Friday, April 19, 2024

Unraveling the invasion paradox  – The Applied Ecologist

EcologyUnraveling the invasion paradox  – The Applied Ecologist


Shortlisted for the 2023 Southwood Prize


Nicholas McMillan details how he and colleagues collected data across eight grassland landscapes to test how an invasive legume affected plant and bird communities at spatial grains ranging from 0.1 m2 to >3,000,000 m2. It was concluded that scale is a central problem in ecology, and defining scale in management objectives is essential for effective biodiversity conservation.

Invasive plants

Invasive plants are managed across much of the globe, in part, because of the suspected negative effect that their spread has on biodiversity and ecosystem function.  This management is not without support: there are decades-worth of data supporting a negative invasion-biodiversity relationship.  Some have gone as far as to suggest that society should work to control invasion for fear of falling into what has been dubbed the “Homogocene Epoch”. 

However, more recently, scientists have also suggested that the relationship between invasion and biodiversity is scale-dependent —a so-called “invasion paradox”—where the negative effect of invasion on biodiversity may only be present at small scales (e.g., < 100 m2), and may be positively related at larger scales (e.g., > 1,000,000 m2). Most of what we know about invasive plants, and how they affect biodiversity is limited to small scales (100 m2 or less), which is grossly different from the scale of management for most landscapes that are important to local or global conservation (i.e., > 1,000,000 m2). 

Tallgrass prairie preserve © Nicholas McMillan

Grasslands are among the most imperiled ecosystems globally. In the United States, management actions attempting to thwart the spread of invaders—and their potential negative effects—are applied across millions of acres every year.  Therefore, unraveling how invasive plants affect patterns of biodiversity, across scales, is critical to our management of those imperiled landscapes and their continued conservation. 

The study

Invasive legume, Lespedeza cuneata © Breanna Barker

The invasive legume Lespedeza cuneata (Dum. Cours.) G. Don. is broadly managed across the tallgrass prairie ecosystem in North America due to its presumed negative effects on biodiversity and livestock production.  We collected vegetation and bird field data across eight replicated, experimental landscapes that were typical of large, workinglands in the region (333-766 ha each) to ask: Does the presumed negative effect of L. cuneata invasion on biodiversity change with spatial scale? 

We found that L. cuneata had a negative effect on plant diversity at small scales, but its effect became neutral with increasing spatial scale.  What’s more, L. cuneata abundance (i.e., how dominant the invasive plant was) had no observable effect on bird diversity across our experimental landscapes at any scale. 

While our research represents a specific case study, it contributes to a 30-year discussion within invasive species ecology suggesting that invasive species effects on biodiversity may be scale dependent—at best—or neutral.  For example, a recent meta-analysis also suggests that invasive species effects, positive or negative, on biodiversity are largely unpredictable regardless of ecosystem, taxa, or spatial scale.

Bird species richness response to Lespedeza cuneata canopy cover (%) at spatial grains ranging from the array (1200 m2) to the landscape (>3,000,000 m2) © McMillan et al, 2023

Our results also suggest that managing large, often complex, landscapes based on expectations gleaned from small-scale experimental studies will rarely lead to predicted outcomes.  Rather, management actions that promote rather than constrain landscape complexity, at all scales, are the most likely to overcome all of the conservation challenges we face in the Anthropocene. 

Plant species richness, exponential Shannon diversity index (eH) and inverse Simpson diversity index (1/D) responses to Lespedeza cuneata canopy cover (%) at the landscape, patch, array and plot scales © McMillan et al, 2023

About the author

The author in the Alps © Nicholas McMillan

I was lucky to have grown up having a ecologist, naturalist, botanist, plant taxonomist for a father; so I have been immersed in the world of ecology (and surrounded by ecologists) for most of my life.  In fact, some of my earliest memories are of trailing behind my mother and father while we did botanical surveys in the Longleaf Pine Savanna – a special ecosystem throughout the coastal plain of the Carolinas.  I grew up in what Reed Noss would call the “forgotten grasslands of the South”, and as an Assistant Professor at the University of Nebraska-Lincoln, I am still fascinated by how ecosystems, especially grasslands, work – my scale has just changed a bit (I am also taller than I was!). 

My ongoing research at UNL surrounds my interest in landscape complexity (heterogeneity) and scale; and how to best apply those findings to address challenges facing North American grasslands, which are often agricultural landscapes. 

In my spare time, my wife (Melanie) and I love to explore our home state of Nebraska, as well as the Great Plains.  We especially like to backpack in the more remote/rural parts of the region, and one of our favorites is the Oglala National Grassland; specifically around Toadstool Geologic Park in Sioux County, Nebraska. 

Nic and Melanie McMillan © Nicholas McMillan

In fact, Oglala National Grassland was one of the landscapes that I first visited in the Great Plains as a teenager – a visit that inspired me to be a grassland ecologist and sparked my passion and intrigue for this wonderful place I call home. 

Read the full article “A plea for scale, and why it matters for invasive species management, biodiversity and conservation” in Journal of Applied Ecology.

Find the other early career researchers and their articles that have been shortlisted for the 2023 Southwood Prize here!

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