Review

Animals and the zoogeochemistry of the carbon cycle

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Science  07 Dec 2018:
Vol. 362, Issue 6419, eaar3213
DOI: 10.1126/science.aar3213

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Animals count

Flux across the carbon cycle is generally characterized by contributions from plants, microbes, and abiotic systems. Animals, however, move vast amounts of carbon, both through ecosystem webs and across the landscape. Schmitz et al. review the different contributions that animal populations make to carbon cycling and discuss approaches that allow for better monitoring of these contributions.

Science, this issue p. eaar3213

Structured Abstract

BACKGROUND

Modern advances in remote-sensing technology are providing unprecedented opportunities to accurately measure the global distribution of carbon held in biomass within ecosystems. Such highly spatially resolved measures of biomass carbon are intended to provide an accurate inventory of global carbon storage within ecosystems. They are also needed to test the accuracy of carbon cycle models that predict how global changes that alter biogeochemical functions—such as carbon assimilation via photosynthesis, carbon losses via plant and microbial respiration, and organic matter deposition in soils and sediments—will affect net ecosystem carbon uptake and storage. Emerging ecological theory predicts that wild animals stand to play an important role in mediating these biogeochemical processes. Furthermore, many animal species roam widely across landscapes, creating a spatial dynamism that could regulate spatial patterning of vegetation biomass and carbon uptake and soil carbon retention. But such zoogeochemical effects are not measured by current remote-sensing approaches nor are they factored into carbon cycle models. Studies are now providing new quantitative insights into how the abundance, diversity, and movement of animal species across landscapes influence the nature and magnitude of zoogeochemical affects. These insights inform how to account for animals in remote-sensing applications and in carbon cycle models to more accurately predict carbon exchange between ecosystems and the atmosphere in the face of global environmental change.

ADVANCES

Zoogeochemical effects have been measured using manipulative experiments that exclude or add focal wild animal species or along landscape gradients where animal abundances or diversity vary naturally. Our review of these studies, which cover a wide diversity of taxa (vertebrates and invertebrates and large- and small-bodied organisms) and ecosystems, reveals that animals can increase or decrease rates of biogeochemical processes, with a median change of 40% but ranging from 15 to 250% or more. Moreover, models that embody zoogeochemical effects reveal the potential for considerable under- or overestimates in ecosystem carbon budgets if animal effects are not considered. The key challenge, in light of these findings, is comprehensively accounting for spatially dynamic animal effects across landscapes. We review new developments in spatial ecosystem ecology that offer the kind of analytical guidance needed to link animal movement ecology to geospatial patterning in ecosystem carbon uptake and storage. Considerations of animal movement will require highly resolved spatially explicit understanding of landscape features, including topography, climate, and the spatial arrangement of habitat patches and habitat connectivity within and among ecosystems across landscapes. We elaborate on advances in remote-sensing capabilities that can deliver these critical data. We further review new geospatial statistical methods that, when combined with remote-sensing data and spatial ecosystem modeling, offer the means to comprehensively understand and predict how zoogeochemical-driven landscape processes regulate spatial patterns in carbon distribution.

OUTLOOK

There is growing interest to slow climate change by enlisting ecological processes to recapture atmospheric carbon and store it within ecosystems. Wild animal species are rarely considered as part of the solution. Instead, it is often held that managing habitat space to conserve wild animals will conflict with carbon storage. Our integrative review offers a pathway forward for deciding when and how conserving or managing a diversity of animal species could in fact enhance ecosystem carbon uptake and storage. Such understanding informs international climate and biodiversity initiatives such as those described by the United Nations Convention on Biological Diversity and national biodiversity strategies and climate action plans. All of these initiatives require better resolution of how biodiversity effects on ecosystem structure and biogeochemical functioning will become altered by global change.

The myriad animal zoogeochemical effects on carbon cycling.

Animals can mediate net carbon sequestration by plants (net primary productivity, NPP) by altering CO2 uptake into (black arrows) and from (red arrows) ecosystems. Herbivore grazing and tree browsing can alter the spatial distribution of plant biomass. Predators can modify herbivore impacts via predation and predator-avoidance behavior. Animal trampling compacts soils and alters soil temperatures by changing the amount of solar radiation reaching soil surfaces (yellow arrows). Animals also change the chemical quality of organic matter that enters the soil pool (orange arrows).

CREDIT: NICOLE FULLER/SAYO-ART

Abstract

Predicting and managing the global carbon cycle requires scientific understanding of ecosystem processes that control carbon uptake and storage. It is generally assumed that carbon cycling is sufficiently characterized in terms of uptake and exchange between ecosystem plant and soil pools and the atmosphere. We show that animals also play an important role by mediating carbon exchange between ecosystems and the atmosphere, at times turning ecosystem carbon sources into sinks, or vice versa. Animals also move across landscapes, creating a dynamism that shapes landscape-scale variation in carbon exchange and storage. Predicting and measuring carbon cycling under such dynamism is an important scientific challenge. We explain how to link analyses of spatial ecosystem functioning, animal movement, and remote sensing of animal habitats with carbon dynamics across landscapes.

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