Effects of fungal pathogens on seeds of native and exotic plants: a test using congeneric pairs.
It has previously been hypothesized that low rates of attack by natural enemies may contribute to the invasiveness of exotic plants. We tested this hypothesis by investigating the influence of pathogens on survival during a critical life-history stage: the seed bank. We used fungicide treatments to estimate the effects of soil fungi on buried seeds of a taxonomically broad suite of congeneric natives and exotics, in both upland and wetland meadows, in Ontario, Canada. Seeds of both natives and exotics were recovered at lower rates in wetlands than in uplands. Fungicide addition reduced this difference by improving recovery in wetlands, indicating that the lower recovery was largely attributable to a higher level of fungal mortality. This suggests that fungal pathogens may contribute to the exclusion of upland species from wetlands. The effects of fungicide on the recovery of buried seeds did not differ between natives and exotics. Seeds of exotics were recovered at a higher rate than seeds of natives in uplands, but this effect was not attributable to fungal pathogens. Fungal seed pathogens may offer poor prospects for the management of most exotic species. The lack of consistent differences in the responses of natives vs. exotics to fungicide suggests few aliens owe their success to low seed pathogen loads, while effects of seed-pathogenic biological control agents on non-target species would be frequent.