Impacts of insect herbivory on cactus population dynamics: experimental demography across an environmental gradient

Abstract

Miller, Tom E. X.; Louda, Svata M.; Rose, Karen A.; and Eckberg, James O., "Impacts of insect herbivory on cactus population dynamics: experimental demography across an environmental gradient" (2009 Abstract. Understanding the role of consumers in plant population dynamics is important, both conceptually and practically. Yet, while the negative effects of herbivory on plant performance have been well documented, we know much less about how individuallevel damage translates to impacts on population growth or whether spatial variation in herbivory affects patterns of plant distribution. We studied the role of insect herbivory in the dynamics and distribution of the tree cholla cactus (Opuntia imbricata), a long-lived perennial plant, across an elevational gradient in central New Mexico, USA, from low-elevation grassland (1670 m) to a grassland-mountain transition zone (1720 m) to the rocky slopes of the Los Pinos Mountains (1790 m). Tree cholla density increased significantly with elevation, while abundance of and damage by a suite of native, cactus-feeding insects decreased. We combined field experiments and demographic models to test the hypothesis that systematic spatial variation in chronic insect herbivory limits the tree cholla distribution to a subset of suitable habitat across the gradient. Our results support this hypothesis.We found that key demographic functions (survival, growth, fecundity) and the responses of these functions to experimental reductions in insect herbivory varied across the gradient. The effects of insect exclusion on plant growth and seed production were strongest in the lowelevation grassland and decreased in magnitude with increasing elevation. We used the experimental data to parameterize integral projection models (IPM), which predict the asymptotic rate of population increase (k). The modeling results showed that insect herbivory depressed k and that the magnitude of this effect was context-dependent. The effect of insect herbivory on population growth was strongest at low elevation (Dk low ¼ 0.095), intermediate at mid elevation (Dk mid ¼ 0.046), and weakest at high elevation (Dk high ¼ À0.0089). The total effect of insects on k was due to a combination of reductions in growth and in fecundity and their combination; the relative contribution of each of these effects varied spatially. Our results, generated by experimental demography across a heterogeneous landscape, provide new insights into the role of native consumers in the population dynamics and distribution of abundance of long-lived native plants.

Balancing anti‐herbivore benefits and anti‐pollinator costs of defensive mutualists

“…Our work was conducted at the Sevilleta National Wildlife Refuge, a long-term ecological research (LTER) site in Socorro County, New Mexico, USA. See Miller et al (2009) for a description of the study area.…”

“…), and a noctuid moth (Cahela ponderosella). Insect exclusion experiments demonstrated that these herbivores depress plant population growth (Miller et al 2009). …”

“…Estimates for positive demographic effects came from previous experimental evidence for the dependence of herbivore damage on ant status (C. opuntiae-tended, L. apiculatum-tended, or antfree) and the dependence of plant growth on herbivore damage (Miller 2007, Miller et al 2009). Together, these data allowed us to quantify the expected growth trajectories of plants in each ant state (C. opuntiae, L. apiculatum, or ant-free; Appendix).…”

“…Insect herbivory depresses plant growth and, because growth is a highelasticity vital rate, population growth (Miller et al 2009). O. imbricata is thought to be an obligate outcrosser that relies on insect pollinators (McFarland et al 1989).…”

Insect herbivory and propagule pressure influence Cirsium vulgare invasiveness across the landscape

“…Experiments that manipulate herbivory across a wide gradient of arriving propagules during the initial The relative importance of propagule pressure and herbivory by native consumers is likely to vary across heterogeneous landscapes. This can result from spatial variation in generalist and specialist herbivore density or feeding activity (Louda and Rodman 1996, Lambrinos 2002, Miller et al 2009), or from the interaction of herbivory with spatial heterogeneity in abiotic conditions and plant community composition (Lambrinos 2002). To date, our knowledge of propagule pressure and ecosystem factors influencing invasions across the landscape is based primarily on observational studies that substitute indirect (ship traffic) or nonindependent (distance to propagule source) measures for propagule pressure (Eschtruth andBattles 2009, Simberloff 2009).…”