The middle-out ecology movement

This blog post is provided by David Jachowski and tells the #StoryBehindthePaper for the paper “Support for the size-mediated sensitivity hypothesis within a diverse carnivore community”, which was recently published in Journal of Animal Ecology. In their paper they explore the role of often overlooked meso-predators in ecosystems.

Popularity of large carnivores has long infiltrated ecology. Open any ecology textbook and you will find a case study from Yellowstone showing  how “wolves change rivers” through trophic cascade, where the addition or removal of top predators have cascading effects down through the food chain. Conversely, if a system isn’t driven by top-down pressures, we often think of systems as being driven by bottom-up processes associated with productivity of plant or herbivore populations. 

In this thinking, small to mid-sized predators are often afterthoughts. The species caught in the middle, reacting to bottom-up and top-down pressures. Ecologists have only recently begun to embrace the potentially valuable reactionary role meso-predators play in ecosystems as a type of biological indicator. 

In a recent review, a group of ecologists from around the globe summarized the multitude of factors influencing small- to midsized or meso-predator dynamics and why they are such good indicators of ecosystems structure and function. In other words, if you want to know the extent to which an ecosystem is changing, monitoring small- to midsize carnivores are your best bet.

In our newly published paper, we provide some of the strongest evidence for this newly coined size-mediated sensitivity hypothesis. While supported by case studies of small carnivore biology from around the world, this hypothesis, which predicts that the smallest carnivore within a community is likely to be the most sensitive to environmental change, has rarely been directly evaluated by comparing the sensitivity of carnivores within a diverse carnivore community. For four years we used camera traps to investigate the carnivore community of the Southern Blue Ridge Mountains, which is a plant and animal biodiversity hotspot in North America that has been identified as the top priority area for conservation action in the US.  

Example photos of the most commonly encountered carnivores within the Blue Ridge mountains of North Carolina, USA.

Using a combination of species occupancy and structural equation modelling to assess factors influencing the occurrence of carnivores across the landscape, we found that within this highly diverse (>10 species) mammalian carnivore community the smaller carnivores were most responsive to changing environmental and anthropogenic conditions.

The smaller the better

There are four primary hypothesized drivers of carnivore community dynamics: interspecific competition/predation, habitat complexity, food availability and anthropogenic disturbance. Our study shows that the spatial occurrence of large carnivores, including feral dogs, are primarily driven by one or two of these factors. By contrast, small- to midsized carnivores were typically influenced by three or more factors. But the clear winner for sensitivity was the smallest carnivore in our study, the eastern spotted skunk.

Structural equation modelling findings based on species occupancy, showing the high relative number of pathways by which smaller carnivores in the system are responding to environmental conditions.

Spotted skunks responded to each of the four hypothesized drivers of carnivore occurrence.  They avoided human disturbance, selected areas with lots of small mammal prey, used areas also frequented by striped skunks and used areas with adequate habitat complexity to avoid being predated upon. This heightened sensitive by spotted skunks in our system is paralleled across the declining range of spotted skunks. In the Channel Islands off of the coast of California, island spotted skunks are excellent indicators of shifts in ecosystem function, including the trajectory of endangered island fox populations. In the Pacific Northwest, western spotted skunks provide critical information on food web linkages between aquatic, terrestrial and arboreal systems.

In western North Carolina, eastern spotted skunks are important indicators of ecosystem change. Credit: Greg Detweiler.

Unfortunately, this newfound appreciation for the utility of small carnivores comes at a time when they are in rapid decline. Recent reviews suggest small carnivores are equally as threatened with extinction as large carnivores globally. What is needed is a new wave of researchers to embark on studies of the highly diverse yet understudied small carnivore communities within ecosystems globally. New technology makes such studies more practical than ever, it just takes more of us to engage in the middle-out ecology movement. By focusing ecological studies less on species located at the bottom or top of the food web within ecosystems, and more on the mid-ranking species (like small carnivores) that are rapidly responding to both top-down and bottom-up pressures, we are likely to gain greater insight into how ecosystems are changing in structure and function in a constantly changing world. 

Read the paper

Read the full paper here: David S. Jachowski, D.S., Marneweck, C.J., Olfenbuttel, C. & Harris, S.N. (2023) Support for the size-mediated sensitivity hypothesis within a diverse carnivore community. Journal of Animal Ecology, 00, 1-14

About the author

David S. Jachowski is an Associate Professor of Wildlife Ecology at Clemson University (USA) and Research Associate at the University of KwaZulu-Natal in South Africa. For over 20 years Dr. Jachowski has been studying the community dynamics of carnivores in a variety of systems around the globe, most recently working with government biologists to address the conservation needs of carnivore species within the Blue Ridge Mountains of North Carolina.

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