Print PageFor more than 50 years, the existence and drivers of population cycles have remained one of the most controversial topics in ecology. Cycles have been studied almost exclusively in animals, while plant populations are thought to be unlikely to cycle. Here we show that density-dependent herbivory in a forest ecosystem regulates population density of a perennial herb, creating cyclic dynamics.
In 2003 we established a replicated deer exclusion experiment in a mixed hardwood forest in western Pennsylvania. Alliaria petiolata, an alleopathic invader that disrupts beneficial fungal mutualisms with native plants, occurs at this site. Exclusion treatments were crossed with A. petiolata removal treatments. All individuals of Maianthemum racemosum (Liliaceae; hereafter “MARA”) were tagged and monitored multiple times each year through 2010 to determine annual density, flowering frequency, and herbivory rates. Probabilities of vegetative dormancy were determined with mark-recapture models.
Results/Conclusions
MARA in the deer exclosures increased in density and flowering frequency. In contrast, density and flowering in the control plots was cyclic. Rates declined from 2003 to 2004, subsequently recovered and reached peak density and flowering in 2008, then declined significantly again in 2009. In 2010, density returned to 2003 levels.
This cycle is driven by a link between herbivory and plant dormancy. Herbivory occurred almost exclusively on flowering plants and tracked flowering rates, reaching ~45% in 2007 when 35% of MARA individuals flowered, and ~75% in 2008 when 45% flowered. Herbivory was density-dependent, remaining <5% in 2003-2006 and 2009-2010 when flowering was <10%. MARA recruitment is low in A. petiolata removal treatment plots and non-existent with the invader present. Changes in plant density in the controls resulted from adult plants emerging from dormancy from 2004-2008, and then returning to dormancy in 2009. The probability of dormancy correlated with herbivory, but with a two-year time lag; dormancy was <15% from 2004-2008 but rose to ~55% in 2009.
Our system has exhibited a single iteration of a deer-regulated population cycle. Like many cyclic animal systems, our population experienced delayed-density dependent regulation. Deer use flowering as an indication of palatability and we propose that the observed density dependence results from a synergistic increase in foraging rates when flowering rates are high. The delay in dormancy is hypothesized to result from disruption of organ preformation, the process whereby plants build their leaves and flowers two years before they are deployed. Future work will determine whether this is a transient or truly cyclic phenomenon.
