Revisiting the effects of climate change on salamander body size: the role of natural history collections

Our recent paper, The relationship between climate and adult body size in redback salamanders (Plethodon cinereus), found that salamanders were larger in warmer parts of their range. We also found that that body size increased significantly in places where the climate had become hotter and drier.

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Image credit: Brian Gratwicke

Small-bodied lungless salamanders breathe through their skin, and tend to come to forage on the surface in cool, damp conditions associated with spring and fall, which is the best time to find them. They have thrived in the cool, temperate climate of the Appalachian Mountains, making this region a global hotspot for salamander diversity. Because of their preference for cool, damp environments, salamander biologists worry that predictions of warmer climates and more intense rainfall events and longer dry spells in between may be bad news for these distinctive creatures.

Several studies using museum specimens found that salamanders in warmer areas have larger bodies, but one recent study suggested that salamanders were actually shrinking in response to climate change. Subsequent papers have dwelled on the challenges of using museum specimens to draw these types of conclusions, but none re-examined the actual phenomenon of the shrinking salamanders. We designed a new study to revisit the question using museum specimens in a way that accounts for some of the previous limitations.

We selected redback salamanders, which are one of the most abundant vertebrates both by number and biomass in forests in the Eastern United States. One classic study by Thomas Burton and Gene Likens at Hubbard Brook Forest in New Hampshire found densities of about 3,000 salamanders per hectare, mostly redbacks.  This wide-ranging, abundant species is also very well represented in museum collections. About 70,000 redback specimens are held in the Smithsonian’s Museum of Natural History, mostly collected between 1950 and 2000.

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A Redback salamander Plethodon cinereus specimen at the Smithsonian Museum of Natural History. Image by Brian Gratwicke

A collection of this size allowed us to pre-emptively select our comparison groups in a way that would eliminate sampling bias, maintain large sample sizes and maximize our power to answer the question.  We corrected our samples for potential sources of sampling bias including seasonal collection bias and potential destructive sampling bias. We found that redback salamander body size actually increased 1.8% in the places that had warmed significantly. Our observations do not really shed any light on whether climate-change is a potential threat to redback salamanders, but there does appear to be a measurable effect on the species.

The nature and culture surrounding natural history collections is changing, and very few redback salamander specimens were lodged after the year 2000, restricting the time period we could analyze. This likely is a product both of the ethics debate surrounding indiscriminate collecting, and the growing popularity of new citizen science tools like iNaturalist which create photographic specimens in publicly accessible databases with critical collection information. We were able to use citizen-science databases like the Maryland Herp Atlas  and iNaturalist to verify that the salamanders are still common in all the counties that have warmed significantly, but body size data were not available. We actively support and participate in these non-destructive efforts, but view them as complimentary to well-curated natural history specimens, rather than a substitute.

Brian Gratwicke is a biologist at the National Zoo’s Smithsonian Conservation Biology Institute in Front Royal, Virginia, USA

Multiple stressors and ecological surprises

The expanding global human population, now about 7.5 billion, is increasing the pressure that we as a species put on the environment.  2016 was the warmest year ever recorded, and temperature records continue to be exceeded. Each year, more natural ecosystems are lost to dam construction, deforestation and urbanisation. Rates of species invasion are increasing, and pollution events continue to pressure native wildlife. Many ecosystems are now threatened simultaneously by these multiple human-caused stressors, yet we still know very little about their combined interactive impacts.

In our paper in Geo (Linking key environmental stressors with the delivery of provisioning ecosystem services in the freshwaters of southern Africa) we review the impacts of multiple stressors on ecosystem services in freshwater ecosystems in southern Africa (e.g. the Okavango Delta; see photo). We chose these systems because freshwaters contribute disproportionately to ecosystem services despite covering less than 1% of the earth’s surface. Freshwater systems are also especially vulnerable to environmental stressors and over exploitation, with water and fish protein growing in importance as commodities, and average species population declines since 1970 estimated at 81% (WWF Living Planet Report, 2016). Communities in southern Africa rely on freshwater ecosystems for critically important provisioning services, such as drinking water and food (e.g. inland fisheries)

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The Okavango Delta

We found evidence that water resources for drinking, agriculture, sanitation and power are declining because of both climate and land use change. In some areas, fish production increased because of dam construction or species invasions, but these stressors can have negative impacts elsewhere. Evidence also suggests that stressors can interact to alter one another’s impacts or promote the proliferation of further stressors.

Multiple stressors often cause impacts which are hard to predict because of both complex interactions between the stressors themselves, and interactions within communities (such as those between species in a food web). These unpredictable impacts have been termed ‘ecological surprises’ and global analyses indicate that they are very common (e.g. http://onlinelibrary.wiley.com/doi/10.1111/gcb.13028). This creates problems for decision makers when prioritising which stressors to manage or control, especially when it comes to the supply of the goods and services which we rely on from natural ecosystems.

We provide a framework to categorise multiple stressor effects on ecosystem services where they can either be additive (i.e. predictable and the sum of their independent effects) or four different types of non-additive ecological surprises.  For instance, nutrient enrichment in Lake Victoria (because of high nutrient inputs from the surrounding catchment) causes low oxygen levels, killing fish (Photo 2). At the same time the nutrients promote growth of invasive aquatic plants (water hyacinth) causing a successive and synergistic multi-stressor interaction whereby the increase in plant biomass triggers further fish kills in the lake. In addition, the introduction of non-native fish (Nile perch) caused a dramatic decline in native fish biodiversity but boosted the overall fishery catch in the lake, benefiting the surrounding populations (see figure below).

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With the growing population, it is becoming difficult to protect biodiversity and rely on our planet’s natural ecosystems for food and water security. Multiple stressors are causing a downward spiral, where our use of ecosystem services threatens the environment and therefore impairs the delivery of these services for future generations.  We need more research into multiple stressors and ecological surprises, and much more needs to be done to reduce the impact that humans have on the environment.

Michelle Jackson is a Researcher at Imperial College London. 

Opening-up (to) the politics of Anthropocene science

A group of scientists working for the International Commission on Stratigraphy recently recommended that the start of the Anthropocene epoch, an age defined by human impacts on the environment, be set at 1950. The concept of the Anthropocene has produced wide-ranging debates across the natural and social sciences. Here, Johannes Lundershausen , PhD candidate at the Tübingen Centre for Ethics, reflects on a recent dialogue in Geo between Mark Maslin and Andrew Barry

Geo: Geography and Environment recently published and thus fostered a timely dialogue between scientists researching the Anthropocene and scholars in science studies reflecting on the practice of this research (Barry and Maslin, 2016). The interlocutors in the published debate are physical geographer Mark Maslin, who has been actively involved in controversial debates between different research groups about the inception of the Anthropocene (Lewis and Maslin, 2015a), and human geographer Andrew Barry (both from University College London).

The starting point of their discussion is that the politics of the Anthropocene do not just relate to environmental governance but also to the ways in which this Epoch is formally defined by geoscientists. They highlight their disagreement about desirable interactions between the ‘formal’ geo-scientific assessment of the Anthropocene and the ‘informal’ engagements of social scientists with the concept. Whereas Mark Maslin holds that these academic realms should debate the Anthropocene in their own terms, Andrew Barry considers closer collaboration between disciplines essential to the study of the Anthropocene.

Copyright Smudge Studio

This difference is intriguing particularly because Maslin and Barry ostensibly agree on the need to ‘open up the geo-scientific debate about the Anthropocene to the social sciences’ and Barry acknowledges that Maslin has taken a step in this direction, for example in  a publication that he co-authored with UCL colleague Simon Lewis (2015b). Nevertheless, Maslin and Barry arguably mean different things when they talk about this interdisciplinary openness.

When prompted, Maslin draws a picture of interdisciplinarity in which social and natural science debates run parallel to each other. Social sciences, from this perspective, are expected to adapt to the requirements of the natural sciences in order to inform the latter about the consequences of their work as well as the social causes of environmental changes. Geoscientists, meanwhile, ‘should not be distracted by the […] valid discussions on the history and politics’ of the Anthropocene (p. 6). This way of ‘opening-up’ is reminiscent of the ‘subordination-service mode’ of interdisciplinarity that Barry and colleagues defined in a seminal paper in 2008 (Barry et al., 2008).

Barry himself comes closer to an ‘agonistic-antagonistic mode’ of interdisciplinarity in which the epistemological and ontological assumptions of established disciplines are challenged. He proposes a collaboration between the social and the geo-sciences that breaks with the existing disciplinary divisions of labour and involves the social sciences directly in the assessment of the merits of different proposals for an Anthropocene inception.

These two approaches, however, are more than different visions of interdisciplinarity. They also make for different ways of dealing with the topic of the debate, i.e. the politics of the Anthropocene, especially as they relate to geo-scientific definitions of the term. For Barry, interdisciplinarity can foster new conceptualisations of the role of politics in the geo-sciences and, reciprocally, the role of the geological in historical and political sciences; whereas for Maslin, interdisciplinary collaboration can or should not affect the constitution of geo-scientific practice.

Ultimately, it could be argued, the two approaches to interdisciplinarity are equipped to deal with very different conceptions of politics in the geo-sciences in particular, and in science more generally. Although Maslin recognizes politics in the interpretation and application of official criteria for establishing formal geological units, these politics are rather different from those that Barry refers to when he highlights the inherently political nature of, for example, applying scientific standards in practice, defining the scope of scientific controversies or deciding which parties are legitimately involved in such controversies (p. 3). Maslin views science as an ideally value-free endeavour in which clearly defined rules and epistemic standards guide the interpretation of data. This ideal is particularly apparent in his criticism of the members of the Anthropocene Working Group, who, he alleges, have been ‘swayed by political considerations’ in their interpretation of stratigraphic evidence (p. 4). Instead of following the remit of their work and keeping politics out of the geo-scientific definition of the Anthropocene, Maslin contends, they have acted as an advocacy group for a post-1945 inception of the Anthropocene.

This difference between Maslin’s ideal of value-free science and Barry’s insistence on the social and historical contingency of scientific practice raises questions about the initially proclaimed agreement of the two interlocutors about the existence of politics in Anthropocene geo-science. One particularly interesting question concerning this emerging disagreement is how to practically deal with social values in scientific practice (including, amongst others, political ones).

Scientists who operate under the ideal of value-free science tend to see social values in scientific practice as a threat to scientific integrity that opens the door to a politicisation of science. But in geo-scientific research on the Anthropocene, as Barry argues, scientific evaluations are underdetermined by evidence (and, I would add, also by epistemic values). In this situation, social values do not necessarily compete with geo-scientific evidence but they can work to complement the available geological evidence by guiding its interpretation and judging its power to support a given proposal for the inception of the Anthropocene.

Involving social scientists in geo-scientific debates, as suggested by Barry, might be a method of operationalising this indirect role of social values in Anthropocene science. I would argue that Maslin’s proposal to adopt public engagement strategies from climatologists, on the other hand, will not suffice to deal with the politics in Anthropocene science (cf. Beck, 2012), nor will his appeal reference to scientific objectivity, which other researchers, who he accuses of practicing politicised geo-science, equally claim for themselves (Zalasiewicz et al., 2015). Opening up to the idea of an appropriate place for social values in geo-scientific practice and the possibility of an interdisciplinary exchange that produces different scientific practices, arguably holds more potential to deal with the ‘mask[ed][…] political views’ (p. 9) and the ‘political ramifications’ (p. 4) that Maslin acknowledges. The question remains as to whether there are ways to do this while maintaining the ‘independence and credibility’ of geo-scientists (p.6) that Maslin calls for. Debates such as this, between physical and social scientists, are vital ways of moving this conversation forward.

Johannes Lundershausen is a PhD candidate at the Tübingen Centre for Ethics.

References
Barry A, Born G and Weszkalnys G (2008) Logics of interdisciplinarity. Economy and Society 37(1): 20–49.

Barry A and Maslin M (2016) The politics of the anthropocene: A dialogue. Geo: Geography and Environment 3(2): e00022

Beck S (2012) Between Tribalism and Trust: The IPCC Under the “Public Microscope”. Nature and Culture 7(2): 151–173

Lewis SL and Maslin MA (2015a) A transparent framework for defining the Anthropocene Epoch. The Anthropocene Review 2(2): 128–146

Lewis SL and Maslin MA (2015b) Defining the anthropocene. Nature 519(7542): 171–180.

Zalasiewicz J, Waters CN, Barnosky AD, Cearreta A, Edgeworth M, Ellis EC, et al. (2015) Colonization of the Americas, ‘Little Ice Age’ climate, and bomb-produced carbon: Their role in defining the Anthropocene. The Anthropocene Review 2(2): 117–127

Journal metrics and linguistic hegemony

Geography is a uniquely international discipline. It is concerned with describing and explaining the world in all its infinite variety. Geographical societies and university departments can be found in all corners of the globe, and the discipline’s practitioners often build careers on internationally collaborative research focused on distant places. Why, then, is the world of geographical publishing and performance measurement so skewed towards the publishing cultures of North America and northwest Europe?

This is the question which arises from a recent paper in Geo: Geography and Environment by Michael Meadows, Ton Dietz and Christian Vandermotten. The authors note the rise and the apparent embedding of a metrics culture in higher education (see for example recent discussions about the role of metrics in the UK’s assessment exercises for research and teaching). Metrics, such as journal impact factors and personal H-index values, have not only become popular ways of trying to describe the impact of publications and their authors – they have also become key adjudicators of academic careers, with measures such as the H-index seemingly holding ever greater sway over promotion and funding decisions.

When metrics become performative, when efforts to describe a system become part of the means by which that system is run, then pre-existing hierarchies and power structures tend to get reinforced. Meadows and colleagues argue that this is particularly the case in academic geography. They point out how the key databases from which the main metrics are derived – Web of Science and Scopus – massively underrepresent research being published outside of the networks of the major commercial publishers, and in languages other than English. Using a newly developed database of geography journals developed by the International Geographical Union, they present some disturbing statistics – of the more than 200 geography journals published in China, not one appears in the international journal rankings produced by these western organisations. Of the 27 geographical journals published in Germany, fewer than ten are represented on Web of Science.  Of the 108 geography journals published worldwide in Spanish, just three appear on Web of Science.

These huge disparities in how ‘quality’ academic research is identified, measured and ranked have significant implications not only for individual career trajectories, but for the discipline as a whole. The concerns and interests of Anglophone geography will continue to dominate so long as metrics and rankings reinforce the dominance of certain publication outlets, at the risk of marginalising alternative paradigms, arguments, or ways of working. As the authors note, “ranking and the dominance of particular leading journals may undermine innovation and alternative and critical thinking” (p5).

What is to be done? The authors note a number of positive developments, including alternative, more inclusive ranking systems such as that being developed at CERES in the Netherlands (see here in PDF). Open access is certainly part of the story as well, with the authors identifying something of a ‘Latin reaction’ to Anglophonic dominance with a widespread move to online, ‘green’ open access publishing models. How to fund open access publishing is still a live question of course, with different initiatives emerging to allocate costs for ‘gold’ open access publishing between research funders, institutions, and individual authors. Geo can be considered part of this broader experimentation.

But returning to the discipline geography more specifically, the authors conclude by addressing the IGU, whose new database underpins the authors’ arguments. They urge the IGU to explore the kind of multi-lingual publishing options being innovated in settings like Conservation Biology, with the organisation’s international reach potentially making it a powerful vehicle for new efforts to promote working and publishing practices which help to break down linguistic barriers. More broadly, the article prompts geographers to reflect on how a discipline so international in reach can make its publication practices more inclusive of linguistic, cultural and intellectual diversity.

Martin Mahony is a Research Fellow in the School of Geography at the University of Nottingham. He also edits the Geo blog.

Drawing, remembering, knowing: natural history and the ecological imagination

By Meredith Root-Bernstein (Aarhus University)

Geo: Geography and Environment recently published my personal essay about how natural history practices have helped me to think about interdisciplinary research and collaborations.  I emphasize in the essay how developing and sharing habits of observing, interpreting, and considering the human contexts of nature can help form shared understandings as the basis for exchanges about social and natural sciences of the environment.  In that essay, I discuss seeing an espino (Acacia caven) with a liana growing on it in central Chile.  My research involves searching for the key problems and solutions for the conservation of a silvopastoral system (“espinal”) and the surrounding shrub and forest habitats in this mediterranean-climate zone.  The most common species in espinal is the espino (Acacia caven).  Yet, I had never seen an espino with a liana, and I became intrigued by trying to understand the potential ecological and social meanings of this unusual species assemblage.  Here, I expand on that essay with a discussion of a sketch of the liana and the espino.  While looking for something else I came across this drawing I made of the espino and its liana:

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I had forgotten about the sketch, and I have also forgotten the exact circumstances of making it.  I am sure that I didn’t make it in situ, and a few days probably elapsed between seeing the tree and making the drawing. The structure of the trunk is hard to read.  First I thought it suggested that the tree is old and perhaps has ridges or hollows, but this doesn’t match the photograph.  I also couldn’t think of any example of thick, undulating or textured espino trunks.  Something was wrong, either with the drawing, my memory, or my knowledge of espinos.  Then, while walking past some trees here in Denmark with ivy on them, I realized by analogy that I had drawn the vines of the liana descending to the ground.

The liana seems to be partly imaginary.  I remember seeing red stems and green leaves, but I am fairly sure that there were no black drupes at the time and that I only saw images of them by looking up the species on the internet.  The drawing thus knits together memory and imagination to represent the way I was thinking about my observation.

The ambiguity of the sketch forced me to think about the visual and structural patterns that things make, and how those map onto our other kinds of knowledge and memory.   There are really two issues here: one is that the sketch was by nature approximate, hasty, and in this case not based on direct observation but rather memory and its own approximations.  All of these aspects confer an abstract nature on the sketch.  It excludes the inessential and retains only an impression, just enough to reconstruct what was seen.  The second issue though relates to my lack of experience thinking about and observing lianas.  This led to what might be a not-so-clear abstraction of a liana growing up a trunk, and certainly created ambiguity in interpretation.  But the ivy I saw that helped me to understand the sketched pattern of something I had only seen once before—a liana on an espino—taught me about lianas and vines in general.

An important part of natural history is personal memory, the accumulation of implicit and tacit knowledge.  How do we make these memories relevant to interpreting the future as well as the past?  Writing, sketching and showing others are all important means of communication, that emphasize different aspects of nature—the narratives and cycles, the structural patterns, the kinaesthetic and embodied knowledge of where, when and how.

It is well-known that natural history drawings have features that photographs do not: they can represent a general or ideal example of something, facilitating recognition, and they can bring attention to particular features or patterns through emphasis, selectivity and abstraction.

As I mention in the article, I think of natural history as seeking patterns, which can be used to interpret the past, but also potentially the future.  In my drawing, I imagined the visual effect of the liana on the espino in a season when it had fruit. In the Anthropocene, it might be interesting to think more about the natural history of the future.  How will places look, behave and feel under climate change? How will we read the landscape of abandoned infrastructures in the future?  What unexpected species pairing will we find somewhere next year, testimony to some casual event yesterday?  These visions don’t have to be apocalyptic—and they don’t have to be written.   Drawings can often be both more subtle and more complex than words.  They have their own logic of organization and representation.

I recently saw a short article in the ESA Bulletin about how ecologists can avoid midlife crises and burnout.  Going into the field from time to time was one suggestion.  I would also add to that that the practice of natural history, and the attempt to communicate it, if only to oneself later on, can be both enjoyable and meaningful.  It was a pleasant surprise to find this sketch that I had forgotten about, and it brought a new angle to what the liana and the espino taught me about the socioecological interactions of central Chile.

Who knows, practicing a little natural history on your days out might even inspire a new line of research, maybe an interdisciplinary one.  You don’t have to try to be serious and professional about natural history, which might take away the enjoyment of being in nature.  Play is an important way to explore the world, and its not just for children.  Many accomplished scientists, among others, take time to have fun with no clear purpose as a way to think better (see here  and here).  Later on, accumulated memories will certainly make something useful out of what you observe in nature for fun, whether it’s a publishable research project or some extra emotional attachment helping you to find satisfaction and motivation.  Indeed, my paper in GEO: Geography and Environment, and this blog post, were written just for fun and have helped me to recognise how important natural history is for my enjoyment of my job.

Meredith Root-Bernstein is a postdoctoral researcher in the Aarhus University Research on the Anthropocene (AURA) project, based in the Department of Bioscience, Aarhus University, Denmark

Uneven geographies of openness and information

By Helen Pallett (University of East Anglia, UK)

Open access to information and data appears to be a cause which has found its moment, with governments, businesses, NGOs and academics queuing up to ratify open access commitments and extoll its virtues. It has variously been heralded as a means of rejuvenating democracy, reforming corrupt institutions, holding big business and business-dealings to account, improving the quality of scientific data available, removing academics from their ivory towers, and changing relationships between publishers, academic journals and authors.

These arguments for the opening up of data and information now seem uncontroversial and have few serious detractors. However, an emerging body of work demonstrates that to take the geographies of information seriously is to add a significant but often-overlooked angle on debates in academia and policy on open access and open data. This is what Mark Graham, Stefano De Sabbata and Matthew A. Zook have done in their recent paper in Geo, ‘Towards a study of information geographies’.

In this paper the authors focus on the different internet-based platforms through which geographic information itself is mediated, hosted and delivered. The potential of the internet and related technologies to facilitate the wide sharing, distribution and processing of information has of course been at the centre of arguments for and models of open data, and even open innovation or open government. Whilst the transformative power of platforms like Wikipedia and OpenStreetMap are evident, the paper draws attention to their uneven geographies in a number of ways.

At a very basic level, geographies of internet access are uneven – less than 20% of the population in countries like India, Tanzania and Guatemala are internet users. But there are also clear barriers to participation in creating and contributing to geographic information platforms which transcend questions of who is connected to the internet, and there are important geographic patterns in what gets represented and what is overlooked. For example, domain names of websites are overwhelmingly located in Europe and North America, and similar patterns are found in the numbers of people contributing information to or coding these platforms such as Wikipedia or Geonames. This geography of participation also has implications for representation, with platforms like OpenStreetMap displaying much denser geographic information for locations in Japan, Europe and North America, as compared to locations in Africa, Australia and much of South America.

 

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Distribution of domain names by country. From Graham et al. (2015).

 

The authors have demonstrated that information has geographies in the way that it is produced, presented and distributed, far from flowing with ease across space. This sounds a note of caution with regards to claims about the democratising and empowering potential of platforms aiming to open up information, including attempts to facilitate access to scientific data, academic papers, and government data. It suggests that enabling open access to information at this general level can only do so much, without addressing existing highly uneven geographies of access to the infrastructures and platforms hosting and transmitting this information. Furthermore, there are not only uneven geographies of access to consider, but also uneven geographies of participation and representation which serve to further shape and limit the data and information which is available to us.

Helen Pallett is a Senior Research Associate in the Science, Society & Sustainability group at the University of East Anglia.

Learning from guano: In search of a paleo-seabird proxy

By Jessica Conroy (University of Illinois Urbana-Champaign, USA)

Take a vacation to the Galápagos Islands and you’re bound to see some of the archipelago’s most colorful denizens perched on guano-splashed basalt rocks, or a sweet ball of puffy white feathers sitting patiently in her nest, waiting for breakfast.

Seabirds like the red-footed and blue-footed booby are some of the unique inhabitants of the Galápagos. Apart from being photogenic, they are impressive animals, with amazing hunting abilities, sometimes flying over 100 km away from their nesting sites in search of food.

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A Genovesa red-footed booby. Credit: Jonathan Overpeck

Seabirds are also harbingers of the large changes sweeping the world ocean. A study of long-term seabird populations shows many species are declining.  Recent research also points to longer-term changes in some seabird populations relative to the last several millennia, likely related to recent human exploitation of the marine ecosystem.

Natural climate and ocean variability can also affect seabird populations. In the tropical Pacific, many species of seabirds suffer the effects of interannual El Niño events, which influences the abundance of seabird prey by altering ocean properties such as temperature and nutrient availability. In the Galápagos, blue-footed booby populations are decreasing, perhaps due to shifts in climate that operate over decades.

It is important to define the natural baseline of these varying populations. In order to place recent changes in a long-term context, scientists must establish the range of natural ups and downs in seabird populations prior to recent changes. But unfortunately, there are very little long-term seabird population data.

Lake sediments have long offered a way to tell us something about past environments– how much it rained, or variations in past temperatures. But, in the right setting, could lake sediments tell us something about seabirds? We thought one such lake may have an interesting story to tell.

Genovesa Island is a small, uninhabited island in the northeastern part of the Galápagos. With no surface water, it is not a tempting site for human settlement; Darwin did not even make it here. But, it is home to thousands of red-footed boobies and lesser numbers of great frigatebirds. According to the only estimate of the island’s bird population in the scientific literature, it is home to the largest colony of red-footed boobies in the world. The birds nest across the island, including the steep crater walls at the center of the island, which shelter a very inaccessible (to humans) crater lake.

The Genovesa seabirds produce large quantities of guano. The crater is streaked in white, and the air has an acrid smell. Some of this guano makes it into the lake, either directly, or by washing in during rainy periods. We hypothesized that there would be a geochemical signal of this guano archived in the lake sediments. One candidate for a paleo-guano proxy was the stable isotopic composition of nitrogen in lake organic matter. The heavier, less abundant stable isotope of nitrogen, 15N, is preferentially sequestered in organisms, with animals higher on the food web containing more 15N relative to 14N. As seabirds sit high on the food web, they, and their guano, have a distinctly high 15N/14N ratio—much higher than the 15N/14N values related to other processes and organisms that are typically found in lake sediments.

We measured 15N/14N in organic matter in Genovesa lake sediments and guano samples. The data, expressed in delta notation, d15N, where the 15N/14N ratio is normalized to the ratio of 15N/14N in air, showed high values in both guano samples and in the sediments, supporting a geochemical signature of seabird presence in the sediment. Although pretty cool, this result has been observed before, in arctic seabird ponds. The most interesting aspect of our study was the variability in d15N values over time.

We found that higher d15N values, which we interpreted as indicating increased seabird activity on Genovesa, coincided with decadal changes in the counts of anchovies and sardines in the eastern Pacific. With more of these fish in the region, d15N was higher, suggesting more seabird activity on the island.  An abrupt increase in d15N also occurred around 1830 AD, right when ocean sediment cores off the coast of Peru and Chile showed increased ocean productivity and nutrients. Another abrupt shift occurred in the mid 1960s. Thus, seabird activity has been highly variable at Genovesa over the last 400 years, increasing and decreasing in concert with decadal and abrupt changes in the ocean environment.

What do these results imply about the seabirds living on Genovesa? They seem to be doing ok, at least in context of last 400 years. Although this year’s monster El Niño event may have a negative impact on seabirds elsewhere in the Galápagos, past observations and our guano proxy suggest Genovesa seabirds don’t seem to feel El Niño events the same way.  But a recent  increase in decade-to-decade ups and downs in the d15N guano proxy record suggests that perhaps Genovesa seabirds are becoming more sensitive to regional ocean and climate changes on the decadal timescale. Or, there may have been a strengthening of climate change on the decadal timescale in this region during the 20th century. We hope to extend our lake sediment record deeper into the past and explore more potential guano indicators in order to find out.

About the author:

Jessica Conroy is an assistant professor in the Departments of Geology and Plant Biology at the University of Illinois Urbana-Champaign.