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Mule Deer & Guzzlers

Here is most of a study on guzzlers and mule deer down in southern California. Enjoy!

Coastal Southern California has lost much of its native habitat to human development. The remaining habitat is often fragmented and home to non-native species. These changes have altered the balance of an ecosystem adapted to thrive in a unique semi-arid strongly influenced by the ocean. In addition, humans have diverted surface water and exploited groundwater in the region which disturbs the natural water cycle. The growing pressures on a unique habitat are further compounded by predictions of increasing droughts due to global climate change (Beuhler, 2003). Currents and future water limitations in coastal Southern California could result in a decline of native wildlife, if water is the limiting resource.

Early conservationists sought to provide water to species on Camp Pendleton like the mule deer as early as the 1940’s by providing supplemental water. Concrete water supplementation sites on base are labeled with an installation date of August 1949. Researchers have studied whether wildlife benefit from supplemental water with mixed or inconclusive results by measuring distribution changes. Artificial water sources in Arizona had a measurable, but minor, influence on mule deer movement during hot, dry condition (Marshal et al, 2006). Radio collared desert mule deer in Arizona remained closer to water during the dry season compared to the wet season (Ordway and Krausman, 1986). If supplemental water affects the density of a species than it may demonstrate that natural water is a limiting factor for that species (Messier 1991). By adding the supplemental water the negative feedback mechanism of a water limitation is removed and density can increase.

However, water may not be the only limiting resource. Precipitation increases the amount of food resources which supplemental water does not.

Because of the moderating effects of the ocean, temperatures at the coast are cooler compared to temperatures further inland, particularly in the dry season when water availability is lowest. In addition, coastal fog provides water to the ecosystem even during drought. If deer are water limited than their supplemental water should attract the species, and increase their occurrences.

A supplemental water conservation program will only succeed if: 1) water is the limiting resource, 2) supplemental water does not create a predator trap, 3) and the landscape’s influence over supplemental water is understood.

Camp Pendleton, with 90% of its 125,000 acres undeveloped, contains oak woodland, chaparral, coastal sage scrub, and grasslands. Camp Pendleton is bordered by the Cleveland National Forest.

Twenty three sites were sampled between January 2007 and November 2008. There are three types of data collection sites: concrete guzzler, fiberglass guzzler, and control sites. Nine additional control sites were added for the fourth season for a total of 32 sites.

There were 13 environmental factors utilized to identify a model that could explain the presence of deer at a study site. Several variables contributed to the most effective models with AIC weights 0.5 or greater: elevation, distance from ocean, presence of predator scat, presence of predator tracks, distance from riparian habitat, and habitat burned in 2007 wildlife. The remaining variables had little influence on effective deer explanation models: slope aspect, distance from development, distance from road, slope, presence of supplemental water, number of data collection visits, and distance from permanent fresh water.

Deer visitation occurred during the dry season and in very low number during the wet season. The best model for explaining deer utilization of water supplementation included elevation, distance from riparian habitat, type of water supplementation, and bobcat detections as variables. The following variables were not useful in producing a model to explain deer water supplementation use: fax visitation, coyote visitation, distance from ocean, distance from development, distance from road, presence of predator scat, slope, distance from permanent fresh water, presence of predator tracks, and slope aspect. Deer visits to water supplementation occurred on average 1439 meter from riparian habitat, were positively correlated with bobcat visitation and fiberglass guzzlers, and occurred at an average elevation of 966 feet. Elevation’s influence was greatest during the dry seasons. During the dry seasons, lower elevation was positively correlated with deer detections.

Discussion:

It appears that standing water is not the limiting resource for deer in coastal Southern California. Deer did not reveal a change in distribution in relation to supplemental water. Deer respond to seasonal variation in temperature and precipitation by shifting their ranges, and this movement likely mitigates the seasonal change in natural water availability. This seasonal movement may be an adaptation that allows mule deer to survive without utilizing supplemental water (Milstead, 2007).

Elevation, not water supplementation, affected deer distribution during the dry season. Seasonal changes in precipitation change food availability based on elevation and habitat, and the deer movements reflect this change in food availability. This is similar to seasonal mule deer movements in Imperial County, California (Marshal et al, 2006), where deer moved to lower elevations during the dry season. This movement contrast with mule deer of the Sierra Nevada which move to higher elevations following the receding snow line and plant growth in the spring and migrate to lower elevations when fall storms begin (Kucera, 1992). However, snow does not occur at the study site, and areas of higher elevation at the study site are characterized by oak savanna habitat, which are warmer and drier during dry seasons than are coastal areas. The grasses in this oak savanna turn brown during the dry season, but flourish during the wet season. The perennial vegetation of the chaparral and coastal sage scrub at lower elevations provide a more consistent supply of food. Deer may move to higher elevations to take advantage of the greening that occurs during the wet season, but will return to lower elevations to eat shrubs that remain green throughout the dry season. In addition to food availability, mule deer may also move lower in elevation to take advantage of the cooler temperatures closer to the ocean during the dry season. The lower temperatures could reduce heat and water stress for the mule deer, reducing their need for supplemental water (Lawrence et al, 2004).

Neither sources of standing natural water nor supplemental water influenced deer distribution. This lack of influence indicates that deer met their hydration needs without visiting supplemental water or known natural water sources. Perhaps unknown natural water sources provide water to the deer, but it’s also possible the water deer receive in the form of their food is adequate to meet their water needs. A third alternative is also possible: deer include a natural water source within their home range but remain mostly away from water during the majority of their movements. Mule deer in northern California changed their ranges seasonally based on food availability and lower temperatures, but remained within 2.5km of a water source (Boroski and Mossman, 1996). This alternative is supported by the landscape analysis of supplemental water utilized by deer.

Food and cooler temperatures, not water, are the likely limiting factors for deer in coastal Southern California. Food availability is linked to precipitation at higher elevations, so seasonal movements occur in response to this precipitation. In addition, there is not a temperature gradient during the wet season so deer can remain further from the coast. Free standing water, whether natural or supplemented, did not affect the distribution of deer.

The motive for a deer to visit supplemental water may be rooted in nutritional requirements. The peak deer visitation to water supplementation coincides with the dry season, the deer’s rut and the deer’s early gestation period (Bowyer, 1991). Both the rut and gestation require additional calories for the buck and doe respectively. If deer are interested in conserving calories they may choose to visit a conveniently placed water supplementation site rather than expend calories to travel to a natural water source. Water supplementations close to riparian habitat and at lower elevations would be in the dry season habitat for deer as well as a long walk from natural water. Deer could save calories and remain near their food and cover by visiting supplemental water instead of venturing to the canyon bottom. This could be very important for pregnant does that are typically found closer to water supplementation in the desert (Krausman and Etchberger, 1995). Nutrition and mass was the best predictor of survival for mule deer fawns and adults in southwest Idaho (Bishop, 2005). Future studies comparing recruitment of water supplemented populations to non-supplemented populations could answer whether water supplementation is beneficial to the species.

Coastal conditions, and not standing water, explained the distribution of deer. During the dry season, coastal areas are cooler, more humid, and have greater food and water availability. This suite of conditions is evidently more important than standing water alone as a factor in determining distribution of deer. Free standing water availability did not explain the distribution of deer either as supplemental water or as natural standing water sources. Consequently, a supplemental water program cannot be expected to increase habitat for deer during the dry season or drought. However, water supplementation may enhance habitat quality for deer. Strategically placed water supplementation sites could allow deer to conserve calories and avoid exposure to predators at natural water sites. A future study focus on whether water supplementation affects recruitment of deer.

The seasonal shift of deer distribution is the greatest lesson for land managers. A patch of habitat that may support the species during the wet season should be connected to a habitat patch that is suitable during the dry season. As habitat becomes more fragmented, land managers should think about the seasonal requirements for the species they wish to preserve.

WishIWasHunting's picture
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Location: Brighton, CO
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There is some interesting

There is some interesting info in that study.  Even though it was focused on California, I would imagine that the findings can be applicable for other areas as well.  Also, it does help prioritize where to look for deer, even during very dry years.  

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