One of the important aspects of whether a species can expand its range or become invasive in a region is propagule pressure—the number of individuals introduced to a region beyond the range. The probability of a new population establishing usually increases most strongly with the number of arriving colonists. However, recent work has shown that the quality of arriving colonists can be an important aspect of establishment success for new populations. Mangroves are undergoing poleward range shifts and have recently transformed herbaceous salt marshes in northern Florida to ecosystems dominated by woody plants. Avicennia germinans is the northernmost mangrove species and is the most abundant mangrove species in northern Florida. Despite documentation of large-scale ecosystem changes due to mangrove expansions, little is known about the propagule pressure of A. germinans at its expanding range edge, which will affect future expansion into saltmarshes in the region.
We assessed variation in flowering effort, and propagule viability in A. germinans along a latitudinal gradient on the Atlantic coast of Florida from 24.8° N -30° N. We also evaluated the effects of eutrophication on propagule production in a fertilization experiment near the northern range limit to assess if addition of a limiting nutrient affected reproductive effort.
Results/Conclusions
The flowering effort of A. germinans plants, measured as flowers/100 shoots, increased with latitude (linear regression, R2=0.73). Range-edge populations (29-30° N) exhibited two to five times the flowering effort found in populations in southern Florida (24-25° N). Propagule viability was not strongly correlated with latitude. Instead, propagule viability was strongly negatively correlated with insect infestation rates (linear regression, R2 = 0.77). Populations at the northern range limit had no insect attacks on propagules. This demonstrates an escape from enemies at the northern range limit for Avicennia germinans, which can increase propagule pressure and lead to colonization of new populations.
In the nutrient addition experiment near the mangrove range limit, trees fertilized with N produced more flowers and more propagules than controls or trees fertilized with P (ANOVA; flowers: F=5.99, p=0.01; propagules: F = 5.15, p = 0.02). Eutrophication and sea-level rise may increase the rate of propagule production in N-limited sites and therefore increase the rate of population growth.