Plant+Competition+Student+Manual

PLANT COMPETITION
- Understand the difference between intra- and interspecific competition - Witness competition in the field - Learn about what a correlation is, what a correlation coefficient means, and how to graph variables that are correlated.
 * Objectives:**

INTRODUCTION
Mutually adverse interactions between individuals may be direct or indirect in nature. Direct interactions such as the release of toxins by plants or physical combat between animals are forms of **interference competition**. Indirect forms of competition include apparent competition and exploitative competition. **Apparent competition** takes place when the abundance of one of the species increases the likelihood that members of the other species will be consumed (Holt 1977). Apparent competition arises when two species share the same consumer. The other form of competition via indirect interactions is exploitative competition; this is the most common form of competition. **Exploitative competition** occurs when individuals are using the same limiting resource. Resources are matter, energy, or space that limit population growth and are consumed by individuals, making them unavailable to others (Tilman 1982). Thus, although increased temperature commonly increases population growth, it is not consumed and is therefore not a resource. The term //competition// typically refers to exploitative competition. Competition, whether direct or indirect, is generally accepted as a significant structuring force in animal and plant communities, but the degree of its importance varies. Gurevitch et al. (1992) reviewed competition studies for the general effect of competition among organisms. They detected small-medium effects among primary producers (i.e., plants) and carnivores and large effects among some herbivores. Although competition varies among organisms in the environment, we often consider isolated sets of species in more controlled conditions when trying to understand its mechanisms and effects. Competition is modeled theoretically after the modified logistic equations of Lotka and Volterra, which assume no spatial or temporal variability in the habitat. Experimentally, we test the relative success of one or more species relative to other species. Is one species driven to extinction, or is a stable equilibrium established? If the latter, then at what densities are the two species compared to their starting (i.e., planted) densities? Given a controlled environment (e.g., beaker, greenhouse, field plot), we can monitor the relative population sizes of two species through time. We could introduce habitat heterogeneity by providing refugia or alternative combinations of limiting resources to determine how competition varies under different conditions. Alternatively, we could remove competitors from their native habitat to understand the effects of competition from that species relative to the control habitats. These types of studies (i.e., partial additive, replacement series, and target-neighborhood; Barbour et al. 1999) are important experimental approaches to understand the effects of individual species, especially invasive species like purple loosestrife (//Lythrum salicaria//; e.g., Weihe and Neely 1997). Controlled experiments, though, cannot include all of the complex interacting effects of the environment in which organisms live, thus we rely on **observational studies**. The value of these studies lies in including all the direct and indirect effects of abiotic and biotic resources and interactions, thus giving us a more realistic impression of species effects. For example, Mack and Harper (1977) found in a study of four dune annual plants that the superior competitor in the greenhouse was not exclusionary in the field due to rabbit herbivory on this species. Comparative studies of habitats with and without a particular species (e.g., purple loosestrife; see Farnsworth and Ellis 2001), can provide correlative evidence of the effects of competition yet cannot address its mechanisms.
 * Competition** is a direct or indirect interaction between individuals in which negative consequences are experienced by both individuals. Competition may take place between individuals of the same species; this form of competition is referred to as **intraspecific competition**. Competition between individuals of different species is called **interspecific competition**.
 * Controlled Experiments**

LITERATURE CITED
Barbour, M.G., J.H. Burk, W.D. Pitts, F.S. Gilliam, and M.W. Schwartz. 1999. Terrestrial Plant Ecology, 3rd Edition. Benjamin/Cummings, Menlo Park, California, USA. Farnsworth, E.J. and D.R. Ellis. 2001. Is purple loosestrife (//Lythrum salicaria//) an invasive threat to freshwater wetlands? Conflicting evidence for several ecological measures. Wetlands 21(2): 199-209. Gurevitch, J., L.L. Morrow, A. Wallace, and J.S. Walsh. 1992. A meta-analysis of competition in field experiments. American Naturalist 140:539-572. Holt, R. D. 1977. Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology 12: 197-229. Mack, R.N. and J.L. Harper. 1977. Interference in dune annuals: spatial pattern and neighborhood effects. Journal of Ecology 65:345-363. Tilman, D. 1982. Resource Competition and Community Structure. Princeton University Press, Princeton, NJ. Weihe, P.E. and R.K. Neely. 1997. The effects of shading on competition between purple loosestrife and broad-leaved cattail. Aquatic Botany 59:127-138.

ACTIVITY
In this laboratory session, we will conduct an observational field study to detect interference competition between species by correlating their size and interplant distance. You can select the species as a group, but select common and similarly sized species, such as sugar maple, pine (white or red), or beech. We will use **correlations to detect both the presence and intensity of intraspecific and interspecific competition**. Our sample area, the Baker Woodlot, is relatively homogenous in resource distribution, but differences surely occur to some extent. In essence, we will measure the size of a randomly located plant of species 1 and the distance to the nearest neighbor of species 2. Each group will sample one 30-meter long transect at least 5m apart from other groups’ transects. Randomly locate the beginning point for your transect using your random number table as you did in Lab 2. Without crossing another group’s transect, run a measuring tape 30m to the end of your transect. Once you have set up your transect line, **identify the most common tree species** around your transect using the identification guides provided. The most common species in Baker Woodlot tend to be sugar maple and American beech, but this depends on where you are sampling. Once you have identified the most common species, label it species 1.

At each 6m transect interval, select the nearest tree of species 1. Record the species and **diameter at breast height** (DBH) with the diameter tape for each overstory (canopy) tree or sapling. **//DBH//** //is defined as the outside bark diameter at breast height, 1.37m above the forest floor, on the uphill side of a tree. For this measurement, the forest floor includes the duff layer that may be present but does not include woody debris that may rise above the ground line. DBH is used as a measurement of tree growth, volume, yield and forest potential.// Next locate and measure the distance to its closest intraspecific and interspecific neighbors (an interspecific neighbor is the closest tree species that is different from species 1). Measure the DBH for these two trees as well. Record your data on the data sheet provided. Make an effort to obtain equal numbers of intraspecific and interspecific pairs, and do not sample the same tree/sapling twice.



After you have collected your data, you need to determine if any competitive interference relationship exists between trees of the same and other species. One method to detect this relationship is by determining if the correlation between the interplant distance and the sums of DBHs, as the measure of fitness in this case, is significant. What could you say if the correlation is significant? Calculate the mean, standard deviation, and correlation coefficient between the sum of the pair sizes and their interplant distance for both intraspecific and interspecific data tables. Then graph your results. In addition, list mean, standard deviation and correlation coefficient (r). Roughly draw the best-fitted line. You should have 2 graphs, one for intraspecific competition, and one for interspecific competition. Pay attention to units, labels, etc. when graphing your data. Below each graph, write a few sentences about your data: is there a correlation between interplant distance and plant DBH? How strong is it? Do your data detect the presence of any exploitative competition within or between species?
 * At the end of the lab, hand in your 2 graphs, answers to the questions, and your calculations to your TA (one copy of each per group). You will be evaluated on your graphs and calculations for this lab.**

** Data Sheet for Plant Competition Laboratory **

Species A =_ Species B =_

||||||||||

** Intraspecific Pairs ** ||

Distance (m) || Species A Plant 1 DBH (cm) || // Interplant Distance (m) // || Species A Plant 2 DBH (cm) || // Sum of Species A’s Plants 1+2 DBH (cm) // ||
 * ||  || x ||   || y ||
 * Transect
 * 6 ||  ||   ||   ||   ||
 * 12 ||  ||   ||   ||   ||
 * 18 ||  ||   ||   ||   ||
 * 24 ||  ||   ||   ||   ||
 * 30 ||  ||   ||   ||   ||
 * Mean ||  ||   ||   ||   ||
 * Standard deviation (s) ||  ||   ||   ||   ||
 * Correlation coefficient (r) ||  ||   ||   ||

|||||||||| **Interspecific Pairs** ||

Distance (m) || Species A Plant 1 DBH (cm) || // Interplant Distance (m) // || Species B Plant 1 DBH (cm) || // Sum of Species (A+B) Plant 1’s DBH (cm) // ||
 * ||  || x ||   || y ||
 * Transect
 * 6 ||  ||   ||   ||   ||
 * 12 ||  ||   ||   ||   ||
 * 18 ||  ||   ||   ||   ||
 * 24 ||  ||   ||   ||   ||
 * 30 ||  ||   ||   ||   ||
 * Mean ||  ||   ||   ||   ||
 * Standard deviation (s) ||  ||   ||   ||   ||
 * Correlation coefficient (r) ||  ||   ||   ||