The effect of neighborhood composition on four mechanisms of associational resistance and susceptibility

Background/Question/Methods

Associational resistance, AR (or susceptibility, AS) occurs when neighboring plants increase (or decrease) the amount of damage to a focal plant.  These effects can arise by neighbors influencing the quality of the focal plant (e.g., the production of plant defenses), or by altering herbivore traits (e.g., feeding behavior, host-plant search strategy, predation).  Both plant density and relative frequencies of plants should influence AR/AS patterns, however, past studies have used experimental designs that confound these two effects which can obscure the mechanisms leading to AR/AS.  In a 2007 field study, we found that leaf damage on a focal plant (Solanum carolinense) was influenced by both the density and relative frequency of neighboring plants (Solidago altissima). For this study, we used a series of experiments simultaneously manipulating the densities and frequencies of the two plant species to examine how plant density and relative frequency (“neighborhood composition”) influenced four potential AR/AS mechanisms. In a field, we examined how neighborhood composition affected (1) herbivore and predator abundances and (2) microclimate (e.g., temperature could influence herbivore feeding behavior).  In the greenhouse and lab, we examined the effects of neighborhood composition on (3) host-plant quality and (4) insect herbivore search strategies.

Results/Conclusions

Neighborhood composition had significant, yet different effects on the various measured variables.  (1) Herbivore and predator abundances: Density of Solidago (heterospecific neighbors) had positive effects on predator and herbivore abundances within the local neighborhood.  Herbivore abundance was marginally affected by the proportion of Solidago. (2) Microclimate: Soil moisture increased with the density and relative frequency of Solanum (focal plant).  Furthermore, the positive effect of Solanum density on soil moisture was stronger at low total density.   Mean light intensity and temperature decreased with the density of Solidago. (3) Host-plant quality:  Plant density had a negative effect on plant quality (measured as the relative growth rate of a specialist Solanum beetle Leptinotarsa juncta), irrespective of the identity of the neighboring plant.  (4) Insect-herbivore search strategies:  Neighborhood composition does not appear to affect searching strategies of L. juncta adults as they easily navigated to host plant patches from afar.  We are currently investigating whether neighborhood composition interferes with host-plant selection at fine spatial scales.  These results suggest that different aspects of the local neighborhood may affect herbivore colonization in complex ways, and that multiple mechanisms may operate simultaneously to influence plant damage patterns.