Palaeo - Basics

Question 1

Trophic Structure:

Answer 1

• The trophic structure of an ecosystem describes the flow of resources from primary producers to higher levels of the food chain.
• Primary producers mark the bottom of the trophic pyramid
o Lock up energy from atmosphere
• As a means of generating biomass, eating is around 10% efficient; as such, higher levels of the trophic pyramid represent progressively rarer components of living communities.
o Loss from inedible parts, heat loss and undigested parts
• Primary productivity is the primary source of organic matter, and by far its most abundant form is photosynthesis.
• Photosynthetic organisms’ dwell in the photic zone, the depth to which light penetrates – up to 200 metres in crystal-clear water, but substantially less (50m?) when water is turbid (i.e. cloudy).
• Photosynthesis is 1% efficient (conversion of sunlight)

Question 2

How might the presence of photosynthesizers in a fossil assemblage help you to interpret water depth?

Answer 2

• Water depth will need to be in a photic zone

o But could be turbid – makes it harder to determining depth

Question 3

What are the primary photosynthesizers in marine communities?

Answer 3

•	Phytoplankton (algae)
o	Photosynthetic floating organisms
o	Can go anywhere because they float
o	~90% of marine photosynthesis
•	Seaweed
o	Macroscopic
o	Grow on sea floor
o	Restricted in a sense
o	Do have a larger size so they can occupy larger amounts of space
o	Does mean that their environment is restricted
•	Fossil Assemblage
o	Inverted chain
o	Higher preservation potential at top

Question 4

What would you expect the biomass pyramid of a fossil assemblage to look like in marine and terrestrial ecosystems?

Answer 4

• In terrestrial ecosystems, typical occupants of the higher levels of the trophic pyramid are herbivores (on level two) and carnivores (whether active predators or scavengers).
• Marine ecosystems differ from terrestrial ones in that there is an abundance of food available in the form of suspended organic matter: these particles include phytoplankton (autotrophic microorganisms that are typically photosynthetic and often unicellular), zooplankton (heterotrophic animals such as krill that feed on phytoplankton), and faecal pellets that rapidly transport organic matter to the sea floor.
• Rule of sea = big eats small (~10% as small)

Question 5

Particles in the sea and its relationship with energy in environments

Answer 5

• The fate of these particles, as with any other sedimentary particle, depends on the energy levels at the sea floor.
• In high energy environments, the particles will typically be suspended in the seawater as waves and currents churn them up.
• In low energy environments, the particles will settle on the sea bed and eventually be buried.

Question 6

Suspension vs deposit feeders and their preferences?

Answer 6

• As such, suspension feeders – which feed on particles in suspension – will be more abundant in high-energy environments
o Better for high energy as nutrients constantly re-suspended and thrown up
• Deposit feeders – which extract organic material (particles and the bacteria that degrade them) from the mud of the sea bed – tend to proliferate low energy environments.
o Better for low energy as more material accumulates

Question 7

Sea floor niches

Answer 7

Sea floor
• Filter feeders eat dying organisms from photic zone which sink and cause ‘rain’
• Therefore, presence of suspension or deposit feeders shows energy of environment
Epifaunal: Live on surface
Infaunal: Live in sediment, i.e. Burrowers
• The proportion of infaunal deposit feeders may reflect oxygen levels within the sediment
- More infaunal = more oxygen in the substrate
• But is particularly vulnerable to distortion by preservational processes
• The nature and abundance of sessile suspension feeders may reflect the firmness of the substrate – soupy sediment lacks attachment points and can only be colonized by ‘floating’ taxa (icebergs / snow shoes).
• A high diversity of predators is indicative of a full and diverse community (typical of a stable and tolerable environment) with more biomass and more opportunity to specialize on particular prey taxa.

Question 8

Trophic guilds and classifying them

Answer 8

Plot ternary plot with
Vagrant detritivores (Wandering animals which eat decaying material)
Infaunal suspension feeders
Epifaunal suspension feeders
• Ideally, such a plot should be based on a survey of the total biomass within a community, rather than the number of individuals or taxa – otherwise small taxa will contribute disproportionately to the count.
• Given the difficulties in precisely estimating biomass, a community can be crudely characterized by its trophic nucleus: the species that make up 80% of the community as a whole.
• As communities are typically dominated by a few taxa, the trophic nucleus may only include three or four separate species that give a good reflection of the depositional environment.

Question 9

Trophic guilds and environments

Answer 9

DETPREDSUSP
All suspension - oyster bank
Middle - middle shelf to inner shelf and bay centre moving up and right and then up centred
Substrate niche:
Epsus: oyster bank - high energy
Centre Bay centre
Top right Middle shelf - Low energy

Question 10

Controls on diversity

Answer 10

• the evenness of the environment: a varied habitat will contain multiple different niches, which different species will be differentially adapted to. Note that diversity itself can increase the variedness of an environment: the arrival of a number of clams might offer opportunities for encrusting or predatory taxa and may provide shelter for yet other taxa.
• comfortable environments give rise to high diversity. Fewer taxa can survive in stressed environments, such as brackish water (a mixture of freshwater and sea water, found for example in estuarine environments) or dysoxic conditions, reducing total diversity. In contrast, areas that are high in resources tend to be associated with higher diversity. Diversity tends to peak at low latitudes: one reason being that higher temperatures improve primary productivity (i.e. more resources), another being the creation of a long temperature gradient between warm surface waters and cool deep waters, making for a more varied environment. Diversity also changes during ecological succession: more on this next time.
• Stability allows for time to diversify, niche creation and specialisation possibility
• Resource heterogeneity in habitats. Areas of greater resources such as trees. Allows for diversity gradients.