The reasons why insect numbers are decreasing

A researcher from Mainz University has published a special issue of Biology Letters on the causes and consequences of the worldwide insect decline as well as potential countermeasures

Peer-Reviewed Publication

JOHANNES GUTENBERG UNIVERSITAET MAINZ

 

IMAGE: THE DARK BUSH-CRICKET PHOLIDOPTERA GRISEOAPTERA IS ONE OF THE MANY DECLINING INSECT SPECIES IN CENTRAL EUROPE. view more 

CREDIT: PHOTO/©: BEAT WERMELINGER

The reasons why insect numbers are decreasing

A researcher from Mainz University has published a special issue of Biology Letters on the causes and consequences of the worldwide insect decline as well as potential countermeasures

Throughout the world we are witnessing not just a decline in the numbers of individual insects, but also a collapse of insect diversity. Major causes of this worrying trend are land-use intensification in the form of greater utilization for agriculture and building development as well as climate change and the spread of invasive animal species as a result of human trade. These are the main conclusions reached in the special feature on insect decline that recently appeared in Biology Letters. One of the three editors is PD Dr. Florian Menzel from the Institute of Organismic and Molecular Evolution at Johannes Gutenberg University Mainz (JGU). "As evidence of an on-going global crash in insect populations increased over the last few years, we decided it was time to edit and publish this special issue. Our aim was not to document insect population declines but to better understand their causes and consequences," said Menzel. Together with forest entomologist Professor Martin Gossner of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and biologist Dr. Nadja Simons of TU Darmstadt, Menzel contacted international researchers in order to collate the information they could provide on insect declines and to stimulate new studies on the subject. The special issue authored by Menzel, Gossner and Simons includes 12 research-related articles, two opinion papers, and an extensive editorial.

More extensive use of land, climate change, and invasive species are the main causes of insect decline

"In view of the results available to us, we learned that not just land-use intensification, global warming, and the escalating dispersal of invasive species are the main drivers of the global disappearance of insects, but also that these drivers interact with each other," added Menzel. For example, ecosystems deteriorated by humans are more susceptible to climate change and so are their insect communities. Added to this, invasive species can establish easier in habitats damaged by human land-use and displace the native species. Hence, while many insect species decline or go extinct, few others, including invasive species, thrive and increase. This leads to an increasing homogenization of the insect communities across habitats.

"It looks as if it is the specialized insect species that suffer most, while the more generalized species tend to survive. This is why we are now finding more insects capable of living nearly anywhere while those species that need specific habitats are on the wane," Menzel pointed out. The consequences of this development are numerous and generally detrimental for the ecosystems. For example, the loss of bumblebee diversity has resulted in a concomitant decline in plants that rely on certain bumblebee species for pollination. "Generally speaking, a decline in insect diversity threatens the stability of ecosystems. Fewer species means that there are fewer insects capable of pollinating plants and keeping pests in check. And, of course, this also means that there is less food available for insect-eating birds and other animals. Their continued existence can thus be placed at risk due to the decline in insect numbers," emphasized Menzel.

In their editorial, Menzel, Gossner, and Simons suggest ways in which we can best respond to the effects that their gathered data has revealed. They advocate a particular approach for future research into insect decline. Standardized techniques should be employed to monitor insect diversity across many habitats and countries, the more so as in many regions of the world we still don't know how the insects are doing. The researchers also propose the creation of a network of interconnected nature reserves such that species can move from one habitat to another. Less heat-tolerant insects would thus be able to migrate from areas where global warming is causing temperatures to rise to higher elevations or cooler regions in the north. Furthermore, we need measures to reduce the dispersal of invasive animal and plant species through our globalized trade and tourism. "This is another problem that has become extremely serious in the last decades," concluded Menzel. One example cited in the current special issue is the invasion of non-native insectivorous fishes in Brazil that has caused a major decline in freshwater insects.

A common red ant (Myrmica rubra) worker carrying a seed of the hollowroot plant (Corydalis cava). The dispersal of seeds is one the many positive effects that ants have in ecosystems.

CREDIT

photo/©: Philipp Hönle

JOURNAL

Biology Letters

DOI

10.1098/rsbl.2023.0007 

METHOD OF RESEARCH

Commentary/editorial

SUBJECT OF RESEARCH

Animals

ARTICLE TITLE

Less overall, but more of the same: drivers of insect population trends lead to community homogenization

Arthropods in high-diversity forests contribute to improved productivity

Peer-Reviewed Publication

CHINESE ACADEMY OF SCIENCES HEADQUARTERS

 

IMAGE: BEF-CHINA EXPERIMENT SITES, HERBIVOROUS CATERPILLARS FEEDING ON TREE LEAVES, AND A PREDATOR WASP PREDATING LEAF-MINING CATERPILLAR view more 

CREDIT: LI SHAN, ANDREAS SCHULDT AND CHEN HUAYAN

An international team of researchers led by Prof. MA Keping from the Institute of Botany of the Chinese Academy of Sciences (IBCAS) has shown that forests with higher tree species richness tend to have greater arthropod diversity.

The researchers showed that higher tree diversity promotes productivity through the suppression of herbivores by enemy arthropods.

These findings, published recently in Nature Ecology & Evolution, underscore the importance of arthropod diversity as a mediator of the effects of tree diversity on forest productivity.

Managing forests for increased productivity will require both increased tree diversity and multitrophic diversity, according to the researchers.

Forests are home to 80% of terrestrial plant and animal diversity, making them a critical component of global biodiversity conservation. However, forest biodiversity is under serious threat from anthropogenic disturbance and climate change. Species-rich groups such as arthropods are declining dramatically due to the degradation of forests and loss of plant diversity.

Most studies of biodiversity–ecosystem functioning (BEF) relationships have focused solely on plant diversity, neglecting the impact of the diversity of other trophic groups. As a result, it remains unclear how the diversity of herbivores and their enemies affects ecosystem functioning. Given the importance of forests in providing essential ecosystem services and global biodiversity, it is vital to understand these relationships and take action to protect them.

Using five years of data on aboveground herbivorous, predatory, and parasitoid arthropods, together with tree growth data from a large-scale forest biodiversity experiment in southeastern China (BEF-China), the researchers showed that the effects of increased tree species richness were consistently positive for species richness and abundance of herbivores, predators, and parasitoids.

This finding is consistent with a previous study from another large grassland biodiversity experiment (the Jena Experiment) and again shows that conserving plant diversity is important to conserving arthropod diversity.

However, in contrast to the bottom-up control of arthropod diversity by plant diversity in the central European grassland study, this new study in the species-rich forests of southeastern China shows that higher tree diversity can enhance the top-down control of herbivores by their enemies, thereby contributing to increased productivity.

"This study emphasizes that arthropod diversity plays an important role in mediating the effects of tree diversity on primary productivity," said Dr. LI Yi, a postdoc researcher at IBCAS and first author of the study.

A previous study conducted at the same sites showed that increasing plant diversity can promote forest productivity directly. The new study shows that increasing plant diversity can also indirectly increase forest productivity by promoting arthropod diversity and trophic interactions.

"It underscores the critical role of conservation efforts aimed at maintaining forest biodiversity," said Prof. LIU Xiaojuan, senior author and leader of the BEF-China management group.

To draw more reliable conclusions, the researchers conducted multi-year sampling. According to Prof. MA, chairman of the BEF-China platform, this approach takes into account the possibility that different arthropod groups may be affected inconsistently from year to year.

"It also highlights the value of the BEF-China platform, not only as the world’s largest experiment on forest biodiversity and ecosystem functioning, but also as an open and international platform for long-term ecological research," he said.

"The BEF-China platform provides valuable opportunities for international research teams and young researchers in different directions," said Prof. ZHU Chaodong, an author from the Institute of Zoology of CAS.

Overall, while several recent studies have documented declines in terrestrial arthropod biodiversity, few have examined the consequences for ecosystems.

"This work fills that gap by demonstrating the important role of arthropod diversity in BEF relationships," said Prof. Bernhard Schmid, an author from the University of Zurich.

In summary, these findings are of great importance in guiding the public to understand the conservation of forest biodiversity, plants, animals, and ultimately their associated microbes.

Relationships between tree diversity and species richness of herbivores (a), predators (b), parasitoids (c) and multitrophic groups (d)

Relationships between species richness of predators (a), parasitoids (b), multitrophic groups (c) and productivity; direct and indirect effects of tree species richness, functional traits, and overall arthropod (d) or herbivore and enemy (e) species richness on productivity

CREDIT

IBCAS

JOURNAL

Nature Ecology & Evolution

DOI

10.1038/s41559-023-02049-1 

METHOD OF RESEARCH

Meta-analysis

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Multitrophic arthropod diversity mediates tree diversity effects on primary productivity

ARTICLE PUBLICATION DATE

27-Apr-2023

When employees leave their jobs, coworkers call it quits: UBC study

Peer-Reviewed Publication

UNIVERSITY OF BRITISH COLUMBIA

People leave jobs all the time, whether they’re laid off, fired, or just quit. But how do their departures affect coworkers left behind? According to a new study from the UBC Sauder School of Business, those exits can lead many others to call it quits.  

The researchers delved deeply into employment data from a major retailer that was experiencing high turnover to find out why. They reviewed data for roughly a million employees — including when they were hired, which store, which position, when they left, and why. 

The study authors also had access to employee performance records, so they could evaluate whether workers were high performers or low performers. 

“We looked at these stores as systems, and looked at the flow of employees in and out of them over time,” explained UBC Sauder assistant professor Sima Sajjadiani (she/her), who co-authored the study with John Kammeyer-Mueller and Alan Benson of the University of Minnesota. “It gave us a great opportunity to look at the immediate, short-term and long-term effects of each exit event, and compare them over time.”

Past studies have looked at how turnover affects remaining employees, but they typically focus on employee performance after others leave — and on the performance of the company overall. The UBC study is the first of its kind to look at the extent to which employee exits lead to even more departures, what types of exits cause the most churn, and the role that the performance level of exiting employees plays.  

The researchers found that layoff announcements have a strong and immediate effect, and boost voluntary turnover among those who survive the layoffs. 

“It’s very bad news for organizations, especially if they are laying off high performers, because if those positions get eliminated, both high and low performers start quitting,” said Dr. Sajjadiani.  “It’s a signal that people’s jobs aren’t secure, and the organization doesn’t care about them, no matter how hard they work. So they think, ‘I should leave as soon as possible.’”

When employees quit their jobs voluntarily, their departures give a more moderate boost to voluntary turnover, and it takes longer for that ripple effect to occur. 

“To high performers, voluntary exits are a positive signal that there are better opportunities elsewhere,” said Dr. Sajjadiani. “So while employees might not leave immediately, they do begin to look for other opportunities.”

When workers are dismissed, however, their departures have a relatively small and fleeting effect — and can even reduce voluntary turnover. 

“Usually these are people who are disruptive or abusive, or aren’t doing their fair share,” said Dr. Sajjadiani. “When they go, high performers tend to stay longer, and the risk of voluntary turnover actually goes down.”

However, when a high performer is dismissed without clear justification, employers not only open themselves to legal headaches, it also sends the wrong message to other high performers. They also start heading for the door. 

According to Dr. Sajjadiani, organizations vastly underestimate the ripple effects of people leaving and the resulting human capital costs. The research also sends a clear message to organizations that they should be extremely careful when they make exit decisions, or they risk destabilizing the whole organization very quickly. 

“Communicating clearly and compassionately, justifying these decisions and trying to avoid the most severe course of actions are better for organizations than simply cutting people,” Dr. Sajjadiani said. 

Interview language: English

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JOURNAL

Academy of Management Journal

DOI

10.5465/amj.2021.1327 

ARTICLE TITLE

Who Is Leaving and Why? The Dynamics of High-Quality Human Capital Outflows

Researchers call for national governments to mandate real-time indoor air quality monitoring

Better ventilated indoor spaces protect against COVID-19 spread

Peer-Reviewed Publication

TSINGHUA UNIVERSITY PRESS

In a response to the COVID-19 pandemic, a team of researchers has published an editorial calling for national governments to consider mandating real-time indoor air quality monitoring in at least all public buildings.

 

Their editorial is published in the journal Building Simulation on 25 April 2023.

 

The three-year-long COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has revealed that there is a global indoor-air crisis. Vaccination alone has not completely controlled the COVID-19 pandemic and the virus continues to threaten human health and life. Scientists now know most if not nearly all transmission occurs indoors in poorly ventilated spaces. The researchers note in their editorial that since more than 6.5 million people have been reported to die globally due to the SARS-CoV-2 infection and people continue to be infected, there is an urgent need to improve ventilation in buildings worldwide.

 

The researchers observe that while two years have passed since it was officially recognized that airborne transmission of SARS-CoV-2 spreads the virus and some efforts have been made, there have been no significant improvements in building ventilation. Some governments have taken steps toward addressing the issue. The U.S. government issued its “Clean Air in Buildings Challenge” in March 2022, and the Hong Kong SAR government set a policy that requires six air changes per hour in 20,000 dine-in restaurants in the city.

 

“In the absence of a worldwide effort to improve building ventilation, it is likely that poorly ventilated buildings will remain common, meaning that airborne transmission of SARS-CoV-2 will continue. Moreover, if another novel and highly contagious respiratory virus emerges in the future, another pandemic is likely to occur,” said Yuguo Li, a professor at The University of Hong Kong.

 

The researchers note that there are likely more than a billion indoor spaces in the world, and identifying those with poor ventilation remains a challenge. Understanding that SARS-CoV-2 is airborne is not enough—for effective improvement, technologies are needed to identify where ventilation is insufficient. They also note that any improvements in ventilation must also take into consideration the buildings’ energy efficiency. This is necessary because energy efficiency is needed to mitigate the effects of climate change.

 

The two key components of building energy performance are thermal performance and ventilation performance. Humans can detect thermal conditions by using a thermometer to measure the temperature. But even though humans can detect odors, they cannot sense or predict a building’s ventilation performance. So humans are not capable of detecting air pollutants, such as infection aerosols. This inability to detect most air pollutants contributes to the indoor air crisis, the researchers note.

 

The researchers suggest that without governments establishing mandatory requirements for building ventilation performance, building owners will unlikely choose to monitor their buildings’ ventilation performance. The researchers also note that monthly or annual data on building ventilation rates is not sufficient. Real-time hourly ventilation rates are needed to determine the ventilation performance of buildings. Taking into account the world’s current population of seven billion people, the researchers suggest there are likely more than one billion homes globally. Along with that number, there are hundreds of millions of other indoor spaces, such as office buildings and movie theaters. It is an unrealistic goal at present to attempt to conduct real-time hourly monitoring for all those indoor spaces.

 

Bringing the changes needed to improve building ventilation is highly challenging. If adequate ventilation data existed then a predictive tool could be used, at low cost, for many buildings. The Internet of Things technologies enable the collection of good quality real-time date in indoor spaces. Integrating predictive tools with Internet of Things, big data, and machine learning approaches would give scientists a way to assess the ventilation performance of buildings.  “We remain optimistic that future innovation will result in advances in economic monitoring and predictive tools for determining ventilation performance in the billions of indoor spaces worldwide,” said Li.

 

The researchers who authored this editorial include Yuguo Li from the Department of Mechanical Engineering and the Faculty of Architecture, The University of Hong Kong; Pan Cheng, Ao Li, and Wei Jia from the Department of Mechanical Engineering, The University of Hong Kong; Li Liu from the School of Architecture, Tsinghua University; and Nan Zhang from the Beijing Key Laboratory of Green Built Environment and Energy Efficient Technology, Beijing University of Technology.

 

The editorial was supported by an RGC collaborative research grant.

 

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JOURNAL

Building Simulation

DOI

10.1007/s12273-023-1019-z 

ARTICLE TITLE

Predicting building ventilation performance in the era of an indoor air crisis

ARTICLE PUBLICATION DATE

25-Apr-2023

The world’s first wood transistor

Electrical current modulation in wood electrochemical transistor

Peer-Reviewed Publication

LINKÖPING UNIVERSITY

 

IMAGE: RESEARCHERS AT LINKÖPING UNIVERSITY, TOGETHER WITH COLLEAGUES FROM THE KTH ROYAL INSTITUTE OF TECHNOLOGY, HAVE NOW DEVELOPED THE WORLD’S FIRST ELECTRICAL TRANSISTOR MADE OF WOOD. view more 

CREDIT: THOR BALKHED

Researchers at Linköping University and the KTH Royal Institute of Technology have developed the world’s first transistor made of wood. Their study, published in the journal PNAS, paves the way for further development of wood-based electronics and control of electronic plants. 

Transistors, invented almost one hundred years ago, are considered by some to be an invention just as important to humanity as the telephone, the light bulb or the bicycle. Today, they are a crucial component in modern electronic devices, and are manufactured at nanoscale.  A transistor regulates the current that passes through it and can also function as a power switch. 

Researchers at Linköping University, together with colleagues from the KTH Royal Institute of Technology, have now developed the world’s first electrical transistor made of wood. 

“We’ve come up with an unprecedented principle. Yes, the wood transistor is slow and bulky, but it does work, and has huge development potential,” says Isak Engquist, senior associate professor at the Laboratory for Organic Electronics at Linköping University.

In previous trials, transistors made of wood have been able to regulate ion transport only. And when the ions run out, the transistor stops functioning. The transistor developed by the Linköping researchers, however, can function continuously and regulate electricity flow without deteriorating.

The researchers used balsa wood to create their transistor, as the technology involved requires a grainless wood that is evenly structured throughout.  They removed the lignin, leaving only long cellulose fibres with channels where the lignin had been.

These channels were then filled with a conductive plastic, or polymer, called PEDOT:PSS, resulting in an electrically conductive wood material. 

The researchers used this to build the wood transistor and could show that it is able to regulate electric current and provide continuous function at a selected output level. It could also switch the power on and off, albeit with a certain delay – switching it off took about a second; on, about five seconds. 

Possible applications could include regulating electronic plants, which is another strong research area at Linköping University. One advantage of the transistor channel being so large is that it could potentially tolerate a higher current than regular organic transistors, which could be important for certain future applications. But Isak Engquist wants to stress something:

“We didn’t create the wood transistor with any specific application in mind. We did it because we could. This is basic research, showing that it’s possible, and we hope it will inspire further research that can lead to applications in the future,” says Isak Engquist.

The study was financially supported by the Knut and Alice Wallenberg Foundation through the Wallenberg Wood Science Center.

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JOURNAL

Proceedings of the National Academy of Sciences

DOI

10.1073/pnas.2218380120 

ARTICLE TITLE

Electrical current modulation in wood electrochemical transistor

ARTICLE PUBLICATION DATE

24-Apr-2023

As the California sea lion population got bigger, so did male sea lions

Unlike other marine mammals, male California sea lions have gotten bigger over the past 50 years as their population has grown

Peer-Reviewed Publication

UNIVERSITY OF CALIFORNIA - SANTA CRUZ

 

IMAGE: CALIFORNIA SEA LION SKULLS USED FOR THIS STUDY AT THE ORNITHOLOGY AND MAMMALOGY COLLECTION OF THE CALIFORNIA ACADEMY OF SCIENCES. view more 

CREDIT: ANA VALENZUELA TORO

Animals tend to get smaller as their populations grow because of increased competition for food resources among members of the same species. That’s not what has happened with California sea lions, however, according to a new study led by scientists at UC Santa Cruz.

Published April 27 in Current Biology, the study found that male California sea lions have gotten bigger as the population grew over the past 50 years, while female body size has remained stable.

“It’s counterintuitive. You would expect that their body size would decrease as dietary resource competition intensified,” said coauthor Paul Koch, professor of Earth and planetary sciences at UCSC.

The number of California sea lions has increased dramatically since the Marine Mammal Protection Act was passed in 1972. In parts of their range, the sea lions may now be approaching the ecological “carrying capacity,” the largest number of animals an ecosystem can support.

In other marine mammal species, including northern fur seals, South American sea lions, and harbor seals, declines in adult body size have been observed as their population size increased, according to first author Ana Valenzuela-Toro. She led the study as a graduate student in ecology and evolutionary biology at UCSC, working with Koch and Daniel Costa, professor of ecology and evolutionary biology and director of the Institute of Marine Sciences at UCSC.

Valenzuela-Toro looked at the size and morphology of sea lion skulls collected between 1962 and 2008 in central California and now held at the California Academy of Sciences. She also analyzed bone samples for clues to changes in the animals’ diets. Stable isotopes of carbon and nitrogen in bone samples can yield information about where in the ocean the animals were foraging and what kinds of prey they were eating.

“We found that male California sea lions have expanded their ecological niche, which means they are now foraging on a more diversified group of prey and expanding the places where they are foraging," Valenzuela-Toro said. “Apparently they are now going farther north than they used to, which is consistent with observations reported by other researchers.”

By expanding the breadth of their diets, the sea lions have been able to get bigger even as their numbers increased. In theory, larger sea lions should be able to travel further, dive deeper, and handle larger prey. In addition, as their breeding sites became more crowded, increased competition between males during the breeding season may have favored larger males over time.

“Body size is very important in competition with other males to control territory at breeding sites. Being bigger also means they can fast longer and stay on the beach to defend their territory,” Valenzuela-Toro said.

Male and female California sea lions differ substantially in body size and foraging behavior. The males generally congregate in colonies only during the breeding season, after which they leave on long foraging trips. Females, meanwhile, stay in the colony to give birth and nurse their pups, so their foraging is restricted to areas near the colony.

“This creates different selection pressures on females and males,” Valenzuela-Toro said.

She noted that prey was abundant during the period covered in this study. That may not continue to be the case in the future as marine ecosystems respond to a changing climate.

“This has been a good period for sea lions, but if warm conditions become more frequent, we could see lower availability of their preferred prey, such as sardines and anchovies,” she said. “Then we might see their population size start to plateau or decrease, and we could even see body size start to decline.”

Costa noted that the study was only possible because of the 167 California sea lion skulls collected over 44 years and maintained at the California Academy of Sciences. “These results are important to help us understand how marine mammals are adapting as their habitat changes in response to a changing climate,” he said.

In addition to Valenzuela-Toro, Costa, and Koch, the coauthors of the paper include Rita Mehta at UC Santa Cruz and Nicholas Pyenson at the Smithsonian Institution’s National Museum of Natural History.

Male and female California sea lions differ substantially in body size and foraging behavior.

CREDIT

Illustration by Sarah Gutierrez

A male California sea lion at Año Nuevo Reserve.

CREDIT

Ana Valenzuela Toro

JOURNAL

Current Biology

DOI

10.1016/j.cub.2023.04.026 

ARTICLE TITLE

Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche

ARTICLE PUBLICATION DATE

27-Apr-2023

Male California sea lions are becoming bigger and better fighters as their population rebounds

Peer-Reviewed Publication

CELL PRESS

 

IMAGE: A PHOTO OF CALIFORNIA SEA LIONS RESTING AT AÑO NUEVO ISLAND. view more 

CREDIT: DANIEL COSTA

California sea lions have managed to maintain—and, in the case of males, increase—their average body size as their population grows and competition for food becomes fiercer. This is in contrast to other marine mammals, whose average body size tends to decrease as their numbers increase. Researchers report April 27 in the journal Current Biology that sexual selection was a strong driving force for males to grow bigger and to strengthen muscles in their neck and jaw that help them fight for mates. Both male and female sea lions evaded food shortages by diversifying their diets and, in some cases, foraging further from the shore.

“Body size reduction is not the universal response to population increase in marine predators,” says lead author Ana Valenzuela-Toro (@Avalenzuelatoro), a paleoecologist at the University of California Santa Cruz and the Smithsonian Institution. “California sea lions were very resilient over the decades that we sampled and were able to overcome increasing competition thanks to prey availability. They’re like the raccoons of the sea: they can consume almost everything, and they can compensate if something is lacking.”

While many marine mammal species have rebounded to some extent since the Marine Mammal Protection Act was passed in 1972, California sea lions are notable for the size and duration of their population increase: the number of breeding females, who have been most consistently counted, has more than tripled in the US since the 1970’s—from around 50,000 to nearly 170,000—and their population growth is only now beginning to plateau.

To explore how California sea lion ecology has changed as their population has grown, the researchers analyzed museum specimens of adult male and female California sea lions collected in central and northern California between 1962 and 2008. To estimate changes in body size, they compared the overall size of more than 300 sea lion skulls collected over the years. They also measured other skull features, such as the size of muscle attachment points, which allowed them to assess changes in sea lion neck flexibility and biting force.

The team additionally took tiny bone samples from some of the skulls and measured their stable carbon and nitrogen isotope composition, which allowed them to make inferences about where the sea lions were foraging and what they were consuming. “Carbon provides information about habitat use—whether they’re foraging along the coast or offshore—and nitrogen provides insights about the trophic level of their prey, for example if they’re consuming smaller or larger fish,” says Valenzuela-Toro.

Overall, the researchers found that male sea lions have increased in size, while female sea lion size has remained stable. This sex difference is probably due to the fact that size matters for male, but not female, mating success. “One male can breed with many females, and males in the breeding colony fight with each other to establish their territory,” says Valenzuela-Toro. “Bigger males are more competitive during physical fights, and they can go longer without eating, so they can stay and defend their territory for longer.”

Male sea lions also increased their biting force and neck flexibility over this same time period. “The neck muscles are really important because they allow them to move their head and neck more agilely, bite harder, and, eventually, win when they are fighting other males in the breeding colony,” says Valenzuela-Toro.

The isotopic analyses indicated that both male and female sea lions managed to meet their nutritional needs by diversifying their diet and feeding on a broader range of prey. Male sea lions also foraged further afield. “Over time, some male sea lions were foraging further north,” says Valenzuela-Toro. “This is consistent with some anecdotal records that they’ve even been seen in Alaska, where they were not known to go in the past.”

Female sea lions consistently had a more diverse diet compared to male sea lions. The authors suggest that this flexibility in food choice is what allowed females to maintain their average body size without foraging further away.

“They remain in a narrow zone around their breeding colony, but they still show a lot of flexibility in what they eat,” says Valenzuela-Toro. “We believe that skull morphology in the rostrum—which is related to the size and shape of the mouth—probably has something to do with this flexible foraging behavior. We found that the size and shape of the mouth of females is related to the size of the prey that they consume.”

However, this flexibility in diet can only take the sea lions so far, and the authors warn that the sea lions’ future may not be so rosy.

“All these dynamics occurred in an environment that was rich in prey: full of anchovies and sardines, two species that are super important for their diet,” says Valenzuela-Toro. “But over the last few years, the populations of these two fish have collapsed, so California sea lions are diversifying their diet to compensate, and apparently they are not doing so well.”

“As climate change progresses, prey availability of sardines and anchovies will decrease even more, and eventually we will have more permanent El Niño-like warming conditions, reducing the size and causing a poleward shift of these and other pelagic fishes,” she says. “It will be a really hostile environment for California sea lions, and eventually we expect that their population size will stop growing and actually decline.”

A photo of California sea lion skulls used for this study, taken at the Ornithology and Mammalogy Collection at the California Academy of Sciences.

CREDIT

Ana Valenzuela Toro

This research was supported by funding from the University of California Santa Cruz Division of Physical and Biological Sciences, the Graduate Student Association, and the Rebecca and Steve Sooy Graduate Fellowship; the Peter Buck Predoctoral Fellowship of the National Museum of Natural History, Smithsonian Institution; and ANID PCHA/Becas Chile.

Current Biology, Valenzuela-Toro et al. “Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche” https://www.cell.com/current-biology/fulltext/S0960-9822(23)00474-8 

Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.cell.com/current-biology. To receive Cell Press media alerts, contact press@cell.com.

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JOURNAL

Current Biology

DOI

10.1016/j.cub.2023.04.026 

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Animals

ARTICLE TITLE

Unexpected decadal density-dependent shifts in California sea lion size, morphology, and foraging niche

ARTICLE PUBLICATION DATE

27-Apr-2023