The role of biotic interactions in mediating population & community responses to environmental change


Sunrise

Sunrise through Quercus muehlenbergii at the Tyson Research Center, Missouri. Photo: J. Myers

Effects of specialist pathogens and predators on global, regional, and local patterns of species diversity

Distance- or density-dependent specialist pathogens, predators, and parasites are thought to maintain biodiversity by limiting abundances of dominant species (a process known as negative conspecific density dependence, or negative CDD).  This process was originally hypothesized to explain why there are so many more species in tropical latitudes compared to temperate latitudes.  Yet we still have little understanding of the ways in which specialized enemies structure ecosystems and promote biodiversity because only a handful of studies have explicitly examined the relationship between CDD and diversity. In collaboration with 24 stem-mapped forest plots in the Smithsonian Forest Global Earth Observatory (ForestGEO), I found evidence that negative density dependent interactions among conspecifics (members of the same species) are stronger at tropical versus temperate latitudes and in more abiotically benign than in more abiotically harsh environments (LaManna et al. 2017 Science, LaManna et al. 2021 Ecosphere, LaManna et al. 2022 Ecology Letters). I also found that these population-stabilizing interactions are especially strong for rare species in the tropics, potentially causing the persistence of many tropical rare species. This study provides some of the first evidence that activities of specialized natural enemies might contribute to one of the most striking biodiversity patterns on Earth.

Activities of specialized natural enemies of trees (e.g. fungal pathogens, insect herbivores) may also depend on large- and local-scale gradients in temperature, precipitation, or soil conditions. Using tree species composition data across a subcontinental primary productivity gradient in the northwestern United States (US Forest Service Forest Inventory and Analysis), I found that CDD is more negative in more productive ecoregions (LaManna et al. 2017 Nature Ecology & Evolution). I have also found evidence that CDD is more negative in more resource-rich environments using data from a stem-mapped temperate forest in the Missouri Ozarks (LaManna et al. 2016 Ecology Letters). These studies suggest that environmental changes in temperature and precipitation may have previously unknown consequences for local, regional, and global patterns of biodiversity. I am now beginning field studies and experiments to test the importance of species interactions to changes in density dependence and biodiversity across environmental gradients.

This research led to NSF funding to determine the best approaches to measure the strength of these conspecific feedbacks and to test the role of plant-fungal interactions in generating gradients of CDD along abiotic environmental gradients. My research using long-term data from the NSF-funded Andrews Long-term Ecological Research (LTER) site in the Cascade Mountains has found similar shifts in the strength of CDD along microclimate gradients in the Cascade mountains (LaManna et al. 2022 Ecology Letters). My lab is currently undertaking field-based reciprocal transplant experiments to determine the biological mechanism through which CDD and other population processes are affected by climate change and gradients in the abiotic environment.

Dusky

Dusky flycatcher (Empidonax oberholseri) nest in the Lewis & Clark National Forest, Montana.

Predation risk mediates bird community responses to environmental change

In a meta-analysis of bird responses to logging at the global scale (LaManna & Martin 2017 Biological Reviews), I found that stronger effects of logging on nest-predation rates appeared to be responsible for greater changes in species composition across latitudes. This analysis highlights the importance of predator-prey interactions to avian community assembly around the world.  This result was further supported by a six-year observational study and landscape-scale conifer-removal experiment I conducted in collaboration with state and federal agencies in western Montana (LaManna et al. 2015 Ecology).  Data from over 2,700 nests combined with experimental removal of conifers from aspen stands demonstrated that effects of forest trees on nest-predation rates were the causal mechanism driving bird abundances, species turnover, and local diversity.  Environmental change has been traditionally thought to influence species composition and diversity via effects on food availability or interspecific competition.  This global meta-analysis and in-depth landscape-scale experiment provide a novel perspective by showing that species interactions across trophic levels, such as predator-prey interactions, play an important role in mediating environment-diversity relationships.

Sugar

Sugar maple (Acer saccharinum) at Tyson Research Center, Missouri. Photo: J. Myers

Do predators mediate the importance of functional-trait diversity?

I am collaborating with researchers from Washington University in St. Louis and the University of California in Riverside to test if mammalian seed predators and herbivores mediate relationships between soil-resource availability, forest fire, and the functional diversity of plant communities.  This landscape-scale experiment will substantially advance our understanding of functional diversity by determining the causal mechanisms that mediate the relationship between functional-trait diversity and community assembly.

 

Carolina buckthorn (Frangula caroliniana) and flowering dogwood (Cornus florida) seeds at Tyson Research Center, Missouri. Photo: J. Myers