Climatic variability and coexistence

(Read a popular article about this work from USU's Research Matters series.)

One of our big challenges is understanding how climate change will influence species diversity.  In arid and semiarid ecosystems, forecast increases in climatic variability may have even greater impacts than changes in mean temperature or precipitation. The importance of variability is clear from theoretical work on "the storage effect," but empirical evidence is lacking.  I have been working with Jonathan Levine at UC Santa Barbara to fill this gap using unique, long-term datasets to evaluate the historical role of variability in maintaining species diversity--a first step towards underestanding the potential effects of climate change.  The datasets are time series (up to 36 yrs) of annually mapped 1 meter-squared quadrats. These maps were collected mostly between the 1930's and the 1970's by early range scientists, some of whom were students of Clements. They are now digitized and stored in a GIS.

Because of the extremely fine spatial resolution and long temporal coverage of the data, we can model individual plant performance and competitive interactions in a variety of years, ranging from severe droughts to extremely wet years, using a hierarchical Bayesian approach. The models provide a link between the raw data and storage effect theory, allowing us to test whether or not the population growth of a particular species will be stabilized by climatic variability, and to quantify the strength of this stabilizing effect. Our results show that the storage effect did in fact have a strong stabilizing effect on three common perennial grass from the 1930's into the 1960's (PNAS 2006). An important lesson from this work is that to understand how climate change will impact biodiversity, we will need to consider changes in both climate averages and variability.

We are now digitizing mapped quadrats from additional sites to evaluate the generality of strong storage effects. We are also designing manipulative experiments to test our model predictions, and reduce their uncertainty.

Patterns of species richness in space and time

The species-area relationships (SAR), often called one of the few "laws" of ecology, shows how species richness scales with area observed. As early as 1960, Preston suggested an analogous relationship between species number and the time period of observation, a species-time relationship. I used the same Kansas time-series mentioned above to show that the SAR and STR are just two special cases of a more general species-time-area relationship (STAR), with species number increasing as a function of area and time observed, as well as their interaction (Ecology Letters 2003). The relationship shows that the rate of species turnover in space decreases with the time span of observation, and species turnover in time decreases with spatial scale. Bill Lauenroth, Ethan White and I recently led an LTER funded working group to test the generality of STRs and STARs. We found a high degree of regularity in STRs from a variety of taxa and ecosystems, and all the STARs we constructed were consistent with the Kansas result (Ecology 2005). We think the STAR has important implications for biodiversity assessment in both conservation and basic research contexts. For example, I found that neutral models could reproduce an observed SARs or STRs from the Kansas dataset, but failed to reproduce both simultaneously with one set of parameters (Ecology 2004).

 

Plant-animal interactions

My dissertation research with Bill Lauenroth at Colorado State University was focused on explaining why some ecosystems are so senstive to livestock grazing, while others appear quite resistant. I compared grazing effects on vegetation and soils in convergent ecosystems of North and South America--the sagebrush steppe of central Washington state, and the Patagonian steppe of southern Argentina. 

  

Similar climate conditions produce similar vegetation types, bunchgrasses and shrubs, in both regions.  While the history of domestic livestock grazing also matches up fairly well, we suspect that the evolutionary history of grazing differs between the two systems.  In Washington we have little evidence of ungulate grazing pressure since the last ice age, but the native guanaco in Patagonia may have imposed strong selective pressure on plants.  Consistent with this story, I found stronger grazing resistant traits in the Patagonian graminoids than the sagebrush steppe graminoids (J. App. Ecol. 2004). The low forage quality of the Patagonian graminoids led to lower levels of utilization by livestock, and correspondingly weaker impacts on primary production and species composition than in sagebrush steppe (Ecol. Apps 2005).  While these results suggest that an ecosystem's evolutionary history of grazing can be important, it is possible that subtle abiotic differences played a role: soils are much sandier in Patagonia leading to greater potential for nitrogen limitation and perhaps explaining the low quality of the Patagonian graminoids (J. Arid. Env. 2006).

I also am interested in how grazing affects the spatial heterogeneity of vegetation.  This research included field work in the shortgrass steppe of Colorado (AVS 2000), along with a literature review and some basic simulation modeling (Oecologia 2001). I recently used a modeling approach to explore how herbivore foraging strategies may influence the development of patterns in forage production and utilization along distance from water gradients (Landscape Ecology 2005).