Friday, March 27, 2009

The evolutionary meaning of autumn colors

ResearchBlogging.orgAs a kid growing up in Ontario, Canada, I have vivid memories of vast expanses of forests set ablaze by their autumn colors. Whole landscapes look like the canvas of a painter whose love of red, orange, gold and yellow are readily apparent. But, like most biologists, I had been taught that these colors are simply the by-product of leaf senescence, nothing more than a biochemical accident. I was amazed to read Marco Archetti's recent work showing that there may actually be adaptive benefits to changing leaf color in autumn and for particular colors. Generally the adaptive benefits involve either protection against abiotic factors or as a response to plant-animal interactions. One of his interesting results is that autumn coloration has evolved repeatedly and cannot be explained by being related to an ancestor who changed colors, rather that there must be some other evolutionary or adaptive explanation. While he suggests a large number of candidate hypotheses, some more plausible than others, I'll list five for example:

1) Sunscreen: Pigments provide photoprotection against photooxidation during the recovery of nutrients.

2) Leaf warming: Colors absorb light and warm the leaves during cooling temperatures.

3) Coevolution: Tells overwintering insects that the tree is not suitable (poisonous or low nutrition) for hibernation.

4) Camouflage: Many insects lack red photoreceptor, making leaves difficult to see -thus protecting trees from overwintering pests.

5) Unpalatability: Pigments (e.g., red -anthocyanins) are unpalatable.

So, we may quibble about particular hypotheses, but the point for me is that there may be deeper explanations as to why certain species produce the vivid colors they do. At a minimum, Archetti provides ammunition to experimental botanists and evolutionary biologists for testing new hypotheses. I'll never look at an autumn forest the same again.

Archetti, M. (2009). Classification of hypotheses on the evolution of autumn colours Oikos, 118 (3), 328-333 DOI: 10.1111/j.1600-0706.2008.17164.x

Archetti, M. (2008). Phylogenetic analysis reveals a scattered distribution of autumn colours Annals of Botany, 103 (5), 703-713 DOI: 10.1093/aob/mcn259

Archetti, M., Döring, T., Hagen, S., Hughes, N., Leather, S., Lee, D., Lev-Yadun, S., Manetas, Y., Ougham, H., & Schaberg, P. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach Trends in Ecology & Evolution, 24 (3), 166-173 DOI: 10.1016/j.tree.2008.10.006

Monday, March 23, 2009

Conserve now or wait for the data?

ResearchBlogging.orgE. O. Wilson, referring to the ethical imperative we should apply to the conservation of life, said “The ethical imperative should be, first of all, prudence. We should judge every scrap of biodiversity as priceless while we learn to use it and to come to understand what it means to humanity” (pg. 351, The Diversity of Life). Although, I would argue we should aim to learn biodiversity’s value, both intrinsic and extrinsic, as opposed to what it solely means to humanity, his point is protect now, study later. The reason being that there is still so much to learn in order to adequately assess the Earth’s biological riches, by the time we inventory and map a fraction of biodiversity, we would have lost numerous unique regions and species. Of course the opposing point of view is that we need detailed information in order to best use limited resources to best protect biodiversity. This is a major philosophical divide. In a recent, important paper by Hedley Grantham and colleagues published in Ecology Letters, the question of how long should we wait to take conservation actions was empirically tested.

The authors used simulations based on 20 years of habitat loss data from the biologically-rich Fynbos region of South Africa and knowledge about spatial distribution of Protea diversity. Protea surveys (The Protea Atlas) have been carried out over 20 years, inventorying 40,000 plots and recording 381 species within the Proteaceae. They began their simulations with no information about Protea diversity patterns and included annually increasing knowledge, set against annual habitat destruction. They showed that waiting to make conservation decisions after only 2 years resulted in species loss, because habitat loss far outweighed any advantage to gaining more information. Further, more detailed information did not appear to increase the effectiveness of conservation decisions over cruder habitat-level maps.

The philosophical divide between protect now-learn later versus the need for detailed information to maximize resources appears bridgeable. It seems that by just accumulating some rough data may go a long way towards making those important conservation decisions. Of course, the irony is that this study needed 20 years of data to adequately assess this.

Grantham, H., Wilson, K., Moilanen, A., Rebelo, T., & Possingham, H. (2009). Delaying conservation actions for improved knowledge: how long should we wait? Ecology Letters, 12 (4), 293-301 DOI: 10.1111/j.1461-0248.2009.01287.x

Tuesday, March 17, 2009

Being a clover isn’t always so lucky

ResearchBlogging.orgHappy St. Patrick’s Day! I thought that covering an article about Trifolium (clover) seemed very appropriate. In a recent paper, Matthias Schleuning and colleagues examine the population dynamics of Trifolium montanum, a species in decline in central Germany. They examined the relative threats of habitat fragmentation and degradation on T. montanum’s population dynamics. They found that both degradation and fragmentation were having serious negative impacts. Degraded habitats in this system mean the shift away from nutrient-poor conditions and include the invasion of taller species that are better light competitors. T. montanum is a poor light competitor and maintains larger populations in mown or grazed habitats that keep taller invaders out. This species also faces the double whammy of fragmented habitats resulting in isolated populations. These isolates have lower reproductive output likely due to greater inbreeding and less genetic transfer, via pollinators, among different populations.

I always think of Trifolium species as being particularly common and widely distributed, but there are some that are threatened and potentially tell us about the threats faced by imperiled plant populations. In fact, while a number of North American Trifolium species have successfully invaded North America, but T. montanum is not, according to the USDA Plants Database. These results reveal that these negative effects affect plants at different stages of their life cycle (growth to maturity vs. recuitment) and that log-term persistence of these populations requires management activities that ameliorate both of these effects.

SCHLEUNING, M., NIGGEMANN, M., BECKER, U., & MATTHIES, D. (2009). Negative effects of habitat degradation and fragmentation on the declining grassland plant Trifolium montanum Basic and Applied Ecology, 10 (1), 61-69 DOI: 10.1016/j.baae.2007.12.002

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