Mangrove range shifts impact ecological interactions

Background/Question/Methods

Climate change is causing a poleward shift in the distribution and abundance of many species and ecosystems. As a result of fewer freeze events at the ecotone between temperate and tropical zones, mangroves are expanding beyond their historical range limits where they are encroaching on saltmarsh ecosystems. Our objective was to determine how this transition from saltmarsh to mangroves is affecting the structure and function of coastal wetlands. We used a space-for-time-substitution approach with a latitudinal gradient as a proxy for time and quantified differences in primary production, arthropod community structure, and rates of herbivory associated with Avicennia germinans (black mangrove). Samples were collected at 15 locations along the Atlantic coast from tropical mangrove forests on Key Largo to temperate mangroves at the northernmost edge of mangrove distribution at Ponte Vedra, FL. We measured plant growth, species diversity, feeding behavior, and leaf damage by three feeding guilds (folivores, leaf gallers, leaf miners).

Results/Conclusions

We found a significant relationship between annual growth rates of A. germinans and latitude with much higher shoot elongation rates at higher latitudes (r²=0.785). The folivores on A. germinans were mainly generalists, including leaf-chewing orthopterans and caterpillars.  The leaf gallers were specialists, including Leuronota maritima, a psyllid that caused rolled-leaf galls, and Meunieriella avicenniae, a cecidomyid gall midge that formed wart galls on the abaxial surface of leaves. The leaf miners were two undescribed agromyzids, also with specialized feeding behavior, including a species that mined the adaxial surface of mature leaves and a second species that mined the inside surface of L. maritima galls. There was variation in annual rates of folivory among the locations, but the differences were not consistent with a latitudinal gradient. However, there was a significant relationship between latitude and the proportion of leaf-bearing shoots with L. maritima galls (r²=0.804), with 4-5 times more galls at the south end of the gradient than at the north.  Leuronota maritima acted as an ecosystem engineer because its gall created a discreet habitat that was used by diverse community of inquilines.  There was a significant relationship (r² =0.529) between latitude and species richness in the inquiline community associated with L. maritima galls with 12-14 species at the tropical end compared to three at the temperate end. Our results suggested that range shifts in foundation plant communities may alter primary production in unexpected ways and have dramatic but cryptic effects on the associated faunal community.