3 CLASSIFYING ASSEMBLAGES

Question 1

What was the general recognition in the 18th and 19th century regarding species’ distribution?

Answer 1

Certain species occur in particular environmental zones or regions

Question 2

What were the observations of Pruvot 1897 regarding the conceptual basis of zoogeographic zones?

Answer 2

• Seafloor not homogenous patch
• Actually composed of series of zones that subtly differ from one another
• These areas reflect variations in environ conditions and faunal distribution, often over space of few m
• Pruvo referred to these sediment-faunal associations as FACIES
• Southern distinguished what we now call infauna and epifauna, so together they recognised importance of interaction betw biotic and abiotic components of the environment

Question 3

What is biocoenosis? (Mobius 1877)

Answer 3

Bi-directional interaction between fauna and sediment biotope

Question 4

What is the generally accepted definition of an assemblage? (Mills 1969)

Answer 4

A group of organisms occurring in a particular environment, presumably interacting with each other & the environment, and SEPARABLE BY MEANS OF ECOLOGICAL SURVEY from other groups

Question 5

What did Carl Georg Johannes Petersen (1860-1928) recognise the limitation of? What did he develop? Why was his work prominent?

Answer 5

Sampling methods at the time, mainly fishing nets/dredges

Recognised that a dredge is a selective sampler which doesn’t sample the whole assemblage, therefore doesn’t provide a representative sample of the assemblage.

He developed the grab to standardise sample retrieval.

He had lots of data to back it, and tested and developed his ideas using the data.

Question 6

What ideas did Petersen develop using his data?

Answer 6

1. Characterising species - assemblages could be described by their characterising species
2. Constancy and dominance - 2 most important features defining a characterising species. Hence, characterising species weren’t seasonal, but constantly present, and were numerically and/or biomass dominant, so regarded as ‘typical’
3. Community definitions - applied his characterising species rules to his data to demonstrate utility of the approach, and defined 7 major community types

Question 7

What are the 7 major community types developed by Petersen?

Answer 7

1. Macoma balthica community
2. Brissopsis community
3. Echinocardium community
4. Astarte-Macoma Abra community
5. Venus community
6. Macoma calcarea community
7. Haploops community

Question 8

What did Jones 1950 suggest to be the most likely factors to affect distribution of communities, and what did he introduce?

Answer 8

Temperature, salinity and sediment type
Introduced zoogeographical regions that define the range of physical conditions within which each identified assemblage exists.

Question 9

What are Jones’ zoogeographical regions?

Answer 9

Arctic (-2 to 7°C, northern)

• Shallow water (Macoma)
• Offshore sand (Venus)
• Offshore muddy sand (Astarte)
• Deep mud (Foraminifera)

Atlantic boreal (3 to 16°C, southern UK to Arctic)
3 tiered hierarchy:
A Shallow water and brackish
B Offshore communities
C Deep communities - hard or soft bottom - up to 3 sed types

Question 10

What were Thorson’s (1957) idea rules for dominant and subdominant species?

Answer 10

1st order species:
Conspicuous, 1 definitive community, 50% of hauls present, 5% total biomass

2nd order species:
Confined to parts of 1 definitive community, 50% hauls present, 5% total biomass

3rd order species:
Not confined to 1 definitive community, found in quantity in others, 70% of hauls present, 10% total biomass

Associated animals:
Common animals, 25% of hauls present, 2% total biomass

Question 11

What are the 7 communities Thorson recognised using this approach?

Answer 11

1. Macoma
2. Tellina
3. Venus
4. Abra
5. Amphiura
6. Maldane-Ophiura
7. Amphipod

Question 12

Explain Thorson’s concept of parallel communities.

Answer 12

If you travel to different parts of the world that share similar environmental conditions, the structure of the community tends to be the same but with different species.

Question 13

What is the issue with Thorson’s theories on parallelism?

Answer 13

Occurrence of parallel communities not as widespread as Thorson thought.
Latitudinal gradients in benthic species diversity (higher diversity in low lats) create difficulties in applying principle universally as Thorson had suggested.