Prey populations: the only thing to fear is fear itself

In reference to the Great Depression, Franklin Delano Roosevelt is famously quoted as stating during his 1933 inaugural speech “the only thing we have to fear is fear itself.” Roosevelt was no biologist, but his words could equally apply to a different type of depression – the decline of animal populations that can be caused by fear.

FDR

Roosevelt’s inauguration in 1933. Credit: Architect of the Capitol.

Ecologists have long known that predators can depress prey populations by killing substantial numbers of their prey. But only in the past two decades or so have they realized that predators can, simply by their presence, cause prey populations to go into decline. There are many different ways this can happen, but, in general, a predation threat sensed by a prey organism can interfere with its feeding behavior, causing it to grow more slowly, or to starve to death. As one example, elk populations declined after wolves were introduced to Yellowstone National Park. There are many factors associated with this decline, but one factor is fear of predators causes elk to spend more time scanning and less time foraging. Also, elk tend to stay away from wolf hotspots, which are often places with good elk forage.

Liana Zanette recognized that ecologists had not considered whether predator presence can cause bird or mammal parents to reduce the amount of provisioning they provide to dependent offspring, thereby reducing offspring growth and survival, and slowing down population growth. For many years, she and her colleagues have studied the Song Sparrow, Melospiza melodia, on several small Gulf Islands in British Columbia, Canada. In an early study, she showed that playbacks of predator calls reduced parental provisioning by 26%, resulting in a 40% reduction in the estimated number of nestlings that fledged (left the nest). But, as she points out, Song Sparrow parents provision their offspring for many days after fledging; she wondered whether continued perception of a predation threat during this later time period further decreased offspring survival and ultimately population growth.

Song sparrow

The Song Sparrow, Melospiza melodia. Credit: Free Software Foundation.

Zanette’s student, Blair Dudeck, did much of the fieldwork for this study. The researchers captured nestlings six days after hatching , weighed and banded them, and fit them with tiny radio collars. They then recaptured and weighed the nestlings within a few hours of fledging (at about 12 days post-hatching) to assess nestling growth rates.

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Banded sparrow nestling with radio antenna trailing from below its wing. Credit: Marek C. Allen.

Three days after the birds fledged, Dudeck radio-tracked them, and surrounded them with three speakers approximately 8 meters from where they perched. For one hour, each youngster listened to recordings of calls made by predators such as ravens or hawks, followed, after a brief rest period, by one hour of calls made by non-predators such as geese or woodpeckers (or vice-versa). During the playbacks, Dudeck observed the birds to record how often the parents visited and fed their offspring, and whether offspring behavior changed in association with predator calls. This included recording all of the offspring begging calls.

BlairRadio

Blair Dudeck simultaneously uses a tracking device to locate Song Sparrows and a recorder mounted to his head to record their begging calls. Credit: Marek C. Allen.

Fear had a major impact on parental behavior. Parents reduced food provisioning vists by 37% when predator calls were played in comparison to when non-predator calls were played. They also fed offspring fewer times per visit, which resulted in 44% fewer meals in association with predator calls.

DudeckFig1

Mean number of parental provisioning visits (in one hour) in relation to whether predator (red) or non-predator (blue) calls were played. Error bars are 1 SE.

Hearing predator calls had no effect on offspring behavior – they continued to beg for food at a high rate, and did not attempt to hide.

Some parents were much more scared than others – in fact, some parents were not scared at all. The researchers measured parental fearfulness by subtracting the number of provisioning visits by parents during predator calls from the number of visits during non-predator calls. A more positive number indicated a more fearful parent (a negative number represents a parent who fed more in the presence of predator calls). The researchers discovered that more fearful parents tended to have offspring that were in poorer condition at day 6 and at fledging.

DudeckFig2

Offspring weight on day 6 (open circles) and at fledging (solid circles) in relation to parental fearfulness.  Higher positive numbers on x-axis indicate increasingly fearful parents.

Importantly, more fearful parents tended to have offspring that died at an earlier age. Based on this finding, the researchers created a statistical model that compared survival of offspring that heard predator playbacks throughout late-development with survival of offspring that heard non-predator playbacks during the same time period. They estimated a 24% reduction in survival. Combined with their previous study on playbacks during early development, the researchers estimate that hearing predator playbacks throughout early and late development would reduce offspring survival by an amazing 53%.

This “fear itself” phenomenon can extend to other trophic levels in a food web. For example recent research by Zanette and a different group of researchers showed that playbacks of large carnivore vocalizations dramatically reduced foraging by raccoons on their major prey, red rock crabs. When these carnivore playbacks were continued for a month, red rock crab populations increased sharply. This increase in crab population size was followed by a decline of the crab’s major competitor – the staghorn sculpin, and the crab’s favorite food, a Littorina periwinkle. Thus “fear itself” can cascade through the food web, affecting multiple trophic levels in important ways that ecologists are now beginning to understand.

note: the paper that describes this research is from the journal Ecology. The reference is Dudeck, B. P., Clinchy, M., Allen, M. C. and Zanette, L. Y. (2018), Fear affects parental care, which predicts juvenile survival and exacerbates the total cost of fear on demography. Ecology, 99: 127–135. 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.

Crawling with caterpillars courtesy of climate change – and ants

In the book of Exodus, Yahweh inflicts upon the Egyptians ten plagues, several of which have biological bases. Plagues two three, four and five are frogs, lice, wild animals and diseased livestock. But it is the eighth plague that is relevant to today’s tale – the locust explosion. As it turns out, insect populations have periodically exploded throughout recorded history (and no doubt before), and for many years ecologists have been trying to understand why insect populations are so variable. Rick Karban has taught a field course at Bodega Marine Reserve, California since 1985, and, as he describes “In some years, the bushes are dripping with caterpillars and in others they are very difficult to find.  The wooly bears (Platyprepia virginalis) are so conspicuous and charismatic that I couldn’t help wondering what was responsible for their large swings in abundance (they are more than 1,000 times as abundant in big years than in lean ones).”

KarbanFig1

Wooly bear caterpillar density during annual surveys conducted in march of each year.

The early stage caterpillars are most common in wet marshy habitats, but as they develop, they move to dryer upland habitats where they pupate, metamorphose into moths and mate. Young caterpillars live in leaf litter, eating vegetation and decaying organic matter.

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Late instar (close to pupation) wooly bear caterpillar feeding on bush lupine. Credit Rick Karban.

Karban and his colleagues recognized that insect populations are sensitive to climate, and wondered whether climate change may be playing a role in Platyprepia population explosions. But there’s much more to climate change than global warming; for example, many areas of the world expect much more variable precipitation patterns, with more big storms and more droughts. Karban and his colleagues wanted to know whether variable precipitation might affect wooly bear populations. So they examined rainfall records between 1983 and 2016, and found that numerous heavy rainfall events (over 5 cm) in the previous year were correlated with increases in caterpillar abundance.

KarbanFig4

Change in caterpillar abundance in relation to number of heavy rainfall events (over 5 cm) during the previous year. Note the y-axis is the natural logarithm of the change in abundance.

Karban and his students explored three hypotheses for why caterpillars increased following a year with numerous heavy rainfall events. First, perhaps more rain causes more plant growth and deeper litter, providing extra food for caterpillars. Second, heavy rains may reduce the number of predacious ground-nesting ants. Lastly, heavy rains may produce deeper denser litter providing refuge from predacious ants.

The researchers tested litter as food hypothesis by comparing caterpillar growth rates during the summer, which usually has very little rainfall. They weighed individual caterpillars, placed them into cages and supplied them with litter from either wet or dry sites. After 30 days, they reweighed the caterpillars and found that all of them had lost weight, and that there were no significant differences in weight change between wet and dry sites. Thus, at least during the summer, there was no evidence that wet sites had better food for caterpillars.

Karban and his colleagues turned their attention to ants.

KarbanFig6

If ants stayed away from wet sites, that would suggest that rainy years may benefit caterpillars by reducing the number of ants in their habitat. To measure ant abundance, the researchers set out bait stations supplied with a sugar-laced cotton ball and 1 cm3 of hot dog. They discovered many more ants, and in particular, many more Formica lasioides (a fearsome caterpillar-killer) ants were recruited to dry sites than wet sites. This suggested that years with numerous rainfall events might reduce ant abundance, at least in the wet areas preferred by young caterpillars.

The researchers tested the ant predation hypotheses by caging caterpillars in plastic deli containers that had either window screen bottoms that allowed ants to enter but prevented the caterpillars from leaving, or had spun polyester bottoms that prevented ant access. At each of 12 field sites, the caterpillars were caged with litter that matched the depth and wetness of litter found at that site. All caterpillars protected from ants survived, while 40% of the unprotected caterpillars from dry sites and 23% of the unprotected caterpillars from wet sites were killed by predators. So ants are clearly fearsome predators, but more so under dry conditions.

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A Formica lasioides ant subdues an early install wooly bear caterpillar within the confines of a deli box. Credit Rick Karban.

But does litter wetness help protect against predacious ants? To investigate this question, the researchers placed caterpillars in deli containers that permitted ant access. At each site, two containers were placed side-by-side; one contained a caterpillar + litter from a wet site, while the other contained a caterpillar + litter from a dry site. Both containers were completely filled with litter and left in the field for 48 hours. The researchers discovered that caterpillars were 26% more likely to avoid predation if they were in a container stuffed with litter from a wet site. This suggests that litter from wet sites acts as a refuge for caterpillars against predators.

KarbanFig7A

Caterpillar survival rate in relation to litter wetness.

Unfortunately, no long-term data on ant abundance are available, so we don’t know the relationship between ant and caterpillar abundance over time. But when ants were excluded, caterpillars survived well, and when ants were present, caterpillars survived best in wet sites with deep litter. It is not clear why caterpillars survive ant predation better in wet litter. One possibility is that caterpillars are more active than ants at cooler temperatures, and may be more likely to avoid them in wet and cool conditions. A second possibility is that dry litter is structurally less complex than wet litter, and ants may be more likely to move efficiently to capture caterpillars in dry terrain.

Given the predictions for more rainfall variability in coming years, Karban and his colleagues expect caterpillar abundance to fluctuate even more dramatically from year to year. In this system, and presumably other insect populations as well, multiple factors interact to determine whether there will be a population outbreak reminiscent of Pharaoh’s experience early in recorded history.

note: the paper that describes this research is from the journal Ecology. The reference is Karban, Richard, Grof-Tisza, Patrick, and Holyoak, Marcel (2017), Wet years have more caterpillars: interacting roles of plant litter and predation by ants. Ecology, 98: 2370–2378. doi:10.1002/ecy.1917. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Changing climate promotes prolific plants and satiated consumers

Plants in Sweden can have a difficult life, but climate change has provided a more benign environment for some of them, including the white swallow-wort, Vincetoxicum hirundinaria. This perennial herb grows in patches in sun-exposed rocky areas, in forests located below cliffs, and along the edges of wooded areas. The plant forms clumps that are heavily laden with flowers in June and July, and creates pod-like fruits in July and August

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Vincetoxicum hirundinaria growing in rocky outcrop (top photo). Vincetoxicum pods releasing their wind-dispersed seeds (bottom photo).

Christer Solbreck has had a lifelong interest in insect populations, and he has been following the insects that eat Vincetoxicum’s seeds for the past 40 years. As he described to me, surprisingly few population ecologists actually measure the amount of food available to insects. I should add that very few people have the resilience to study the same population of insects for 40 years, either. And interestingly, though this paper discusses the effect of a changing climate on seed production and seed predation, it was not Solbreck’s intent to consider climate change as a variable when he began, as climate change was not a concern of most scientists in the 1970s.

But climate change has happened in southeastern Sweden (and elsewhere), and has affected ecosystems in many different ways. Ecologists can quantify climate change by describing its effect on the vegetation period, or growing season (days above 5°C), which has increased by about 20 days since the mid 1990s.

Solbreck2B

Length of growing season (vegetation period) in southern Sweden.

During the same time period the abundance of Vincetoxicum has increased sharply.

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Vincetoxicum abundance, measured as area of the research site covered, during the study.

You will note that “Vincetoxicum” has the word “toxic” in its midst; the seeds are toxic to most consumers, and are important food sources for only two insect species. Euphranta connexa females lay eggs in developing fruits of the host plant, with the emerging larva boring through the seeds and killing most of them. Lygaeus equestris is an all-purpose seed predator; both larvae and adults suck on flowers, on developing seeds within the fruits, and on dry seeds they find on the ground up to a year later.

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Euphranta connexa female lays eggs in an immature seed pod.

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Lygaeus equestra larva feeds on a fallen seed.

Solbreck teamed up with biostatistician Jonas Knape to analyze his data. From the beginning of the study, Solbreck suspected that annual variation in weather – particularly rainfall – might influence Vincetoxicum seed production, and consequent population growth of the two insect species. They discovered something quite unexpected; the dynamics of seed production shifted dramatically in the second half of the study, alternating annually from very high to very low production over that period. This dynamic shift coincides with the extension of the growth season as a result of climate change.

Solbreck2Cyes

Seed pod abundance by year.

The researchers argue that there is a non-linear negative feedback relationship of the previous year’s seed production on the current year’s seed production. Negative feedback occurs when an increase in one factor or event causes a subsequent decrease in that same factor or event. In this case, an increase in seed production uses up plant resources, leading to a decrease in seed production the following year. But the effect is non-linear, and does not come into play unless Vincetoxicum produces a huge number of seeds, as shown by the graph below,

Solbreck3yes

Seed production in the current year in relation to seed production in the previous year. Note that both axes are logarithmic. The curve represents the expected seed pod density generated by the statistical model, with the shaded area representing the 95% credible intervals. Open circles are data for 1977-1996, while closed circles are data for 1997-2016.

The researchers also found that high rainfall in June and July increased seed production.

So how do these wild fluctuations in seed production affect insects and the plant itself? One important finding is that in high seed production years, the proportion of seeds attacked by insects plummets because the sheer number of seeds overwhelms the seed-eating abilities of the insect consumers. Ecologists describe this phenomenon as predator satiation.

Solbreck4yes

Seed predation rates in relation to seed pod density.  Note that both axes are logarithmic. The curve represents the expected predation rate generated by the statistical model, with the shaded area representing the 95% credible intervals. Points are E. connexa predation rates while triangles are combined predation by both insect species.

As a result of predator satiation, there were, on average, seven times as many healthy (unattacked) seed pods in 1997-2016 than there were in 1977-1996. Presumably, this increased number of healthy seeds translates to an increase in new plants becoming established in the area. An important takehome message is that the entire dynamics of an ecosystem can change as a result of changes to the environment, in this case, climate change. More long-term studies are needed to evaluate how common these shifting dynamics are likely to become in the novel environmental conditions we humans are creating.

note: the paper that describes this research is from the journal Ecology. The reference is Solbreck, Christer and Knape, Jonas (2017), Seed production and predation in a changing climate: new roles for resource and seed predator feedback?. Ecology, 98: 2301–2311. doi:10.1002/ecy.1941. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Powdery parasites pursue pedunculate oak

Studying disease transmission is tricky for many reasons. Most humans frown on what might seem like the easiest experimental protocol – release a disease into the environment and watch to see how it spreads. For his doctoral dissertation in 2006, Ayco Tack settled on a different experimental protocol – bring the potential hosts to the disease. In this study, staged in Finland, the hosts were pedunculate oak trees, Quercus robur, and the disease was the powdery mildew parasite, Erysiphe alphitoides. Almost 10 years later, Adam Ekholm continued research on the same system, with Tack as his co-supervisor.

Ayco Tack

Trees on the move. Credit: Ayco Tack.

But before moving trees around, the researchers first needed to see how the disease moved around under field conditions.  Within a tree stand, powdery mildew success will depend on how many trees it occupies, how many trees it colonizes in the future, and how many trees it disappears from (extinction rate). The researchers measured these rates over a four year period (2003 – 2006) on 1868 oak trees situated on the island of Wattkast in southwest Finland. They also measured spatial connectivity of each tree to others in the stand. In this case connectivity is a measure of the distance between a tree and other trees, weighted by the size of the other trees. So a tree that has many large neighbors nearby has high connectivity, while a tree with a few distant and mostly small neighbors has low connectivity. Results varied from year-to-year, but in general, the researchers found higher infection rates, lower extinction rates, and some evidence of higher colonization rates in trees with high connectivity.

Mildew_Adam Ekholm

Oak leaf infected with powdery mildew parasite. Credit: Adam Ekholm.

The importance of connectivity indicated that the parasites simply could not disperse efficiently to distant trees. But perhaps the environment might play a role in colonization rates as well. For example, fungi like powdery mildew tend to thrive in shady and humid environments. Thus a tree out in the open might resist colonization by powdery mildew more effectively than would a tree deep in the forest. To test this hypothesis, Tack and his colleagues placed 70 trees varying distances (up to 300 meters) from an infected oak stand. On one side of the oak stand was an open field, while the other side was closed forest. Thus two variables, distance and environment, could be investigated simultaneously.

Ayco Tack inspecting a potted tree_Tomas Roslin

Ayco Tack inspects an oak tree placed in an open field. Credit: Tomas Roslin.

The researchers collected infection data twice; once in the middle of the growing season (July) and a second time at the end of the growing season (September). Not surprisingly, infection rates were higher by the end of the growing season. In general, infection rates, and infection intensity (mildew abundance) were higher in the forest than in the field, indicating a strong environment effect. In the July survey, trees further from the oak stand had lower infection intensity, but as infection rates increased over the course of the season, the effects of distance diminished, particularly in the forest.

EkholmFig3

Upper two graphs show the impact of habitat type on (a) proportion of trees infected and (b) mildew abundance. The lower two graphs are the influence of distance from parasite source on mildew abundance of trees set in (c) a forest habitat and (d) an open field. Mildew abundance was scored on an ordinal scale with 0 = none and 4 = very abundant.

Ten years later, Adam Ekholm, as part of his PhD dissertation that studies the effect of climate on the insect community on oak trees, added a third element to the mix – the influence of genes on disease resistance. He wondered whether certain genotypes were more resistant to powdery mildew infection. The researchers grafted twigs from 12 large “mother” trees, creating 12 groups of trees, with between 2 – 27 trees per group (depending on grafting success). Each tree in a given group was thus genetically identical to all other trees within that group.

Ayco Tack

Oak tree placed in the forest. Credit: Ayco Tack.

The researchers chose a site that contained a dense stand of infected oaks, but was surrounded by a grassy matrix that contained only an occasional tree. To study the impact of early season exposure, Ekholm and his colleagues divided the trees into two groups; 128 trees were placed in the matrix at varying distances from the infected stand, while 58 trees were placed directly in the midst of the stand for about 50 days, and then moved varying distances away. The researchers scored trees for infection at the end of the growing season (mid-September).

 

Trees that spent 50 days within the oak stand had much higher infection frequency and intensity than trees that were initially placed in the matrix. Some genotypes (for example genotype I in graphs C and D below) were much more resistant to infection than others (such as genotypes D and J). Finally trees further from the source of infection were less susceptible to become colonized over the course of the summer (data not shown).

EkholmFig2

Proportion of trees infected (A) and proportion of leaves infected (B) in response to early season exposure to stand of oaks infected with the powdery mildew parasite (oak stand) or no early season exposure (matrix). Proportion of trees infected (C) and proportion of leaves infected (D) in relation to tree genotype. Genotypes are labeled A – L; numbers in parenthesis are sample size for each group.

These findings illustrate how dispersal, host genotype and the environment influence the spread of a parasite under natural conditions. The parasite exists as a metapopulation – a group of local populations inhabiting networks of somewhat discrete habitat patches. Some populations go extinct while others successfully colonize each year, depending on distance from a source, tree genotype and environment. Ekholm and his colleagues encourage researchers to use similar experimental approaches in other host-parasite systems to evaluate how general these findings are, and to explore how multiple factors interact to shape the dynamics of disease transmission.

note: the paper that describes this research is from the journal Ecology. The reference is Ekholm, Adam; Roslin, Tomas; Pulkkinen, Pertti and Tack, Ayco. J. M. (2017). Dispersal, host genotype and environment shape the spatial dynamics of a parasite in the wild. Ecology. doi:10.1002/ecy.1949. The paper should come out in print very soon. Meanwhile you can also link to Dr. Tack’s website at www.plantmicrobeinsect.com Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Is your goose cooked? Climate change and phenological mismatch.

As an undergraduate at the University of Manitoba, Megan Ross investigated nutrient reserves stored up by Lesser Snow Geese before reproduction in southern Manitoba. These reserves of fat and protein are critical to female geese, who then fly thousands of kilometers north to breeding grounds above the Arctic Circle, where they lay eggs and raise their young. For her Master’s thesis (this study), Ross investigated how nutrient levels influence adult reproductive success and recruitment of new goslings into the population.

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Lesser Snow Goose on its nest. Credit: Megan Ross

As climates become warmer and more variable, there is a danger that goose reproduction may fall out of synchrony with the availability of high quality food in the feeding grounds – a phenomenon called phenological mismatch. If eggs don’t hatch until substantially after the grass is well-established, then grass nutritive value may not be high enough to raise a goose family. The problem is that even if geese had the ability to adjust the timing of their migration, it would be very difficult for them to know what feeding conditions are like thousands of kilometers away. Ross and her colleagues explored several questions regarding phenological mismatch and how successfully Snow Geese and Ross’s Geese (a smaller relative of Snow Geese – not named after our senior author) raise their broods.

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Ross’s Goose paddling about. Credit: DickDaniels (http://carolinabirds.org/)

Those of you who hang out near ponds, golf courses, or farms probably know that goose populations are thriving. Over the past few decades, geese at Karrak Lake in Nunavut, Canada, have increased sharply in population, though the growth rate has leveled off in recent years (see graph below).

RossLakePop

Combined estimate of Ross’s Goose and Lesser Snow Goose population size at Karrak Lake. Credit: Megan Ross.

Measuring recruitment of goslings into a population of a million birds is not the easiest task. The researchers used helicopters to herd the birds into portable geese corrals, and then simply calculated the proportion of juveniles as their measure of recruitment. They also weighed, measured and banded all of the captured birds before releasing them, unharmed back into the environment. For each year of the study, they calculated phenological mismatch as the difference between the mean annual hatch date and the NDVI50 date (which stands for the date of 50% annual maximum Normalized Difference Vegetation Index). To calculate NDVI50, researchers use satellite images to estimate the date at which the environment achieved 50% of maximum green-up for the year.

Thecolony

Small portion of the goose population near Karrak Lake. Credit: Megan Ross

Several factors were related to recruitment. For both species, recruitment was very low in years with considerable phenological mismatch (Graph a). High recruitment was associated with high levels of protein in both species (Graph b), and high levels of fat in snow geese (Graph d). Recruitment was also greatest if nests were initiated early in the year (Graph c).

RossFig1

Mean annual values for Ross’s Geese (gray circles) and Lesser Snow Geese (black circles) in relation to (a) phenological mismatch, (b) female body protein index, (c) ELI (early late index) – a measure of nest initiation date (for example, ELI = -5 means that nests were initiated 5 days earlier than average on that particular year), and (d) female body fat index.

The number of eggs per clutch, and the number of nests that produced at least one gosling (nest success) were highest when geese initiated their nests earlier in the year. Snow Geese laid, on average, more eggs, than did Ross’s Geese, but Ross’s Geese had somewhat higher nesting success than did Snow Geese. But nutrients also figure into this increasingly complex picture. In years when females stored up more protein, they tended to lay more eggs.

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Three Lesser Snow Goose goslings huddle together. Credit: Megan Ross.

Not surprisingly, the researchers also demonstrated that warmer springs were associated with earlier vegetation growth (NDVI50). The geese were able to adjust somewhat to earlier NDVI50 by initiating nests earlier in the year. However, these adjustments were only partial at best, so that phenological mismatch was very high in years that greened-up early (the distance along the x-axis between the data points and the bold dotted line in the graph below).

RossFig4b

Mean annual hatch date for Ross’s Geese (gray circles) and Lesser Snow Geese (black circles) in relation to the date of the year that NDVI50 is reached.  The bold dotted line is the expected value if there were no phenological mismatch.

From these data, you might ask why don’t the geese migrate earlier in the spring? One problem is that they need enough protein and fat to migrate, to produce eggs and to incubate the eggs during the cool Arctic spring. Another part of the problem is that gonadal development is determined by day length – not temperature – so there is a limit to how early in the year the geese are able to begin courtship and breeding activities. The concern is that if, as expected, environmental warming continues, phenological mismatch could become more extreme, further reducing juvenile recruitment, and putting a seemingly robust population at risk.

note: the paper that describes this research is from the journal Ecology. The reference is Ross, Megan V., Alisauskas, Ray T., Douglas, David C. and Kellett, Dana K. (2017), Decadal declines in avian herbivore reproduction: density-dependent nutrition and phenological mismatch in the Arctic. Ecology, 98: 1869–1883. doi:10.1002/ecy.1856. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Languishing Leatherbacks

Leatherback turtles, Dermochelys coriacea, are the largest of all sea turtles, tipping the scales at up to 900 kg. Unlike other sea turtles, the leatherback lacks a carapace covered with scutes; instead its carapace is covered by thick leathery skin that is embedded with small bones forming seven ridges running along its back. This turtle has a wonderful set of anatomical and physiological adaptations, such as huge flippers and an efficient circulatory system, that make it a powerful swimmer and deep ocean diver. Males spend their entire lives at sea, while females usually return to their birthplace along sandy beaches to dig nests and lay eggs.

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Leatherback female on the beach at Las Baulas National Park. Credit: Karla Hernández.

Unfortunately, from the perspective of conserving awesome animals in our world, some populations of leatherbacks are declining rapidly, and many are now listed as critically endangered by the IUCN Red List. Pilar Santidrian Tomillo wanted to know why leatherback populations in the Eastern Pacific Ocean have declined so much in recent years. Working at Las Baulas National Park in northwestern Costa Rica since 1993, Tomillo and her colleagues have tagged 1927 nesting females so they could measure survival and return rates to the nesting shoreline. They discovered an alarming trend of sharp decline as described by the graph below.

TomilloFig1Tomillo and her colleagues knew that many leatherbacks were killed every year as a consequence of bycatch – capture by fishing nets or lines cast by fishermen who are targeting other species. But leatherback bycatch is very difficult to monitor accurately, as few fishermen keep accurate records of dead turtles, and turtles may die after being entangled and subsequently freed. The researchers also suspected that climate variability could influence leatherback population size. El Niño Southern Oscillation (ENSO) is a large-scale atmospheric system that affects global climate. In leatherback foraging areas, El Niño years are associated with high atmospheric pressure and warm sea temperatures, while La Niña years are associated with low atmospheric pressure and cool sea temperatures. Importantly, cool sea temperatures stimulate upwelling of nutrient-rich water to the surface, increasing production of phytoplankton, thereby increasing the abundance of  jellyfish and other favored leatherback food items. So the researchers hypothesized that the leatherbacks might do better in La Niña years than in El Niño years.

But what do they mean by doing better? There are two important factors influencing population growth: survival and reproduction. Either one could be affected by climate. By recapturing marked individuals, Tomillo and her colleagues were able to measure both survival and one important aspect of reproduction, which is how often females return to lay eggs. Reproduction is a very energetically demanding process for leatherback females, as they must migrate long distances (often thousands of kilometers) from their feeding grounds, and their eggs are large and plentiful, so females require a huge investment in resources to reproduce. Consequently, at Tomillo’s field site, only 4.5% of females reproduced in consecutive years, while the average interval between reproductive events was 3.65 years.

Let’s consider leatherback survival. As you can see from the data below, annual survival probability is very variable from year to year, ranging from about 30% in 2012 to near 100% in several years. Disturbingly, the long-term trend is downward, and the overall mean adult survival rate of 0.78 is very low in comparison to viable populations of sea turtles. If survival rates do not increase, the future is very bleak for this population.

Tomillo Fig4

Annual survival probability of adult females tagged at Las Baulas National Park. Vertical bars indicate 95% confidence intervals.

How does climate variation influence survival and reproduction? The Multivariate ENSO Index (MEI) measures ENSO strength, with positive numbers (X-axis on graphs below) indicating El Niño years (with poor food availability), and negative numbers indicating La Niña years (with good food availability). The researchers found no climate effect on survival (top graph below), but a high reproductive rate associated with La Niña events (bottom graph below).

TomilloFig5

The question remains, why is survival so low? Climate does not appear to affect survival, so that brings us back to human impact. Tomillo and her colleagues recommend reducing bycatch levels and implementing beach conservation measures to eradicate egg poaching. They also warn us that increases in global temperatures reduce egg hatching success, and pose a severe stress to this and other critically endangered leatherback populations throughout the world.

note: the paper that describes this research is from the journal Ecology. The reference is Santidrian Tomillo, P., N. J. Robinson, A. SanzAguilar, J. R. Spotila, F. V. Paladino, and G. Tavecchia. 2017. High and variable mortality of leatherback turtles reveal possible anthropogenic impacts.  Ecology 98: 2170–2179. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Limpet larvae and their fantastic voyage

As he began his PhD program, Takuya Yahagi was puzzled by some laboratory findings. Juvenile red blood limpets, Shinkailepas myojinensis, seemed to survive and grow extraordinarily well at temperatures between 15-25° C. Adult limpets live in deep sea vent communities, where temperatures generally range between 6-11° C.

limpet photo

Adult Shinkailepas myojinensis.  These are approximately 6 mm in length. Credit: Takuya Yahagi.

Yahagi and his colleagues wondered why limpets are making babies that survive and grow at much higher temperatures than they are likely to experience after hatching.

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Deep sea hydrothermal vent community at 795 meters depth at Myojinsho Caldera in the northwest Pacific. White patches on the rocks are vast communities of chemosynthetic bacteria which are being grazed by purple/pinkish limpets. You can also see the white feathery feeding legs of a barnacle population in the upper portion of the photo. Credit: JAMSTEC

Yahagi reasoned that perhaps, in the natural world, the limpet juveniles live in different (warmer) environments than do their parents. If they migrated closer to the sea surface, their world would be somewhat warmer. But limpet babies are microscopic, so capturing them near the sea surface (and knowing that you had captured them!) is very challenging. Working with three other researchers, Yahagi decided to collect indirect evidence to test the hypothesis that baby limpets migrate to the surface where they feed and grow before returning to the ocean depths.

YahagiFig3

Larval S. myojinensis limpet 156 days after hatching. sh=shell, f =foot, e=eye, vl=velar lobe.

Initially, the researchers needed to determine what temperatures these growing limpets preferred. With the help of a remotely operated submarine, they collected adult limpets laden with egg capsules, and placed newly hatched larvae into separate containers under different conditions. Some larvae were fed and raised at one of six different temperatures: 5, 10, 15, 20, 25 and 30° C. Other larvae were starved at 5, 15 or 25° C to see how long they survived at different temperatures. If the larvae were migrating upwards to warmer waters, it was important to see how long they could survive until they arrived at the richer food sources near the surface.

Starved larvae survived up to 150 days at the lowest temperature, and for more than three weeks at 25° C, which provided ample time for upward migration (even at very mellow baby limpet swimming speeds). Fed larvae grew much more quickly at warmer temperatures, with best growth at 25° C, and no growth at 5-10° C, which is the approximate temperature at hydrothermal vents.. Larvae initially grew quickly at 30° C, but long term exposure to that temperature killed them.

YahagiFig4

Growth (shell length) of fed larvae at different temperatures.

These temperature profiles corresponded to temperatures at the sea surface down to about 100 meters, which ranged between 19-28° C. This correspondence supported the hypothesis that juveniles migrated upwards in the water column after hatching. But could Yahagi and his colleagues find any direct evidence for this vertical migration? To answer this question, they video-recorded new hatchlings in a clear plastic bath, and measured how fast these limpets swam, and what direction they preferred. They discovered that new hatchlings constantly swam upward in their test bath, and swimming speed was considerably faster at warmer temperatures.

The sea surface is a wonderful place to find food, because sunlight is abundant, so there are abundant phytoplankton to satisfy even the most voracious juvenile limpets. But sea surfaces also have very strong currents which can whisk juvenile limpets hundreds or thousands of kilometers away. The upshot is that vertical migration and wide dispersal of juveniles by ocean currents can introduce new genes into far-away limpet populations.

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A hot vent animal community at 700 meters depth at Minami-Ensei Knoll in the northwest Pacific. Prevalent groups include lobsters (white), two species of shrimp, mussels and two different limpet species. Credit: JAMSTEC.

Gene flow – the movement of genes from one population to another – has some important genetic impacts. Without gene flow, two populations that are separated from each other can become genetically distinct. But the mixing of genes from long-distance dispersal can prevent this from occurring. The researchers compared 1218 base pairs of the COI gene from 77 adult limpets that were collected from four different sites which were separated, in some cases, by more than 1000 kilometers. In support of the gene flow hypothesis they found no evidence of any genetic differentiation among the four populations.

Yahagi Fig1

Hydrothermal vent fields in the northwest Pacific Ocean.  Black squares are limpet collection sites for this study.  Notice the vast distances separating these populations.

Gene flow requires long distance dispersal, and the adult limpets travel very little along the sea floor. This finding of no genetic differentiation among the geographically separated populations supports the hypothesis that the juveniles migrate upwards, feed on abundant phytoplankton, and are carried to new distant environments. There, they mature and settle into new ocean vent communities where they can feed on the superabundant chemosynthetic bacteria associated with the ocean vents. But we still don’t know how limpets find a new ocean vent community – do they migrate, checking out possible vent habitats, while they are still juveniles and still capable of swimming? Do they have sense organs that pick up environmental cues such as hydrogen sulfide content, water temperature, turbulence or noise from vent emissions, to help them complete their fantastic ocean voyage?

note: the paper that describes this research is from the journal Ecology. The reference is Yahagi, Takuya, Hiromi Kayama Watanabe, Shigeaki Kojima, and Yasunori Kano. 2017. Do larvae from deep‐sea hydrothermal vents disperse in surface waters? Ecology 98: 1524-1534Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.