GEOL2301: Palaeoecosystems

Rapidly-dispersing generalists can be discriminated mathematically from slow-and-steady specialist competitors based on the formula

where:

• N = number of individuals
• r = reproductive rate, and
• K = carrying capacity of the environment.

The equation can be expressed verbally as:

The change in the number of individuals per unit time (dN/dt) is equal to the number of offspring per individual (r), multiplied by the number of individuals (N), multiplied by the proportion of the carrying capacity that is unoccupied (K − N / K).

The equation predicts that the population of an environment will grow exponentially until it approaches the carrying capacity, at which point it will level off.

When an environment is empty (perhaps a storm deposit has just smothered the pre-existing community), the value of r is the dominant control on population size.

When an environment is ‘full’ (N ≈ K), conversely, the population size is controlled by the value of K; successful taxa are those that can best compete for the resources that limit carrying capacity.

On this basis, one might distinguish two biological strategies. One strategy, the r-strategy, is to make sure that you are the first on the scene, and can reproduce rapidly once you get there (i.e. have a big r). Then you’ll quickly rise to dominance. The opposite strategy, the K-strategy, is to play the long game, and be a good competitor; it might take you longer to get a foothold, but when population size is limited by K rather than r, you will be better suited to compete for the resource that is limiting the carrying capacity. Next time you take a train journey, you’ll notice r-strategists (silver birch, buddleia) colonizing disused sidings and abandoned platforms, whilst K-strategists (holly, oaks) typify ancient woodland that’s had plenty of time to mature.

An age-frequency curve plots the relative frequency of individuals of a certain age in death assemblages. Pragmatically, it's usually much easier to measure a dead organism's size than to calculate its age, so you'll typically see size-frequency plots as a proxy.

• Remember, by looking at dead organisms we get a picture of typical life expectancy, rather than the average age of members of a living community.

• Use your mouse to sketch size-frequency curves for each strategy:
• • • • • ×
• Clear the canvas by double-clicking the white ×.

Survivorship curves

A survivorship curve is another way of depicting the life history of a species. Survivorship rate plummets rapidly in taxa with high infant mortality, but individuals that survive to adulthood can expect a long life ahead (Type III). Taxa in which the probability of survival is independent of age sit along a linear Type II curve. Taxa in which most juveniles survive to adulthood sit upon a Type I curve.

• Select the appropriate survivorship curve for a typical r-selected taxon
• Type I
• Type II
• Type III
• Live fast, die young: r-selected taxa have zillions of offspring, and only a lucky few find a suitable place to thrive and reach adulthood. This corresponds to the Type III curve.
• Select the appropriate survivorship curve for a typical K-selected taxon
• Type I
• Type II
• Type III
• Slow and steady wins the race: K-selected taxa invest heavily in giving their offspring the best start in life, to maximise their chance of reaching adulthood. This is a Type I trajectory.

Optional consolidation exercise

You've learned a lot of new concepts so far. Why not make yourself a coffee and watch Sir David expound the life cycle of corals (a group with a magnificent fossil record)?

Once you're done, draw a size-frequency (or age-frequency) and survivorship curve for this coral species.

• • • • ×

• Are these corals K- or r-strategists?
• r-strategists
• K-strategists

• Large numbers of tiny offspring with a tiny chance of survival, looking for fresh, uncolonized sites at an early stage of succession to occupy? Hopefully you recognized this as an excellent example of r-selection at work.

  Your size-frequency and survivorship plots should look similar to those you drew for r-selected species above.

• How would your size-frequency curve differ if you were looking at the fossil record of this coral, rather than a complete death assemblage?
• Sketch your answer on the plot above before revealing the answer.
• Larvae have no mineralized or even recalcitrant parts; they have almost zero fossilization potential. The fossil record will only preserve those individuals lucky enough to find a suitable substrate and start secreting a skeleton. But we should suspect a pretty complete record of such individuals, as components of a cemented reef are not particularly prone to size-selective transport. So we may well still pick up the distinctive r-strategist size-frequency shape.

Recap

Let's recap what we've learned so far, with our sedimentary environments hat on – remember, we are interested in what we can learn about original habitat based on our ecological observations.

r-selected taxa:

• Rapidly colonize new environments
  • What factors might render a particular place suitable for colonization?
• Produce loads of offspring
  • High infant mortality
  • Wide dispersal
  • Bloom rapidly when resources allow

K-selected taxa:

• Masters of competition in stable environments
  • What factors characterize stable marine environments?
• Equip offspring with resources to survive until competitive
  • Few, large offspring
  • Minimal dispersal – specialized to existing environment

Pioneer communities are typically dominated by a low diversity of r-strategists, whereas climax communities usually host a more diverse suite of K-strategists.

As such, pioneer communities typically have a small trophic nucleus (the majority of individuals belong to just one or two species that boast a very high reproductive rate), whereas climax communities have a larger trophic nucleus incorporating a wide variety of specialists.

• How many species are in the trophic nucleus of the pictured assemblage?
• The first eight bars (species) are the trophic nucleus: they account for 80% of the total number of individuals.
• What does this suggest about the successional stage of this community?
• This is a large trophic nucleus, comprising more than a third of species. This suggests that there are lots of specialist taxa, each doing quite well at doing their own thing – characteristic of an established community at the later stages of succession.

Dimensions of diversity

Pioneer and climax communities also differ in two different aspects of diversity:

• Species richness addresses the number of species, relative to what might be expected;
• Dominance measures the degree to which the majority of individuals belong to a small number of taxa – how a sample of individuals is distributed between species.

The stacked bar charts on the right depict two communities: each block represents a separate species; larger blocks contain more individuals.

 

• Which community has the higher species richness?
• The left-hand community
• The right-hand community
• Neither
• The left-hand assemblage contains seven species, so is less species rich than the right-hand assemblage with its nine species.

• Which community has the higher dominance?
• The left-hand community
• The right-hand community
• Neither
• Most of the individuals in the right-hand assemblage belong to a single dominant species, so this assemblage is more dominated; the left-hand assemblage has a higher evenness.

• Which community is more diverse?
• The left-hand community
• The right-hand community
• Neither
• Now the answer depends on your definition of diversity! Although the right-hand community contains more species, most individuals belong to just one of these species. Though there are more species there in the right-hand community, you really have to look hard to find any besides the most abundant. Do these extra species really have much of an ecological impact?
  This is why it's useful to distinguish these two aspects of diversity.

Rank-abundance plots

A rank-abundance plot (sometimes called a Whittaker plot) shows how diversity is spread between species. It's a more sophisticated way of depicting species richness alongside dominance. Taxa are ordered by the number of observed individuals, with the rank order plotted on the x axis, and the logarithm of the number of individuals plotted on the y axis.

Use the two rank-abundance plots to answer the following questions:

• Which community has the higher species richness?
• The left-hand community
• The right-hand community
• Neither
• The left-hand assemblage contains seven species, each represented by a point in the plot. This makes it less species rich than the right-hand assemblage with its nine species.

• Which community has the higher dominance?
• The left-hand community
• The right-hand community
• Neither
• The steep slope of the right-hand assemblage indicates that the most abundant species contains many times more individuals than the next most species, and so on. (Remember that the plot is logarithmic.) A perfectly even community would be represented by a flat line.

• Which community is more diverse?
• The left-hand community
• The right-hand community
• Neither
• You might have noticed that these communities are essentially the same as those depicted by bar plots. Do the rank-abundance plots provide a more intuitive picture of how the different aspects of diversity are interrelated?

Extension: Octave plots

With me so far? Things get a bit more complicated with octave plots. These are very useful, and enhance the degree and sophistication of analysis that you can attempt: but if you're already starting to feel out of your mathematical depth, then you should be able to get by without them.

An octave plot is a histogram that provides an indication of how many species are high, low, or intermediate diversity. As with any histogram, the y axis denotes frequency; unusually, however, the x-axis is log transformed, such that the lower bound of each interval (‘octave’) is twice the lower bound of the last (by analogy with the frequencies of a musical scale). The first bar thus shows the number of taxa known from only one individual, the next bar, the number of taxa known from 2–3 individuals, then subsequent bars from 4–7, 8–15, 16–31, individuals.

Geometric model of diversity

A straight line on a rank-abundance plot fits with a geometric model of species diversity. One way of visualizing this model is to imagine that the first species to arrive consumes a given proportion (half, perhaps) of the available resources. The next species to arrive consumes the same proportion of the remaining resources, and so on for each species. When ordered, each species thus contains a half (or whatever fraction characterizes that assemblage) of the biomass as the next most abundant species. The slope of the line determines what proportion of the ‘remaining’ resources each species consumes; a shallow slope indicates that resources are spread more evenly between species, indicating a more diverse community. The geometric model applies well to ecosystems in the early stages of succession, or in stressed environments.

Log-normal model of diversity

An alternative model of species diversity is the log-normal model. This model produces an S-shape on a rank-abundance plot: only a few species are very abundant or very rare; the majority of species are intermediate in abundance. When plotted on an octave plot, a population with a log-normal diversity distribution produces a bell-shaped curve – though because taxa known from fewer than one individual are not observed, the left-hand side of the bell curve will be truncated in small sample sizes.

Recap

Let's recap what we've learned so far, with our sedimentary environments hat on – remember, we are interested in what we can learn about original habitat based on our ecological observations.

Early succession: a few weeds dominate

• Low diversity
  • Few species
• High dominance
  • Small trophic nucleus
  • Dominated by fastest reproducer!
• Geometric distribution of species abundance
  • Linear Whittaker (=rank-abundance) plot

Late succession: complex trophic structures in stable environments

• High diversity
  • Many species, scope for symbioses
• Low dominance, high evenness
  • K-dominated; lots of competitive strategies
• Log-normal distribution of species abundance
  • Niche sizes log-normally distributed?

Maximum diversity is thought to be maintained when an environment is intermittently disturbed, allowing the co-existence of r- and K- strategists.

• What sort of factors might disturb marine communities?
• Some examples, across a range of timescales:
  • Submarine landslides
  • Storms
  • Stranding at extreme tides
  • Freshwater input
  • Earthquakes changing water depth
  • Volcanic activity
  • Eutrophication / algal blooms
  • Periods of dysoxia
  • Predation?

A single site that is periodically disturbed may contain a succession of low diversity biotas (representing pioneer communities) and high diversity biotas (representing climax communities), despite no change in the local environment. In contrast, a stable site (perhaps one dominated by the buffering effect of the ocean) is likely to record a uniformly high diversity.