“I do not believe than any animal in the world performs an action for the exclusive good of another of a distinct species, yet each species tried to take advantage of the instincts of others.”

Charles Darwin (1859), On the Origin of Species, Chapter Six

Granivore effects on plant density

Vertebrate and invertebrate granivores consume many seeds, but does this affect adult plant density? If most seeds germinate in sites or at times unsuitable for seedling establishment, then post-dispersal seed consumption might have little or no effect on plant population density. However, if viable seeds persist in the soil for long periods, then seed consumption might strongly affect plant recruitment. Does seed predation more strongly affects seedling recruitment in plants with dense soil seed banks?

Collaborator John Maron and I examined whether post-dispersal seed predation affected seedling emergence by yellow bush lupine (Lupinus arboreus) in two habitats at Bodega Marine Reserve (Maron & Simms 1997). We estimated seed bank density by counting and removing seeds from sand used to set up experimental seed removal plots. We followed the fate of seeds in placed in experimental plots that were either protected or exposed to rodent granivores.

Significantly more seeds were removed by rodents in dunes than in grassland. In dunes, where we also found a sparse seed bank, granivory significantly reduced seedling emergence, suggesting seed-limited seedling recruitment. The grassland seed bank was 43-fold denser than the dune seed bank. Here, seed removal by rodents only marginally influenced seedling recruitment because most seeds that survived predation remained dormant. Longer studies would be needed to understand the effects of granivores in the grassland.

In our next experiment, we monitored patterns of seedling emergence and survival over three years from seeds sown into control plots and plots that excluded rodent granivores (Maron & Simms 2001).

Evolutionary responses to Consumers

Evolutionary theory predicts that stabilizing selection, resulting from a balance between the benefits of reducing herbivory and the costs of defenses, should maintain intermediate levels of resistance (Simms & Rausher, 1987; Simms, 1992). Our early work on herbivore resistance in tall morning glories (Ipomoea purpurea) found no evidence for stabilizing selection on resistance to any insect herbivores (Rausher & Simms, 1989; Simms, 1990; Simms & Rausher, 1993). Additional experiments revealed no evidence for costs of resistance to damage by either insects (Simms & Rausher, 1987; 1989) or pathogens (Simms & Triplett, 1994) in the tall morning glory.

In addition to evolving resistance to damage, plants may also evolve tolerance, defined as the ability to sustain damage with little or no fitness decrement (Simms & Triplett, 1994; Simms, 2001). As with resistance, theory predicts that costs of tolerance would cause stabilizing selection favoring an intermediate level of tolerance. We found genetic variance for tolerance to damage by the fungal pathogen, Colletotrichum dematium. Further experiments showed that tolerance was costly (Simms & Triplett, 1994). Morning glory families with high fitness in the presence of the fungus were less fit in a disease-free environment.