Grand Fog Mountain Park, located in West Virginia, hosts a diverse variety of fireflies that attract thousands of visitors each year. The complex environment and vast variety of plants have allowed fireflies to find niches and speciate (the evolutionary process of creation of new species); each firefly species has a specific plant on which they specialize. In recent years the number of firefly species has significantly declined. This has prompted research to explain the loss of firefly populations. The three leading firefly experts include Dr. Bush, Dr. Chew, and Dr. Brillient each provided a hypothesis for anthropogenic causes of population decline. Dr. Bush hypothesized that the replacement of native plants by an invasive honeysuckle bush has removed a vital habitat that many fireflies need to complete their life cycle. Dr. Chew has hypothesized that the introduction of a large invasive firefly species that preys on female fireflies of different species has led to the decline of species diversity. Finally, Dr. Brillient has hypothesized that light pollution produced by the increasing number of suburban areas is interfering with firefly mating. The three hypotheses each provide a valid argument to illustrate why there has been such a drastic decline in firefly species. This report will explain the scientific basis for each of these researchers’ hypotheses could lead to the decline of firefly species diversity.
First, I would like to address Dr. Bush’s hypothesis. He believes that the replacement of native plants by an invasive honeysuckle bush is the culprit behind the decline of firefly species. The native plants used by fireflies are important for reproductive purposes for several reasons: male advertisement, egg laying location, and larva development. Male fireflies preferentially display on particular plants to attract females. The process that drives this display is called sexual selection. Sexual selection is a form of natural selection that is based on the principle that an individual chooses a mate based on preference for a certain characteristic (1). In this case, the preferred characteristics are the pattern, intensity, and hue of flashing displayed by the males. After the mating process has taken place, females will oviposit on the plant, and the eggs eventually turn into larvae (1). The native plants are also the sites where the larvae have the greatest success foraging and developing. The honeysuckle bush outcompeting the native plants used by fireflies could be a reason for the decline in firefly species assuming that the fireflies cannot or will not use the honeysuckle bushes for their normal behaviors; this equates to a loss of habitat to complete their life cycle. Also, of the matings that do take place, females have less space to oviposit, and larvae have fewer options for foraging area, which leads to increased competition. Another potential impact of losing their native plants are females becoming less choosey when it comes to finding a mate since fewer males have the proper means to attract them. This could lead to hybridization between the firefly species. The reason that hybridization can reduce species diversity is because of outbreeding depression. This means that hybrid offspring are less fit to survive in their environment than their parents (2). Years of evolution have made the pure species adapted for a specific environment (specific plants); however, when the species produce hybrid offspring, these adaptations may be lost and reduce the offspring’s chance of survival. Also, even if the hybrid offspring do possess the tools to adequately survive in their environment, they may be sterile and cannot further reproduce (2). If fireflies are hybridizing, the hybrid offspring are probably less fecund, and even if they are fecund, hybridization by definition reduces genetic diversity in the population.
Dr. Chew has suggested that the decline in the number of firefly species is due to the introduction of a nonnative firefly. This nonnative species eats other female fireflies by using aggressive mimicry. Aggressive mimicry is when an animal exploits the sensory biases of other animals for their own benefit (1). In this case, the nonnative species knows that the native firefly species is looking for potential mates, which is done so by specific flashes of light. In order to exploit this sensory bias, the nonnative species will flash his light pretending to be a potential mate. However, when the female approaches, she is eaten by the much larger nonnative species. This type of aggressive mimicry has already been shown to take place by the female firefly of the genus, Photuris. In the research completed, it was found that the males searching for mates by flashing were more than five times more likely to be answered by the imposter than an actual potential mate (3). Depending on the population size of this nonnative species, they may be a cause for the decline in firefly species diversity. If females are being depredated at unnaturally high rates, then fewer females are available for mating. This could lead to a decrease in the number of firefly species since less reproduction is taking place.
Because fireflies are among a specialized group of animals that uses phosphorescence as a means to communicate and attract mates, they are naturally attuned to perceiving light sources including polarized light. Dr. Brillient believes that anthropogenic light pollution and polarizing man-made structures are interfering with the female fireflies’ ability to select a mate. Polarized light is the process where normal three-dimensional light waves strike a surface and are reflected back in only a horizontal or 2D orientation (4). Courtship from males to females is primarily to communicate species identity and secondarily to communicate male fitness (5). Studies on butterflies, frogs, crabs, and fish all suggest that the intensity of stimulation received by the female is critical to mate choice, and that any interruption to this communication greatly affects the ability of the female to choose (5). Consequently, a female firefly in search of an attractive mate, will be attracted to the more intense manmade light source, and miss opportunities for a successful mating. This is an example of an evolutionary trap, the phenomenon where an adaptive trait suddenly becomes maladaptive through a rapid environmental change (1). Though not always in the form of light, other studies have suggested that any sensory pollutant, be it light, acoustic, or otherwise, has similar detrimental effects (6). Many anthropogenic sources, such as glass panes or asphalt roads, share the same polarizing characteristics as the surface of natural bodies of water. It is common in many animals, and especially insects, to use polarized light patterns in the sky as a means to orient themselves for dispersal or migration. Likewise, nocturnal insects who exhibit phototactic behavior (orientation towards light sources) often are attracted to artificial sources of light and become trapped there, subject to increased predation, maladaptive migration behavior, structure collision, altered resource allocation, and decreased opportunity to exhibit natural behaviors such as foraging and mating (4), all of which can affect the diversity and distribution of populations. Being so sensitive to light sources, fireflies are susceptible to evolutionary traps caused by light pollution.
The loss of species diversity in Grand Fog Mountain Park has raised concerns for many people namely because the park attracts thousands of visitors to view the impressive array of firefly mating displays. The three leading firefly experts Dr. Bush, Dr. Chew, and Dr. Brillient hypothesized that human activity is causing the decline. Dr. Bush suggested that the introduction of an invasive honeysuckle bush that outcompetes multiple species of plants is inhibiting genetic divergence and isolation as well as reducing available habitat for reproduction. Dr. Chew believes a different invasive species, an invasive firefly predator, is using aggressive mimicry to lure and depredate on female fireflies, attracted by the false display. Dr. Brillient's hypothesis states that anthropogenic sources that produce and/or polarize light are disrupting the fireflies’ ability to communicate and select mates. All of the hypotheses involve the interruption of the fireflies’ unique and complex reproductive behavior, but in different ways. It is possible that, due to the park’s location to human development and the known introduction of two invasive species, the decline in firefly species diversity has multiple causes.
Citations:
1) J. Maerz, Personal communication.
2) Rhymer, J. M. 2006. Extinction by hybridization and introgression in Anatinae ducks. Acta Zoologica Sinica 52 (Supplement): 583-585.
3) El-Hani, Charbel N., Joao Queiroz, and Frederik Stjernfelt. "Firefly Femmes Fatales: A Case Study in the Semiotics of Deception." Academia.edu. Springer Science & Business Media B.V, n.d. Web.
4) Horvath, et all. 2009. Polarized light pollution: a new kind of ecological photopollution. The Ecological Society of America: 317-324.
5) Ryan, M. 1990. Sexual selection, sensory systems and sensory exploitation. Oxford surveys in evolutionary biology 7: 157-195.
6) Kight, Caitlin & Swaddle, John. 2011. How and why environmental noise impacts animals: an integrative, mechanistic review Ecology Letters, 14: 1052–1061.