Facilitative, or positive interactions that benefit at least one participant and cause harm to neither,have been described from plant communities around the globe. Although the study of negative interactions such as competition has dominated the literature in the past, in the last 2 decades facilitation has been recognized as an important driving force in plant community assembly. Positive plant-plant interactions can occur when one plant makes the microenvironment more favorable for another plant either directly (by regulating temperature, moisture or nutrient stress) or indirectly by eliminating competing plant species or deterring predators (Bruno et al. 2003).
The bulk of evidence for facilitative effects between plants comes from severe environments, such as deserts, arctic or alpine systems, tundra systems, or salt marshes (Brooker et al. 2008). Bertness and Callaways seminal 1994 paper proposed that increases in the degree of abiotic stress would increase the frequency of positive interactions, so understanding the balance of positive and negative interactions can be best understood by comparing them along gradients of abiotic stress.
Nurse plants are often the focus of studies of facilitation. A nurse plant is one that benefits other plant species through mechanisms such as stress amelioration, herbivore protection, and substrate modification (Callaway 1995). Many examples of plant-based facilitation come from arid and semi-arid environments where dominant desert shrubs facilitate annual species that ordinarily would not be able to grow there. This photo illustrates the nurse plant effect quite well…you can see here a mat of green annual vegetation underneath this desert shrub while annual vegetation is nearly absent from adjacent open areas.
Nesting plant-animal interactions into known plant-plant interaction networks is rare. Positive plant-plant interactions are an important current topic and provide a great opportunity to explore the extended importance of these facilitative interactions to other taxa. For plant-animal interactions, especially those nested within positive plant-plant interaction networks, variation in abiotic conditions is likely very important. So, it’s important that these studies also incorporate gradients to examine whether the sign, strength, or frequency of interactions is dependent on variation or ‘stress’ within the environment. Chapters 2 and 3 of my thesis project involve testing aspects of this interaction.
For the first chapter of my thesis I will be conducting a systematic review regarding the positive impacts of shrubs on animals to identify any areas or mechanisms that have been extensively studied as well as any possible research gaps. There are no reviews that I am aware of that specifically target positive shrub-animal interactions. I am only including those papers testing positive effects of shrubs focusing on the animal side of the interaction. I searched the web of knowledge database using the search terms shrub* and animal* limiting the search to relevant research areas. The resulting 844 articles were exported into Microsoft Excel and reviewed for inclusion criteria resulting 122 retained articles. Data extracted from each article includes the type of interaction (direct/indirect); nature of interaction; evidence of any reciprocal interactions; the shrub and animal species examined; climate data; and GPS coordinates. To identify the geographic distribution of these studies GPS coordinates were extracted from each article. Work for this systematic review is ongoing. I have to finish extracting data and I will be adding additional search terms to ensure I have identified all of the relevant research on the topic to date.
Here is a map of the geographic distribution of those studies. Although the studies are widespread around the globe the bulk of the studies have been conducted in North America especially in the mid-west and south-west United States as well as in eastern Europe. Currently I am finishing extracting all other relevant data from these studies.
Field work for chapters 2 and 3 of my project will take place within the Mojave National Preserve which is located in the Mojave Desert of San Bernardino County, California.
My field site is located along Kelbaker Road, a 56 mile paved road that runs through the reserve. The vegetation in this area is dominated by the creosote shrub Larrea tridentata, and to a lesser extent Ambrosia dumosa(Pavlik 1989; Beatley 1974). Yucca brevifolia (the Joshua tree)also makes up a large portion of the vegetation at higher elevations (Brooks and Matchett 2006). The dominant shrub Larrea tridentataand it’s effects on resident kangaroo rats will be the focus of this research.
The genus Dipodomys is a group commonly referred to as kangaroo rats due to their bipedal locomotion. They are nocturnal, burrowing rodents of the family Heteromyidae, endemic to western North America. They are primarily granivorous (store seeds in cheek pouches) but will occasional consume vegetation. Krats have been shown to be important components of desert ecosystems directly through soil disturbance (burrow creation increases soil nitrogen and moisture levels by mixing surface soils with subsurface soils) and indirectly through preferential seed selection (select seeds based on size and moisture content which leads to the promotion of some species and possible suppression of others). These animals may be reciprocally impacting resident shrub and annual plants, so a better understanding of their use of shrubs is important in understanding the ecology of the system. 3 kangaroo rat species are found in the Mojave National Preserve: Dipodomys deserti (the desert kangaroo rat), Dipodomys Merriami(Merriam’s kangaroo rat), and Dipodomys microps (the chisel-toothed kangaroo rat) (The IUCN Red List of Threatened Species).
The 2nd chapter of my project will consist of an extensive survey of kangaroo rat activity and annual vegetation in shrub and open microsites along an elevational gradient (located on Kelbaker rd) to examine the relative impact of desert shrubs and desert annuals on kangaroo rat activity and how differences in elevation effects these interactions. The study site will be broken up into 5 sub-sites that differ in elevation by approximately 200 meters. I hope to survey at least 100 shrub/open pairs per elevational sub-site. In order to assess the relative important of shrubs vs. annuals on animal activity I will be removing all annuals from 50 shrub/open pairs per elevational subsite. Animal measurements include counting scat, burrows, tracks, trails, and determining total disturbance. Shrub dimensions and annual measurements such as the number of annual species, % cover, and the number of individuals will also be recorded as well as environmental measures such as elevation, temperature, and humidity.
Animal measurements include counting scat, burrows, tracks, trails, and determining total disturbance. Shrub dimensions and annual measurements such as the number of annual species, % cover, and the number of individuals will also be recorded as well as environmental measures such as elevation, temperature, and humidity.
I conducted a pilot survey in January of this year. I surveyed 125 shrub/open microsites along the elevational gradient. Although I haven’t finished analyzing the data the difference in the number of burrows between the two microsites really stood out to me, so I ran a t-test to see if the means of the shrub/open microsites differed significantly, and as you can see for the shrubs I surveyed there are significantly more burrows located in shrub microsites in comparison to open microsites (t=-13.59; p<0.001).
Chapter 3 of my study will involve an experiment testing the effect of Larrea tridentata and associated annual vegetation on the foraging patterns and giving up densities of kangaroo rats along an elevational gradient. The giving up density method (Brown 1988) predicts that a forager will stop depleting a patch when the cost of foraging outweigh the benefits of foraging at a particular patch. The giving up density is the density or mass of a resource remaining after a set amount of time. For this experiment within each elevational sub-site there will be 4 treatments: shrub with annuals intact; open with annuals intact; shrub with annuals removed; and open with annuals removed.
Seed trays will be constructed using reinforced aluminum foil trays, sifted sand, and a known mass of unhusked millet seeds (4 grams). The seeds will be mixed with the sand and the surface will be flattened to facilitate the identification and counting of kangaroo rat tracks and other signs of activity (scat; digging). Seed trays will be positioned under the north side of each shrub approximately 20-40 cm from the base and 1-1.5 meters from shrub edge in adjacent open area. The trays will be set up shortly before sunset and left out until just before sunrise. 10 motion triggered wildlife cameras will also be used during this experiment to help examine foraging patterns and to help with species identification. Before sunrise trays and cameras will be collected and the contents of the trays will be sieved in order to calculate the remaining seed mass.
1. Amelia Lamb
1st Progress Report
March 19th 2013
Shrub and understory annual plant
effects on resident kangaroo rat
species in a desert annual
Facilitation in plant communities
MacIntire and Fajardo 2014; Pugnaire et al. 2011; Soliveres et al. 2011; Holmgren and Scheffer 2010; Brooker et al. 2008; Bruno et al.
2003; Holmgren et al. 1997; Callaway and Walker 1997; Pugnaire et al. 1996.
The Stress Gradient Hypothesis (SGH)
Michelet et al. 2013; Holmgren 2010; Malkinson 2010; Maestre et al. 2009; Brooker et al. 2008; Kawai et al. 2007; Lortie and Callaway
2006; Maestre et al. 2006; Callaway and Walker 1994.
Armas and Pugnaire 2005; Gomez-Aparicio et al. 2004; Flores and Jurado 2003; Holzapfel 1999; Pugnaire et al . 1996; Callaway
6. Systematic Review
• Purpose: summarize literature to identify extensively studied
• Criteria: papers testing positive interactions/animal side of
• Search: Web of knowledge (shrub*+animal*)
• 844 articles
• 122 articles retained
• Data: direct/indirect; nature of interaction; reciprocal
interactions; study species; climate; GPS
7. Chapter 1
Geographic Distribution of Previous Studies
8. Study Site and Species
9. Study Site and Species
10. Study Site and Species
• Purpose: examine the
impact of shrubs and
annuals on animal
activity along a
• 5-point gradient- sites
differ by 200 meters
• Microsites- shrub vs.
• Replicates- 100
removed (50 pairs)
annuals intact (50
14. Kangaroo rat foraging patterns and GUD
• Purpose: examine the
impact of shrubs and
annuals on foraging
patterns and GUD
• 5-point gradient: sites
differ by 200 meters
• Microsites: shrub vs.
• # of Trials: 10-15
• Treatments: annuals
removed; annuals intact
• Trays: reinforced aluminum foil; sifted sand; millet seed
• Shrub side: positioned under north side near base and adjacent
• Set up: before sunset; flatten surface; set up cameras; collect
trays/cameras before sunrise; record activity; sieve contents; weigh
• Pilot study: krats ate seeds; cameras recorded krat activity.
16. Current Objectives
• Finish extracting systematic review data
• Begin writing review
• Finish analyzing data from pilot survey and GUD trials
• Field (April 15th – May 30th)
• Extensive survey of Larrea tridentata and associated annual vegetation and
animal activity (at least 100 paired shrub/open site per elevational sub-site)
• 10-15 GUD trials; 4 treatments; 3 replicates per treatment in each
elevational sub-site (60 trays/trial)