BIOL1003 Ecology Module ? Week 8 Analysis of Communities
LEARNING OBJECTIVES:
1. To experience field sampling techniques that are widely used in ecology;
2. To understand the concept of species diversity and its measurement; and
3. To experience the use of dichotomous key for identification of biological samples.
WHAT TO BRING:
1. Covered Footwear – suited to rough/dirty/muddy conditions (old sneakers are good)
2. Outdoor protective clothing (hat, sunscreen, long pants etc.)
3. Insect repellent
4. Mobile phone
5. Lab coat and safety goggles
6. Dissection Kit
7. Calculator, logbook and pen
SPECIFIC SAFETY HAZARDS:
This fieldtrip will involve the examination of the diversity and abundance of leaf litter invertebrate species found at two sites on the Callaghan campus (Site 1: Bushland behind Chancellery; Site 2: Red Ash Gully in Don Morris Walk). At the sites, leaf litter will be sampled using quadrats and transported back to the laboratory. Invertebrate species will then be recovered from leaf litter by sifting, counted and identified. Major risks of this fieldtrip are described below:
1. Abrasions created by spines and thorns of plants. Appropriate clothing (e.g. long sleeve shirt and long pants), footwear (i.e. fully covered shoes with strong soles) and work gloves must be worn at all times. First aid kit should be available.
2. Bites from dangerous bush animals. All persons must be aware of possible dangerous animals (e.g. poisonous snakes and spiders) and do not approach them if found or touch them with bare hands. Communication devices (e.g. mobile phone) should be available. First aid kit should be available.
3. Separation from work groups. All persons should bring along a campus map to the sites and a mobile phone for communication. Reflective vests should be worn at all times in the field trip. A sign-in-and-out system will be used to record the attendance of all participants.
4. Exposure to inclement weather (sun, cold, etc.). Always review weather forecasts prior to departure. All students must be aware of weather conditions and be equipped with the necessary protection (i.e. sunscreen, hat, protective clothing, insect repellant, wet-weather gear, etc.).
5. Minor irritation caused by contact with 70% ethanol (used for preserving biological samples). Preserving alcohol should not be drunk or inhaled, and care should be taken not to spill it on skin. If alcohol is spilled, any affected skin should be washed thoroughly with water. Protective Equipment (PPE) including gloves (double nitrile, 8 mil nitrile, latex), lab coat and goggles and closed toed shoes should be worn.
ORGANIZATION:
Students in each group will be divided into two halves; one half (2?3 students) will sample at Site 1 (Bushland behind Chancellery) and the other half (2?3 students) will sample at campus Site 2 (Red Ash Gully in Don Morris Walk).
Tasks:
• Field sampling
• Recovery of invertebrates from leaf litter, sorting, counting and identification of invertebrates
• Pooling class data
• Data analysis (done after class)
BACKGROUND:
A community is an assemblage of interacting populations that constitute a relatively self-sufficient ecological unit. Often we consider only a portion of a community, that is, an assemblage of interacting populations that lack self-sufficiency. This is called a subcommunity. For example, we may examine the ground subcommunity or a rotting log subcommunity within a deciduous forest community.
Although a community does not always have easily delineated spatial boundaries, it is a basic ecological concept. Communities show organization and homeostasis, and they have unique structural and functional attributes not possessed by their individual component populations.
Graphical representations of certain aspects of community structure are often helpful in community analysis. Rank-abundance diagram may be prepared using density, coverage, biomass, frequency, productivity, or importance value. One ranks the species in a sequence from 1 to s, where s is the total number of species being considered. The most abundant species (or the one with the greatest coverage, biomass, etc., depending on the measured variable) is assigned rank 1, the second most abundant is given rank 2, and so on, with the least abundant receiving rank s. Then, the abundance (or biomass, coverage, etc.) is plotted against the corresponding rank, as in the figure below. Logarithms permit convenient placement of a large range of values on the graph and result in a curve such as C in the figure being a straight line.
Fig. 1. A relative-abundance curve, also called a dominance-diversity curve, or a species-importance curve. Curve A exemplifies the highest diversity and lowest dominance; curve D is the lowest of the four in diversity and highest in dominance.
A community with a high degree of diversity will tend to have more species and a more even abundance in each species than will a community of low diversity. Communities with low species diversity and/or a high degree of dominance tend to have very steep curves on such a graph. Those with high species diversity and/or low dominance assume a more horizontal aspect.
Species diversity
Species diversity, a characteristic unique to the community level of biological organization, is an expression of community structure. The most useful measures of species diversity incorporate consideration of both the number of species (richness) and the distribution of individuals among the species (evenness).
A community is said to have a high species diversity if many equally or nearly equally abundant species are present. On the other hand, if a community is composed of a very few species, or if only a few species are abundant, then species diversity is low. For example, a mixed stand of ten tree species, all fairly common, is considered more diverse than a community with an equal number of tree species present, but with 95% of the individuals belonging to one species.
High species diversity indicates a highly complex community, for a greater variety of species allows for a larger array of species interaction. Thus, population interactions involving energy transfer (food web), predation, competition, and niche apportionment are theoretically more complex and varied in a community of high species diversity. This is still the subject of considerable discussion; some ecologists have supported the concept of species diversity as a measure of community stability (the ability of community structure to be unaffected by disturbance of its components), while others have concluded that there is no simple relationship between diversity and stability.
Once species diversity was recognized as a parameter that could be used to compare different ecological communities, several indices or mathematical measures were proposed for use. There has been a great deal written about which index should be used under what circumstances. Several authors have questioned the usefulness of species diversity indices on a number of grounds. Even when comparing many indices (including simple measure of species number) are influenced by sample size and the kind of samples made. Other objectives are made on the statistical grounds since procedures for measuring significant differences have not yet been worked out.
Shannon Diversity Index
This index assumes that individuals are randomly sampled from an ‘indefinitely large’ population. The index also assumes that all species are represented in the sample. It is calculated from the equation:
s
H = ? - (Pi * ln Pi) i=1
where:
H = the Shannon diversity index
Pi = fraction of the entire population made up of species i S = numbers of species encountered
? = sum from species 1 to species S
Note: The power to which the base e (e = 2.718281828.......) must be raised to obtain a number is called the natural logarithm (ln) of the number.
To calculate the index:
1. Divide the number of individuals of species #1 you found in your sample by the total number of individuals of all species. This is Pi
2. Multiply the fraction by its natural log (P1 * ln P1)
3. Repeat this for all of the different species that you have. The last species is species “s”
4. Sum all the - (Pi * ln Pi) products to get the value of H
For example:
Organism ni pi ln pi pi*ln pi
Subterranean Termite 5 5/24 -1.568 -.3267
Thrip 3 3/24 -2.079 -.259
Ground Beetle 1 1/24 -3.18 -.132
Phalangidae 1 1/24 -3.18 -.132
Swallowtail 1 1/24 -3.18 -.132
Springtail 12 12/24 -.693 -.3465
Caddisfly 1 1/24 -3.18 -.132
N = 24 H = 1.4602
High values of H would be representative of more diverse communities. A community with only one species would have an H value of 0 because Pi would equal 1 and be multiplied by ln Pi which would equal zero. If the species are evenly distributed then the H value would be high. So the H value allows us to know not only the number of species but how the abundance of the species is distributed among all the species in the community.
MATERIALS:
Field sampling
20-50 m measuring tape flags permanent marker 1-m2 quadrat sealable plastic bags work gloves (1 pair per participant) reflective vests
Animal sorting sifting pan white tray
insect aspirator soft forceps disposable gloves
Identification of specimens petri dishes fine forceps stereoscopic microscope with light source
70% ethanol cotton ball
plastic disposable pipettes
1 large jar with lid for discarding used specimens, debris, used alcohol
EXPERIMENTAL PROCEDURES:
1. Two on-campus biological communities, Site 1 (Bushland behind Chancellery) and Site 2 (Red Ash Gully in Don Morris Walk) will be used for sampling invertebrates living in leaf litter. We will meet at the laboratory for a briefing before the trip.
2. At each site, lay out two transects (16 m in length) perpendicular to the trail at random locations. Five lab groups will work on one transect and the other lab groups five lab groups will work on the other transect.
3. Take samples with a 1-m2 quadrat at 4-m intervals along the transect (i.e., each group will sample one quadrat per site). Collect all the surface leaf litter (down to a depth of approx. 2 cm) and place it inside a bag. Take the samples to the lab.
4. Place the litter in a sieve for sifting. Shake sample in the sieve 20 seconds for each 100 grams of mass above a shallow pan by two of the team members while the other three team members participate as timers and watch for organisms attempting to escape. Use aspirators to collect small organisms. Place collected organisms in petri dishes with a cotton ball and ethanol.
A -cat litter- pan with sifting pan.
5. Identify collected organisms to Order with the aid of the provided dichotomous key (Appendix) and then sorted by morphological type using a stereoscope. Count the number of individuals of each morphospecies.
6. Tabulate your result. Calculate the Shannon diversity index (H) of the two sampling sites using your group data.
QUESTIONS FOR DISCUSSION:
1. Compare the Shannon diversity indices for the two sites. Which site is more diverse according to the index? What is the implication of your finding?
2. Is there any difference in species richness and evenness between the two sites (Hint: Abundance curves can tell)?
3. Which animals were found in the largest numbers in the two locations? Suggest reasons why some species dominate in one location but not the other (Hint: Relate habitat characteristics to species patterns).
REFERENCES:
1. Brower, J.E., Zar J.H. & von Ende C.N. 1997. Field and laboratory methods for general ecology. 4th edition. WCB/McGraw-Hill.
2. Krebs, C.J. 1999. Ecological Methodology. 2nd edition. Benjamin/Cummings.
Appendix