Lecture 12: March 23

• Chapter 8 textbook; emphasis on pages 141-end.
• Wells, M., 1998. How old is a fish? In: Civilization and the limpet, p. 28-34
• Wells, M., 1998. The dilemma of the jet  set. In: Civilization and the limpet, p. 188-194
• S. J. Gould, 1994. The evolution of life on Earth. Scientific American, October, p. 63-69; the text without figures is available on the web: 'Contingency of Evolution'.

• Dinosaurs: were they warm-blooded?
• Chemosynthesis: hydrothermal vent faunas

• Focused on one type of organism: autecology (as discussed in Functional Morphology, see lecture 11).
• Focused on community: synecology

Communities are living on a small geographic scale, short time scale. Community-scale processes not well expressed in the fossil record. Focus of study shifted to larger-scale processes: interaction of whole ecosystems and their environments.

• Biosphere
  • Ecosystem
    • Community: how tightly knit? How strong are interactions?

At level of organism:

• Habitat: physical environment of an organism.
• Niche: physical, chemical, biological environmental limits. Difficult to define, does it exist if no organisms fill it?

• Temperature
• Oxygen content (water dwellers)
• Precipitation (land dwellers)
• Salinity (ocean dwellers; plants on land)
• Substrate (sea), soil type (land)
• Nutrients (primary producers); food (heterotrophs)

How to define ecological terms? Marine terminology:

• By where organisms live: shallow-deep zones
• By how organisms feed: e.g, filter feeders, detritus feeders, hebrivores, carnivores, scavengers
• Energy flow within ecosystems (trophic dynamics)

Food webs: complex interaction of organisms, flow of energy through ecosystems. Complex in present world: how to do this for ancient worlds?

Base of all food webs: primary producers

• Dominantly: photosynthesis (use light energy)
• Also: chemosynthesis (use chemical energy): hydrothermal vent faunas.

• Shape of 'food pyramid' on land and in sea.
• Endothermic/Ectothermic food pyramids (dinosaurs)
• Community succession
• Competition

• Difficult to reconstruct communities (local community of organisms). Succession; replacement, competition. Ecological processes occur on space-time scales which are not easily caught in fossil record.
• Shift to larger-scale processes: interaction of whole ecosystems and their environments.

• Life on Earth:  at least since 3.5 Ga.
• Enormous increase in the number of species on Earth, as well as in the disparity: Early life on Earth: Archaea and Bacteria only;  life in oceans only.
• How and when present great biological diversity?

Difficulties in estimating of diversity of life  over time:

1. Fewer old rocks than young rocks preserved; deep-sea floor subducted (oldest deep-sea floor ~ 200 Ma, Jurassic).
2. State of preservation generally decreases with greater age.
3. Amount of fossil-bearing rocks does not simply decrease with time:  periods of low sea level had small regions of shallow oceans, few fossil bearing rocks.
4. 'Monograph effect': some intervals if time better studied than others
5.  Soft-bodied species  do not usually fossilize; any time period for which such a locality exists with special preservation (Lagerstatten, see lecture 1) will appear to have an unusually high number of species.

In reconstructing past diversity reliably, we can thus only use some groups, the most common of which are all marine, shallow-water invertebrates with shells (e.g., Brachiopoda, Mollusca. Cnidaria, Echinodermata).

Which taxonomic units do we use in trying to study changes in diversity over time?

• Number of species ideal, for most cases not known; use genera or families (see discussion in lecture 8) on mass extinctions,
• Only a single of a larger unit (say, a family) needs to be found as a fossil to recognize the whole family as 'present'. We have thus a pretty good chance of finding at least one species of such a large grouping.

Diversity of Life during the Phanerozoic (textbook pages 140-141):

• Marine organisms (left ): at the beginning of the Cambrium (about 545 Ma) hard-shelled animals became common (lecture Cambrian explosion). Abundances of species and higher units increased until the end of the Silurian Period (with some periods of decrease), remained overall stable during the Paleozoic, then  a sharp drop at the mass extinction at the end of the Paleozoic (lecture 8). Diversity recovered gradually during the Triassic Period, kept increasing, and went up sharply during the Cretaceous. At the end of the Cretaceous the second largest mass extinction set diversity back again. During the Cenozoic there has been a steady increase.
• Land organisms (right ):  life on land (insects, amphibians, plants) started toward the end of the Silurian, increased during the Devonian Period, and really took off during the Carboniferous. The mass extinction at the end of the Paleozoic also set back diversity on land (lecture 8). After recovery, diversity increased during the Mesozoic, with a strong increase in the Cretaceous, a small fall-back at the end of the Cretaceous (mass extinction, lecture 8), and strong increase ever after until today. 

Family-level diversity of marine and continental organisms, showing a strong overall increase over the Phanerozoic (S. M. Kidwell and J. J. Sepkoski: The Nature of the Fossil Record. Paleontological Society Spec. Publ. 9, 1999, p. 61-76); the lower curves indicate the knowledge in 1982, the upper curves in 1992.

Overall seen as resulting from various biotic innovations, biotic interactiuons, interrupted by mass extinctions.

Innovations

Largest scale: Cambrian explosion, establishment life on land:

Major innovation in marine realm: tiering.  'Layers' of habitats on ocean floor, with organisms sticking out above the sediment and collecting food by sieving sea water at various levels. Animals crawl over the mud, and burrowing to various depths. In the beginning of the Paleozoic this tiering did not exist, and animals lives only on  and shallow in the sediment (no deep burrowers).

Mass extinctions: main causes probably external to the biota lecture 8. Mass extinctions may be followed by periods of not just recovery, but also reorganization of ecosystems. Example: biota living in shallow oceans (inner shelf) move off-shore. Trilobites after extinctions at end Cambrian replaced by brachiopods/crinoids; these were after end Permian extinction replaced by molluscs.

Do alternations between  stasis  and rapid change involve communities of organisms and whole ecosystems? Strongly debated by paleontologists; ecosystems varying from local to global, and time scales from a few million years to the whole Phanerozoic.

Evolutionary Faunas have been recognized over the last 545 Ma (Jack Sepkoski; text book p. 140-141). In such EFs we describe the occurrence of guilds, groups of animals that make their living in specific ways. We distinguish, for instance, organisms that sieve their food from the water (suspension feeders), organisms that eat mud and digest the organic matter  (detritivores), carnivores, scavengers.

1. Cambrian Evolutionary Fauna: 540 to 510 Ma; trilobites, inarticulate brachiopods, and ancient snail-like creatures. Low diversity, mainly detritivores and carnivores; few suspension feeders; few planktonic and swimming organisms; simple  food webs were probably simple. Tiering  little developed.
2. Paleozoic Evolutionary Faunas: 510 through 250 Ma; corals, articulate brachiopods, rich Echinodermata, mollusca (clams, snails and squid) and floating graptolites. Increased diversity, suspension feeders common, in addition to detritivores and carnivores, tiering; planktonic and swimming organisms common; food webs of intermediate complexity.
3. Modern Evolutionary Fauna: 250 Ma-now. Common mollusca(particularly snails, and clams), fish in the water column, sea urchins as most common Echinodermata, marine reptiles/mammals. Diversity high, suspension feeders, detritivores and carnivores, Tiering is very complex (but epifaunal tall orgnisms died during out during mesozoic; text book p. 143), planktonic and swimming organisms common,  food webs  highly complex.

These Evolutionary Faunas reflect increased diversity as organisms detected or invented more and more different ways to make a living, although set back by mass extinctions. Note major increase in diversity during Mesozoic: Mesozoic Marine Revolution. Note that during these revolutions life became more and more energy-intensive: faster swimming, more active life styles, more predators.

• EVOLUTIONARY FAUNAS: 3 in 540 Ma, global. Evolution of diverse groups; appearance of main guilds. Diversification of life, discovering and inventing new niches to live in and new ways to make a living; disrupted by large extinctions.

• 'Incumbency': if someone is in place, (s)he is difficult to dislodge. The incumbent species has the advantage over a species trying to migrate in  because there are already so many specimens of these species present. Immigrant species have small populations, which can easily become extinct as a result of relatively small environmental disturbances.
• 'Ecological locking':  interactions between existing species make it more difficult for immigrant species to become established because the immigrant species does not easily fit into the existing network of connections between species, and thus can not compete. Immigrants can move in successfully only when the existing ecosystem has been disturbed.

The response of ecosystem to changes in the environment is thus not simple: for long periods, continual changes have little effect on the biota which can tolerate the changes, until a very small additional change is the 'straw that broke the camel's back', and the system collapses.

Relatively rapid changes in the environment that have been implicated include changes in climate, changes in sea level, changes in the amount of oxygen dissolved in sea water, changing ocean currents as continents move and collide, the number of continents, impacting meteorites and erupting volcanoes: the same causes as invoked for mass extinctions (lecture 8).