Disturbance has both direct and indirect effects on individuals that may result in measureable effects at the population level. The direct effects of disturbance are often detrimental for individuals and result in damage to tissue or mortality, but they also have the potential to facilitate persistence, or even spread, of a species at the population level. This is possible if these effects result in increased reproductive effort, as is sometimes the case in plants. We considered this phenomenon by developing a conceptual model where a tradeoff in resource allocation results in increasing flowering biomass and decreasing vegetative biomass as disturbance increases, up to a maxima where increasing mortality decreases both flowering and vegetative biomass as disturbance increases further. We tested this model by examining the direct effects of disturbance on reproductive allocation in the invasive dwarf eelgrass Zostera japonica Aschers. & Graebn. across different sediment disturbance regimes in Yaquina Bay, Newport, Oregon, USA. We pair data from 2 yr of monitoring at 6 sites with a field experiment manipulating sediment deposition at 3 sites to examine potential for the direct effects of disturbance to facilitate the persistence and spread of this invasive species.
Both the observational and experimental studies showed that there was a negative linear relationship between vegetative biomass of Z. japonica and sediment deposition (our measure of disturbance) among the sites; the experiment showed a 50% decline of vegetative biomass at sediment deposition levels of ~1 cm mo-1 and nearly a 100% decline at rates above ~3.5 cm mo-1. The experiment also showed that flowering responded unimodally to increasing sediment deposition; flowering increased up to sediment deposition rates of ~0.75 cm mo-1 and declined at ~1.5 cm mo-1. For rates up 1.0 cm mo-1 flowering biomass was negatively correlated with vegetative biomass, suggesting a tradeoff was occurring. The results show that there are tradeoffs in resource allocation triggered by the direct effects of disturbance, favoring sexual reproduction as a potential escape response to increasing disturbance severity. At sites with moderate levels of disturbance, the limited mortality and increased flowering biomass may potentially facilitate the persistence and spread of this invasive species.