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I've been wondering about insular dwarfism, and would like to know if it only affects certain animals (Since some animals grow larger upon colonizing an island, often due to the absence of predators.) Do only big animals get smaller, and if so, how big is "big"? Are there any taxonomic groups that are more or less prone to insular dwarfism?

The second thing that I'd like to know is the minimum size of an island without insular dwarfism could be. Also, I'm not sure if resources, biodiversity or climate affects it too (It wouldn't surprise me, or those who support Bergmann's rule.) This may sound like a lot of questions, but the main components of an answer I'd like are shown in the title - Who and Where.

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    $\begingroup$ Have you asked biology.SE? $\endgroup$
    – RonJohn
    Apr 3, 2018 at 18:20
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    $\begingroup$ @RonJohn This question is perfectly fine here. As a reminder you are welcome to suggest other options to users posting question but it is their choice where the question should be. This question is not off topic for WB. Also please keep in mind how experienced users are when addressing them. $\endgroup$
    – James
    Apr 3, 2018 at 18:36
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    $\begingroup$ @James then what question isn't fine in WB? $\endgroup$
    – RonJohn
    Apr 3, 2018 at 18:43
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    $\begingroup$ @RonJohn Now you're getting it. The thing with building a world is the best thing we have to go on is the real world we inhabit. Anything we create or ask questions on is based on how the real world works. If I am setting up a world and need to understand how something works in reality world building is a good place for those questions. $\endgroup$
    – James
    Apr 3, 2018 at 19:26
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    $\begingroup$ @James This is yet another example of "what if..." science questions which have no Worldbuilding aspect at all. I do not think this question is "fine" here - I think it's an example of how WB SE is becoming a dumping ground for questions of this "what if..." type. That's not what Worldbuilding is for. $\endgroup$
    – StephenG - Help Ukraine
    Apr 3, 2018 at 20:25

2 Answers 2

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Dwarfism is only island based because of the resources and competition not the island size. Many islands do not have it. Many islands have gigantism instead in some species. Some have both. Both dwarfism and gigantism occur on continents as well depending on habitat and competition.

Foster did a study and came to many interesting conclusions, basically some sorts of animals eg,. rodents tend to grow bigger, others shrink. Biologists call this 'Fosters Rule'.

Birds seem to be all over the place in terms of size though. NZ had many huge species and several small ones. The largest eagle the Haast Eagle, Moa's, the largest parrot as well I think.

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You've got two forces at work here: Insular dwarfism vs Island gigantism. This balances out population numbers with available calories.

So let's say the hard limit on resources means there can only be 100 plaid-tailed lemurs in the island at their current size. If there's evolutionary stress (limited resources, high predation), the tendency is towards smaller but more numerous. If there is little to no evolutionary stress, then size is no longer on the list of traits selected-for, so gigantism can occur in exchange for fewer members of the species.

Strangely, smaller animals have a tendency to get bigger, while bigger animals tend to get smaller. Some theorize it's due to the fact that smaller species (like rodents) are able to self-regulate population sizes, whereas bigger ones tend to overstrip their ecosystems and crash towards extinction.

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    $\begingroup$ Considering what you said about evolutionary stress, does that mean that on a food-rich island apex predators would not experience dwarfism (With ample prey and 0 predation.)? $\endgroup$
    – SealBoi
    Apr 5, 2018 at 9:59
  • $\begingroup$ I don't think we can categorically say, but yes, the odds appear to be in its favor. $\endgroup$
    – Carduus
    Apr 5, 2018 at 13:40

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