Palaeo – Histories and Succession Flashcards
Ecological succession
• Over the course of a number of years, any fresh environment will rapidly be colonized by generalist taxa with a high dispersal rate that can grow rapidly in any environment. Eventually, specialists that are better adapted to that environment will gradually arrive, ultimately displacing the original generalists.
• Note that these timescales are too short for evolution to be a significant factor. We’ll explore how the character of ecosystems has changed over geological time (millions of years) later in the course.
o Ecological timescale: 10s-100s years
1. Pioneer species colonise the land surface
2. Abiotic conditions are hostile - pioneer species are able to survive as they are well adapted
3. Pioneer species change the environment/ abiotic conditions
4. Conditions become less hostile
5. New species form
6. Pioneer species are no longer well adapted and so die
7. Different plants that are better adapted out-compete those that aren’t and are more dominant
8. Ecosystem becomes more complex - biodiversity increases
9. Climax community forms
10. Ecosystem is stable
THE EQUATION
PLZ LOOK
distinguish two biological strategies
- 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 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.
Pioneer community traits
- r selected species (when large K-N)
- Reproductive rates dominate survival of a species in certain scenarios
- Little competition
Climax Community traits
• K selected species (when low K-N)
• Reproduction rate controlled by how much space is available + resources
• Leads to competition
• Good competitors survive
• Reproductive rates lower
o Take more time nurturing a more specialised offspring rather than many simple ones
Properties of r and K-strategists
r-strategist • Low complexity (seaweed) • Many eggs • Little amounts of nurturing and specialisation; generalists) • Filter feeders • Little competition • Rapid growth k-strategist • High complexity (whales) • Few eggs • Low infant mortality • Nurtured and specialised • Predators • High competition • Slower to reach maturity
Size frequency and r and k strategists
A size-frequency curve plots the relative frequency of individuals of a certain size (a proxy for age) in death assemblages. 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.
r- strategists - a bell curve in the middle - what is ideal is a decreasing slope
K-strategists - a bell curve towards the end of the spectra (older/larger bodies) - ideal is increasing slope
Survivorship curve
- 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) - (r-Strat -Poorly adapted for survival but those who survive (youth) will go on to survive for a while)
- 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 (best chance - k-strat)
Measuring diversity + trophic nucleus
Pioneer communities are typically dominated by a low diversity of r-strategists, whereas climax communities usually host a more diverse suite of K-strategists
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.
Species past trophic nucleus are dominant species (80% of population). How high this point is shows diversity + dominance.
• 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
Rank-abundance plot
(sometimes called a Whittaker plot) shows the way that diversity is spread between species.
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. A straight line on a rank-abundance plot fits with a geometric model of species diversity - The geometric model applies well to ecosystems in the early stages of succession, or in stressed environments.
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 - climax community
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.
Geometric model
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
Disturbance
In fact, maximum diversity is maintained when an environment is intermittently disturbed, allowing the co- existence of r- and K- strategists
What sort of factors might disturb marine communities?
• Eutrophication
• Turbidity current
• Storm
• New predator
• Environment changes
o Ocean acidification
o Temp change
• Hydro thermal vents
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.
Evolutionary succession
Sepkoski’s Cambrian, Palaeozoic and Modern faunas are characterized respectively by high, medium and low origination and extinction rates.
The Cambrian fauna contains many generalists, whereas many members of the Modern fauna have more specialized ecologies.
