Factors Affecting Population Change

Are you Dense…ity-Dependent? There are a variety of factors that can cause a population to change in some manner. These factors can be grouped into two main categories: 1.Density-dependent factors – factors that influence a population more and more as the population increases. 2.Density-independent factors – factors that influence a population regardless of population density.

Density-Dependent Factors Factors that depend upon the population size may include… –Intraspecific competition – the fight for resources between you and members of your own species. –Predation – more predators means less prey. –Disease – Communicable diseases spread more rapidly through more densely populated regions. –Allee effect – your numbers become so small that finding a mate becomes next to impossible and your species may not survive in the ecosystem. The Allee effect is experienced when a population dips below the minimum viable population size.

Density-Independent Factors Factors that affect the population regardless of its size may include… –Climate – rainfall and temperature – these are going to be plentiful or not – high or low – regardless of population numbers. –Pesticides/Toxins – man applies these and they will affect all organisms in the area – especially if it is a broad-spectrum pesticide. –Limiting factors – essential resources required to live – if they are not there in large enough quantities, you will suffer.

Interactions Within Communities

Know Your Role!!! A community is comprised of all the populations in a given ecosystem at a given time. With many different species living within the same community – it is important to recognize that there are a number of different ecological niches present and it is the differences between these niches that allow all of the species to live together in harmony. An ecological niche is an organism’s biological role within the ecosystem. This includes the organisms usage of both abiotic and biotic resources. A fundamental niche is the characteristics of and organism and the resources it would use under ideal conditions. The realized niche of an organism is the characteristics and resources used under the prevailing environmental conditions.

Interactions Between Species Competition – competing for resources required to survive – may be between two different species or within the same species. Predation – A predator species hunts and eats a prey species. Symbiosis – two species maintain a close association with each other – there are three types of symbiotic relationships. 1.Parasitism – One gets a benefit and the other experiences a negative. (+/–) 2.Mutualism – Both benefit from the interaction. (+/+) 3.Commensalism – One gets a benefit while the other is unaffected. (+/o)

Symbiotic Relationships Parasitism (+/–) Mutualism (+/+) Commensalism (+/o)

Interspecific Competition Interspecific competition occurs when members of different species compete for a common resource. There are two forms of interspecific competition: –Interference competition – The two species go head-to- head and fight for control of the resource. An example may be two species of birds competing for space in a tree. –Exploitive competition – The two species consume the same resource and the consumption of that resource by one species limits the availability of the resource to the other species. An example may be two species of carnivore that both compete for antelope in Africa – as one eats more deer, there is less for the other. The amount of competition increases as the amount of niche overlap increases. A limit in the amount of a desired resource also increases the level of competition.

Effects of Competition There are several possibilities resulting from interspecific competition: –The weaker species declines, or is eradicated, from the area. –One species may have to change its behaviour or switch to a different resource. –One species has to move to a new area. Resource partitioning is an option that can reduce competition. In this, the competing species occupy different parts of the same resource. There may be several birds needing resources from the same tree but they may occupy different parts of the tree – trunk, understory and canopy. Another result of interspecific competition may be the increased rate of adaptation and evolutionary change within a species. You are pushed to survive and the traits that allow you to do so become more prevalent in the species.

Resource Partitioning

Predation Predation is an interspecific interaction involving a predator species (hunter) and a prey species (hunted). The two populations are dependent upon one another. A.As the predator numbers increase – the prey numbers go into decline. (Too many killers!) B.When prey go into decline – the predators begin to go into decline. (No food to eat!) C.When predators decline – the prey increase their numbers. (Few killers killing!) D.Prey numbers increase so predators increase too! (Lots of food to eat!) Now the process repeats! This phenomena between the predator and prey populations is known as predator-prey cycling.

Predator-Prey Cycling

Defence Mechanisms A variety of defence mechanisms have evolved in many species as a result of predator-prey interactions. Plants have both morphological and chemical defence mechanisms designed to keep them from getting eaten. –Morphological defences include physical features such as thorns, hooks and needles. –Chemical defences include substances that are distasteful or unpleasant. Some may even mimic hormones of other organisms that scare off the thing that wants to eat the plant! It is interesting to note that as the plants change to defend themselves, the herbivores in turn, change to be able to continue feeding on the plants. It is an interesting case of coevolution in some ways.

Defence Mechanisms Animals Animals may have passive defences such as hiding or active defences such as fleeing from their predators. There is a greater energy cost to the animal that practices active defences. Camouflage is passive and helps some animals blend into their surroundings so they go undetected by predators. Other animals use bright colourations or audible warnings that make them conspicuous, but, they often have very strong venoms. Mimicry may also be employed – this is when one species takes on the appearance of another species that its predator finds very unpleasant. The viceroy butterfly looks like the monarch butterfly which birds find very distasteful.

Defence Mechanisms Plant – Thorns Plant – Needles Plant – Sap Animal – Fleeing Animal – Hiding Animal – Camouflage Animal – Warning Animal – Mimicry

Equilibrium – A Delicate Balance Communities live in balance – they do not exceed their carrying capacities. The animals that are sustained by the community are able to survive for many generations because the environment rarely changes (and even if it does it is usually a very gradual change). The term used to describe the balance in an ecosystem is equilibrium. The introduction of an exotic species can disturb the equilibrium in an ecosystem. The exotic species may: –Have no predators – so it flourishes. –Drains a resource so it is not available for others. –Outcompetes a native species – it flourishes and the native dies. –Economic effects as well – destruction of crops or livestock.

FIN!