BOT 332, Plant Ecology Lab Exercise IV: Competition1 Objectives: 1. Become familiar with the basic tenets of experimental design, analysis, and interpretation. 2. Set up an experiment to measure the relative effects of inter- and intra-specific competition on plant growth and size hierarchies for plants grown at different densities and proportions. Background: Increased density of individuals is expected to increase competition among plants for resources such as space, water, light, and nutrients. Competition may be defined by the mechanism, i.e. the capacity for resource capture and rapid growth when resources are readily available; or defined by the outcome, i.e. ability to maintain growth and reproduction when resources are limited, due to a low equilibrium resource requirement. Generally speaking, competition can be classified as either intraspecific (within a species) or interspecific (among species). Both within and among species, high competitive ability can lead to skewed size distributions, decreased average growth, reduced or absent reproductive output, or even mortality of other plants. There is no doubt that some species are better competitors than others, yet when we look outside we do not see a world dominated by a single dominant species. Why hasn’t the “best” competitor replaced all the other species? Clearly, a species’ competitive ability, as well as the population/community level effects of competition will be dependent upon numerous ecological and environmental factors. The intensity of effects of competition can be influenced by plant density, spatial arrangement, soil nutrient heterogeneity, intensity of other ecological factors (e.g. herbivory), resource availability, and numerous other factors and processes. We will use a multiple deWit replacement design to test for competition within and between species. The multiple replacement design varies both density of plants and proportion of plants of each species. Growing each species in monoculture at different densities allows us to measure the strength of intraspecific competition. Growing pairs of species in different proportions allows us to measure the strength of interspecific competition. Comparing intra- to inter- specific competition in this way allows us to estimate their relative importance. The use of a multiple deWit replacement design will also allow us to test how planting density affects size hierarchies of individual plants within each pot. Many studies have found that as plants grow and compete for resources, the distribution of plant sizes becomes increasingly skewed, with a few large individuals and many smaller individuals. This effect has been hypothesized to be caused by asymmetric competition, where the few large individuals have disproportionate competitive effects as they prevent their smaller neighbours from obtaining essential resources such as light. We will grow each species alone, and in different proportions with the other species. Each density/proportion treatment will be replicated. Throughout the semester we will measure a variety of response variables, including plant height and final aboveground 1This lab was designed by Elizabeth Elle, Simon Fraser University and modified for this course. http://www.sfu.ca/biology/faculty/elle/ biomass. These data will be used to construct DeWit plots, estimate size hierarchies at different densities, and test the relative importance of intra- and inter-specific competition. Write-up: You will write-up the results of this experiment as a scientific paper. In doing so, you will review results of other competition experiments done on other species and compare your results with those obtained by other workers. Consult your lab manual (p.18-25) for instructions on how to write a scientific paper, and follow Ecology format when preparing your lab report. The report should be 10-12 pages in length (doublespaced). See the course web page for a link to detailed information on how to prepare this report. You should address the follow issues in your write-up: 1. Compare the strength of inter- vs. intra-specific competition, size hierarchies and growth rates for our two species at: i. different planting densities in monoculture ii. different planting densities in two species 'communities' iii. different relative abundances 2. For each of these different questions, discuss whether and how the responses you observed differed depending on how size was measured (height vs. biomass) or the time of measurement (height at 3 weeks vs. 5 weeks). 3. Discuss the relevance of your findings with respect to previously published studies and theory. How are the strength of inter- vs. intra-specific competition, size hierarchies or growth rate predicted to vary with time, density or relative abundance? Did your findings support these predictions? 4. Give a critique of the experimental method and suggest ideas for further research. Readings: For your write-up, you will need to include a minimum of five papers from the primary literature. Chapter 10 of the course textbook provides a good introduction to the study of competition in plant ecology and will point you in the right direction for finding more studies, but note that textbooks and websites are not considered primary literature sources! Experimental Design and Setup: We will use a multiple deWit replacement design to test for competition within and between species. The multiple replacement design varies both density of plants and proportion of plants of each species. Growing each species in monoculture at different densities allows us to measure the strength of intraspecific competition. Growing pairs of species in mixture allows us to measure the strength of interspecific competition. Comparing intra- to inter- specific competition in this way allows us to estimate their relative importance. We will grow each species alone, and in mixture with the other species. Each density/proportion treatment will be replicated multiple times. At the end of the semester, we will measure our response variables, plant height and above-ground biomass, and use this information to construct deWit plots, measure size asymmetry and perform the statistical analyses documented here. Basic design: Density = 4 0 % Bean : 100 % Oat 25 % Bean : 75 % Oat 50 % Bean: 50 % Oat 75 % Bean: 25 % Oat 100 % Bean: 0 % Oat Density = 8 0 : 100 25 : 75 50 : 50 75 : 25 100 : 0 Density = 16 0 : 100 25 : 75 50 : 50 75 : 25 100 : 0 Methods: A. Preparation Fill 15 pots to the lip with the soil provided. Label each pot with your group name, the species names, pot density and the Bean:Oat proportion.ch density, for one native/invasive pair. For example, one label might read: Bean:Oat 25:75 density = 4 Ted & Alice Transplant seeds into the pots, as designated on your labels. Make a small hole in the soil, insert the seed, and gently push over the hole. Seeds can be haphazardly arranged in the pots. Bring the pots up to the greenhouse, and place on the designated bench. B. Growth Measurements To estimate plant size, we will take measurements repeatedly during the semester. We will calculate the average values for individuals of a species within a pot. Individuals within a pot are not independent, and so although we are interested in how individuals respond, we must determine the average response. You will be assigned the pots to measure so that all students have some low and high density pots to work with. Be sure that you can distinguish the different species within pots, and be careful in recording data and in transferring the data to the class data sheet to avoid any errors. For each pot, note the ID, and copy the species names and number of individuals of each species planted in the pot from the label (some plants may die over the experiment, so you will need to note how many you actually measure as well). Please be careful that you measure EVERY PLANT. Measure the length of the longest leaf to the nearest mm (the length of the main stem to the apical meristem for the bean). Be GENTLE with the grasses; if you damage them, it may affect your results! Make a list of all your lengths and leaf numbers in the data sheet provided for you in lab. From these numbers, you will calculate averages for each variable for each species in each pot C. Final Measurements In addition to one final size measurement, we will measure final plant biomass for plants in each pot. You will be assigned pots to work with. After recording the species and measuring the height of each plant, cut the plant off at the soil surface, and note whether each individual is still alive (note that an ‘individual’ is one rooted genet; multiple stems are not multiple individuals). Using the balances in the lab, get a fresh biomass reading for each plant within each pot (remember that some pots have individuals from just one species present). Write your data in the table provided for you in the lab. D. Data analysis Your TA's will provide you with a copy of the pooled class data in spreadsheet form. These data can be analyzed both descriptively and statistically. Basically, you are interested in whether the average size of plants, size hierarchies within a pot, or plant growth rate change with density and with whether plants are grown in monoculture or mixture. To visualize and interpret the data, you will construct DeWit plots and plots of growth rate, and calculate coefficient of variation of biomass within each plot. To determine whether apparent differences on your plots are significant, you will perform both t-tests and ANOVAs. 1. DeWit plots For each species pair and density, you will construct a DeWit plot. Density is on the X axis (for the purposes of these plots, ignore variation in survival), and mean mass per pot and species is on the Y axis (traditionally termed ‘yield’). There are two lines plotted per species pair/density combination. You should also draw the expected line if interspecific and intraspecific competition are equal—this line joins zero to the yield of each species in monoculture. Your graphs will look like this: 12 Species J Species I 10 Yield per pot (g) 8 6 4 2 0 0.0 0.5 1.0 Proportion Species J If yield is above the expected line, then interspecific competition is likely weaker than intraspecific competition (the case for species I). If yield is below the expected line, then interspecific competition is likely stronger than intraspecific competition (the case for species J). This remains a tentative conclusion until you perform t-tests to determine whether your data are significantly different from the expected value, at which point you can state them with confidence. See the discussion of competition in your textbook for more information regarding the interpretation of DeWit plots. 2. Size hierarchies Symmetrical Low CV Asymmetrical High CV You will calculate how asymmetrical the size distributions of plants are at different planting densities using the coefficient of variation (CV) of plant sizes in each pots. CV for a given pot is calculated as: CVpot = Standard Deviationpot / Meanpot In this case, the mean is the average height or biomass of all plants in the pot, and the standard deviation is a measure of the variability of plant heights or biomasses in that pot. As the size hierarchy within a pot becomes stronger (a few large plants dominate numerous smaller plants), the CV will increase. You will test for differences in CV of height and biomass among different planting densities using an ANOVA, with density as the independent variable and CV in each pot as the dependent variable. 3. Growth Rate A simple way to visualize the change in plant size over time is to plot your size variables over time. It’s best to do each species within each treatment separately. You can also calculate growth rate using the equation for population growth, with lambda defined as the relative growth rate: lambda = N t+1 / N t In this case, N t+1 will be the mean height for the plants the second time we measured them, and N t will be their height the first time we measured them To estimate the effect of interspecific competition on growth rate, subtract lambda for each species in mixture from lambda from that species in monoculture. To estimate the effect of intraspecific competition, subtract lambda at high density from lambda at low density. 4. Statistical Analysis We will do several different types of analyses of our data, ranging from qualitative assessments of figures to statistical tests. Your TAs will provide you with a spreadsheet with the raw data and statistical tests set up for you. The specific details of these tests will be discussed when the data spreadsheet is provided. You will use t-tests, ANOVAs, and plots of your data to examine the effects of density and inter- vs. intra-specific competition on: a. Yield in monoculture/mixture at different densities (deWit plots, t-tests, ANOVAs) b. Size hierarchies (histograms, CV, ANOVAs) c. Growth rates (plots, t-tests, ANOVAs) The spreadsheet will be set up with examples of all of these tests; it’s important that you understand both how to perform them and how to interpret your results!! Please do not hesitate to contact your TAs if you have any questions.