Destroying volcanoes:Hawaii volcanoes are huge piles of mechanically weak rock. As eruptions build up a volcanic edifice, the force of gravity works to destroy it in two distinct ways:
Hawaii rift zones from USGS
Holei Pali from Chain of Craters Road
Weathering and Erosion: As soon as volcanic rock forms, agents of weathering attack it. These can include the mechanical forces (name examples) and chemical ones. Indeed, igneous rock tends to be vulnerable to the slight acidity of rainwater. (Why is rainwater acidic?)
Basalt polygons etched by rainwater at Whittington Beach, HI
- Climate: Warm and wet maximizes opportunities for weathering reactions, however, climates with frequent freeze-thaw cycles experience more ice-wedging.
- Drainage: For weathering reactions to proceed, products must be removed, otherwise equilibrium will be reached and weathering will stop. Thus well drained environments experience more continuous weathering.
- Relief: Increases speed of drainage and mass-wasting and discourages development of soils.
These weathering products are transported downhill by the various agents of transport. In Hawaii, gravity and running water are particularly important. The result is that older volcanoes are incised by steep valleys. A survey of the island's topography shows that these canyons are not distributed randomly.
What two factors might influence their distribution?
Canyons of Hamakua District from Shutterstock
Because chemical weathering by low pH rainwater is such an important factor, weathering tends to be concentrated in places receiving the most rainfall - High elevations on the windward side of the island, resulting in the erosion of amphitheaters that drain to the ocean through narrow canyons. In places with shallower slopes and less rainfall, soils can form.
Waipio Valley from Wall Street Journal
Paradox: It all seems very simple until you start thinking about global climate change. In Hawaii's most eroded regions, we see broad, flat-bottomed valleys like Waipio (right) and Pololu, whose floors are filled with sediment that forms the best agricultural soils on the island. How did they form?
Picture the scene 20,000 years ago at the last glacial maximum, when sea-level was up to 80 m lower than today. The canyons actually cut much deeper, but filled with sediment as sea level rose to modern levels.
In all of this, we see that Hawaii geography is connected to global systems that govern their overall conduciveness to habitation by organisms.
Kipuka from Wikipedia
Island Biogeography is the study of the parameters effecting the species diversity of insular habitats. These may be:
- actual islands of land in the ocean
- parks in an urban landscape
- kipukas (forest islands isolated by lava flows)
- fresh water lakes separated by land
- or any other habitable region separated from others by inhospitable environments.
It's basic principles were presented in the 1960s by Robert MacArthur and E. O. Wilson. In a nutshell, the diversity of species on an island is determined by:
- Immigration rate
- Extinction rate
At the crudest level, these factors are determined by:
- Distance from the source of immigrants (the mainland or another island)
- This varies for different kinds of immigrants. Birds can cross longer distances than frogs.
- Character of dispersal agents like prevailing winds and ocean currents
- Diversity on the mainland.
- The habitability of the island, a function of
- Its size
- The diversity of habitats on it
- The roster of organisms already inhabiting it.
Native land vertebrates of Hawaii
- Native amphibians: 0
- Native reptiles: 0
- Native mammals: 1 (the Hawaiian hoary bat, which can disperse by air.)
- Native freshwater fish: 6, all with close salt-water relatives
Ohia lehua (Metrosideros polymorpha)
Plants: 255 original endemic species tend to:
- reproduce by spores (ferns), which can be dispersed thousands of miles by wind (1.4%)
- have very small seeds that can be spread long distances (E.G.: the Ohia lehua - right)
- have fleshy fruit that is appetizing to birds (39%)
- have hooks on seeds or other adaptations that enable them to cling to birds (36%).
- have propagules that can disperse over water (14%)
- are capable of rafting over water (9%)
Hawaiian fruit flies in combat from School of Ocean and Earth Science Technologies
- Arthropods (primarily flies) (5163)
- Land snails (99 historically, now 25)
- Birds (71 historically, now 48)
- Bats (2 historically, now 1)
Expanding to native fauna, we gain some migratory sea birds and shore birds.
And yet the Hawaiian Islands have many introduced invasive species:
- Amphibians: 6
- Land/ fresh water turtles: 3
- Squamates (lizards and snakes): 18
- Birds: 62
These lists reveal a remarkable pattern of extinctions and recent introductions. How did this happen?
Hawaiian honeycreepers from Douglas Pratt
The roster of endemic plants and animals of Hawaii reflects the demands of dispersal over thousands of miles of ocean - effectively impossible for any non-flying animal. No surprise that ancient Hawaii's faunas were dominated by birds. Hawaii hosts endemic adaptive radiations of:
- Honeycreepers: Related to seed-eating cardueline finches of Eurasia, but adapted to various feeding strategies including nectar eating (i'iwi, apapane), insectivory (Hawaii amakihi), and granivory (palila)
- Monarch flycatchers: Related to generalists of the southern hemisphere. Includes the Hawaii elepaio.
- Thrushes: Related to northern hemisphere insectivores including the American robin. Includes the puahiohi
Hawaiian Honeyeaters from Fleischer et al., 2008
- Hawaiian "Honeyeaters:" Recently extinct. Physically and ecologically similar to the proper honeyeaters (Meliphagidae) of Australasia. Entirely extinct, including the Hawaii o'o, whose yellow feathers were valued in the manufacture of the traditional regalia of Hawaiian nobility.
Hawaiian Anseriformes - all extinct except the nene (right) from Safarizona
- Moa-nalo: "Lost fowl." Extinct giant flightless dabbling ducks known from fossil remains. Including:
- Chelychelynechen quassus (Kauai)
- Ptaiochen pau (Maui)
- Thambetochen xanion (Oahu)
- Thambetochen chauliodous (Maui, Lanai, Molokai)
These were accompanied by an assortment of flightless birds including:
- Chelychelynechen quassus (Kauai)
The Hawaiian goose (Nene)
Alongside them live land birds with close relatives in other places:
The Hawaiian scene just prior to discovery at the National Zoo from Studia Mirabilium
Evolutionary Pattern I - FlightlessnessHawaii is not the only island where immigrating birds have lost the power of flight. Consider:
Flight is useful for:
- gaining access to food in inaccessible places
- escaping predators
- traveling long distances
Moreover: When your home is a small island in the middle of the ocean, the ability to fly becomes a hazard because it allows you to be blown out to sea.
Blue-faced honeyeater (a true meliphagid)
Hawaii O'o (a Hawaiian honeyeater)
Evolutionary Pattern II - Convergent EvolutionFrom their discovery until Fleischer et al., 2008, ornithologists considered the honeyeaters of Hawaii to be members of Meliphagidae - the Australasian honeyeaters, based on morphology and behavior. Imagine our surprise when molecular phylogenetic analyses showed that they had been close relatives of New World silky flycatchers and waxwings.
The discovery of Hawaii from John Harrington
What Happened?Prehistoric Hawaii suffered two ecological blows, each a one-two punch:
The Polynesian Discovery of Hawaii - 300 - 600 CEPrior to the modern age, the Polynesians were the most widespread nationality on Earth. Polynesian success stemmed from the combination of:
- Sophisticated navigational techniques and sea-going vessels based on dugout canoes.
- Agriculture based on taro, yam, breadfruit, chickens, dogs and pigs; supplemented by fishing and hunting.
- Moa nails and other large slow-moving flightless birds quickly ended up in the stomachs of the settlers or their domestic mammals.
- The conversion of forest to farming disturbed habitats inhabited by specialist animals
- Pigs are especially adept at disturbing habitats by creating wallows. Previously, mosquitos had little foothold in Hawaii, but pig-wallows made avian malaria a threat to native fauna. Today, native land birds hold on at higher, mosquito-hostile, elevations.
- The Polynesian rat
- Various destructive insects
Gold dust day gecko in Kona District - descended form escaped pet
The World's Discovery of Hawaii - 1778 to presentBritish Capt. James Cook made the first documented (!) outside contact with Hawaii. This roughly coincided with the establishment of a unified Hawaiian government, which allowed widespread foreign settlement. This drastically accelerated the trends set in motion by the Polynesians, with the conversion of land to farming and ranching, and the wholesale importation of invasive species, by design or accident.
In the last century, insult has been added to injury through the widespread importation of invasive birds, mammals, reptiles, and amphibians. Many of these are ecological generalists or disturbed habitat specialists who are right at home in Hawaii's. Sometimes they arrive by accident (stowaways in shipments) and sometimes by human folly (deliberate well-intentioned introductions and released pets). Some bad actors:
- The Indian Mongoose: Deliberately released in the Hamakua District in 1883 in an attempt to control invasive rats. Alas, the mongooses were diurnal, the rats nocturnal, so the predators didn't meet their intended prey. Instead, they became major predators on land and sea birds.
- The Coqui: Accidentally introduced to the island of Hawaii from Puerto Rico in the 1990s, this noisy frog has become a concern to plant exporters, amid fears that it will eat defenseless insect pollinators.
- Feral cats: fierce predators of birds.
- Introduced birds: Do an image search for "birds of Hawaii." Most of what comes up will be an invasive exotic.
- Jackson's chameleons.
In short, the Hawaiian archipelago is a kind of anti-Galapagos. The setting of one of the world's most disturbed and transformed habitats, with no equilibrium in sight.