The “gall” of it all – how a latitudinal gradient in the diversity of oak gall formers contributes to the outbreak of a range-expanding species

This blog post is provided by Dylan Jones and Kirsten Prior and tells the #StoryBehindthePaper for the paper “Latitudinal gradient in species diversity provides high niche opportunities for a range-expanding phytophagous insect“, which was recently published in the Journal of Animal Ecology. In their study they explore how gall wasp communities vary from north to south in the United States of America, and how reduced diversity closer to the poles can allow some species to become “invasive” as they are no longer competing with other species.
The Background

When species undergo northern range expansions in response to environmental and climatic changes, they may encounter less diverse communities. This is because communities commonly experience a loss of diversity at higher latitudes (known as the Latitudinal Diversity Gradient). If low diversity communities have weaker interactions, such as among competitors or predators, range-expanding species may experience “release” from interactions that keep their populations in check, causing them to outbreak or become “invasive.”  In our recent paper, Latitudinal gradient in species diversity provides high niche opportunities for a range-expanding phytophagous insect, in the Journal of Animal Ecology, we use a study system of oak gall wasps on a dominant oak, Garry oak – Quercus garryana, in western North American oak ecosystems to uncover if this community shows a reduction in species diversity in northern latitudes. One community member, “The jumping gall wasp” – Neuroterus saltatorius, experienced a range expansion northward and is outbreaking, damaging oaks in its northern expanded range. We ask if lower diversity communities provide weaker competition that might contribute to the jumping gall wasp outbreaking in its expanded range. To ask these questions, with funding from the National Geographic Society, we surveyed gall wasps on Garry oaks throughout most of the tree’s range, from northern California to Vancouver Island, British Columbia. In total, we found and visited 18 sites and found 23 oak gall wasp species.

From East to West

Our study system is far from our home institution – Binghamton University, New York. This means when we were preparing for this large-scale study, and we had to coordinate with landowners and identify study sites from across the country – this was no easy task! Luckily, we have worked at some of our study sites in Washington and Vancouver Island, British Columbia, for many years. However, as we aimed to document the diversity of oak gall wasps throughout most of the range, we needed to search for sites in areas we had never worked in before in Oregon and California. Finding new study sites and coordinating with landowners afar for 18 sites was no easy task. We did extensive research – looking at papers, websites, parks, and iNaturalist to find oak habitats and working with a diversity of landowners, from private landowners to the Bureau of Land Management, the Nature Conservancy, and state and municipal parks and preserves. We are incredibly grateful to landowners for supporting research on their property and for many who do important conservation or restoration work in this critical ecosystem.  

Then the fun part began. We drove across the country in our lab truck, eventually reaching our home base in Southern Oregon. However, the adventure was just beginning. Once out west, we needed to verify the suitability of our new sites. Sometimes this meant a pleasant day with a landowner. In other instances, we would find ourselves driving down unpaved roads (some that brought us to the side of cliffs!) with no phone service. Sometimes we struck out, finding inaccessible or burned oak patches. However, there is nothing quite like driving for hours and stumbling upon a beautiful open oak savanna ecosystem in the middle of a dense coniferous forest. The sites were all unique and beautiful in their way.

Pictures of oak savanna ecosystems from northern California to Vancouver Island, BC.

After days of driving and hiking to confirm our field sites, we started with our gall collections. Days were spent hiking around sites and staring closely at leaves to look for as many galls species as possible, and we were always ecstatic to add a new species to our list!

The Science

Galling insects deposit eggs in living plant tissue that stimulates plant growth, forming a gall structure where the larvae develop and feed on plant tissue. A suite of gall wasps that specialize on oaks (Cynipidae, Cynipini), with approximately 700 species, are found in North America. Oak gall wasps make amazingly diverse galls on various plant tissues (leaves, stems, acorns, roots) that vary in size (from as small as a pinhead to as large as an apple), shapes (from perfectly round to looking like a plant organ), and texture (woody, fleshy or hairy). Oak gall wasps are hosts to a diverse community of natural enemies (largely parasitoid wasps).

Map showing the range of Q. garryana (in gray) with field sites (circles = native range; stars expanded range) color coded by region. Additionally featured are authors – Dylan G. Jones, Kirsten M. Prior, and Julia Kobelt performing fieldwork. 

Given gall wasp’s sedentary nature on trees (meaning they stay on the tree) for a big part of their lifecycle, we assume interactions occur most strongly when they co-occur on trees. We categorized our 23 gall wasps into three guilds – those that create small, detachable leaf galls; woody galls on stems; or fleshy, succulent galls integrated within leaf tissue. The jumping gall wasp creates small detachable leaf galls.

As expected, we found that oak gall wasps show lower diversity at northern regions. This was primarily due to a decline in detachable leaf gall wasps that are likely important competitors for the jumping gall wasp in the native range but missing in the expanded range. A lack of interactions with competitors on trees may contribute to release of the jumping gall wasp in the expanded range. Given that latitudinal diversity gradients are commonly found in nature, poleward range-expanding species are likely moving into low diversity communities.

A) Gall wasp richness on Q. garyanna. In the blue box are detachable leaf gall wasps (13 species) (* is N. saltatorius), purple is integral leaf gall wasps (2), and orange is stem gall wasps (9). B) Gall wasp richness decreases at higher latitudes. C) Gall wasp species co-occur on trees with the range expanding species (N. saltatorius, NSA) in the native range, but D) not in the expanded range. 

This work was funded by the National Geographic Society and the National Science Foundation, and we were lucky to be able to spend our summer hiking in these spectacular landscapes uncovering the beauty of the diversity of oak inhabitants! Our observations and collections also contribute to a collaborative effort with other North American gall researchers. Recent publications including, Ward et al. 2022a, Ward et al. 2022b, and Zhang et al. 2022.

Who we are

Kirsten M. Prior is an Assistant Professor in the Department of Biological Sciences at Binghamton University. She is a community ecologist whose research focuses on the ecology and evolution of antagonistic and mutualistic interactions. She is particularly interested in how anthropogenic change alters intra- and interspecific interactions, and diversity and functioning of communities. Website, twitter: @kirstenmprior.

Dylan G. Jones is a PhD candidate in the Department of Biological Sciences at Binghamton University working under Dr. Kirsten M. Prior. Like Kirsten, Dylan is also interested in species interactions. Dylan would like to use the knowledge he gains in his PhD research to transition to applied science to use his community ecologist skills as a means to mitigate and prevent species outbreaks. Twitter: @DylanGJones92.

Read the paper

Read the full paper here: Jones, D. G., Kobelt, J., Ross, J. M., Powell, T. H. Q., & Prior, K. M. (2022). Latitudinal gradient in species diversity provides high niche opportunities for a range-expanding phytophagous insect. Journal of Animal Ecology, 00, 1– 13.

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