In a grassland system in Connecticut, Schmitz (2009) and his colleagues described a series of interactions among trophic levels. The red-legged grasshopper, Melanoplus femurrubrum, prefers to graze the grass Poa pratensis (smooth meadow-grass). However, if the ambush predator Pisaurina mira (nursery-web spider) is present, the spider waits in Poa pratensis. As a result, M. femurrubrum move from grasses to goldenrod (Solidago rugosa), a less preferred food plant. While on goldenrod, the grasshoppers graze the goldenrod and suppress its density. This allows a variety of small herbaceous plants to persist in the field, and allows Poa pratensis to become dominant.

Without grazing, goldenrod is the superior competitor. It outcompetes the other vegetation and becomes strongly dominant in the field. Small herbaceous plants and grasses become less common.

a. Draw a connectedness web and a functional web to illustrate this system. Use dotted lines to indicate indirect effects, and label positive and negative effects.

b. What is likely to happen to the vegetation community if Pisaurina mira is removed? What will happen to plant species richness?

c. This community exhibits alternative stable state behavior. If P. mira is removed for one year and then reintroduced, the field can return to its original state. If P. mira is absent for more than two years, reintroduction does not return the community to its original state. Speculate: why might this be the case? What are possible mechanisms that keep the community in its altered state, even after reintroduction?