Tuesday, June 23, 2015

Dinosaurs of the Deserts

"Because the history of evolution is that life escapes all barriers. Life breaks free. Life expands to new territories. Painfully, perhaps even dangerously. But life finds a way." -Michael Crichton, Jurassic Park

The Collared Lizard, a facultative biped like early dinosaur ancestors

Jurassic Park has long been a favorite of mine, both the books and the movies. Dinosaurs perhaps first sparked my fascination with the natural world. With the revival of the franchise in the new Jurassic World, a couple thoughts have been floating around in my head regarding how dinosaurs relate to desert ecology. I think a few connections and parallels can be drawn between dinosaurs (or plot points in Jurassic Park) and modern day reptiles native to the North American Deserts.

One of the most fascinating aspects of reptile biology to me is the evolutionary history of this group, which includes birds, crocodiles, snakes, lizards, and a few other weird and lesser known groups. While many focus on the events that led up to the extinction of dinosaurs (cough* dinosaurs aren't actually extinct), I believe that the evolutionary origins of dinosaurs are far more interesting and far more mysterious.

One of the close relatives of the ancestral archosaurs (the group which included dinosaurs, birds, and crocodiles) was a small reptile called Euparkeria. This reptile had hind limbs which were slightly longer than its forelimbs, meaning that this animal likely exhibited some form of bipedalism (runs on two legs) at times. Bipedalism is a characteristic that is typical for the identification of the major dinosaur group which gave rise to modern birds.

For some reason I can't help but see Euparkeria and other early dinosaur ancestors whenever I see a Collared Lizard sprint on its hind legs after its prey, which will often be other lizards. Though this connection is admittedly superficial, its fun to imagine. Evolution has once again shaped a reptile that is facultatively bipedal and superficially resembles certain dinosaur ancestors.

Crotaphytus collaris, note the muscular hind limbs and short forelimbs

Another important plot point in the original Jurassic Park books and movies is the idea that a population composed entirely of female dinosaurs is able to successfully reproduce once they escape from their enclosures. In order to make this plausible Michael Crichton and Steven Spielberg invoke that genetic material from frogs were used in the cloning of the dinosaurs, and that some frogs are able to spontaneously change sexes from female to male. 

Certain desert lizards have been able to long maintain viable, reproductive populations in the wild when all individuals are in fact female. No spontaneous sex change is necessary. In fact this trait, the ability to go through "parthenogenesis" or self cloning, is not uncommon among reptiles, and is the norm for many species of Whiptail Lizard.

Parthenogenesis has been documented in many species of reptile including Komodo Dragons, Pythons, and Geckos. It seems that many reptiles have the ability to reproduce in this matter, but that it hasn't yet caught on in the same way it has with Whiptail Lizards. 

So the idea that a population composed entirely of female reptiles is able to reproduce successfully is perhaps the least far-fetched biological plot point of the Jurassic Park series. However, you don't need to invoke sex changing frogs to get there, diverse groups have reptiles have often been able to reproduce asexually under the right conditions. Could certain dinosaurs too have been able to reproduce asexually like our native Whiptails?

The Checkered Whiptail, Aspidocelis tesselata
Photo by Vicente Mata-Silva, no changes made, https://creativecommons.org/licenses/by/3.0/legalcode

References and further reading:

Snyder, Richard C. "Adaptations for bipedal locomotion of lizards." American Zoologist (1962): 191-203.

Maslin, T. Paul. "Conclusive Evidence of Parthenogenesis in Three Species of Cnemidophorus (Teiidae)." Copeia 1971.1 (1971)

Price, Andrew H. "Comparative behavior in lizards of the genus Cnemidophorus (Teiidae), with comments on the evolution of parthenogenesis in reptiles."Copeia (1992): 323-331.

Tuesday, June 16, 2015

Seasonal Life Cycles

"Summer time and the livings easy..." -Sublime

During the summer its easy to forget that mere months ago the desert looked like this

One of the quickest ways to forget about your day to day worries is to go for a walk in the Mojave Desert mid-day, during the summer. Of course this is not recommended for the unprepared, or for anyone really. Immediately you will notice the bright sun and feel its heat on your skin. In a few minutes you will start to become fatigued, and will start to look for quick retreats from the heat. Looking for shade, looking for liquid, trying to find the quickest way to the indoors and the safety of an air conditioned environment. Earlier in my life I would hike in the desert during the summer, and I still do from time to time. During these hikes I would be walking before the sun rose, and would be done hiking for the most part before 10 AM. Even by that early hour the temperature would often be approaching or in excess of 110 degrees Fahrenheit (43 C).

This bighorn sheep has succumbed to the harsh conditions

The summer in the desert is extreme, and it is upon us. So how do animals and plants survive conditions which can only be described as oppressive? The answer for many organisms may seem counter-intuitive. There are many adaptations that exist to deal with heat, from waxy leaves that minimize water loss in plants to shifts in behavior to become strictly nocturnal in animals and body shapes which allow for more efficient dissipation of heat and minimized water loss. However, adaptations like this are not the norm. Many organisms simply don't attempt to deal with these conditions. Summer living for them is "easy" because they aren't alive during the summer at all (who knew the band Sublime was so fond of desert ecology?).

Most people are familiar with the terms annual and perennial plants. I prefer the word "ephemeral" for a more apt description of "annual plants", especially in the desert. These plants, which have life cycles which are extremely fast from seed to death make up about 13% of plant diversity worldwide. That is 13% of plant species in the world are annual or ephemeral in their life cycles. Now lets look at deserts. Across the North American Deserts ephemeral plants make up 40% of all plant diversity. The more extreme the desert, the greater the proportion of short-lived ephemeral plants. In Death Valley, the hottest place in North America, over 90% of all plant species are ephemeral! The most successful way that plants survive in Death Valley is by being short-lived and avoiding the summer altogether. The seeds of the next generation can then establish themselves when conditions are more mild.

This "life by death" or "life by avoidance" strategy is not unique to plants. Animals also have been able to survive bad conditions by avoiding them altogether. Fairy shrimp or Triops are crustaceans which live in temporary rain ponds, their eggs can survive years of desiccation on a dry lake bed. Desert Caddisflies also undergo quick development time and leave the Desert altogether during the summer, returning in the fall to breed when conditions are better. Desert locusts also swarm in droves after heavy rains, feeding on the ephemeral plants which grow under the wet conditions.

Vertebrates too also exhibit seasonality in their life history traits. Whiptail lizard densities can as much as double following a wet year due to increase in arthropod abundance. The Spadefoot toad and its aestivation during unfavorable conditions and the short development time of its tadpoles was discussed previously. Birds in hot deserts have also exhibited high mortality rates during heat waves, the likelihood of these die-offs will increase due to climate change.

Despite the diversity of adaptations that animals have to desert environments Summer is still an extremely vulnerable time period for desert organisms. More work needs to be done, but I tend to think that mortality is very high even for well-adapted organisms during the summer months. The old saying goes, "If you can't stand the heat, get out of the kitchen". It seems that avoiding the kitchen, or the desert during summer is perhaps one of the most effective ways to persist here. Its easier to live and die during the spring than to even attempt to deal with the oppressive summer conditions.

An aquatic frog, Rana yavapaiensis (center left) clings to life in the only pool of water left in this desert canyon, weeks later this pond was gone and presumably so was the frog.

Sources and further reading:

Colburn, Elizabeth A. "Diapause in a salt-tolerant desert caddisfly: the life cycle of Limnephilus assimilis (Trichoptera) in Death Valley." American Midland Naturalist (1984): 280-287.

Brown, Lon R., and Lars H. Carpelan. "Egg hatching and life history of a fairy shrimp Branchinecta mackini Dexter (Crustacea: Anostraca) in a Mohave desert playa (Rabbit Dry Lake)." Ecology (1971): 41-54.

McKechnie, Andrew E., and Blair O. Wolf. "Climate change increases the likelihood of catastrophic avian mortality events during extreme heat waves."Biology Letters (2009).

  • Sowell, John, 1958. Desert Ecology: An Introduction to Life in the Arid Southwest. University of Utah Press, 2001.
  • Cloudsley-Thompson, J. L. Ecophysiology of Desert Arthropods and Reptiles. Springer-Verlag, 1991.

Saturday, June 6, 2015

Kit Foxes

"The desert will still be here in the spring. And then comes another thought. When I return will it be the same? ... Will anything ever be quite the same again?" -Edward Abby

Kit Fox, Vulpes macrotis. Photo by U.S. Fish and Wildlife Service

Kit Foxes are pretty cool little creatures. Often while driving desert roads at night one can see them darting across the road, sometimes looking back at the headlights of the car in amazement. It is found throughout the Southwest United States and is the smallest member of the family Canidae in North America. It is carnivorous, but a generalist generally eating whatever small prey items it can find, from lizards to jackrabbits. They are also monogamous, forming pair bonds which often last for life.

These animals are nocturnal, and though they are monogamous Kit Fox pairs do not coordinate their night time movements. This may allow for sharing of information about the predators or the environment and for greater chance of success when hunting by covering twice as much ground during nightly activity. Some studies have shown that Kit Foxes near urban areas live longer and are healthier than foxes in more natural areas due to the constant high calorie food supply that humans often provide in their trash cans.

Many desert subspecies of the Kit Fox are endangered. Coyotes act as predators and competitors with these small foxes, both preying upon them and competing for overlapping dietary needs. Management practices that increase the range of coyotes have not been helpful to Kit Foxes. One subspecies of Kit Fox, the Southern California Kit Fox has already gone extinct. It is important that heterogeneous habitats are maintained in deserts in order to conserve these small canids; unfortunately loss of heterogeneity is likely to be a major effect of climate change in the Southwest.

Like many desert animals, the Kit Fox obtains most of its water requirements from its food. Unlike the Coyote, it is also able to maintain minimal water loss to its environment across seasons, and counts mainly on its food for its water requirements. Its water loss is only about 65% of what would be expected based on the weight of the animal during the summer months, for comparison Coyotes water loss during summer in the desert is about 155% of what would be expected based on its weight. So when it comes to water utilization the Kit Fox does appear to have a leg up on Coyotes at least in that respect.

Human caused alterations in deserts have a huge influence on the dynamics of desert systems. The cute Kit Fox is only one species that may be effected. Desert systems are not as "tough" as once assumed. Their biological inhabitants are often highly localized and often near the limits of their physiological tolerances.

According to a Nature Serve report prepared for the Nature Conservancy regarding the state of biological diversity in the U.S., four of the five top states for overall levels of Biodiversity are states with deserts. Desert states also make up three of the top five in terms of extinction risk (Hawaii and its highly localized community and disturbed systems ranks first). Desert states also make up three out of the top five states in terms of endemism (species found only in that state), and two of the top five in overall existing extinctions. These ranks for extinction risk and overall extinction are despite the fact that desert states (with the exception of Texas) also rank highest for preserved natural areas. The Kit Fox is but one species that is restricted mainly to arid regions, and whose subspecies are threatened to some degree with extinction. 

Although it may not seem so at the surface desert biotic communities are diverse, unique, and threatened. Change that is caused and exacerbated by humans may cause these unique communities to be almost as ephemeral and temporary as the water sources they depend on. 

Sources and further reading:

  • White, P. J., Katherine Ralls, and Donald B. Siniff. "NOCTURNAL ENCOUNTERS BETWEEN KIT FOXES." Journal of mammalogy 81.2 (2000): 456-61.
Meadows, Robin. "Fast Food Boosts Urban Kit Foxes." Frontiers in Ecology and the Environment 8.8 (2010)

Nelson, Julia L., Brian L. Cypher, and Curtis D. Bjurlin. "Effects of Habitat on Competition between Kit Foxes and Coyotes." The Journal of Wildlife Management [H.W.Wilson - GS] 71.5 (2007): 1467.

Golightly, Richard T., and Robert D. Ohmart. "Water Economy of Two Desert Canids: Coyote and Kit Fox." Journal of mammalogy 65.1 (1984)

State of the Union: Ranking America's Biodiversity. http://www.natureserve.org/library/stateofunions.pdf