Invasive crayfish hit the self-destruct button

One important feature of a biological invasion is that invaders can change an entire ecosystem in a substantial way.  A possible outcome of this change is that, in theory, an invasive species could inadvertently make an ecosystem less suitable as a habitat for itself.  Does this happen, and if so, under what circumstances?  One reason invasive species are so successful is that they usually can increase in population size very quickly.  Ecologists have discovered that species with the potential to increase very quickly may also have the potential to decline equally rapidly and then increase again, going through repeated boom-bust cycles of population size.  Thus if an invasive species starts to decline, it does not always mean that this decline will continue over time. Consequently, monitoring a biological invasion for only a few years may give a misleading picture of long-term prognosis for the invasive species and the ecosystem.

Eric Larson was able to address these problems when he began his postdoctoral research with David Lodge at the University of Notre Dame in 2014. Lodge (and John Magnuson before him) has studied the rusty crayfish (Faxonius rusticus) invasion in 17 northern Wisconsin lakes since the 1970s, using the same bait (beef liver) and the same traps on the same days each year.

Gantz_BeefLiver

Crysta Gantz prepares to bait a trap with beef liver, which the crayfish love, but she – not so much! Credit: Eric R. Larson.

Three graduate students (the other co-authors of the paper) had continued data collection and done extensive mapping of the lake bottoms.  When Larson joined the research program he had about 40 years of data and 17 well-described lakes.  He knew that rusty crayfish were declining in some lakes and not others, and he and his colleagues were ready to explore whether these declines could be tied in to some environmental variable that the crayfish were influencing in some lakes, but not others.

AllequashLake

Allequash Lake. Credit Eric R. Larson

As an avid fisherman (more in my mind than in actuality), I have, on many occasions, caught a nice bass only to have it regurgitate the contents of its stomach, which usually includes bits of crayfish.  As it turns out, predacious fish such as bass love to eat crayfish, and crayfish are more likely to survive in environments that provide hiding places such as rocks or luxurious macroalgae that grow in sand or muck. The problem is that crayfish enjoy dining on macroalgae, so they can do themselves a disservice by eating their shelter from predators, effectively changing their environment so that their invasion is no longer sustainable.  Does this actually happen?

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Two rusty crayfish discuss the issues of the day. Credit: Eric R. Larson.

Larson and his colleagues continued collecting data on 17 lakes, and used their long-term data set to evaluate whether rusty crayfish populations were not declining (steady or increasing), declining or occupying an ambiguous gray zone where there was no clear trend in how the population was changing. The analysis showed that three lakes were not declining since the rusty crayfish invasion, eight lakes had declined substantially and six lakes were ambiguous.

LarsenFig1

The researchers turned their attention to the lake-bottom substrate.  Were rusty crayfish more successful in rocky bottom lakes that gave them continuous predator protection?  Their analysis indicated that the three lakes where the invasion was going strong had the rockiest substrate, while the eight lakes experiencing population declines after the rust crayfish invasion were significantly less rocky.

LarsenFig2

Proportion rocky substrate in lakes whose rusty crayfish populations are in decline (red), have an ambiguous trend (black) or are not in decline (blue). The horizontal line within each box is the median value, box bottom and top are 25th and 75th percentile, and whiskers are the 10th and 90th percentile. Non-overlapping letters above the bars (a and b) indicate significant differences between the groups.

The researchers conclude that in the absence of rocky substrate, the rusty crayfish is eating the aquatic macrophytes that grow from the sandy lake bottom, thereby exposing itself to predators.  Larson and his colleagues recommend simultaneous surveys of crayfish populations and density of aquatic macrophytes to see whether lakes may oscillate between states dominated by one or the other.

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Captured crayfish. Photo Eric R. Larson

Researchers want to know how commonly invasive species modify habitat in a self-destructive way.  A literature review of invasive species declines failed to find much evidence, but there are not enough long-term data sets to get a sense of how frequently this occurs. The problem is that researchers need to monitor the invasive species population and the relevant habitat variables for an extended time period.  The jury is still out on this question and only time (and careful data collection) will tell.

note: the paper that describes this research is from the journal Ecology. The reference is Larson, E. R.,  Kreps, T. A.,  Peters, B.,  Peters, J. A., and  Lodge, D. M.  2019.  Habitat explains patterns of population decline for an invasive crayfish. Ecology  100( 5):e02659. 10.1002/ecy.2659. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Invasive crayfish depress dragonflies and boost mosquitoes

Paradoxically, obliviousness and intense focus can be two sides of the same coin, as the following story highlights.  As a new graduate student at the University of Minnesota, I took a field ecology course at the University’s field station at Lake Itasca (famously known as headwaters of the Mississippi River).  One afternoon we watched dragonflies at a small pond; the male dragonflies were obviously patrolling territories and behaving thuggishly whenever intruders came by, and amorously whenever females of their species approached.  Surprisingly, territorial males chased off male intruders of any species, even though they posed no reproductive threat to them.  Why, I wondered…  So I sat there for many hours and kept very careful track of who chased whom, and for how long.  Big focus time. Ultimately, these observations blossomed into my doctoral dissertation.  Unfortunately, these observations also blossomed into the most virulent case of poison ivy known to humanity, as my intense focus on dragonflies obliviousized me to the luxurious patch of poison ivy, which served as my observation perch.

Anax junius Henry Hartley

Anax junius dragonflies in copula.  The male has the bright blue abdomen.  Credit: Henry Hartley.

Despite this ignoble incident, dragonflies remain one of my favorite animal groups.  They are strikingly beautiful, brilliant flyers, and fun to try to catch. In addition, they have so many wonderful adaptations, including males with penises that are shaped to scoop out sperm (previously introduced by another male) from their mate’s spermatheca, and females who go to extremes to avoid repeated copulation attempts, for example, by playing dead when approached by a male. Thus I was delighted to come across research by Gary Bucciarelli and his colleagues that highlighted the important role dragonflies play in stream ecosystems just west of Los Angeles, California.

Back Camera

Captured dragonfly nymph.  Dragonflies require from one to four years to develop in aquatic systems, before they metamorphose into terrestrial winged adults. As nymphs, they are fearsome predators on aquatic invertebrates. As adults, they specialize on winged insects, though there are stories of them killing small birds. Credit: Gary Bucciarelli

Bucciarelli and his colleagues came up with their research question as a result of working in local streams with students on a different project.  They wanted to know if invasive non-native crayfish (Procambarus clarkii) affect the composition of stream invertebrates and whether removal of crayfish could lead to rapid recovery of these invertebrate communities.

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Invasive crayfish, P. clarkii, sits on the stream bottom. Credit: Gary Bucciarelli

The researchers collected stream invertebrate samples and noticed a dramatic pattern – in all the streams with crayfish there were numerous mosquito larvae, but in all of the streams without crayfish there were no mosquito larvae and much greater numbers of dragonfly nymphs. This led them to formulate and test two related hypotheses. First, dragonfly nymphs (Aeshnaspecies) are more efficient predators on mosquitoes (Anopheles species) than are the invasive crayfish. Second, crayfish interfere with dragonfly predation on mosquitoes in streams where crayfish and dragonflies are both present.

Field Sampling

Student researchers collect stream samples. Credit: Gary Bucciarelli

Bucciarelli and his colleagues systematically sampled 13 streams monthly from March to October 2016 in the Santa Monica Mountains. Eight streams have had crayfish populations since the 1960s, while four streams never had crayfish, and one stream had crayfish removed as part of a restoration effort in 2015. Overall, streams with crayfish had a much lower number of dragonfly nymphs than did streams without crayfish.  In addition, streams with crayfish had substantial populations of Anopheles mosquitoes, while streams without crayfish (but much higher dragonfly populations) had no Anopheles mosquitoes in the samples.

BuccTable1

Number of mosquito larvae (MSQ) and dragonfly nymphs (DF)  by month in streams with crayfish (CF – top row of data) or without crayfish (CF Absent – bottom row)

This field finding supports both of the hypotheses, but the evidence is purely correlational.  So the researchers brought the animals into the laboratory to test predation under more controlled conditions.  They introduced 15 mosquito larvae into tanks, and exposed them to one of four treatments: (1) a single crayfish, (2) a single dragonfly nymph, (3) one crayfish and one dragonfly nymph, or (4) no predators. The researchers counted the numbers of survivors periodically over the three day trials. As the graph below indicates, dragonflies are vastly superior consumers of mosquito larvae compared to crayfish.  However, when forced to share a tank with crayfish, dragonflies stop hunting, either huddling in corners or actually perching on the crayfish.  By 36 hours into the experiment, all of the dragonflies had been eaten by the crayfish.  After three days, mosquito survival was similar when comparing tanks with crayfish alone with tanks that had both a crayfish and a dragonfly.

BuccFig2A

Mean number of surviving mosquito larvae in tanks with a lone dragonfly (DF), a lone crayfish (CF), one crayfish and one dragonfly (CF+DF) in comparison to controls with no predators.

Bucciarelli and his colleagues conclude that dragonfly nymphs are much more efficient predators of mosquito larvae than are crayfish. But when placed together with crayfish, dragonfly foraging efficiency plummeted. Field surveys showed a negative correlation between crayfish abundance and dragonfly larvae, and much greater mosquito larva populations in streams with crayfish.  This supports the conclusion that invasive crayfish cause mosquito populations to increase sharply by depressing dragonfly populations and foraging efficiency.  This is a complex trophic cascade because crayfish increase mosquito populations despite eating a substantial number of mosquito individuals.

The researchers argue that crayfish probably relegate dragonfly larvae to inferior foraging habitats, thereby limiting their efficiency as mosquito predators. As such, ecosystem services provided by dragonflies to humans are greatly diminished.  Recently, several new mosquito species that are disease vectors have moved into California.  Thus the loss of dragonfly predation services could pose a public health threat to the human population.  Bucciarelli and his colleagues recommend removing the invasive crayfish to restore the natural community of predators, including dragonflies, which will then naturally regulate the increased number of potential disease vectors.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Bucciarelli, G. M., Suh, D. , Lamb, A. D., Roberts, D. , Sharpton, D. , Shaffer, H. B., Fisher, R. N. and Kats, L. B. (2019), Assessing effects of non‐native crayfish on mosquito survival. Conservation Biology, 33: 122-131. doi:10.1111/cobi.13198. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Intertidal tussles: a shifting balance

As an omnivore with a research-oriented palate, I delight in consuming many different food types.  High on my list are crustaceans – in particular the American lobster, Homarus americanus.

BaillieLobster

A juvenile American lobster, Homarus americanus. Credit: C. Baillie.

However, another crustacean, the invasive Asian shore crab, Hemigrapsus sanguineus, threatens to disrupt my epicurean delight, by interfering with the growth and development of juvenile lobsters in the low intertidal zone in the north Atlantic. Christopher Baillie and Jonathan Grabowski have explored interactions between these lobsters and crabs to unravel how they might be influencing each other.

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The invasive Asian shore crab, Hemigrapsus sanguineus. Credit: Rhode Island Marine and Estuarine Invasive Species Site.

The Asian shore crab was first detected off the New Jersey coast in 1988 and quickly spread from North Carolina to Maine. Their increase has coincided with a sharp decrease in the abundance of their rival green crabs over the same range. Baillie and Grabowski were concerned that the Asian shore crab could also be adversely affecting lobster populations. They did monthly surveys (May-October) of both lobster and crab densities in Dorothy Cove in Masachusetts, USA, between 2013 and 2017, and discovered that crab populations were increasing sharply at the same time that lobster populations were decreasing steadily.

BaillieFig1

Annual average densities of Asian shore crabs (dark gray) and American lobsters (light gray) from surveys at Dorothy Cove, Nahant, Massachusetts, USA, between 2013 and 2017. Error bars are 1 standard error.

The researchers wanted to know whether the increased number of Asian shore crabs was responsible for the lobster decline. Perhaps the two species competed with each other for shelter. Baillie and Grabowski set up experimental tanks, each containing a wire mesh bottom with a rectangular opening cut in the center, so that a burrow could be excavated.  They then introduced a single lobster or crab to the tank, and allowed it to dig a burrow in the cutout center (we’ll call this individual the resident).

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In one shelter experiment, the researchers compared the behavior of larger (mean carapace length = 24.7 cm) and smaller (mean carapace length = 9.3 cm) juvenile lobsters in the presence and absence of a variable number of crabs. They discovered that both larger and smaller lobsters spent most of the time in their burrow when no crabs were in the tank. However, introducing crabs was a major disruptor to their mellow existence, with both lobster size classes being more likely to abandon their residences when crabs were present.

BaillieFig3final

Mean (+ standard error) percentage of time spent in shelter by large juvenile lobsters (top graph) and small juvenile lobsters (bottom graph) in relation to absence (Control) or presence of different numbers of crabs.  Different letters above the bars indicate that the means are statistically different from each other.

The reasons for the decline in residence time were very different for large vs. small lobsters.  In an experiment with one large lobster pitted against one crab, resident lobsters initiated an average of 18.00 attacks against crabs, while resident crabs initiated an average of only 0.20 attacks against lobsters. Even if crabs were allowed to establish residency, when a lobster was introduced, it usually picked a fight with the resident crab. So large resident lobsters left their burrows to challenge intruding crabs. Lobsters managed to kill and eat two intruding crabs.

In contrast, smaller lobsters had a much different experience. Crabs attacked resident small lobsters and were able to displace them from their burrow. This was particularly the case when a greater number of crabs were added to the tank.  When eight crabs were added, the poor lobster was kicked out of its burrow, on average, almost 20 times within a six-hour trial.  Under these conditions, crabs attacked the resident lobster almost 40 times per trial.

BaillieFig4

Crab behavior towards a resident lobster in relation to the number of crabs (heterospecific competitors) introduced into the tank. (A) Mean number of times the lobster is displaced. (B) Mean number of fights initiated by an intruder crab. Error bars are 1 standard error. Different letters above the bars indicate that the means are statistically different from each other.

Baillie and Grabowski also conducted feeding trials – but only with a larger lobster pitted against an individual crab (a blue mussel – a preferred food item for both species – was the prey).  Lobsters were much more successful feeders than crabs, and actually increased their feeding rates in the presence of crabs, presumably having no interest in sharing the mussel with its competitor. Taken together, the shelter and feeding experiments suggest a reversal in dominance structure occurs over the course of lobster development.  The abundant Asian shore crab outcompetes small juvenile lobsters for shelter, but once lobsters attain a certain size, they can outcompete crabs for both shelter and food. We still don’t know, for sure, whether the decline in lobsters in the low intertidal zone at the study site was caused by the increase in crabs; the Asian shore crab may still be expanding its range, so it may be possible to more directly study changes in distribution at other sites both north and south of its current range. Fortunately for lobsters (and for lobster consumers), juveniles can also grow and flourish in deeper ocean waters, where Asian shore crabs are much less of a threat.

note: the paper that describes this research is from the journal Ecology. The reference is Baillie, C. J. and Grabowski, J. H. (2018), Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster. Ecology, 99: 2067-2079. doi:10.1002/ecy.2432. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Indirect effects of the lionfish invasion

I’m old enough to remember when ecological studies of invasive species were uncommon.  Early on, there was a debate within the ecological community whether they should be called “invasive” (which conveyed to some people an aggressive image akin to a military invasion) or more dispassionately “exotic” or “introduced.” Lionfish (Pterois volitans), however, fit this more aggressive moniker. Native to the south Pacific and Indian Oceans, lionfish were first sighted in south Florida in 1985, and became established along the east Atlantic coast and Caribbean Islands by the early 2000s. They are active and voracious predators, consuming over 50 different species of prey in their newly-adopted habitat. Many population ecologists study the direct consumptive effects of invasive species such as lionfish.  In some cases they find that an invasive species may deplete its prey population to very low levels, and even drive it to extinction.

Lionfish

A lionfish swims in a reef. Credit: Tye Kindinger

But things are not always that simple. Tye Kindinger realized that lionfish (or any predator that feeds on more than one species) could influence prey populations in several different ways.  For the present study, Kindinger considered two different prey species – the fairy basslet (Gramma loreto) and the blackcap basslet (Gramma melacara). Both species feed primarily on zooplankton, with larger individuals monopolizing prime feeding locations at the front of reef ledges, while smaller individuals are forced to feed at the back of ledges where plankton are less abundant, and predators are more common.  Thus there is intense competition both within and between these two species for food and habitat. Kindinger reasoned that if lionfish depleted one of these competing species more than the other, they could be indirectly benefiting the second species by releasing it from competition.

Basslets

Fairy basslet (top) and blackcap basslet (bottom). Credit Tye Kindinger.

For her PhD research, Kindinger set up an experiment in which she manipulated both lionfish abundance and the abundance of each basslet species.  She created high density and low density lionfish reefs by capturing most of the lionfish from one reef and transferring them to another (a total of three reefs of each density).  She manipulated basslet density on each reef by removing either fairy or blackcap basslets from an isolated reef ledge within a particular reef.  This experimental design allowed her to separate out the effects of predation by lionfish from the effects of competition between the two basslet species.  Most of her results pertained to juveniles, which were about 2 cm long and favored by the lionfish.

KindingerTable

Alex Davis

Alex Davis captures and removes basslets beneath a ledge. Credit Tye Kindinger.

Kindinger measured basslet abundance in grams of basslet biomass per m2 of ledge area.  When lionfish were abundant, juvenile fairy basslet abundance decreased over the eight weeks of the experiment (dashed line) but did not change when lionfish were rare (solid line).  In contrast, juvenile blackcap basslet populations remained steady over the course of the study, whether lionfish were abundant or rare. Kindinger concluded that lionfish were eating more fairy basslets.

KindingerFig12A

Abundance of juvenile fairy basslets (left) and blackcap basslets (right) as measured as change in overall biomass. Triangles represent high lionfish reefs and circles are low lionfish reefs.

Competition is intense between the two basslet species, and can affect feeding position and growth rate.  Kindinger’s manipulations of lionfish density and basslet density demonstrate that fairy basslet foraging and growth depend primarily on the abundance of their blackcap competitors. When competitor blackcap basslets are common (approach a biomass value of 1.0 on the x-axis on the two graphs below), fairy basslets tend to move towards the back of the ledge, and grow more slowly.  This occurs at both high and low lionfish densities.

KindingerFig1BC

Change in feeding position (top) and growth rate (bottom) of fairy basslets in relation to competitor (blackcap basslet) abundance (x-axis) and lionfish abundance (triangles = high, circles = low)

In contrast, blackcap basslets had an interactive response to fairy basslet and lionfish abundance. Let’s look first at low lionfish densities (circles in the graphs below).  You can see that blackcap basslets tend to move towards the back of the ledge (poor feeding position) at high competitor (fairy basslet) biomass, and also grow very slowly.  But when lionfish are common (triangles in the graphs below), blackcap basslets retain a favorable feeding position and grow quickly, even at high fairy basslet abundance.

KindingerFig2BC

Change in feeding position (top) and growth rate (bottom) of blackcap basslets in relation to competitor (fairy basslet) abundance (x-axis) and lionfish abundance (triangles = high, circles = low)

By preying primarily on fairy basslets, lionfish are changing the dynamics of competition between the two species. The diagram below nicely summarizes the process.  Larger fish of both species forage near the front of the ledge, while smaller fish forage further back.  But there is an even distribution of both species.  Focusing on juveniles, they are relatively evenly distributed in the rear portion of the ledge (Figure B).  When fairy basslets are removed experimentally, the juvenile blackcap basslets move to the front of the rear portion of the ledge, as they are released from competition with fairy basslets (Figure D).  Finally, when lionfish are abundant, fairy basslets are eaten more frequently, and juvenile blackcaps benefit from the lack of competition (Figure F)

KindingerFig3

Kindinger was very surprised with the results of this study because she knew the lionfish were generalist predators that eat both basslet species, so she expected lionfish to have similar effects on both prey species.  But they didn’t, and she does not know why.  Do lionfish prefer to eat fairy basslets due to increased conspicuousness or higher activity levels, or are blackcap basslets better at escaping lionfish predators? Whatever the mechanism, this study highlights that indirect effects of predation by invasive species can influence prey populations in unexpected ways.

note: the paper that describes this research is from the journal Ecology. The reference is Kindinger, T. L. (2018). Invasive predator tips the balance of symmetrical competition between native coral‐reef fishes. Ecology99(4), 792-800. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Too much of a good thing is killing Monarch butterflies

There was a time in the mid-Pleisticine when a photo of an ecological event was an awesome novelty, and a movie of an ecological event even more so.  Dodderers of an ecological bent (myself included), can vividly recall viewing a series of photos or a movie, either in a seminar or in an ancient ecology text, of a blue jay consuming a monarch butterfly, Danaus plexippus.  Consumption is immediately followed by explosive vomiting, as the cardenolides within the monarch butterfly claim another victim.  The monarch sequesters these cardenolide toxins from its larval food (milkweed), and incorporates them into its tissues as a means of protecting itself from predators – presumably blue jays learn from this very aversive experience.  I should point out that the individual sacrificial butterfly enjoys no fitness from this learning event – which raises some evolutionary questions we will not explore at the present.

Karen Oberhauser

Five instars (stages of development) of monarch caterpillars on a milkweed leaf. Credit: Karen Oberhauser

Rather we turn our attention to the relationship between milkweed, monarchs, and climate change. In several places in this blog we’ve talked about how climate change has influenced the behavior or physiology of a single species. For example, my first blog (Jan 31, 2017) discusses how increasing temperatures create more females in a loggerhead turtle population. But there are fewer studies that explore how climate change influences the ecological landscape, ultimately affecting interactions between species.  Along these lines, Matt Faldyn wondered if increased air temperature would change the chemical constitution of milkweed in a way that might influence monarch populations.  As he describes, “With milkweed toxicity, there is a ‘goldilocks’ zone where monarchs prefer to feed on milkweed that produce enough toxins in order to sequester these (cardenolide) chemicals as an antipredator/antiparasite defense, while also avoiding reaching a tipping point of toxicity where feeding on very toxic milkweeds negatively impacts monarch fitness.” He expected that at higher temperatures, milkweed would become stressed, and be physiologically unable to sustain normal levels of cardenolide production.

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Monarch butterfly feeds on a native milkweed, Asclepias incarnata. Credit: Teune at the English Language Wikipedia.

For their research, Faldyn and his colleagues worked with two milkweed species.  Asclepias incarnata is a common, native milkweed found throughout the monarch butterfly’s range in the eastern and southeastern United States.  Asclepias curassavica is an exotic species that has become established in the southern United States.  In contrast to A. incarnata, A. curassavica does not die back over the winter months; consequently some monarch populations are no longer migratory, relying on A. curassavicato provide them with a year round food supply.

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The exotic milkweed, Asclepias curassavica. Credit: 2016 Jee & Rani Nature Photography (License: CC BY-SA 4.0)

To protect against herbivory, milkweeds have two primary chemical deterrants: (1) the already-mentioned cardenolides, which are toxic steroids that disrupt cell membrane function, and (2) release of sticky latex, which can gum up caterpillar mouthparts and actually trap young caterpillars.

field_noborderii.jpgThe researchers wanted to simulate climate change under field conditions, so they created open-top chambers with plexiglass plates that functioned much like mini-greenhouses, into which they placed one milkweed plant that was covered with butterfly netting.  This setup raised ambient temperatures by about 3°C during the day and 0.2°C at nighttime.  Control plots were single milkweed plants with butterfly netting. Half of the plants were native milkweed, and the other half were the exotic species.

For their experiments, Faldyn and his colleagues introduced 80 monarch caterpillars (one per plant) and allowed them to feed normally until they pupated.  Pupae were brought into the lab and allowed to metamorphose into adults.

MattGood

Matt Faldyn holds two monarch butterflies in the laboratory. Credit Matt Faldyn.

At normal (ambient) temperatures, monarchs survived somewhat better on exotic milkweed.  But at warmer temperatures, there is a strikingly different picture. Monarch survival is unaffected by warmer temperatures on native milkweed, but is sharply reduced by warmer temperatures on exotic milkweed (top graph below). The few that managed to survive warm temperatures on exotic milkweed grew much smaller, based on their body mass and forewing length (middle and bottom graph below)

FaldynFig1

Survival (top), adult mass (middle) and forewing length (bottom) of monarch butterflies raised under normal (ambient) and warmed temperatures.  Error bars are 95% confidence intervals.

Both milkweed species increased production of both types of chemicals over the course of the experiment. But by the end of the experiment, the exotic species released 3-times the quantity of latex and 13-times the quantity of cardenolides than did the native milkweed species.

FaldynFig2

Average amount of latex released at the beginning and end of the experiment.  Error bars are 95% confidence intervals.

FaldynFig2

Average cardenolide concentration at the beginning and end of the experiment.

The researchers argue that the exotic milkweed, Asclepias curassavica, may become an ecological trap for monarch butterflies, in that it attracts monarchs to feed on it, but will, under future warmer conditions, result in dramatically reduced monarch survival. Interestingly, these results are not what Faldyn originally expected; recall that he anticipated that temperature-stressed plants would reduce cardenolide production. The tremendous increase in cardenolide production in exotic milkweed at warmer temperatures may simply be too much toxin for the monarchs to process. The researchers predict that as climate warms, milkweed ranges will expand further north into Canada, and lead to northward shifts of monarch populations as well.  They urge nurseries to emphasize the distribution of native rather than exotic milkweed, so that monarchs will be less likely to become victims of this ecological trap.

note: the paper that describes this research is from the journal Ecology. The reference is Faldyn, M. J., Hunter, M. D. and Elderd, B. D. (2018), Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology. doi:10.1002/ecy.2198. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.