Restoration Ecology

Quantification of demographic responses to soil nutrient availability and population-level measures of competitive ability

In the context of restoration ecology, the tallgrass prairie provides a useful model system in which to test trade-offs between factors that maximize competitive ability and those that affect population growth. Reestablishment of prairie-natives in abandoned agricultural areas is often hindered by the greater relative growth rate of prairie-invasives in response to artificially high levels of available soil nutrients, nitrogen in particular. Tilman (1997) found that trade-offs between competitive ability and recruitment ability present one strategy that allows inferior competitors to persist in a community despite poor utilization efficiency with limiting resources. Such trade-offs between competitive ability and demographic performance have direct relevance to community structure and grassland restoration success.

Because ecological restoration involves the establishment of populations, the initial success of species is likely determined by characteristics such as greater germination success, rapid growth, and early maturation. These traits would allow successful populations to rapidly preempt available space and therefore increase population growth rate. Consequently, understanding the competitive responses of seeds, seedlings, and immature plants of desirable species to soil resources will help facilitate successful ecological restorations. The consequences of competitive responses of immature stages raise a number of important ecological questions that build on our current understanding of plant competition.

•  Given variation in reproductive potential, what improvement in average individual growth rates is required to shift the mere persistence of inferior competitors at a site to positive population growth?

•  To what extent does resource use versus recruitment determine population trajectories?

•  To what extent are tradeoffs in demographic parameters such as growth and fertility mediated by soil fertility?

•  Can mechanistic responses to resource availability serve as indicators of demographic performance and population trajectory?

We will use projection matrices to model population growth and to conduct prospective and retrospective analyses of population dynamics. Sensitivity and elasticity analyses are prospective analyses, which forecast the importance of life-history stage or demographic transition given experimental treatment (Horvitz et al. 1997) . These analyses have been applied in a number of ecological studies to inform population management (e.g. Crouse 1987, Menges 1990) . Life table response experiments (LTREs) are retrospective analyses that can be used to determine life-history stages or demographic transitions that contribute to observed differences between populations (Horvitz et al. 1997, Caswell 2000) . LTREs have been used in a variety of basic and applied ecological contexts including determining the relevance of facilitative versus competitive interactions for population growth of a desert plant (Miriti et al. 2001) , the importance of pollinator limitation for the population growth of an invasive weed (Parker 2000) and the importance of social structure on killer whale demography (Brault and Caswell 1993) .

References

Brault, S., and H. Caswell. 1993. Pod-specific demography of killer whales ( Orcinus orca ). Ecology 75 :1444-1454.

Caswell, H. 2000b. Prospective and retrospective perturbation analyses: their roles in conservation biology. Ecology 81 :619-627.

Crouse, D. T. 1987. A stage-based population model for loggerhead sea turtles and implications for conservation. Ecology 68 :1412-1423.

Horvitz, C., D. W. Schemske, and H. Caswell. 1997. The relative "importance" of life-history stages to population growth: prospective and retrospective analyses. Pages 247-271 in S. Tuljapurkar and H. Caswell, editors. Structured-population models in marine, terrestrial, and freshwater systems. Chapman and Hall, New York.

Menges, E. S. 1990. Population viability analysis for an endangered plant. Conservation Biology 4 :52-62.

Miriti, M. N., H. F. Howe, and S. J. Wright. 2001. Neighbor effects and population dynamic of a common desert perennial shrub ( Ambrosia dumosa ). Ecological Monographs 71 :491-509.

Parker, I. M. 2000. Invasion dynamics of Cytisus scoparius : a matrix model approach. Ecological Applications 10 :726-743.

Tilman, D. 1997. Community invasibility, recruitment limitation, and grassland biodiversity. Ecology 78 :81-92.