UMD Study Shows Tree Species Diversity Is Good for Spiders—and Us

Often maligned—and feared—as creepy nightmare critters, spiders in reality are some of the most environmentally friendly pest regulators. They actively feed on flies, moths, mosquitoes and roaches, eliminating parasites and many other vectors of disease, protecting both humans and plants from harm.

A new University of Maryland-led study published online in the journal Ecology found a simple way to take advantage of this natural ecosystem service: Give tree-dwelling spiders more diversity of trees to make their homes in.

“Spiders really like complex habitats, so having a large variety of tree species with different structural features like height, canopy cover and foliage density will help increase spider abundance and also the natural pest regulation they provide,” said Karin Burghardt, senior author of the study and assistant professor of entomology at UMD.

The study is part of the BiodiversiTREE project, a series of large-scale, long-term experiments aimed at restoring diverse coastal forests along compromised shorelines of the Chesapeake Bay. Conducted by the Smithsonian Environmental Research Center and collaborators from institutions including UMD, BiodiversiTree investigates the role of tree species diversity in forest responses to climate change from the molecular to the watershed levels.

The forest plots where the experiments are taking place were planted in 2013 at the Smithsonian center in Edgewater, Md., where formerly agricultural land was reforested with 16 tree species native to Maryland, including white oak, red maple and black gum trees. Each of 75 plots measures 35 square meters and was planted with 255 trees. On the plots, the researchers planted the same tree species or four or 12 species.

Burghardt’s team, which included study lead author Elizabeth Butz ’21 and postdoctoral associate Lauren Schmitt, began the spider study in 2019. After repeatedly sampling 540 trees in these plots by counting the number of spiders found in each one, the team found that plots with higher tree species diversity also hosted greater spider populations. By the end of the project in 2021, plots containing four or 12 different tree species supported approximately 23% to 50% more spiders than single-species plots.

“Plots with more variation in tree species tend to have more canopy cover than plots with only a single tree species,” Burghardt explained. “More canopy cover and tree shade may mean more water retention, cooler temperatures, additional hiding spaces from predators and better web-making environments for spiders—all features that influence spider distribution.”

The researchers found the strongest relationship between tree diversity and spider populations during the late summer months when average temperatures were at their highest, which may relate to spiders’ extreme sensitivity to heat.

“Maintaining certain microclimate conditions is essential to helping to maintain spider population levels and their natural pest control services, which allow managers to minimize the use of toxic chemical pesticides,” Burghardt said.

As average temperatures continue to rise due to climate change, populations of tree-feeding insect pests and occurrences of deadly diseases transmitted by pests like mosquitoes and ticks will likely also steadily increase. Designing landscapes that support spiders and other beneficial bugs can offer economically and environmentally sustainable ways to counter this trend.