Andrew M. Liebhold

SPATIAL SYNCHRONY IN FOREST INSECT OUTBREAKS: ROLES OF REGIONAL STOCHASTICITY AND DISPERSAL

Spatial synchrony, that is, correlated population fluctuation over wide geo- graphical areas, has been detected in diverse taxa and over various geographical scales. The most commonly suggested mechanisms to explain spatial synchrony include dispersal and regional stochasticity (i.e., the Moran effect). We analyzed landscape-scale historical outbreak data for six forest insect species: spruce budworm (Choristoneura fumiferana), western spruce budworm (C. occidentalis), larch bud moth (Zeiraphera diniana), forest tent caterpillar (Malacosoma disstria), mountain pine beetle (Dendroctonus ponderosae ), and gypsy moth (Lymantria dispar). We used a recently developed statistical method (the nonparametric covariance function) for quantifying the magnitude and spatial range of synchrony in both outbreak and corresponding weather data. The varying dispersal capa- bilities of the species enabled us to speculate on the relative importance of dispersal vs. the Moran effect as potential mechanisms behind the observed patterns. Our results indicated that spatial synchrony was not directly associated with dispersal capabilities at the spatial scales considered. In contrast, the spatial correlation in weather variables was high enough to account for the levels of synchrony observed in the outbreak data. Therefore, the Moran effect appeared to be the more dominant process affecting the spatial dynamics of these species at the landscape scale. In general, however, the synchrony in outbreaks declined more steeply with geographical distance than the correlation in the weather variables, breaking with the predictions of Morans theorem. A more detailed analysis of gypsy moth outbreak data showed that local dynamics varied considerably in a spatially dependent manner. The existence of such variation violates one of the assumptions of the Morans theorem, namely, that the dynamic properties of disjunct populations are identical. We used a simple theoretical model to demonstrate that such geographical variation in local popu- lation dynamics may indeed force synchrony to decline more rapidly with distance than the correlation in the environment.

1200 years of regular outbreaks in alpine insects

The long-term history of Zeiraphera diniana Gn. (the larch budmoth, LBM) outbreaks was reconstructed from tree rings of host subalpine larch in the European Alps. This record was derived from 47u200a513 maximum latewood density measurements, and highlights the impact of contemporary climate change on ecological disturbance regimes. With over 1000 generations represented, this is the longest annually resolved record of herbivore population dynamics, and our analysis demonstrates that remarkably regular LBM fluctuations persisted over the past 1173 years with population peaks averaging every 9.3 years. These regular abundance oscillations recurred until 1981, with the absence of peak events during recent decades. Comparison with an annually resolved, millennium-long temperature reconstruction representative for the European Alps (r=0.72, correlation with instrumental data) demonstrates that regular insect population cycles continued despite major climatic changes related to warming during medieval times and cooling during the Little Ice Age. The late twentieth century absence of LBM outbreaks, however, corresponds to a period of regional warmth that is exceptional with respect to the last 1000+ years, suggesting vulnerability of an otherwise stable ecological system in a warming environment.

What causes outbreaks of the gypsy moth in North America

Abstract The gypsy moth has been present in North America for more than 100 years, and in many of the areas where it has become established outbreaks occur with varying degrees of periodicity. There also exists extensive spatial synchrony in the onset of outbreaks over large geographic regions. Density-dependent mortality clearly limits high-density populations, but there is little evidence for strong regulation of low-density populations. Predation by small mammals appears to be the major source of mortality affecting low-density populations, but because these are generalist predators and gypsy moths are a less preferred food item, mammals do not appear to regulate populations in a density-dependent fashion. Instead, predation levels appear to be primarily determined by small mammal abundance, which is in turn closely linked to the production of acorns that are a major source of food for overwintering predator populations. Mast production by host oak trees is typically variable among years, but considerable spatial synchrony in masting exists over large geographic areas. Thus, it appears that the temporal and spatial patterns of mast production may be responsible for the episodic and spatially synchronous behavior of gypsy moth outbreaks in North America. This multitrophic relationship among mast, predators, and gypsy moths represents a very different explanation of forest insect outbreak dynamics than the more widely applied theories based upon predator–prey cycles or feedbacks with host foliage quality.

EFFECTS OF PERIODICAL CICADA EMERGENCES ON ABUNDANCE AND SYNCHRONY OF AVIAN POPULATIONS

We used 37 years of North American Breeding Bird Surveys to test for effects of periodical cicada (Magicicada spp.) emergences on the abundance and spatial synchrony of 24 species of avian predators in hardwood forests of the eastern United States. Fifteen (63%) of the bird species exhibited numerical changes in abundance apparently associated with emergences of the local periodical cicada brood, and intraspecific spatial synchrony of bird abundance was significantly greater between populations sharing the same cicada brood than between populations in the ranges of different broods. Species exhibited at least four partially overlapping temporal patterns. (1) Cuckoos ( Coccyzus spp.) occurred in high numbers only during emergence years and subsequently declined in abun- dance. (2) Red-bellied Woodpeckers (Melanerpes carolinus), Blue Jays (Cyanocitta cris- tata), Common Grackles (Quiscalus quiscula), and Brown-headed Cowbirds (Molothrus ater) increased significantly 1-3 years following emergences and then declined. (3) Red- headed Woodpeckers (Melanerpes erythrocephalus ), American Crows (Corvus brachyryn- chos), Tufted Titmice (Baeolophus bicolor), Gray Catbirds (Dumetella carolinensis), and Brown Thrashers (Toxostoma rufum) were found in significantly low numbers during emer- gence years, underwent significant numerical increases in the following year, and then stabilized. (4) Wood Thrushes (Hylocichla mustelina), Northern Mockingbirds (Mimus po- lyglottos), Northern Cardinals (Cardinalis cardinalis), and House Sparrows (Passer do- mesticus) exhibited significantly deviant population numbers 1-2 years prior to emergences, below the long-term mean in the first two species and above the long-term mean in the latter two. These results suggest that the pulses of resources available at 13- or 17-year intervals when periodical cicadas emerge have significant demographic effects on key avian predators, mostly during or immediately after emergences, but in some cases apparently years following emergence events.

Canopy herbivore community structure: large-scale geographical variation and relation to forest composition

Abstract.u2002 1.u2002Geographical distributions of individual foliage‐feeding forest herbivore species have been found to be aligned closely with the distribution of their host trees, however little is known about the extent to which broad herbivore communities are geographically associated with distinct host communities.