Monday, March 16, 2009

A roadmap to generalized linear mixed models

In a recent paper in TREE, Ben Bolker (from the University of Florida) and colleagues describe the use of generalized linear mixed models for ecology and evolution. GLMMs are used more and more in evolution and ecology given how powerful they are, basically because they allow the use of random and fix effects and can analyze non-normal data better than other models. The authors made a really good job at explaining what to use when. Despite the fact that you need more than basic knowledge of stats to fully understand this guide, I think that people should take a look at it before starting to plan their projects, since it outlines really well all the possible alternatives (and challenges) that one can have when analyzing data. This article also describes what is available in each software package; this is really useful since is not obvious with program in SAS or R you need to use when dealing with some specific GLMMs.

Bolker, B., Brooks, M., Clark, C., Geange, S., Poulsen, J., Stevens, M., & White, J. (2009). Generalized linear mixed models: a practical guide for ecology and evolution Trends in Ecology & Evolution, 24 (3), 127-135 DOI: 10.1016/j.tree.2008.10.008

Friday, March 6, 2009

Salamaders and climate change -impending extinctions?

ResearchBlogging.orgBy the now the evidence of a global frog decline, perhaps even an extinction crisis, has been well documented. But what about salamanders? They are normally less abundant and less-studied compared to frogs, but is there evidence of the same general pattern of declining population sizes? According to Sean Rovito and colleagues, the answer is unfortunately yes. They repeated a plethodontid (lungless) salamander survey done in the 1970’s in Central America and found that many species have declined. In fact they failed to find a couple of previously very abundant species. They also found that species declines were phylogenetically non-random and so these declines may result in the loss of whole clades of species, meaning that the evolutionary history of these salamanders is at risk.

The authors attempted to determine the cause of these declines and found that neither habitat loss or the chytridiomycosis fungal disease implicated in other declines explained these salamander declines. The authors hypothesize that these declines are a direct result of climate change –namely changing temperature and humidity. If so, we may be witnessing some of the first extinctions that are directly caused by climate change.

S. M. Rovito, G. Parra-Olea, C. R. Vasquez-Almazan, T. J. Papenfuss, D. B. Wake (2009). Dramatic declines in neotropical salamander populations are an important part of the global amphibian crisis Proceedings of the National Academy of Sciences, 106 (9), 3231-3236 DOI: 10.1073/pnas.0813051106

Post script:
We had a comment questioning the use of climate change as an explanation and here is my response.

Science works by finding parsimonious explanations, until through experimentation or observation another, better explanation emerges. The previous explanations of habitat loss or fungal infections were not supported. These habitats, known as cloud forests, are very humid. The lungless salamanders have no lungs and instead breath through their skin, which must be kept moist. These forest are becoming drier, hence the probable connection. Here's a quote from the paper:

"Pounds et al. (25) used modeling to show that largescale warming led to a greater decrease in relative humidity at Monteverde compared to that caused by deforestation. Species of cloud forest salamanders that can still be found rely at least
in part on bromeliads. Bromeliads depend on cloud water deposition and are predicted to be articularly vulnerable to climate change (26, 27)."

Doesn't sound like "magic" to me, rather a robust hypothesized mechanism worthy of more testing. Given that species are going extinct, it is important to suggest likely mechanisms, providing an impetus for more research.

For those that think that scientists use climate change as boogey man to scare up more research funding (i.e., Crichton), please read the science. You'll discover honest, hardworking folks that are trying to understand this changing world and whose research can only benefit you , me and the salamanders.

Sunday, March 1, 2009

Phytoplankton motility and morphology might influence red tides

ResearchBlogging.orgThin layers, intense congregations of phytoplankton that can extend horizontally for kilometers, can be either a boon or a bust to marine food webs. On the one hand, these layers can stimulate the food web from the bottom up by providing elevated concentrations of marine snow (e.g., protozoa and organic detritus), bacteria, and plankton. On the other hand, because many of the phytoplankton species found in thin layers can be toxic, these layers can disrupt grazing, cause zooplankton and fish die-offs, and seed algal blooms at the ocean’s surface that can generate red tides. Understanding the processes driving the formation of thin layers is crucial for predicting their occurrence and ecological impact.

Although thin layer formation was previously thought to be solely influenced by abiotic forces, a recent paper in Science by William M. Durham and colleagues suggests that plankton’s swimming and shape play a role. Many phytoplankton species swim upward against gravity. When the water is calm, they swim up in a straight path. But add ocean currents to the equation, and the plankton start to encounter vertical shear where layers of faster- and slower- moving water meet. These shear forces can cause the plankton to tumble and spin instead of swimming straight up. The tumbling plankton become trapped in these regions of high shear, accumulating in a thin layer. The strength of the shear forces interacts with the morphology of the plankton to determine which species get trapped. For instance, bottom heavy species require higher shear to knock them off their straight path. Durham et al.’s findings suggest that vertical shear and cell morphology could be important predictors of red tides.

W. M. Durham, J. O. Kessler, R. Stocker (2009). Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers Science, 323 (5917), 1067-1070 DOI: 10.1126/science.1167334

Friday, February 27, 2009

Peace and conservation biology


A recent study published in Conservation Biology found that conservation hotspots are also hotspots for wars. Most of the wars (90%!) from 1950 to 2000 have been on countries that have these key areas for conservation, and 80% of these conflicts took place directly within the conservation hotspots. The authors mention many problems, such as the destruction of vegetation (i.e. the application of Agent Orange to detect enemies), poaching by soldiers, or that the cost of war could come at the expense of other government projects, such as conservation programs. But, there is not all bad news regarding biodiversity conservation (of course is Always bad news for humans), since these wars have created spaces with very low human impact (such as buffer areas with no human activity), or reduces economic activity that can make wartime a recovery period for some exploited species, and other unique situations. They conclude that since most of our biodiversity is in unstable regions, plans to conserve biodiversity should be also active in these regions, which are not the most appealing to work in, but may the most important. Also, that we need to integrate conservation biology into military and humanitarian programs that operate in these conflict zones.

These news are really sad, and I hope that we can live in peace for the benefits of all the species living in here (including, of course, Homo sapiens).

THOR HANSON, THOMAS M. BROOKS, GUSTAVO A. B. DA FONSECA, MICHAEL HOFFMANN, JOHN F. LAMOREUX, GARY MACHLIS, CRISTINA G. MITTERMEIER, RUSSELL A. MITTERMEIER, JOHN D. PILGRIM (2009). Warfare in Biodiversity Hotspots Conservation Biology DOI: 10.1111/j.1523-1739.2009.01166.x

Sunday, February 22, 2009

The incredible spreadable weeds

ResearchBlogging.orgResearch into the spread of non-native species usually assumes a long time lag between introduction and rapid spread, and many studies cite 50 years as the lag time. The reason for believing this is that it is thought that there needs to be sufficient time for adaptations to fine tune the fit between the exotic and its new environment, or that densities are so low to start with, finding mates and buffering populations from stochasticity (i.e., Allee effects) takes time. However, Curtis Daehler at the University of Hawaii, collected information on purposeful plant introductions into Hawaii in the 1920s. 23 of those planted have become serious invaders and the herbacious species showed a lag time of 5 years and 14 years for woody species. Knowing that lag times can be much shorter then we previously thought means that monitoring and management activities need to much more aggressive. It seems we can no longer assume a period of relative safety after a new species in introduced, new records of non-natives needs to be followed active assessment and perhaps intervention.

Curtis C. Daehler (2009). Short Lag Times for Invasive Tropical Plants: Evidence from Experimental Plantings in Hawai'i PLoS ONE, 4 (2) DOI: 10.1371/Journal.pone.0004462

Friday, February 20, 2009

Increased access to science, but who gets to publish?

ResearchBlogging.orgWhat role will open access (OA) journals play as science publishing increasingly moves to the internet and involves a more diverse array of participants? In a recent short article in Science, Evans and Reimer tried to answer this using citation rates from 8253 journals and examine trends in citation rate shifts. They found that researchers from wealthier countries were not likely to shift to citing OA journals while researchers from poorer countries did. The authors conclude that the overall shift to citing OA journals has been rather modest, but these journals have increased inclusion for researchers at institutions in poorer countries that cannot afford commercial subscriptions. However, there is an unfortunate flip side to the OA model -paying to publish. Most OA journals recoup the lack of subscription earnings by placing the financial onus on to the publishing scientists. This means that while researchers from poorer countries can now read and cite current articles in OA journals, they still are limited from publishing in them. True, most OA journals allow for deferring costs for researchers lacking funds, there is usually a cap to the frequency in which this can be done.

J. A. Evans, J. Reimer (2009). Open Access and Global Participation in Science Science, 323 (5917), 1025-1025 DOI: 10.1126/science.1154562

Wednesday, February 18, 2009

Functional traits and trade-offs explain phytoplankton community structure


After attending the presentation by Elena Litchman at the ASLO Aquatic Science Meeting in Nice three weeks ago I came across this paper. Although it was published already two years ago, this works need to be highlighted! Marine phytoplankton is important. It contributes approximately 50% to world primary productivity. Among other factors phytoplankton communities are structured by competition for limiting nutrients (mainly for nitrate and ammonia) in the ocean. Litchman et al. base their paper on the presumption that phytoplankton organisms can achieve higher competitive ability (Tilman’s R*) by different strategies. That is, the organisms can either increase their maximum nutrient uptake and/or growth rate or they decrease the minimum cell quota, the half saturation constant for nutrient uptake and/or their mortality. Litchman et al. tested if they can find constraints and trade-offs on the evolution of better competitive abilities (lower R*) in major phytoplankton groups. Specifically they asked if there is a positive relationship between maximum growth rate and R* which would show a gleaner-opportunist trade-off.
The authors show positive relationships between measurements for growth and nitrate uptake which can constrain the evolution on competitive ability. Indeed major groups of phytoplankton group along these trade-off curves. Whereas coccolithophores e.g. show low nitrate uptake rates and low half-saturation constants, diatoms and dinoflagelates show the opposite nitrate uptake strategy with high uptake rates and high half-saturation constants. A gleaner-opportunist trade-off, i.e. a positive correlation between maximum growth rates and R*which would result in a super species, could not be found across major groups but within the diatoms. The paper gives more results about trait differences among taxonomic groups and allometric scaling relationships. Trade-offs and different strategies in nutrient uptake are discussed in a very concise way either from a mechanistic physiological view as well as from the evolutionary history perspective.


Elena Litchman, Christopher A. Klausmeier, Oscar M. Schofield and Paul G. Falkowski (2009) The role of functional traits and trade-offs in structuring phytoplankton communities: scaling from cellular to ecosystem level. Ecology Letters. DOI: 10.1111/j.1461-0248.2007.01117.x

Sunday, February 15, 2009

The next generation of invasive plants

The study of plant invasions has usually focused on plants adapted to colonizing areas immediately after a disturbance (early successional species or “weeds” by some definitions). An example of this, is the seminal work of Herbert Baker, with his list of traits to explain why some species become weeds and others do not (e.g. fast growth, production of lots of seeds, good dispersal.). So why do late-successional species (for example, forest understory species) not invade? The answer is: we did not look close enough - they indeed invade!! This is the point of a paper by Patrick Martin and collaborators, who study forests – which are usually known as “invasion resistant systems” – and the colonization of exotic shade-tolerant species in them. Their central point is that there have been larger numbers of introductions of early successional species compared to late successional species (or shade tolerant), and that forest dynamics are much slower than other systems (e.g. a forest gap must be created for a species that need disturbances to colonize). And it is for these reasons that we associate invasive plants solely with early successional species, and we see forests as invasion-resistant systems. We are now observing many highly invasive plants that are not disturbance dependent. These may be a lot harder to control and could have important detrimental effects on native communities.

Patrick H. Martin, Charles D. Canham, Peter L. Marks (2009). Why forests appear resistant to exotic plant invasions: intentional introductions, stand dynamics, and the role of shade tolerance Frontiers in Ecology and the Environment DOI: 10.1890/070096

Friday, February 13, 2009

40% believe in evolution, but only 25% do not!

Gallup released a poll, that coincides with Darwin's birthday, which examines American's belief in biological evolution. It is a great poll, breaking down belief patterns across education attainment, age , religiousness, etc.

However, several reports and blogs about this poll disparage Americans for their lack of scientific sophistication, but I think that the results are far more positive then I would have guessed. Only 25% outright deny evolution! I would have thought a clear majority would take this stance as was shown in 2005. A further 36% do not have an opinion, and as scientists and educators, these folks are the reason why we educate and hold events like Darwin Day. Thank you to all those who work so tirelessly promoting science education and literacy, like those at NCSE.

Wednesday, February 11, 2009

Charles Darwin, founder of evolution AND ecology

Perhaps a good alternative title should be: “Why we need a second modern synthesis”

Darwin is rightfully seen (or vilified in some quarters) as the founder of modern evolutionary biology. He gave the naturalists of that era an observable and testable mechanism explaining species change and for understanding the similarities and differences among species. As we celebrate Darwin’s 200th birthday and the 150th anniversary of the publication of the Origin of the Species, it seemed right to think about Darwin’s contributions beyond just evolutionary change, namely ecological patterns and processes.

I’ve read Origin probably half a dozen times now and as an ecologist, I am always amazed by the depth and breadth of Darwin’s insights. Every time I read it, there are passages that directly relate to what I happen to be thinking about or working on at the time, which leads me to the conclusion that he thought a lot about what scientists would come to call ecology. Though the word “ecology” wouldn’t be invented for another seven years (by Ernst Haeckel in 1866) and the first ecology text book didn’t appear until 1895 (by Eugenius Warming, and which includes interesting Lamarckian invocations in the last chapter), Darwin thought and wrote about ecology extensively.

In the Origin (1st edition), Darwin makes predictions about ecological patterns. On page 109, he states, “a … larger number of the very common and much diffused or dominant species will be found on the side of larger genera”. That is community dominance likely relays on inherited traits linked to species success. This certainly sounds like the result of some recent, interesting papers (e.g., Strauss et al.*).

Almost the whole discussion in the Struggle for Existence chapter is about ecological interactions and the severity of negative interactions, which stems from the fact that populations, if unchecked, will increase exponentially (i.e., page 116). We all know from work by ecologists such as Connell and Huston that those negative, deterministic interactions can be overridden by non-equilibrium processes, especially disturbances. Here again Darwin’s observations lead him to this conclusion; “If turf which has long been mown …be let to grow, the more vigorous plants gradually kill the less vigorous” and he observes that diversity in a plot goes from 20 species to 11 when the disturbance is removed.

Further, we often think of Darwin’s view of the environment as a selective pressure (e.g., fur thickness), but he also saw the environment as a determinant of species interactions. Lush places support a lot of species and the control of populations is due to competitive interactions, whereas in harsh places, populations are controlled by “injurious action” of the environment (e.g., page 121). Thus there is a shift from biotic to abiotic controls on ecological processes.

I think that we have collectively forgotten that evolution directly informs our expectations and predictions of ecological patterns and processes. While ecological geneticists drove much of the modern synthesis in the mid 1900’s by incorporating ecology (namely selection) into evolutionary processes, the reverse, bringing evolution into ecology is only now really starting to happen. Lets hope this second modern synthesis completes Darwin’s vision.