Background
Marine copepods are small crustaceans that are found in the world's oceans and form a key trophic link between phytoplankton and fish. Some of them are planktonic and drift in sea water, but more of them are benthic and live on the ocean floor.

What was done
The authors developed the first global description of geographical variation in the diversity of marine copepods in relation to ten environmental variables.

What was learned
Rombouts et al. report that "ocean temperature was the most important explanatory factor among all environmental variables tested, accounting for 54 percent of the variation in diversity." Hence, it was not surprising that "diversity peaked at subtropical latitudes in the Northern Hemisphere and showed a plateau in the Southern Hemisphere where diversity remained high from the Equator to the beginning of the temperate regions," which pattern, in their words, "is consistent with latitudinal variations found for some other marine taxa, e.g. foraminifera (Rutherford et al., 1999), tintinnids (Dolan et al., 2006) and fish (Worm et al., 2005; Boyce et al., 2008), and also in the terrestrial environment, e.g. aphids, sawflies and birds (Gaston and Blackburn, 2000)."

What it means
"Given the strong positive correlation between diversity and temperature," the six scientists say that "local copepod diversity, especially in extra-tropical regions, is likely to increase with climate change as their large-scale distributions respond to climate warming." This state of affairs is much the same as what has typically been found on land for birds, butterflies, and several other terrestrial lifeforms, as their ranges expand and overlap in response to global warming. And with more territory thus available to them, their "foothold" on the planet becomes ever stronger, fortifying them against forces (many of them human-induced) that might otherwise lead to their extinction.

References
Boyce, D.G., Tittensor, D.P. and Worm, B. 2008. Effects of temperature on global patterns of tuna and billfish richness. Marine Ecology Progress Series 355: 267-276.

Dolan, J.R., Lemee, R., Gasparini, S., Mousseau, L. and Heyndrickx, C. 2006. Probing diversity in the plankton: using patterns in Tintinnids (planktonic marine ciliates) to identify mechanisms. Hydrobiologia 555: 143-157.

Gaston, J.K. and Blackburn, T.M. 2000. Pattern and Process in Macroecology. Blackwell Science Ltd., Oxford, United Kingdom.

Rutherford, S., D'Hondt, S. and Prell, W. 1999. Environmental controls on the geographic distribution of zooplankton diversity. Nature 400: 749-753.

Worm, B., Oschlies, A., Lotze, H.K. and Myers, R.A. 2005. Global patterns of predator diversity in the open oceans. Science 309: 1365-1369.