As a Watershed is Urbanized, Damselflies Show Declines
By John P. Roche, Ph.D.
The damselfly Hetaerina americana, sometimes known as the American rubyspot, lives in river environments in dry regions of the United States, Mexico, and Central America. Many of H. americana‘s habitats are undergoing habitat degradation as a result of urbanization, and this habit change is a concern for populations of damselflies and their relatives the dragonflies.
Because damselflies have both an aquatic larval stage and a terrestrial adult stage and are thus reliant on two different habitats, land-use change has the potential to magnify negative effects on these species. Alex Córdoba-Aguilar, Ph.D., and Maya Rocha-Ortega, Ph.D., of the National Autonomous University of Mexico investigated if urbanization in riverine habitats is affecting populations of H. americana in central Mexico, and their findings were published in late June in the open-access Journal of Insect Science.
Two byproducts of human land-use changes in riverine environments are reductions in total vegetation and reduction in water quality resulting from wastewater effluents. Both have the potential to negatively impact damselflies. To test for effects of these variables on damselflies, Córdoba-Aguilar and Rocha-Ortega surveyed Hetaerina americana in riverine habitats in Mexico in 2002 and 2016, and surveyed vegetation and wastewater outlets in 2002, 2003, 2004, and 2016.
Their field site was the Amacuzac River in Central Mexico. They created six 600-by-100-meter plots that ran longitudinally along the river, and they counted each tree and shrub occurring in these plots in 2002, 2003, 2004, and 2016. They also recorded the number of wastewater outlets found in the study area in those four years.
Córdoba-Aguilar and Rocha-Ortega collected last instar larvae of American rubyspots with an aquatic net every 34 meters at 15 sampling spots along the river for 23 days in 2002 and for 23 days in 2016. They also sampled abundance of adult American rubyspots along two 600-meter transects within the study area, one on each side of the river, every five days for a total of 16 days per year in 2002 and 2016. When they are in direct sunlight, damselflies have to expend more energy regulating their temperature; for this reason, Córdoba-Aguilar and Rocha-Ortega measured the amount of time damselflies spent in direct sunlight along the same transects in 2002 and 2016. They also recorded the proportion of eggs that hatched from 20 sample females in 2002 and from 20 sample females in 2016.
The investigators observed that in 2016 there were significantly fewer shrubs and trees in the study plots than in 2002. They also found that there was a significant increase in the number of wastewater outlets from 2002 to 2016. In 2016, there were significantly fewer H. americana adults and larvae than in 2002. In addition, the damselflies that were sampled in 2016 had significantly lower lipid mass and significantly lower muscle mass than those sampled in 2002. Time spent in direct sunlight was significantly higher in 2016 than in 2002. Furthermore, the hatchability of eggs was significantly lower in 2016 than in 2002.
What does this all mean? Vegetation offers several benefits to damselflies. It provides shelter from sun, reducing energy expenditure needed to regulate their temperature. It also provides protection from predation from birds and flying insects such as bees and dragonflies. Thus, lower amounts of vegetation could tend to reduce the physiological quality and survival of damselflies, relationships that have been observed in other studies, in other locations as far-flung as Iowa and Papua New Guinea. An increase in the number of wastewater outlets could negatively impact damselflies by reducing water quality, which is critical for the aquatic stage of their life cycle. This could in turn reduce damselfly health, hatchability, or survivorship. However, the potential effect of wastewater on damselflies is more difficult to determine, and it has been less well established in the literature.
“A follow-up study,” Córdoba-Aguilar says, “would be to measure body temperatures and see whether areas with less vegetation lead to higher body temperatures. The same goes for food with deforested areas producing fewer food items. In fact, our recent results comparing damselfly feeding patterns indicate that damselflies living in urbanized areas have less available food.”
Damselflies and their close relatives in the order Odonata, the dragonflies, are ecologically important to riverine communities. Being dependent on both aquatic and terrestrial habitats, they can be important indicator species. They are also beneficial as predators of pest species such as mosquitoes. Additionally, damselfly nymphs provide food for amphibians and fish, and their adults provide food for terrestrial arthropods and vertebrates, making damselflies important structuring components for both aquatic and shoreline communities. Thus, reductions in damselfly populations could cause ripple effects, including allowing an increase in mosquitoes, an outcome that could contribute to an increased risk of the spread of arboviral diseases. Reductions in damselflies could also cause disruptions to food webs in aquatic and terrestrial communities, making those communities less resilient and less species-rich. As a result, reductions in populations of damselflies in the face of urbanization are concerning.
In their paper, the authors write, “We believe that an explanation for detecting—or not detecting—the negative effects of urbanization on odonates may be a problem of scale.” That is, simply measuring the presence of a species in a habitat would miss the information contained in measuring the size of populations of a species in that habitat, because reductions in population size can have profound ecological significance.
Regarding future research questions, Córdoba-Aguilar says, “The next steps are to find a link between polluted water and reduced larval survival and between a reduction in vegetation and reduced adult survival. These steps need to be experimental, such as placing larvae in polluted water and counting surviving animals.”
Future research in the field could also examine potential changes in condition, hatchability, and abundance in damselfly populations of other species and could work on determining which specific effects of reducing vegetation may most impact damselfly condition and abundance. Then, such research could explore potential measures that could allow the economic growth and varied land-use associated with urbanization in a way that conserves key habitats and key species such as damselflies. Two possibilities would be leaving some vegetation along rivers and having more treatment of wastewater prior to its entry into river environments. As of yet, only a small number of species of damselflies are endangered in North America, and the American rubyspot is not among them. But, with ongoing urbanization along rivers in many areas, continuing research and a search for practical steps to minimize adverse impacts will be essential to preserve the species richness and ecosystem function we enjoy from riverine habitats.
Journal of Insect Science
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.