Nonadditive effects among threats on rare plant species.

The current loss of biodiversity has put 50,000 plant species at an elevated risk of extinction worldwide. Conserving at-risk species is often complicated by covariance or nonadditivity among threats, which makes it difficult to determine optimal management strategies. We sought to demographically quantify covariance and nonadditive effects of more threats on more rare plant species than ever attempted in a single analysis. We used 1082 population reports from 186 populations across 3 U.S. states of 27 rare, herbaceous plant species collected over 15 years by citizen scientists. We used a linear mixed-effects model with 4 threats and their interactions as fixed predictors, species as a random predictor, and annual growth rates as the response. We found a significant 3-way interaction on annual growth rates; rare plant population sizes were reduced by 46% during the time immediately after disturbance when populations were also browsed by deer (Odocoileus virginianus) and had high levels of encroachment by woody species. This nonadditive effect should be considered a major threat to the persistence of rare plant species. Our results highlight the need for comprehensive, multithreat assessments to determine optimal conservation actions.

Efectos No Acumulativos entre las Amenazas para las Especies Raras de Plantas Resumen La actual pérdida de biodiversidad ha colocado a 50,000 especies de plantas en un riesgo elevado de extinción global. La conservación de las especies en riesgo frecuentemente se complica por la covarianza o la no acumulabilidad entre las amenazas, lo que dificulta la determinación de las estrategias óptimas de manejo. Buscamos cuantificar demográficamente la covarianza y los efectos no acumulativos de más amenazas sobre un mayor número de especies raras de plantas, lo más que se ha intentado en un solo análisis. Utilizamos 1,082 reportes poblacionales tomados de 186 poblaciones en tres estados de los Estados Unidos. Estas poblaciones fueron de 27 especies raras de plantas herbáceas recolectadas a lo largo de 15 años por ciudadanos científicos. Usamos un modelo lineal de efectos mixtos con cuatro amenazas y sus interacciones como pronosticadoras fijas, las especies como pronosticadoras aleatorias y las tasas anuales de crecimiento como las respuestas. Encontramos una interacción significativa de tres vías en las tasas anuales de crecimiento; el tamaño poblacional de las plantas raras se redujo en un 46% durante el tiempo transcurrido inmediatamente después de una perturbación, cuando a la vez las poblaciones también eran ramoneadas por venados (Odocoileus virginianus) y tenían niveles altos de invasión por parte de especies leñosas. Este efecto no acumulativo debería considerarse una amenaza mayor para la persistencia de las especies raras de plantas. Nuestros resultados resaltan la necesidad de tener evaluaciones completas que consideren amenazas múltiples para así determinar las acciones óptimas de conservación.

Effects of above- and belowground herbivory on growth, pollination, and reproduction in cucumber.

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant-herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.