Your name: __________________________ Partner’s name:______________________LS 1 – Ecology LabIntroductionEcology is the scientific study of the interactions betweenorganisms and the environment. It is these interactionsthat determine the distribution and abundance oforganisms. Ecology is an interdisciplinary field,incorporating aspects of evolution, genetics, physiology,behavioral sciences, geology, geography, natural history,and more. Ecological studies take place on severaldifferent scales, from the organism to the entirebiosphere, and each level contributes to ourunderstanding of the complexity of biotic and abioticinteractions on our planet. Ecology often requires long-term observational studies to establish a clear picture ofbiotic and abiotic patterns, as well as manipulativeexperiments to determine organisms’ relationships andthe causal mechanisms for these relationships. In this lab, we will explore some of the principles of Population Ecology and Community Ecologyusing computer simulations. After completing this lab you should be able to:1. Identify exponential and logistic growth patterns, and describe how initial population size,growth rate, and carrying capacity affect patterns of population growth.2. Estimate an unknown population size using the Mark & Recapture method, and identify the benefits and restrictions of using this method.3. Describe the interactions between organisms in a simple food web and predict potential outcomes based on these interactions. 4. Apply the Scientific Method (develop a question, formulate hypotheses, design and execute an experiment, and interpret the results of your experiment) to investigate interactions between populations in a simple food web. Population EcologyPopulation ecology focuses on how populations change over space and time, and the reasons for these changes. Knowledge of population dynamics is an integral component of all aspects ofecology, as well as the study of evolution since natural selection acts on individuals to change populations. Logistic GrowthAll populations have the ability to grow infinitely large, but the availability of resources such as food and space limit population size. Exponential growth occurs when populations grow unrestricted, when there are excess available resources. However, resources such as space and food are never infinite. When resources become limiting, populations experience logistic growth. The availability of resources determines the carrying capacity (K), which is the maximum number of individuals a particular habitat can support over a sustained period of time. The 1carrying capacity can change depending onchanges to the pool of total available resources.For example, if nutrients are added to pond, algalbiomass could increase, at least until some otherresource (e.g. light availability or space) becomelimited. Per capita growth rate (r) is the change in birthrate versus death rate per individual over time.When r is positive, the per capita birth rate isgreater than the death rate and the population isgrowing. When r is negative, the per capita deathrate is higher than the birth rate and the population is shrinking. During periods of exponential growth, r does not change over time. Under logistic growth conditions, r decreases as the population reaches the carrying capacity. Estimating population sizeIn order to study interactions within and between populations, we must know something about the size of each population. Determining population size can be extremely challenging, labor and time intensive, and sometimes expensive! Ecologists employ many different techniques to estimate the size of populations, including attempting to count every single individual in a population. However, most of the time this is not possible. Instead, we must subsample the population and estimate the actual population size based on the subsample. The most appropriate method to select to estimate a population size depends on what type of organism you are measuring. You would select a different technique to count trees in a forest than you would use to count fish in a lake. One estimation technique that works well fororganisms that stay mostly in one place is thetransect/quadrat method. A transect line is runthrough the center of a habitat and quadrats areused at randomized intervals along the transectline to count all of the individuals in a smallerknown area that is representative of the largerarea that the population inhabits. The counts forthe smaller area can then be scaled up toencompass the entire habitat. For example, ifyou were trying to determine the abundance ofdifferent sessile organisms in the rocky intertidalzone of a section along a southern Californiabeach (such as a mussel or a barnacle), it wouldlook like the images to the right.Another technique used to estimate the size of a population is the Mark and Recapture method. First, we capture individuals from a population, mark (tag) them in some way, and release them. Then, we resample the population by capturing individuals again and seeing how many of theseindividuals from the second group were part of the original group (i.e. how many are tagged). Bycalculating the proportion of the recaptured group that has tags and comparing this to the number of individuals that were initially tagged, we can estimate the overall size of the population. 2Community EcologyA community refers to the biotic components of an ecosystem, and includes all of the species that interact in an area. Community ecology focuses on species’ interactions. Individuals interactwith individuals of the same species (intra-) and with individuals of other species (inter-). These interactions shape the structure of the community, and can affect the distribution and abundance of each species. Species interactions also affect the fitness of each individual and can influence natural selection. A key concept in community ecology is energy transfer across trophic levels, or who eats whom.Food chains can be controlled from the bottom-up and from the top-down. In bottom-up control, an increase (or decrease) in the base of the food chain producesand increase (or decrease) in every trophic level above it. Forexample, if coyotes eat rabbits and rabbits eat plants, an increasein the plant population will lead to an increase in both the rabbitand then the coyote populations. In top-down control, the highertrophic level influences each level below it in an alternatingdirection.