GEOL2301: Palaeoecosystems

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 (50 m?) when water is turbid (i.e. cloudy).

Here are some questions to help you develop your understanding. The video won’t give you the answers, but should teach you enough that you can attempt an intelligent answer.

• Don’t worry about being 'right': instead, develop a reasoned argument that shows that your answer is sensible, based on the knowledge that you have to hand.
• How might the presence of photosynthesizers in a fossil assemblage help you to interpret water depth?

• Simple answer: Photosynthesis requires shallow water: < 200 m if water is clear (i.e. low sedimentary energy), much less if water is turbid (e.g. high sedimentary energy, or perhaps eutrophic conditions?)

• Caveat: Were the photosynthesizers truly living in the envinroment in which they were deposited? Could they have been washed in, or sunk from shallower overlying waters?

• What are the primary photosynthesizers in marine communities?
• Most photosynthesis is conducted by phytoplankton. Though individually tiny, they can live anywhere in the top 200 m of the oceans – an enormous volume, in contrast to macroalgae, which can only live close to shallow sea floors – i.e. fringing continents. Seaweeds are thus individually large and locally important, but only account for a couple of percent of the total carbon fixation of the global oceans.
• What would you expect the biomass pyramid of a fossil assemblage to look like?

• TLDR: Many biases might cause a fossil assemblage to deviate from the true biomass pyramid. Palaeontologist beware!

• Benthic primary producers might be under-represented as they lack mineralized components.

  Phytoplankton might be under-represented as they are tiny and thus dissolve easily – though where conditions are right (e.g. Cretaceous Chalk) their mineralized skeletons, lacking in nutrients, can dominate sediment, whereas consumers might be more attractive to scavengers.

  Higher levels of the pyramid may be under-represented, as individuals are scarce and fossilization of any single individual is improbable.

  If prey items are mineralized (as protection against predators?) then their empty shells might still sit around for hundreds of years, and thus be over-represented.

  You can probably think of more potential biases of your own…

• Think carefully about each question, and write your answer down, before clicking on the question to reveal its answer.
• Note any questions you'd like to discuss further in the Q & A session.

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 micro-organisms 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.

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.

As such, suspension feeders – which feed on particles in suspension – will be more abundant in high-energy environments, whereas 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.

Where primary consumers live

The proportion of infaunal consumers may reflect oxygen levels within the sediment, but is particularly vulnerable to distortion by preservational processes. (Why might this be? You should think about this again after the Taphonomy session).

The nature and abundance of epifaunal consumers may reflect the firmness of the substrate – soupy sediment lacks attachment points and can only be colonized by ‘floating’ taxa (icebergs / snow shoes).

Quiz

Hopefully you are starting to see how the observed abundance of organisms in different sediment niches can be used to infer the environment. Let’s put this to the test!

• A community contains many suspension feeders, and few deposit feeders. Is it more likely to live in:
• The quiet, still waters of a tropical lagoon
• Hmm... In calm waters, food particles quickly settle to the sea floor. This means that there aren’t enough particles suspended in the water to keep suspension feeders well fed. But deposit feeders can feast on the particles that settle out into the muds – so these will be more abundant than suspension feeders.
• The turbulent waters of a tidal, storm-ridden basin
• Correct! Storms and tidal currents will frequenly lift food particles out of the reach of deposit feeders, suspending them in the water column.
• A community lives in a setting that periodically experiences low oxygen conditions. Are its consumers likely to be:
• Infaunal
• With little oxygen dissolved in the seawater, it’s difficult to meet the oxygen requirements for metabolism whilst buried below the sediment – oxygen’s rate of diffusion is low, so a lot of energy must be expended to keep burrows ventilated.
• Epifaunal
• Correct! Low oxygen levels pose a big obstacle to living within the sediment.
• A community lives below storm wave base on the continental slope. Is it likely to be dominated by:
• Suspension feeders
• Hmm... Below storm wave base, it’s hard to imagine a source of energy that will consistently create currents to suspend food particles.
• Deposit feeders
• Correct! Below storm wave base, it’s hard to imagine a source of energy that will consistently create currents to suspend food particles. What little food rains in from above will settle into the sea floor sediments.
• A community contains many epifaunal individuals, but few infaunal. Is it more likely to inhabit:
• A rocky foreshore
• This highly erosive setting is likely to be amply oxygenated to allow for infaunal organisms: the challenge they’ll face is burrowing into the rocky substrate, which may host a rich diversity of epifaunal suspension feeders and vagrant grazers.
• A sandy beach
• Beach waves introduce plenty of oxygen into the water, and a firm sand can be a great substrate for burrowing – but wave action makes dwelling on the surface less attractive. The balance here favours infaunal organisms.

In view of the environmental constraints listed above, different proportions of suspension feeders, deposit feeders (detritovores) and predators correspond to different depositional environments. Further resolution can be obtained by subdividing suspension feeders according to whether they are epifaunal and infaunal, which provides some indication of the nature of the underlying sediment.

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.

• Don’t worry yet if you’re struggling to take all this in; we’ll cover ternary plots and their interpretation at a gentler pace in the synchronous session.

Diversity itself is controlled by a number of factors. A fundamental control is 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.

On the whole, 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.

Quiz

• All else being equal, which is likely to contain a higher proportion of predatory species:
• A community of 12 species
• Hmm… a low diversity suggests an adverse environment, though I did say that all else was equal. Typically, low-diversity assemblages are especially low in predators.
• A community of a thousand species
• Correct! There are a number of reasons that higher diversity might correspond to a higher proportion of predators. High diversity suggests stability, and gives predators more opportunities to specialise to prey on specific species – creating more niches, and thus allowing more species to be sustained. And each species of predator might in turn be prey to its own species of hunter: more species means that food chains can be longer, adding extra tiers of predators (and thus more species) to the top of trophic pyramids.
• Neither – there’s no reason for one to have a higher proportion of predators
• Think again. How might the number of species influence the height of a trophic pyramid and the length of its constituent food chains?
• Given the differences in the number of species between the assemblages above, the "all else being equal" claim is pretty unlikely. How are the environments of the two assemblages likely to differ?
• A higher total diversity suggests a stable, resource-rich, low-stress environment in which many different specialist species can prosper at sufficient population sizes that finding a mate is never a problem.
• Which of the two communities below is likely to have a higher diversity?
• A community where most taxa are generalists

• Generalists tend to do well in unpredictable environments, where they need to be ready for whatever tomorrow throws at them; and to high-stress environments, when specialists divert too many resources to specialization and so don’t have any spare to cope with stressful conditions. This doesn’t sound like a setting that favours diversity.

  From a more pragmatic perspective, generalists tend not to be adapted to specific niches, suggesting a homogeneous and thus low-diversity ecosystem.

• A community where most taxa are specialists
• Correct. Specialists can occupy niches that are more tightly defined, thus allowing more species to be packed into a given gradient of resource availability, temperature, etc.