Why Midway violates ecological laws and what it means for conservation in the long-term
E.O. Wilson, the avuncular, silver-haired ant biologist, conservationist, and ambassador for biodiversity is probably the most famous ecologist the world has ever known, which maybe isn’t saying much. But if you are or were a student of biology you might remember what put him on the ecology top hits charts back in the 1960s: a manuscript titled The Theory of Island Biogeography in which he and Robert MacArthur argued that that the number of species found on any given island was the result of two simple things: the rate if colonization (new species arriving on the island) and extinction (the rate at which species wink out due to random population fluctuations, disease, etc.). The closer an island is to the mainland, the higher the rate of colonization. The bigger the island, the larger the populations and therefore the lower the extinction rates. So large islands close to continents should have way more species than small islands far from continents. It all makes sense and the empirical evidence support the theory pretty well. For example, Cuba, a large island (over 100,000 square kilometers or 42,000 square miles) is a just 160 km (100 miles) or so from the shores of North America and has a whopping 8,000 species of flowering plants while the much smaller St. John, Virgin Islands (50 square kilometers or 19 square miles) sitting far out in the Caribbean far from any continent has just 1,000 species.
So what does this have to do with Midway?
The three islands of Midway Atoll total just under 6 square km (about 2.25 square miles) and are at least 3,500 km (2,800 miles) from either Asia or North America and over 1,900 km (1200 miles) from Honolulu. These are some of the smallest and most isolated islands in the world and according MacArthur and Wilson's theory, one would expect that the number of plant species found here would be very small. And indeed, this is what ornithologist William Alanson Bryan found when he arrived on Midway Atoll in 1902. Bryan reported finding just eight species of plants on Sand Island and ten on nearby Eastern Island. Today, a visitor to Midway could expect to find nearly 200 species of plants! What happened?
|Native plants of Midway Atoll. Clockwise from upper left: Nohu (Tribulus cistoides), Emoloa (Eragrostis variabilis), Naupaka (Scaevola taccada), Ilima (Sida fallax).|
It all started around the turn of the 20th century when the Commecial Pacific Cable Company decided that Midway was a critical link in their efforts to establish a telegraph line across the Pacific Ocean. Once the decision was made to route the cable through this remote atoll, a station had to to be established and along with it a small settlement to accommodate the personnel needed to maintain it. Prior to this the only visitors to Midway were native Polynesians who occasionally traveled through, shipwrecked sailors, and feather hunters. Never before had people actually intentionally settled the islands of Midway Atoll!
When the cable company began construction of their outpost, Sand Island was “an uninhabited shimmering white pile of sand” but it didn’t take long for the newly arrived inhabitants to begin to reshape the island to meet their expectations. The ships that the cable station with food and other provisions also brought with them soil and plants to establish vegetation more to the inhabitant's liking. In October 1903 the ship Whalen brought 400 tons of soil to Midway from Honolulu as well as plants and grass seeds. Plants included coconut, milo, banana, passion fruit, and banana. Ironwood trees (Casuarina equisetifolia) were also imported with the intention of providing the new residents shade from the sub-tropical sun as well as species such as beach morning glory (Pohuehue or Ipomea pes-caprae ssp. brasiliensis) which were hoped would stabilize the shifting sands.
Over the years, many tons of soil and hundreds of plants were intentionally transported to Midway, including an assortment of vegetables and fruits grown to enhance the diets of the residents. In addition to desirable plants, some weedy species were brought in inadvertently, some of which proved to be a real nuisance later on. Midway’s rich history as cable station, air transport stopover, military outpost, and cold war surveillance outpost, also made it a hotspot for non-native plant introduction as each new cohort of occupants brought with them plant species they found interesting or useful. This has included turfgrasses for golf courses, palm trees for tropical ambiance, food plants such as papaya, and ornamental flowers for their beauty.
By importing so many species of plants to Midway over the past 100 years, residents have “short-circuited” MacArthur and Wilson’s Theory of Island Biogeograhy. Despite the fact that Midway is about as far from any continent as it could be, immigration rates (i.e., the rate at which new species arrive) have been relatively high. The end result is that Midway Atoll has much higher plant species diversity than would be expected based on purely “natural” factors. Today, Midway has about ten times the number of plant species as there were a little more than 100 years ago when the first inhabitants arrived.
The inflated plant species richness of Midway Atoll has its pros and cons. Many of the plant species that have been introduced here have proved to be invasive. That is, they do so well as to dominate areas to the point of causing problems for native species. A good example of this is golden crownbeard (Verbesina encelioides), a sunflower-like plant which grew so well here that it covered 70% of the islands soils and degraded habitat for both Laysan’s and Black-footed albatross. The US Fish and Wildlife Service worked hard to reduce the amount of this weed and now it covers less than 1% of the islands. Ironwood has also proven to be problematic as it dominates areas so completely that nothing can grow in its shade.
|Some introduced species such as ironwood (Casuarina equisetifolia; upper left) prove to be invasive requiring management while others, for example, sea grape (Coccoloba uvifera; lower right)) appear to be benign or even beneficial.|
Prior to the habitat restoration efforts undertaken by the US Fish and Wildlife Service, vegetation on Midway was nearly exclusively non-native and provided sub-optimal conditions for seabirds and other wildlife. Over the past 15 years much progress has been made in bring back the natives. Restoring native plant species is important not only for wildlife but for the plant species themselves, as many have seen their populations drastically reduced on other islands through the introduction of non-native animals such as rabbits and goats. In this new era, Midway Atoll, despite the fact that it has been intensively utilized for a variety of uses for over 100 years, has the potential to contribute significantly not only in the conservation of seabirds but also for native plants of the Northwestern Hawaiian Islands. The work to be done here at Midway does not fit neatly into what might be regarded as “restoration” as the islands have been modified so extensively over the past century that going back to what once was is simply not possible. Instead, we are challenged to managing these islands to their highest purpose, something that requires a solid understanding of the past, an acute awareness of the current needs of both plants and wildlife, and a creative vision for the future.
Acknowledgements: Thanks to William Alanson Bryan for having the forethought to visit and document Midway Atoll way back in the early 1900s. Much appreciation to Forest and Kim Starr who have returned to Midway three times in the past two decades to document the vascular flora of Sand, Eastern, and Spit Islands. Meticulous documentation of the plant species growing on Midway is certainly not an easy or glamorous job but were essential in the writing this blog post. Thank you all!