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This is one of a series of questions centered around how an isolated group of people would survive. Each question focuses on a single aspect of survival. Details about the peoples' situation are below:

In a novel I am developing, a village's worth of people is living on a peninsula. The isthmus connecting the peninsula to the mainland is very narrow, and spanned by a wall, which prevents the people from leaving (there are deterrents preventing them from climbing the wall or otherwise circumventing it). They also cannot swim around the wall. This also means that no land-based animals can cross onto the peninsual from the mainland. The inhabitants have to live with what they have. For the sake of details, assume the peninusla is roughly the size, shape, and location of Mahia Peninsula.

This question revolves around the fauna on the island. What kind of animals would be present? Obviously there would be fish, and birds would be plentiful. I'm focused mainly on the land-based animals. If they can't get past the wall in any way (including burrowing beneath it), and are trapped on the peninsula just like the people, will they thrive, or slowly die out?

What kinds of animals would you find on this peninsula?

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There are several things which need to be considered:

  1. Human impacts on ecosystem: Humans alter ecosystems in tremendous ways. In your case, the smaller the island and the larger the population, the higher the impact. Impacts can include:

a) Tree logging: Generally, it can be assumed that trees (if present) will be gone pretty fast. Timber has always been used excessively due the its importance for so many different things (house/ship etc. building, firewood). One example are the Easter Islands (https://en.wikipedia.org/wiki/Easter_Island). This being said, all animals which depend on forest/trees will be gone probably before the last tree has been logged. This includes for example tree nesting birds, many insects etc.

b) Hunting: Hunting is a big issue, especially if the need of the human population for food exceeds the reproductive capability of the animal populations. Usually, predators will be gone first as they compete for food sources, may often be harmful to humans and usually have the lowest population size. They may also be negatively impacted by the decreasing numbers of prey available (see next sentence). After that, animals which are slow, naive (not afraid of humans), large and tasty will be gone next. Examples are the Moa , Dodo and some large tortoise species found on the Galapagos Islands. When those are gone, humans will start hunting for smaller, less nutritious animals (again, see Easter Islands)

c) Invasive species: Humans usually bring along (knowingly or unknowingly) species which might impact the local flora and fauna. The best example are rats, which often have devastating impacts, such as the impact on the Kakapo. Similar impacts can be expected by introduced pigs, cats and snakes; the brown tree snake on Guam is a good example (https://en.wikipedia.org/wiki/Guam). Both are devastating bird and small mammal (also small reptile) populations.

d) Agriculture: Agriculture leads to the loss of natural habitats and therefore the loss of flora and fauna. The extend and intensity of the agriculture will decide how severe the impact will be. Of course, the larger the human population gets, the larger the impacts will be. If, for example, crops which are mainly wind pollinated, such as wheat or barley, replace natural habitats, this will have a strong negative impact on pollinator populations.

  1. Island biogeography: Island biogeography tells you how species rich your island was before humans arrived and to which degree organisms will be able to recolonize the island. Also, read metapopulation theory in this context.

a) Distance from mainland: The further away your island is from the mainland the less species can be found on it.

b) Size of Island: The larger your island, the more species will be found on it (species-area-curve).

c) Age of Island: The longer the island existed the more species are likely to be found on it (even though this might not be 100% true). The idea is, the longer the island existed, the more time organisms had to arrive on that island and establish a population and/or diversify (evolve into new and different species. One example is the island dwarfism, which has already been mentioned. But I do not consider this an issue in your case). There is also a specific order: Usually plants are the first organisms to establish populations, followed by herbivores followed by predators. The higher the trophic level, the later the organism will establish a population. Seabirds, however, might use islands as breeding grounds in the meantime.

  1. Inbreeding: Inbreeding and the already mentioned bottlenecks might negatively impact populations in the long term. However, I do not consider these to be to severe as there are many counter examples where organisms still survive besides bottlenecks in the past (e.g., Californian condor, cheetah). Other scientists might disagree on this point, but this is my personal opinion. Read about metapopulation theory in this context.

Further things to consider are e.g. pollution and topography, e.g. habitats at higher elevations usually stay pristine for a longer period of time as they are less accessible.

So as a short prediction from my side: Large vertebrate animals will be gone soon, followed by smaller vertebrate animals. The only "larger" vertebrate animals which will be present in the long run are rats, if they have been introduced.

P.S. Sorry I had to remove most of the links as I am only allowed to post two links with my reputation. But I am sure you can find the examples and theories on wikipedia.

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  • $\begingroup$ Thank you for your detailed answer. Keep up the good work and you'll have the reputation in no time. $\endgroup$ – Thomas Myron Nov 21 '16 at 16:02
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Aside from birds, you're missing the other major type of animal that can easily live on the island: insects. Assuming there's enough vegetation to feed them, bugs would easily thrive on an isolated peninsula. With the birds feeding on the bugs to balance out their population, that would be a fairly sustainable ecology.

Rats would be a very likely occurrence. Rats can almost always be found near or in human settlements. Once the humans unintentionally bring rats onto the peninsula, there's a very good chance of a population boom.

Why can't humans swim past the wall? Unless the water is poisonous or insanely treacherous, I'd imagine certain amphibious creatures wouldn't mind visiting the peninsula. Especially if there are no other major predators, turtles might favor the beaches there as a safe place to lay their eggs (although the rats might put a stop to that plan).

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  • $\begingroup$ Magic is preventing all access between the peninsula and the mainland. I did not specify magic in the OP because it didn't contribute anything to the question. $\endgroup$ – Thomas Myron Nov 18 '16 at 1:00
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The peninsula you have mentioned in your post (Mahia) is nearly about 240 sq km. The fauns of the peninsula depends primarily upon a few factors which you have not explained in your post:

1- where is the peninsula located? Specifically the latitude, which would determine the climate of the region.

2- what is the population of humans on the peninsula?

3- how long has the wall/divide been present between the peninsula and the mainland?

Let us assume that before the wall was present, the peninsula was normally connected to the mainland. In this case, you would expect all kinds of animals in the peninsula, including mammals, reptiles and amphibians, alongside birds and insects. This assortment would depend on the location of the peninsula.

Once the wall is erected, the land-based animals would gradually shrink in size in the long run, due to a phenomenon known as insular dwarfism. This would happen over a long time, taking at least several thousand years. Thus the history of wall erection is important in this context.

Predator populations are the ones most severely affected when constrained to a small area. You would have all types of problems going for them, inbreeding, lack of food, infighting and whatnot. I would expect that with time, either the predators' population would shrink to a bottleneck or get extinct completely. Notice that I am talking about large mammalian populations here, like tigers, lions, jaguars, bears and wolves. Things like snakes would continue as usual, only perhaps slightly reducing in number, as their diets are more diverse than mammalian predators' diets.

Lastly, there is the impact of human activities on the habitat. How many people are there? What is their technology level? Are they hunters? Are they familiar with domesticating animals? Do they know how to grow crops? Do they have sufficient numbers of fruit trees available on the peninsula to avoid extincting the fauna through excessive hunting?

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