Mechanisms for the keystone status of kangaroo rats: graminivory rather than granivory?

Graminivory by kangaroo rats (Dipodomys spp.) was investigated as a potential mechanism for the keystone role of these rodents in the dynamics of desert grasslands. Experiments confirmed that Ord's kangaroo rats (Dipodomys ordii) cut and consumed a large proportion of the tillers of three Chihuahuan Desert tussock-forming grass species. Field observations indicated that the characteristically cut grass tillers were absent from all-rodent and medium-sized kangaroo rat exclosures, but were frequent in large-sized kangaroo rat and rabbit exclosures, indicating that the medium-sized kangaroo rats (D. ordii, D. merriami) were responsible for grass cutting. Tiller waste as a percentage of peak standing crop ranged from 7% in grassland habitats to 0.7% in Flourensia cernua shrubland. Of the 13 species of perennial, tussock-forming grasses measured, only one, Muhlenbergia porteri, had no tillers cut by kangaroo rats. This study demonstrates that the keystone role of kangaroo rats in Chihuahuan Desert grassland ecosystems is probably the result of their graminivory. Received: 28 October 1996 / Accepted: 26 February 1997     

Why do kangaroo rats (Dipodomys spectabilis) footdrum at snakes?

We examined alternative hypotheses for the benefits of footdrummingin the presence of snakes by the banner-tailed kangaroo rat,Dipodomys spectabitis, by testing whether the target of thesignal includes conspecifics, the predator or both. Footdrummingrecorded in the field revealed that rats altered their footdrummingsignatures when drumming at snakes. In playback tests, however,neighbors failed to show any measurable change in behavior tobroadcasts of the snake drumming pattern, but mothers footdrummedsignificantly more than nonmothen in the presence of a tetheredsnake. Gopher snakes, Pituophis melanolsucus affinis, respondedto footdrumming vibrations created by a mechanical thumper.Nonhungry snakes avoided footdrumming, while hungry snakes approachedthe seismic footdrumming. Snakes decreased stalking rates asfootdrumming increased, but they spent more time stalking drummingthan nondnimming rats. We conclude that D. spectabilis footdrumsin individual defense and in parental care, rather than to warnadult conspecfics. Footdrumming deters pursuit by informingthe snake that the rat is alert and the chances of predationare low. We find little evidence that footdrumming startles,confuses, or harasses the snake. Hungry gopher snakes, however,may locate prey by eavesdropping on territorial footdrumming     

Burrowing activities of kangaroo rats and patterns in plant species dominance at a shortgrass steppe-desert grassland ecotone

Abstract. Our objective was to evaluate the effects of burrowing activities by banner-tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off-mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis-dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda-dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas. Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small-scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.     

Spatial population structure in the banner-tailed kangaroo rat,Dipodomys spectabilis

I attempted to characterize spatial units of local dynamics and dispersal in banner-tailed kangaroo rats (Dipodomys spectabilis), to determine if spatial structure influenced population dynamics in the way predicted by current metapopulation models. D. spectabilis exhibited a hierarchical spatial structure. Local populations that appeared as discrete entities on a scale of kilometers were subdivided into clusters of mounds on a scale of meters. This structure, however, cannot be characyerized in terms of the discrete habitat patches envisioned by the metapopulation models. Occupied areas were statistically distinguishable from the surrounding matrix, but this difference was only quantitative. There were no discrete boundaries between occupied areas and the matrix. Habitat within occupied areas was heterogeneous, and occupied areas in different locations were statistically distinguishable from each other. High heterogeneity within occupied areas, and high contrast among them, make it difficult to define what is a suitable habitat patch for D. spectabilis. On a smaller spatial scale, there was significant aggregation of resident mounds within occupied areas. These aggregations, however, do not correspond to discrete habitat patches. Rather, they appear to result from an interaction between fine-scale habitat heterogeneity and limited dispersal due to natal philopatry and low adult vagility. These complications make it difficult to identify habitat patches independent of the species' distribution. For species like D. spectabilis that are patchily distributed but do not occupy discrete habitat patches, a patch occupancy approach does not seem appropriate for describing spatial structure. Hierarchical spatial structure underscores the need for a framework that incorporates multiple scales of spatial structure, rather than one that pre-imposes a single spatial scale as being important for population dynamics. A framework that (i) considers patchiness as a combination of both habitat heterogeneity, and life-history and behavioral characteristics, and (ii) incorporates hierarchical spatial structure, appears to be the most suitable for conceptualizing spatial dynamics of behaviorally complex vertebrates such as D. spectabilis.     

Rapid genetic restoration of a keystone species exhibiting delayed demographic response

Genetic founder effects are often expected when animals colonize restored habitat in fragmented landscapes, but empirical data on genetic responses to restoration are limited. We examined the genetic response of banner‐tailed kangaroo rats (Dipodomys spectabilis) to landscape‐scale grassland restoration in the Chihuahuan Desert of New Mexico, USA. Dipodomys spectabilis is a grassland specialist and keystone species. At sites treated with herbicide to remove shrubs, colonization by D. spectabilis is slow and populations persist at low density for ≥10 years (≥6 generations). Persistence at low density and low gene flow may cause strong founder effects. We compared genetic structure of D. spectabilis populations between treated sites and remnant grasslands, and we examined how the genetic response to restoration depended on treatment age, area, and connectivity to source populations. Allelic richness and heterozygosity were similar between treated sites and remnant grasslands. Allelic richness at treated sites was greatest early in the restoration trajectory, and genetic divergence did not differ between recently colonized and established populations. These results indicated that founder effects during colonization of treated sites were weak or absent. Moreover, our results suggested founder effects were not mitigated by treatment area or connectivity. Dispersal is negatively density‐dependent in D. spectabilis, and we hypothesize that high gene flow may occur early in the restoration trajectory when density is low. Our study shows genetic diversity can be recovered more rapidly than demographic components of populations after habitat restoration and that founder effects are not inevitable for animals colonizing restored habitat in fragmented landscapes.     

Search for lunar periodicity in the bannertail kangaroo rat (Dipodomys spectabilis)

Abstract

The endogenous activity cycle of the nocturnal bannertail kangaroo rat was investigated. Although bannertail activity is a function of the lunar day as well as the solar day, all ten subjects exhibited free‐running activity periods of solar‐day length; there was no evidence of an endogenous lunar‐day cycle. Animals were provided with a burrow system and a small pseudo‐desert, a laboratory facility in which animal activity data closely resembled measurements taken in the field. Several analytical techniques for quantifying the data were utilized, and one, the mean interval of activity, is recommended to other investigators.     

Deterring rodent seed‐predation using seed‐coating technologies

With many degraded environments undergoing restoration efforts, there is a growing need for the optimization of direct seeding practices. Seeds planted on wildlands are often consumed by rodents, leading to reduced plant establishment. Coating seeds in rodent aversive products may prevent seed‐predation. We tested 10 seed‐coating formulations containing products expected to deter rodents, namely: ghost and cayenne pepper powders; essential oils from bergamot, neem, and pine; methyl‐nonyl‐ketone, anthraquinone, activated carbon, beta‐cyclodextrin, and a blank coating containing no rodent deterrents to serve as a control treatment. Each treatment was applied to Pseudoroegneria spicata (bluebunch wheatgrass) seeds. These seeds germinated similarly to uncoated control seeds unless the coating contained methyl‐nonyl‐ketone which reduced germination. When seeds were offered to Ord's kangaroo rats (Dipodomys ordii), they strongly avoided the treatments in favor of uncoated control seeds. Notably, the blank coating, lacking active ingredients, still elicited 99% avoidance. However, these results indicated behavior when alternative food sources are readily available, a scenario rare in nature. To address this, a second feeding experiment was conducted to observe D. ordii's behavior under calorie‐restricted conditions. D. ordii were subjected to a fast period, then offered only one treatment. Under these conditions, many subjects chose to consume coated seeds, but to a lesser degree than subjects offered control seeds. Seeds coated in ghost pepper, neem oil, and activated carbon reduced consumption by 47–50%. Given these lab results, we would expect these treatments to increase native plant establishment following the direct seeding of wildlands by protecting seeds from rodent predation.     

Characterization of nine microsatellite loci on endemic kangaroo rats Dipodomys simulans peninsularis from southern Baja California Peninsula

Nine polymorphic microsatellite loci were isolated to investigate the effects of habitat fragmentation on gene flow and genetic variation of the peninsular kangaroo rat Dipodomys simulans peninsularis on agricultural landscape from the Baja California Peninsula. The markers had an average of 9.11 alleles per locus (range 2–14), with mean observed and expected heterozygosities 0.646 (range 0.333–0.900) and 0.788 (range 0.284–0.932), respectively. No evidence of linkage disequilibrium was found across pairs of loci, and only one locus exhibited evidence of null alleles.