Monday, December 14, 2015

The Sonoran Desert Toad

"Bart: 'Dad, are you licking toads?'  Homer: I'm not NOT licking toads." - The Simpsons

The Sonoran Desert Toad, Bufo alvarius, note enlarged paratoid glands

The Sonoran Desert Toad is a large amphibian native to the lower Colorado River area of Arizona's Sonoran Desert. It is large, green, and can be quite aggressive if provoked. It has a special surprise for predators that try to make a meal of a what would at first glance be a feast for many a desert animal. 

Look but don't touch, and definitely don't lick this toad. 

The Sonoran Desert Toad has been made famous because of its potent toxins. Dog owners report seizures, high fevers, and rapid heart beat in their dogs following unfortunate encounters with this toad.

In fact the poison is so toxic that the Sonoran Desert Toad is responsible for more canine deaths per year than Rattlesnakes. Remember to keep an eye on your dog during summer monsoons so that you can avoid any unpleasant experiences with this amphibian. 

Because of its potent toxins the Sonoran Desert Toad has made its way into pop culture. Any references to "licking toads" are often referencing this species, which produces copious amounts of the potent neurotoxin in its enlarged paratoid glands (the fat pockets behind the ear in the first picture). So yes, this desert amphibian has even been referenced on an episode of The Simpsons, though the writers did not identify the Sonoran Desert Toad specifically as the one Homer "did not not lick". 

Some anthropologists have suggested that ancient peoples of mesoamerica used a toad as a ritualistic hallucinogen, citing mythological representations of toads. If the peoples of mesoamerica did use toads in ritualistic ways, it was likely this one due to the potent, hallucinogenic nature of its toxin. In the past some suggested that the Cane Toad, found further South, could have been used in this way, but its toxin is more a pure poison than a hallucinogen. 

 Many media outlets have reported that people have resorted to extracting this toad's toxin and smoking or ingesting it to gain a desired hallucinogenic experience. Given the high toxicity of this toxin and the contracting range of the species this action can be both illegal and stupid. 

Interestingly, the toad's toxin is not a banned substance but drug enforcement officials have prosecuted drug offenders by inciting bans on the exportation of this toad(in States where the toad is not native). The Sonoran Desert Toad is protected across some portion of its range, so officials have used the legal protection of this toad to prosecute drug offenders who would use the toxin for a cheap high.

References and Further Reading:

Musgrave, M. E., and Doris M. Cochran. "Bufo alvarius, a poisonous toad."Copeia 173 (1929): 96-99.

Hanson, Joe A., and James L. Vial. "Defensive behavior and effects of toxins in Bufo alvarius." Herpetologica (1956): 141-149.

Weil, Andrew T., and Wade Davis. "Bufo alvarius: a potent hallucinogen of animal origin." Journal of ethnopharmacology 41.1 (1994): 1-8.

Wednesday, December 2, 2015

Endangered Habitats: Joshua Tree National Park

"But those trees! Those Trees!... All my life I've been searching for trees such as these!" -Dr. Seuss, The Lorax

Lost Palms Oasis in Joshua Tree National Park

Today I'm launching a new and regular feature of this blog. A focus on habitats and special areas in desert systems which are threatened. Without complete and intact habitats we would not know much that we currently do about how the world functions. I've focused much on adaptations which desert animals possess and some ecological interactions in desert systems; but from time to time it is important to remember that without relatively undisturbed, intact habitat for these organisms we would lose so much of what we value in these unique systems.

Importantly, when it comes to conservation of intact ecosystems we must often fight many unique battles to preserve these special places. The stakes are especially high in these battles as margin for error is minimal. It is not easy to restore and ecosystem and it is impossible to undo extinction. Too many times in conservation do we forget that sometimes losing even a single battle amounts to losing the war. In this sense, conservation can be likened to evolution where the stakes are equally high for individual organisms.

I'll start this habitat spotlight with the place where I first experienced a true desert wilderness. Joshua Tree National Park in California's Mojave Desert.

Joshua Tree supports a diverse group of desert organisms, being the meeting point and melting pot for two vastly different systems, the Sonoran Desert (Colorado subsection) and the Mojave Desert. The boundary of the two is readily apparent. In the higher sections of the park in the Mojave Desert, the namesake plant dominates. In lower sections Creosote dominates the landscape.

Fan palm oases dot the park and provide unique habitats to creatures which otherwise would not be able to live in the harsh landscapes. Water bubbles to the surface and provides critical habitat to Red Spotted Toads and California Treefrogs as well as native California Fan Palm Trees.

Even a place that is formally protected such as Joshua Tree is still faced with a plethora of threats externally and internally.
The Joshua Tree, Yucca brevifolia

First and foremost is the threat of climate change which has the potential to drastically reduce the range of Joshua Trees. In fact warmer temperatures have the potential to push Joshua Trees to extinction within Joshua Tree National Park.

Ironically the people who love Joshua Tree National Park so much also have the potential to alter the systems here. The park is a geologic wonderland and is popular with rock climbers who visit from all over the world to climb here. Surveys of popular climbing routes show that ecological communities of plants and birds in and around popular climbing areas are altered compared to similar communities which are not heavily frequented. Loving a natural area to death is a very real threat, especially in fragile desert parks where footprints can mark the landscape for years.

What can be done to address these threats? Acting on climate change will be important if we hope to protect the biological feature the park was originally intended for. All of the formal protection in the world will not save a plant from a system that will no longer support it biologically. Making sure that certain critical ecological areas are subject to limited use by climbers and hikers could also be important for the future of this park which is only a two hour drive from almost 20 million people.

References and further reading:

Cole, Kenneth L., et al. "Past and ongoing shifts in Joshua tree distribution support future modeled range contraction." Ecological Applications 21.1 (2011): 137-149.

Camp, Richard J., and Richard L. Knight. "Rock climbing and cliff bird communities at Joshua Tree National Park, California." Wildlife Society Bulletin(1998): 892-898.

Camp, Richard J., and Richard L. Knight. "Effects of rock climbing on cliff plant communities at Joshua Tree National Park, California." Conservation Biology12.6 (1998): 1302-1306.

Thursday, November 5, 2015

Canyon Tree Frogs and Cryptic Species

"Trust not too much in appearances" -Virgil

A Canyon Tree Frog calling to a potential mate, photo by Jessica Phelps

What does "home" sound like? For me, the sound of home is the call of a Canyon Tree Frog. A rather odd call which could be described as a cross between the sound of a domestic sheep and a duck. I recall one night camping alone on a remote, burned over plateau in the Great Basin Desert. I went there to explore and for perspective. Sometimes nothing gives you perspective better than getting as close as possible to the natural forces that shaped us. This is probably why I love the deserts so much, as it seems here those forces are especially apparent even to casual observers. I remember the sun setting and feeling perhaps unsettled; similar to how people often do when they are alone after dark. A few minutes later I heard the call of a single frog, easily identifiable as a Canyon Tree Frog. Eventually several joined, then more, and even more. Eventually I could hear the calming call of hundreds of "Tree Frogs", though there was no living tree in site. I've since returned and camped here several times and have never found the frogs, though their incessant calls tell me they are there. Though I admittedly haven't tried very hard, perhaps worried that gazing upon frogs that no human has yet seen will somehow diminish them.

Okay. I'm getting a little too poetic and emotional for a science blog right?

You get the point. I am sentimental about these frogs. I love them. 

A "tree" frog where there are no trees, the Canyon Tree Frog has found a way to make an arboreal living in an often terrestrial landscape. These semi-aquatic frogs climb on canyon walls instead of vegetation, their coloration often varying depending on the color of the canyon they inhabit (pinkish brown if sandstone, grey if granite). These wall-climbing frogs also seem to be fairly intelligent, moving large distances between suitable bodies of water in patterns not typical of random dispersal. In other words, they KNOW where they are going.

This widespread "species" also allows us to delve into another paradox in evolution. Sometimes two species can look the same but can be quite different. These are called "cryptic species", species that look the same morphologically but cannot interbreed; hidden species. The Canyon Treefrog as we know it might actually be several cryptic species. 

Canyon Tree Frogs are widespread but their range is also choppy and intermittent in nature because of the desert springs and streams they occupy. They are found throughout the American Southwest and into Mexico but these regions have diverged quite substantially based on genetic evidence. 

In addition to this genetic differentiation, there are also subtle differences in the calls of these lineages which may not be enough for a human ear to pick up on. To a female frog though, these differences mean the world. A frog found in the southern lineages strongly prefer the calls of her own locality, and don't like the calls of other Canyon Tree Frog localities. Even something as simple as not speaking the same "language"; could be considered a barrier to reproduction and thus an example of speciation. These data are supported by the genetic data which supports strong divergence between geographical localities of Canyon Tree Frogs.

Sadly, these frogs are threatened by many of the same forces threatening amphibians worldwide. A changing climate, extreme variability and drought, and disease are all threats to this and other amphibian species. For my sake though, I hope that every time I return to that camp site I will be able to hear my frogs.

Further reading and references:

  • Kay, David W. "Movements and Homing in the Canyon Tree Frog (Hyla Cadaverina)." The Southwestern Naturalist 34.2 (1989): 293-5. Web.
  • KLYMUS, KATY E., SARAH C. HUMFELD, and H. CARL GERHARDT. "Geographical Variation in Male Advertisement Calls and Female Preference of the wide‐ranging Canyon Treefrog, Hyla Arenicolor." Biological Journal of the Linnean Society 107.1 (2012): 219-32. Web.
  • Barber, PAUL H. "Phylogeography of the Canyon Treefrog, Hyla Arenicolor (Cope) Based on Mitochondrial DNA Sequence Data." Molecular Ecology 8.4 (1999): 547-62. Web.
  • Klymus, Katy E. "Phylogenetic and Behavioral Differentiation in the Canyon Treefrog, Hyla Arenicolor." ProQuest Dissertations Publishing, 2011. Web.
  • Bradley, GA, et al. "Chytridiomycosis in Native Arizona Frogs." Journal of Wildlife Diseases 38.1 (2002): 206-12. Web.

Saturday, October 17, 2015

Metapopulation Dynamics

"When we try to pick out anything by itself, we find it hitched to everything else in the universe." -John Muir

Zion National Park has suffered many more extinctions of mammal species than the much larger Yellowstone National Park. Probably because Zion's area is so much smaller than Yellowstone's.

How can scientists explain the persistence, movement, and dispersal of populations of organisms in a complex and dynamic landscape? How much habitat and how interconnected must habitats be in order to sustain healthy self-sustaining populations of native species? In order to better describe and test the dynamics of populations in complex environments Richard Levins described the idea of a "metapopulation" in 1970, which he described as "a population of populations". In this "population of populations" some habitats are occupied by a species, others are not. Some populations go extinct and are perhaps re-established later, while others persist for long periods of time in spite of the constant flux of individuals.

Why is this idea central to the understanding of desert organisms? The answer lies in the variability of desert environments, even at small scales. Take for instance a desert fish, one which is isolated in only one small stream system. Movement of individuals into other populations is rare, sometimes absent. Perhaps heavy rains and flooding make it possible for this fish to travel long distances and colonize new habitat? The reality of this fishes' world is that there is a small area of suitable habitat(its stream), and large areas of non-existent habitats(the desert), perhaps with small and rare opportunities for dispersal (heavy rains and flash floods connecting drainages).

Think now of a large predator, perhaps a Mountain Lion or bear whose habitat is a high mountain range surrounded by miles of barren desert. The same sort of situation exists here at a larger scale. In order to move into other habitat these animals must traverse miles of unsuitable habitat before finding solace in another desert mountain range.

This is the crux of metapopulation theory. Dynamic populations in complex environments; some populations waning from mortality and emigration while others are increasing from recruitment and immigration. You can now realize why this is so valuable for understanding desert systems; where crucial habitat is often interrupted by areas that are completely unsuitable for the life cycle of certain species. Be this a desert spring surrounded by arid lands, a mountain range surrounded by a sea of cacti, or wild lands interrupted by a large city. Desert species are often composed of many populations which may interact with each other infrequently, but these infrequent events may be crucial to the maintenance of those populations.

In some cases humans have interrupted these important processes and interactions. In many populations of Desert Bighorn Sheep that have been isolated from other populations due to a human improvements (roads, cities, etc) there has been a marked decrease in genetic diversity. This lack of diversity could be a problem for these populations going forward.

Desert Bighorn Sheep have experienced a decrease in genetic diversity where their populations have been isolated by human establishments.

It is often crucial that habitat is of sufficient size in order to maintain healthy population (and metapopulation dynamics). For instance, if conserved habitat is of insufficient size and separated from other important habitat by a large spatial distance local extinctions are likely to occur. The Western National Parks provided an important model to test this idea. Zion National Park, a relatively small protected area has suffered five times as many mammalian extinctions as the much larger Yellowstone National Park. Scientists have found that protected area (or suitable habitat) size is often the best predictor of extinction risk, rather than park heterogeneity or differences in elevation. When it comes to protecting habitat, size does matter. Larger, intact habitats are more likely to enable persistence of populations. Populations in smaller, more
disconnected habitats are less likely to recover from losses due to unforeseen circumstances.

So when you visit your local natural area there is often more at work than meets the eye. Protection of a small area often holds no guarantee that healthy populations will persist there, especially if the area is not connected to other important habitats. Increased variability and habitat changes due to climate change could threaten many desert species and populations. Where can bears and other predators go as the deserts climb up the mountains? What about endemic aquatic species found in desert springs as their habitats dry and change?

References and further reading:

Hanski, Ilkka. "Metapopulation dynamics." (1997).

Epps, Clinton W., et al. "Highways block gene flow and cause a rapid decline in genetic diversity of desert bighorn sheep." Ecology letters 8.10 (2005): 1029-1038.

Newmark, William D. "Extinction of mammal populations in western North American national parks." Conservation Biology (1995): 512-526.

Fagan, William F., et al. "Rarity, fragmentation, and extinction risk in desert fishes." Ecology 83.12 (2002): 3250-3256.

Hellgren, Eric C., David P. Onorato, and J. Raymond Skiles. "Dynamics of a black bear population within a desert metapopulation." Biological conservation122.1 (2005): 131-140.

Wednesday, October 7, 2015

Saguaro Cacti

"You're gonna go far kid" -The Offspring

A mature Saguaro, Arizona

Saguaro Cacti are emblematic of the Sonoran Desert. These desert plants can grow to over 50 feet tall and live for well over 100 years, but first they have to make it through the first few years of life.

Even the most regal Saguaro had to start somewhere. In the first few years of a Saguaro's life survivorship is incredibly low. The high temperatures, intense sun, wind, and browsing animals can all be potential threats to young Saguaros.

Because young Saguaros are so vulnerable to these threats most successful Saguaros take shelter under "nurse plants" like brittlebush and creosote. These plants provide a microclimate for the young Saguaro complete with shade and protection from grazing animals. The use of a nurse plant could be important for other species of cacti as well. Would you consider the use of nurse plants to be a type of parasitism?

One might be apt to call a young Saguaro a "parasite" when it utilizes the micro-habitat provided by the nurse. In many cases the nurse plants will die entirely after as the cactus grows and starts to utilize the majority of the water and other resources present in that area.

Because of this there is an evolutionary pressure for shrubs in the desert to NOT become nurse plants. Being a nurse plant will ensure that there is competition for resources and water over your lifetime, a competition that a nurse plant might not win. In fact nurse plants OFTEN lose the competitive battle for resources as young Saguaros mature.

A famous "50 armed" Saguaro in Central Arizona

The Creosote Bush, Larrea tridentata, might be a good example of a plant which has evolved to avoid becoming a nurse to Saguaros and other cacti. Creosote bushes exhibit allelopathic interactions with neighboring plants. Allelopathy is the inhibition of the growth of neighboring plants by disrupting them in some way. It is thought that Creosote roots exude a toxin that is detrimental to the growth of nearby plants. Creosote bushes also provide limited shade, it is possible that this lack of shade makes them bad nurse plants as well. Research shows that shade is absolutely critical to the survival of young Saguaros. Whatever the reason, Saguaros tend to not take root underneath Larrea tridentata as readily as they would under other plants.

Like many other desert species, climate change is likely to have tremendous impacts on Saguaro Cacti including dramatic range shifts. The Saguaro's range may diverge tremendously due to climate change, with new habitat opening to the east and to the west while current habitats becomes less and less favorable. Will this potential divergence create two distinct type of Saguaros? What will happen to Saguaros if their future range fails to coincide with the range of important nurse plants?

References and further reading:
Turner, Raymond M., et al. "The influence of shade, soil, and water on saguaro seedling establishment." Botanical Gazette (1966): 95-102.

Drezner, T. D. "Plant facilitation in extreme environments: the non-random distribution of saguaro cacti (Carnegiea gigantea) under their nurse associates and the relationship to nurse architecture." Journal of Arid Environments 65.1 (2006): 46-61.

Callaway, Ragan M. "Experimental designs for the study of allelopathy." Plant and soil 256.1 (2003): 1-11.

Shafer, Sarah L., Patrick J. Bartlein, and Robert S. Thompson. "Potential changes in the distributions of western North America tree and shrub taxa under future climate scenarios." Ecosystems 4.3 (2001): 200-215.

Tuesday, September 15, 2015

The Devils Hole Pupfish

"Living wild species are like a library of books still unread. Our heedless destruction of them is akin to burning the library without ever having read the books." -John Dingell

Devils Hole Pupfish. Photo by U.S. Fish and Wildlife Service

The Devils Hole Pupfish is but one member of an extraordinary group of fishes. Fishes which may at first glance seem unremarkable. Looks can often be deceiving, and this fish has had profound impacts on the history of the United States, and its story is puzzling to many biologists. Many years ago this fish had its day in front of the United States Supreme Court when groundwater pumping threatened its only natural habitat. Ultimately the Court sided on the side of conservationists, stating that the Federal Government owned the water rights associated with Devils Hole in order to preserve the resources therein. 

The evolutionary history of this fish and its ecology are far from unremarkable. The story of this fish merges biology, politics, anthropology, and conservation. Remember that the desert region of the Southwest is in terms of earth's history very new. A few thousand years ago huge lakes covered much of the region. Nevada, now the driest state in the United States was covered by parts of two gigantic lakes: Lake Bonneville and Lake Lahontan. The climate was generally much wetter and cooler in the Southwestern United States. Death Valley was covered by Lake Manly and Tecopa Lake. Many fish lived in these large lakes. 

The entire area started to warm and dry due to climatic factors. The lakes mentioned above started to disappear, leaving behind only remnants of their former selves. Fish in these lakes became caught in small thermal springs as deserts formed and started on new evolutionary tracts shaped by the living and non-living forces present in their unique habitats. Eventually these fish became distinct from their relatives in other springs. This is the story of most desert pupfish, this group of fish constitute some of the most endangered animals on the planet. Many species in this group have already been forced into extinction. Presumably the Devils Hole Pupfish were then trapped in their cavernous warm spring as Lake Manly receded in Death Valley?

Devils Hole, Nevada. 

Except that there is much evidence that current drainages and springs with other pupfish were at a time interconnected. There is NO EVIDENCE that Devils Hole was ever connected to another water source within the last 100,000 years. This presents a problem for many biologists who suggest that the fish separated from its ancestors well less than 100,000 years ago and some even guess that this separation might have been as recent as 360 years ago. This fish somehow colonized a secluded spring relatively recently with significant separation from other water sources and ZERO geologic evidence that it was ever connected to another water source during this time frame.

How then did this fish get into Devils Hole? Given that no evidence suggests that there was ever a way for this fish to cross a kilometer of dry land and make it into Devils Hole many biologists refuse to discount the hypothesis that humans, perhaps native peoples of the area, moved fish from another nearby spring into Devils Hole. The harsh environment of Devils Hole then provided the pressures which made this fish different from its relatives.

Recently biologists have begun to question whether the environment provided the pressure for these changes in the form of natural selection or whether the conditions at Devils Hole alter the development of this fish in a meaningful way (fish here have large heads, small bodies, and tend to lack pelvic fins). When environmental forces alter the expression of genetic material in the overlying organism we call it "phenotypic plasticity". Phenotypic plasticity could be an important reason why these fish look so different from their close relatives.

How did fish get into Devils Hole? How did they become so different from their close relatives? What role did the environment play and over what time period?

Better examination of the biology of this fish may help scientists better understand exactly what constitutes a 'species' and the role that environmental stresses play in organismal development and the evolution of populations. 

The Devils Hole Pupfish has the most restricted range of any vertebrate, and the entire population of this species is often less than 100 individuals. Luckily we began to "read the books in this library", but we almost didn't get the chance. The continued efforts of conservationists and ultimately the decision of the Supreme Court ensure that we are able to continue reading these books which may hold the answers to big questions regarding ecology and evolution.

References and further reading:

Reed, J. Michael, and Craig A. Stockwell. "Evaluating an icon of population persistence: the Devil's Hole pupfish." Proceedings of the Royal Society of London B: Biological Sciences 281.1794 (2014): 20141648.

Riggs, Alan C., and James E. Deacon. "Connectivity in desert aquatic ecosystems: The Devils Hole story." Conference proceedings. Spring-fed wetlands: important scientific and cultural resources of the intermountain region. Vol. 11. 2002.

Deacon, James E., Frances R. Taylor, and John W. Pedretti. "Egg viability and ecology of Devils Hole pupfish: insights from captive propagation." The Southwestern Naturalist (1995): 216-223.

Phillips, Kathryn. "WHAT MAKES DEVILS HOLE PUPFISH SPECIAL?."Journal of Experimental Biology 209.18 (2006).

Lema, Sean C. "The Phenotypic Plasticity of Death Valley's Pupfish Desert fish are revealing how the environment alters development to modify body shape and behavior." American Scientist 96.1 (2008): 28-36.

Thursday, August 27, 2015

Kangaroo Rats

"He that has satisfied his thirst turns his back on the well." -Baltazar Gracian

A Kangaroo Rat, note the large hind limbs.

Kangaroo Rats are an interesting little desert rodent, with a variety of adaptations to desert living.  Perhaps the most readily observable is their morphology (body shape) and their method of locomotion. These animals have large, powerful hind limbs. Just like the Collared Lizards we previously discussed Kangaroo Rats never miss leg day at the gym. They exploit bipedal motion just like the Collared Lizard. Although rather than bipedal running the Kangaroo Rats tend to hop, like their Aussie namesakes. They leap about 1-2 feet per hop when they are in a hurry. No small feat for a small animal such as this. These powerful hind limbs also allow them display impressive vertical leaps when frightened or threatened by a predator. One kangaroo rat was observed to jump 5 feet horizontally and over 2 feet vertically. Based on their body size this is the equivalent of a six foot human leaping 60 feet on the long jump while simultaneously reaching 24 feet vertically. Now that's a leap!

Kangaroo Rats also have some pretty awesome adaptations to live in desert environments. They have very low metabolic rates and a low thermal conductance compared to a closely related species found in wetter environments. This helps them thermoregulate and minimize water loss. This might indicate that their adaptations to desert life happened relative late in their evolutionary history. Which makes sense because North American deserts haven't been around for very long relatively speaking.

Perhaps most amazingly is that the Kangaroo Rats REFUSE to drink, even when water is available to them. Seems counter-intuitive considering they live in some of the driest areas in the world. How can Kangaroo Rats do this without succumbing to their harsh environment?

Kangaroo rats gather dry seeds and then bring them back to their burrow to store and consume them later. Their burrows are much more humid than the outside environment, and that atmospheric water moves into the stored seeds. When the Kangaroo Rats eat the seeds they are full of water from their humid burrows. The behavior and physiology of the Kangaroo Rat complement each other in such way that help it easily survive in the desert, even without drinking a drop of water in their lives.

References and further reading:

Schmidt‐Nielsen, Bodil, and Knut Schmidt‐Nielsen. "A complete account of the water metabolism in kangaroo rats and an experimental verification." Journal of Cellular and Comparative Physiology 38.2 (1951): 165-181.

Bartholomew, George A., and Herbert H. Caswell. "Locomotion in kangaroo rats and its adaptive significance." Journal of Mammalogy (1951): 155-169.

McNab, Brian K. "Climatic adaptation in the energetics of heteromyid rodents."Comparative Biochemistry and Physiology Part A: Physiology 62.4 (1979): 813-820.

Wednesday, August 12, 2015

Divergence and Speciation

"There is grandeur in this view of life, with its several powers, being originally breathed into a few forms or into one... from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." -Charles Darwin, On the Origin of Species

A Diamondback Rattlesnake, 13 different species of Rattlesnake inhabit Arizona. Why so many? What forces led to the diversification and speciation of Rattlesnakes in the Southwest?

The question of how and why species diverge and differentiate is one of biggest questions in evolutionary biology. Basic understanding of speciation and divergence in many way's starts to address life's big questions, providing insight into why their is such a diversity of life on earth, and what forces led to the evolution of humans and humanity. 

First we must ask the question, "What is a species?". This question has been subject to much debate over the years. Most biologists subscribe to the "biological species concept" which defines a species as a group of potentially interbreeding individuals. Even this widely accepted view can be problematic at times, for instance what constitutes a species in groups of asexually reproducing organisms such as bacteria or Whiptail Lizards? Is geographic isolation enough e.g. if two lizards are separated by a canyon, or 500 miles can we consider them separate species simply because they are technically "reproductively isolated" due to geology or geography? You can see that defining a species can be problematic, but nevertheless the concept of species is an important tool for biologists. For our purposes we will consider reproductive isolation from other groups of organisms to be paramount in the discussion of speciation. How does reproductive isolation arise between two populations? 

There area many possibilities that can lead to the separation of one species into two (or many), below we will discuss a few which probably have played major roles in Desert regions and systems.

1.) Geologic barriers inhibiting or stopping gene flow
The Desert regions of the United States are famous for their geologic heterogeneity (variability). A high mountain range, an extremely low and hot valley, a Grand Canyon all have the potential to make interbreeding physically impossible between groups of otherwise closely related individuals. If this separation of the two groups is prolonged genetic and morphological differences can accumulate at random (due to genetic drift) and due to similar or differing selective forces (natural or sexual selection). 

Abert's Squirrels and Kaibab squirrels are separated by the Grand Canyon. The Colorado River eroding the canyon and the uplift of the Colorado Plateau made interbreeding of the squirrels on the South Rim and North Rim of the canyon impossible. Mutations and genetic differences accumulated in the North Rim and South Rim squirrels. Now these two squirrels look quite different. 

This type of divergence has been common in aquatic desert organisms as stream flow has changed and pleistocene lakes receded leaving many aquatic organisms to be isolated from each other in distinct drainages or springs separated from other groups by miles of barren desert.

2.) Ecological Speciation
Australian deserts have a large diversity of lizards, but no obvious geologic barriers currently exist or have been known to be present in the desert regions there. How then have lizards diverge there despite a lack of a physical barrier? 

The answer likely lies in the diversity of habitats that exist in deserts there. Rocky habitats, sand plains, and shrub-acacia deserts are a few of the habitats found here. As lizard populations adapt to localized conditions they can often become obligates to a certain habitat type. In other words the barrier that exists between lizard species is ecological rather than geologic. Lizards under differing selective pressures have become separated here because selection has forced them down quite different evolutionary roads.

This same diversity of habitats is present in the American Desert regions: bajada and arroyo habitats, creosote flats, sand dunes, rocky habitats, and wetland habitats all exist here. Many of these habitats do have one or two lizard species typical of that habitat. So perhaps ecological separation has also shaped lizard evolution here as well.

3.) Hybridization
Ahh.... the Whiptail Lizards of the Desert Southwest. Ubiquitous throughout the desert and sometimes uninteresting in their appearance. These are not Gila Monsters (big, colorful, and venomous) or Collared Lizards (bright blue and eat lizards). I have at times been guilty of glancing at a lizard darting in a jerky motion through the bushes and thinking "It is JUST a Whiptail". 

However, when it comes to their reproduction, evolution, and classification it is hard to think of a more magnificent lizard. Again, what constitutes a species when we are talking about individuals which reproduce asexually? 

In this group of lizards there are multiple species which reproduce asexually and multiple species which reproduce sexually. A closer look at their evolutionary relationships proves even more confusing. Often times the closest relatives of an asexual Whiptail are a group of sexual Whiptails. In other words asexual reproduction has arisen AND persisted multiple times in Whiptail Lizards.

The likely cause of rapid speciation in this group of lizards is likely hybridization. One species of Whiptail mates with another species of Whiptail, and the daughter is a fully asexual NEW species! This means that the separation or speciation takes place in a single generation; speciation that is spontaneous! You see evolution can be quite rapid, it doesn't have to take thousands or millions of years. We can test it experimentally and observe it in our lifetimes. In the case of Whiptail Lizards a single mating event can sometimes spur an entirely new reproductively isolated species of lizards. 

I can't help but end in the same way I began, with a wonderful quote from Darwin's masterpiece which I think provides a simple observation regarding speciation and evolution, "....from so simple a beginning.... ENDLESS forms most beautiful and wonderful".  

A Fence or Spiny Lizard, genus Sceloporus, this genus of lizard is incredibly diverse.

References and further reading:

Knott, Jeffrey R., et al. "Reconstructing late Pliocene to middle Pleistocene Death Valley lakes and river systems as a test of pupfish (Cyprinodontidae) dispersal hypotheses." Geological Society of America Special Papers 439 (2008): 1-26.

Nosil, Patrik, Luke J. Harmon, and Ole Seehausen. "Ecological explanations for (incomplete) speciation." Trends in Ecology & Evolution 24.3 (2009): 145-156.

Santucci, Vincent L. "Historical perspectives on biodiversity and geodiversity."Geodiversity & Geoconservation 22.3 (2005): 29-34.

Blackwelder, Eliot. "Lake Manly: an extinct lake of Death Valley." Geographical Review (1933): 464-471.

Pianka, Eric R. "Zoogeography and speciation of Australian desert lizards: an ecological perspective." Copeia (1972): 127-145.

Cole, Charles J., et al. "Laboratory hybridization among North American whiptail lizards, including Aspidoscelis inornata arizonae× A. tigris marmorata (Squamata: Teiidae), ancestors of unisexual clones in nature." American Museum Novitates 3698 (2010): 1-43.

Goldman, E. A. "The Colorado River as a barrier in mammalian distribution."Journal of mammalogy 18.4 (1937): 427-435.

Tuesday, August 4, 2015

Arthropods in the Desert. A physiological paradox?

"The Spider's touch, how exquisitely fine! Feels at each thread, and lives along the line." -Alexander Pope

A tarantula in the Sonoran Desert.

When outsiders think of the desert, many immediately picture a landscape covered with all sorts of intimidating looking arthropods (which includes insects and arachnids among other things). This vision wouldn't be at all wrong. Arthropods are the most abundant and diverse organisms in most places, and deserts prove to be no exceptions.

However, arthropods surviving in the desert may be of at least some surprise at least from a physiological perspective. In fact, arthropod diversity is lower in extremely dry areas, compared to areas which have higher humidity. Recall from the recent post about water in the desert that for some groups of organisms wetlands increase species richness in deserts not by supporting communities that are more diverse than arid communities, but by supporting unique communities which could not exist in more arid areas. For arthropods in the desert, wetter areas tend to support a higher number of species in addition to supporting taxa which could not live in dryer areas.

Deserts test arthropods in a number of ways, so their survival here should be more of a surprise than an expectation.

Test 1: Arthropods in many cases cannot exploit evaporative cooling unlike many desert organisms.

Dogs, humans, sheep, and camels are all animals which can exploit evaporative water loss as a mechanism for regulating body temperature. Recently it was shown that Gila Monsters use evaporative cooling through their cloaca to regulate temperature. The larger your size, the better you are able to exploit this mechanism for thermoregulation. Insects and other arthropods have been thought for many years to be too small to be able to thermoregulate in this fashion. It would require more water than they are able to expend for many species of small insect. Being unable to exploit this process could be a huge disadvantage for some desert arthropods. Though recently some have argued that the body size limit for cooling in this manner is smaller than previously thought, and there is evidence that desert cicadas do use evaporative cooling. It is still difficult for many arthropods to use evaporation to maintain body temperature due to their small size.

Instead of exploiting evaporative cooling many arthropods thermoregulate behaviorally by moving between shade and sun during the course of the day or entering and exiting burrows.

Test 2: High respiratory and cuticular(surface) water loss.

Insects and arachnids have respiratory organs which are quite different from our own. Insects breathe through spiracles, tiny pores on their body which allow oxygen to diffuse in. When these spiracles are open insects are incredibly sensitive to water loss to their environment.

In wet environments many arthropods exhibit very high rates of water loss. Desert insects exhibit water loss that can be as little as 1/5 of the rate of loss in related species found in wet environments. Desert arthropods often have waxy surfaces and lipids on their exoskeleton  which prevent water loss.  Scorpions exhibit much lower rate of water loss during the summer than they do in the winter because as temperature increases they ramp up production of these protective coatings.

Next time you are thirsty try holding your breath, this is what some insects do, perhaps in order to minimize respiratory water loss.
Some insects have also evolved "discontinuous gas exchange". Instead of keeping their spiracles (respiratory organs) open all the time they open and close them in a cyclic manner. Keeping spiracles closed does seem to reduce water loss and minimizing water loss could be one adaptive advantage of discontinuous gas exchange in insects. Though how and why discontinuous gas exchange evolved in insects is not completely resolved.

Test 3: Prolonged high temperatures and low thermal tolerances.

Extreme heat is a test for every desert organism, and arthropods are no exception. The average summer high temperatures in many hot deserts is higher than the critical thermal maximum for many arthropod species we consider synonymous with deserts, including the American Cockroach. In fact for 4 months of the year desert high temperatures are at or above the critical maximum for this species. Many other insect species have critical thermal maxima that are near or the same as the American Cockroach. No wonder desert dwellers find cockroaches and other insects so often have taken refuge in their air conditioned homes in the summer months.

Arachnids are however more tolerant of very high temperatures. The sun spider (Solpugidae), which looks like it evolved in a nightmare, has a critical thermal maximum of 50 degrees Celsius (122 F). Good luck killing that horror.

So next time you're in the desert and happen upon an unnerving arthropod don't be so quick to judge. That these small animals can survive at all in the deserts, despite the plethora of challenges they are presented with (just by virtue of being an arthropod) deserves our respect and perhaps our admiration.

Arachnids are already the stuff of nightmares for many. They are remarkably well adapted to high temperatures compared to other arthropods.

References and further reading

Lighton, John RB. "Discontinuous gas exchange in insects." Annual review of entomology 41.1 (1996): 309-324.

Schmidt-Nielsen, Knut, and Bodil Schmidt-Nielsen. "Water metabolism of desert mammals." 

Prange, Henry D. "Evaporative cooling in insects." Journal of Insect Physiology42.5 (1996): 493-499.

Tigar, Barbara J., and Patrick E. Osborne. "Patterns of arthropod abundance and diversity in an Arabian desert." Ecography 20.6 (1997): 550-558.

Edney, E. B. "Water balance in desert arthropods." Science 156.3778 (1967): 1059-1066.

DeNardo, Dale F., Tricia E. Zubal, and Ty CM Hoffman. "Cloacal evaporative cooling: a previously undescribed means of increasing evaporative water loss at higher temperatures in a desert ectotherm, the Gila monster Heloderma suspectum." Journal of experimental biology 207.6 (2004): 945-953.

CLOUDSLEY‐THOMPSON, J. L. "Lethal temperatures of some desert arthropods and the mechanism of heat death." Entomologia experimentalis et applicata 5.4 (1962): 270-280.

Monday, July 27, 2015

Your Body and Heat

"The thermometer nailed to a post reads 110 degrees F., but in the shade, with a breeze and almost no humidity, such a temperature is comfortable, even pleasant." -Edward Abbey

Devils Golf Course in Death Valley

I will have to respectfully disagree with  Mr. Abbey on this point. I've spent much time in the desert, and not once have I found 110 degrees to my liking under any circumstances. Perhaps an exaggeration on his part? As even animals that evolved here avoid those sorts of extremes at all costs.

I recently found myself  in this condition though. Hot desert sun baking me, bare rock beneath my feet radiating that heat back towards my body. Another point I will contest Abbey on is where he exactly expects one to find shade, in many parts of the hottest deserts there is none. In my case the shade of boulders existed ephemerally, disappearing and shifting as the sun danced across the sky. What is one to do when no amount of liquid can quench your thirst? When drenching yourself with provides only a moment's relief, and when the sun becomes your worst enemy? Scramble for shade. Get wet. Get indoors are the solutions which come to mind.

More importantly for our purposes, is the question of what is happening inside our bodies and cells when we (or any other animal) are tested with these extremes in temperature.

First lets focus on the brain. Our most important organ, the control center for our body. What happens to our cognitive abilities during heat stress? Well it turns out that it depends on a number of factors. In general, when you are challenged by heat, your cognitive performance will decrease. The degree to which it is negatively affected depends on many things including difficulty of the task, gender (females are perhaps less susceptible to cognitive decline during heat stress), and hydration (the better hydrated you are the less steep the decline when challenged with heat). This cognitive decline is notable because your ability to make smart decisions decreases when you are challenged with heat, potentially leading to mistakes which could make a bad situation worse.

What about in your muscles? As you work in hot conditions, your muscles become fatigued more quickly. You will be unable to exert as much force following long periods of exercise in hot conditions. This affect on muscles seems to be mainly seen during long periods of exertion, short term physical exertions are not influenced as much unless they come immediately following a period of prolonged exercise in hot conditions. 

What about at the cellular level? Well, the cellular response to heat stress is perhaps one of the most well preserved responses throughout evolution. Following exposure to high temperatures your cells will start producing a special type of "heat shock proteins". Organisms needed a way to deal with the consequences of stresses like heat, and these special proteins could be evolution's answer. These proteins are recruited to make sure that your cells don't succumb to the often deadly affects of heat. That much is clear, but more remains to be understood regarding their mechanism of action within a cell.

There may be some temporary benefits of being challenged by heat. Rabbits that were challenged by heat had hearts that were less susceptible to other environmental stresses 24 hours later. It seems that dealing with heat, may equip your body to deal with other environmental stressors within a short time frame. Though this advantage seems to disappear the longer an organism is removed from the heat.

Decreased brain function, muscles that fatigue more quickly, and cells functioning differently than they would otherwise. All are consequences of heat exposure. Stay safe, stay hydrated, and stay cool.

References and further reading:

Hancock, P. A., and Ioannis Vasmatzidis. "Effects of heat stress on cognitive performance: the current state of knowledge." International Journal of Hyperthermia 19.3 (2003): 355-372.

Gopinathan, P. M., G. Pichan, and V. M. Sharma. "Role of dehydration in heat stress-induced variations in mental performance." Archives of Environmental Health: An International Journal 43.1 (1988): 15-17.

Morimoto, Richard I. "Regulation of the heat shock transcriptional response: cross talk between a family of heat shock factors, molecular chaperones, and negative regulators." Genes & development 12.24 (1998): 3788-3796.

Currie, R. W., R. M. Tanguay, and J. G. Kingma. "Heat-shock response and limitation of tissue necrosis during occlusion/reperfusion in rabbit hearts."Circulation 87.3 (1993): 963-971.

Nybo, Lars, and Bodil Nielsen. "Hyperthermia and central fatigue during prolonged exercise in humans." Journal of applied physiology 91.3 (2001): 1055-1060.

Lindquist, Susan. "The heat-shock response." Annual review of biochemistry55.1 (1986): 1151-1191.

Wednesday, July 15, 2015


"Nowhere is water so beautiful as the desert, for nowhere else is it so scarce." -Edward Abbey

Salt Creek in Death Valley National Park. Home to the endemic Salt Creek Pupfish

One of the greatest paradoxes of the desert is that its most limiting resource is simultaneously the most powerful force shaping the the landscape; geologically, biologically, and socially. When the rains do come here, they come furiously. Eroding clay, dirt, and rock. The meticulous planning of water resources makes it possible that people are able to live in the desert, and fights over water rights still often constitute the biggest political and legal battles in desert regions (follow the link to learn about an ongoing legal battle regarding water rights in Nevada).

Water and its availability or lack thereof also has exceptional consequences for biotic communities throughout the desert. For perspective, it is estimated that about 0.4% of Arizona's surface area is a wetland, or riparian habitat. Nevertheless about 80% of vertebrate species in Arizona depend on riparian habitat at some point in their lives. 80% of species depend on 0.4% of total land area for the completion of their life cycles. Birds are especially dependent upon riparian habitat. Protection of a river, or stream can significantly increase bird biodiversity throughout an entire area.

Lost Palms Oasis, Joshua Tree National Park

Additionally, these riparian areas are important because the biological community in them is often quite different from the surrounding area. One need look no further than Ash Meadows National Wildlife Refuge to confirm this, this small area dotted with multiple springs in Nevada's Mojave Desert is home to the highest concentration of endemic species (species found nowhere else) in the World. The endemic species found at Ash Meadows include plants, snails, and some very special fish.

Wildlife that depends on these habitats are also disproportionately imperiled. Over one dozen freshwater fish species went extinct during the last century from desert regions of the United States and Mexico.  Extinction threatens 40% of desert pupfish species today. These numbers are higher when other groups that depend on these habitats, like amphibians, are included.

Water and its management will continue to shape the social, geological, and biological landscape in deserts going forward. Can we strike a balance between managing water for the well being of humans, and managing it in such way that no more distinct evolutionary lineages are lost forever?  Exceptionally unique species assemblages still exist where water reaches the surface, though much was lost, much still remains. Whether or not what is left will remain in the next century, depends largely on

References and further reading:

Sabo, John L., et al. "Riparian zones increase regional species richness by harboring different, not more, species." Ecology 86.1 (2005): 56-62.

Zaimes, George, et al. "Understanding Arizona's Riparian Areas." (2007).

Brown, James H., and C. Robert Feldmeth. "Evolution in constant and fluctuating environments: thermal tolerances of desert pupfish (Cyprinodon)."Evolution (1971): 390-398.

Williams, Jack E., et al. "Endangered aquatic ecosystems in North American deserts with a list of vanishing fishes of the region." Journal of the Arizona-Nevada Academy of Science (1985): 1-61.

Miller, Robert R., James D. Williams, and Jack E. Williams. "Extinctions of North American fishes during the past century." Fisheries 14.6 (1989): 22-38.