Invasive Species and the Call to Christian Environmental Stewardship
Submitted in partial fulfillment of the requirements for tenure by
Todd T. Tracy, Ph.D.
Department of Biology, Northwestern College, Orange City IA 51041
Invasive species: So what?
“Invasive species”… words that strike fear in the hearts of… well, almost nobody. Despite the fact that invasive species are one of the largest and most easily preventable causes of the loss of biodiversity on our planet, most Americans have never heard of them and are completely unaware of their existence, except perhaps for the dandelions1 in their lawn. Unfortunately, what we don’t know can hurt us, and as we in our ignorance do nothing about invasive species, their spread across the globe accelerates, wreaking ecological havoc everywhere from the alpine tundra of Colorado to the icy terrain of Antarctica.
What is an “invasive species”?
An “invasive species” is a rapidly spreading species introduced, either directly2 or indirectly3, intentionally2 or unintentionally4, into a new geographic area by humans, with negative ecological and/or economic consequences (GISD 2005a; Brennan & Withgott 2005 p. 152; Lockwood et al. 2007 pp. 7-8). Because the new environment lacks the interspecific interactions (i.e., competition and predation) that kept their populations controlled in their native areas, invasive species tend to ‘take over’ their new area, supplanting native species and, in many cases, causing the extirpation5 or extinction of these natives (GISD 2005a; Lockwood et al. 2007 pp. 7-8). The spread of invasives ranks second only to habitat destruction in the global loss of biodiversity (Brennan & Withgott 2005 p. 466; GISD 2005a).
A classic example of an invasive species is the cane toad Bufo marinus. A native to Central and northern South America, this species was introduced onto myriad sugar-producing islands around the world, including Australia in 1935, as a “biocontrol” for the sugar cane beetle, whose larvae were destroying sugar cane crops. The plan in Australia and elsewhere was for the toads to eat the beetle larvae, but instead the toads focused their voracious appetite on more delectable treats such as native invertebrates, amphibians, reptiles, and bird eggs and hatchlings (e.g., Australia’s endemic rainbow bee-eater Merops ornatus; Boland 2004). Adding insult to injury, cane toads exude a neurotoxin from their paratoid and other epidermal glands when grabbed, killing almost any species attempting to consume them. Cane toads present an ever-present danger to unsuspecting children attracted to them, and human and pet fatalities have been reported as a result of ingestion of eggs6, tadpoles, and toads (Lever 2001). From the original release of 101 toads at a site near Gordonville, Queensland, the cane toad’s range now exceeds 1.2 million km2 in northern and eastern Australia, with numbers exceeding 200 million individuals (UQ IMB). Given the cane toad’s invasion history and prolific nature (one female toad can produce upwards of 50,000 eggs per year), some expect the cane toads to double the size of their current range in Australia (Urban et al. 2007) and further effect the extinction of several Australian endemics (Murray & Hose 2004).
While most invasive species are trans-oceanic transplants, “home-grown” non-natives can be just as detrimental and thus may be just as worthy of the “invasive” designation. For example, eastern redcedar Juniperus virginiana is generally classified as an invasive species in the central U.S., despite the fact that it is a native tree. This species has become problematic in that it is spreading (via birds passing seeds through their digestive tracts) across the grasslands and rangelands of the central U.S., areas that historically have burned frequently enough and/or been sufficiently trampled by buffalo to prevent woody species from taking hold. Until recently, redcedar’s range was limited to river bottoms and cliff faces where trampling and fires were less likely to have occurred. The facilitation of the spread of the trees into upland areas where they would not have naturally occurred has been greatly aided by farmers, ranchers, and state Departments of Transportation planting the species as a windbreak (Wisconsin DNR; USDA NRCS; Drake & Todd 2002). Although the species is useful for windbreaks and building material, economic losses from lost rangeland and lost hunting leases, decreased stream water quality, and lowered water tables appear to outweigh the benefits of the species in many areas (Drake & Todd 2002). The pastureland owned by Inspiration Hills (the RCA retreat center in northwestern Sioux County) is seriously impacted by eastern redcedar, and the camp is losing profits from its pastures because of the decreased grass production caused by the invaders. Terra Nova and our ecology and environmental science students have spent many hours eradicating redcedar from prairie remnants at Oak Grove Park and from the mixed-grass prairie at the Nature Conservancy’s Niobrara Preserve in north-central Nebraska.
Another example of a home-grown invasive is the house finch Carpodacus mexicanus, a species of which I have grown quite fond, as I did my Ph.D. dissertation work on the species in Colorado and have studied the songs of Sioux County house finches during the summers of 2003 and 2005. The eastern half of the U.S. was house finch-free until 1940, when a pet-store owner in New York City transported a handful of finches from California to sell illegally as pets. As federal agents moved in to arrest the store owner, he set the evidence free. This population of house finches barely survived the next few years, but by 1951 it was apparent that these birds had become a permanent part of the avifauna of New York City (Elliott & Arbib 1953). The last half of the 20th century found the eastern house finches slowly, then rapidly, expanding their range westward, and in 1989, the eastern and original populations are believed to have initially converged in central Oklahoma (Tyler 1992). The house finch is now found in all 48 contiguous United States. As far as invasive species go, the house finch does not stand out as being particularly insidious, and their warbling song and bright red coloration are admired by many backyard birders. Furthermore, the spread of the species in the eastern U.S. appears to have led to a steep decline of the much-maligned invasive house sparrow Passer domesticus (Cooper et al. 2007), a serious agricultural pest7 intentionally imported from Europe in the 1800’s. House sparrows steal nests and nest sites from eastern bluebirds Sialia sialis and purple martins Progne subis, causing sharp declines in those species’ numbers in the 20th century (GISD 2005n). It was actually the arrival of the house finch in Chicago in the mid-1980’s that piqued my interest in ornithology (the bird was refreshingly different from the crows, robins, and house sparrows that I’d grown up with), led me into the realm of research on bird songs, and may have in fact affected my decision to pursue an advanced degree in zoology rather than chemistry. Although I am reluctant even to classify them as invasive, house finches are classified as such by many because of evidence that they have displaced native purple finches Carpodacus purpureus from the southern edge of their range (Hill 1993), and because of their negative economic impact, as they have a penchant for fruits, grain, and other crop seeds (GISD 2005c). However, house finches also help control weeds by eating weed seeds and dandelion flowers (Hill 1993), and they have helped control the house sparrow, an even more serious agricultural pest. I would furthermore argue that the species would have eventually spread throughout the eastern U.S., as the Great Plains seems no longer to be an effective barrier for other species and subspecies whose ranges had previously been restricted by the Great Plains (e.g., red-shafted and yellow-shafted flickers Colaptes auratus, and Baltimore Icterus galbula and Bullocks orioles Icterus bullockii). Of course, the Great Plains are no longer an effective barrier because humans have changed the habitat and food availability in the region8, and any species expanding into and negatively affecting new areas as a result of human habitat alteration would still be considered invasive, so even if house finches had made it to the eastern U.S. on their own, they might still be classified as invasive.
Even pets and livestock can become feral and wreak havoc on native ecosystems. The red-eared slider turtle Trachemys scripta, bullfrog Rana catesbiana, domestic cat Felis catus, domestic rabbit Oryctolagus coniculus, pig Sus scrofa, and goat Capra hircus all rank among the world’s 100 worst invasive species (Lowe et al. 2000). The cat, rabbit, and pig have had particularly profound impacts on islands onto which they were intentionally introduced (GISD 2005a). For example, the black stilt Himantopus novaezelandiae in New Zealand and the Cayman Island ground iguana Cyclura lewisi are threatened with extinction because of feral cats (GISDh), while pigs introduced onto the island of Hawaii kill native trees by felling and barking them, and they uproot large areas of land, threatening not only native vegetation, but also native birds, as the bare, uprooted land becomes a breeding ground for invasive mosquitoes that carry diseases against which native birds have no defense. In his book Hawai’i: the Islands of Life, Gavan Daws (1989) summarizes the effects of feral pigs on the Hawaiian Islands as follows:
“To the Hawaiian rainforest, the pig is death: consuming ground-cover plants, churning the rich ground into foul muck, the forest dies from the bottom upward and the rains wash the soil away to smother coral reefs with silt.”
Are invasive species mentioned in Scripture?
Although there are many references to weeds, thornbushes, thistles, etc., mentioned in scripture, I could find only a single scriptural reference specifically mentioning non-native species being brought into Israel:
You have forgotten God your Savior; you have not remembered the Rock, your fortress. Therefore, though you set out the finest plants and plant imported vines [KJV = strange slips], though on the day you set them out, you make them grow, and on the morning when you plant them, you bring them to bud, yet the harvest will be as nothing in the day of disease and incurable pain. (Isaiah 17:10-11)
While the disease and pain prophesied by Isaiah appear not to have been directly precipitated by the imported plants themselves, it appears that God’s people cared more about their exotic plants than they did for God himself. As explained by Matthew Henry (1706) in his Bible commentary:
“The destruction itself [was] aggravated by the great care they took to improve their land and to make it yet more pleasant. Look upon it at the time of the seedness, and it was all like a garden and a vineyard; that pleasant land was replenished with pleasant plants, the choicest of its own growth; nay, so nice and curious were the inhabitants that, not content with them, they sent to all the neighbouring countries for strange slips, the more valuable for being strange, uncommon, far-fetched, and dear-bought, though perhaps they had of their own not inferior to them. This was an instance of their pride and vanity, and (that ruining error) their affection to be like the nations. Wheat, and honey, and oil were their staple commodities (Ezekiel. 27:17); but, not content with these, they must have flowers and greens with strange names imported from other nations…”
Interestingly, although the passage is not explicitly about invasive species during Biblical times, it does describe the invasive-species scenario that has played out many times in human history. Humans, unsatisfied with the native flora and fauna of an area, bring in seductively attractive species from elsewhere, only to find out the hard way that such was not a wise thing to do. Kudzu Pueraria montana, initially introduced to the southern U.S. in the 1930’s as an attractive vine useful in controlling erosion, now covers 2 to 3 million hectares in the southeastern U.S. Kudzu out-competes native vegetation, completely covers and collapses trees, and costs over $500 million in lost productivity and control efforts annually (GISD 2005o; NPS). Kudzu is also an alternate host of soybean rust Phakopsora pachyrhizi and could facilitate a major outbreak of the disease in the future, devastating soybean crops in the southern U.S. and potentially farther north (MSU 2005). The “beautiful” water hyacinth Eichhornia crassipes threatens the lives and livelihoods of thousands of people along the shores of Lake Victoria (see below). Miconia Miconia calvescens, a beautiful palm-like tree with purple undersides to its leaves, has been intentionally introduced onto several Pacific islands. Consequently, it now covers more than 70% of the island of Tahiti, directly threatening between 40 to 50 rare endemic plant species, and it is now spreading rapidly on the islands of Maui, O’ahu, and Hawaii (GISD 2005l; National Geographic). On the attempts to “improve” the Hawaiian islands, Clements and Corapi (2005) state that it “is not a little ironic that so many people who have come to “paradise” have sought to “improve it” by modifying the native flora and fauna.” There are 1,131 native plant species on the Hawaiian Islands, and humans have introduced over 10,000 non-native vascular plants to the islands, with over 1000 of these species now naturalized. (Clements and Corapi 2005).
Why should we care about invasive species?
a) We should care because species are being forced into interactions that would not have otherwise occurred.
While it is true that competitive interactions and predator-prey relationships occur naturally in all biological communities9, the involvement of humans in spreading invasive species is causing species that otherwise would never have contact with each other to directly interact with and/or compete with each other. One of the classic questions in ecology is why a certain species is found in one area and not another. Sometimes the answer relates to physiological limitations. For example, red-eared slider hatchlings cannot supercool10 and cannot tolerate the freezing of their body fluids and thus are limited to areas of the southern U.S., while painted turtle hatchlings Chrysemys picta, a closely related species, can supercool their body fluids and thus range as far north as south-central Canada (Packard et al. 1997). In other cases, the answer relates to the dispersal capabilities of the organisms themselves. For example, why were there no House Sparrows in North America before humans introduced them? The environmental conditions in North America were not beyond the physiological capabilities of the species, but dispersal onto this continent was beyond the capability of the species, and without our help, the species likely would never have settled North America. Just as the Romans forced interactions between early Christians and lions, humans are now forcing (albeit sometimes unintentionally) interactions among species that otherwise would not naturally have come into contact with each other, so the argument that “invasive species are natural and that therefore we should not be concerned about them” is not very persuasive. Rather than introducing new species wherever we want them (or wherever we, out of ignorance, accidentally introduce them), a more appropriate approach is to honor the adaptations of organisms in an area by not introducing species to which the native organisms have not adapted. For example, in reference to the human-facilitated introductions of non-native predators onto the Hawaiian Islands, Holmes Rolston III (1994) suggests that, rather than considering it “catastrophically tragic” not to have charismatic megafauna such as mammals and reptiles on the remote islands, that “Hawaii be an especially remote test of oceanic mobility,” thus honoring the adaptation of birds that have made it to Hawaii on their own by not forcing these birds to compete with species that we introduce to the islands to “enrich” the landscape. (pp. 116-117)
b) We should care because invasive species lead to the loss of biodiversity and homogenization of our environment.
Species on many islands have had no contact with, and therefore have evolved no response to, ground predators, so they have no defense against ground predators introduced onto their islands. For example, the accidental introduction of the brown tree snake Boiga irregularis onto the island of Guam has directly caused the extinction of 9 out of 12 native species of birds and 2 out of 11 native species of reptiles (GISD 2005b). The small Indian mongoose Herpestes javanicus was brought to the West Indies, Hawaii, and other Pacific Islands in an attempt to control rats in sugar cane fields, but instead the mongoose has decimated endemic populations and in Hawaii threaten the nene goose Branta sandwicensis, Hawaiian crow Corvus hawaiiensis, Hawaiian dark-rumped petrel Pterodroma phaeopygia, and several other species with extinction. Mongoose-induced extinctions and imperilments have likewise occurred in Costa Rica and on the Virgin Islands, Amani Island, Jamaica, Honduras, Japan, Haiti, Hispaniola, Grenada, Fiji, Viti Levu Island, Antigua and Barbuda, Bahamas, and Bermuda (GISD 2005j). I list and describe the values of biodiversity (and all of creation) in chapter 2 of this paper.
c) We should care because invasive species directly affect humankind’s welfare and ability to use and enjoy the Earth’s natural resources.
While concerns about the induced unnatural interspecific interactions and the loss of biodiversity caused by invasive species are great among scientists and most environmentalists, the majority of Christians are not particularly interested in such issues and thus show little concern about invasive species, and indeed, about environmental degradation in general. What is overlooked, however, is the fact that most, if not all, invasive species have some negative impact on humans. Invasives can cause increased disease infection rates in humans and livestock11, either directly or indirectly inhibit food production, outcompete valuable native species, and even impact our ability to enjoy God’s Creation.
In some cases, the impacts of an invasive species are readily apparent and are directly related to the activity of the invasive. For example, until Hurricane Katrina laid waste to much of New Orleans, Formosan subterranean termites Coptotermes formosanus were already doing so, albeit more slowly than Katrina did. The termites, inadvertently brought over from the China Sea in shipping crates after World War II, left their own trail of destruction within the city and eventually throughout the South as they trees and destroyed wood-frame homes (National Geographic 2005; GISD 2005d). After Katrina, a major fear in termite-free areas of the country was that the termites would spread to new areas via cheap wood mulch produced from the debris in the New Orleans area. Costs from damage and control of the termites in the U.S. exceeds $1 billion annually (GISD 2005d).
While some invasive species present direct negative effects that are readily apparent, other invasives present themselves initially as being quite innocuous, and their indirect effects on humans often initially go unnoticed. The water hyacinth, an attractive aquatic plant native to South America, was initially brought to Africa for ornamental purposes (GISD 2005g; National Geographic 2005). The plant first showed up in Lake Victoria in 1990, and by 1995 over 90% of Uganda’s shoreline along Lake Victoria was covered with the hyacinth, with profound negative economic and health effects on the people of the area (Williams). Incidence of human schistosomiasis12 infections rose because of the increase in snails associated with the increase in hyacinth (National Geographic 2005). The hyacinth also caused a spike in the incidence of malaria, as the stagnation of water created by the plant provided perfect a breeding environment for Anopheles mosquitoes13 (Williams; National Geographic 2005). Rotting vegetation contaminated water supplies, and with the vegetation clogging the shoreline, fishing in the lake became a challenge. Even a hydroelectric plant was forced to close because of the invasion (GISDa; Twongo & Balirwa 1995 as cited by Williams; Plummer 2005).
Another invasive species whose impact on humans might not be so readily apparent is the European buckthorn Rhamnus cathartica14, a shrub/tree initially brought to the U.S. from Europe in the 1800’s for ornamental purposes (GISD 2005p). European buckthorn in the understory of a forest alters soil properties (Heneghan et al. 2002, Heneghan et al. 2006), outcompete native saplings for light and other resources (Fagan & Peart 2004), increases songbird nest predation (Schmidt & Whelan 1999), and may possibly even poison other plant species via the plant’s allelopathic capabilities15 (Vincent 2006). Areas of forest invaded by the closely related and invasive glossy buckthorn Rhamnus frangula have been shown to have reduced herbaceous groundcover growth (potentially increasing erosion) and native sapling recruitment relative to uninvaded areas of forest (Frappier et al. 2003, Frappier et al. 2004). In essence, once the trees of the native overstory die, there would be few trees left in the forest other than buckthorn because native saplings cannot survive beneath buckthorn.
In the parable of the seeds (Matthew 13), Jesus describes the effects of thornbushes on herbaceous cover and sapling recruitment, although he does not use those exact terms:
3 Then he told them many things in parables, saying: "A farmer went out to sow his seed. 4 As he was scattering the seed, some fell along the path, and the birds came and ate it up. 5 Some fell on rocky places, where it did not have much soil. It sprang up quickly, because the soil was shallow. 6 But when the sun came up, the plants were scorched, and they withered because they had no root. 7 Other seed fell among thorns, which grew up and choked the plants. 8 Still other seed fell on good soil, where it produced a crop--a hundred, sixty or thirty times what was sown. (Matthew 13:3-8)
While European Buckthorn is not native to Israel, a closely related species called the Palestine buckthorn R. palestina is native to the area and may be described in this passage, although the term “akanthos” translated as thorns is thought to refer to a “generic” thornbush (Bible Plants). On the other hand, it is believed by many that the Hebrew word “’atad” (translated in NIV as “thornbush”) as used by Jotham to describe Abimelech in Judges 9:14-15 does specifically refer to the Palestine buckthorn (NeXtBible; Bible Plants), and knowing what happens to plants growing in the shade of buckthorn brings new insights into the passage:
Finally all the trees said to the thornbush, 'Come and be our king.' The thornbush said to the trees, 'If you really want to anoint me king over you, come and take refuge in my shade; but if not, then let fire come out of the thornbush and consume the cedars of Lebanon!'
So how does buckthorn affect humans? Beyond its myriad ecological effects, European buckthorn also manifests agricultural impacts as well. Buckthorn is an alternate host for oat crown rust Puccinia coronata, which can cause 10-40% oat crop loss in heavily infected areas and total crop failure in individual fields (USDA ARS). Buckthorn is also a wintering host for the soybean aphid Aphis glycines, an invasive from China that first appeared in the U.S. in 2000, probably as a stowaway on someone’s undeclared plant material sneaked into O’Hare Airport (Hartzler & Pope 2001; Regional Pest Alert; UM Extension). Aphid-related soybean crop losses and treatment costs in Minnesota alone are estimated at $200 million per year (UM Extension). The Asian lady beetle Harmonia axyridis was introduced to the U.S. in 1916 as a biocontrol for native aphids, and the beetles has been shown to be an effective predator of the soybean aphid as well. Unfortunately, as is the case with many early biocontrol attempts, the introduced beetles became invasive, causing declines in native ladybird beetles, eating fall fruit crops16, and creating a nuisance for homeowners and almost anyone out of doors (GISDi 2005).
Beyond their negative ecological and economic impacts, invasive species can also negatively affect our recreational activities. For example, Eurasian watermilfoil Myriophyllum spicatum, a freshwater aquatic plant thought to have been originally imported to the U.S. for use in aquariums, has now taken up residence in lakes and water courses in 45 states (USDA NAL). Among its undesirable characteristics is the plant’s propensity to inhibit recreational use of our lakes by forming unsightly, dense floating mats of vegetation that interfere with boating, fishing, and swimming. The plant also out-competes native aquatic plants and diminishes resources for fish, thereby further impacting recreational fishing (MPRB). Watermilfoil is currently spreading into uninfected bodies of water via dirty fishing gear, boats, and boat trailers, as small plant fragments transported in bait buckets or adhering to the boat, trailer, or equipment are capable of rooting and can survive for some time out of water. Millions of dollars are spent annually controlling this weed, with chemical and mechanical control costing $200 to $2000 annually per acre treated (MPCA). The inhibition of recreation activities has severe economic impact on the tourism and recreation industry, as people do not want to vacation in areas where the watermilfoil grows out of control.
d) We should care because God calls us to care for His creation.
Invasive species and environmental degradation affect people. Since Christians are called to care for our fellow human beings, the mere fact that invasive species have profound negative impact on humans should be sufficient reason for us to care about invasives. But beyond the human-centered reason for Christians to care about invasives, Christians should also care because God has given us a stewardship responsibility toward his creation. This would be a pretty short tenure paper if the idea of “creation care” were an easy sell, but I realize that many Christians are unaware of God’s stewardship directive and probably need a little convincing that they should care about (and for) the environment. In order to understand God’s stewardship directive, one must first understand that creation exists for reasons beyond use by humans. In the next chapter, I discuss God’s purposes for his creation and explore reasons for the general apathy toward the environment among Christians. I expound on one of these reasons for apathy in chapter 3, and in my final chapter I examine our call to stewardship, with particular consideration of how invasive species relate to this call.