During his life, Wallace was a well-known writer for his book The Malay Archipelago, which described his time in the Indo-Pacific (Indo-Australian) region. In this region is the division between Australian fauna and Asian fauna that Wallace observed. This zone of division is now referred to as the Wallace Line. Since the study of the distribution of organisms among regions is the science of biogeography, Wallace’s studies in this region led to him being named the father of biogeography.
Several ideas have come out of the study of biogeography. A few of them are called rules or sometimes principles.
Bergmann’s Rule:
Body size among similar homeothermic organisms is larger in colder climates than in warm climates. Thus, a related bird species should be larger in Greenland than in Costa Rico. The idea has to do with surface-to-volume ratios and size in general. If you are small, your body temp will cool down much faster than a similar large-sized creature.
Island Biogeography
Robert MacArthur and E.O. Wilson famously studied species diversity among islands, developing several rules. One rule is that the larger the island, the higher the biodiversity. Their work opened up the study of isolated “island” habitats.
Biodiversity is higher in the tropics than in temperate zones and less diverse near the poles. There are several hypotheses for why this exists.
One is that less sunlight = less energy coming into the zone. Thus, arctic regions have fewer organisms due to less energy flowing through the systems. In contrast, the tropics have lots of sunlight energy, which flows into the systems, allowing many more organisms to survive.
Another hypothesis is that the tropics, being older regions, have had more time, and thus, more species have evolved.
This observation is called the latitudinal diversity gradient. Regardless of the reason behind the differences, they exist… mostly. One of the fascinating things to study in biology is systems or species that don’t appear to follow the rules. First, it’s interesting, and finding interesting things to do, see, and read is life’s goal. Second, the exceptions may indicate another pattern or rule of law of biology that helps us understand the world better.
This leads to observing an ecosystem that does not appear to follow the higher biodiversity in the tropic rule. A recent paper, 2023, minted this month, has the provocative title, Marine latitudinal diversity gradients are generally absent in intertidal ecosystems. Let’s explore.
The lead author had previously published a similar article in 2021, the abstract of which ended with the line: “Polar and tropical intertidal data were sparse, and more sampling is required to improve knowledge of marine biodiversity.” Thus, the setup for another look. A second 2021 paper, with the same first author (Thyrring) set about aiding the dearth of data by collecting intertidal data throughout Greenland.
In their 2021 paper, they scoured (forgive the word) published studies and ended up with data from 433 intertidal sites around the planet. They analyzed northern hemisphere and southern hemisphere data separately. They found a peak of species richness (AKA alpha diversity) at mid-latitudes (30-55 degrees) in the northern hemisphere and a slight trend toward tropical diversity highs in the southern hemisphere (Figure 1).
Stronger associations were seen among environmental variables; salinity was the strongest. Ice-scoured sites had more diversity than non-scoured sites, and sites with macroalgae cover had greater diversity than those with a canopy.
The researchers also looked at functional diversity; producer (algae) proportions increased with latitude, while more predators and suspension feeders were abundant at lower latitudes. As with the general trends, data scarcity at some latitudes hampered the results.
The takeaway from the 2021 study is that local conditions (salinity, ice-scour) influence species richness (alpha diversity) while there is no latitudinal trend.
On to 2023, but not much of it since I’m traveling and tired, and they report the same result.
The 2023 study found latitudinal trends in subtidal benthic and pelagic zones but no intertidal gradient in any ocean or hemisphere. They use the acronym LDG for latitudinal diversity gradient, which gets messy because others have used LGR for latitudinal gradient of richness. I’ll not go into a rant, which I do occasionally, on acronyms and some science terminology that hampers understanding.
The contrast between intertidal richness, which holds little correlation, and subtidal richness, which does show a pattern with latitude, is interesting, as the authors point out.
Last, a quick note back to Wallace: In 1903, he published Man's Place in the Universe. In it, he contemplates life on other planets, specifically Mars. Now I'll have to find this book and make a blog about it.
Sources and Further Reading:
Bergmann C. 1847. Ueber die verhältnisse der wärmeökonomie der thiere zu ihrer grösse. Göttinger Studien 3: 595–708.
Darwin CR and Wallace AR. 1858. On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.
Darwin CR, and Wallace AR. On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection. Journal of the Proceedings of the Linnean Society of London, Zoology 3(9): 45-62.
Darwin C. 1859. On the origin of species by means of natural selection, or, the preservation of the favoured races in the struggle for life. London, J. Murray.
MacArthur RH, and Wilson EO. 1967. The theory of island biogeography. Princeton, N.J: Princeton University Press.
Thyrring J, Peck LS. 2021. Global gradients in intertidal species richness and functional groups. eLife 10:e64541
Thyrring J, Wegeberg S, Blicher ME, Krause-Jensen D, Høgslund S, Olesen B, Jozef W Jr., Mouritsen KN, Peck LS, and Sejr MK. 2021. Latitudinal patterns in intertidal ecosystem structure in West Greenland suggest resilience to climate change. Ecography, 44: 1156-1168.
Thyrring J, and Harley CDG. 2023. Marine latitudinal diversity gradients are generally absent in intertidal ecosystems. Ecology. Accepted Author Manuscript e4205. https://doi.org/10.1002/ecy.4205
Wallace AR. The Malay Archipelago; The Land of the Orang-utan and the Bird of Paradise; A Narrative of Travel With Studies of Man and Nature. 2 volumes. Macmillan & Co., London, 9 March 1869.
Wallace AR. 1889. Darwinism; An Exposition of the Theory of Natural Selection With Some of Its Applications. Macmillan & Co., London & New York. 494pp.
Wallace AR. 1903. Man’s place in the universe: A study of the results of scientific research in relation to the unity of plurality of worlds. New York. McClure Philips and Co.