California Sage Scrub Research Articles

Effects of artificial light on foraging behavior of the Pacific Kangaroo Rat (Dipodomys agilis)

Abstract Artificial light at night is increasingly recognized as a cause of habitat degradation, and as a conservation threat to wildlife species. We studied the behavioral response of Pacific kangaroo rats (Dipodomys agilis) to natural and artificial light in coastal sage scrub in southern California, United States. We used digital trail cameras to monitor visits to seed trays in shrub and open, intershrub microhabitats. Under natural-light conditions, kangaroo rats were more active on full- than new-moon nights, and concentrated foraging beneath shrubs. Surprisingly, kangaroo rats frequently interacted with other rodents in seed trays. Near to an artificial light source, kangaroo rats removed fewer seeds, had fewer foraging bouts, and spent less time in trays—especially in brightly lit open microhabitats—compared to distances farther from the light. Differences between the first and second nights of trials during both experiments suggested that kangaroo rats learned locations of resource-rich seed trays and modified their behavior accordingly. Our results are similar to recent studies showing that artificial light depresses foraging activity of rodents in the areas of highest illumination, and adds to evidence of the need to mitigate impacts of urban lighting in fragments of coastal sage scrub, especially along the urban–wildland interface.

Struggling to survive: A comparison of Vanessa cardui larval survivorship on putative host plants

AbstractThe painted lady butterfly (Vanessa cardui) is a generalist herbivore with a global distribution. In North America, over 100 species have been identified as V. cardui larval host plants. This cosmopolitan species is commercially available in all life stages and can be reared on an artificial diet. As a result, V. cardui commonly serves as a model organism for education and outreach. However, gaps in our knowledge remain with regard to V. cardui ecology and larval host plant suitability for supporting complete development of larvae to reproductive adulthood. In a laboratory setting, we tested host plant suitability of five reported host plants commonly found in California sage scrub ecosystems by assessing survival of V. cardui caterpillars in its entirety. We also assessed how commercially sourced larvae survived on artificial diet: both raising the caterpillars exclusively on artificial diet as well as switching diets from artificial to thistle (a commonly reported host plant) and switching from thistle to artificial diet. We found that all commercially sourced caterpillars exhibited both high larval mortality and low pupation rates when reared on host plants, although larvae reared on Malacothamnus fasciculatus and Sphaeralcea ambigua survived longest. Moreover, M. fasciculatus and S. ambigua were the only host plants tested that supported successful pupation. This contrasted with 63% higher survival of wild‐collected larvae. Our findings suggest that commercially obtained V. cardui may struggle to utilize wild host plants and that future investigations into host plant suitability in wild populations are needed. Additionally, future research using commercial larvae should consider the implications for interpreting host plant suitability as larvae may exhibit adaptations to artificial diet or the loss of adaptation to consuming plant material.

An Illusion of Barriers to Gene Flow in Suburban Coyotes (Canis latrans): Spatial and Temporal Population Structure across a Fragmented Landscape in Southern California

Carnivores with large home ranges are especially vulnerable to habitat fragmentation. As coyotes (Canis latrans) are often found living in highly modified landscapes, it is unclear how urban and suburban development impact gene flow between their populations. This study evaluated gene flow among coyotes inhabiting California sage scrub fragments within the highly developed Pomona Valley, California. We genotyped microsatellites from scat samples collected from four study sites to examine population structure between coyotes separated by a major freeway, coyotes separated by suburban development, and finally, coyotes in contiguous, natural habitat sites over 15 months. Though coyotes from all four sites were genetically distinct, near-complete turnover of individuals in sites and examination of temporal genetic structure and relatedness within one site indicated the movement of family groups through natural fragments over time. Thus, we argue that solely examining spatial genetic structure may create the illusion of genetic barriers among coyote populations where they may not exist, and that incorporating temporal components of genetic variation is critical to understanding gene flow across space and time in highly mobile animals. Understanding how to better study and manage coyotes, an apex predator, is key to the conservation of the endangered California sage scrub ecosystem.

Effects of Mustard Invasions on Soil Microbial Abundances and Fungal Assemblages in Southern California

Although mustards (family, Brassicaceae) are common across southern California, research has not focused on the effects of type-conversion of native California sage scrub (CSS) to areas dominated by invasive mustards. To better understand how mustard invasions, primarily the short-pod mustard, Hirschfeldia incana, impact soil microbial assemblages, we examined microbial abundance and assemblages from intact CSS and adjacent mustard-dominated soils at three sites. We also explored if germination rates for various plant species differed between CSS and mustard soils. We found that mustard invasions reduce soil microbial abundances by more than 50% and alter soil fungal assemblages. Fungal richness, diversity, and evenness did not differ between habitats, highlighting that these habitats harbor unique microbial assemblages. While mustard allelopathy is predicted to be the primary driver of these changes, mustard invasions also increased soil pH. Although functional consequences of these shifts are unknown, low mustard germination in CSS soils supports biological resistance to mustard invasion in CSS. Overall, our results demonstrate that mustard invasions, H. incana in particular, exert a strong selecting force on soil microbial assemblages, which can influence effective CSS restoration and preservation of ecosystem services.

Herbivore Influence on Post-Fire California Sage Scrub Plant and Soil Microbial Assemblages

California sage scrub (CSS) is an endangered, shrub-dominated, southern California ecosystem type threatened by increasing fire frequencies and conversion to non-native grasslands. As CSS harbors a rich endemic flora and fauna and is critical for maintaining key ecosystem services; significant attention has been focused on understanding how many key factors (N-deposition, drought, and competition with non-natives) influence post-fire CSS recovery. Unfortunately, few studies have examined the role of herbivores. For five years, we investigated the influence of herbivores on post-fire CSS succession by annually surveying plant assemblages in control (herbivores allowed access) and experimental (herbivore excluded) plots at the Bernard Field Station following a fire in May 2017. We found that herbivores reduce the cover of both invasive grasses and native shrubs, and negatively impact Artemisia californica survivorship. Consequently, post-fire plant restoration efforts should not discount the effects of herbivores. While native forb cover was elevated in plots that permitted herbivores, plots near the fire edge experienced increased herbivory pressure and lower cover of most plants. Despite modifications in plant assemblages, we did not observe differences in soil microbe abundances, fungi assemblage structure, or nutrient concentrations. We suggest that restoration practitioners consider caging ‘shrub regeneration’ areas to facilitate post-fire CSS recovery.

Flower-Visiting Insect Assemblages on Fall-Blooming Native California Sage Scrub Shrubs

Pollinator studies in the endangered California sage scrub ecosystem have focused on spring insect assemblages, when most plant species bloom. Consequently, the insect assemblages using common fall-blooming sage scrub shrubs Lepidospartum squamatum, Ericameria pinifolia, and Baccharis pilularis remain undescribed. Our study aimed to: (1) document flower-visiting insect assemblages on fall-blooming shrubs, (2) assess the efficacy of three sampling techniques in inventorying insect assemblages, and (3) explore, using DNA metabarcoding, which plants are utilized and the extent to which surrounding suburban habitats’ plants are also used. While elevated sampling is required to inventory flower-visiting insects, we describe a diverse assemblage consisting of 123 species. Insect assemblages differed between L. squamatum and B. pilularis, as well as, E. pinifolia and B. pilularis, but not between L. squamatum and E. pinifolia. Direct sampling approaches (netting and photo documentation) collected 115 species not collected by passive malaise traps, highlighting that active observations are required to describe flower-visiting insect assemblages. Sequencing the ITS2 region of pollen from abundant visitors revealed that a majority of pollen is from the sage scrub ecosystem, highlighting its value. Our results indicate that the presence of fall-blooming shrubs may be critical for maintaining diverse sage scrub insect and pollinator assemblages.

Community Science Reveals High Diversity of Nectaring Plants Visited by Painted Lady Butterflies (Lepidoptera: Nymphalidae) in California Sage Scrub.

California's sage scrub habitats support a diversity of nectar and host plants for migrating and resident populations of painted lady butterflies (Vanessa cardui) throughout all seasons. North America experiences spring V. cardui migrations involving butterflies totaling in the millions in some years. These irruptive years are thought to be driven by winter weather patterns at breeding grounds near the US-Mexico border and due to their irregularity, it is difficult to study floral resource use along the migration route. Here we used the community science platform iNaturalist to quantify patterns in V. cardui nectar resource use in sage scrub over time and space during irruptive and nonirruptive years. We identified over 329 different nectaring plant species of varying functional types (72% native to California) visited by adult V. cardui, 195 of which had not been previously identified as known nectar plants for V. cardui. Vanessa cardui butterflies were observed in similar locations regardless of whether an irruptive migration occurred, indicating the presence of either sparse migrants or resident populations across California. Moreover, irruptive years were positively correlated with warmer and wetter local conditions at observation locations. Our results provide new insights into patterns of floral resource use by North American V. cardui by harnessing the power of community science data and while highlighting the factors associated with its North American migration.

Soil bacterial assemblage responses to wildfire in low elevation southern California habitats.

Understanding how wildfires and modification in plant assemblages interact to influence soil bacteria assemblages is a crucial step in understanding how these disturbances may influence ecosystem structure and function. Here, we resampled soil from three study sites previously surveyed in spring 2016 and 2017 and compared soil bacterial assemblages prior to and six months after (spring 2019) the 2018 Woolsey Fire in the Santa Monica Mountain National Recreation Area using Illumina sequencing of the 16S rRNA gene. All sites harbored both native California sage scrub and a non-native (grassland or forbland) habitat, allowing us to examine how fire influenced bacterial assemblages in common southern California habitats. Most results contrasted with our a-priori hypotheses: (1) richness and diversity increased following the fire, (2) heat/drought resistant and sensitive bacteria did not show consistent and differing patterns by increasing and decreasing, respectively, in relative abundance after the fire, and (3) bacterial assemblage structure was only minimally impacted by fire, with no differences being found between 2017 (pre-fire) and 2019 (post-fire) in three of the six habitats sampled. As sage scrub and non-native grasslands consistently harbored unique bacterial assemblages both before and following the fire, modifications in plant compositions will likely have legacy effects on these soils that persist even after a fire. Combined, our results demonstrate that bacterial assemblages in southern California habitats are minimally affected by fire. Because direct impacts of fire are limited, but indirect impacts, e.g., modifications in plant compositions, are significant, plant restoration efforts following a fire should strive to revegetate sage scrub areas to prevent legacy changes in bacterial composition.

Factors Affecting the Presence and Abundance of Amphibians, Reptiles, and Small Mammals under Artificial Cover in Southern California

Artificial cover objects (ACOs) are known to attract small terrestrial vertebrates, but the actual parameters that attract species to ACOs can vary across geographic regions and climates. For this study ACOs were placed in coastal sage scrub and grassland habitats in southern California and surveyed weekly for small terrestrial vertebrates over a 4-yr period. We observed 1643 individuals of 34 taxa during 143 survey sessions totaling 16,312 ACO days. Overall species richness and abundances under ACOs were highest from February to April. Our results showed that the probability of encountering a reptile under an ACO was highest in February and March under large wooden ACOs containing moist soil, when temperatures under the ACO were relatively mild, and when minimum air temperatures were low. At the community level, encounter probabilities for small mammals were highest for large wooden ACOs and ACOs with low soil moisture, with several species differences. Amphibians of three species were only captured in low numbers primarily under wooden ACOs from November to March. Use of ACOs in research and monitoring is an inexpensive and simple way to document and capture a wide variety of small terrestrial vertebrates. Species richness and abundance can be maximized over short periods using ACOs and can be equally important for long-term monitoring, particularly once the factors that make ACO use effective for small terrestrial vertebrates of a certain region have been investigated.

Virus discovery in winter-growing perennial plants of southern California sage scrub habitat

Plant viruses cause serious disease and losses in domesticated crops. However, we know little about plant viruses outside of agriculture. One reason for this is the lack of symptoms of virus infection in wild plants to promptly diagnose and identify with targeted methods. This is now changing with the availability of untargeted “next-generation” sequencing technologies to analyze the viromes of asymptomatic wild plants and study virus impacts on plant health. In this study, we determined whether key winter-growing perennials in local reserves are infected with crop-associated viruses. A previous study in the same locations found that summer-growing perennials host co-infections by multiple crop-associated viruses, but winter-growing plants have not been explored. To enrich for virus sequences, we extracted doubled-stranded RNA, a unique feature of virus replication. We sequenced this material using the Illumina NextSeq platform, then assembled and identified viruses using Galaxy software. In contrast to summer-growing plants, we detected just one crop-associated virus in winter plants: the generalist, Cucumber mosaic virus (CMV). Phylogenetic analysis shows this CMV is in subgroup IA, the most prevalent in the United States. Our results suggest that winter-growing plant viromes have minimal overlap with summer-growing plant viromes and that winter plants may be less exposed to crop-associated viruses.

Granivory in California sage scrub: implications for common plant invaders and ecosystem conservation

Granivory in California sage scrub: implications for common plant invaders and ecosystem conservation

Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub.

Non-native plant invasions, changes in fire regime, and increasing drought stress all pose important threats to biodiverse mediterranean-climate shrublands. These factors can also interact, with fire and drought potentially creating opportunities for non-native species to establish dominance before native shrubs recover. We carried out post-fire demographic monitoring of the common native shrub Artemisia californica in a southern California sage scrub fragment for 7 years, including several with very low rainfall. Experimental removals of non-native plants were included for the first 4 years. We quantified A. californica post-fire crown resprouting and seedling emergence, and tested effects of precipitation, non-native plants, and their interactions on seedling and adult survival. Only 7 A. californica were confirmed as resprouts; almost all individuals established after the fire from seedlings, with 90% of emergence occurring in the second growing year after fire (spring 2015). Higher spring precipitation increased both adult and seedling survival. Non-native grasses and forbs rapidly recolonized control plots, but the removal treatment reduced non-native cover by nearly 60%. For seedlings, non-native removal reduced the probability of dropping leaves by start of summer drought and increased survival both directly and through positive interactions with rainfall. Non-native removal also reduced mortality in smaller adult plants. By 2020, mean A. californica canopy area was nearly four times greater in non-native removal plots. These findings reinforce the high vulnerability of sage scrub habitat to post-fire loss of shrub cover and potential type conversion, particularly with increasing drought frequency and in stands and species with limited crown resprouting. Yet they also illustrate the potential for targeted management of non-natives immediately after fire to promote recovery of native shrubs in this increasingly endangered community.

Type conversion from native California sage scrub to non-native grassland accelerates decomposition processes

Predicting how plant invasions influence decomposition processes is difficult, as multiple factors change with plant community alteration. Here, we examined the importance of various abiotic and biotic factors (litter quality (C:N), UV radiation, and macroinvertebrate access) in driving early decomposition processes in native California sage scrub (CSS) and in adjacent non-native grassland habitats. Many findings from our experiment were consistent with studies from semi-arid regions (e.g., UV radiation drove decomposition processes and high quality litter decayed more rapidly). However, the acceleration of litter decomposition in the non-native grassland relative to native California sage scrub (CSS) was unexpected and contrasts with studies of partially invaded CSS systems. Our results highlight that studies of invaded systems that still retain native species diversity should not be extrapolated to type-converted systems. We hypothesize that high grass cover in the type-converted grassland created conditions that enhanced soil moisture, bacterial abundance, and macroinvertebrate influence relative to the CSS, accelerating decomposition and reducing regional C storage.

Vertebrate herbivory on shrub seedlings in California sage scrub: important but understudied interactions

Herbivory is poorly characterized in the endangered California sage scrub ecosystem, because previous studies were conducted only in winter and spring. To better understand vertebrate herbivore impacts, we first examined if and when vertebrate herbivores influence Artemisia californica seedling survivorship by examining differences in mortality between caged and uncaged seedlings. Then, we examined: (1) vertebrate herbivore preference of common sage scrub shrub species, and (2) which vertebrate species constitute important herbivores in the ecosystem. We found that mortality for uncaged A. californica seedlings was twice that of caged seedlings, and occurred in the summer and early fall, when previous studies in the system were not conducted. Short leaved shrubs, Artemisia californica and Eriogonum fasciculatum, were more susceptible to herbivory than broader leaved Salvia apiana and Eriodictyon trichocalyx. However, susceptibility likely depends on the herbivore assemblage, as the three main herbivore species [Sylvilagus audubonii (desert cottontail), Zonotrichia leucophrys (white-crowned sparrow; a bird, not a mammal), and Otospermophilus beecheyi (California gray squirrel)] all had different shrub preferences. Our results contrast with previous studies that suggests that native vertebrate herbivores facilitate shrub seedling survivorship by highlighting that late-season vertebrate herbivory may negatively affect shrub seedling survivorship. We demonstrate the complex but understudied role herbivores play in the sage scrub ecosystem, and highlight that further study of these key ecological interactions is warranted.

Impacts of invasive annuals on soil carbon and nitrogen storage in southern California depend on the identity of the invader.

Non‐native plant invasions can alter nutrient cycling processes and contribute to global climate change. In southern California, California sage scrub (hereafter sage scrub), a native shrub‐dominated habitat type in lowland areas, has decreased to <10% of its original distribution. Postdisturbance type‐conversion to non‐native annual grassland, and increasingly to mustard‐dominated invasive forbland, is a key contributor to sage scrub loss. To better understand how type‐conversion by common invasive annuals impacts carbon (C) and nitrogen (N) storage in surface soils, we examined how the identity of the invader (non‐native grasses, Bromus spp.; and non‐native forbs, Brassica nigra), microbial concentrations, and soil properties interact to influence soil nutrient storage in adjacent native and invasive habitat types at nine sites along a coast to inland gradient. We found that the impact of type‐conversion on nutrient storage was contingent upon the invasive plant type. Sage scrub soils stored more C and N than non‐native grasslands, whereas non‐native forblands had nutrient storage similar to or higher than sage scrub. We calculate that >940 t C km−2 and >60 t N km−2 are lost when sage scrub converts to grass‐dominated habitat, demonstrating that grass invasions are significant regional contributors to greenhouse gas emissions. We found that sites with greater total C and N storage were associated with high cation exchange capacities and bacterial concentrations. Non‐native grassland habitat type was a predictor of lower total C, and soil pH, which was greatest in invasive habitats, was a predictor of lower total N. We demonstrate that modeling regional nutrient storage requires accurate classification of habitat type and fine‐scale quantification of cation exchange capacity, pH, and bacterial abundance. Our results provide evidence that efforts to restore and conserve sage scrub enhance nutrient storage, a key ecosystem service reducing atmospheric CO2 concentrations.

Distributions of Invasive Arthropods across Heterogeneous Urban Landscapes in Southern California: Aridity as a Key Component of Ecological Resistance.

Urban systems often support large numbers of non-native species, but due to the heterogeneity of urban landscapes, species are not evenly distributed. Understanding the drivers of ecological resistance in urban landscapes may help to identify habitats that are most resistant to invasion, and inform efforts to model and conserve native biodiversity. We used pitfall traps to survey non-native ground-dwelling arthropods in three adjacent, low-elevation habitat types in southern California: California sage scrub, non-native grassland, and suburban development. We found that non-native species were fewer and less widely distributed in the sage scrub and grassland habitats. Due to the proximity of our sites, differences in propagule pressure is an unlikely explanation. Instead, we suggest that the absence of water subsidies in the sage scrub and grassland habitats increases those habitats’ resistance to arthropod invasions. Comparisons to studies conducted at fragments closer to the coast provide further support for the relationship between aridity and invasibility in southern California. Our findings highlight that inland fragments are important for conserving native arthropod diversity, that models of non-native species distributions in arid and semi-arid urban systems should include aridity measures, and that reducing resource subsidies across the region is critical to mitigating spread of non-natives.

Assessment of Topsoil Salvage and Seed Augmentation in the Restoration of Coastal Sage Scrub on Santa Catalina Island, California

The Catalina Island Conservancy manages over 22,000 acres of coastal (California) sage scrub (CSS) habitat off the coast of Southern California, including much of the infrastructure, roads, and recreation associated with habitat disturbance. Restoration trials were initiated to establish best management practices for roadside and disturbed site revegetation under local site conditions. Site treatments compared excavated native subsoil and surface soil (±20 cm) reapplication practices with and without seed augmentation. Evaluation of native and nonnative germinants indicated that salvaged surface soil alone may not be sufficient to restore native cover. Seeding improved native cover and reduced establishment of nonnative species on unirrigated sites. Evaluation of water potential of seedlings indicated that high-frequency irrigation may favor establishment of nonnative annual grasses. Most compelling for land managers is the indication that CSS habitats may contain a significant nonnative seed bank in spite of the composition of aboveground vegetation, such that disturbance may facilitate habitat-type conversion without seed augmentation or weed control.

Fire Impacts on Ant Assemblages in California Sage Scrub

Few studies have examined impacts of fire on invertebrates in southern California ecosystems despite the fact they harbor diverse invertebrate assemblages with many narrowly endemic species. California sage scrub, an endangered ecosystem type of low-elevation areas in southern California, is increasingly threatened by altered fire regimes and type conversion to non-native grasslands often resulting from fire disturbances. The effect of fire on ant assemblages in a patch of recovering sage scrub was examined by using adjacent intact sage scrub and non-native grassland habitats as checks. While short-term effects in ant activity potentially associated with temporary changes in habitat structure and abiotic conditions were observed, ant assemblages in the burned area did not differ between summer 2013 (pre-fire) and fall 2013 (post-fire), or between spring 2013 (pre-fire) and spring 2014 (post-fire). Results indicated that either the ant assemblages recover quickly, or more likely, pre-existing ant assemblage remained and were not directly impacted by fire. Our results, combined with the few other studies that examined impacts of fire on sage scrub invertebrates, are consistent with an emerging hypothesis that ground-dwelling ant/invertebrate assemblages are little affected by direct impacts of fire in semi-arid systems and areas where fires are common.

Carbon and nitrogen in the topsoils of Inceptisols and Mollisols under native sage scrub and non-native grasslands in southern California

Understanding how invasive plants influence terrestrial carbon (C) and nitrogen (N) budgets is important in the context of global climate change. In southern California, type-conversion, a process in which native California sage scrub is type-converted to non-native grassland, is thought to negatively impact total C and N storage in surface soil horizons. To better understand the extent to which type-conversion influences regional nutrient storage, we examined C and N concentration (%) and quantity (g/m2), key soil properties, and microbial abundances and assemblages in sage scrub and non-native grassland habitats at three sites that represent varying environmental conditions. Type-conversion decreased soil C concentration, but did not influence C quantity. Differences between these two metrics were driven by a higher aggregate soil density in the non-native grassland habitat compared to the sage scrub habitat at one site. Contrary to previous studies, we found that type-conversion did not impact total N storage, even in a site previously found to have increased soil N quantities under sage scrub. Sage scrub habitats contained more active fungi, and differences in microbial assemblages were found between habitat types. Despite the vast number of microbial OTUs, habitats harbored unique communities of microbial taxa with some species consistently more abundant in one habitat type across sites. Our results demonstrate that type-conversion negatively impacts topsoil C concentrations, but accurate modeling of nutrient stocks requires consideration of the links between vegetation structure, soil properties such as soil density, and microbial communities that vary significantly across small spatial scales. Collectively, we demonstrate that invasive grasses alter microbial communities and reduce soil C storage capacity in the region.

Body size, not phylogenetic relationship or residency, drives interspecific dominance in a little pocket mouse community

The role of interspecific aggression in structuring ecological communities can be important to consider when reintroducing endangered species to areas of their historic range that are occupied by competitors. We sought to determine which species is the most serious interference competitor of the endangered Pacific pocket mouse, Perognathus longimembris pacificus, and more generally, whether interspecific aggression in rodents is predicted by body size, residency status or phylogenetic relatedness. We carried out simulated territory intrusion experiments between P. longimembris and four sympatric species of rodents (Chaetodipus fallax, Dipodomys simulans, Peromyscus maniculatus, Reithrodontomys megalotis) in a field enclosure in southern California sage scrub habitat. We found that body size asymmetries strongly predicted dominance, regardless of phylogenetic relatedness or the residency status of the individuals. The largest species, D. simulans, was the most dominant while the smallest species, R. megalotis, was the least dominant to P. longimembris. Furthermore, P. longimembris actively avoided encounters with all species, except R. megalotis. One management recommendation that follows from these results is that P. longimembris should not be reintroduced to areas with high densities of D. simulans until further research is carried out to assess the fitness consequences of the interactions. Our finding that the species least similar in body size is the most serious interference competitor of P. longimembris highlights an important distinction between interference and exploitative competition in rodent communities.