Prior to 1967, the environmental extremes of the deep were thought to limit life. The deep sea is dark (can’t-see-your-hand-in-front-of-your-face dark), cold (only-four-degrees-above-freezing cold), and under an extreme amount of pressure (one-elephant-on-each-square-inch-of-your-body pressure).  This suite of factors should make survival challenging, and thus for a century, scientists assumed the deep sea was biologically a desolate wasteland.  Even after the discoveries of animals living at extreme depths in the late 1800s, Victorian scientists expected that there could not be a diverse array of animals surviving in the deep sea.  Enter Robert Hessler and Howard Sanders who in 1967 used newly developed sampling devices to discover that the deep sea is shockingly diverse, and perhaps just as diverse as tropical shallow-water habitats. 

Scientists were completely baffled as to how high diversity could occur in such a bleak place.  They began to throw out theories, but they were limited by the little data that had been gathered from a poorly explored deep ocean.  The scientific publications of this time on deep-sea diversity read like there were a few people in a room with a whiteboard, writing everything they remember from their ecological textbooks, talking through each theory, slowly crossing off possibilities, and working their way down the list.  

Howard Sanders began by writing “Specialization” on the whiteboard with his paper introducing the Stability-Time Hypothesis in 1968.  He suggested that because the deep sea is monotonous and predictable (i.e., it is stable), populations have the evolutionary time to become newly specialized in how they feed. Over time, these populations become so specialized they evolve into totally new species, eventually driving diversity up.  Further research and explorations indicated that the premise of this argument was wrong- the deep sea is actually not that stable.

Then, Paul Dayton and Robert Hessler walked up to the board and scratched off the “Specialization” idea with their paper in 1972 entitled “The role of biological disturbance in maintaining diversity in the deep sea.”  The pair do not argue against the idea that the deep sea is predictable and stable.  In fact, they favor the idea… except for the part where they proved that deep-sea species are actually not more specialized than shallow water species.

“Specialization” got a strikethrough on the whiteboard, and Dayton and Hessler wrote “Predation” below it.   The duo introduced a specific type of predation pressure they labelled “biological cropping.”  No, biological cropping is not deep-sea animals learning agricultural techniques… but a combination of predation and deposit feeding.  Animals can eat other animals either intentionally (e.g. hunting down prey) or unintentionally (e.g. stuffing everything you come across into your mouth and it just so happens that you get a live one).  This “cropping,” whether accidental or not, reduces competition by preventing one or a few abundant species from monopolizing the resource.  These species get knocked out, allowing far more species to get a piece of the proverbial pie. Nobody gets sent into extinction by competition.  Dayton and Hessler’s idea is not necessarily that diversity is driven to be high in the deep sea, just that it is not limited.

Dayton and Hessler’s “Predation” idea never got fully scratched off the list, but the difficulty of testing the idea and conflicting results have led many to write large question marks next to it.   Many other ideas now are situated below “Predation,” including: “Disturbance,” “Patchiness,” and “Successional Dynamics.”

Ultimately, those of us in the deep-sea scientific community are still today standing around the dry erase board bouncing many of these same ideas off each other.  Sometimes we manage to cross one off the list, or add one, or at least add to our understanding of the ideas.  One thing is clear though, we still haven’t gotten it all figured out.  So… anyone have a dry erase marker?

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The Notorious B.I.G., Mase, and Puff Daddy understand. Increase one variable in a system and another variable rises en suite. For the B.I.G. this was money and problems. It’s like the more money we come across. The more problems we see. In the biological realm, increasing the food available increases the number of species. More food, more species.

In the case of B.I.G., Mase, and Puff how more money becomes more problems is clear. The trio “rock” and “sell out in the stores” which leads to more money. Bag a money much longer than yours. This leads to more purchases. Gotta call me on the yacht. The success and belongings are coveted by others who try to bring the trio down in an effort to elevate themselves. Know you’d rather see me die than to see me fly. But scientists are much less clear about how mo’ food leads to mo’ species. Scientists have erected dozens of hypotheses to explain this rather simple pattern.   Enter a deep-sea experiment that I dedicated 10 years of my life to.

Mo’ individuals, mo’ species hypothesis

Wright posed the more individuals hypothesis. The basic ideas is that low food supports smaller populations of species; any species is likely to be represented by just a few individuals. This makes these species more susceptible to being wiped out locally by a catastrophic event like a storm or predator. If in low food environments species are often going locally extinct, exacerbated by their low population numbers, then these environments are likely possess far less species overall. Wright’s hypothesis is ultimately a no food, mo’ problems, no species hypothesis.

Nothing-special hypothesis

Tilman, in one of the most influential papers in ecology, proposed the resource-ratio hypothesis. To simplify his elegant idea, few species are biologically equipped to deal with any resource at low availability. Mo’ food, mo’ species that can occur. Tilman actually proposed that species in resource-limited areas were just subsets of those living in high-resource areas. This is because any species can benefit with a little mo’ food, but conversely not every species can live with a little less. Tillman took these ideas a step further and actually predicted that at very high food availability the number of species should decrease because another resource would become limiting, i.e. high food habitats are not some beautiful utopia where everything, e.g. habitat space or other nutrients, is abundant.

The diva species/unique and special snowflake hypothesis

Of course this is not the real name of the hypothesis (none of the headings are). Several ecologists have converged on the idea that mo’ food allows for more specialized species. These diva species are very particular in their food type requirements. At low overall food availability, these specific food types are rare and cannot support a diva species. To restate, mo’ food allows species to be specialized. No food and species need to be generalists and take what they can get.

Mo’ food, mo’ prey hypothesis

Another ideas is that mo’ food allows for mo’ prey. This in turns supports mo’ types of predators, thereby increasing diversity. A more sophisticated variant of this is that mo’ complex food webs, containing mo’ species, can occur at higher food availabilities.

Mo’ food, mo’ giants and miniatures

This is a hypothesis of my own creation. Basically, there is “right” size for a given animal to be. This optimal size reflects a balancing of constraints. For example, too big and a species requires too much food. Too small and species does not have enough fat reserves to weather starvation. This suggests that areas with little food would only possess species of this intermediate and optimal size. Mo’ food and these caloric constraints are released and and species can get away with not being an optimal size. Thus both large- and small-sized species are allowed increasing diversity

Tourist hypothesis

Chase proposed another hypothesis that is fundamental to the mo’ food, mo’ species pattern; this pattern can only exist when low and high food habitats are isolated. If migration by adults or larvae can occur from high food to low food, diversity will be artificially elevated in low-food habitats. These tourist species from high-food areas cannot sustain themselves in low-food areas without consistent visits of individuals from these high food areas. Cut the flow of tourists and the diversity of low-food habitats diminishes.

Wood fall, the experiment

Scientists have published lots of creative studies testing aspects of these ideas. However, studies are rare that experimentally alter the food supply to a habitat and observe what happens. It’s not obvious how nor is it easy to increase the amount food at a coral reef or tropical rain forest. Mesocosm experiments, in which scientists creates an artificial system like a miniature ocean in a beaker or aquarium, provide exciting opportunities. My friend and colleague, Allen Hurlburt, conducted once such experiment in which he manipulated the amount of banana in containers.  Fruit flies collected in the rainforest where then allowed to colonize. It remains a beautiful and elegant experiment demonstrating the importance of food in controlling diversity. Allen’s study served as the inspiration for the wood-fall experiment.

Wood falls are the perfect experimental system to test mo’ food, mo’ species hypotheses. Each of the dead pieces of wood on the deep-sea floor represent little food islands. The background and typical deep-sea, muddy bottom is a food desert. The species occurring on wood falls are ultimately dependent on only the wood for nutrition. By ultimately controlling the size of the wood fall, we can control the amount of food the community of species receives.

In 2005, Jim Barry and I chunked 32 Acacia log into the deep ocean off the Central California coast. In actuality, we placed them with an ROV at spot over 3 kilometers deep.

Then we waited.

Five years later we collected half of the wood falls. Two years after that we returned for the other half.

Ten years after initially deploying the wood falls, the main paper from this work is now available as preprint. The nearly decade this experiment took to realize actually results in part reflected the length of the experiment.  However, even once collected a considerable amount of effort was need.  In the last three years, I spent countless hours meticulously sorting all the animals, nearly 13,000 individuals, from the wood falls. Taxonomists, all coauthors on the paper, spent many hours identifying these to species. With the analyses taken over a year plus the writing of the manuscript…well it adds up.

Wood fall, the results

Thankfully, with increased wood-fall size, i.e. increased food, the number of species actually increased. Strikingly, no individual hypothesis was the smoking gun for this increase in diversity.

Rather the mo’ food, mo’ species relationship reflects a combination of routes. In accordance with the mo’ individuals, mo’ species hypothesis, the total number of individuals increased with wood fall size, and was concordant with rises in the number of species. As predicted by the nothing-special hypothesis, the species on smaller wood falls, i.e. food poor, were just subsets of those species occurring on larger wood falls, i.e. mo’ food.   Increasing wood-fall size also lead to increased rare species, supporting the diva species/unique and special snowflake hypothesis. Increased larval connections between small and large wood falls also seemed to ameliorate the mo’ food, mo’ species relationship in conjunction with the tourist hypothesis.

I am just finishing examining body sizes of all the wood-fall species, but interestingly my pet hypothesis about miniatures and giants does not seem to hold. The pattern is far more interesting. Thanks to the many who supported my crowdfund project (I still love that video), David Honig and I are beginning to construct the food web through stable isotope analyses.

Notorious B.I.G., Mase, and Puff Daddy lamented the rise of problems with more money. However, to all three of these artists the reasons why this occurred were pretty straightforward. Haters gonna hate. People gonna covet your yacht. The biological world is much more complex. As simple as mo’ food, mo’ species is, the reasons why this elegant pattern exists represents a variety of interacting processes, only some we are beginning to understand.

McClain, C., Barry, J., Eernisse, D., Horton, T., Judge, J., Kakui, K., Mah, C., & Warén, A. (2015). Multiple Processes Generate Productivity-Diversity Relationships in Experimental Wood-Fall Communities Ecology DOI: 10.1890/15-1669.1

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Maybe if Red Bull funded marine research, we could send a skydiving human icebreaker, whose parachute doubles as an otter trawl and niskin bottle, to crash through polar seas in the winter and collect scientific samples. (Felix Baumgartner, CALL ME!)

No, marine research is funded by government agencies – they hold the dolla$ for the ships. Winter sampling-by-stuntman would be a little too risky for their tastes, and so research in polar regions by default has to happen in the summer (one of the reasons the Antarctic program was almost screwed by the government shutdown). My point? Our knowledge of polar regions is almost exclusively based on research done during ONE season. But according to a recent study, that seasonal bias is really messing with our understanding of biology.

In a badass new paper, Ladau et al. (2013) looked at diversity in bacterial communities around the globe, comparing patterns across seasons, and at the equator versus the poles. Although we generally think of the tropics as “biodiversity hotspots” for larger organisms, bacteria swimming in surface ocean waters are way to hipster to follow such mainstream diversity patterns.

Because scientists always sample high latitudes during summer months, previous data seemed to give the appearance of higher bacterial species diversity in tropical waters. But this isn’t actually true! The bacteria all go party at high latitudes in the WINTER – you know, that time of year when there are NO SCIENTISTS doing any sampling. Bacterial diversity shows huge, seasonal winter peaks (in December at temperate and high latitudes in the Northern Hemisphere, and in June at temperate latitudes in the Southern Hemisphere), making tropical biodiversity look  pretty LAME in comparison.

Ladau et al. used a modeling approach to compensate for the persistent sampling bias in time and location – they were able to extrapolate the predicted species distributions of bacteria based on real environmental datasets (rRNA genes sequences). They pummeled the data from every possible angle – changing models, changing parameters, subsampling their data, using other datasets, and even looking at error rates. Nothing could mess with their results – the predicted biodiversity patterns stood up to every kind of test.

Why does bacterial diversity show seasonal peaks during the wintertime in both hemispheres? We’re still not sure, but it might be due to vertical mixing which brings nutrients (and species?) to the surface. Another theory is that bacteria could migrate across latitudes. Models are no substitute old fashioned fieldwork, but they’re important for letting us look at biology from a different perspective. In this case, it shows we need to collect way more wintertime samples.

Reference:

Ladau J, Sharpton TJ, Finucane MM, Jospin G, Kembel SW, Dwyer JOA, et al. (2013) Global marine bacterial diversity peaks at high latitudes in winter. The ISME Journal, 7:1669–77.

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But in general, it turns out we were pretty wrong about that hypothesis. The presence of a mid-ocean ridge doesn’t exaggerate biomass and biodiversity at all in the North Atlantic–not even close, in fact. This is the cool result from the ECOMAR project, a large multi-year study focused on the Charlie-Gibbs fracture zone in the northern Mid-Atlantic Ridge (Priede et al. 2013). I actually went to sea on the 2009 cruise, and as a scientist it’s pretty awesome to read a paper and exclaim “hey, I know those sample sites!”

So if the Mid-Atlantic ridge isn’t a biodiversity hotspot, what’s so interesting about this paper? Well…

Meaning, the relationship between depth and the amount of “living stuff” equates to a constant number. Like the speed of light in Physics! And that calculation of total biomass will be the same at most places in the world’s oceans. For any given depth, we can determine the total biomass of animals based on a graph that looks like this:

So at the Mid-Atlantic Ridge, the seafloor is raised up higher than the surrounding abyssal plains. This means that the amount of pelagic biomass in the overlying water column is lower (fish, plankton, all those swimmy things), but there’s way more stuff living on the ridge seafloor.

You can think about this as dividing a bag of jellybeans between two jars. You have a fixed amount of jellybeans (constant total biomass);  you can divide them equally between both jars (water column biomass equals benthic biomass), or perhaps put one-quarter of your jellybeans into jar 1 and three-quarters into jar 2 (our Mid-Atlantic ridge example, where the seafloor biomass is jar 2). Note: you’re not allowed to eat any jellybeans in this example.

This is just one neat study to come out of the ECOMAR project, and I expect we’ll be seeing more exciting results in the very near future. Stay tuned!

Reference:

Priede IG, Bergstad OA, Miller PI, Vecchione M, Gebruk A, Falkenhaug T, et al. (2013) Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity? PLoS ONE, 8(5):e61550.

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