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Climate and species are a-changin’

8 Oct

This summer was a perfect example of environmental changes. In the northern states, some waterfowl species never migrated south last fall, and this spring’s surveys revealed that some birds migrated north earlier than normal. Conversely, milkweed (Asclepias spp.) emerged one to two weeks early and migrating monarch butterflies arrived two to three weeks late in all the northeastern states. Areas in a few northeastern states were stricken with a historic “100-year or more drought.” On the other hand, many areas in the semi-arid southwest experienced historical devastating floods.

Is this a fluke? Or is this the new norm? Perhaps somewhere in the middle, but more likely these weather and climatic changes are the ‘new normal.’  The events and data support that assessment.*

How do living organisms respond?

Published studies by biologists have been documenting the impact of climate change on the environment, especially species that are adapting and not adapting. We can learn about impacts on organisms  by examining changes in cyclic and seasonal natural phenomena of plants and animals in relation to climate. These seasonal changes and cycles are known as phenology. Noting the times of year that specific plants bloom, or when birds migrate are two examples. Comparing the phenology of many species over a period of time can reveal informative clues on how changes in climate may affect them. Many studies along this model of investigation demonstrate that the living environment is indeed impacted.

United Kingdom researcher Stephen Thackeray(1) and his colleagues analyzed the phenology of a wide range of species. They used 10,003 phenological data sets to determine if and how much species’ phenology have changed over a minimum of 20 years. The analysis revealed that phenology has shifted in unequal rates in different species groups. Thus, climate change leads to disruptions of the phenological match between species, which often impacts ecological relationships.

Another question the researchers asked was how sensitive events in their life cycles are to the two most common variables in climatic change: temperature and precipitation. Both variables have changed in an uneven process over the flow of seasons. How does this impact species relationships? Some periods of the year have warmed faster than others, which may affect two interrelated species with equal temperature sensitivities but at different times. This could shift their phenological events at different rates and cause a mismatch in their relationship.

For example, milkweed plants emerging and flowering much earlier than normal resulted in sub-optimal conditions for the late-arriving monarch butterflies to use the plants for breeding. Additionally, the persistent hot and humid weather in the northeast could impact monarch larva (caterpillar) by either accelerating or arresting development.

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Trophic levels

The study authors also discovered a difference of sensitivity to temperature variations at different positions of the food chain (referred to as trophic levels). Species at different levels did not differ in the time of year at which they were sensitive to annual variations in temperature. But they did vary in how sensitive they were.

 

Species in higher levels of the food chain (the secondary consumers) are less sensitive to temperature changes than species at the bottom (the producers and primary consumers). These species are twice as sensitive to temperature changes than upper level species. Secondary consumers are also less sensitive to precipitation variations.

The authors then combined the species sensitivities with a future climate scenarios. They forecast that primary consumers -birds, insects, small mammals, etc- will shift the timing of their phenological events by twice as much as will species at other levels of the food chain. One reason their response varies is because species at different tropic levels respond differently to exactly the same temperature cue. Species respond differently to temperature during various times of the year.

The above example of the milkweed and monarch butterfly mismatch could impact the breeding success and thus population numbers of the butterflies. Both species have different physiological mechanisms that determine their phenological events and use different cues to determine their timing. Although these cues will be correlated to some extent, the cue used by the consumer -in this example, the monarch butterfly- is less reliable than that of the the plant they rely on. This cue unreliability in the consumers may mean that they will evolve with less temperature sensitive phenology than those species at the trophic level they rely on.

Ecologist Marcel Visser (at Netherlands Institute of Ecology) calls attention to moving from conventional two-species interaction research to a more holistic approach: investigating the effects of climatic change on the entire food-web. In a review(2) of the Thackeray, et al. study, Visser additionally proposes that impacts by phenological mismatches could be buffered by other mechanisms in their ecosystems.

To help us understand the consequences of phenological mismatches and thereby form predictions, he proposes questions that should be considered in studying changes in climate changes and relationships:

How are the strengths of the links in a food web affected by phenological mismatches? What happens if the phenology of species at one trophic level shifts more than that of species at another? Does this lead to the loss of some links and the formation of others? Does this destabilize the web? Such analyses would be a stepping stone from studying the phenological shifts of species to understanding the effects of
climate change on ecosystem function.(2)

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Stink bug preys on larva.

An example for a holistic ecosystem approach is field observations (my own and in the literature) that have suggested that as prolonged temperatures increase, depredation and parasitism of monarch larvae and adults increase. Is this a function of differences in phenology of  monarchs and its predators, or changes in all vegetation and species interactions (a complex of one or more phenological overlapping and mismatches)  in the habitat? Do temperature mismatches in other members of the monarch habitat increase risk or rates of depredation?

One research team suggested that migration of monarch butterflies may have evolved as an adaptation to decrease depredation and parasitism in their breeding habitats. If monarch adults were to delay or ignore cues to migrate because of changing climate, how would that impact their overall population?

Adding to the complexity, climate sensitivity in species is not fixed. Phenological mismatches can lead to selection on the timing of phenological events. Resilience to environmental challenges can alter phenology, but over time can also result in genetic changes to sensitivity, thereby fixing phenological changes. Conventional theory on temperature range sensitivity of monarch adults and larvae states that it quite narrow. However, some observations(3) of their coping mechanisms with prolonged high temperatures in the Pacific Northwest sub-population questions if this sensitivity range is more flexible than conventional thought, or if this could be a developing adaptation.

Some researchers are already investigating genetic changes accompanying phenological adaptations to climate change (e.g. genetic alterations in melanin associated with plumage and physiology in European owls that have adapted to changing ecosystems). Such complex studies must be conducted to forecast the impacts of climate change and phenological responses and ecosystem function.

Research by Thackeray, Visser, and other colleagues demonstrates that long-time series of data are essential for such investigations. They also applaud and encourage professional and citizen scientists to continue collecting and submitting observations to add to the data pool. As Visser commented, “The additional advantage is that observing phenological shifts in, sometimes literally, your own backyard drives the message of global climate change home.”


(1) Thackeray, SJ, et al. “Phenological sensitivity to climate across taxa and trophic levels”. Nature 535, 241–245 (14 July 2016)
(2) Visser, ME. “Interactions of climate change and species”. Nature 535, 236–237 (14 July 2016)
(3) Anecdotal observations by Dr. David James, Washington State University entomologist, in central Washington and myself at Malheur National Wildlife Refuge, eastern Oregon.

* The main difference between weather and climate is time. Weather is the atmospheric local events over a short period of time.  Climate is an average of the weather over much longer time in a region or globally. Sure, we can agree that weather and climate is cyclic, with highs and lows historically up and down. Also, a few episodic variances from the average can be expected.  But climate does not vary as greatly as weather. The trends clearly demonstrate that climate is changing. Modern paleoclimate technologies can now add to the 70-year human records of climatic changes, both which confirm that climate change is a reality. Those changes have accelerated, more than any other equal span of time in historical evidence.

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Wings of mimicry

17 Sep

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The early rising sun greeted us with a visitor in the damp grass one morning of a camping trip near New York’s Thousand Islands. As sunlight glistened on the blanketing dew, this large winged visitor rested on the grass waiting for moisture to evaporate off its wings and the sun to warm its body. It reminded me of another large moth, the luna moth (Actia luna), that I knew well during my life in the Maine woods.

The moth found on the wet grass that morning was a polyphemus moth (Antheraea polyphemus). Both moths are of the Giant Silkmoth family (Saturniidae). With a wingspan of up to six inches or more, the polyphemus moth is about the same size as a luna moth. These two species are the largest moths in continental America and may be found from Canada to northern Mexico.

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Male polyphemus moth (antenna are larger than female’s)

Polyphemus moths are generalists, which means they do not require a specific species of plant for the larvae to develop and survive. Females lay flat brown eggs on many species of decidous trees: elm, birch, willow, maple, beech, locust and a variety of Prunus species (cherry, plum, peach, etc). Like many Lepidoptera, polyphemus larvae develop through five stages and molts (instar). Unlike monarch butterflies, of which the instars are very similar in coloration, these moths have slightly different coloration with each instar. The fifth and final instar is an average of four inches long and a bright green color with silver spots on its sides. A caterpillar can devour about 86,000 times its weight from emergence to full development in two months.

From the photos of the adult moth below one can see hair-like body scales, small head and mouth parts, and the eye spots on the wings. Because of their small mouth parts, adults do not eat and only live for a week or less, during which their entire purpose is to avoid depredation and reproduce.

Mimicry

Mimicry throughout the animal kingdom is an example of natural selection in evolution. Ranging from mammals to tiny insects, mimicry may increase survival of individuals in their environment. Or it may reduce survival in another environment.

Lepidoptera are fascinating examples of how mimicry enables survival. One tactic is to mimic another insect that may be undesirable prey. Another tactic is the patterns and structures on butterfly and moth wings that mimic a component of their environment to hide from depredation. These tactics may be adaptive defense mechanisms (or artifacts of other patterns of coloration) in response to threats. Our polyphemus moth will serve as an example of mimicry as a defense mechanism.

Distraction Pattern

Like many saturniids the polyphemus moth has large ‘eye spots’ on its hind wings. These wing eye spots are translucent ‘windows’ which may be surrounded by bright colors. The pair of eye spots on the polyphemus hind wing are bordered by bright colors and, with the entire wing pattern, may resemble eyes of a predator. These are distraction patterns, which is a form of mimicry. They may resemble eyes of a different animal and confuse or deceive potential predators.

Wing eye spots can be a form of self-mimicry and a distraction pattern: to draw a predator’s attention away from the most vulnerable body parts or to appear as an inedible or dangerous animal. When threatened, adult polyphemus moths flash their  wings exposing the large hind wing eye spots to distract, startle, or scare off potential predators.

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The centers of the eye spots lack scales, so they are transparent.

Another example of distraction pattern in mimicry is camouflage which helps avoid detection by predators. Eye spots and wing color patterns on adult polyphemus can serve as blending camouflage (color matching) and pattern camouflage (pattern matching) in their environment.

Unrelated to mimicry, these eye spots may also play a role in mate attraction, but this has not been conclusively confirmed.

dscn1811-sMimicry is also exhibited by the polyphemus caterpillars. They can be protected from predators by their cryptic green coloration (another example of what kind of distraction pattern?). When threatened the caterpillars often raise the front part of the body up in a threatening pose. If attacked, the caterpillars make a clicking noise with the mandibles.  This clicking is associated with a distasteful fluid exuded by the caterpillars which can cause regurgitation by the attacker. Some animals (squirrels, birds, other insects) are deterred by the ingestion and regurgitation and the clicking may serve as a warning.

Mimicry and names

Since one of my interests is the etymology of animal binomial names (simply put, the naming of things), mimicry also plays a part in this moth’s name.

The four silkmoth species in the New World (the Americas) were assigned to either Telea or Metosamia genus. The polyphemus silkmoth in the Americas was first described and named by Dutch naturalist Pieter Cramer in 1776 as Telea polyphemus. Jacob Hübner, a German entomologist (1761-1826), assigned the Old World (endemic to Asia and Europe) silkmoths to the genus Antheraea in 1819.  In 1952, American entomologist Charles Duncan Michener (1918-2015) systematically categorized the Telea and Metosamia in with Antheraea classification. All the silkmoths are now in one genus classification.

The Modern Latin genus name Antheraea likely derives from the Greek anthēros, meaning brightly colored, brilliant, or flowery.  The Lepidoptera Antheraea type species (the species on which the description of a genus is based on, and with which the genus name remains associated during any taxonomic revision) is the beautiful and vibrantly-colored tasar silkworm (Antheraea mylitta, formerly Phalaena mylitta), named and characterized in 1773 by English entomologist Dru Drury. Although not a silkworm like the tasar species, the polyphemus is colorful and has similar eye spots.

Cramer’s choice of the species name was based on Polyphemus, the giant cyclops from Greek mythology who had a single large, round eye in the middle of his forehead. Cramer may have been reminded of the name because of the large eye spots in the middle of the hind wings.

And the commonly used name ‘sphinx’ moth?  It could have arisen because of the behavior of threatened larvae. When they raise their heads and thoraxes up, the pose superficially resembles Egyptian sphinxes. Someone had imagination.

Of course, the family name Saturniidae  also peaked my curiosity.  The consensus is that it was based on the eye spots of some members of the family that contain concentric rings reminiscent of the planet Saturn. I’ll take that, too.

 

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