TOPIC ECOSYSTEM

Part 4: Relationships, adaption, evolution, natural selection, & survival.

Looking back at the food web in Part 3, each food chain has an interaction

within the biotic (living) factors and the abiotic (environment) factors.

Biotic factors refer to the interactions between the

living organisms of the same or different species.

These may include:

» human activities;

» competition;

» prey-predator;

» parasitism;

» commensalism and

» mutualism.

Abiotic factors refer to the non-living components found in an ecosystem. Among them are:

» soil pH;

» humidity/temperature;

» light intensity;

» water availability;

» oxygen availability;

» space.

Remember that the ecosystem is dynamic and no number of

individuals within a population remains constant.

There will be a change in the population number because of the

interactions between the biotic and abiotic factors.

Figures 1 to 6 on this page not only shows the biotic factors in action but also how it interacts with

the abiotic factors. Consider the condition of the environment they are living in and you will

understand why the inter-relationship between the living and non-living beings.

Figure 1: Mutualism relationship between the clown

fish and the coral reef. The crown fish eats the algae

growing on the coral reef while the coral provides

protection to the clown fish from predators.

Figure 2: A commensalism relationship

between a Killdear Bird and a Cape

Buffalo. The buffalo is unaffected and the

birds gain a mode of transport and

protection.

Figure 3: The Raflessia (commonly found in

the Borneo island, Malaysia) is a parasite that

absorbs nutrients from roots of trees. As a

result, the trees do not obtain the proper

nutrients for growth.

Figure 4: A pack of hyenas and a lioness

competing over food.

Figure 5: (A) Interspecies competition between the H. pilosella

and P. lanceolata. Notice that the latter competes and outdoes

the former. (B) H. pilosella species. (C) P. lanceolata species.

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GROWTH CURVE MODELS

Figure 6 highlights a typical pattern of population in an environment.

This graph is known as the logistic growth model.

A » Individuals are slowly adapting to a new environment especially after birth.

» Individuals well adapted will survive to reproduce; others just die.

B » Abundant resources allow rapid increase in numbers of adapted types.

C » Predation and competition for resources and survival keep the numbers more or less stable.

» Resources can be limited but the individuals can still be sustained.

D » Disease kills many of the population.

» Only the toughest survives i.e. stronger characteristics will be naturally selected and

passed down to the surviving species.

E » Numbers recover as survivors reproduce.

» There is also a possibility that organisms will evolve to improve from their current state.

Change in population number

A change in population numbers:

» is strongly influenced by food availability;

» can be predicted by examining feeding

relationships & the consequences of food changes

on populations that are not directly interacting but

are linked to each other via food chains.

» need to consider changes in population numbers

above and below the initial trophic level which

underwent change.

» can also be caused by diseases and a combination of

biotic and abiotic components which reduces the

number.

In the food web, assume that the number of mice has decreased:

» The population of snakes will decrease as there has been a reduction in the supply of food.

» The population of grasshoppers and rabbits will increase because there is less competition for the

grass; the selection pressure from the mice has been removed.

» The population of hawks will slightly decrease but not significantly as there are other food sources.

Figure 6: The typical pattern for a population in an environment.

Figure 7: A food web in a grassland.

Populations are always interacting with each other.

As a consequence, predator and prey numbers are very closely linked and tend to track each other.

Figure 8s might be a typical pattern of the population changes occurring between prey (mice) and

predators (hawks):

Figure 8: Interactions between prey and predator will influence the population size in an ecosystem.

Communities and diversity

Diverse communities are better able to successfully respond in a sustainable way to environmental

changes which impact on population numbers and species survival.

» Therefore, diverse communities will continue to exist and the species within the community

continue to exist.

Relationships and interactions between both biotic and abiotic factors allow organisms to adapt,

evolve, naturally select (due to pressure points) & survive through the inheritance of strong

traits.

Meiosis in reproduction allows variation & natural selection of traits.

A dynamic environment exists in a diverse community and there is the ‘survival of the fittest’.

Species extinction is very often linked to communities of organisms which have a low level of

diversity.

One of the most important factors that influence the dynamics is the food availability.

The ‘Species Diversity Index, d; is a quantitative tool to determine the abundance and diversity in a

community.

d =

N = the TOTAL number of all organisms in the community

n = the number of organisms of a particular species

∑ = the sum of

» A large diversity index indicates a well-established, stable community with many successful

species.

» A small diversity index suggests a less stable and less well established community of just a few

species.

(mice)

(hawks)