Primary Research Projects

Contingency in community assembly: priority and year effects in restoration (PRYER)

Two alternative but potentially complementary paradigms shed light on the conceptual basis of community ecology for ecological restoration. Succession theory suggests that ecological restoration is the task of assisting, reestablishing, or accelerating natural succession toward a desired (climax) state. Some versions of assembly theory suggest that there may be multiple alternative ecological states that can be derived from a given species pool, making restoration efforts ambiguous and difficult. At the heart of these models is the idea of priority effects, in which chance variation in early arrival by different species changes the course of community development through preemption of space or other limiting resources. We are examining the strength of such priority effects and possible mechanisms to encourage or suppress them in restoration settings, often in the context of grassland communities. In particular, we are testing both temporal priority effects (planting less dominant species a year earlier than more dominant species) and spatial priority effects (planting in small-sale monocultures versus seed mixes). We are also examining a related form of contingency- year effects, in which year of establishment(or restoration) has profound effects on the trajectory of community development. (see Restoration Publications).  Our California grassland restoration research has been supported by NSF and by CalFed through Audubon (pdf of an overall summary of the CalFed research, by our group and others).


Kenya Long-term Exclosure Experiment (KLEE)

My main research interest in Africa is represented by a long-term, large-scale herbivore exclusion experiment in Laikipia, Kenya.  On arid and semi-arid range lands worldwide, livestock share the landscape with native plant and animal biodiversity.  Much of this land suffers from degradation caused by inappropriate land management.  Although there has been considerable research on the effects of stocking densities on range ecosystems, there has been relatively little research on the separate and combined effects of different herbivore guilds on each other and on the vegetation they share.  As the value of biodiversity climbs, and the profitability of livestock production declines, we are seeking more innovative ways to manage and restore these landscapes.  In 1995, we established 18 four-hectare exclosures that allow herbivory by six different combinations of cattle, wildlife, and mega-herbivores (elephants and giraffes).  My students, collaborators, and I are monitoring soil, plant, invertebrate, and vertebrate responses to these experimental treatments (see KLEE Publications), These exclosures also allow us to address fundamental issues in ecology, including induced defence, compensation, and competition. We have also engaged (with Corinna Riginos, Kari Veblen and Lauren Porensky) in a long-term study of restoration techniques in degraded rangelands (breaking soil crusts, enrichment seeding, fertilization), designed to restore both livestock productivity and native biodiversity. We are increasingly interested in the causes and consequences of environmental heterogeneity in this ecosystem.


Secondary Research Projects

Planting issues in ecological restoration (PIER)

Our lab is engaged in several projects designed to inform more effective ecological restoration:

  • We are testing the cost-effectiveness of various planting techniques in restoration settings. We know that irrigation, tree shelters, and planting container stock each increase establishment success. However, all of these are also expensive, and it is not clear whether their greater establishment effects are worth their greater cost. We are carrying out a series of trials examining these issues. For example, Alex Palmerlee’s data from a wide range of woody species strongly suggests that direct seeding is more cost-effecive than planting container stock, especially for large-seeded species. Even though container stock has greater survivorship that direct seeding, its far greater cost outways this.
  • In collaboration with Dr. Save’s lab in Catalonia, Jeffrey Clary, Kurt Vaughn, Richard Evans and I have been examining the ecophysiological bases of competitive differences among native California plants and invasive Mediterranean plants, in the context of differing rainfall patterns. One of our more fascinating results is that California perennial grasses are physiologically intermediate between Mediterranean annual grasses (which have more profligate water use) and Mediterranean perennial grasses (which have greater drought efficiency).
  • Fire is increasingly used as a weed control technique in restoration, but may be detrimental to planted woody species. We have been examining these processes in a series of replicated controlled burns (with Katherine Holmes and Kari Veblen), using a dominant restoration species in the Central Valley, Quercus lobata (Valley Oak), and several native perennial grasses. Our results suggest that although weed-control burns often result in top-kill of small planted individuals, both valley oaks and native grasses are resilient to these burns, and regrow vigorously. (see Restoration Publications)


The maintenance of biodiversity in a model system: Ant Coexistence and Competition in Africa (ACACIA)

I have been a junior collaborator with Todd Palmer (University of Florida), Maureen Stanton (UC Davis) and Dan Doak (University of Wyoming) on a research project that gets at the conceptual heart of biodiversity conservation.  One of the central questions in ecology is, “Why are there so many species?” The success of conservation and restoration projects is measured most fundamentally as species richness, and yet we still have only a rudimentary notion of what limits species diversity within and among communities. Our ability to restore and maintain biodiversity is limited by our understanding of how nature itself maintains species diversity. We are studying mechanisms of species co-existence in a system of acacias and their symbiotic ants uniquely suited to experimental, descriptive, and modeling approaches- the four ant species that inhabit Acacia drepanolobium of Laikipia, Kenya (see ACACIA Publications, and also this external summary). This group is also studying the ecology of mutualisms in this system.


The evolution of semelparity (and plant population biology)

Some of my more theoretical, early research was on life history evolution and plant population biology. In particular, I was interested in the evolution of semelparity– the life history characterized by a single, massive, fatal reproductive episode. Organisms exhibiting semelparity range from the spectacular (certain salmon, bamboos, cicadas) to the mundance (many invertebrates), and include all of our grain crops. Biologists have long pondered how evolution could favor death after first reproduction. A key clue is in the observation that semeplarous organisms produce more offspring (or seeds) in their single reproductive event than do closely related iteroparous species in any one of theirs. My past research on Mount Kenya Lobelias and more recent work with Peter Lescia on Arabis fecunda suggest that a simple mathematical model explains the demographic conditions under which this increase in reproductive output more than compensates for the loss of potential future reproduction. I continue to dabble at the edges of these questions (see Life History Publications)