Nutrients, pathogens, and herbivores influence plant diversity by different means

Photo Credit

Robert W. Heckman

A photo series of the fertilization impacts on experimental plant communities.

Maintaining or increasing diversity in plant communities requires seedlings to establish and grow. In perennial-dominated systems, this can be challenging because the resident vegetation is often able to suppress new seedlings, especially when the resident vegetation is able to reduce the availability of resources like light and water or support large pathogen and herbivore populations that can attack vulnerable seedlings. Decades of research has shown that two factors—nutrients and consumers—often contribute substantially to the diversity of plant communities, which includes impacting seedling establishment. Yet, much less research has examined whether the combined effects of nutrients and consumers are greater than their individual effects and if so, why.

To better understand the importance of nutrients and consumers (here, insect herbivores and fungal pathogens) for promoting diversity in abandoned agricultural fields  dominated by perennial plants, this study used fertilizer application to increase soil nutrient availability and pesticide application to exclude insect herbivores and fungal pathogens. The impacts of these treatments on the availability of other resources, like light and water, and on attributes of the plant community, were assessed and related to seedling establishment of species that were added as seed.

Nutrients and consumers each had strong impacts on seedling establishment, but these effects are largely independent of one another. Nutrient addition reduced seedling establishment indirectly by reducing the availability of light and water to seedlings; excluding consumers reduced herbivory and disease to the resident vegetation, allowing them to better suppress seedlings. Human-induced nitrogen deposition increases soil fertility worldwide, making the impacts of soil nitrogen on communities an important determinant of our ability to predict and promote community diversity.