Literature Review: Tepui endemism and climate envelopes

Literature Review For “Is the Lost World Lost? High Endemisms of Amphibians and Reptiles on South American Tepuis in a Changing Climate”. Dennis Rodder, Andreas Schluter, Stefan Lotters From the Book Relict Species: Phylogeny and Conservation Biology, J.C. Habel and T Assmann. 2010 Springer-Verlag Berlin Heidelberg page 401

Introduction.

The authors published the work “Is the Lost World Lost?” as a detailed review of tepui biogeography under changing climatic conditions. The purpose of the work is to analyze the phylogenetic characteristics of herpetofaunal communities since the Long Glacial Maximum, and to assess their stability under Anthropogenic Climate Change, commonly known as Global Warming. They utilized a series of computer models to analyze the effects of climate change with Climate Envelope Distribution Models (CEDMs) as the primary methodology of mapping herpetofaunal communities under varying paleoecological conditions to assess their distribution. The thesis of the work is that reptile communities evolved under the a specific climatic condition characterized by lower temperatures, and may face extinction by Anthropogenic climate change, which varies markedly from previous long term temperature changes.

Summary

The work is divided into five parts. Section one is an outline of the biogeography of the Pantepui region in section, section two details problems with data sampling, while sections 3-5 address the CEDM methodology and the implications of the models. The work delineates the issues of tepui endemism by moving from the general to the specific, discarding certain models of tepui speciation in sections one and two in favor of a specific model known as the Cool Climate hypothesis.

While rejecting fictionalized stories of the tepui ecosystems, the authors remark that high levels of endemic organisms ( 77.3% for reptiles, 54.8% for amphibians)¹ constitute a kind of “lost world” for herpetofauna. They cite the geomorphology of the Pantepui as the cause of this endemicity. The origin of the tepui ecosystem is analyzed in four models of establishment: the Distance Dispersal hypothesis, The Habitat Shift hypothesis, the Lost world hypothesis, and the Cool climate hypothesis.

The models are now explained in brief.

Distance Dispersal refers to avifauna, proposing that Andean birds reached tepui summits while crossing the Orinoco and Negro Rivers.² In a type of 'island hoping' migration, populations became established on western tepuis and migrated to the eastern tepuis, and speciated. Similar climate conditions in the Andes and the tepuis encouraged species with Andean affinities and adaptations. The distance dispersal method can only apply with highly mobile organisms, and is generally restricted to avifauna.

The Habitat Shift model assumes that the highland and summit species developed from lowland species that adapted to highland habitats and have since speciated. This model was developed to explain certain elements in avifauna endemicity ( more that 38% of endemic avifauna can be accounted for in this way.), as well as elements of the herpetofauna community.³ Given that the highlands uplift and erosion occurred over a long period of time, this hypothesis seems reasonable to explain elements the unique ecosystems.

The Lost world model states that the high levels of endemicity are the result of millions of years of isolation from the lowlands. Certain herpetofaunal elements such as the bufonid Oreophrynella suggest that certain faunal components considered to be paleofauna may have developed in isolation on the tepui summits, and have since gone extinct or radiated beyond recognition in the lowlands.⁴

The Cool climate model is based upon alternating vertical displacement during glacial cycles during the Quaternary. ⁵ The model states that the primary diversification occurred during cold glacial periods, when the tepui ecosystem migrated down from the summits to escape cooling. The dispersal into lowland areas allowed biological exchange from one tepui to another, vastly expanding the ranges of summit biota. As warming trends appeared during interglacial periods, tepui biota was forced back onto the higher elevated summits and midlands, driving adaptive radiation.

The publication focuses on herpetofauna in an effort to determine which model best fits known reptile communities on the tepuis, utilizing these communities to infer general tepui evolutionary dynamics. The authors contend that “herpetofaunal communities are perhaps better suited for biogeographic analysis...since in general these animals are less mobile than birds and most mammals and may be closely tied to specific habitats”.⁶ Comparison between different tepuis indicates that a good deal of herpetofauna overlaps, indicating a variety of connections between proximal tepuis. This data can be used to indicate the extent of gene flow between tepuis, as well as the origin of the fauna itself. Precise relationships between summit fauna has not been established due to a lack of phylogenetic analysis.

An alternative method for determining overlap of faunal elements and gene flows between the tepuis is the use of GIS based Climate Envelope Distribution Models (CEDMs). Utilization of CEDMs to evaluate gene flow in the GH rely on the Cool Climate model as a critical factor determining the origin and genesis of tepui biota. The approach relies on the assumption that climate and temperature tolerance determines the distribution of animal species, within a short enough time frame to eliminate adaptation and evolutionary change as a variable. ( Ie restricted to thousands of years, not millions). Technical work has been done by Peterson and Nyari (2008) and Marnaval and Moritz (2008) to test the location and existence of refugial stable regions, both contemporary, future, and prehistoric. The principles which CEDMs utilize are based entirely on known environmental data, cross referenced with the “environmental envelope” of target species ( their tolerance and current ranges), which is then computed using specific algorithms to determine the distribution of that species under less than their ideal conditions.

Modeling the CEDMs for pantepui organisms is difficult due to the degree of endemicity in the tepuis, where species are known from single tepuis, and are not widely dispersed. As a result of this high endemicity, CEDMs for many species can not be created, as their range is far too restricted. To solve this problem, Carnaval and Moritz (2008) preformed CEDMs for habitats rather than species to locate refugia during the LGM in the Brazilian forests. The current authors utilized a similar methodology to establish a Pantepui CEDM. The algorithm used was MaxEnt 3.2.1, a maximum entropy based machine learning algorithm. 10,000 randomly assigned distribution points from the region were generated with GIS data from DIVA-GIS 5.4. Altitudinal points were assigned two classes; upland and tepui, representing 800-1500 meters and +1500 meters respectively. Climate data was obtained from WorldClim database. Paleoclimate modeling utilized the General circulation Model (GCM) from the Community Climate System Model based on lower temperatures provided by R.J Hijmans at the Community Earth Systems Model website. A series of precise parameters and programs were utilized to determine the change in ecosystems during the Pleistocene and projected into the future.

Results from climate modeling indicate that the current upland and tepui climates were far more extensive during the LGM, allowing for the potential movement of fauna between tepui summits and into the highlands. Thus, the possibility of a downward migration and intermixing of tepui biota during the LGM and similar cold periods is possible. What is interesting is that the similarities between summit herpetofauna ( such as the similarity between the Eastern summits of Los Testigos, Aprada, Chimanta and Auyan) is supported by a paleoenvironmental envelope extending between these summits during the LGM. Effectively, the Pantepui region became almost entirely composed of tepui and upland climatic conditions 21,000 kya, with tepui summit conditions occupying the entire currently defined uplands. During this period of time, the two climatologically similar areas of the Eastern and Western tepui could be classified as two separate islands, irrespective of the geographical boundaries. This leads to a grouping of two herpetofaunal assemblages, one in the west and one in the east, which supports the morphological data regarding summit herpetofauna. This grouping could represent two subsets of tepui fauna, indicating a divergence in the overall trend of having distinct GH fauna, when compared to other South American herpetological assemblages.

The authors conducted a CDEM utilizing the same methods used for LGM spatial analysis, but with data from the Special Reports on Emissions Scenarios (SRES) by the IPCC. The climate models were based on CCCMA, CSIRO, and HADCM3 climate and temperature models. (Flato et al 2000, Gordon et al 2000). The SRES has developed different models for future emissions, based on four basic scenarios.⁷ The study discussed utilized only scenario A2a and B2a. Both A2a and B2a represent environmentally conscious future realities, with B2a utilizing regionalized solutions for development and environmental sustainability, while A2a is a less environmentally conscious reality. The results indicate that nearly all tepui climate envelopes will disappear by the year 2080, with appreciable reduction in tepui summit ecosystems by 2050. This prediction is based on the restricted altitude of the tepuis, making vertical migration impossible due to the lack of spacial movement upwards. The issue of extinction is therefore very real, as the adaptability of tepui herpetofauna is unknown, as is their climate tolerance. However, if the vertical downward migration and intermingling of GH herpetofauna during the LGM is accurate, it can be assumed that these species thrive in colder climates, and do poorly in warmer conditions. The adaptability of herpetofauna over a short period of time (ie hundreds of years) to adapt to a 1.8-5 degree Celsius temperature change is unknown, but seems unlikely. The average temperature change during post glacial periods is 0.025 degrees Celsius, several orders of magnitude below what is predicted to occur during the next hundred years.

Analysis

The pertinent question arising from this analysis of biogeography is which of the models is the most accurate. It is unlikely that any single model can account for the sum totality of tepui biota. One must consider the tepuis in terms of their recent geological history, with in the paleoecosystems of South America. First, the tepuis are relatively depauperate of large vertebrates due to the heavy precipitation which leads to rapid soil erosion. Secondly, their origin and subsequent differentiation has created a variety of habitats ranging from the stone forests of Roraima to the dense forests on the summit of Auyan. Based upon the assumption that all tepuis share a common origin in the Guyana massif, a cladogram was constructed by Dr. Funk in 1990 detailing the isolation of the various tepuis into individual summits. The differentiation begins with the establishment of the Rio Caura and Rio Paragua. The establishment of these rivers systems in their current drainage pattern has been linked to the Miocene uplifts of the Andes.⁸ The tributaries of the Orinoco river such as the Ventuari and Caroni differentiated the larger Roraima group into smaller tepuis such as Neblina and Duida in th west, and Chimanta and Roraima in the east. While the change in the orientation of the Orinoco river (from the Maracaibo to its current position) is well known, the exact date of the establishment of current tepuis is widely variable. Funk superimposed a cladogram for the monophylitic Stenopadus over the tepui cladogram, and the match indicates that the ancestral species was distributed at a high elevation, and the current forms differentiated through vicariance and allopatric speciation. The single Stenopadus anomaly in the cladogram concerns a lowland species found between Chimanta and Roraima. This cladogram indicates that summit establishment creates individual species which do not hybridize. This indicates a long isolation of derived species in most cases, with the lowland species near Roraima suggesting that vertical displacement resulting in speciation is also a factor. A critical element in determining which models fit the tepuis is the relative age of tepui establishment. Obviously, the tepui ecosystems are not divorced from the geological processes which effect South America. The process which started the uplift of the Guiana shield are the same tectonic forces which split up Pangaea, and created the Andes mountain range. This process began in the Jurassic, and by the Cretaceous northern South America was firmly located in the Intertropical Convergence Zone. The break-up and erosive forces which carved the tepuis accelerated from the Cretaceous into the Miocene,(?) and slowed during the the last thirty million years.⁹ With the rise of the Andes mountains, the westward flow of air currents was stopped, creating a more seasonable climate. Furthermore, the entire drainage patterns of the Orinoco river delta changed with the uplift of the Andes, and the creation of rivers to accelerate the erosion of the GH into the current tepuis. It has been established that the Amazon and Orinoco basins originated as early as 100-80 mya.¹⁰ Thus, tepui biota at the time was contemporary with the the biota of South America as the tepuis developed, to some degree. Given this observation, the lack of paleofauna on the tepui summits indicates that the process of biological establishment and extant biological communities on the tepui summits are in no obvious ways divorced from evolutionary processes of the rest of South American fauna. If, as suggested by Maguire, tepui biota was established around the time of the uplift and erosion of the Guiana Shield, we could expect to see elements of paleofauna and paleoflora. However, with the possible exception of Oreophrynella, no such organisms exist that any survey team has yet collected. The genus Oreophynella was speculatively assigned paleofaunal status by both Rivero (1970) and Hoogmoed (1979), as the distribution of the toad occurs exclusively in the Guyana Highlands, and seems to have speciated primarily on the tepuis.¹¹ The indication of hybridization and gene flow between tepui summit communities suggests that vertical migration the genus has occurred in the past. Phylogenetic analysis is not presently available on these organisms, although techniques of comparative analysis show some gene flow between populations on proximal tepuis. The absence of paleofauna, which to a large degree invalidates models indicating long term isolation, is a matter of two questions: Are the tepuis too small to maintain a sustainable population of larger organisms, or does the vertical displacement of organisms play a larger role than previously thought?

Tepuis vary in size dramatically, but the largest one is Auyan tepui, with a relatively continuous summit topography of 900km2.¹² In his work “Lost world of the Guiana highlands”, describes the tepuis as being rain deserts, where the erosion of soil due to continuous heavy rain washing sediment down to the lowlands inhibits the growth of rich forests.

The implications of this are that some members of GH fauna may represent localized speciation of relative antiquity compared to the rest of South America. Among the groups which may have been part of the initial faunal radiation are Oreophrynella, Pristimantis, Anomaloglossus and the snake Thamnodynastes. These groups seem to have distinct related species on separate tepuis, and could comprise a group established in situ over a long period of time, rather than representing lowland invaders who established themselves on the tepui summits. If the climate models are correct, then their ranges down upland regions was more expansive during colder climatic conditions. The establishment in specific niches would then limit the expansion of invading lowland species from the tepuis, preserving the native diversity from expansions of amphibian fauna from the Andes and the amazon.¹³ Now that the paleoclimatological models have been assessed, the tepui ecosystems must be monitored for predicted Anthropogenic climate change and warming. The conservation of the region has been noted, and Pantepui is recognized as one of the WWF/IUCN Neotropical Plant Diversity centers. More than 70% of the GH is under official protection.¹⁴ However, the nature of global climate change does not preclude the GH from extinction events which can be mitigated by land conservation alone.¹⁵ Rulls' estimation of a 30%+ extinction rate over 100 years with a temperature change of 2-4 degrees necessitates new techniques in conservation to be undertaken to avoid unacceptable biodiversity losses.

Conclusion

The work is very timely; it coincides with the plethora of research and analysis by Rull and other scientists who strive to understand the dynamics of the pantepui region. It confirms the models of vertical migration which for the most part analyzed flora as opposed to fauna. As such, the article is a welcome addition to the little understood evolution of tepui fauna. That being said, the authors indicate that further research will need to be conducted to assess both the phylogeny of the tepui herpetofauna as well as the potential impacts of global warming on the region. If their climate envelope models are accurate, then this will need to be done rapidly to avoid short term catastrophic extinctions in the region. Quite possibly, and international effort may need to be undertaken to protect the tepuis, and other mountain ecosystems across the world.

1McDiarmid & Donnelly, 2005 reference

2Mayr and Phelps 1967

3Mayr and Phelps 1967, Hoogmoed 1979

4Maguire 1970, Brewer Carias 1978

5Steyermark & Dunsterville 1980, Rull 2004, 2005 etc.

6Page 404. References to Hoogmoed 1979, McDarmid and Donnely 2005, MacCulloch at al. 2007

7 The SRES developed a variety of scenarios for future energy production and human narratives, which are used to determine future environmental impact based on predicted human behavior. See the IPCC website SRES Emission Scenarios at http://sedac.ciesin.columbia.edu/ddc/sres/

8Andean tectonics as a cause for the changing drainage patterns in the Miocene northern South America. Carina Hoorn et al.

9Book, Lost world of the GH

10Edmond et al 1995

11Josepha Clesa Senaris, Papies Zoologocios.

12Verify this fact

13Santos JC et al 2009.

14Berry at al 1995

15Rull- just about everything.