Evolution and Ecology Lab (BIOL 3113)

Spring 2003


Abiotic factors regulating population size

Click here to access the pre-lab questions for this lab.

Introduction

In the absence of environmental constraints, any population of organisms reproducing at their full potential would cover the earth in a relatively short period of time. The maximum rate of growth that can be achieved by a population under ideal conditions is known as the intrinsic rate of increase. Fortunately, under natural conditions, limiting factors come into play and population growth falls below the intrinsic rate. Resources that might be limiting for any given population include sunlight, space for growth, nutrients, pollinators, refuges from harsh weather, hiding places from predators, as well as many others. The availability of these resources determines the carrying capacity, the maximum population size that can be sustained by that species given the resource base. In this experiment we will investigate how the availability of nutrients (specifically, nitrogen) within the ecosystem may affect the population growth of a floating aquatic plant species.

Few plants are suitable for studying continuous population growth because most plants have life cycles with discrete jumps in population size, their reproduction is seasonal, and they respond to changes in population density by changing size and shape instead of population number. However, free-floating aquatic plants such as duckweeds (Lemna spp.) or water ferns (Azolla and Salvinia) undergo continuous growth and therefore are excellent models for quantifying aspects of population growth. These plants are stemless and have only one to four leaf-like structures called thalli (singular = thallus), if they are flowering plants, or fronds, if they are ferns. Roots from the thallus hang free in the water. Duckweeds and water ferns can reproduce sexually but seldom do. More commonly they reproduce asexually by producing a new thallus or frond directly from an old one. When a new thallus has grown large enough and has roots, it breaks loose from its parent plant and grows on its own as a separate plant. The growth of a population can be followed by counting thalli or measuring changes in biomass (dry weight) or percent cover.

If a pond or lab beaker is inoculated with one or two thalli and conditions are favorable, the plants commence exponential growth (Phase I in Fig. 1). The growth rate of the population under these conditions is density independent; the population grows unimpeded by resource limitation or competition. We can estimate the intrinsic rate of growth (r; click here for a list of equations relevant to the study of population growth) by measuring the unlimited growth of low density populations.

As thalli accumulate, the population becomes crowded and limited by the available resources. For a period, growth appears constant (Fig. 1, Phase II) as the width and thickness of the mat of floating plants increases. Eventually the beaker or pond fills with floating plants (Fig. 1, Phase III) and the population reaches a steady state. At this point, for every new thallus that appears, an existing one is shaded and dies, i.e., the population size is stable. The logistic growth curve (Fig. 1) illustrates all three phases.

We will study how nutrient levels influence the population growth of the floating aquatic plants Landoltia punctata ("Spotted False Duckweed") in a space-limited environment. Landoltia punctata is a small, non-native, floating plant that can grow into dense masses in stagnant water bodies. A Landoltia plant usually has two leaves attached together. The leaves are shoe-shaped, purplish underneath, with 2-5 descending roots. In this experiment you will monitor the growth of populations of Landoltia punctata grown in the laboratory with various concentrations of nutrients. By comparing the population size over time in these differing nutrient conditions, you may be able to determine the role that nutrients play in enhancing or limiting population growth in this species.

Procedures

Each bench group will prepare three replicates (i.e., three film canisters) of six different nitrogen levels: 1/2 strength (mixed according to manufacturer's instructions in distilled water), 1/4 strength, 1/8 strength, 1/16 strength, 1/32 strength, and no nitrogen (control). Use the randomization file on your bench's laptop to assign treatments to canisters. Each canister should be filled with 20 ml of the appropriate fluid. Place a label on each canister so that the top of the label is approximately at the 20 ml mark; on this label, record the nutrient treatment, as well as your name and lab section.

The most commonly used method of measuring growth of duckweed is to count thalli. Most duckweed research and phytotoxicity tests depend on thallus counts. However, counting thalli is deceptively tricky. To count the thalli in just a few containers can take considerable time. Thallus counts will be proportional to biomass only if thalli in different treatments share the same average measurements (the same geometry) and same density. The effects of the different treatments may negate these assumptions. When counting thalli, it is the accepted procedure to count every visible thalli, even the tips of small new fronds that are just beginning to emerge from the pocket of the mother frond (for example). A magnifying glass or a stereomicroscope (10x is good) is necessary for good thallus counts. It is all too easy to miss thalli or count them twice. To start the experiment, place 10 thalli in each canister. Please note: the duckweed samples were freshly collected from Bird Pond, Bullock County, and include several species, including water meal (Wolffia) and mud-midget (Wolffiella). Make sure you use only Landoltia plants. Click here to see these plants. Do not break plants apart to get exactly 10 thalli; instead, simply find another plant with the desired number. Enter the data in one of the laptops at your bench.

Next week, record the number of Landoltia thalli in each canister, and enter the data in one of the laptops at your bench. Refill each canister with distilled water to its original 20ml level. Think about how you will analyze this data to test your hypothesis. Your first lab report will include a complete results section for this lab.


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