Population Ecology ZLY 101 Dr. Wasantha Weliange

POPULATION ECOLOGY branch of ecology We can study 1.structure & dynamics of populations 2.the variables that determine the abundance & distribution of a population a)in time & space b)with regard to environment

Population is a group of organisms, of the same species, which roughly occupy the same geographical area at the same time.

Different definitions In human demography – a population is a set of humans in a given area. In genetics – a population is a group of interbreeding individuals of the same species, which is isolated from other groups. In population ecology – a population is a group of individuals of the same species inhabiting the same area.

Spatial scale population Local populations can occupy very small habitat patches like a puddle. A set of local populations connected by dispersing individuals is called a meta-population.

Scale Level Population Regional level – population of the region Island level – population of an island Continental level – population of the African continent Global level – Even the entire species can be viewed as a population

Continent level populations

Differ in their stability Some populations are stable for thousands of years Some persist only because of continuous immigration from other areas Populations on small islands get extinct, but then these islands can be re-colonized.

Characteristics of a population 1.Population size 2.Population density 3.Patterns of dispersion 4.Demographics 5.Population growth 6.Limits on population growth Above characteristics of a population are shaped by the interactions between individuals and their environments on both ecological and evolutionary time scales, and natural selection can modify these characteristics in a population.

Population size is the number of individuals living in the defined area Grassland, lake, pond, river, tundra increases and decreases over time – resource availability – Competition – Parasitism – Predation – climate – Natality / motality Population density The number of individuals of a species in an unit area

Natality & mortality used to calculate the dynamics of a population key factors determining size of a population over time Natality – is the greatest influence on a population’s increase Crude birth rate – number of births per 1000 population/year

Dispersion Dispersal is movement of individuals or their offspring into or out of an area regulates population size mixing of genes between populations is important for evolution Dispersal is accomplished through – immigration (into a population) – emigration (out of a population) – Migration (frequent movement into or out of a population area).

Dispersion

Patterns of dispersion Individual members of populations may be distributed over a geographical area in a number of different ways including: 1.Clumped distribution (attraction) - AGGREGATED 2.Uniform distribution (repulsion) - REGULAR 3.Random distribution (minimal interaction/influence) - RANDOM

Clumped distribution (attraction)

Uniform distribution (repulsion)

Random distribution (minimal interaction/influen ce)

Age structure = Population pyramid = age pyramid graphical illustration shows the distribution of various age groups in a population two back-to-back bar graphs Population - X axis Age - Yaxis Males on the left Females on the right

POPULATION Growth rate Population growth occurs when birth rates exceed death rates or immigration exceeds emigration. Population size regulates by – physical factors Weather Water nutrient availability – biological factors food availability Predators Parasitoids Competitors diseases

Factors affecting a population 1.density-dependent – Competition for resources – Food – Parasitism – Predation – Diseases – Toxic wastes 1.density-independent 1.Flood 2.Drought 3.Fire 4.climatic conditions 5.most pest control actions

Shortage of Food – As food will become limited when the population reaches a size, the death rate will increase until the food supply is sustainable for the population to be survive Population Regulation in an Ecosystem Density Dependant Factors

Predators – increase in population size will result more prey for the species' predators – therefore more chance of that species' members of being killed Population Regulation in an Ecosystem Density Dependant Factors

Diseases – Diseases spread rapidly across the densely populated area – especially across the same members of a species Population Regulation in an Ecosystem Density Dependant Factors

Toxic waste – important for organisms living in a small-enclosed ecosystem – a large population produce large amount of waste – increases the risk of disease, and reduces hygiene Population Regulation in an Ecosystem Density Dependant Factors Salton Sea in imminent danger of drying up, becoming toxic waste dump

Population Regulation in an Ecosystem Density Independent Factors occur regardless of a population size environmental disasters 1.forest fire 2.tidal wave 3.Tsunamis 4.Land sides

Pollution – Density independent factor that is not determined by a species' population, but by its external environment. Population Regulation in an Ecosystem - Density Independent Factors

Human intervention also has increasingly played a part in determining the population of species – via domestication – consequence from our actions – introductions