Demographic Patterns of Mortality of Espeletia grandiflora, an Index Plant of the Colombian Andean Páramo

Luis Ernesto Beltran-Forero, Purdue University

Abstract

The Páramo is a high elevation mountain ecosystem in the tropical Andes Mountains. This ecosystem provides 70% of the water consumed by inhabitants in Colombia at lower elevations. Espeletia grandiflora is a long-lived, caulescent plant that is the index plant of the Páramo in Chingaza National Natural Park (CNNP). A technical report from the Colombian Meteorological Institute and information from park rangers at CNNP reported recent increases in both E. grandiflora mortality and herbivory by insects, possibly from adjacent agricultural fields. The main aim of this research was to provide scientific documentation of the spatio-temporal patterns of plant demography and mortality at this site, as well as to assess the effects of insect herbivory on that mortality and demography. The central hypothesis tested posed that insect herbivory, especially from invasive agricultural pests, on E. grandiflora is associated with the increased plant mortality rate. Predictions tested included that warmer areas (lower rather than higher elevations), and those with historical or current agricultural use, will exhibit higher herbivory pressure and plant mortality rates. Along an elevational gradient that encompassed elevations from 3100 to 3700 m.a.s.l., 15-100m2 plots were established for monitoring plant mortality and insect herbivory during February 2014 to July 2015. This sampling period included two dry seasons (November to February) and two wet seasons (March to October), as well as including a severe El Niño-Southern Oscillation (ENSO) climate event in 2015, where mean temperatures increased 0.6 - 0.8 0C beyond the mean of the last half of the 20th century. A total of 1843 plants were censused in February 2014 and classified in four size classes that differentiated non-reproductive and reproductive individuals of increasing age. To link herbivory and plant mortality it was necessary to identify the arthropod community associated with E. grandiflora using mtCOI DNA barcoding techniques. Barcodes of 77 known agricultural pests from Colombia were compared to determine the presence of agricultural pests on this Páramo community. Finally, the incidence and amount of herbivory, as judged by assessing the amount of leaf tissue removed, was assessed at each census, by elevation and size class. Results showed a dramatic increase in overall mortality from pre-ENSO value of 2.7% to 18.0% and 19.4% during ENSO dry and wet seasons, respectively. More than one-third (N = 658; >35%) of these perennial plants died during the censuses. Unexpectedly, this mortality was proportionally and homogeneously distributed across elevations and size classes, even though plant density and the proportion of juvenile and smaller reproductives increased with elevation. Both the elevational pattern of demography and the observed mortality indicated that E. grandiflora has been responding to increased temperatures for several decades. From the 1812 arthropods collected, 56 families of arthropods representing all trophic guilds were determined as present in the aerial parts of E. grandiflora, but herbivores represented 72% (N= 1295) of all arthropods collected. From this community four herbivorous families were the most abundant during the four censuses: Pteriphoridae and Melyridae (Lepidoptera), Membracidae and Pseudococcidae (Hemiptera). Based on the comparative analysis of mtCOI barcodes, none of individuals collected were crop pests, making the entire observed community completely endemic. The severity of herbivory (amount of leave tissue removed by these insects) exceeded the proportions reported in lowland tropical rainforest and tended to decrease with the higher temperatures recorded during the ENSO censuses. Non-reproductive size classes exhibited the lowest severity while all reproductive size classes were equally affected by measured herbivory damage. Nonetheless, survival analysis showed that mortality in E. grandiflora is not associated with any direct measure of herbivory. The possibility, however, remains that a disease vectored via an endemic insect herbivore may be contributing to the mortality increase associated with higher temperature stress during ENSO. The data collected in this study indicated the vulnerability of this Páramo plant community to abiotic changes as increases in temperature as well as potentially altered biotic interactions like herbivory. Future mechanistic studies that incorporate eco-physiological variables in Espeletia spp and their associated insects, with environmental variables, such as drought, will contribute not only to understand the complexity of this problem but also will help to propose mitigation that could be adopted in a regional scope from the northwestern part of the tropical Andes. By example, we learned that, at CNNP, SC4 class plants at low elevation have the significantly greatest survivorship in the entire gradient, i.e., at the site of the historically greatest temperature and moisture stress and the lowest overall plant density. Restoration strategies might well utilize these plants as the genetic source stock rather than the smaller/younger plants more abundant at other elevation

Degree

Ph.D.

Advisors

Levy, Purdue University.

Subject Area

Ecology|Demography

COinS