Invasive species are the second biggest threat for native biodiversity, after habitat destruction. Once introduced to a new habitat, exotic species can reduce native species richness, abundance, and overall diversity. How it is possible? For instance, in new habitats they lose their specific herbivores and pathogens and only deal with generalist herbivory in the invaded range. However, they will lose this advantage over time, starting to accumulate new enemies. In addition, herbivory and pathogen impacts may depend on plant density and life stage.
There is one hypothesis about density dependent regulation of population growth, called the Janzen-Connell hypothesis. It considers that natural enemies prevent recruitment of offspring near their parent plants, thus facilitating coexistence among species, so it’s also a possible explanation of forest’s biodiversity. This hypothesis predicts more seeds close to the mother plant, but probability of survival increasing at a distance away from the mother. In this study, we wanted to test the Janzen-Connell hypothesis, testing whether the negative density dependent effects on invasive seedlings changed over time as the enemy release effects diminished over time. We expected that in newly invaded areas, there would be more seedlings close to the mother plants (loss of Janzen-Connell) than near the source of invasion.
We develop this study at Las Cruces Biological Station due to the invasive ginger Zingiber spectabile presents an ideal system to test the effects of enemy release and Janzen-Connell. Previously, we knew where the source (the Wilson Botanical Garden), and the invasion front are located. The spatial distribution of the invasive ginger was used as a chronosequence (space for time substitution) to test the variation in the strength of density-dependent seedling herbivory. We used the native ginger Renealmia cernua for comparison with the invasive one, expecting it to present stronger density-dependence akin to the Janzen-Connell hypothesis. Our methodology consisted in sampling 5 individuals of Z. spectabile and 3 of R. cernua, at the source and edge of the invasion. Placinga mother plant at the center, we surveyed seedlings radially every 1m outward to 8m, counting the number of seedlings, as you can observe in the images.
Our results showed the Janzen-Connell prediction near the source of invasion, implying that well-established individuals of Z. spectabile have started accumulating new enemies!!! On the other hand, we did not find the same pattern at the edge of the invasion. More specifically, in the source of invasion we saw more seedlings between 6–7 m from the mother plant, and fewer seedlings between 0–1 m. At the edge of the invasion we reported more seedlings between 2–3 m, and fewer between 4–5 m. However, unexpectedly we did not find Janzen-Connell in the native ginger, perhaps because of its low density throughout the surveyed area, or also because we did not capture the ideal distance from the mother plant to survey the seedlings. We did not find seedlings between 0–1 m both at the edge and source of invasion, but we counted a few more seedlings between 7–8 m at both sites.
Although the evidence shows that invasive gingers near the source are presenting signs of density dependent factors, the density at the source is larger than at the front. This could imply that it is still too early to see the outcomes of population regulation in this invasive plant. Future studies on this system will inform us about the dynamics of population regulation of well-established invasive plants.