualitative (presence/absence, also known as binary) or quantitative data, in which the number of individuals for each species are counted. Small mobile animals such as insects are usually captured using traps or nets, while plants are usually visually identified in the field.
Standardization
Assuming that the data are to be compared or analyzed, sampling for a survey must be kept consistent, not just between different surveyors but also from site to site and day to day. Standardization ensures that differences between sites are significant and not the result of uneven sampling.
Sampling Effort
Determining the sampling effort is important and difficult, because it requires a balance between time and effort and interpretability of data, assuming that not all organisms in the area can be correctly identified due to time and labor constraints.
Sampling effort can be expressed many ways: as search time per site, as search within a given distance of a reference point or line, or as total number of sites or replicates needed to find a pattern.
For example, the Audubon Christmas Bird Counts will select one 24-hour period where investigators will try to find as many breeding species in an area as possible within 24 hours. Setting a definite time limit also allows the survey to be more standardized and results can be compared from year to year. For a survey to be considered scientific, it must be random; that is, the sites should be selected independent of factors such as the number of organisms found at a site or proximity of a site to the laboratory; although these seem to be valid reasons for spending more effort at a site, this only serves to make the sites unevenly sampled with the end result that differences in diversity between sites cannot be convincingly attributed to something other than the difference in sampling effort.
Scale
The scale at which the survey will be done depends upon the goals of the project and on the unit of biodiversity being used. The scale should be appropriate to the organisms being surveyed; a one-meter scale would be ineffective for full-grown trees, whose bases are often greater than a meter in area; it may be a good choice for ferns. A large scale may also be needed for motile organisms, such as caribou, or large oceanic fish, which have a much larger habitat.
Sampling grids
Large areas may be divided into bio-geographic regions or landscape types, but conducting surveys along these categories is problematic because of the differing sizes of regions or landscapes, especially when the variation within them is examined, or when sharp boundaries must be delineated. Therefore, biodiversity surveys are usually performed using a grid of some sort, as the diversity of an area is usually what is of concern. Point data are not as useful because they are not consistent and the diversity at a point depends upon chance encountering of aggregations. Linear measurements are rarely useful because most patterns of biodiversity are two- or three-dimensional.
A grid should cover the entire area of interest and aggregates of the shape used should form the same shape to allow different scales to be easily compared. Squares and hexagons are the most often used shapes, their dimensions making it simple to sample in one quadrat without inadvertently wandering into another. Grids superimposed over large areas may have problems due to the curvature of the planet (making some quadrats bigger or smaller than others), and so equal-area or almost equal-area grids are used, where the grid is modified such that every quadrat has the same area, although the shape may differ somewhat.
Monitoring
Monitoring is a special type of survey used extensively in conservation work. Monitoring involves repeated surveys of an area over time, which allows examination of effects of change through time as well as of change through space. In fact, the implementation of long-term monitoring of forest ecosystems, St. Lawrence River ecosystems and species at risk is one of the major aims of the Québec Biodiversity Strategy. Monitoring programs are being set up all over the world and span from regional surveys (e.g. Mont St. Hilaire) to global ones.
Quantitative data are better for monitoring, as they allow changes in the population to be measured, instead of the population simply being recorded as present or absent; it would be helpful to know if a population were in danger before it disappears. Knowledge of the population structure is also very important in conservation, so sex ratios and ratios in different life stages are also needed, which is possible if quantitative measurements are made. Monitoring allows research into not just changes of population size and structure, but also ranges of variation in population size and structure.
Frequency of surveys
The frequency of monitoring depends largely upon the goals of the project and the life history of the species; population changes that may be the result of regular cyclical fluctuations may appear drastic if the cycle is not known. In Canada, population cycles of many larger animals follow a 10-year period while those of small herbivores and their predators follow a 4-year period. Consideration of such natural cycles is important to the monitoring of populations.
To prevent an area from being overly disturbed, surveys should not repeatedly sample the same areas, but some area should be sampled in the subsequent survey as well, so that observed changes can be separated into the degree to which they result from sampling new areas and the more universal changes that have occurred since the last survey.
Species-Area Curves: In general, the bigger the area sampled, the more species found. This relationship between species and area can be plotted to generate a species/area curve. Such a plot can give us useful information such as the total number of species in a region (which is the number at which the curve levels off with increasing area indicating that no more species are found), and the rate of species increase with area between different regions (calculated from the slope of the curve).
Such species area curves have been described for a wide variety of organisms including vascular plants, birds, mammals, fishes, and terrestrial and aquatic invertebrates.
Productivity
Naturally, you do not always find the same number of species in areas of the same size because some areas support far more individuals than others. There are more species in a hectare of tropical forest, for example, than there are in a hectare of tundra. The capacity of an area to support growth is called its 
productivity.
Productivity, defined as gross primary productivity, is the solar energy that is captured and converted to carbon compounds in an ecosystem.
In general, more productive areas support more species. But the pattern is usually more complicated than this. In many systems, the relationship between primary productivity and diversity has been shown to be unimodal, or hump-shaped: diversity is highest at intermediate levels of productivity. Such a pattern has occurred at regional scales in many biomes and for many groups of animals and plants, including desert rodents, tropical mammals, marine benthic communities and megafauna, freshwater plankton, montane ferns and bryophytes.