How periodic cicadas evolved their timing

Every few years, the periodic cicadas come into the news, when they simultaneously complete their life cycles and emerge from the ground as winged adults. They swarm over large parts of the eastern United States and attract both curiosity and alarm from residents and the media. After a few weeks, their mating and egg-laying over, they disappear just as quickly as they appeared. Of course, they are not completely gone: their juveniles live underground, having perhaps the longest maturation of any insect, to emerge as adults after a period of 13 or 17 years.

13-year cicada from Brood XIX. Via Wikimedia Commons.

Why 13 or 17 years? These prime-numbered periods have puzzled more mathematically-minded biologists for ages, with one suggestion being that a prime-numbered life cycle would minimize the number of predator life cycles that could synchronize with it (because a prime number has no factors but itself). But how did this situation evolve?

A new paper published in PNAS (open access) from a group of Japanese scientists looks at the phylogeny and population genetics of the known species of periodic cicadas. The periodic cicadas fall within the genus Magicicada, within which are species partly defined by the length of their period. M. tredecim for example is a 13-year species, while M. septendecim is a 17-year species. Within each species there are also multiple "broods", representing different cohorts have the same emergence and mating cycles. One brood may encompass multiple species. Siva blogged here about one such brood in 2004, the ominously-named Brood X, which had an unusually large emergence (the "X" is actually just a roman numeral). The different species fall within three species groups, each with both 13- and 17-year species.

Contrary to expectations, the old species, defined by morphology and period, do not correspond to the evolutionary history as uncovered by molecular phylogeny and haplotyping. The three big species groups are still supported, representing two evolutionary splits at about 3.9 and 2.5 million years ago (Mya). The splits within the species groups, however, are relatively recent, mostly less than 0.5 Mya. Furthermore, the splits correspond more to geographical regions than to life cycle period. The split between 13- and 17-year periods have also evolved multiple times. To quote from the paper:

Our results are broadly consistent with the previous idea that an ancestor of all Magicicada diverged into three species allopatrically, and later, the three became sympatric and each species independently diverged into 13- and 17-y cicadas. Surprisingly, however, the divergence of 13- and 17-y cicadas was asynchronous among the species groups and occurred repeatedly even within a species group. This finding is all of the more interesting given that each species group shows similar eastern, middle, and western phylogeographic divisions similar to post-Pleistocene patterns observed in other North American taxa, suggesting that the three Magicicacda groups shared multiple refugia during the last glacial maximum.

This is a nice surprise, and as the authors point out, the repeated switching between 13- and 17-year forms suggests that there is a single genetic "switch" involved, because it is unlikely that a complex mechanism could be repeatedly gained and lost in such a manner.

Keystone species, Keystone teacher?

Keystone species are members of ecological communities that have a disproportionate effect on the community if they are removed. They need not be the most abundant element, but may play an important role, e.g. in keeping down the population of some other species that would otherwise dominate.

The classic experiment was performed in the 1960s by ecologist Bob Paine, who removed a species of sea star Pisaster ochraeus from a rocky shore intertidal environment. In months, the community became dominated by mussels and decreased in diversity. Because of this, he coined the term "keystone species" for the role that this sea star was playing in this community.

Pisaster ochraeus (via Wikimedia)

Paine himself has had a disproportionate influence on the academic community in the field of ecology. This profile article describes his career and students, and illustrates well how a single teacher can not only be a good mentor to his or her students, but also go beyond that to build a scientific family.

The agave's big bang

One of the condominiums near my home has a bit of fancy horticulture at the entrance. Lined up along the road is a row of big agave plants, and lately four of them have been at various stages of flowering. If you haven't seen an agave plant (also called "century-plants") flowering, it's a sight to behold: the succulent leaves are thick and waxy, forming a rosette close to the ground, and from the middle emerges a spike-shaped stalked reaching taller than a person, from which the flowers will eventually unfurl.

Agave plants flowering in Portugal (Wikimedia)

The agave plants (see the Agavaceae webpage for more photographs) are also known as century plants because of a misconception that they only flower once a century. It is true however that it can take up to several years or even decades before an individual plant flowers. Once it does so, it apparently withers away and dies. For this reason, probably, in some of the plants that I saw, the flower stalks were cut away soon afterwards.

Why would any organism only flower once in its lifetime? Doesn't natural selection reward those who have more offspring? Isn't having only one crop of offspring akin to putting all your eggs in one basket?

Immigration and invasion - biological xenophobia?

One of the hottest political topics in Singapore today is the issue of immigration: your 'foreign talent' may be my 'job-stealer', as the debate continues on just how much immigration is desirable for a small country like ours.

In the field of biological conservation, a similar conflict has popped up, as reported recently in the Boston Globe. Conservationists have traditionally spoken of 'introduced' vs. 'native' species, with the least desirable introduced species being termed 'invasive'. But a number of vocal critics have spoken up to say that this is simply nativism - an unthinking bias in favour of the supposedly native. They contend that the division between native and non-native is arbitrary and unscientific, in part because many familiar species were originally imports from elsewhere, and also because the environment is constantly changing, and species are continually migrating, and to try to freeze a community in time is akin to gardening nature. The author Michael Pollan went as far as to say that species-nativism was "xenophobic".

Classical examples of species invasions include the rabbit and cane toad in Australia, and the European starling in the USA (for which Shakespeare, bizarrely enough, is to blame). In Singapore, Clidemia hirta or Koster's Curse (a native of the Americas) has a tenacious foothold on forest fringes and clearings, preventing the re-establishment of native vegetation.

However, much of our modern landscape is precisely the product of introduced species. The grand arching wayside tree canopies that provide shade and shelter are often rain trees, Samanea saman, which are native to Brazil. These are perhaps the 'foreign talent' side of the argument.

We justifiably take pride in our native species, and among the popular Science Centre guidebook series is a volume on cultivating native plant species. But for an island as small as Singapore, does it make sense to speak of 'native' species? What hope is there of eradicating non-native species, anyway?

Ultimately, it may simply be a matter of rhetoric. The disagreement is not over the substance of the matter but the language that we use to talk about it. As the Globe article says:

Just as most ecologists accept that only a fraction of non-native species are harmful, the anti-nativists, when pressed, will admit that unequivocally destructive species like the Asian longhorned beetle should be reined in. Their disagreement lies more in how we should talk about the issue, how we justify our interventions and how we label the species we want to eradicate.

That is to say: when we call a species an 'invasive species', it's not enough simply to say that it's non-native and hence must be eradicated for that reason alone. We must be clearer about what 'invasiveness' means, and be able to justify our assertions about their harmfulness and undesirability.

Work on Bukit Timah Eco-Link begins

Two years ago, Cheng Puay blogged about the announcement of an ecological 'bridge' to be built across the Bukit Timah Expressway (BKE) to connect Bukit Timah Nature Reserve and the Central Catchment area. Construction of the BKE 25 years ago had caused the two forested areas to be separated from each other, preventing animals from moving freely and making them run the risk of becoming roadkill.

Today the Straits Times reported that work on the Eco-Link@BKE has finally begun, at a projected cost of $17 million, to be completed around December 2013. It will certainly be an exciting experiment to see whether man-made forest corridors of this sort can be effective at allowing migration and genetic exchange between two forest fragments.

Incidentally, the previous blog post is the most often-visited post on this blog. Those who have read that article may hence be glad to have this update.

In other news, the ST today also reported on a growing poaching problem in forested areas around Singapore, including wild boar traps which have inadvertently maimed stray dogs. Nature aficionados who habitually walk through the undergrowth should watch out!

Snails surviving being eaten by birds

Japanese scientists have found that small land snails of the species Tomatellides boeningi can survive being eaten by birds. They fed live snails to Japanese white-eyes and a brown-eared bulbul, and found that about 15% of the snails remained alive after passing through the birds' guts.

Just as birds are important dispersers of plant seeds, it appears that they can contribute to the dispersal of small animals too.

Wild populations of T. boeningi on Hahajima Island in the W Pacific show genetic heterogeneity within populations and no evidence of isolation, and there is a statistically-significant positive correlation between genetic variation and the density of Japanese white-eyes, lending support to the idea of bird-borne dispersal.

Sources:

• S Wada, K Kawakami, S Chiba. 2011. Snails can survive passage through a bird's digestive system. Journal of Biogeography published online 21 June 2011, doi: 10.1111/j.1365-2699.2011.02559.x
• BBC News article

History of Amazon Basin Shaped by Andes

Here's something that has relevance to the name of this blog. The traditional model for the history of Amazonian biodiversity held that much of it weathered the Pleistocene period in forest fragments, or refugia, and from there spread out again to repopulate the basin when the climate again became amenable. These so-called diversity centers, however, have been shown to be artifacts of sampling, rather than a real biotic phenomenon, pushing back the origins of diversity in the Amazon to earlier periods of geological history.

Amazonia today. From Hoorn et al. 2010 (Link)

A recent review in Science magazine synthesizes what is known about the geological and biological past of the Amazon, to reconstruct a scenario where the uplift of the Andes mountains, which are relatively young, is responsible in part for the present-day patterns of diversity in the basin. Prior to the Andes uplift (which began in the Paleogene after the end-Cretaceous 65 Mya, and continues to the present day), the geology of Amazonia was 'cratonic', i.e. it was dominated by the rocks of the continental center. The rising mountain range changed both the climate, e.g. by intensifying rainfall to the East of the range, and also the landscape, as sediment eroded off the mountain faces and came to be deposited in the basin below. Extensive, inland wetlands (the Pebas system) first appeared in correlation with intensified uplift and then disappeared, replaced by a riverine, fluvial landscape.

The present-day biota of the Amazon is very much terrestrial. Therefore, the disappearance of the extensive wetlands, which formerly fragmented the terrestrial forests, was a prerequisite for the appearance of modern Amazonian fauna and flora. Furthermore, diversity as measured today for both mammals and tree species is highest where the soils are of Andean origin (swept down by erosion and deposited by the rivers), as opposed to where the original cratonic soils remain at the surface. Perhaps the nutrient levels are linked to both forest productivity and resultant diversity, but the climate engendered by the topographic relief may also have a role in patterns of diversity.

This is very much a 'big picture' review, and is worth reading to think about how the literal shape of the Earth has a role in defining the life that dwells as a thin film on its surface.

Ammonite Jaws Reconstructed in 3D with X-rays

Ammonites are common fossils with distinctive chambered spiral shells – so common that they're probably some of the first fossils most of us encounter, as decorative objects sitting prettily in someone's home or workplace. They're cephalopods, extinct relatives of the octopus, squid, and cuttlefish, but unlike these, have an external shell. The still-extant Nautilus is a cephalopod with an external shell, and is often confused with the ammonites, but these two actually represent two different lineages within the cephalopods.

Like most molluscs, ammonites had a (presumably) chitinous feeding apparatus comprising jaws and a radula–a rasping row of teeth-like structures unique to molluscs. These have been recovered occasionally in the past, but using X-rays, these have been studied in-situ in the ammonite Baculites from the Upper Cretaceous of South Dakota, allowing them to be reconstructed in breathtaking, 3-dimensional detail (paper in Science).

What's more, the food of these creatures can also be detected and identified. Within the buccal mass of these ammonites were fragments of small isopods, also called pill-bugs, which are crustaceans that, as their name suggests, look like rounded pills.

Unlike Nautilus and other nautiloids, the jaw structure and the kind of food they ate suggest that Baculites fed on small prey in the water column (i.e. zooplankton), instead of larger prey on the sea floor (benthic). They hypothesize that this preference for planktonic food may be responsible for the extinction of ammonites at the end of the Cretaceous, when the composition of planktonic communities changed considerably, but this theory will certainly have to be tested with more fossil data, as the commentary article points out.

NY Times New Field Scientist Blog

Science! As it happens! The New York Times website has a new blog, called Scientist at Work, the aim of which is, as they put it, "to give scientists in the field a chance to describe what they do as they are doing it."

Of course most day-to-day science is abysmally boring. It's interesting to think about science and talk about it, but to actually watch someone carry out a hundred PCR reactions or measure the second tarsi of countless pinned beetles for an afternoon is... not very entertaining. So trust the field biologists to jump at the chance to portray themselves as modern-day explorers, as the ones who truly get to see the action. Lots of breathtaking photographs too, such as this latest entry on Philippine nudibranchs written by Terry Gosliner of the California Academy of Sciences.

Don't expect to see too much ink spilled about the months of permit-haggling, visa-stamping, and grant-writing that bracket any field expedition, though. And why should there be, anyway? It doesn't interest the public, and those who would know about the boring bits don't need to be reminded. Even seasoned scientists and naturalists need to take a step back from the minutiae, every now and then, and reaffirm that what they study is truly exciting and awesome.

Urban ecology: Revisiting weeds?

Urban landscapes seem very different from natural, 'pristine' landscapes, but many similar principles apply to understanding both of them. Those of us who are city-dwellers from birth, like most Singaporeans, have a very skewed vision of what urban vegetation 'should' be like, conditioned by heavily manicured and landscapes parks and plantings. We tend to look down on weeds and wildflowers as plant pests and ugly eyesores to be managed rather than celebrated. But weeds, or to use a more neutral term, 'spontaneous vegetation', have an important function in the urban environment, as Peter del Tredici argues.

Del Tredici is a scientist at the Arnold Arboretum and lecturer at the Harvard Graduate School of Design, and has just written a book titled Wild Urban Plants of the Northeast: A Field Guide, where he explains his philosophy of urban ecology in detail, along with descriptions of common species in the American Northeast. I attended a talk he gave in conjunction with a booksigning event, and was struck by how many of the things he observed about urban vegetation also applied to what we find in Singapore:

• Plants that thrive in urban open lots tend to be native to harsh environments. In the US, the 'Tree of Heaven' (Ailanthus altissima) is a cliff-dwelling species from East Asia and common in cities like Boston; in Singapore, Adinandra dumosa, which is common on wasteland, is actually native to limestone hills on the Peninsula.
  
• Bird-dispersed species with small seeds spread quickly and aggressively. Most of the successful species in urban landscapes are early successional species in 'nature'.
  
• The growth of spontaneous vegetation is inversely proportional to how wealthy the neighborhood is. In shrinking cities with declining economies, like Detroit, abandoned lots are being left to be grown over by wild grass and shrubs, and some companies even run pheasant hunts in the city because those birds are starting to colonize these spaces.
  
• Wayside trees planted for ornament and shade can escape and invade native habitats, displacing native species. Norway Maple is a big problem in Massachusetts, where it's illegal to plant this species, just like Albizzia might be in Singapore.
  

Ultimately what he argues for is a change of perspective - we can see these as ugly weeds, or we can incorporate them into our appreciation of the urban landscape, using and manipulating them to suit our needs. Even in their unmanaged form, urban wild plants provide many important services, not least among them shade and temperature regulation, soil consolidation, and even phytoremediation. With the economic downturn, less money is available for urban landscaping and planting, but if we can make use of these erstwhile invasive species, by weeding out the uglier and less cooperative species, making paths and borders, we could produce wild urban woodlands that enhance the urban experience, instead of detracting from it.

Looking at the plant list in his book, it is also striking how many of the species listed are actually common to both places. At the booksigning, he wrote in my copy that I "won't find any of these in Singapore," but at the genus or family level, there are quite a number of familiar names: Composites, Legumes, and Crucifers for sure, but also Euphorbiaceae, Scrophulariaceae, and Vitaceae. Familiar genera abounded especially among the grasses: Eragrostis, Digitaria, Poa. I'm sure someone has had similar thoughts before, but it would be instructive to look at the comparative community phylogenetics of weeds around the world - are certain groups of plants particularly predisposed to be successful in the urban landscape? If so we should start getting familiar with them, because these are the species that will be most familiar to most of the human population in time to come, if not already.

HOME - Yann Arthus-Bertrand

HOME is a beautiful one and a half hour documentary by Yanns Arthus-Bertrand. Narrated by Glen Close, the story of how life began and how humans changed the environment as civilisation progress is beautifully told.

You can also watch it on YouTube.

Ecological Corridor for Bukit Timah and Central Catchment Nature Reserve, Singapore

Finally there are plans for an ecolink connecting Bukit Timah and Central Catchment Nature Reserve.

This conservation corridor will offset the impact of fragmentation caused by the Bukit Timah Expressway which dissected the original continuous forest area into 2. Built over the Bukit Timah Expressway, the Ecolink is hourglass shaped and will be 50m at its narrowest point.

Some readings on conservation corridors

Simberloff, D. & J. Cox, 1987. Consequences and Costs of Conservation Corridors. Conservation Biology, 1(1): 63-71.

Fleury, A. M. & R. D. Brown, 1997. A framework for the design of wildlife conservation corridors with specific application to southwestern Ontario. Landscape and Urban Planning, 37(3-4): 163-186.

Article in The Straits Times 6 Sept 2009 by Grace Chua.

Bridge to link two nature reserves by 2013
By 2013, there will be a new green link between the Bukit Timah Nature Reserve and Central Catchment Area.

Conservationists had long bemoaned the separation of the two reserves by the Bukit Timah Expressway (BKE) since its construction in 1986.

The road prevented plant and animal species from moving between the two forest tracts.

The new eco-passage will arch over the 50m-wide BKE and be sited at a suitable narrow point between the reserves. Its construction was announced yesterday at the opening of the National Parks Board's Dairy Farm Nature Park.

A tender will be called at the end of this year and construction will start next year.

The bridge, 50m wide at its narrowest point and planted with dense trees resembling a forest habitat, could help populations of animals like the critically-endangered banded leaf monkey to recover.

Four to six years ago, there were thought to be fewer than 20 of the small dark-furred monkeys - a number too small to be sustainable - but now there are about 30 living around the Central Catchment Area.

The eco-link could help them migrate to Bukit Timah Nature Reserve to find other food sources.

A hiking trail at one edge will also enable humans to move between the reserves.

National University of Singapore conservation biologist Navjot Sodhi said of the bridge: 'Every plan to connect habitats is a good plan but how it pans out - only time will tell.'

'I hope NParks will do surveys to see how species are moving between the reserves,' he added.

Also launched yesterday was a national document which outlines Singapore's strategy for protecting its plants, animals and ecosystems.

The eco-link and Dairy Farm Nature Park are key components of the National Biodiversity Strategy and Action Plan, which describes five strategies:

- Safeguarding biodiversity.
- Taking biodiversity into account in policy-making.
- Improving knowledge of the natural environment.
- Raising public awareness of biodiversity.
- Strengthening local and international partnerships.

'It's not just scientists and government. Every individual plays a part in making conservation work in Singapore,' said Dr Lena Chan of NParks' conservation division.

Marine biologist Chou Loke Ming of the National University of Singapore said the plan recognises the importance of biodiversity and sets up a framework to protect it.

The plan is available online at www.nparks.gov.sg/nbsap.

Grace Chua

Parasite makes ant mimic fruit

Parasites frequently modify the behavior of their hosts to encourage the infection of new hosts. For example, see this video (taken from the Planet Earth documentary) of the fungus Cordyceps that makes insects climb to the top of grass stems, and then erupts its fruiting body from the host's body, and disperses its spores over more hapless hosts from this elevated position. Yanoviak et al. (Am Nat 2008. Vol. 171, pp. 536–544; DOI: 10.1086/528968) describe a case of parasite-induced mimicry in the ant Cephalotes atratus. A nematode infection causes the gasters (rear portion of the abdomen) to become bright red and swollen, resembling a berry fruit, where normally it is black and inconspicuous. The infected gasters are also full of parasite eggs. Birds that feed on berries would then pop off these packets of parasite propagules, and pass out the eggs in their faeces. Ants congregate around bird faeces, which represent food resources to them, and collect them to feed to their brood, completing the cycle.

Here's the lesson from all this, kids: don't eat dung.

Fungal Endophytes

Is a leaf all that it seems to be -- just a leaf? It turns out that fungal endophytes - fungi that live inside plants without harming them - are common and to be found in almost all leaves of tropical forest plants. It seems that the more we learn about organisms and their interactions, the more the idea of the 'free living organism' turns out to be a big lie! Not only are all organisms dependent upon others in obvious ways such as for food and nutrient cycling, but virtually all macroscopic creatures seem to have microscopic organisms living in them too (and frequently even the microscopic ones are hosts to yet smaller ones.)

We commonly understand fungi to adopt a saprobiontic lifestyle, acting as decomposers in the soil, in wood, in leaf litter, or perhaps as pathogens and parasites such as rusts and smuts. But the diversity of these lifestyles may be matched by the diversity of fungal endophytes, which live in the leaf tissues of plants without showing any symptoms or substantially impairing host productivity. They offer benefits to their hosts' fitness, such as preventing pathogenic infection, and possibly making leaves less palatable to herbivores. At the same time the leaves offer a substrate for the fungi to grow upon and provide food substances for their growth and reproduction. Most of these endophytes are transmitted horizontally rather than vertically, though the most well-studied case, that of grass endophytic fungi, are vertically transmitted.

Check out the webpage of Betsy Arnold from the University of Arizona. Her lab does a lot of work on fungal endophyte diversity and interactions with their host plants. Their diversity is still barely understood, especially in the tropics, and culture methods are relatively simple. This could be a pretty cool project for those of you living in places where the leaves aren't all falling off the trees!

Cure for amphibian chytridiomycosis?

"Frog killer fungus 'breakthrough,'" by Kim Griggs. BBC News, 30 Oct 2007.

New Zealand scientists have claimed that the common antibiotic chloramphenicol can be used to cure amphibians infected with chytridiomycosis, the fungal disease which is causing widespread worldwide decline of amphibian populations and is pushing many endangered frog species to the edge of extinction.

Update: Poster from the team which did the research

Counteracting [phosphorus-induced] phytoplankton blooms with filter-feeding fish

Yang, S.L., 1982. Fish culture and reservoir management in the Republic of Singapore. In: Soewardi, K.Mahar, C. (eds.), Proceedings of the Seminar on Production and Exploitation of Open waters (with special emphasis on reservoirs), Bogor, 15-17 June, 1982. SEAMEO-BIOTROP, Bogor (Indonesia), 1987. Biotrop Special Publication (Indonesia) (ISSN 0125-975X), No. 27. Pp. 89-101.

Abstract - The primary function of impounding reservoirs in the Republic of Singapore is the collection and storage of rain water for the nation's water supply. The quality of reservoir water is a prime concern and the Public Utilities Board of the Republic has a responsibility to ensure that any activity in these reservoirs does not lead to deterioration of water supply. The objective of fish culture and reservoir management, apart from protection, is therefore to improve the quality of the raw water in these reservoirs.

A Biology Unit was established toward the end of 1970 to undertake these biological activities. The project on fish culture in reservoirs was initiated in 1972 at Seletar Reservoir. The feasibility of rearing the bighead carp (Aristichthys nobilis) in floating net-cages without supplemental feed was studied and the growth rates of the carp at different stocking rates were compared.

The stocking rate of 300 carp per net-cage of 6 m (length) x 4 m (width) x 5 m (depth) was found to have an optimum yield of about 680 kilograms at Seletar Reservoir. At this stocking rate, the carp attained an average weight of 2.0-3.0 kilograms during a grouping period of 12 months. A pilot project of 48 net-cages introduced at Seletar Reservoir in 1973 resulted in a harvest of 20.000 kilograms of carp in 1974. The project was then expanded at Seletar Reservoir and extended to other reservoirs. By the end of 1981, a total of 342 net-cages were operated in 4 reservoirs with a total harvest of 88.000 kilograms of carp.

The bighead carp which feeds solely on planktonic organisms in the reservoirs acts as a natural biological filter and thus helps to remove the excessive nutrients in the reservoirs. The carps, after growing big, are sold. Proceeds from the sales help to defray the cost of operating the Biology Unit.

----

I thought this was a pretty neat exercise. We used to gaze out to cages in Kranji Reservoir and hear about it and later tell students, so its nice to see this abstract; the NUS Science Library is digging this up and there might be a couple more papers that I hope to get. Now that I'm using it in class, I'd like to cite a paper, since its possible, rather then my memory!

It was not the only measure exercised to reduce eutrophication in our reservoirs, of course. Besides relocation of farms near rivers, the government even discouraged phosphorus-rich residential soap water runoff into drains as that eventually led to the catchment water. So I remember them removing backyard tap heads in the 1970's! The backyards of residential area were where pre-washing machine clothes were laundered, and the spent soap water was poured into drains. By removing the tap heads in backyards, families redesigned their backyard areas. From then on, waste soap water was directed into the sewage system instead.

Nowadays you can find biodegradable, phosphorus-free soap powder for sale.

Links

• "Historical perspective of the phosphate detergent conflict," by Chris Knud-Hansen.
  Working Paper 94-54, February 1994.
• "Shoreline Stewardship for Landowners." NSLandsCo.com. Has a nice illustration.
• "Province's dish-soap step too timid, critics charge," by Mary Agnes Welch. Winnnipeg Free Press, 05 Sep 2007.
• FAO: Cultured Aquatic Species Information Programme: Aristichthys nobilis

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