Lesson 5 - Population Ecology PDF

Lesson 5 : The Ecological Population “If we do not take charge of our population size, then nature will do it for us.” - David Attenborough Population density is the number of individuals per unit area How crowded a population is Varies depending on the species...

Lesson 5 : The Ecological Population “If we do not take charge of our population size, then nature will do it for us.” - David Attenborough Population density is the number of individuals per unit area How crowded a population is Varies depending on the species & ecosystem Population – is the summation of all organisms of the same group of species which live in the same geographical area and have the capability of interbreeding. Characteristics of Population 1. Size 2. Density 3. Distribution/Dispersion 3 1. Size ▫ pertains to the number of individuals in a population ▫ it may change when individuals enter or leave the population ▫ there are several factors that can affect the size of the population : emigration, immigration, natality and mortality 4 2. Density ▫ it is the number of the individuals of species living in a particular area of that population ▫ increases when factors are favorable, decreases when they are unfavorable ▫ Example: the number of goats which is 120 in every hectare, it may vary from year to year and is determined by the external factors such as availability of space, heat, water, light, food and of course predators 5 6 What causes population growth? Four factors: - Birth rates - Death rates - Immigration - Emigration Which result in population growth? Which result in population decline?  Changes in a population’s size are determined by immigration, births, emigration, and deaths. ▫ The size of a population is always changing. ▫ Four factors affect the size of a population. - immigration - births - emigration - deaths 3. Distribution/Dispersion ▫ it is another characteristic of population that tells how individuals are located in a particular area ▫ it simply the arrangement or how organisms belong in a population is organized within a particular space\ ▫ there are 3 ways that organism can be distributed: uniform, random and clumped 9 How do individuals of a population arrange themselves in the space? ▫ Depends of the type of species & environment ▫ 3 types: uniform, random, clumped Dispersion ▫ is the pattern of spacing among individuals within the boundaries of the population ▫ a pattern could be clumped, uniform or random 11 a. Organisms are concentrated in an area, may offer the population protection from enemies (school of fish) b. Organisms are evenly distributed over an area c. No specific order, organisms is spread throughout the are without an overall pattern (shrubs) 12 Population Ecology ▫ is the study of populations and their interactions with their environment ▫ the environment influences populations in terms of its density and distribution , age-structure, and size ▫ the environment also imposes a limit on populations ▫ Population ecology gives an insight into the factors affecting fluctuations of populations in the context of environmental supporting capacity 13 Population Dynamics ▫ is the study of how,when and why populations change over time ▫ it deals mainly with population density , which refers to the numbers of individuals per unit area or volume Population Density = (Births + Immigration)- ( Deaths + Emigration) 14 Factors Affecting Population Dynamics ▫ all living things need food, water, shelter and space to survive ▫ there are factors that may affect the increase of population and growth of an organism ▫ Limiting factors – are the forms of environmental resistance that limits the population 15 a. Limiting Factors ▫ there are biotic and abiotic factors that restrict the numbers, reproduction or distribution of organisms ▫ because of the limiting factors , each ecosystem has a finite capacity for growth connected to its carrying capacity ▫ carrying capacity is the maximum number of organisms of single species that an area can hold ▫ limiting factors are generally classified : density independent and density dependent 16 Classification of Limiting Factors ▫ Density - independent factors – can affect the population no matter what its density is, ▫ Examples: natural disasters, temperature, sunlight , human activities, physical characteristics and behaviors of organisms ▫ Density dependent factors- can only affect a population when its reaches a certain density ▫ Examples: competition, parasitism, predation, crowding : stress and disease 17 Limiting Factors and Population Dynamics ▫ given that immigration and emigration rates cancel out, a population grows when the birth rate exceeds the death rate or declines when the death rate exceeds the birth rate ▫ Density-independent : a birth rate or death rate does not change with population density ▫ Density –dependent : birth rate decreases and death rate increase at higher population density 18 ▫ Density-dependent factors have amplified effect as the population increases and a diminished effect as the population decreases ▫ Density- independent factors affect populations indiscriminately Example: Density-dependent : contagious disease is more likely acquired by large population than small Density-independent : are usually environmental related , in winter some organisms will experience a drop in population due to the harsh conditions, regardless of its initial population density 19  Ecological factors limit population growth. ▫ A limiting factor is something that keeps the size of a population down. Density-dependent limiting factors are affected by the number of individuals in a given area. ▫ Density-dependent limiting factors are affected by the number of individuals in a given area. – predation – competition – parasitism and disease ▫ Density-independent limiting factors limit a population’s growth regardless of the density. – unusual weather – natural disasters – human activities Population size is affected by density-dependent and density-independent factors Factors that influence population size are either density-dependent or density-independent. ▫ Density-dependent factor A factor that influences an individual’s probability of survival and reproduction in a manner that depends on the size of the population. ▫ Density-independent factor A factor that has the same effect on an individual’s probability of survival and the amount of reproduction at any population size. b. Resource Abundance ▫ the availability and abundance of environmental resources like food, water and space is a critical determinant of population dynamics ▫ If a population has unlimited access to these resources , the population will exhibit exponential growth plotted as J- shaped curve ▫ If population abundance is high, the limited environmental resources impose a ceiling for growth, population demonstrate logistic growth plotted as an S- shaped curve 24 Population growth is based on available resources. ▫ Exponential growth is a rapid population increase due to an abundance of resources. ▫ Logistic growth is due to a population facing limited resources. ▫ Carrying capacity is the maximum number of individuals in a population that the environment can support. A population crash is a dramatic decline in the size of a population over a short period of time. The Exponential Growth Model The exponential growth model. When populations are not limited by resources, their growth can be very rapid. More births occur with each step in time, creating a J- shaped growth curve. The Logistic Growth Model The logistic growth model. A small population initially experiences exponential growth. As the population becomes larger, however, resources become scarcer, and the growth rate slows. When the population size reaches the carrying capacity of the environment, growth stops. As a result, the pattern of population growth follows an S-shaped curve. Exponential growth is plotted as a J-shaped curve, showing the slow initial increase because the number of reproducing individuals is small and then gradually losing its steepness due to the exponential increase in reproducing individuals. Exponential growth can continue up to an organism’s biotic potential as long as the environment can provide resources for them to grow indefinitely. However, very few populations exhibit this type of growth. When population size is low, the population grows exponentially because birth rate exceeds death rates, but as population size increases , the environmental resources become limited, increasing the death rate and thus slowing the rate of population growth. K – carrying capacity , is the upper limit on population size , it is a theoretical maximum number of individuals of a given species that the environment can sustain indefinitely. As the population reaches carrying capacity, competitions for resources intensified, death rates increase and birth rate decreases. The logistic growth stabilizes just around or below the carrying capacity. 30 31 c. Life- history patterns ▫ organisms vary in their reproductive pattern ▫ r-selection : reproduce rapidly and produce may offsprings within a short-period of time, rapid life-history pattern, common in species that have small body size, reproduce early , highly mobile and have a short-life span that do not reach sexual maturity ( insects, mosquitos), once environmental conditions change and unfavorable , population drops dramatically 32 ▫ large species like mammals and birds that live in more stable environments have a slow life-history pattern called K- selection ▫ K selected organisms have long life-span that reach sexual maturity , larger body size , reproduce later in life and produce few offspring ▫ In this pattern, environmental resistance specifically competition for resources , plays a key role in maintain population size near the carrying capacity. 33 The Logistic Growth Model ▫ If food becomes scarce or other conditions change, a population can experience fluctuations. ▫ Overshoot When a population becomes larger than the environment’s carrying capacity. ▫ Die-off A rapid decline in a population due to death. Species have different reproductive strategies and distinct survivorship curves ▫ K-selected species A species with a low intrinsic growth rate that causes the population to increase slowly until it reaches carrying capacity. ▫ r-selected species A species that has a high intrinsic growth rate, which often leads to population overshoots and die-offs. Reproductive Strategies  Survivorship Curves ▫ Survivorship curve A graph that represents the distinct patterns of species survival as a function of age. There are three types of survivorship curves: ▫ Type I survivorship curve A pattern of survival over time in which there is high survival throughout most of the life span, but then individuals start to die in large numbers as they approach old age. ▫ Type II survivorship curve A pattern of survival over time in which there is a relatively constant decline in survivorship throughout most of the life span. ▫ Type III survivorship curve A pattern of survival over time in which there is low survivorship early in life with few individuals reaching adulthood.  Survivorship Curves Survivorship curves. Different species have distinct patterns of survivorship over the life span. Species range from exhibiting excellent survivorship until old age (type I curve) to exhibiting a relatively constant decline in survivorship over time (type II curve) to having very low rates of survivorship early in life (type III curve). K-selected species tend to exhibit type I curves, whereas r-selected species tend to exhibit type III curves. d. Environmental Conditions ▫ variability in environmental conditions impacts population density ▫ population dynamics may fluctuate , example organisms that change in abundance according to the seasons in a year ▫ availability of water and light are among the most crucial as these affect populations of producers at the base of the food chain ▫ favorable climatic conditions lead to an increase in population density whereas severe climate can have negative impact ▫ changes in temperature and humidity as well as natural disasters lik fires and floods can alter population dynamics significantly 39 e. Organism Interactions ▫ Interactions amomg organisms in a community are the main limiting factors in keeping population sizes below the environmental capacity Cooperative interactions – mutualism and commensalism , allow more efficient use of resources , reduce death rates , encourages population growth Predation- yields a negative regulatory effect on both predator and prey Competition- for limited environmental resources regulates population size Territoriality- limits population density as territory space becomes the desired resource for competing individuals Crowding – can lead to stress which impacts the population negatively 40 Some species interactions cause negative effects on one or both of the species ▫ Community ecology The study of interactions between species. Throughout the world, species have symbiotic relationships. ▫ Symbiotic relationship The relationship between two species that live in close association with each other. Negative species interactions include: ▫ Competition ▫ Predation ▫ Parasitism ▫ Herbivory Competition ▫ Competition The struggle of individuals to obtain a shared limiting resource. ▫ Competitive exclusion principle The principle stating that two species competing for the same limiting resource cannot coexist. ▫ When two species have the same realized niche, one species will perform better and drive the other to extinction.  Competition Competition for a limiting resource. When Gause grew two species of Paramecium separately, both achieved large population sizes. However, when the two species were grown together, P. aurelia continued to grow well, while P. caudatum declined to extinction. These experiments demonstrated that two species competing for the same limiting resource cannot coexist. Competition ▫ Resource partitioning When two species divide a resource based on differences in their behavior or morphology. The evolution of resource partitioning. (a) When two species overlap in their use of a limiting resource, selection favors those individuals of each species whose use of the resource overlaps the least with that of the other species. (b) Over many generations, the two species can evolve to reduce their overlap and thereby partition their use of the limiting resource. Predation ▫ Predation An interaction in which one animal typically kills and consumes another animal. ▫ Parasitoid A specialized type of predator that lays eggs inside other organisms—referred to as its host. ▫ To avoid being eaten or harmed by predators, many prey species have evolved defenses. Parasitism ▫ Parasitism An interaction in which one organism lives on or in another organism. ▫ Pathogen A parasite that causes disease in its host. ▫ A single parasite rarely causes the death of its host. Herbivory ▫ Herbivory An interaction in which an animal consumes a producer. ▫ When herbivores become abundant they can have dramatic effects on producers. ▫ Many species of producers have evolved defenses against herbivores. Other species interactions can cause neutral or positive effects on one or both species. ▫ Mutualism An interaction between two species that increases the chances of survival or reproduction for both species. ▫ Commensalism A relationship between species in which one species benefits and the other species is neither harmed nor helped. Species Interactions 50