Lecture 3: Geographies of biodiversity 4

Question 1

Complexity in Succession

Answer 1

Now accepted that succession is very complex and contains many elements of randomness or stochasticity (e.g. propagule availability, interspecific and intraspecific competition, site characteristics, disturbance)

• Allogenic succession = externally induced
• Autogenic succession = internally induced

Question 2

Ecosystem changes

Answer 2

There are several important changes in plant communities and their ecosystem during the seral process

1. In plant morphology and organisation:
   • Increasing height of dominant vegetation • Increasing variation in growth form
   • Stratification of communities

Question 3

Ecosystem changes- abiotic environment

Answer 3

soil characteristics (breakdown of parent material , soil formation, increasing organic matter, pH, texture, structure and horizon development)

• light intensity (increased shade)
• relative humidity/hydrological cycle • temperature

Question 4

Climax concepts

Answer 4

‘Climax community’ represents the endpoint of the successional process, after which further changes in community do not take place

• There were (historically) two main arguments regarding climax stages – Clements maintained that all successions within a climatic region would approach the same climax stage (mono climax)
• Other views are variations of Tansley’s theme that other abiotic factors (e.g. soil, topography, moisture) would also influence the climax stage (polyclimax model and its variations)

Question 5

Mono climax succession

Answer 5

‘Vegetation in stable equilibrium with climate and soil, given undisturbed conditions and free drainage’

• Concept of the plant community as an organism developing into a mature state, which would be in equilibrium provided the environment was stable
• The same climax would be achieved for a given climatic region no matter what the starting point (e.g. oak woodland in Southern Britain)

Question 6

Poly climax succession

Answer 6

Refuted the concept that communities behaved as organisms, and that there would be one climatic climax in a given location

• Other environmental factors would be strong enough to hold a community stable for long periods of time so that a climax was not reached (e.g. soil drainage/chemistry, topography, or the influence of humans and other animals)
• In a given climatic region these factors would mean that different plant communities would be in a stable equilibrium for long periods of time

Question 7

Mosaic Succession

Answer 7

Similar to the polyclimax theory, except that patterns in environmental and biotic pressures are observed which relate to climax pattern, and so a mosaic of ‘climax’ vegetation types is formed

• Noted that 40% of plant communities could not be placed in climax communities and that a gradient existed between community boundaries – these are ecotones, and are a very important concept in ecology

Question 8

Cyclical Climax succession

Answer 8

Explains the mosaics observed in climax vegetation as being due to cyclical patterns of growth and decay (based on observations of tropical rainforests)