Author Abstract: There is evidence from the fossil record to suggest that latitudinal gradients in taxonomic diversity may be time-invariant features, although almost certainly not on the same scale as that seen at the present day. It is now apparent that both latitudinal and longitudinal gradients increased dramatically in strength through the Cenozoic era (i.e. the last 65 my) to become more pronounced today than at any time in the geological past. Present-day taxonomic diversity gradients, in both the marine and terrestrial realms, are underpinned by the tropical radiations of a comparatively small number of species-rich clades. Quite why these particular taxa proliferated through the Cenozoic is uncertain, but it could be that at least part of the explanation involves the phenomenon of evolutionary escalation. This is, in essence, a theory of biological diversification through evolutionary feedback mechanisms between predators and prey; first one develops an adaptive advantage, and then the other. However, there may also have been some form of extrinsic control on the process of tropical diversification, and this was most likely centred on the phenomenon of global climate change. This is especially so over the last 15 my. Various Late Cenozoic (Neogene) vicariant events effectively partitioned the tropics into a series of high diversity centres, or foci. It has been suggested that, in the largest of these in the marine realm (the Indo-West Pacific or IWP centre), a critical patterns of islands acted as a template for rapid speciation during glacioeustatic sea level cycles. The same process occurred in the Atlantic, Caribbean and East Pacific (ACEP) centre, though on a lesser scale. Tropical terrestrial diversity may also have been promoted by rapid range expansions and contractions in concert with glacial cycles (a modified refugium hypothesis). We are beginning to appreciate that an integrated sequence of Neogene tectonic and climatic events greatly influenced the formation of contemporary taxonomic diversity patterns.
Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.
Review: Today we start looking at specific examples of applied biogeography now that we have an understanding of several schools of biogeography. The first paper we look at this week falls into the applied historical biogeography category. This paper looks at the case of latitudinal diversity gradients (the observation that the majority of taxonomic groups have a higher proportion of species in tropical, lower latitudes than higher latitudes) throughout the fossil record to determine whether "latitudinal gradients in taxonomic diversity may be time-invariant features."
The author found that both for the "marine and terrestrial realms" that a high profusion of tropical species today compared to higher, temperate latitudes is the result of "the tropical radiations of a comparatively small number of species-rich clades."
The author examined the fossil record over the last 100 million years to come to his conclusions. While he found a gradient throughout the past, the author notes that only from the Cenozoic era (i.e. the last 65 million years) did major shifts in the diversity gradient occur, particularly as a result of new radiation and speciation "following the K-T mass extinction event." What is interesting in this paper is the finding that "taxonomic diversity gradients must have steepened substantially" since that die off of organisms.
The model marine taxonomic group chosen by the author to examine latitudinal gradients throughout the last 100 million years and to compare to present-day latitudinal gradients of extant (living) taxa is the molluscs. They were chosen in part because of their excellent fossil record as well as because they are one of the largest living groups (in terms of diversity, or numbers of species) of marine organisms today. The model terrestrial taxonomic group examined by the author is the angiosperms (flowering plants).
In both groups examined, the author shows that large percentages of total group diversity today are reflected in relatively few family clades within each major group, with those clades being relatively young (< 65 million years since first fossil record) and principally lower latitude, tropical groups.
The author makes a small note about other taxonomic groups with major latitudinal diversity gradients showing highest diversity in tropical, lower latitudes (especially equatorial): "all major insect groups," birds, mammals, teleost fish, and snakes.
All groups discussed show a latitudinal diversity gradient in the fossil record, though none to the extent seen in the present day. As a result, the author concludes that "there must have been a dramatic increase in the scale of this [latitudinal diversity gradient] feature between the latest Cretaceous and Recent."
One hypothesis the author presents for why the diversity gradient is more pronounced today than in the fossil record (besides noting that the fossil record is still incomplete and not entirely understood) is linked to Global Climate Change, where a "temperature maximum occurred between 19.5 and 17 Ma, followed by an abrupt cooling trend between 17 and 14 Ma linked to the rapid expansion of the East Antarctic ice cap." 65 million years ago there were no polar ice caps, so it is suggested that perhaps a major die off of organisms occurred in the poles as "polar temperatures deteriorated rapidly to their present day levels" compared to the tropics, which "were much less affected." As major clades of organisms "continued to expand" in the tropics, the same radiation wasn't seen in areas of rapidly deteriorating temperatures.
The author concludes a continued tropical radiation "is of considerable significance because it suggests that the process of tropical diversification was a prolonged one that coincided with phases of both global warming and cooling."
And tying in with our panbiogeography seminar series last week, the author also notes that "over the last 100 my [million years] the continental land masses have become unusually emergent, fragmented and dispersed; long, north-south-trending continents and continental shelves have become progressively partitioned along latitudinal environmental gradients" [as a result of plate tectonic movements]. As populations of organisms became geographically fragmented, speciation increased in part due to reproductive barriers between populations from such geographic separation.
The author focuses on abiotic factors (such as continental movement and climate change) because he notes that "it is still open to question as to precisely how local ecological interactions scale up to determine origination and extinction rates. Many biotic replacements [die-offs of some groups and radiation of others] within the fossil record could equally well be interpreted as the replacement of incumbent taxa by fundamental environmental change as by ecological interactions [e.g., predator-prey interactions or feedback mechanisms]."
The author also notes "one further important phenomenon to bear in mind. Rather than being evenly spread geographically throughout the tropics, diversity is very strongly concentrated within discrete centres, or foci. This is particularly so within the marine realm, where two major foci are traditionally recognized: an Indo-West Pacific (IWP) one and an Atlantic, Caribbean and East Pacific one (ACEP)."
From a fossil perspective, the author brings a fresh view on whether the IWP is "an evolutionary centre of origin." The author disagrees with such a view, noting that "there is now an increasing volume of evidence to suggest that what we may in fact be looking at is a homogenous, early Cenozoic, pan-tropical (Tethyan) fauna that has been disrupted by a series of essentially Neogene tectonic and climatic events."
The author notes that "rapid speciation" is often linked in the fossil record with "rapid climate change" and that as sea levels rose and fell or tropical regions expanded or contracted, the most tropical and climatically-stable regions (equatorial) would have acted as a "refugium" that would periodically be infused with new species on the peripheries of the tropics as climatic conditions were favorable.
The author concludes that "latitudinal gradients in taxonomic diversity can be traced back in the geological record to well within the Palaeozoic era (i.e. at least 400 my, and perhaps substantially more" but that "all the available evidence suggests that, from the Late Palaeozoic to Early Cenozoic, they were on nothing like the scale of those seen today in either the marine or terrestrial realms." Examining the fossil record and comparing diversity to today, the author suggests that present day levels are at a "maximum or near-maximum" for diversity.
Next we'll continue our seminar series with a look at track analysis on the fossil record, followed by a track analysis of forests in Mexico and how applying the results of such analyses can influence conservation priorities.
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