Population Attributes PDF

## Population - A group of interbreeding or potentially interbreeding organisms of the same species occupying a particular space at the same time. - Are characterized by: - **Density:** Number of individuals occupying a definite unit of space. - **Age structure:** The ratio of one age class...

## Population - A group of interbreeding or potentially interbreeding organisms of the same species occupying a particular space at the same time. - Are characterized by: - **Density:** Number of individuals occupying a definite unit of space. - **Age structure:** The ratio of one age class to another. - **Population growth:** Gains and losses of members through birth, death, and emigration. **Population ecology has different components** - **Demography:** deals with the vital statistics of a population like age structure, density, birth rates, deaths, growth, and reproduction. - **Population genetics:** concerned with the breeding structure and genetic composition of a population. - **Ontogeny:** Population consist of either unitary organism or modular org. **Components of population** - The deer is a unitary organism with a discrete growth form. - The tree are probably modules arising from the original parent tree surrounding tree appear as buds or horizontal roots to form clones. **Individual production** - Individuals are produced in dut derived from the parent gamet or gamets. - They can give rise independently of the parent genet and can enist the same genetic constitution of the parent genet. - By producing ramets, the parent genet can spread over a considerable area. **Population consists of** - Interbreeding organisms. - Gene pool - consists of all genetic information carried by all individuals of a population. - Gene flow - the exchange of genetic information between populations. - May also be considered as evolutionary units. - Evolution results from changes in gene frequency in a given gene pool over a period of generations. - An outcome may be changes in the physical expression of all the population, reflecting the genetic constitution. - These changes in genetic constitution often result from selective pressure brought to bear by the environment on the individuals of the population. - Consists of numerous subpopulation or 'demes' of varying densities. ## Characteristics/Attributes of population - **Density:** The size of a population in relation to a definite unit of space is density. - The measure of the number of individuals per unit area is called crude density. - Populations do not occupy all the space within a unit because it is not suitable habitat. - Biologists estimate the no. of deer per square mileage but omit the no. of deer per square because of human habitation and land use practices. - **Dispersion/Distribution:** Organisms in a population that are distributed in space. - **Random distribution**: Distribution is random if the position of each individual is independent of the other. - It is rare for it to occur only where the env. conditions are uniform, equally available throughout the environment, and there is no interaction among members of the population. - Some invertebrates appear to be randomly distributed. - **Uniform/regular distribution**: It is the more even spacing of individuals resulting in regular patterns of distribution. - Particularly, distribution results from interspecific competition among members of a population. - Eg- spiders can produce uniform distribution. - **Clumped distribution**: Most common type of distribution also called clustered, contagious or aggregated distribution. - **Temporal dispersion**: Organisms in a population are distributed not only in space but also in time. - Temporal distribution can be circadian, relating to daily changes in light and dark. - Eg- Nector feeding insects seeking patches of open flowers. - Daily movement of plankton from deeper to upper layers of water. - Withdrawal and emergence of nocturnal and diurnal animals. - Seasonal changes in humidity and temperature cycles relate to changes in seasonal differences. - Eg- Blooming of flowers in spring and in the sequential bloom and the return and departure of migrant animals. ## Dispersal movements - Most organisms disperse at some stage in their life cycle. - They leave their immediate env. either permanently or seasonally for more far habitats such as - The movements are essential for survival. - Esp- of the young are essential for fitness by moving to a new territory. - Dispersal leads to colonization of suitable areas, expansion of species range, and the spread of genes. **Immigration -** One way in to a habitat with no return trip. **Emigration** - one way out of one habitat without a retur trip **Nigration** - One way into a habitat with no return trip, but usually return to the place of origin. - Emigrants from one area become immigrants to another. - For mobile animals, dispersal is achieved by many means of various displacement such as - **Active dispersal**: Occurs when animals move under their own power such as bird, mammal, etc. - **Passive dispersal**: Animals dispersal range depends upon the distance their dispersal agent travels, eg. - clearly wind carried seeds. **Migratory types are divided into three categories.** - **Repeated return trips / round trip migration**: - Occurs when animals make movements to the same destination every day and more often 70 downs to surface at night. - These movements may be daily, annual or short range. - **Annually return trip migration:** Eg- Pacific Salmon. - Young hatch and grow in head waters. They move downstream and out to the ocean where they hatch and grow up to the young stage. - They return to their home streams to spawn and may reach there as adults. - **No return trip:** Eg- Monarch butterfly. They are fall migrants that donot return to the north but their offspring do. ## Age Structure/Age Distribution: - Age distribution influences both growth rate and death rate. - The ratio of various age groups in a population determines the current reproductive status of the population and indicates what may be expected in the future. - A rapidly expanding population usually has a large proportion of young individuals, a stable population will contain a larger proportion of adults, and a more evenly distributed age class. - A declining pop" will have a large no. of old individuals. ## Sex ratios - It is the proportion of males to females in a population. - Most tend towards a 1:1 sen ratio. It is also of two kinds - **Primary sex ratio**: Ratio at conception. It also tends to be 1:1 - **Secondary sex ratio**: Ratio at birth. - Among mammals, it is often weighed towards males but the population shifts towards females in the older age groups. - In humans, males exceed females at birth, but as age increases the ratio shifts in favor of females. - Among birds the sex ratio is higher towards males. - Physiological and behavioral patterns affect mortality of different sexes. - During the breeding season, the mole elk battles other males for dominance of the territory. They defend their harem from rivals and mates, consuming considerable energy but leave little to do in the breeding season. - Often, ends the breeding season in poor physical condition. - Among birds, the females may help the male defend territory, but she sits on the nest and often incubates the eggs, and feeds the young while the male is much more dimorphic to the female with sexual display and to promoting and the progress of the nest. - In adult male and 4 month old she, she, adult females, thus depend more energy are vulnerable to predation and in dangers. Their higher vulnerability to mortality is consistent with the imbalance in the sex ratio in the older ages. ## Mortality and Natality - The age structure of a population reflects two continuous processes in a population: deaths and births. - **Mortality** is often expressed as crude death rate, usually the no of deaths/1000 in a given period. - **Death rate:** The no. of deaths in a given interval divided by the average pop" at an instantaneous rate. - Eg- The pop" size at the beginning of the period is 1000 and the no alive at the end of the period is 600. The average size of the pop" = 1000+600 = 800. The no of deaths = 400, so the death rate = 400 = 0.5 - **Probability of dying:** The number that died during a given time interval divided by the no. alive at the beginning of the period; ie: 400/1000 = 0.4. - **Probability of surviving:** The no. of survivors divided by the number alive at the beginning of the period ie 600/1000 = 0.6 - **Life expectancy:** The average no of years to be lived in the future by members of a given age in a pop". **Natality may be expressed as physiological or realized.** ***Physiological*** or ***Absolute/Maximum natality:** - Max. possible no of births under ideal env. conditions. - No biological limiting factors (reproduction being limited only by physiological factors). - It is constant for a given pop. ***Realized*** or ***Ecological natality*** - Amount successful reproductive that actually occurs over of period. - It is influenced by env. condition, nutrition, and density of the pop <start_of_image>- **Minimum mortality** is the min loss under ideal or non-limiting conditions. It is constant for a given pop. - **Ecological/realised mortality** is the loss of individual under given env. conditions. It is not constant for a pop" but varies with env conditions. **Natality may also be expressed in terms of pop" size: ** - **Crude birth rate:** Eg- 50 births/ 1000 population. - **Specific birth rate:** It is expressed relative to a specific criterion such as age. It is usually expressed as an age-specific schedule of births, the no. of offspring produced per unit time by females in different age classes. ## Survivorship and mortality curve - **The life table:** - A clear and systematic picture of mortality and survival in a pop" is best provided by a life table. - It is a useful device to analyze probabilities of survivorship of individuals in a pop". - To determine ages most vulnerable to mortality and to predict pop" growth. - Pop" ecologists have adopted the life table for the study of natural pop". - The life table consists of a series of columns, each of which describes an aspect of mortality statistics of the members of a pop" according to age. - Figures are presented in terms of a standard number of individuals, all born at the same time called a cohort. - By convention, the initial no. of ind. in a cohort is set at 1000. The columns include- - **_(n)_** : The unit of age or age level. - **_(lx)_**: The no. of individuals in a cohort that survive at a particular age level. - **_(dx)_:** The no. of individuals in a cohort that die in an age interval *n* to *n+1* (from one age level to the next). - **_(qx)_**: The probability of dying/ age-specific mortality rate. - **_(mx)_**: This rate is determined by dividing the no. of died during an age interval by the no. alive at the begining of the age interval. - **_(sx)_**: Survival rate. Calculated from 1-qx - **In order to calculate life expectancy (ex), two additional characteristics are needed.** - **_(Lx)_**: _av. no of years lived._ It is obtained by sunspring the no. alive at each age interval and donbling the sum of the no. at time n+1, and donbling the sum of the no. at time n, and dividing the sum by 2. - **_(Tx)_**: The no. time units left for all ind. to live from age *n* onwards. - It is calculated by summing all the values of *lx* from the bottom of the table upwards to the age interval of interest. - **Life expectancy (ex)** = Tx for a particular age class /lx ## Survivorship and Mortality Curves - The life table is a very useful tool in the analysis of pop" dynamics. From it we can derive survivorship and mortality curves. - Based on *_(lx)_* column - to determine the age at which a particular org. most often dies. - These curves enable us in determining the causes of death and the process that affect the pop" dynamics of a given species. ## Survivorship curves - It depicts the age-specific survivorship of a particular age group by plotting no. of individuals against time. - They can be classified into 3 hypothetical types. - **Type I curve (convex):** It is common for a pop" whose ineti tend to live out their long potential life span. - **Type II curve (linear):** Typical of organisms with constant morality rate. - Eg- Adult mammals such as rodents, many birds and some plants. - **Type III curve (concave):** Typical of org. with extremely high mortality rates in early life. - Eg- Invertebrates, fish, and some plants. - Among plants, life table curves depict the features not only to individual but also to the env. - Life tables and corresponding curves indicate the nature of a pop" under diff. env conditions, a stable or under-perturban, continuous stable age distribution. ## Mortality curve - They are obtained by plotting *_(qx)_* or mortality rate column of the life-table against age. - It consists of 2 parts - **juvenile phase:** In which the rate of mortality is high. - **post- juvenile phase:** In which the rate frequently decreases and then increases with age after a low point in mortality. - Most pop" have roughly a J-shaped curve. ## Fecundity tables - Natality is a measure of birth rate and is used for expressing the no. of offspring produced per unit-time. - The pop" increase mostly depends upon the no. of offspring produced by females, particularly age specific birth rate to a - *Fecundity table* schedule is obtained by determining the mean no. of newborn born in each group of females (mx). - We can take the *_(lx)_* column, from the life table and age-specific mx values, we can construct a fecundity table. - **Reproductive value (Vx)** - The concept of reproductive value was given by R.A. Fisher (1930). In a pop" of constant size, he defined the reproductive value as the age specific expectation of future offspring. - **Vx = my/lyfx** - **lyfx = probability of surviving from age *x* to age *y*.** - **my = reproductive success of an individual at age y.** - **Vx = Reproductive value.** - For newborn indi. in a pop" that is neither ↑ nor ↓, the Rep. Value is the same as net reproductive rate, R=1. - The Vx (RV) of a new boen indi. is influenced by the state of the pop". - In an expanding pop", the RV (Vx) is low for 2 reasons. - **In an increasing pop", the probability of death before reproduction may also increase. ** - **the future breeding pop" of a young will be a part will be larger, they will contribute less to the overall gene pool than offsprings currently being born.** - **Conversely new borns into a declining pop" will contribute more to the future than the present progeny.** ## Population ecology II (Growth) - The pop" is a changing entity. The study of changes in the absolute number of organisms in a pop" and the factors explaining these changes is termed as POPULATION DYNAMICS. - Ecologists are more interested in how and at what rate the pop" is changing than in its absolute size and composition at any given moment. - The growth rate of the pop" is **_dN/dt_**. - **_dN_** = the change in the no. of org. - **_dt_** = change in time or time period. - The rate of change in the no. of org. (min or max) respect to time change. - **_dN/dt_**: The rate of change in the no. of org. teme/org. when pop" of the specific size are to be compared. - **_NAE_**: This is the specific growth rate. ## Population growth - Pop" show characteristic patterns of increase, termed as pop" growth forms. - Two basic types of growth forms can be studied: - **J-shaped growth form.** - **S-shaped or sigmoid growth form** ### Exponential growth (J-shaped growth form) - If a pop" were suddenly presented with an unlimited environment, it would tend to expand geometrically. - Eg- Small no. of bacteria, non-native plants or animals introduced into a suitable but unoccupied habitat. - Assuming there were no movement in or out of the pop" or no mortality, then birth rate would account for changes in pop" numbers. - Under these conditions, the pop" growth is a continuous increase called exponential growth. - **_dN/dt = rN_ ** - **_r_** = (b-d) - **_dN/dt_**: rate of increase. - It is proportional to pop" size and growth rate (r). - r = difference between instantaneous birth rate and instantaneous death rate (d).. - It can also be expressed as. - **_Nt = Noert_** - The J-shaped growth curve (growth form), the density (ie no. of ind.) increases rapidly in exponential fashion. - The growth stops abruptly and then as env. resistance or other limiting factors become effective. - The history of growth is at first influenced by the age of the org. such as the age at beginning of reproduction, the no. of litters produced during the lifetime of each female, the no. of young produced, survival of the young, and rungth of reproductive period. - Regardless of the initial no. of young produced, as the pre-reproductive age class enter the reproductive stage and more young are produced, the no. of births would increase until the pop" would enter a population growth curve and, if the environment would now hold a characteristic of many org. introduced into a new and unoccupied env. ### Logistic growth (S-shaped/ sigmoid growth form): - Exponential growth is not biologically realistic: no food and resources are not constant and the pop" growth cannot continue forever without limiting factors. - As pop" increase and density increases, eventually pop" unty their rate of growth slows down. - Determined by competition for available resources, it approaches carrying capacity. - Even intu the pop" is theoretically in equilibrium with its env. - **Called CARRYING CAPACITY** It is expressed as **K**. - The concept that the decline of exponential growth rate of the pop" in a particular habitat can be sustained without exceeding a certain density of pop". - Francos Verhulst in 1838. In 1920, Raymond Pearl and I.J. Reed of John Hopkins University published a nearly identical version of the growth pattern of decline in the pop" size as the pop" size increased. They gave a modified eq: _dN/dt= rN (K-N)/K_ - **_dN/dt_** = instantaneous rate of change. - **_K_**: Carrying capacity. - **_(K-N)/K_**: unutilized opportunity for pop" growth. - The eq. says that the rate of increase of a pop" over a unit of time is equal to the potential increase of the pop" times the unutilized portion of the resources. - When **N** is low (**K-N**) is **≈ K** (Most resources are unutilized). - When **N** is **≈ K** (**K-N**) **≈ 0** (Most resources are utilized). - When **N > K** (**K-N**) = negative (Over-exploitation of resources) - In the S-shaped growth form, the pop" shows the following pattern: - Growth is exponential (establishment lag phase). - Then accelerates (exponential growth phase ), but it then slows down gradually as env. resistance increases (maximal and acceleration phase / lag phase). - The pop" will enter a steady-state and is maintained until equilibrium is reached. - The point in the logistic growth curve where pop" growth is maximal, K/2 is inflection point. - From this point on, the pop" growth slows as the density approaches K, but the rate of growth declines. - **_dN/dt_** is low when **N ≈ 0** and **N ≈ K** and highest when **N=K/2**. - The logistic equation is also called the Verhulst equation and has several assumptions. - Age distribution is stable. - No emigration- immigration takes place. - Increasing density depresses the rate of growth without any time lags. - The relationship between pop" size and rate of growth is linear. - When plotted logarithmically the rate of growth declines directly as pop" increases. - In the logistic growth form. environment al resistance created by the growing pop" itself brings about an increase in reduction in the potential reproduction rate as the pop" size approaches carrying capacity. - In pop" of higher plants and animals, which have complicated life histories and long periods of individual development, there are likely to be delays in the time it takes for the impact of env. factors to increase density and these delays means my result such cases. - The Max carrying capacity **(Km)** is the max. density that the resources in a particular habitat can support, without exceeding a certain density of pop". - The optimal **C.C (ko)** is the lower level density that can be sustained in a particular habitat without living on the edge regarding resources, such as food or space.. - There are different phases in a logistic G.F. - ** Lag phase** - The time req to acclimate for a pop" to become acclimated to its environment. - Once pop" acclimed to an env. where resources such as food, cover, and space are abundant, these pop" reproduce at an exponential rate of increase (log phase). - The may. rate of increase is termed as point of inflection. Beyond this point, the rate of increase begins to decelerate. The reason is that in the env. there is a set of resources becomes limiting or resources are slowing of pop" growth due to limiting resources being the env. conditions. - **Resistance phase** - the sigmoid growth. Finally, the pop" reaches carrying capacity conditions, in this rate of pop" increase is zero. And the pop" density is maxed. - **Crash** - The pop" strongly overshoots K and may drop below K, recovers slowly or may go extinct or may go low and return to eq. ## r - Selection and K- Selection - There are patters of difference among the reproductive boom in organisms. - Some species are weed, wild insects, they are small, have high reproductive rate and are short lives. - Other species like trees and deer, are large and have low reproductive rates and long lives. - Ecologists call the former r-species and the latter Ksp. - The pop" of the **r-species** grow rapidly and do not seem to remain/reach the carrying capacity. The pop" of the K species attain and more or less remain around carrying capacity - The concept of **r and k** was given by MacArthur and Wilson in 1967 and elaborated by Pianka (1970). - Empty habitats (places) are colonised by a variety of emigrant/ most successful of which would be species with the best mechanism for dispersal and the best ability for rapid population growin in unfavorable conditions. - Natural Selection would favor individuals able to achieve rapid pop" growth in uncrowded-resource-rich environments. - However, as pop" increased, conditions became crowded and pop" competed for resources. - NS would favor the most competitive individuals, those that were able to continue pop" growth at higher sustained densities near carrying capacity. - Mac Arthur and Wilson considered the former as **r-selected** sp. and the latter as **k-selected**. - Mortality in these esp. is largely density independent. - They considered the latter as **k-selected** b/k they are able to maintain their densest pop" at **k/2**. - **r-species** are able to compete affectively for food and other resources in uncrowded - resource rich environment. - Morehu as the result of density dependent factory, the pop" declines. - The theory of r- and K-selection predicts that species in these diff. env. will differ in life history trait such as size, fecundity, age at first reproduction no. of reproductive events during a lifetime and total lifetime. ## Features (r-strategists/ r-sp.) - **Short-rived.** - **Selection favors those genotypes that confer high reproductive rate at low popn.** - **Early reproduction.** - **Rapid development.** - **Small body size.** - **Large no. of offspring.** - **Minimal parental care (but with low survival).** - **They have the ability to make use of temporary habitats.** - **Many inhabit unstable or unpredictable environments where catastrophic mortality is environmentally caused and relatively independent of population density.** - **Env. resources are rarely limiting and they are able to exploit relatively uncompetitive situations.** - **Tough and adaptable.** - **Wide dispersal.** - **Good colonizers.** - **Respond rapidly to disturbance.** ## Features of k-strategists/ k-species - **Competitive sp.** - **Selection favors genotypes that confer a long life and slow growth rate at low pop" density.** - **They can cope with physical and biotic pressures.** - **Greater longevity and repeated reproduction.** - **Possess been delayed developmental.** - **Large body size, parental care for the young.** - **Among animals, parental care, stored food that gives offspring a strong start.** - **Among plants, a shorter life cycle.** - **They are specialists, efficient users of a particular enw. But their pop" are of near K. and are resource limited.** - **Poor colonisers, lack the means of wide dispersal. ** ## Model for r- and K-selected species: - (By Mac Arthur & Wilson) - A model of r- and k-selection involving the rates of increase of two genes *x*1 and X2. - In region A, where density is low and food (or sunlight and nutrients) are high, it is r-selection. - Allele *x*1 wins out: Because *x*1 is the fast growing sp. - In region B, species *x*1 is growing faster than *x*2 and allele *x*2 wins, thus it is **_k_** -selection. - Where the lines cross at point *C*, r-selection switches to **_k_** -selection. ## Attributes of r-selection and K-selection | Attribute | r-Selection | K-Selection | |---|---|---| | Climate | Unpredictable | Constant in time | | Pop size | Variable in time | Slow dev | | r & K selection factors | - Rapid development - Early reproduction - Small body size - Many offsprings - Short - Productivity | - Delayed reproduction - Large body size - Few offsprings - Long - Efficiency | | Length of life | Short | Long | | Leads to | Rapid reproductive rate | Longevity | **Small specie with rapid reproductive rates, such as insects and mites, are categorized as r species and long lived up like oak trees, deer, and elephants are characteristic as k species.** ## The concept of r-selection, and k- selection assumes deterministic environments. - One that is unpredictable. - Another that is randomly fluctuate over the life of an organism. - Environments that are subjected to sudden shifts in adult survival or in the two components of natality: fecundity and juvenile-death. - According to this hypotheses, in a volatile environment, in which adult mortality is low, and juvenile mortality is high, selection should favor early maturity, larger reproductive effort, and more young to replace the adults. - The same conditions if adult mortality is low and juvenile survival is high, then no need selection should favor late maturity; as no need exists for either early maturity or high production of young. - Thus, a volatile environment that affects juvenile survival more than adult survival should lead to lower reproductive effort each year and increased iteroparity (k-traits). - However, if the valuable environment affects adult survival more than juvenile survival, then it should lead to higher reproduction each year and reduced iteroparity (K-traits). -