last update January 28, 1999
Results
Detailed descriptions of each community are listed below. A summary of theinformation recorded in the vegetation transects are found in Table 1. The sum ofpercent cover of individual species in an area may exceed 100% due to canopyoverlap.
Plant Community Descriptions
1) SuedaThe Sueda community covers approximately 5 acres or 1.5 % of the park and islocated in the northeast quadrant of the park. This community is dominated by Suedamoquinii. (sueda) which covers 34% of the shrub layer. Density of Sueda is relativelyhigh at 136 plants per hectare. This area is surrounded by tamarisk and a fewscattered mesquite. Off Highway vehicle(OHV) activity in this vegetation region is mildto moderate with activity confined to well used trails.
2) Creosotebush scrub This vegetation community, dominated by Larrea tridentata (creosote bush), occurs in two different vegetation regions. The largest region occurs in the southwest portion of the park covering approximately 47 acres. The smaller second portion, located along the central western edge of the park, covers 5.6 acres. Both portions together cover roughly 15.5 % of the park. Larrea percent cover in the larger southern region is 53% while in the smaller region coverage was slightly higher at 63%. A large number of Oligomeris linifolia were observed in the under story, but did not occur on the transect. The larger southern region had densities of 2120 and 1720 plants per hectare for Larrea tridentata and Oligomeris linifolia respectively. The smaller western section had a density of 640 plants per hectare for Larrea tridentata.
A disturbance gradient is readily apparent in the larger southern Larrea vegetationregion. It appears that heavy OHV disturbance has reduced the size and density ofLarrea shrubs located along the northern edge of this region nearest the highly usedcentral sand dune/tamarisk area. The larger less disturbed Larrea shrubs can beseen as a darker band on the southern edge of this vegetation zone. Both Larreavegetation regions contain scattered tamarisk and numerous arroweed shrubs.
3) Arroweed/Baccharis
This vegetation community occurs in only one vegetation region which is a long,narrow section along the eastern edge of the park. The community coversapproximately 14 acres, or 4% of the park, and is dominated by Pluchea sericea(arroweed) and Baccharis emoryi (baccharis). Additional species of interest foundwithin this vegetation region, but not on the transects, are Prosopis glandulosa(mesquite), Populus fremontiii (Fremont cottonwood) and Salix gooddingii(Goodding's black willow). Total relative percent cover was 100 % and includedoverlapping shrub canopies of Pluchea at 48% and Baccharis at 52%. While notrepresented in the vegetation transect data, this area is being invaded extensively bytamarisk. OHV activity in this vegetation region is nearly impossible due to theextremely dense vegetation. Baccharis density was estimated at 2400 shrubs perhectare and Pluchea at 2560 shrubs per hectare.
4) Arroweed/Saltbush
Structurally the Arroweed/Saltbush community is very similar to theArroweed/Baccharis vegetation complex, with 100% percent cover in bothcommunities. The main differences in the Arroweed/Saltbush community is a highpercent cover of Atriplex lentiformis and tamarisk. Heber Dunes has three vegetationregions that contain variations on the Arroweed/Saltbush vegetation complex. Thelargest lies in the southwest corner of the park and covers approximately 16.5 acres.Two smaller regions lie on the western edge of the park and cover 2.5 and 3.4 acreseach. The sum of Arroweed/Saltbush vegetation communities cover approximately6.6% of Heber Dunes. The relative percent cover in the largest Arroweed/Saltbushregion includes: 29% Atriplex lentiformis, 40% Baccharis emoryi, 8% Pluchea sericeaand 22% Tamarix ramosissima. As in the Pluchea/Baccharis community, OHV activityin the Arroweed/Saltbush regions is nearly impossible due to the extremely highdensity. The following shrub densities were calculated: 640 Atriplex shrubs perhectare; 1360 Baccharis shrubs per hectare; 400 Pluchea shrubs per hectare; and320 Tamarix shrubs per hectare.
5) Saltbush The Saltbush community occurs in three different vegetation regions covering almost 7% of the park. The largest vegetation region containing the Saltbush community is found at the south western tip of the park and covers approximately 19.5 acres. To the north are found two smaller Saltbush regions which cover 3.1 and 0.2 acres. This community is dominated by Atriplex lentiformis. The largest region contains 29% Atriplex lentiformis, 11.8 % Pluchea sericea and 3.5% Baccharis emoryi. In this vegetation community, Atriplex also had the highest density with 2920 shrubs per hectare, Pluchea the second highest at 1200 shrubs per hectare and Baccharis the lowest with only 40 shrubs per hectare. OHV activity in the Saltbush community is moderate with most activity confined to numerous well worn trails.
6) Tamarisk/Sand dune
The Tamarisk community, located in the central section of the park, is dominated by Tamarix ramosissima and covers 181 acres, or 53% of the total acreage of the park. Tamarix was observed to be the principal plant species present from a ground survey of the community. The remains of several annual herbaceous species in the under story were observed, including; Tiquilia plicata, Cryptantha sp., Atriplex elegans, Salsola tragus and Schismus barbatus. The large scale and large distances between individual tamarisk trees in this community made 50 meter vegetation transects impractical. Percent cover of Tamarix was therefore estimated at 30% from the aerial photograph. Intense OHV activity in this area along with the sandy soil substrate has resulted in large bare sandy expanses with only a scant covering of native and exotic annuals.
Discussion
Five communities, Sueda, Creosotebush scrub, Arroweed/Baccharis,Arroweed/Saltbush and Saltbush scrub, contain all of the native perennial plantbiodiversity at Heber Dunes. Taken together these vegetation communities accountfor roughly 160 acres or 47% of the park. The Tamarisk/Sand dune communityaccounts for the remaining 53% of the park. The Tamarisk/Sand dune community isvirtually devoid of native perennial plant diversity, due in part due to OHV activity andthe windblown sandy substrate.
Sampling in early June, after the springtime flush of annual species had passed, dictated that this project would serve only as a preliminary survey focusing on perennial plant biodiversity. Of the annual plant species encountered, most were past accurate identification due to advanced senescence and only a few could be identified. It does appear that the Tamarisk/Sand dune community contains endemic dune species, such as Abronia villosa and Oenothera deltoides, and should be resampled in the future under appropriate conditions to gain a more accurate flora of the community.
The boundaries for each community were determined from a 1998 aerial photographwith ground surveys and may be arbitrary where no clear cut boundaries exist. Six 50meter transects were used to describe the vegetation communities identified in thisreport, but, because of the large area involved, adding more vegetation transects mayshift the relative coverage occupied by each species and result in a more accuratepicture of each community composition. Additional transects placed in some of thesmaller vegetation regions identified in this report could potentially shift boundaryedges or influence management decisions regarding preservation.
BIRDS
Results
Twenty-seven different species were observed during the sampling of Heber Duneswith a total of 332 individuals counted. A comprehensive list is seen in Table 3. Ofthese twenty-seven species, eight were identified during non-sampling times andwere not including in determining species density and diversity. Also, two species,the cattle egret and the white-faced ibis, were observed as transient flyovers and werealso not included in the determination of either density or diversity. By far the mostabundant bird was the red-winged blackbird with a total of 100 individuals sighted onall three plots. The next most abundant species was the mourning dove with a total of58 individuals sighted, also on all three plots. The only other species with more than20 sightings was the rough-winged swallow with 24 individuals observed.
The Saltbush/Creosotebush scrub plot had the most species observed with a total of14 different species and a total individual count of 94. The Arrowweed/Baccharis plothad 13 different species with an individual count of 161. The centrally locatedTamarisk/Sand dune plot had the lowest number of both species and individualsobserved with 6 and 37 respectively.
Four species were observed on all three plots: ground dove; brown-headed cowbird;mourning dove; and red-winged blackbird. Four species were observed at only oneplot: black phoebe and western bluebird were seen only at the Arrowweed/Baccharisshrub plot while Gambel's quail and western meadowlark were seen only at theCreosote/Saltbush scrub plot.
Bird density ranged from a high of 84 birds per hectare at the Saltbush/Creosotebushscrub plot to a low of 21 birds per hectare at the Tamarisk/Sand dune plot. TheArrowweed/Baccharis plot had a density of 54 birds per hectare. Species diversitywas highest in the Saltbush/Creosote scrub habitat with a diversity index of 2.33.Arrowweed/Baccharis shrub habitat was next with a diversity index of 1.90 with theTamarisk/Sand dune habitat having the lowest diversity index of 1.27.
Discussion
Two of the three plots, Saltbush/Creosote scrub and Arrowweed/Baccharis shrub,demonstrated both high species composition and high total number of individualssighted. The common denominator between these two plots is that the majority of thevegetation in both these habitats is native, unlike the exotic tamarisk trees that formthe vast majority of the vegetation in the Tamarisk/Sand dune plot. The greaternumber of plant species and overall amount of cover provided by theSaltbush/Creosote scrub and Arrowweed/Baccharis shrub habitats is probably theprimary reason for the relatively large number of both species and individualsobserved in these two plots.
Both the Saltbush/Creosote scrub and Arrowweed/Baccharis shrub plots also hadmuch greater density and diversity numbers than did the Tamarisk/Sand dune plot,again demonstrating that the two habitats dominated by native vegetation are themost productive bird habitats at Heber Dunes. Density numbers for both native plotsare very high in relation to those found even in the most productive habitats at theOcotillo Wells State Vehicular Recreation Area which had had densities of 42 and 35birds per hectare at San Felipe Wash and Barrel Springs respectively(McClenaghan,et al. 1997). Such high densities at Heber Dunes is not unexpected considering itprovides the only native habitat in an area dominated by agricultural fields for miles inevery direction.
Diversity indices for the Saltbush/Creosote scrub and Arrowweed/Baccharis shrubhabitats are very similar to the highest indices found at Ocotillo Wells SVRA; 2.36 forSan Felipe Wash and 1.86 at Quarry Wash. This would appear to indicate thatspecies diversity for the two native plant habitats at Heber Dunes is normal for aColorado Desert site.
It appears that Heber Dunes is serving as an oasis in the middle of an agriculturaldesert for a large number of birds, in both number of species and number ofindividual birds. This can be seen in the very high number of species and individualsobserved in the two native habitats, even at a time when migratory species hadalready passed through the area. The native habitat provided by Heber Dunes, smallas it is, provides food, nesting sites, perching sites and protection from predators fora large number of resident birds and, in all probability, for a large number of migratorybirds as well during the spring and autumn months.
REPTILES
Results
A total of 5 species were found throughout the four sites sampled, all of which were observed during the sample periods. No incidental species were observed while surveying, however, at least three different snake tracks were observed in the Creosotebush scrub and the Saltbush scrub sites. One type of track was identified as a sidewinder (Crotalus cerastes) while the other track was most likely a gopher snake (Pituophus melanoluecus). The total number of specimens recorded throughout the four sites was 25.
The Saltbush scrub site had the highest number of reptiles observed with 10 sightings. Most of the sighted reptiles were western whiptails (Cnemidophorus tigris). The only other reptile observed was one side-blotched lizard (Uta stansburiana). The Creosotebush scrub had the next highest number of reptiles observed with a total of 7 sightings, 5 of which were long-tailed brush lizards (Urosaurus graciosus), one western whiptail (Cnemidophorus Tigris), and one long-nosed snake (Rhinocheilus lecontei). The Tamarisk/Sand dune and Arrowweed/ Baccharis sites both had lower numbers of species observed, however the Tamarisk/ Sand dune was the only site where sidewinders were observed.
The most abundant species appears to be the western whiptail (Cnemidophorus Tigris) with a total of 11 sightings over three of the four sites. A total of 6 Side-blotched Lizards (Uta stansburiana) were recorded over three of the four sights. All other species were observed two or less times with the exception of the long-tailed brush lizard (Urosaurus graciosus) which was recorded 5 times, but was only found in the Creosotebush Scrub.
Density ranged from 3 reptiles/hectare to 11 reptiles/hectare over the four sitessampled. The Arrowweed/ Baccharis site had the lowest density (3 reptiles/hectare).The Saltbush scrub had the highest density (11 reptiles/ hectare). The other two sites(Tamarisk/Sand dune, Creosotebush scrub) had densities of 6 and 8 reptiles/hectarerespectively.
Diversity, measured using the Shannon-Wiener function, ranged from H=0.32 toH=0.80. The Saltbush scrub had the lowest diversity (H=0.32) since thewestern whiptail comprised the majority of sightings within that habitat. TheCreosotebush scrub exhibited the highest diversity (H=0.80) of all sites because itwas the only site where more than two reptile species were observed.
Discussion
Overall, the reptile survey produced a relatively low number of species, with the total number of reptiles observed at each of the sites ranging from 3 to 10. Three of the four habitats supported just 2 reptile species, with the Creosotebush scrub habitat supporting 3 species. However, based on observations it is likely that the Saltbush scrub site supports more than two reptile species. During survey times many snake tracks were observed within this habitat, and at least 2 different tracks were identified (Crotalus cerastes, Pituophus melanoleucus). Altogether a total of 5 reptile species were observed within the Heber Dunes Park.
The most productive sites, measured in terms of density and diversity, were theCreosotebush scrub and the Saltbush scrub. Although the diversity of the Saltbushscrub was low, the results do not reflect the high occurrence of snake tracks whichare evidence that more species of reptiles utilize this habitat. It is also likely that thetwo habitats (Creosotebush scrub, Saltbush scrub) share the same speciescomposition. Although the results of the survey do not portray this trend, the habitatpreferences of the types of species observed in the two scrub sites are similar, andthe two habitats are located adjacent to each other. One exception are the long-tailedbrush lizards (Urosaurus graciosus). They most likely occur within the Creosotebushscrub only. These lizards are exclusively arboreal and typically inhabit creosote.
The Tamarisk/Sand dune site and the Arrowweed/Baccharis site exhibited similardiversity measurements ranging from H=0.63 to H=0.67. The low density/abundanceof reptiles on the Arrowweed/Baccharis site was somewhat unexpected, since thevegetation cover was extensive and many arthropods were found within this habitat.However, this site was somewhat difficult to search for reptiles due to the vegetationcover. Pit-fall traps similar to those utilized by Case and Fisher (1995) would be abetter method of sampling for reptiles in this habitat.
MAMMALS
Results
Small Mammals
The live-trapping survey consisted of 425 trap-nights of trapping effort. During thesurvey, 39 different individuals were captured a total of 44 times for a trappingsuccess rate of 10.4%. A total of three species of small mammals were capturedduring the study. None of the species captured have been designated as"endangered" or "threatened" by either federal or state agencies.
The desert pocket mouse (Chaetodipus penicillatus) was the most abundant species at the Heber Dunes site, accounting for 82% of all individuals captured; the remaining captures were evenly distributed between the deer mouse (Peromyscus maniculatus, 10%) and the cactus mouse (Peromyscus eremicus, 8%). Chaetodipus penicillatus was also the most widely distributed species, with individuals of the species being captured in all four habitat types. Peromyscus maniculatus was captured in the Saltbush and Tamarisk/Sand dune habitats, while P. eremicus was captured in the Saltbush and Creosotebush habitats.
Numbersof rodent species captured in each habitat were generally low, varying between 1 and3 species. Similarly, species diversity indices (H) reflect the low numbers of speciespresent, ranging between 0.000 (1 species, Arrowweed/Baccharis habitat) to 0.908 (3species, Saltbush habitat). Pooling all data, H for the Heber Dunes site wasestimated at 0.593.
Relative abundances for each habitat type were estimated as the number of individuals capture/100 trap-nights. Rodent abundances were highest in the Saltbush and Creosotebush communities, with estimated abundances of 14.7 and 10.7 individuals/100 trap-nights, respectively. Rodents were considerably less abundant in the Tamarisk/Sand dune and Arrowweed/Baccharis communities (6.0 individuals/100 trap-nights in both habitats). The abundance and diversity estimated for the Tamarisk/Sand dune habitat solely reflects animals living in the tamarisk which were captured in traps set along the dune-tamarisk interface.
Large Mammals
In addition to rodents, several other mammalian species inhabit the Heber Dunes site. The presence of these species was confirmed either through direct observation of the animals or through observation of signs left by animals. Audubon's cottontail (Sylvilagus audubonii) was abundant at the site, particularly in habitats adjacent to neighboring agricultural fields. Black- tailed jackrabbits (Lepus californicus) were present, though much less abundant than the cottontails. Several roundtailed ground squirrels (Spermophilus tereticaudus) were observed running down burrows along the perimeter road in the southern part of the site and tracks of coyotes (Canis latrans) were observed in the Tamarisk/Sand dune and Creosotebush habitats. The presence of the spotted skunk (Spilogale putorius) is based upon the finding of the tail of a dead individual in the Saltbush habitat.
Discussion
Probably the most striking pattern to emerge from this survey is the low diversity andabundance of rodents at the Heber Dunes site. Rodent communities at the siteconsisted of only 1-3 species and diversity indices (H) were less than 1.000 in all fourhabitats. Several species frequently found in desert habitats, such as the little pocketmouse (Perognathus longimembris) and the desert wood rat (Neotoma lepida),appear to be absent from Heber Dunes. Particularly noteworthy in this regard is theabsence of kangaroo rats (genus Dipodomys). Kangaroo rats are perhaps the mostcommon rodents occurring in Creosotescrub and Tamarisk/Sand dune habitats andsome species, such as D. deserti, are blow-sand specialists that are restricted todunes and other areas with soft, sandy soils. The absence of kangaroo rats andother rodent species from Heber Dunes was unexpected since suitable habitat forthese animals is fairly abundant at the site.
The most likely explanation for this pattern is the extinction of species following thespatial isolation of Heber Dunes from the rest of the Colorado Desert. Heber Dunesis a "habitat island" surrounded by a "sea" of agricultural land. Extinction rates onislands are a function of island size; small islands typically support small populationsthat are more susceptible to extinction through random processes or "demographicstochasticity" (sensu Gilpin and Soule, 1986) than large islands. In addition,colonization rates of islands by non-volant mammals are generally low andconsiderably less than rates of extinction (Brown, 1971).
With an area of 364 acres, Heber Dunes is clearly a "small island" for mammals and its small mammal community has probably been subjected to extinctions since its isolation. It is not possible to unambiguously identify what processes may have contributed to these extinctions. The impact of off-road vehicle activity is probably negligible, given that this activity is largely confined to unvegetated dunes which probably never supported rodent populations. One possible factor contributing to the extinction of small mammals such as Dipodomys may be competition from the dense ant populations which inhabit Heber Dunes (pers. obs.). Brown and Davidson (1977) and others have shown an inverse relationship between ant and rodent diversity and abundance in desert ecosystems, reflecting the fact that these taxa compete interspecifically for food (seeds).