Population Growth and Competition in Lemna sp. and Spirodela sp.

Population Growth and Competition in Lemna sp. and Spirodela sp.1

Joane C. Tampoco
Group 3 Sec. U-1L

August 8-29, 2009

_____________________ 1A scientific paper submitted in partial fulfillment of the requirements in Ecology laboratory under MS. Faith Maranan, 1st sem., 2009-2010.
INTRODUCTION

Population growth is the change in population over time, and can be quantified as the change in the number of individuals in a population using "per unit time" for measurement (Wikipedia.com). A population can grow in an exponential or logistic growth pattern. Exponential population growth is the geometric increase of a population as it grows in an ideal, unlimited environment. For a continuously reproducing population, exponential growth is an excellent first-approximation of population growth. When resources are not limiting, and interspecific competition is at a minimum, many populations of organisms grow exponentially. This generally occurs when populations are at densities far below their environmental carrying capacity, which is the maximum number of individuals a given environment can sustain (IUC. edu). The outcome of n exponential growth can be determined by the intrinsic rate of increase which is the difference between the birth rate and death rate. If the rate is greater than zero, the population increases exponentially. If it is less than zero, the population exponentially decreases, and if it is equal to zero, the population remains constant.
In reality, populations do not appear to increase in an unlimited manner. Instead, populations are limited in their numbers by the availability of the resources or by the carrying capacity. With high number of species, intraspecific competition, that is, competition among similar species, intensifies. While the intrinsic rate of increase was assumed to be constant in the exponential growth equation, when intraspecific competition is considered, it decreases when the number of species is high and increases if the number of species decreases. Since the effects of intraspecific competition are dependent upon the number of species, this factor is referred to as density dependent factor. The exponential growth equation, if modified to incorporate the effects of the intraspecific competition, is the logistic growth pattern. This indicates that the growth will level off as the population reaches its carrying capacity. This new equation has three possible outcomes: if the number of species is less than the carrying capacity, the growth is positive, population increases and reaches a plateau, which is the logistic curve, if the number of species is less than the carrying capacity, growth is negative, population decreases and reaches a plateau, and if the number of species is equal to the carrying capacity, growth is zero and the population remains constant.
In the presence of another species, individuals of a population may suffer reduction in fecundity, growth and/or survivorship as a result of interspecific competition, that is, a form of competition in which individuals of different species vie for the same resource in an ecosystem (Wikipedia.com). Members of different species may compete by exploitative competition or by interference competition. Interspecific competition is an important factor limiting the population size of many species. Interspecific competition is an important factor limiting the population size of many species. The growth rate of a species will be limited as the population size of their competitors increases, either because they have access to fewer limiting resources (exploitative competition) or because of the negative effects of the direct interactions of their competitor (interference competition). In addition, interspecific competition can limit the number of species that can coexist in a community and affect the phenotypic characteristics of organisms in an attempt to reduce the effects of competition (Eoearth.org). If the individuals of the other species (Species 2) are not competitive enough, the other population (Species 1) may be minimally harmed. However if the individuals of the other species (Species 2) are highly competitive, in due time, the population growth of the other population (Species 1) may not only stop but may even decline until it becomes extinct. Same also happens with Species 2. In some instances, both species may co-exist and reach equilibrium over time.
Population growth and competition, therefore, affects the fate of an individual’s population; it can decline, grow or remain constant depending on the carrying capacity and competitors present in the environment. This study was designed to determine the effects of the interspecific competition and the environment on the dynamics of the population of Lemna sp. and Spirodela sp. This study was conducted t Room C-205, Institute of Biological Sciences, UPLB on July 8 to July 29, 2009.

MATERIALS AND METHODS

In determining the effects of different types of environment and interspecific competition between Lemna sp. and Spirodela sp., we first prepared eight jars; three replicates for treatment A which is 50 ml tap water and for treatment B which is 5 g of soil plus 50 ml water. We introduced eight fronds of Lemna to one set of treatment A and B. To another set of the treatments, we introduced the same number of fronds of Spirodela. These represent pure cultures of Lemna and Spirodela where the initial population is eight. We put four fronds of Lemna sp. and four fronds of Spirodela sp. to the remaining set of treatments. This represents the mixed culture of Lemna sp. and Spirodela sp. where the initial population is also eight. We covered the jars with paper and punched a few small holes on the paper for aeration. We kept the cultures in the glass house and counted the total number of fronds of Lemna sp. and Spirodela sp. every other day until the frond count levels off. We recorded the observations in Table 3C.1. We then constructed graphs of pure cultures of Lemna sp. and Spirodela sp. grown in the different treatments. There were two graphs for each treatment. On the first graph, we plotted frond count versus time for Lemna grown as pure culture and mixed cultures. On another graph, we did the same for the pure and mixed cultures of Spirodela. We then computed for the instantaneous rate of growth (r) at each observation interval in all cultures using the formula below and entered all computed values in Table 3C.2.

Where λ is the annual finite rate of increase, is the population in time t, and is the population at the next observation day.