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Causal inferences from experimental data are often justified based on treatment randomization. However, inferring causality from data also requires complementary causal assumptions, which have been formalized by scholars of causality but not widely discussed in ecology. While ecologists have recognized challenges to inferring causal relationships in experiments and developed solutions, they lack a general framework to identify and address them. We review four assumptions required to infer causality from experiments and provide design-based and statistically based solutions for when these assumptions are violated. We conclude that there is no clear demarcation between experimental and non-experimental designs. This insight can help ecologists design better experiments and remove barriers between experimental and observational scholarship in ecology.

The effects of altered nutrient supplies and herbivore density on species diversity vary with spatial scale, because coexistence mechanisms are scale dependent. This scale dependence may alter the shape of the species–area relationship (SAR), which can be described by changes in species richness (S) as a power function of the sample area (A): S = cAz, where c and z are constants. We analysed the effects of experimental manipulations of nutrient supply and herbivore density on species richness across a range of scales (0.01–75 m²) at 30 grasslands in 10 countries. We found that nutrient addition reduced the number of species that could co-occur locally, indicated by the SAR intercepts (log c), but did not affect the SAR slopes (z). As a result, proportional species loss due to nutrient enrichment was largely unchanged across sampling scales, whereas total species loss increased over threefold across our range of sampling scales.

Many species may face multiple distinct and persistent drivers of extinction risk, yet theoretical and empirical studies tend to focus on the single largest driver. This means that existing approaches potentially underestimate and mischaracterize future risks to biodiversity. We synthesized existing knowledge on how multiple drivers of extinction can interact to influence a species' overall extinction probability in a probabilistic model of extinction risk that incorporated the impacts of multiple drivers of extinction risk, their interactions, and their accumulative effects through time. We then used this model framework to explore how different threats, interactions between them, and time trends may affect a species' overall extinction probability. Multiple small threats together had potential to pose a large cumulative extinction risk; for example, 10 individual threats posed a 1% extinction risk each and cumulatively posed a 9.7% total extinction risk. Interactions among drivers resulted in escalated risk in some cases, and persistent threats with a small (1%) extinction risk each decade ultimately posed large extinction risk over 100 (9.6% total extinction risk) to 200 years (18.2% total extinction risk). By estimating long-term extinction risk posed by several different factors and their interactions, this approach provides a framework to identify drivers of extinction risk that could be proactively targeted to help prevent species currently of least concern from becoming threatened with extinction.Muchas especies pueden enfrentarse a múltiples impulsores distintivos y persistentes del riesgo de extinción, aunque los estudios teóricos y empíricos tienden a enfocarse en el impulsor más relevante. Esto significa que las estrategias existentes tienen el potencial de subestimar y caracterizar erróneamente los riesgos para la biodiversidad en el futuro. Sintetizamos el conocimiento existente sobre cómo los múltiples impulsores de la extinción pueden interactuar para influir sobre la probabilidad general de extinción de una especie en un modelo probabilístico del riesgo de extinción, el cual incorporó los impactos de los múltiples impulsores del riesgo de extinción, sus interacciones y sus efectos acumulativos a través del tiempo. Después usamos este modelo para explorar cómo las diferentes amenazas, las interacciones entre ellas y las tendencias temporales pueden afectar la probabilidad general de extinción de una especie. El conjunto de múltiples amenazas pequeñas tuvo el potencial de representar un gran riesgo de extinción acumulativo; por ejemplo, cada una de diez amenazas individuales representó 1% de riesgo de extinción, y acumuladas representaron un riesgo total de extinción de 9.7%. Las interacciones entre los impulsores resultaron en un riesgo escalado en algunos casos, y las amenazas persistentes con un riesgo pequeño (1%) de extinción durante cada década al final representaron un gran riesgo de extinción después de 100 (9.6% del riesgo total de extinción) y 200 años (18.2% del riesgo total de extinción). Mediante la estimación del riesgo de extinción a largo plazo que presentan los diferentes factores y sus interacciones, esta estrategia proporciona un marco para identificar los impulsores del riesgo de extinción que podrían focalizarse proactivamente para ayudar a prevenir que las especies que actualmente están en menor riesgo se conviertan en especies amenazadas.

In most plant communities, the net effect of nitrogen enrichment is an increase in plant productivity. However, nitrogen enrichment also has been shown to decrease species richness and to acidify soils, each of which may diminish the long-term impact of nutrient enrichment on productivity. Here we use a long-term (20 year) grassland plant diversity by nitrogen enrichment experiment in Minnesota, United States (a subexperiment within the BioCON experiment) to quantify the net impacts of nitrogen enrichment on productivity, including its potential indirect effects on productivity via changes in species richness and soil pH over an experimental diversity gradient. Overall, we found that nitrogen enrichment led to an immediate positive increment in productivity, but that this effect became nonsignificant over later years of the experiment, with the difference in productivity between fertilized and unfertilized plots decreasing in proportion to nitrogen addition-dependent declines in soil pH and losses of plant diversity. The net effect of nitrogen enrichment on productivity could have been 14.5% more on average over 20 years in monocultures if not for nitrogen-induced decreases in pH and about 28.5% more on average over 20 years in 16 species communities if not for nitrogen-induced species richness losses. Together, these results suggest that the positive effects of nutrient enrichment on biomass production can diminish in their magnitude over time, especially because of soil acidification in low diversity communities and especially because of plant diversity loss in initially high diversity communities.

Summary Recent studies have revealed many potential benefits of increasing plant diversity in natural ecosystems, as well as in agroecosystems and production forests. Plant diversity potentially provides a partial to complete substitute for many costly agricultural inputs, such as fertilizers, pesticides, imported pollinators and irrigation. Diversification strategies include enhancing crop genetic diversity, mixed plantings, rotating crops, agroforestry and diversifying landscapes surrounding croplands. Here we briefly review studies considering how increasing plant diversity influences the production of crops, forage, and wood, yield stability, and several regulating and supporting agroecosystem services. We also discuss challenges and recommendations for diversifying agroecosystems. There is consistently strong evidence that strategically increasing plant diversity increases crop and forage yield, wood production, yield stability, pollinators, weed suppression and pest suppression, whereas effects of diversification on soil nutrients and carbon remain poorly understood. Synthesis. The benefits of diversifying agroecosystems are expected to be greatest where the aims are to sustainably intensify production while reducing conventional inputs or to optimize both yields and ecosystem services. Over the next few decades, as monoculture yields continue to decelerate or decline for many crops, and as demand for ecosystem services continues to rise, diversification could become an essential tool for sustaining production and ecosystem services in croplands, rangelands and production forests.

It is now well established that biodiversity plays an important role in determining ecosystem functioning and its stability over time. A possible mechanism for this positive effect of biodiversity is that more diverse plant communities have a greater capacity to respond to environmental changes through shifts in species dominance and composition. In our study, we utilized data from five long-term grassland biodiversity experiments located in North America (three studies) and Central Europe (two studies), in which plant species richness and global change drivers were manipulated simultaneously. The global change drivers included warming, drought, elevated atmospheric CO2 concentrations, elevated N inputs, or intensive management. Across drivers, functional change over time was significantly greater for communities of high plant diversity than that of low diversity because of a higher functional and phylogenetic richness and mostly associated with a dominance by species with a ‘slow and tall’ strategy. Community functional shifts in response to global change drivers were, however, relatively weak and mostly not influenced by diversity. The exception to this was warming, where diverse communities showed stronger shifts than species-poor communities. Our results confirm the hypothesis that diverse communities have a greater capacity for functional change than species-poor communities, particularly in their successional dynamics, but also potentially in their responses to environmental change. This capacity could underlie the positive biodiversity-stability relationship and buffer ecosystem responses to environmental change.

Tens of thousands of species are threatened with extinction as a result of human activities. Here we explore how the extinction risks of terrestrial mammals and birds might change in the next 50 years. Future population growth and economic development are forecasted to impose unprecedented levels of extinction risk on many more species worldwide, especially the large mammals of tropical Africa, Asia and South America. Yet these threats are not inevitable. Proactive international efforts to increase crop yields, minimize land clearing and habitat fragmentation, and protect natural lands could increase food security in developing nations and preserve much of Earth's remaining biodiversity.