As many of you know, I will be traveling to the Hawaiian Islands to complete my Young Birder Odyssey big year. I thought this trip would also be a good way to revive my evolutionary biology series that I've skimmed the surface with in my posts on redpolls and feathers, now I'm going to discuss how islands affect the evolution of the animals living there, and I will primarily be using various examples from the Hawaiian Islands to help provide examples of what I discuss, including giant waterfowl, long-legged owls, and of course the native honeycreepers that rival Darwin's finches as an example of adaptive radiation from a common ancestor (sorry if you were expecting me to write a post on them). Because Hawaii unfortunately lacks the dwarf elephants, monitor lizards, tortoises, ratites, lemurs, azdharchids, tiny iguanodonts, and some of the other animals featured in Trey the Explainer's Biology on Islands video, which I have watched numerous times in preparation for this post, so I will borrow examples from Madagascar, New Zealand, Indonesia, the Galapagos, Mediterranean, West Indies, California's Channel Islands, and others to supplement the Hawaiian examples when helpful.
One sixth of all land area on earth is geographically separated from everything else. Unlike continents, islands are small and secluded. This isolation means only a few selective organisms can exist on them, if they can get there in the first place. There are three main ways animals get to islands: by flying or swimming there, by crossing natural land bridges that are now underwater, or as castaways of storms. Many birds were able to fly from the mainland to islands, often blown off course or intentionally, and establish themselves there. Seals are long-distance travelers that in many cases can also swim to islands if they need to. Another interesting case is of the Komodo Dragon (Varanus komodoensis), which is theorized to have originated in Australia and moved north to escape the receding forest habitat as deserts took over following a landbridge to New Guinea and Indonesia. However, the islands inhabited by dragons today were never connected to Australasia by the landbridge, so scientists theorize the dragons colonized them by swimming (they are surprisingly good swimmers). Inclement weather can blow flying animals off course on migration, leaving them stranded on islands as well. This is how the ancestors of the native bird species and subspecies got there, as well as those of the native arthropods and Hawaiian subspecies Hoary Bat. In short, most of the native wildlife in Hawaii got there by accident.
A land-bridge is an area of a continent that is exposed when sea levels are lower, allowing animals to travel between the future islands and the mainland. This is how life has been able to cross between the Americas and Eurasia during the many intermittent periods in history from the Cretaceous to the Pleistocene when the Bering Land bridge connecting Alaska and Russia was open due to lower sea levels. Another interesting land bridge relevant to island biogeography is one that existed in northwestern Europe called Doggerland. During the last ice age, sea levels were lower, and the British Isles were connected to the rest of Europe by a grassy plain called Doggerland that will one day rest at the bottom of the North Sea. These plains supported a variety of European megafauna including mammoths, bison, horses, lions, Megaloceros, rhinos, reindeer, and nomadic humans. Over time, as the climate warmed, these humans and animals were forced to migrate to higher elevations in Britain and the Netherlands as sea levels rose due to melting ice sheets and a tsunami off the coast of what is now Norway.
The rafting theory states that animals sometimes trapped mats of vegetation that blow out to sea during storms, and when they reach the nearest island, they are able to colonize the new land. This is how all of Madagascar's native land mammals got there as well as the ancestors of the iguanas and tortoises in the Galapagos.
Foster's rule, also known as the island rule or the island effect, is a biological rule stating that members of a species get smaller or bigger depending on the resources available in the environment. The rule was first stated by J. Bristol Foster in 1964, in which he compared 116 island species to their mainland varieties. He proposed that certain island creatures evolved into larger versions of themselves while others became smaller. He proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed because of the absence of some of the predators of the mainland, and larger creatures become smaller when food resources are limited because of land area constraints.
The more famous residents of definitely the giants. In the absence of predators or competition for resources, animals living on islands have grown enormous, such as the moas and Haast's Eagles of New Zealand or the Galapagos Tortoises. Despite the presence of kiwis in New Zealand, the closest relatives of the giant moas are a clade of South American paleognaths called tinamous, which are still capable of flight. Despite their size, moas had one predator before the arrival of humans: Haast's Eagle, which had a 3 meter wingspan. Like the moas, they became extinct shortly after humans arrived. The Galapagos Tortoises are the most famous of the living giant tortoises, but another species of giant tortoise, the Aldabra Tortoise, lives on an island northwest of Madagascar with which it shares it's name. Earlier, I mentioned that Komodo Dragons evolved in Australia and moved northward, there was an even bigger monitor lizard related to the dragons to inhabit Australia known as Megalania or Varanus priscus, which became extinct due to climate change along with its preferred prey of giant kangaroos and wombats. Even Hawaii had its own giant flightless birds (a common theme on islands in the Indo-Pacific). In addition to living and extinct species of Hawaiian Goose or Nene, the Southeast islands were home to four species of flightless geese known as Moa-nalos and the Giant Hawaiian Goose (Branta rhuax).
Islands can also decrease the size of the animals living there due to limited resources and space. One of the most nocticable examples are the Channel Island Fox and Pygmy Mammoth, smaller relatives of the Gray Fox and Columbian Mammoth that live on the mainland in California. I've included other examples, such as Giant Anteater-sized ground sloths and pygmy chameleons
Convergent evolution is when two unrelated animals evolve similar appearances in response to similar environments. One example of convergence that I find most fascinating is one of two Island species imitating each other. In the absence of rodents on New Zealand, a group of ratites shrunk to fill the role of nocturnal opportunists. Kiwis traded flight for a longer bill and an enhanced sense of smell to hunt for insects and worms on the floor of the temperate rainforest, at the cost of good eyesight. What I find most interesting, possibly even more interesting, is that Hawaii has its own version of a kiwi! The Kaua'i Mole Duck (Talpanas lippa) was a flightless species of Duck related to modern-day stifftail ducks (Ruddy, Andean, Lake, Maccoa, Blue-billed, and White-headed, genus Oxyura) that like the kiwis and Kakapo, also gave up flight to hunt for smaller animals on the forest floor at night. Talpanas is unfortunately extinct, but if they were still around, finding one would have been on my list of birding priorities once I got there. Some other cases of convergent evolution in Hawaii include stilt-owls (genus Grallistrix) which evolved long legs similar to resemble those of phorusrhacids, Secretarybirds, and giant flightless Cuban owls of the genus Ornimegalonyx (unlike these owls, Grallistrix kept the ability to fly); and the Hawaiian Honeyeaters, which resemble the honeyeaters of Australasia so closely, they were considered to be part of the same family before elevated to full family status (Mohoidae)
New Zealand isn't just home to flightless birds and monstrous raptors, the islands also act as a time capsule from the age of the dinosaurs. Dense forests of tree ferns and podocarps similar to those in the Lord of the Rings series are found on both islands, and are where many episodes of Walking With Dinosaurs were also filmed. These ancient forests are home to two ancient creatures from the Mesozoic: the Giant Weta and the Tuatara, both coincidentally appearing in Spirits of the Ice Forest (episode 5). The Tuatara looks like a lizard, but is from an unrelated order called Rhyncocephala. Competition from lizards elsewhere has driven Tuataras into extinction, and now survives only on a few small islands off New Zealand’s coast.
Adaptive radiation is the diversification of a clade from one common ancestor to fill a variety of niches and exploit the abundance of food sources in their new environment. This is the reason I chose to use Hawaii as my example location in this blog post. Most people typically think of the tanagers of the Galápagos when adaptive radiation comes to mind, but I’ve chosen the Hawaiian honeycreepers not just out of personal bias, but also because their bill shapes reflect a greater divergence than that in the Galapagos and they’re more colorful (reds, greens, and yellows are more appealing than different shades of gray and brown, sorry Darwin). About 4 million years ago, the ancestors of the drepanidid finches, most likely a flock of rosefinches from the Asian mainland based on genetic analysis, was blown or flew naturally to Hawaii. Some of these finches had genes that gave them large grosbeak-like bills, others had those for long, thin bills, and others had genes for short, straight bills. Over time, finches would mate with birds that had bills which would best enable them to feed themselves and provide food for their young until they could only mate with birds of similar bill shape. This is called speciation. In Hawaii, most of the native finches can be divided into five categories: Generalists like the ‘alauahios and ‘Anianiau; nectarivores like the I’iwi, mamos, ‘apapanes, ‘Ākohekohe, and ‘Ula‘aihāwane; frugivores like the Rhodacanthis grosbeaks, koa-finches, palilas, ‘Ō‘ū, Telespiza finches, and Lanai Hookbill; gleaning insectivores like common ‘amakihis, ‘ākepas, ‘Akeke‘e, and Greater ‘Amakihi, and bark-picking insectivores like the ‘Akialoas, nukupu’us, ‘Ākiapōlā‘au, Kiwikiu, ‘Alawī, ‘Akikiki, and Po‘ouli. Some, such as the ‘akialoas, ‘amakihis, and ‘Ula‘aihāwane blur the lines between niches, exploiting multiple roles based on avalibility of food, and the Laysan and Nihoa finches have even been known to eat the eggs of seabirds when seeds and insects are scarce
Living on an island can not only alter the physical appearance of a species, but also their behavior. Island species
However, being less responsive to predators can also work against a species, and that is what I will talk about in part 2...
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