2.3 Population level

• A population is defined by Mader (1998) as the existence of all organisms of the same species within a specified area.

• An emphasis is based on the factors that affect growth and regulation of population size.

• Ecology = studies factors affecting growth + regulations of populations size and distribution of organisms (where & why in a particular place at a particular time)

2.3.1 Characteristics of Populations

• At any one point in time, populations have a certain size• So, population size is defined as the number of individuals contributing to the

population’s gene pool

2.3.1.1 Factors affecting the size of a population

• Population depends upon a balance between natality (births), mortality (deaths), emigration (movement of individuals out of a defined population) and immigration (movement of individuals into a defined population).

• Look at the diagrammatic representation of factors influencing population size, increase in pop size (natality, immigration), decrease in pop size (mortality, emigration)

• Life either add individuals or removes them (quite gross but a fact) for balance

Change in Population size = (Natality + Immigration) – (Mortality + Emigration)

The Fundamental Equation of Ecology – Harper 1977

Δ N = B – D + I – E

Change in Number = Births – Deaths + Immigration - Emigration

John L. Harper – 1925-2009

A graph showing exponential population growth

2.3.2 Species interaction

• Species interact in different ways, and may have impact in the relative distribution and abundance

• Member of two different populations may interact in a symbiotic relationship• There are three forms of symbiotic relationship that may exist: parasitism,

commensalism and mutualism • Parasitism: similar to predation in that an organism called a parasite derives

nourishment from another called host just as predators derived nourishment from their prey e.g. viruses (HIV), protists (malaria), bacteria (Streptococcus sp.) etc.

• Commensalism: is a symbiotic relationship where one species is benefiting and the other is neither benefiting or harmed e.g. provision of home, transportation, protection etc.

• Mutualism: both species benefit from the relationship e.g. bacteria that reside in the human gastro-intestinal tract are provided with food and they in turn provide us with vitamins

2.4 Community level

• Populations do not occur in isolation, they form part of a community• A biological community is define by Mader (1998) as an assemblage of populations

interacting with one another within the same environment. What are the examples of a community within the campus?

2.4.1 Characteristics of a community

• A community is characterised by four elements i.e. size, boundary, diversity and abundance

• Communities differ from each other in terms of size• No two communities can have the same size• Community does not remain the same through out its existence• It fluctuates in size and composition with time• It is sometimes difficult to have clear delineating boundaries between different

communities• Composition of a community refers to the listing of the various species in that

community• Diversity includes two elements i.e. the number of species and the relative

abundance of individuals of different species

2.4.2 Factors maintaining composition and diversity of species

• “it goes without saying that species in a community may change both in time and space which can affect the number of species and their abundance”

• Some of these factors include competition (C) and predation (P)• C & P particularly by dominant species over others may lead to their possible

extinction or removal from the community• Competition: species utilize same food source and exist no specialization between

species• Predation by dominant and efficient species may monopolise all available prey and

also by preventing one prey species from eliminating other through competition (phenomenon not always the case)

• What maintains species diversity and prevents one or more species in a trophic level from eliminating the other through competition? 1. when species are in potential competition for a food supply, the resource may be subdivided in some way, e.g. specialization. Different species may feed on a particular range within the community or even during different times. Variety of habitats in a community characterised by its own diversity may attract different species

• Another cause of diversity may be the creation of different habitats within a region by periodic disturbance

• A community that forms if the land is undisturbed and that perpetuates itself for as long as no disturbance arises is called the climax community. When this climax community is disturbed – either by natural forces or man-made activities such as floods and fires or removal and overexploitation respectively, it begins the process of returning to its original state by a process known as ecological succession that produces a climax community again.

• There exist two types of ecological succession: primary and secondary succession• Primary succession takes place in a barren or lifeless land with limited soil or no

soil at all (very slow process)• Secondary succession is the series of community changes that takes place in

disturbed areas that have not been totally stripped off their soil. It is the most widely occurring phenomenon than primary. It is important to note that ecological succession is a process that takes place over relatively long periods with secondary succession being the least with about a few years to a hundred (Arms & Camp, 87)

• The third cause of diversity of species within communities lies with the climate.