Palaeo - Productivity and macroevolution Flashcards
Controls on body size
• Maximum body size is an oft-discussed property of the palaeontological record.
• Cope’s law asserts (imprecisely and unprofoundly) that maximum body size tends to increase within a given lineage over geological time.
• But is there a physical limit to maximum body size, and are whales at it?
o Oxygen circulation limit?
o Evolution should find a way.
• One upper bound on body size is given by the flow of energy up food chains.
o The largest animal needs to be able to find enough food to sustain it. And in the planktic realm (open oceans), predators are always larger than their prey – specialized prey-capture appendages would cause prohibitive drag, so swallowing is the only means of eating.
• Law of the sea = big eats small
o Prey length = ~7% of predator length
o Predator volume = 3000x prey volume
o 17-fold size differential
o Volume differential = 1000-fold
What factors add to or reduce the amount of biomass at a certain size class (i.e. level in the trophic pyramid)?
- Predator eats prey
- Birth rate
- Death and decomposition
- Evolutions
- Excretion
- Parasite – breaks the law of the sea
Sea trophic pyramid
Sea = skyscaper
• Biomass is the same at every level
• Population decreases up every level
• Gene pool becomes too small at a point so there is a size limit despite biomass not being the limiting factor
Growth efficiency (scaling of growth rate with mass)
- Metabolic rates decrease up pyramid
- Doubles its biomass in time proportional to its size because more inefficiencies when getting bigger
- Larger animals live slower – slower heart beats
- Smaller animals – quick – quick breeding – quick feeding – short lifespan
Biomass changes through time
• Taller skyscrapers increase the standing crop of biomass ‘for free’: each additional storey can be sustained with no additional primary productivity.
• Diversity increases through Phanerozoic
• Biomass has increased through time
• Cenozoic organisms are at least as common as Palaeozoic organisms were (at least if we consider the most common groups of organisms from each time period)
• Cenozoic organisms had, on average, more biomass than Palaeozoic organisms – larger body size, and even at same body size, more of the body is ‘biomass’ rather than ‘space’
• Does this indicate an increase in primary productivity?
o Default hypothesis
o Or could it simply be that Cenozoic orgnaisms were more efficient than Cambrian ones?
• Increase in ‘meatiness’ over time
o Trilobita = just skeletal
o Gastropod = muscular
Improved efficiency argument
Evidence for improved efficiency: increase in deposit feeding in deep (nutrient poor) waters – Nereites ichnofacies – attributed to evolution of new hyper- efficient biological solutions.
• Counterargument: simply less food in these settings in the Cambrian (because less productivity).
o Nutrients in mud are scarce
o Burrowing s energetically expensive
o Depth and frequency of burrowing increases in Phanerozoic
Less complex therefore
Increase in efficiency less likely
• Support for counterargument: Deep-water burrowing occurs in Cambrian where nutrients supplied.
• In modern oceans, burrowing depth tracks surface productivity.
Efficiency does allow larger body sizes when resources are scarce (less energy lost to metabolism). But where resources are freely available, efficient organisms have been displaced by energetic organisms.
Efficiency: The ‘Starbucks Effect’
- Active modes of life have displaced efficient modes of life to nutrient-poor (oligotrophic) refugia – to see more fuel-efficient cars, just make fuel scarcer (dearer).
- “High energy” lifestyles include active burrowing (pushing sediment aside is wasted energy), predation (failed pursuits waste energy).
- These lifestyles can incidentally complicate the ‘easy’ lives of more efficient organisms.
- Interestingly, lineages do not tend to change their energeticness / efficiency / effectiveness of nutrient capture very much as they evolve. Rather, new strategies for effective resource use evolve suddenly as a ‘new major group’ develops in a new trophic guild (which came first is an interesting question).
- Indeed, lineages seem to show a trend to decreasing evolvability through time: all the ‘low hanging’ evolutionary fruit have been picked, and subsequent mutations are more likely to have a smaller effect. (cf. ‘key innovation’ model of evolution.)
- As such changes in energetics of organisms tend to represent the arrival of new, ‘caffeinated’ lineages (‘bullies’), which displace existing lineages into oligotrophic refugia.
Biological bulldozing:
• Hyper-efficient slow lifestyles can be disrupted by high energy lifestyles e.g. burrowers churning up sediment
• Hyper-energetic becomes more effective as you’re getting more food
• When resources become scarce the lifestyle is more costly than efficient organisms
• Hyper-energetics are selected against
• Efficiency drive: efficient lineages stay efficient
Indeed, the Recent Fauna (III) predominantly comprise high-energy taxa.
Efficiency-diversity trend
• Early on in organism’s history:
o Burst of diversity
o Slowly lose the less efficient and species become less diverse
o Organisms in I were more efficient and rarely moved to get resources, by II they are moving to get more resources
Increase in competition
More niches
Decrease in efficiency
Increase in biomass over time
• Nutrient source: Land
o Land plates in Ordovician
o Forests in the Devonian
o Rocks weathered by roots = nutrients
o Did this spur increase in primary productivity?
• Influx of Super-Noval Fe60 at Pilocene-Pleisotocene boundary = nutrients
• More productivity = more diversity
Ediacaran–Cambrian transition
o Ediacaran ecosystems are dominated by sessile primary producers.
o By the second epoch of the Cambrian, ecosystems are much more modern in character, incorporating motile burrowers, predators, suspension feeders and more.
o Many factors have been proposed for this overhaul of life’s biodiversity.
