3 million years of a marine latitudinal diversity gradient

Perhaps the largest, most visible, macroecological trend on this planet is that there are generally more species in the tropics (low latitudes) than the poles (high latitudes). This pattern has been observed in both terrestrial and marine systems, across hugely varied groups of plants and animals. Despite the fact that this has been known for awhile now, no unified model exists that explains how this latitudinal species gradient came about or what mechanism(s) act to maintain it (intriguing hypotheses abound, however, including faster speciation in the tropics). What the structure of this gradient was like in the past is uncertain as well; for example, how stable this pattern is through time?

A recent paper in Ecology Letters by Dr. Yashuhara and colleagues uses paleotemperature information and diversity data of tiny planktonic critters called formaminfera (forams, if you’d like fit in) to investigate this latitudinal species gradient at various snapshots through time, over the past three million years.

Planktonic (residing in the water column while alive, rather than at the bottom of the sea in the benthos) forams as a groups are hugely abundant; deep-sea sediment in some parts of the world can comprise of almost 70% of forams, particularly their tests, or tiny calcium carbonate shells. Scientists have been studying forams in depth for some time now, in no small part due to their importance in studying past climate and oceanic conditions, and as a result their taxonomy is well-known. So if you’d like to take a detailed look at the latitudinal diversity gradient in the ocean over time, this is a good model system for that.

Using past datasets consisting of foram diversity and reconstructed sea-surface temperatures, the researchers investigated four specific temporal snapshots: modern, 18,000 years ago (Last Glacial Maximum), 120,000 years ago (Last Interglacial), and 3.3-3.0 millions years ago (in the Pliocene). They found that unimodal (think a relationship with a single hump-shaped peak) relationship between species diversity and latitude for all time points, with diversity generally peaking in the subtropics and falling off rapidly towards the poles. Additionally, used the reconstructed sea-surface temperature dataset, they found that species diversity tracks closely to temperature. For example, diversity was highest in the mid-Pliocene samples when it was the warmest of the time points considered here, and lowest during the last glacial maximum, the coldest. The authors even point out that the latitudinal diversity gradient was steeper during the last glacial maximum (i.e. the diversity in this set of samples decreased faster–at a steeper slope–towards the poles than the other time points), but this is likely due to the fact that the temperature is known to change more drastically with latitude during this time period.

This is important because it indicates that temperature has a big role to play in determining what species (and how many) live where. It also shows, as other studies have, that temperature is an important predictor of diversity in the ocean. This work’s central finding shows that this relationship has been robust at these various time points throughout the last three million years, even when examined at very high taxonomic resolution. Paleoecological work is really interesting and hugely useful in this vein; it can show us how things like climate can affect biodiversity over large timescales, and allow a different perspective of how large ecological patterns work.

Yasuhara, M., G. Hunt, H. J. Dowsett, M. M. Robinson, and D. K. Stoll (2012). Latitudinal species diversity gradient of marine zooplankton for the last three million years. Ecology Letters DOI: 10.1111/j.1461-0248.2012.01828.x

Crushing predators reinvade the Antarctic benthos

In Gotham, Batman drives a batmobile that shoots fire out the back and has all sorts of mechanical wizardry so he can catch fiends in style.  Or something close to that, unless my childhood was dreadfully misinformed.  He isn’t supposed to turn up in a St. Patrick’s day parade in New Jersey, pedaling away on a two-wheeled crime-fighting vehicle adorned with no fewer than 13 (count them!) bat symbols.  I feel that witnessing that would be strange–similar to the feeling you get when you discover your keys in the refrigerator next to the milk.  Simply out of place.

Recently, Antarctica has its own version of things showing up in the wrong place.  King crabs, predators that the Antarctic underwater shelf has not seen in over 40 million years, appear to be making a rapid comeback.  The endemic (unique to a specific, defined locale) nature of Antarctic shelf organisms is the result of a massive climatic cooling event in the middle Eocene, approximately 41 million years ago (Ma)1.  From 41 to 33.5 Ma, coastal sea surface temperatures decreased as much as 10°C, even before the onset on glaciation; this led to the eventual extinction of shell-breaking (durophagus) predators, such as modern bony fish, decapod crustaceans, and most sharks and rays. These groups have not returned due to their lack of an ability to physiologically cope with magnesium, one of the major cations present in seawater, at low temperatures.  Under one degree C or so, these magnesium ions are lethal to these organisms1.  Due to the fact that in the Antarctic, shallower seawater is slightly colder than that of the deep, they are effectively shut out of the shallows.

Distribution of epifaunal suspension feeders before and after the Eocene cooling at 41 Ma. The graph on the left shows temperature data derived from oxygen isotope values in bivalve shells. The schematic on the right shows the relative abundance of fossil concentrations of brachiopods, stalked and unstalked crinoids, and ophiuroids. Aronson et al. 2009, PLoS ONE.

Paleontological findings on Seymour Island, near the Antarctic Peninsula, reveal that dense populations of ophiuroids (Ophiura hendleri) and crinoids (Metacrinus fossilis and Notocrinus rasmusseni) were present on the soft substrate after the 41 Ma cooling event, but not prior1.  Both ophiuroids and crinoids are vulnerable to durophagy, and thus reduced predation pressure is implied after the Eocene cooling event.  This is quite straightforward:  if the things that normally eat you are no longer there, the size of your population increases, and you can invite the folks down the way to come over and watch Buffy the Vampire Slayer and enjoy your mean gin and tonics with a decreased sense of doom2.

Even today, these and other suspension feeders are abundant across the Antarctic shelf3.  However, in the past 50 years, sea surface temperatures off the Antarctic Peninsula have risen 1°C4, and as a result, predatory crabs and duropaguous fish may be able to enter this isolated shelf environment.  Anomuran king crab populations have already been found in slightly warmer, deeper waters nearby5 and it was reported on Sunday by the Washington Post that a recent expedition observed hundreds, potentially primed for invasion into the shallows of the continental shelf.  Dr. Sven Thatje and colleagues are currently searching thousands of seafloor images for evidence that predation by these crabs is ongoing.

Current climatic warming is essentially opening a physiological door for these polar predators to reclaim their place in the Antarctic benthic community via range extensions and human-induced introductions5.  This reinvasion has the potential to drastically alter ecological relationships, perhaps even eliminate populations of dominant suspension feeders and homogenize the unique Antarctic nearshore benthos with higher latitude communities.

Images/figure:  1) Michael Bocchieri/Bocchieri Archive, from Flickr user Foto Bocch (cc).  I have been itching to find an excuse to use it since I saw it as NPR’s photo of the day. 2) From Aronson et al. 2009, PLoS ONE (cc).

1. Aronson RB, Moody RM, Ivany LC, Blake DB, Werner JE, & Glass A (2009). Climate change and trophic response of the Antarctic bottom fauna. PloS one, 4 (2) PMID: 19194490
2. I’m actually unaware of any invertebrates that enjoy Joss Whedon shows or G and T’s.  Pity for them.
3. GILI, J., ARNTZ, W., PALANQUES, A., OREJAS, C., CLARKE, A., DAYTON, P., ISLA, E., TEIXIDO, N., ROSSI, S., & LOPEZGONZALEZ, P. (2006). A unique assemblage of epibenthic sessile suspension feeders with archaic features in the high-Antarctic Deep Sea Research Part II: Topical Studies in Oceanography, 53 (8-10), 1029-1052 DOI: 10.1016/j.dsr2.2005.10.021
4. Clarke, A., Murphy, E., Meredith, M., King, J., Peck, L., Barnes, D., & Smith, R. (2007). Climate change and the marine ecosystem of the western Antarctic Peninsula Philosophical Transactions of the Royal Society B: Biological Sciences, 362 (1477), 149-166 DOI: 10.1098/rstb.2006.1958
5. Thatje, S., Anger, K., Calcagno, J., Lovrich, G., Pörtner, H., & Arntz, W. (2005). CHALLENGING THE COLD: CRABS RECONQUER THE ANTARCTIC Ecology, 86 (3), 619-625 DOI: 10.1890/04-0620