Temporary enemy release: Quantifying arrival rates of exotic fungi on introduced host plants

Background/Question/Methods

Host species may escape their natural associates, including pests, pathogens and mutualists, following introduction to a new region, but over time these associates can in turn be introduced and catch up with hosts in the new range. How rapidly do associates arrive and accumulate on host plants that have been introduced to a new region? The answer is important to understanding the window of opportunity that introduced plants may have in escaping their natural enemies, and in understanding the risk of pest and pathogen arrival for economically important host plants. Here we use data on host plant-fungal associations recorded in New Zealand over the last 130 years (>13000 records from >1000 introduced host plant species and >1000 species of exotic fungi) to quantify the rate at which exotic fungi have arrived and accumulated on introduced host plants. The rate at which new fungi were recorded on host plants was positively correlated with sampling effort, which increased over time. To account for this sampling bias we modelled both the underlying rate of fungal arrival and the discovery process as a function of sampling effort.

Results/Conclusions

The arrival rate of exotic fungi associated with introduced host plants in New Zealand has accelerated over the last 110 years, increasing from about 3 species per year in 1900 to 20 species per year in 2010. The pattern is similar for fungi classified as pathogens, although the increase in arrival rate is lower: from 2.5 to 11 species per year from 1900 to 2010. While just over 1000 species of exotic fungi have been recorded on introduced host plants, of which 671 are classified as pathogens, we estimate that a further 200 species have arrived but are as yet undiscovered, including about 100 pathogens. The average rate of arrival of exotic fungi on 59 host plant species having at least 15 species of fungi recorded on them varied substantially, from 0.05 species per year since the first recorded discovery on a host, to over 1 species per year, although our model indicates that many host plants had already accumulated fungi by the time they were first recorded as having any. Our results show that exotic fungi continue to accumulate on introduced host plants in New Zealand, implying there is a limited temporal window for escape from natural enemies.