Tuesday, January 26, 2016

Grazing in the Deserts

"Temper tantrums, however fun they may be to throw, rarely solve whatever problem is causing them." -Lemony Snicket

A Desert Tortoise from the area where Cliven Bundy's cattle have been grazing illegally for years.

I decided to delve into this topic in part because of current events that should be startling to conservationists, resource managers, people who recreate in deserts and ranchers themselves. That is the closure of Malheur National Wildlife Refuge in Oregon. Since the refuge is located in the Great Basin Desert of Oregon, and the same family who has shut down the refuge hails from my home state I felt that it would be interesting to delve into the science behind the point the Bundy's are trying to make.

Much of their conflicts with land management agencies revolves around disputes over grazing rights in certain areas. The Bundy family famously owing over $1 million in grazing fees and fines to the federal government. They have refused to pay these fees and have now occupied an important wildlife refuge in Oregon.

A central claim that the Bundy's have raised is that grazing is good for wildlife. They have stated that wildlife on ranches and areas where grazing is allowed is more abundant than in Malheur National Wildlife Refuge itself. They famously destroyed a boundary fence between the refuge and a ranch, claiming that the Ranchers had been treated unfairly by the federal government. It should be noted that the Ranch owner, did not want the fence removed or his livestock to roam onto the wildlife refuge which encompasses 100,000 acres.

But what about the science behind these claims? Do the Bundy's have a point? Is grazing good for wildlife and wildlife habitat?

Well, like many things in nature. The answer is "it depends". In areas where plants evolved with some level of livestock grazing it is known that some levels of disturbance by grazing can promote diversity in plant communities.

It should be noted that studies on grazing can be inherently difficult. It is difficult to separate variables from each other in many of these systems. It is difficult to distinguish between certain management techniques. For instance, does season of grazing matter? How does one objectively distinguish between different grazing intensity levels?

Some studies can also become biased because certain areas, though technically open to grazing are not grazed by livestock due to other factors.

Unfortunately, some claims in this field of study are often overstated and anecdotal. One paper from  Rangelands states that Desert Tortoises are found in highest abundance where cattle are also found in highest abundance (perhaps a coincidence of both relying on similar food sources?). Other studies have found that livestock trample young tortoises, destroy burrows and remove important forage.

However, one thing is clear. Overgrazing is hardly ever beneficial, especially in deserts and riparian areas (Malheur National Wildlife is both). Cattle in riparian areas can alter stream flows and make water less available to plants, which could alter the entire riparian community. They also impact soils in these fragile areas which desert wildlife are dependent on.

Riparian habitat near where Cliven Bundy's cattle graze in Nevada

It isn't just the plants though. Overgrazing effects people too. Trout from streams where grazing is not allowed or has been eliminated tend to be much larger and more abundant, overgrazing eliminates bird species richness in riparian areas, even snakes are more abundant in areas where grazing is limited. This is bad news for the fishermen, hunters, and wildlife viewers who also own Malheur National Wildlife Refuge if the Bundy's do get their wish.

Ironically, overgrazing also reduces edible forage for cattle. So it even hurts the ranchers, which is after all why grazing is regulated in the first place. If you don't do it right you hurt ecological communities and you hurt people.

References and further reading:

Milchunas, D. G., O. E. Sala, and W_K Lauenroth. "A generalized model of the effects of grazing by large herbivores on grassland community structure." American Naturalist (1988): 87-106.

Waser, Nickolas M., and Mary V. Price. "Effects of grazing on diversity of annual plants in the Sonoran Desert." Oecologia 50.3 (1981): 407-411.

Fleischner, Thomas L. "Ecological costs of livestock grazing in western North America." Conservation biology (1994): 629-644.

Bostick, Vernon. "The desert tortoise in relation to cattle grazing."Rangelands (1990): 149-151.

Webb, Robert H. "Recovery of severely compacted soils in the Mojave Desert, California, USA." Arid Land Research and Management 16.3 (2002): 291-305.

Thursday, January 7, 2016

Side-Blotched Lizards

"Remind yourself that you don't have to do what everyone else is doing" -Rajat Dogra

A Side-blotched Lizard in the Sonoran Desert

The Side-blotched Lizard, Uta stanbursiana, is the most widespread and common lizard in the North American Deserts. One can observe them throughout the year, even during the cool winter months. Though they can be quite colorful, otherwise nothing jumps out about this typical lizard. I recall one guide book referring to them as "the popcorn of the desert", because they provide snacks to so many desert predators; snakes, birds, coyotes, and other lizards.

With this abundance, comes an increased chance that lizards of this species will cross paths and interact. This common lizard, which a seasoned herpetologist might ignore due to its abundance is part of a complex social hierarchy which lends insight into important ideas in evolution and physiology.

Males of this species are polymorphic (multiple "types" that look and behave differently). There are large males which control and defend large territories, smaller males which defend smaller territories, and males which control and defend no territories and resemble female Side-blotched Lizards. Females

Selective forces should shape evolution of males towards big, territorial males, able to control and monopolize large territories and thus female lizards. So why do the other two morphs exist?

If very large males become to abundant, they struggle. They compete with each other and have trouble defending the territory and the females in their respective territories. The smaller males are more able to defend their small territories and mate with the females there.

If either of the two males that defend territories become abundant the males that resemble females have an advantage. They are able to "sneak" into other males territories and mate with females unnoticed as the larger males fight with each other.

But when these "sneakers" become most common the large males are able to identify them and chase them away easily.

In other words, the relative fitness of each type of male is related directly to its relative abundance within the population. This is called density dependent natural selection. If one type of male becomes too abundant it increases the fitness of the other two types, and this is how all three are able to persist in the population. This is the hipster form of natural selection, you're most cool (fit) when you're doing things that no one else is doing.

Another interesting part of this social hierarchy deals with the length of each lizards tail. Recall that an important defense mechanism for many lizards is that they are able to drop their tails when threatened by a predator and that they eventually regenerate them. The predator might be then left with a tail, but no lizard.

Losing a tail appears to have social costs in Side-blotched Lizards. When a dominant lizard loses its tail, subordinate lizards recognize this and behave more aggressively towards the formerly dominant lizard, perhaps recognizing that this lizard no longer has the social status it once did due to the unfavorable encounter. The length of the tail and whether or not it was regenerated appears to be especially important for status in female lizards.

References and further reading:

Sinervo, Barry, et al. "Testosterone, endurance, and Darwinian fitness: natural and sexual selection on the physiological bases of alternative male behaviors in side-blotched lizards." Hormones and Behavior 38.4 (2000): 222-233.

Ferguson, Gary W. "Mating behaviour of the side-blotched lizards of the genus Uta (Sauria: Iguandidae)." Animal Behaviour 18 (1970): 65-72.

Sinervo, Barry, and Curt M. Lively. "The rock-paper-scissors game and the evolution of alternative male strategies." Nature 380.6571 (1996): 240-243.

Fox, Stanley F., Nancy A. Heger, and Linda S. Delay. "Social cost of tail loss in Uta stansburiana: lizard tails as status-signalling badges." Animal Behaviour 39.3 (1990): 549-554.