Biosphere Contents

Biosphere - Development - Page 16

Question 1 answers

(i) An environmental gradient is a continuum upon which a plant species can have an optimum point for growth, and tolerance zone delimiting the conditions it can withstand. E.g. environmental gradients could include mean annual precipitation, soil fertility, etc.

(ii) No.
For instance, a fatal stress for one species (such as too high a salt
concentration in estuaries for freshwater plants) could enhance
favourability for another (e.g. salt loving estuary plants).

Question 2 answers

These can include

- avoidance of drought through rapid flowering and fruiting in the favourable periods following rainfall, with dormancy during the long arid periods;

- tolerance of drought by reducing the water needs of the plant, through short stature (thus avoiding wind related water loss), by leaf rolling, and by reducing leaf surface area to minimise water loss through transpiration

- storage of water in succulent stems or leaves (e.g. cacti) ;

- the development of long rooting systems to reach lowered ground water tables.

Question 3 answers

The rate of environmental change must be less than the rate at which an organism can respond to the change.

Question 4 answers

In an ecological community, interactions (i.e. inter- and intraspecific) between individuals can affect their survival. Group behaviour enhances survival for both prey (e.g. fish shoals) and predators (e.g. lion prides). In certain species of social insects, colonies behave as a large organism with differentiation in the form and function of individual members that are specialised to perform particular tasks.

Question 5 answers

A microclimate is the localised climatic conditions experienced in a particular site. This relates to the 'active surface' in the first few metres above the ground - it could be 5 metres thick over water or bare ground or 50 metres thick in forested areas.

Question 6 answers

Microhabitat refers to the localised climate whilst the microclimate refers to the localised habitat.

Question 7 answers

Succesion is the sequential development of a plant community towards an equilibrium community. It is also associated with change in the animal community.

Question 8 answers

Sugar cane was the first land plant to colonise the island.
It was followed by casuarina, forming patches of dense forest,
This was slowly replaced by 'secondary' rainforest species, such as
the fig tree.
It may take hundreds of years for a truly 'primary' rain forest to
appear.
See Topic 9 Figures 1 to 4

Question 9 answers

a) Classic theory - 'relay floristic' or the 'facilitation model'. As pioneer species able to tolerate harsh environmental conditions grow, they alter these
conditions, favouring other species which are better competitors.
b) Initial floristic composition theory or 'inhibition model' - the species
composition largely depends on who gets there first; these species
then try to maintain their hold on the site.
c) Tolerance model - replacement species are those which can
tolerate lower levels of resources than earlier species. The earlier species have little effect on these later arrivals.

Question 10 answers

A sere is a stage in the sequential development of a plant community at a site over a period of
time, i.e. succession at any particular site will progress as a set of seres.

Question 11 answers

i) A climax state is an end point of succession in which the vegetation community has achieved a
dynamic equilibrium with its environment.
ii) In monoclimax theory, only climatic climax communities are seen as true climax communities. These are determined by prevailing climatic conditions.
In polyclimax theory; a wider range of factors, including geology and soils, are seen as being as important as climate in producing distinct climax communities.