My friends and I are jointly writing a story set on a huge island that contains dinosaurs. How the dinosaurs got there has not yet been determined in the story; for now, let it suffice to say that the dinosaurs are natural, true-breeding dinosaurs from every continent and age. The island itself is large and varied: The landmass is the size of Madagascar, with smaller islands ranging in a circle around the island for about 100 miles. About 120 miles from the main shore, a huge coral reef that surrounds the entire island, making it difficult for any large creature or any boat to enter or leave. The islands themselves range in size from small outcrops of rock to the size of some of the Hawaii islands. They aren’t especially thick, but if you were to “island hop” from one to the next, you could go all the way around without ever going over 10 miles. Most of these would be small, though, and if you were looking for a major island, you could go 70 miles with only small ones.
The main island is mostly similar to the African Savanna, only wetter. Annual rainfall is higher, the vegetation is thick and green all year round around most of the island. After you cross the midpoint of the island, it starts getting warmer and dryer, with the final third being more like the dry savanna, with the final 30 miles or so being a hot desert. But the majority of the island is lush. To the north and scattered throughout are thick, small jungles, and the whole island except for the desert is full of rivers and lakes. The center of the island has a volcanic mountain range which stretches from the beginning of the desert all the way to the north. The small islands that surround the main island are mostly as follows:
The ones to the South are mostly barren desert. As you head north, the islands become thick jungles. All but the smallest islands are thick forest most of the year, but the type of vegetation is almost entirely fast-growing trees and huge bushes, for reasons that will be discussed bellow. To the very north, the islands become barren and rocky, and a bit colder. The northernmost island is the only Temperate area throughout the land.
NOTE: Next section contains assumptions about dinosaur behavior. These assumptions are not necessarily accurate, but make good cinematic.
Now, my real question: How many dinosaurs can this land sustain? How many:
Small, medium, and large Hardosaurus in huge herds, like what we have in Africa with Wildebeest?
Sauropods in elephant-like family groups, ranging in sizes from Stegosaurus-sized dinosaurs to Brachiosaurus, Titanosaurus, and the like?
Triceratops, Stegosaurus, Anklyosaurus, and other similar animals of similar size?
Small herbivores, such as Protoceratops?
Large predators, such as Tyranosaurus, Carcharodontosaurus, and the like?
Large fish-eaters like Spinosaurus, Baryonyx, and the like?
Medium carnivores in packs like Deinonychus?
Small carnivores like Microraptor, Velociraptor, and such?
And finally: On the outer islands, a large herd of some type of Hardosaurus goes from island to island, eating vegetation and then leaving. The herd works its way around the island in a circle.
How long should this cycle take?
What type of Hardosaur (Or any dinosaur that makes this more likely) should be there?
Is this even somewhat realistic/possible?
Sorry about the long question, and thanks for reading!
Start with some basic calculations. In terms of biomass, herbivores need 10x their own weight in vegetation. Carnivores need 10x their own weight in prey.
It looks like plant biomass ranges through the year, averaging something like 120 g per square meter. Only 20% of that is alive, though. So 24 g/m^2. If we are talking about a 1 ton dinosaur, we need 10,000,000 g of biomass to keep it alive, which requires or 416,666 m^2 - about half a square kilometer. A herd of a thousand of them would require 500 km^2 of productive grassland.
Predators are where the demands really show up. If you want a minimally viable breeding population of at least 100 T-Rex, you need 7,000 tons of herbivores and 3,500 km^2 of grazing land.
Admittedly, Madagascar is about 580,000 km^2, so there’s a lot of potential room for these guys, but not all of that is productive grassland. In your own description, you’ve got a chunk of land area tied up in desert and volcano/mountain.
I think at this point, it’s mostly a math equation. Add up the critters you want, make sure to add some biomass for non-interesting but essential critters (you know, the rats, bees, fungi, ants, flies, etc.) and then see whether the numbers permit what you want.
Sorry for double post, but I can’t find any way to edit my message.
I just wanted to add that the animals on the island OTHER THEN dinosaurs are a mix of prehistoric Mesozoic fauna, and some of the smaller, faster modern creatures: Rats, birds, modern snakes and lizards.
EDIT: There we go, I can edit this one. Now, I am not sure HOW to do these calculations. Could you clarify, or link to a website that will?
EDIT2: One more thing: What size would a true-breeding self-sustaining population need to be? 100 seems small, since when Cheetahs fell pretty low, there was all this talk about their genetics getting messed up, and I wouldn’t want that here.
Lets say 15 grams of foliage per square meter, due to being wetter then the Savanna.
15X1,000=15,000 grams per square kilometer. Madagascar is 580,000 square kilometers, but we loose a bit for mountains and desert. However, we also gain some from thicker jungles. So, 550,000 square kilometers of lush vegetation.
That is 8,250,000,000 grams of plant material. Now, 1/100th of this can be transfered to the next level. Therefore, we have 825,000,000 grams of primary consumers, 82,500,000 grams of predators, and 825,000 of Alpha Predators.
This seems a bit small… Have I made a mistake somewhere?
Babale, I think you’re forgetting to square some of your conversions. A square kilometer is 1,000,000 times a square meter (1,000x longer, x 1,000x wider). So 150 g per m^2 is 150 million g per km^2 (or 150,000 kg or 150 tons to convert units to something more useful).
But 150 is awfully high for the figures I found on savanna. At the peak wet season, it was 200 g/m^2 and only 40 g/m^2 was living. So I’d stick with a number between 20 and 40 for the rich grasslands.
If I were you, I’d set up an Excel spreadsheet and make three lists.
List 1: Terrain types. Establish the size (in area) and edible vegetation density. Add another column for the total edible vegetation so that Excel can do the work. If you want to get fancy, have colums for vegetation density in the wet seasons vs dry seasons, and you’ll be able to use Excel to determine the need for migrations.
List 2: Herbivores. Establish a list of herbivores in each terrain type and make a note of the desired population, the weight for each, and the total weight of the population to be supported. By summing up the herbivores in each terrain type, you’ll know how many can be supported there (by making sure that herbivores are a tenth of the vegetation supported by the terrain).
List 3: Carnivores. This will look the same as the list of herbivores, with a separate list for each terrain type. You can sum up the weight of the carnivores and make sure the number of herbivores can support them (again, by making sure that carnivores are a tenth of the herbivores.
As far as minimum viable populations go… I’ve heard 100 is a minimum to prevent inbreeding and lack of diversity in a population. This was in reference to African cats, if memory serves. Bigger is better, but I think the idea is that 100 is at least big enough so that you don’t have to breed with a sibling or cousin.
I think the oceans and lakes are the only place where alpha predators primarily eat other carnivores. On land, even top predators mostly eat herbivores.
Of course, you do also have omnivores…
If I were you, I’d just keep it simple and do the calculations based on the assumption that everything either eats plants or eats the herbivores.
After all, your fantasy world only needs basic plausibility, not detailed food webs accurate to two decimal places.
How long have they been there? If it’s long enough for significant evolutionary changes, you’d see some major adaptations. Island species tend toward either gigantism or dwarfism at unusually high rates.
Not really. With some exceptions it has to be a fairly small island. And in most cases large animals become small (those over the size of a rabbit), while small animals become large. There are many instances of dwarf mammoths or elephants evolving on small islands. Large animals evolving small size is probably due to a more limited food supply on small islands.
I don’t follow. How is this in any way similar to African savanna if it is wetter with thick green vegetation year round? By definition savannas are characterised by prolonged annual dry seasons and a canopy sufficiently open to support an uninterrupted understorey of grass. IOW by definition a savanna can never be thick and it can never be green all year round.
You do realise that this is pretty much impossible in reality, right? Your island is only about 1000 kilometres across, so you are describing a tropical situation where it gets both warmer and drier with increasing proximity to the ocean. That’s not really possible. It’s either going to get colder and drier or warmer and wetter. It’s also very difficult to imagine how it is ever going to be drier in the 45km adjacent to the ocean than it is in the hinterlands. Oceans are a source of moisture and it takes some really exceptional conditions of altitude conspiring with stable prevailing winds to make coastal areas more arid than subcoastal.
For your island to be believable you’re going to need a significant area of highlands to produce as a rain shadow, and event then area immediately behind that rain shadow will be the driest point, rather than the 45km adjacent to the ocean.
OK, so that situates your islands in northern hemisphere with a prevailing northerly wind.
Ecologically this won’t work. You never discussed this below, but I’m assuming it’s got something to do with your bands of migratory browsers. The problem is that if your browsers are knocking down the trees they won’t evolve into “fast-growing trees and huge bushes”. That’s just a waste of energy. Any woody plant that tries to adapt to regular defoliation by rapidly replacing the woody tissue is doomed to extinction.
In such situations the plants have two options. Either they will develop defences to make them unplatable or they will adopt a growth form that leads to less competitive loss as a rsult of defoliation. In the real world your islands will rapidly become a savanna type system co-dominated by prickly, hard leafed or poisonous woody plants with an understorey of herbaceous plants that can rapidly regenerate biomass following defoliation.
This is very difficult to justify. To get a temperature gradent from tropical to temperate in only 1000 km you really need something to funnel either air or water currents, either a continental coast or uninterrupted parralel mountain ranges. Without those features the temperature gradient will necessarily be much more gentle. Since you can’t have mountain ranges on an island chain, you’ve essentially just placed your island within 1000 kilmetres of a continental coast
Hadrosaur. Not Hardosaur. And the answer is “not many”. There’s a lot more to this than simply multiplying primary productivity by biomass consumption.
The trouble is that you’ve created a fractured ecosystem because of the rapid eco-climatic gradients. With your largest island being only 1000 km across your largest environment can’t be more than 700 km on a side. That’s a tiny range for a large animal. Then you need to factor in the amount of available vegetation. With an environment wetter and greener than a savnna you’re talking about a tall woodland or closed forest community, and that is a terrible palce for a large animal to try and survive. There are lots of good reasons why there is a far higher magafunal density in savannas than in forests, but basically forests suck in terms of finding food and evading predators.
Realistically you’d be lucky to sustain a tonne of large animal biomass per 100 hectares in the type of environment you’ve described. If we assume a single species of small hadrosaur weighs 3 tonnes and they can maximally exploit the environment with zero disease, starvation or predation than we get around 160 thousand individuals. In the real world by the time we factor in multiple competing species, predation etc. that probably equates 1, 500 or so animals.
Your environment could probably support a population of the smaller creatures, maybe as large as 10 tonnes. They’d either be competing with your hadrosaurs or else occupying an alternative niche, which means reducing your hadrosaur niche. Either way your population total of 1, 500 animals remains about the same, just split it betweent the various groups.
But the titanosaurs, forget it. You’re never going to sustain a breeding population of animals that size on an island 700 km across.
Probably none at all. In these sorts of locales large predators are at a huge disadvantage because they need lots of live food regularly. Omnivores and small predators that can act as facultative scavengers are much more competitive in these situations. IOW thinks dogs and bears rather than tigers.
Are these supposed to be marine fishers, or are you going to further reduce your island’s size by having major river systems?
Quite a few, but they’ll likely be acting as scavengers and preying mostly on animals smaller than themselves. That’s not to say they won’t attack adult dinos when the opportunity presents itsef, but it’s going to be tough too sustain both predator and prey populations if such a condition is common. IOW they’ll be filling a niche akin to coyotes.
The first problem I have with this is how it can possibly be competitive with sedentary grazers? If this group defoliates and island and then moves on, but there is a population of smaller sedentary browsers that are permanent residents, how does this migratory pattern give the dinos any more food? Why wouldn’t they return and find that the stay-at-homes have prevented the vegetation recovering?
Bison or wildebeest migrate because the food supply is seasonal. They chase the rains basically. But your dinosaurs can’t be doing that if they are moving cyclically on a small island chain.
In the real world, forever. What you are describing works well for cell grazing, where people fence out sedentary competitors, but in reality the sedentary residents will prevent the vegetation from recovering and will very rapidly settle into a stable relationship.
If there is some magical force that prevents the establishment of sedentary populations then it still makes no sense because the vegetation will respond as described above and you will very rapidly establish a savanna. In such a situation the herbaceous plants will recover in a month or so given sufficient rainfall and the woody plants will either not be touched or will become extinct.
First-Thank you very much for the help. Now, let’s see if we can’t make some of my ideas better.
What I meant was really Temperate Grassland which in this case COULD grow into a woodland, but doesn’t. The forests themselves are not JUNGLE as I’ve accidentally said, but more like Canada’s great rainforests. The reason the forest doesn’t spread into the grassland is the same as in the savanna.
What about other methods of making the area warmer? Maybe not desert, but since there is a volcano, would geysers like in Yellowstone work? We can work with a geyser area.
What about non-woody plants like ferns, tall grasses, and bamboo? These all grow fast, and would quickly rush in to replace them, no?
No argument here. Erasing the whole rocky islands part.
With a temperate grassland, would there be more room for animals? And if the herds are mostly stationary, or if they slowly travel in a large circle around the island, would that not be enough space for them?
Will my fixed environment be better?
Alright.
Not even a token population of 100 or 200?
I was under the impression that Tyrannosaurus, being a reptile, would have to eat less often then a mammal of similar size. And what about if Tyrannosaurus was a scavenger as often suggested? Would that make him fit the “Bear” niche? (NOTE: I personally do not believe he WAS mostly a scavenger, but I would rather not turn this thread into a flame war over this.)
And what about replacing Lions and Tigers with Cheetahs and Pumas? As in, T. rex vs. Allosaurus?
Assuming this won’t work: How big does the island need to be for a breeding population of T. rex?
Good point. Could they have hunted large fish by the shore? I somehow doubt it, but…
Alright. Works for me.
What if they migrate because they need a unique food resource? Like, say, pandas eating only bamboo. What if these guys only eat 1 type of plant, that is found in medium quantities on these islands, so they go through, eat it, and destroy everything else?
We really don’t know exactly where T-Rex fit in the “food pyramid”.
Nor are we sure how many are need to sustain a species. Some breeding programs seem to have been successful with fairly small numbers. 100 used to be a widely used “guesstimate”. But the Whooping Crane (still “Endangered”)seems to have rebounded from a start of only 21, and CA Condors (still “Critically Endangered”) from 22.
Birds are closely related to Dinos (or are Dinos, depending how you look at it), and thus those figures may be enough. At least they aren’t flat out bad science.
Which is why I used the term "seems"and even italicized it. Honestly we don’t know,it likely differs for each species and what sort of breeding stock you started with.
If tyrannosaurs were endotherms (loosely, “warm blooded”), as other theropods are believed to have been by many paleontologists (and as their close relatives, modern birds are today), they would have similar energy requirements to a mammal or bird of equivalent size.
IMHO it’s rather unlikely that tyrannosaurs were scavengers. There are few if any non-flying animals that are purely scavengers today; flight may be necessary to cover enough ground to find enough carcasses.
Regarding population sizes of large animals, an isolated population of Steller’s Sea Cow survived in the Commander Islands off Siberia for (probably) thousands of years after the rest of the species had gone extinct. The largest island in the archipelago is 95 km long. The sea cow was roughly the size of an African elephant (8-10 tons). Its population was estimated at 1,500 at the time of its discovery. Offhand, I don’t know how the productivity of the seaweeds on which it fed would have compared with terrestrial environments.