When enemies catch up – declining invasive impacts in hogweed?

Dostál P., Müllerová J., Pyšek P., Pergl J. & Klinerová T. (2013). The impact of an invasive plant changes over time. Ecology Letters, Early View.

Invasive species are a major ecological issue in this age of global connectivity. Many ecosystems are unrecognizable today after invasion, and invasive species have considerable impacts on agriculture, tourism, aquaculture, forestry, and native biodiversity. Some invasive species have been present in novel environments for decades or centuries, but many are more recent introductions. What we are still trying to understand is whether invasive species impacts are constant through time. Do ecosystems and communities adapt to invasive species, and how do they do so? Some examples of how the impacts of invasions can decline through time exist: for example,  the efficacy of allelopathic chemicals produced by garlic mustard (e.g. Alliaria petiolata) may lessen as native species adapt. Another hypothesis is that specialized pathogens or herbivores, absent at the start of the invasion, will increase with time, reducing populations of invasive species and allowing the recovery of native species.

Do specialized pathogens
find escaped species eventually?

If specialist enemies eventually catch up with invaders, this could reduce invasion impacts eventually, but long-term empirical data showing this type of effect is almost nonexistent. Dostal et al. (2013) provide some of the first  strong evidence that specialist pathogens catch up with and reduce the impact of giant hogweed (Heracleum mantegazzianum), a species from the Caucasus now found throughout Europe and North America. Hogweed invades successfully for a number of reasons, including its ability to form large, dense stands that reduce light availability, as well as through the production of allelopathic chemicals.

Giant hogweed was first introduced to the Czech Republic as an ornamental in 1863, but became invasive when grassland management ceased during WWII. The authors took advantage of aerial photographs taken since 1964 to locate hogweed presence. They identified a chronosequence of sites invaded for at least 48 years, 42 years, 28 years, 11 years or never (hogweed-free sites). These sites were on average 1.3km apart, and shared similar management history and environmental conditions. The authors then surveyed the sites, recording current hogweed percent cover, native biomass, and native species richness.

The researchers also collected soil samples from these sites. These samples were used in a common garden experiment to test whether soil pathogens that affected hogweed success might differ between the sites with different ages of invasion. Soil from the sites with different ages of invasion history was either sterilized to kill living pathogens, or left unsterilized. To these different types of soil (sterilized or not, 0, 11, 28, 42, or 48 years of invasion), they planted 1) 1 hogweed seedling, 2) a mix of native species seed, or 3) 1 hogweed seedling and the mix of native species together. They then looked at how well the hogweed seedlings survived.

Results from the observational data showing changes in native productivity and richness,
and hogweed cover as a function of time since initial invasion. (Dostal et al. 2013)

Hogweed showed decreases in biomass as the length
 of invasion history in the soil increased, but
 only if the soil was not sterilized. (Dostal et al. 2013) 

Hogweed, as expected, had very negative effects on the native species in these sites: on average, hogweed invasion decreased native species richness by ~10 species, and similarly decreased native biomass. These two sets of experiments however, showed clearly that as hogweed invasion age increased, hogweed cover declined. Further, native species richness and biomass increased once invasions were more than 28 years old. The common garden experiment suggested that this might be due to hogweed-specific soil pathogen accumulation through time. Non-sterilized soil from sites with longer histories of invasion had a significantly more negative effect on hogweed biomass than the sterilized soil. This suggests that living components of the soil -presumably pathogens - differed between soils with different invasion histories. Directly quantifying or identifying soil pathogens would be the obvious and important next stage.

Dostal et al. (2013) suggest that for these sites in the Czech Republic, hogweed dieback due to increased specialist pathogen load will prove key for native species recovery. They make it clear that this is not the same as advocating for a lack of response to the invasion. Ignoring established sites is providing a source for seed for new sites, puts rare native species at risk, and may leave management concerns such as declining ecosystem functions untouched. The other issue is that density dependent, stabilizing processes like increases in specialist pathogens, may lead to boom and bust cycles – initial rapid invasion might be followed by declining invasive populations as specialist enemies increase, but declining invasive populations would lead to declines in specialist enemies, and increased invasive pressures could start anew. Such a situation wouldn’t lead to a general decline in invasive species, but rather a new (non-equilibrium) state for the community.