Given similarities in climate why dont all tropical

Given similarities in climate, why don’t all tropical regions have the same species or, at least, the same types of species, and communities that have the same proportions of primates, birds, reptiles, etc. ?

History! In other words, ecological processes (for example, the inability of a taxonomic group to withstand cold temperatures, or competition with a closely related species) can’t explain everything about the distribution and abundance of particular taxonomic groups.

Some groups unique (endemic) to southeast Asian tropics Gibbons Orangutans

African isolation resulted in radiation of mammalian clade Afrotheria which includes the orders Proboscidea (elephants) Hyracoidea (hyraxes) Macroscelidea (elephant shrews) Tubulidentata (aardvark) Afrosoricida (golden moles and Madagascan tenrecs) Sirenia (dugongs and manatees)

Madagascar primates are lemurs—a unique (endemic) group of primates

Endemic mammals of the Neotropics (New World tropics) Sloths Armadillos Anteaters

Australian mammalian fauna has many marsupials (likely initially evolved in North America) Because many placental mammals either never made it to Australia (except for recent invasives and bats) or didn’t establish a strong foothold, marsupials radiated

Monkeys are common residents of all the world’s rainforests except those of New Guinea and Australia (With the exception of some invasive crabeating macaques on New Guinea)

Most tropical areas originated in Gondwanaland

Many tropical areas separated from each other between 160 and 50 million years ago, when many present-day taxonomic groups originated Thus the distributions of these taxonomic groups often demonstrate these past geographical relationships

Each geographic region has a unique history, leading to a unique biota

Specific histories—Southeast Asia Composed of several continental blocks that separated from Gondwanaland between 400 and 160 million years ago Exchange of organisms with Indonesia has been increasing as Australia moves northward and Indonesia archipelago continues to form

Specific histories—Africa and Madagascar 160 mya Africa and South America split from the rest of Gondwanaland 105 mya South America and Africa split Africa connected to Asia for the last 20 my but the dry regions of the Sahara and middle East have prohibited exchange of many humid tropical organisms Madagascar has been isolated from Africa for about 90 my

Specific histories--India Over 130 mya Madagascar and India split from Antarctica-Australia 40 mya India fused with Asia

Specific histories—South America 105 mya South America and Africa split. South America split from Antarctica after this date and so was isolated for about 70 my In the last 3 my intermittent archipelagoes have existed between Central and South America

Specific histories—Central America Conglomerate of pieces of land of various ages and origins The Great American Interchange (3 mya—when Central America joined with South America because of the development of land connections) has had a great influence on shaping the region’s biota

Specific histories--Australia 100 mya Australia and New Guinea split from Antarctica and are continuing to move northward Australia and New Guinea should become connected to Asia in about 40 my

Although the climates of the tropical regions have many similarities, history has played a large role in shaping the unique biotas of each region

Mammal faunas of different tropical regions result from different geographical/geological histories and then evolution within tropical environments

The Great American Interchange was an historic phenomenon that shaped North, Central and South American biotas

From 65 to 30 mya, mammals evolved and speciated in South America, both marsupials and placentals

South America’s biota during the Tertiary included Glyptodonts (placental)--hippopotamus-size armadillos Toxodons (placental)--similar to rhinos and hippopotamuses Thylacosmilus--a sabertooth marsupial cat. Pyrotheres (placental)--some similarities to elephants Borhyaenids (marsupials)--resembled dogs, cats, mustelids, and shrews

Panamanian landbridge connected North to South America about 3 mya Great American Interchange occurred

Species from NA that moved into SA Jaguars, peccaries, tapirs, coatis, kinkajous, giant otters, llamas, mastodons

Species from SA that moved into NA Giant sloths, armadillos, possums, glyptodonts, porcupines, anteaters, toxodons

Marsupial cats of SA were replaced by placental cats Toxodons of SA were replaced by tapirs and deer Some unique groups of SA persisted--monkeys, tree sloth, anteaters

50% of mammal species of SA have NA ancestors 10 -20% of NA species have SA origins Why?

Hypotheses to explain pattern described on previous slide Northern mammals were better migrators? Northern mammals more prone to speciation? Northern mammals outcompeted Southern mammals? North mammals more tested? South American forms larger?

Refugial hypothesis of Amazonian speciation We’ve been examining historical influences on the scale of millions of year Now we’ll move to thousands of years and theory that Pleistocene refugia have led to speciation in tropical areas

Idea of Pleistocene refugia first developed by Haffer (1969) The idea was used to explain some of the great species richness of the Amazon—how did so many closely related species evolve?

Haffer’s theory “…during several dry climatic periods (glacial periods) of the Pleistocene and post. Pleistocene, the Amazonian forest was divided into a number of smaller forests which were isolated from each other by tracts of open, nonforest vegetation”. Haffer, J. 1969. Speciation in Amazonian forest birds. Science 165: 99 -105

Haffer’s theory “The remaining forests served as "refuge areas" for numerous populations of forest animals, which deviated from one another during periods of geographic isolation”. Haffer, J. 1969. Speciation in Amazonian forest birds. Science 165: 99 -105

Hypothesized forest refugia during dry periods of the Pleistocene

Haffer’s theory “The isolated forests were again united during humid climatic periods when the intervening open country became once more forestcovered, permitting the refuge-area populations to extend their ranges”. Haffer, J. 1969. Speciation in Amazonian forest birds. Science 165: 99 -105

Haffer’s theory “This rupturing and rejoining of the various forests in Amazonia probably was repeated several times during the Quaternary and led to a rapid differentiation of the Amazonian forest fauna in geologically very recent times”. Haffer, J. 1969. Speciation in Amazonian forest birds. Science 165: 99 -105

This theory has not been uniformly accepted and has engendered some other potential theories

The rise and fall of the Refugial Hypothesis of Amazonian speciation: a paleoecological perspective Mark B. Bush; Paulo E. de Oliveira Biota Neotrop. vol. 6 no. 1 Campinas 2006

Bush and Oliveira reviewed previous palynological (pollen) data to see whether they supported the idea that savannas had separated areas of forest during dry periods.

Very few data exist, given the lack of many lakes, from which to obtain pollen samples, in the Amazon

Pollen diagram from 2013 paper by Velasquez and Hooghiemstra

Pollen diagram in previous slide shows Proportions of different types of plants (for example parano vs. subandean) change with depth of the soil. The deeper the soil, the further back in time. Changes in proportions of different plant types is likely linked to climate changes in the past.

Bush and Oliveira argue that these data may show local changes to savannas during dry periods (more grasses and fewer trees) but not wide-spread changes.

Bush and Oliveira suggest previous data show many grasses existed during Holocene, as well as Pleistocene, but refugia weren’t suggested to be in place in Holocene. Also, they suggest the grasses may only be in local, not regional environment They also say the dry period in the Pleistocene, from 26, 000 to 15, 000 years before the present, wasn’t sufficient time for speciation

Sites in Ecuador and Brazil have pollen patterns that show cooling during ice ages, but not necessarily the changes in precipitation patterns predicted by the refugium hypothesis

Amazon fan (area in square) acts as a pollen trap for the whole Amazon basin. Pollen data from the fan show about the same proportion of grasses today as in the dry, glacial periods of the Pleistocene. Bush and Oliveira argue that this provides evidence of a lack of grasslands separating forest into refugia during the Pleistocene, because much of Amazon is forested today.

Haffer argues pollen from fan only represent pollen from river systems and are not representative of whole Amazon basin

Bush and Oliveira ultimately reject refugium hypothesis

Another investigator, Colinvaux, argues, based on palynological data, that, although cooling occurred, this did not result in changes from forest to savanna and that Amazonian region has been continuously forested. Colinvaux also argues against the refugium hypothesis

Some climatological changes, with accompanying vegetation changes did occur in some areas of tropical South America, but evidence for these types of changes in the Amazon isn’t strong so evidence for refugium hypothesis isn’t strong Interestingly, these data indicate that drying occurred more during interglacial than glacial periods (one would expect the opposite)

Another interesting pattern is that molecular data demonstrate that many taxonomic groups did undergo speciation during the Pleistocene, in accord with Haffer’s ideas. However the mechanism of refugia isn’t strongly supported.

Amazon is very large, data are sparse, and it’s difficult to draw definitive conclusions about the refugium hypothesis

In contrast, data from Africa (Anthony et al. 2007) show that Pleistocene refugia may have led to differentiation in gorillas. Africa may be a more likely region forest refugia during dry periods because it is drier to start with than the Neotropics—this means it is closer to the threshold point at which savannas may largerly replace forests

Anthony et al. (2007) also suggest a potentially important role for rivers in gorilla diversification (at the population level if not at the full species level)

(Gorilla data) Geographic distribution of major mitochondrial haplogroups across central Africa. Important rivers are also indicated with an arrow. CR, Cross River; SA, Sanaga River; OG, Ogooué River; IV, Ivindo River; SG, Sangha River; UB, Ubangui River; CG, Congo River. The locations of two published museum samples (22, 57) are indicated by “? . ”

Other hypotheses to explain speciation patterns in the Amazon Marine incursions hypothesis—flooding of parts of South America with sea water from 10 -15 mya would have meant that terrestrial organisms had to inhabit the Andes, the Guyana shield, or the Brazilian shield, leading to speciation patterns congruent with this geographical pattern.

Marine incursions occurred 10 -15 mya

Other hypotheses to explain speciation patterns in the Amazon Riverine hypothesis: tropical rivers isolate populations, leading to divergence and speciation

Solomon et al. 2008 Expected distributions of leaf-cutter ants according to different speciation hypotheses

Solomon’s results, based on molecular data, for leaf-cutter ants, show that Pleistocene refugia and marine incursions may have been important in diversification of the ants, but that rivers were not. (Solomon et al. 2008)

Dependence on particular barriers for speciation may vary depending on taxonomic group. Perhaps Pleistocene refugia were important to the speciation of particular groups and not others.