As the number of plant species in a grassland increases, the amount of productivity ______.

The statement seems to be incomplete. However, based on ecological studies, as the number of plant species in a grassland increases, the amount of productivity generally increases. This is due to the fact that different plant species often have different resource requirements and growth patterns, which allows them to effectively share resources and space, leading to an overall increase in productivity. This concept is known as the biodiversity-productivity relationship.

Here are the steps to understand this:

1. Biodiversity: This refers to the variety of life in a particular habitat or ecosystem. In this case, it's the variety of plant species in a grassland.

2. Productivity: In ecology, productivity refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of energy.

3. Biodiversity-Productivity Relationship: This is a concept in ecology that suggests that there is a positive correlation between biodiversity and productivity. As biodiversity increases, so does productivity. This is because different species often have different resource needs and growth patterns, allowing them to share resources and space effectively.

4. Application to Grasslands: In a grassland ecosystem, increasing the number of plant species can lead to an increase in productivity. More plant species means a greater variety of resource use and growth patterns, which can lead to more efficient use of resources and space, and thus higher overall productivity.