Large effects of white-tailed deer (Odocoileus virginianus) upon individual plants, plant populations and communities have been documented in a number of studies. However, well-supported experimental measures of the magnitude and geographical extent of these effects are still surprisingly scarce. Deer-caused changes in stem morphology and reductions in plant growth rates are well-documented in some parts of the North America. Furthermore, deer have been shown to affect the composition of several plant communities in the north-central and northeastern United States. There are some documented cases of deer-caused reductions in plant survival; most of these are tree seedlings and saplings. However , many studies have detected no effects on plant survival or fecundity, or have found that negative effects occur only in a fraction of years, seasons, sites or deer densities. Little is known about population-level or ecosystem-level impacts. Many regions and plant communities with large deer populations have not been studied. Whereas deer density is clearly important in determining spatial and temporal variation in the presence and magnitude of deer effects, other factors that may modify the effects of deer density are poorly understood.

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

The problem of invasive species has reignited interest in biological control as a management tool. Classical biological control involves deliberate release of exotic natural enemies into new environments in an attempt to limit the density of an invasive species. Persistent, sustained limitation of invasive species by coevolved natural enemies is a seductive concept. Evidence now suggests, however, that biological control through the release of natural enemies can carry unanticipated ecological risks. There have been ecological side effects of distributing a deliberately introduced weevil (Rhinocyllus conicus) and an adventitious weevil (Larinus planus) for the biological control of exotic thistles. Both weevils have had major direct effects on key population-growth parameters of native thistles, and R. conicus has had an indirect effect on the interaction between a thistle and a native insect. These findings led us to review how ecological risk is evaluated, and to ask whether pre-release tests can predict the types of ecological effects documented. We conclude that, when done thoroughly, the tests used can determine host specificity by identifying physiological host range, but the usual tests cannot be relied upon to predict the ecological host range or impact on populations of less-preferred but accepted native species. Our data provide support for suggestions that the behavioral and developmental data now taken need to be supplemented with additional data on population parameters to better predict field-host use, population growth, interaction strengths, and ecological outcomes for native species that are potential hosts. Invasividad de Algunos Insectos de Control Biológico y Adecuación de Su Evaluación de Riesgo Ecológico y RegulaciónResumen El problema de las especies invasoras ha despertado nuevo interés en el control biológico como una herramienta de manejo. El control biológico clásico implica la liberación deliberada de enemigos naturales exóticos en nuevos ambientes con la intención de limitar la densidad de una especie invasora. La limitación persistente, sostenida de especies invasoras por enemigos naturales coevolucionados es un concepto seductor. Sin embargo, la evidencia ahora sugiere que la liberación de enemigos naturales para el control biológico puede acarrear riesgos ecológicos no anticipados. Hubo efectos ecológicos secundarios por la introducción deliberada de un gorgojo (Rhinocyllus conicus) y de un gorgojo adventicio (Larinus planus) para el control biológico de abrojos exóticos. Ambos gorgojos han tenido importantes efectos directos sobre los parámetros clave del crecimiento poblacional de abrojos nativos, y R. conicus ha tenido un efecto indirecto en una interacción abrojo -insecto nativo. Estos hallazgos nos condujeron a revisar como se evalúa el riesgo ecológico y a preguntar si las pruebas previas a la liberación pueden predecir los tipos de efectos ecológicos documentados. Concluimos que las pruebas utilizadas, cuando están bien hechas, pueden determinar la especificidad del...

To measure the effects of white-tailed deer ͑Odocoileus virginianus) herbivory on seeds, seedlings, and young saplings of Quercus buckleyi on the eastern Edwards Plateau of central Texas, USA, experimental fenced deer exclosures were constructed. Acorns or small Q. buckleyi transplants were placed in each exclosure and in each unfenced control plot. Deer did not significantly affect acorn survival and germination, but did significantly reduce transplant survival and growth rates. The results support the hypothesis that deer are responsible for the failure of recruitment into adult size classes in Q. buckleyi populations in this region. Without adult recruitment of Q. buckleyi, the species composition and possibly even the physiognomy of woodlands on the eastern Edwards Plateau will change markedly. The results of this experiment also indicate that, although juniper ͑Junipe-rus ashei͒ and Q. buckleyi presumably compete for water, light, and nutrients, in the presence of deer junipers can have a positive effect on seedlings and saplings of Q. buckleyi, a case of facilitation. In the presence of deer the transplants increased in height significantly more slowly away from juniper saplings than they did beneath juniper saplings, probably due to the physical protection from browsing that junipers provided to the transplants.

., "Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore" (2005). Faculty Publications in the Biological Sciences. 58. http://digitalcommons.unl.edu/bioscifacpub/58 AbstractSpatial and temporal variation in insect floral herbivory is common and often important. Yet, the determinants of such variation remain incompletely understood. Using 12 years of flowering data and 4 years of biweekly insect counts, we evaluated four hypotheses to explain variation in damage by the Eurasian flower head weevil, Rhinocyllus conicus, to the native North American wavyleaf thistle, Cirsium undulatum. The four factors hypothesized to influence weevil impact were variations in climate, weevil abundance, phenological synchrony, and number of flower heads available, either on wavyleaf thistle or on the other co-occurring, acquired native host plant (Platte thistle, Cirsium canescens), or on both. Climate did not contribute significantly to an explanation of variation in R. conicus damage to wavyleaf thistle. However, climate did influence weevil synchrony with wavyleaf flower head initiation, and phenological synchrony was important in determining R. conicus oviposition levels on wavyleaf thistle. The earlier R. conicus was active, the less it oviposited on wavyleaf thistle, even when weevils were abundant. Neither weevil abundance nor availability of wavyleaf flower heads predicted R. conicus egg load. Instead, the strongest predictor of R. conicus egg load on wavyleaf thistle was the availability of flower heads on Platte thistle, the more common, earlier flowering native thistle in the sand prairie. Egg load on wavyleaf thistle decreased as the number of Platte thistle flower heads at a site increased. Thus, wavyleaf thistle experienced associational defense in the presence of flowering by its now declining native congener, Platte thistle. These results demonstrate that prediction of damage to a native plant by an exotic insect may require knowledge of both likely phenological synchrony and total resource availability to the herbivore, including resources provided by other nontarget native species.

2001 -2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0 -220 m) from a musk thistle patch. Neither the level of R. conicus oviposition on C. undulatum, nor the strength of the distance effect, were predicted by local musk thistle patch density or by local C. undulatum density (< 5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the co-evolved, preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both plant proximity and herbivore density strongly affected the herbivore-mediated indirect interaction, and that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants. (240 words)

Abstract. Understanding spatial and temporal variation in factors influencing plant regeneration is critical to predicting plant population growth. We experimentally evaluated seed limitation, insect herbivory, and their interaction in the regeneration and density of tall thistle (Cirsium altissimum) across a topographic ecosystem productivity gradient in tallgrass prairie over two years. On ridges and in valleys, we used a factorial experiment manipulating seed availability and insect herbivory to quantify effects of: seed input on seedling density, insect herbivory on juvenile density, and cumulative impacts of both seed input and herbivory on reproductive adult density. Seed addition increased seedling densities at three of five sites in 2006, and all five sites in 2007. Insect herbivory reduced seedling survival across all sites in both years, as well as rosette survival from previous year's seedlings. In both years, insecticide treatment of seed addition plots led to greater adult tall thistle densities in the following year, reflecting the increase in juvenile thistle densities in the experimental year. Seedling survival was not densitydependent. Our analytical projection model predicts a significant long-term increase in adult densities from seed input, with a greater increase under experimentally reduced insect herbivory.While plant community biomass and water stress varied significantly between ridges and valleys, the effects of seed addition and insect herbivory did not vary with gradient position. These results support conceptual models that predict seedling and adult densities of short-lived monocarpic perennial plants should be seed-limited. Further, the experiment demonstrates that even at high juvenile plant densities, where density-dependence potentially could have overridden herbivore effects on plant survival, insect herbivory strongly affected juvenile thistle performance and adult densities of this native prairie species.

No abstract