Double or nothing

As I finished my undergrad career and started thinking about graduate school, I was totally infatuated with the chromosomal speciation of treefrogs in the genus Hyla. Hyla versicolor and H. chrysoscelis, the 'gray treefrogs', have similar geographic distributions and look almost identical - except that one is a tetraploid version of the other. The increase in genome size is associated with a slight increase in cell size, which has trickle-down effects into physiology, the sound of their call, and other ecological factors, and of course they are reproductively isolated. As it turns out, Margaret Ptacek and colleagues were unraveling this mystery at the genetic level just as I was learning of it, and while I was disappointed not to be able to explore this for my graduate work, Margaret made up for it by paying for all the drinks when I visited Clemson a few years back.

So it was with considerable interest that I stumbled across one of the first tables of contents of the new year, in BMC Evolutionary Biology. Two co-occurring populations of the diatom Ditylum brightwellii, it turns out, differ in genome size. In this case, the belief is that there is a single taxonomic species harboring a very recent genome duplication polymorphism (which are likely cryptic species). Of course, a species by any other name... well, that's the problem isn't it? In the world of diatoms, according to Koester and colleagues, the 'barcode' standard is to use the 18S rDNA gene sequences and silica cell wall morphology in diagnosing species. However, already armed with evidence that two substantially distinct populations could be identified with the more rapidly-evolving ITS gene region, these researchers explored how differences in reproductive rates and size distributions might be associated with genome size.

See, diatoms are the petri dishes of the natural world. In order to reproduce, each side of the interlocking silica case separates and generates a new nested case. One of the offspring of this fission will be the same size as the parental individual, the other will be slightly smaller - the smaller of the two original cell walls, with an even smaller one nested within. At least that is how I understand it. Over time, these clonal lineages reduce substantially in size, and cell size is eventually limited by genome size; sexual reproduction then allows them to regain a larger cell size and the process repeats. So, the life history of this species requires an interesting interaction among the genome (which places a lower bound on cell size, and a lower bound on reproductive rate) and the population.

In Ditylum, Koester et al. were able to show that there are not only two very distinct genetic lineages, but that the one that is regionally localized to Puget Sound appears to have been generated through genome duplication. That is, there is a cosmopolitan species, and an offshoot lineage that was formed through some form of genome duplication, with concomitant changes in cell size, rates of population growth, and reproductive isolation. Koester et al. conclude that these lineages are cryptic species, and that this form of isolation may be common in marine diatoms.

More generally, this shows another way in which our understanding of biodiversity is changing rapidly thanks to molecular diversity analysis. The latest term to be coined by John Avise, biodiversity genetics, reflects the fact that we must now consider all of the new ways in which this technology can accelerate the rate of discovery in our natural world. Taxonomists trained in the morphology and phenotypic diversity of life are few; certainly too few to keep up with growing scientific collections, and the bottleneck in describing species can be a difficult one for management and conservation. The '18S or bust' approach in diatoms may be one standard that will change as more studies like this one, out of Armbrust's lab at Washington, illuminate how dynamic biological diversity can be.

Predicting invader success requires integrating ecological and land use patterns.

Disclaimer, this was modified from an editorial I wrote for the Journal of Applied Ecology.

In the quest to understand species invasions, we often try to link the abundance and distribution of invaders to underlying ecological processes. For example, oft-studied are the links between exotic diversity and native richness or environmental heterogeneity. Seemingly independently, research into how specific land use or management activities affect invasion dynamics is also fairly common. While both research strategies are of fundamental importance, not often recognized, or at least explicitly studied, is that both ecological patterns and management activities simultaneously affect invasion success. Thus a truly integrative approach to understanding invader success must take into account variation in ecological communities and abiotic resource avalibility as well as land use patterns at multiple spatial scales. Such an approach is necessary if ecologists wish to predict potential invader abundance, spread and impact.

Diez et al. Examine how environmental and management heterogeneity interact to influence patterns of Hieracium pilosella (Asteraceae) inasions in the South Island of New Zealand. The spread of H. Pilosella in New Zealand is threatening native habitats (tussock fields) and the livestock grazing industry. Diez et al. Asked how environmental and management regimes affect H. Pilosella abundance and distribution across six large farms on the South Island. This is an interesting and important question, not just because they are examining how human-caused and ecological variation interact to affect H. Pilosella dynamics, but also because these sources are heterogeneity are realized at different spatial scales.

Diez et al. show that the abundance and distribution of H. Pilosella was significantly affected by the interaction of habitat type (i.e., short vs. tall tussocks) and farm management strategies (i.e., fertilization and grazing rates). At larger scales, H. Pilosella was more abundant in tall tussock habitats and was unaffected by fertilization, while in short tussocks, it was less abundant in fertilized patches. At small scales, H. Pilosella was less likely to be found in short tussocks with high exotic grass cover and high productivity (measured as site soil moisture and solar radiation). Conversely, in tall tussocks, H. Pilosella was more likely to be found on sites with high natural productivity. Diez et al. were able to tease these complex causal mechanism apart by using Bayesian multilevel linear models, for which they included example R code in an online appendix.

While it is a truism in ecology to say that heterogeneity affects ecological patterns, this paper deserves mention because they convincingly show that the spread of noxious exotic plants in a complex landscape, can potentially predicted by understanding the invader success in different habitat types and land management strategies. In their case they show how human activities, which were not designed to affect H. Pilosella, can strongly affect abundance in different habitat types. This type of approach to understanding invader dynamics can potentially arm managers with the ability to use existing land use strategies to predict how and where further invader targeting would be most useful.


Diez, J., Buckley, H., Case, B., Harsch, M., Sciligo, A., Wangen, S., & Duncan, R. (2009). Interacting effects of management and environmental variability at multiple scales on invasive species distributions Journal of Applied Ecology DOI: 10.1111/j.1365-2664.2009.01725.x

1st anniversary!

The Eeb and Flow is now a year old. Thanks to all our contributors and we hope readers will continue to find our posts interesting, informative and timely. Hopefully, with sustained effort on our part, we can surpass the 20,000 reads from this year and make 2010 a great year for readers and contributors.

Thanks!

Parasite competition enhances host survival

Contracting a parasite is bad. But is getting colonized by multiple parasitic species worse? This is an interesting and important question. The host is a resource, which can support a limited number of parasitic individuals, and so how does competition affect parasitic species and host mortality?
This was the premise of a recent paper by Oliver Balmer and colleagues, studying trypanosome infection of mice hosts. They engineered two transgeneic strains of the protozoan parasite, Trypanosoma brucei (African sleeping sickness), to fluoresce different colors in order to assess infections. They infected mice with each strain separately and together and measured host survival and parasite density.

They found that when both strains were present, they competitively suppressed each other and that the level of suppression depended on the initial density of each strain. One of the strains was more virulent than the other, and infection by both strains reduced mortality by 15% compared to infection by the virulent strain only. This is due to the suppression of the virulent strain by the low virulent strain.

The authors argue that strain source and intraspecific genetic diversity can have an important effect on host mortality. I would also argue that understanding interspecific interactions and within-host niche differences, would also be critical.

What a cool use of molecular technology to test basic hypotheses about disease ecology.

Balmer, O., Stearns, S., Schötzau, A., & Brun, R. (2009). Intraspecific competition between co-infecting parasite strains enhances host survival in African trypanosomes Ecology, 90 (12), 3367-3378 DOI: 10.1890/08-2291.1

Understanding wildlife-friendly ecolabels

These days, it seems like nearly everything in the supermarket is good for the environment in one way or another. Over the past decade, more and more companies have started using ecolabels to collect a premium on products that claim to contribute to environmental protection.

But not all ecolabels are created equal. The credibility of their claims varies widely, ranging from environmentally meaningful to downright exploitative.

A recent study by Adrian Treves and Stephanie Jones provides a model for policy-makers and consumers to discriminate between claims.

“In a nutshell, [we] were looking for a way to analyze this cloud of ecolabels out there, all of them claiming to be the best thing for a given species or the best thing for a given ecosystem,” said Treves in an interview.

In the early stages of their research, Treves and Jones realized that wildlife friendly ecolabels can be split along the same lines that have divided debating groups of conservationists. They drew upon these divergent perspectives to partition wildlife friendly ecolabels into three categories.

“Supportive” ecolabels such as Endangered Species Chocolate donate some percentage of revenues to conservation organizations. Verifying the claims for this category is compromised by the transfer of funds to a third-party recipient who is usually not accountable to consumers.

“Persuasive” ecolabels claim to improve production methods in a way that eliminates threats to wildlife, but do not assess actual conservation of wildlife. Although the persuasive category is more transparent and environmentally effective than the supportive one, this type of ecolabel bases its certification requirements on assumptions about threats to wildlife without testing how reduction of perceived threats impacts wildlife. Tuna labeled as Dolphin Safe is an example of a persuasive ecolabel.

“Protective” ecolabels certify wildlife conservation by assessing whether reduction of threats enhances wildlife populations. The Marine Stewardship Council certifies fisheries under a protective ecolabel. This category is the most meaningful to wildlife because it matches the recommendations of the latest conservation science. By following the scientific method, protective ecolabels can verify that they actually help humans and wildlife coexist.

Just as conservation is often pitted against economic interests like agriculture or development, ecolabels must balance a trade-off between consumer confidence and producer incentive.

Protective ecolabels gain the most consumer credibility but also require the greatest verification effort. Proving that producers conserved wildlife is costly, time-consuming, and logistically challenging. Wild animals habitually ignore property boundaries and can die or disperse for reasons unrelated to producer activities. Often, the costs associated with these challenges outweigh the economic incentive of being labeled as “green.”

Treves, A. and S. M. Jones. 2009. Strategic trade-offs for wildlife-friendly eco-labels. Frontiers in Ecology and the Environment. DOI:10.1890/080173

(Image courtesy of kateboydell at flickr under a Creative Commons license)

Taking below-ground processes seriously: plant coexistence and soil depth

Some of the earliest ecologists, like Eugen Warming and Christen Raunkiaer, were enthralled with the minutia of the differences in plant life forms and how these differences determined where plants lived. They realized that differences in plant growth forms corresponded to how different plants made their way in the world. Since this early era, understanding the mechanisms of plant competition is one of the most widely-studied aspects of ecology. This is such an important aspect of ecology because understanding plant coexistence allows us to understand what controls productivity in the basal trophic level for most terrestrial food webs. There are a plethora of plausible mechanisms for how plants are able to coexist, and most involve above-ground partitioning strategies (such as different leaf shapes) or phenological differences (such as germination or bolting timing). Yet, below-ground interactions among plants as a way to understand competition and coexistence have been making a strong resurgence in the literature lately. This resurgence has been driven by new hypotheses and technologies.In what is probably the best hypothesis test of the role for below-ground niche partitioning, Mathew Dornbush and Brian Wilsey reveal how soil depth can affect coexistence. They seeded 36 tallgrass prairie species into plot that were either shallow, medium or deep soiled, and asked if species richness and diversity were affected after 3 years. They found that species richness significantly increased with increased soil depth, revealing that deeper soils likely had greater niche opportunities for species. Not only did deeper soils harbor greater richness, but compositions were non-random subsets. The species inhabiting shallow soils were a subset of medium soils, and medium a subset of deep. This means that increasing depth opened new niche opportunities, unique from the ones for shallow soils.

This study is the first field-based experiment of soil depth and coexistence, that I know of and the results are compelling. Plant species are segregating below-ground niches, and perhaps we look for other partitioning strategies for species that inhabit the same soil depth.

Dornbush, M., & Wilsey, B. (2009). Experimental manipulation of soil depth alters species richness and co-occurrence in restored tallgrass prairie Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01605.x

Other notable recent papers on below-ground processes:

Bartelheimer, M., Gowing, D., & Silvertown, J. (2009). Explaining hydrological niches: the decisive role of below-ground competition in two closely related species Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01598.x

Cramer, M., van Cauter, A., & Bond, W. (2009). Growth of N-fixing African savanna species is constrained by below-ground competition with grass Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01594.x

Meier, C., Keyserling, K., & Bowman, W. (2009). Fine root inputs to soil reduce growth of a neighbouring plant via distinct mechanisms dependent on root carbon chemistry Journal of Ecology, 97 (5), 941-949 DOI: 10.1111/j.1365-2745.2009.01537.x