Trophic Levels and Dinosaurs?

acurro

New member
Great article!

I have a question not related to reef tanks, for once. If the trophic levels as they exist now do so because the number of primary producers can only support so many predators, why were animals so much larger several millions of years ago? Dinosaurs are so much larger than modern elephants, sharks were larger, even mammouths existed in cold environments. Icythyosaurus, for example, existed eating fish. Was primary production larger then? I have three thoughts, I am not sure if any are correct:

1) So many nutrients have become locked beneath thermoclines over the ages that there are decreasing number of nutrients to fuel primary production.

2) There was more carbon dioxide in the atmosphere then, fueling primary production.

3) Biological dice rolling!
 
Actually I seem to recall that the oxygen levels were higher, which would help oxygenate those massive bodies.
 
If oxygen levels were higher it would result from a higher level of carbon dioxide, since free oxygen in the atmosphere is created by plants, which would support what I think happened of higher levels of primary production at that time.
 
Certainly there was a lot of biomass during the age of the giants: lot of algae. The day/night oxy co2 cycle would demand it, and the oceans were likely quite full of plankton, massing probably larger than the population of giants, keeping it all going---but quite fragile in its way: a biological decline, such as at the alleged asteroid impact, would hit such dependent systems hard, and an oxygen crash would change things pretty fast.

Not to mention the breakdown products like methane and nitrogen.
 
One of the points of the article is that the number of trophic levels existing are limited by the primary producers the higher levels are dependent on. E.g. (and solely for clarification), 1000 lbs. of phtyoplankton makes 100 pounds of zooplankton, making 10 pouts of sardines, making 1 pound of tuna flesh. Why in the time of dinosaurs was there, for example (and again for clarification), 10000 lbs. of phytoplankton making 1000 pounds of zooplankton, making 100 pounds of sardines, making (for example) 10 pounds of tuna, making 1 pound of icthyosaur. What was different then from today that allowed for more trophic levels? Was there any difference at all?
 
Wow, so you guys have got your thinking caps on I can see. Good for you :thumbsup:

If the trophic levels as they exist now do so because the number of primary producers can only support so many predators, why were animals so much larger several millions of years ago?

But this varies from group to group. The largest animals to ever exist (as far as we know) exist now--cetacians like blue whales are far, far larger than any dinosaur or anything that has existed before (as far as we know).

Dinosaurs are so much larger than modern elephants, sharks were larger, even mammouths existed in cold environments. Icythyosaurus, for example, existed eating fish.

Yes, some dinosaurs, most especially many of the sauropods, were incredibly large as land animals go and without doubt were much larger than any living land animals, but they were mostly gone or on their way out near the end of the Jurassic. Most of the dinosaurs during the Triassic and Cretaceous did not attain nearly these proportions. A lot of the larger fauna alive now and especially within the last, say, 3 million years (e.g. the recently extinct megafauna) were comparable in size to the dinosaurs during these periods. Megaladon was definitely bigger than extant sharks, but most large shark species over the fossil record aren't necessarily any bigger than extant species like white sharks. Also, let us not forget about the massive whale shark which is proportionally similar in size to Megaladon, though a planktivore. Icthyosaurs were ecologically equivalent to modern cetacians, and many modern cetacians are much larger than any ichthyosaurs.

Was primary production larger then?

Probably, but not by that much (ca. 10% if I recall correctly, and I may not be so don't quote me on that). I'll explain below.

1) So many nutrients have become locked beneath thermoclines over the ages that there are decreasing number of nutrients to fuel primary production.

Good thought, but not the case. Going back to the first article we see that the C and N cycles truly are cyclical. The P cycle is in some places cyclical and in others sort of a one-way trip, depending on geologic activity. Falling below the oceanic thermocline (and to a lesser extent the halocline at the salinity maximum in the tropics) does not bury those nutrients forever but merely makes fast access to them by phytoplankton up in the photic zone impossible. These nutrients are brought back to the photic zone/atmosphere in areas where the water is upwelling (e.g. especially around the poles, but other places too). The nutrients are not lost--they are never lost--they are simply hetergenously arranged in the environment.

2) There was more carbon dioxide in the atmosphere then, fueling primary production.

There was actually, though primary production was only very slightly higher (see above). The fertilization effects of increased CO2 on primary production are really not that dramatic. A bigger effect was the generally warmer climate which extended growing seasons. However, as I said, primary production was only a bit higher.

3) Biological dice rolling!

Probably has a lot to do with it ;) It is very difficult to say why a group of organisms grows larger or smaller over evolutionary time in most cases. For example, dwarfism amongst groups sometimes occurs amongst groups on islands due to the reduced amount of space available and the ultimate requirement to maintain a large enough population (otherwise the species simply dies out). For example, on some of the Aleutians in Alaska there was a race of mammoths that grew to be only 5 ft. at the shoulder--much smaller than those living on the mainland. On the other hand, sometimes species produce giants when they reach islands because a potential niche is open and they are able to evolved to fill it. So, why does a species develop in one way and not another? The simple answer is selective pressures, though figuring out what those pressures are is a different story.

Actually I seem to recall that the oxygen levels were higher, which would help oxygenate those massive bodies.

Not in the time period we're talking about. Oxygen concentration has been pretty stable over the last 500 million years, if I recall. Efficient circulation would have taken care of this requirement (look at cetacians).

Why in the time of dinosaurs was there, for example (and again for clarification), 10000 lbs. of phytoplankton making 1000 pounds of zooplankton, making 100 pounds of sardines, making (for example) 10 pounds of tuna, making 1 pound of icthyosaur. What was different then from today that allowed for more trophic levels? Was there any difference at all?

Nope. There was relatively similar trophic structure amongst ecosystems then as compared to now.

One thing I do mention in the article, however, is that an assimilation efficiency of 10% is a good estimate for most species, but that it is actually dependent on complex physiological interactions and truly only an estimation.

Something worth considering is that the rate of metabolism of an organism within a group varies allometrically with the size of that organism such that smaller organisms have faster basal metabolisms (and thus need more food and energy) relative to their size than do large organisms.

For example, a vole might need to eat twice its weight in grass every week to maintain itself. A white rhino might need to eat 1/3 its weight in food to maintain itself. Over a given area a lot more voles can live than can rhinos, numerically, but about 6 times as much rhino biomass can survive as can vole biomass. Thus, getting larger requires more food for each individual but the efficiency of the usage of this food is increased. This relationship seems to hold for every group of organims studied so far. So, on a given amount of vegetation one might be able to feed 500 bison but only 1 large sauropod, but the sauropod might have 3 times the mass of the 500 bison...or something like that.

Besides this one should consider where on the spectrum these extinct species and extant species lie with regards to endothermic and ectothermic metabolisms. A lion the same size as a Nile crocodile requires about 16 times as much food to maintain itself. Dinosaurs were probably somewhere intermediate between what we would consider endothermic and ectothermic with some species being downright endotherms, but this could also increase the carrying capacity for these creatures.

Best,

Chris
 
Back
Top