Succession and Species Diversity in a Plant Community
Ecological Succession can be defined as: the orderly process of community change; it is the sequence of communities which replace one another in a given area over a given time span. Typically, the development of a community in a barren area begins with the establishment of colonizing species which have short life cycles and a high reproductive output (i.e., weeds, grasses, vines, etc.). These pioneer communities are then replaced by or give way to a series of more mature communities characterized by larger, longer lived species (i.e., large shrubs, fast growing trees, etc.). The climax community is the final stage in the process of long-lived, slow growing species Examples of plant and animal succession are given in Ricklefs, in Chap. 46. Read this primarily descriptive chapter before attending this lab.
Many ecologists feel that succession is an orderly, unidirectional process which invariably proceeds from a mixture of colonizers in the pioneer stage to the climax community. For example, one worker lists the following trends or progressive developments which underlie most successional processes:
1) There is usually progressive development of the soil, with increasing depth, increasing organic content, and increasing differentiation of layers or horizons toward the mature soil of the final community.
2) The height, biomass, and stratification of the plant community increase.
3) Productivity, the rate of formation of organic matter per unit area in the community, increases with increasing development of the soil and of community structure and increasing utilization by the community of environmental resources.
4) As height and density of above ground plant cover increase, the microclimate within the community is increasingly determined by characteristics of the community itself.
5) Species-diversity increases from the simple communities of early succession to the richer communities of late succession.
6) Populations rise and fall replacing one another along the time gradient in a manner analogous to stable communities along environmental gradients. The rate of this replacement in many cases slows through the course of succession as smaller and shorter-lived species are replaced by larger and longer-lived ones.
7) Relative stability of the communities consequently increases. Early stages are typically unstable, with populations rapidly replacing one another; the final community is usually stable, dominated by longerlived plants, which maintain their populations with community composition no longer changing directionally.
It's not as simple as this, however! Succession is a concept in ecology which has generated much discussion and controversy. This controversy seems to be based around two questions:
1) Does succession as a process really exist?
2) What is the "driving force" behind successional changes in communities?
Most of the debate sterns from the fact that there is actually very little data, either empirical or experimental probably because, at least for plants, the process of succession from pioneer to climax may take 25 to 150 years to complete. The second question is even more controversial. With the insight you will gain from this lab, you should be able to formulate and substantiate several of your own hypotheses.
Species Diversity
Species diversity is a concept which can be used to compare different successional stages as to the number and abundance of species within each stage. The "usual" pattern is for pioneer communities to be less diverse than climax stages. Why do you think this would be so? Can you think of any biological causes behind this trend?
There are several ways to measure species diversity. One can simply count the number of species in a community and say that a community with 20 species is more diverse than one with 5 species. But this method ignores the relative abundance of each species. Thus a community with 5 species, each with 20 individuals, can be thought of as more diverse than a 5 species community where 1 species consists of 96 individuals and the 4 remaining species are only represented by 1 individual each. What we need is a measure which takes into account both number of species and relative abundances. In addition, our diversity measure should be a large number when the species diversity is high and a low number when diversity is low. Furthermore, we want the measure to assume a large value when there are lots of species in the community and a smaller value when there are few species. The Shannon-Wiener function, borrowed from information theory, behaves in precisely this manner. The form of the function is:
n
H' = - å Pi ln pi
i = 1
where H' is the species diversity measure
pi is the proportion (from 0 to 1.0) of the total number of species that belong to the ith species.
And n is the total number of species
A large H' value indicates an area that is more diverse than an area giving a smaller H' value.
Another means of quantifying successional stages is to look at the %-cover of various plant species. That is, what percentage of the total number of plants does species 1, 2, 3, 4, etc. contribute. Obviously this is directly related to how abundant a plant species is. One difficulty with counting plants is recognizing individuals. Within a clump of grass, how many individuals are there? How do you tell, tear up the clump and look at roots? The techniques of %-cover and point sampling were designed to allow people to sample plant populations quickly and without destroying the plants themselves. The technique will be explained in the Methods section.
This week's lab will focus again on using various sampling techniques in the field to obtain sufficient data for analysis. We will be looking at two stages of a successional plant community: a burned area and an adjacent, unburned area on Mauumae Ridge. The fire took place in 1971. Our purpose is to analyze and compare the plant populations of these two areas, and to set up the first part of a yearly census of the burned area in order to quantify the successional changes from pioneer to climax vegetation.
Methods
%-Cover: We will use a technique called Point sampling to determine the %-cover in both successional areas. We will run several transects through the area and locate sampling points at regular intervals 1 meter apart along the transect line. We will use regular intervals instead of random intervals because there is the possibility of an environmental gradient (slope) influencing the plant distribution. At each sampling point, lay a meter stick across the transect line so that about 50 cm sticks out on both sides. Every 10 cm along the stick, hold your finger about waist high and run it straight down until t either touches a plant or the bare ground. The first plant touched is the species recorded for that subsampling point. There are thus 10 such subsampling points at 10 cm by each sampling point along the transect. Keep a record of the location of the sampling point along the transect and the species encountered.
Is there a gradient effect as a function of slope? What are the %-cover estimates for each species? How do the two successional areas compare? What is happening at the edge of the burned areas? Look for burned trees and logs to determine the actual edge of the original burn.
When using the %-cover technique, grasses are treated differently from all other plants. A grass is recorded only if your finger falls within the basal area of the grass tussock:
Basal Area
Why was this measure devised? What biases does it supposedly get rid of?
For the species diversity estimates we will randomly pick the locations for several 2 x 2 meter quadrats and record all the plant species and the number of individuals within each quadrat. The sum of all these quadrat counts for each area will give us the data for determining P~ values. How do the H~ values for the two successional areas compare? Do they follow the trend listed on page 3? Why or why not? Try for an in depth explanation here. What does a single ~ value mean? What does H' mean biologically? What is the usefulness of diversity measures?
In addition to these measures of succession, we will also look at the Koa trees and how they influence the community development. We will get size estimates of several marked Koa trees in an area. We can get growth rate data by making repeated measurements of height and girth over 1 year intervals of time. In addition we will look at possible competition between Koa and ferns and grasses.
References
Drury, W. and Nisbet, I. 1973. Succession Journal of the Arnold Arboretum. Vol. 54 (331-368).
Margalef, R. 1958. Information theory in ecology. Gen. Syst. #3.
Pielou, F. C. 1966. The measurements of diversity in different types of biological collections. J. Theor. Biol. 13:145-162.
Scowcroft, P. C. and H. B. Wood. 1976. Reproduction of Acacia koa after fire. Pacific Science 30:177-186.
Vogl, R. J. 1969. The role of fire in the evolution of Hawaiian flora and vegetation. Proceedings. Annual Tall Timbers Fire Ecology Conference, April 10-11, 1969.