Population Ecology ZLY 101 Dr Wasantha Weliange POPULATION

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. 2. 3. 4. 5. 6. Population size Population density Patterns of dispersion Demographics Population growth 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

Population Regulation in an Ecosystem Density Dependant Factors • 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 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

Population Regulation in an Ecosystem Density Independent Factors • 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