An interesting piece of research by a large international team led by the Forest Service of the US Department of Agriculture (USDA), has shown how herbivory by leaf-eating insects is part of a dynamic forest ecosystem, as the results of herbivory releases nutrients back into the soil from where they are recycled back into the forest canopy.
It is a lovely illustration of how, in contrast to what creationists claim is evidence that Earth is fine-tuned for life, it is actually life that fine tunes itself to fit the environment by an evolutionary process. There is nothing magical about it; it is simply the operation of basic laws of chemistry and physics, and the differential survival of those best fitted to live and reproduce within the ecosystem.
How the team did this research is the subject of an open access research paper in Nature Communications and is explained in a press release from USDA:
What are the elements of a forest ecosystem, including herbivory by leaf-eating insects? A forest ecosystem is a complex and dynamic system composed of various biotic and abiotic elements that interact with each other. Here are the main elements of a forest ecosystem, including herbivory by leaf-eating insects:
- Biotic Components
a. Producersb. Consumers
- Trees and Shrubs: Dominant vegetation that forms the canopy and understory.
- Herbaceous Plants: Ground-level plants such as ferns, grasses, and wildflowers.
- Mosses and Lichens: Non-vascular plants that thrive on trees and forest floors.
c. Decomposers and Detritivores
- Primary Consumers (Herbivores): Animals that feed directly on producers. This includes:
- Leaf-eating Insects: Such as caterpillars, beetles, and aphids that consume leaves and play a significant role in nutrient cycling and energy flow.
- Other Herbivores: Deer, rabbits, and small mammals that graze on vegetation.
- Secondary Consumers (Carnivores): Animals that prey on herbivores. This includes:
- Insectivorous Birds: Species that feed on leaf-eating insects, helping control their populations.
- Small Predators: Such as spiders, lizards, and small mammals like shrews.
- Tertiary Consumers: Apex predators that prey on secondary consumers, such as hawks, owls, and larger mammals like foxes and bears.
- Fungi and Bacteria: Break down dead organic matter, returning nutrients to the soil.
- Detritivores: Organisms like earthworms, beetles, and millipedes that consume decomposing plant and animal material.
- Abiotic Components
- Soil: Provides nutrients and a medium for plant growth. Soil composition affects the types of vegetation that can thrive in a forest.
- Water: Essential for all living organisms, influences plant growth and distribution.
- Air: Supplies oxygen for respiration and carbon dioxide for photosynthesis.
- Sunlight: Primary energy source driving photosynthesis and influencing temperature and climate conditions.
- Climate: Temperature, humidity, and precipitation patterns that define the forest's environment.
- Ecological Interactions
a. Herbivory
b. Symbiotic Relationships
- Impact on Vegetation: Leaf-eating insects can significantly influence plant health and growth. Heavy infestations can lead to defoliation, reduced photosynthesis, and sometimes plant death.
- Nutrient Cycling: Herbivory contributes to nutrient cycling by accelerating the breakdown of plant material and returning nutrients to the soil through insect waste.
- Population Control: Predators and parasites help control leaf-eating insect populations, maintaining ecological balance.
c. Competition
- Mutualism: Interactions where both species benefit, such as mycorrhizal fungi with tree roots.
- Parasitism: One species benefits at the expense of another, such as parasitic insects laying eggs in or on host plants.
- Commensalism: One species benefits while the other is neither helped nor harmed, like birds nesting in trees.
- Intraspecific: Competition within a species for resources like light, water, and nutrients.
- Interspecific: Competition between different species for the same resources.
- Human Impact
- Deforestation: Logging and land conversion reduce forest area and biodiversity.
- Pollution: Air and water pollution can damage forest health.
- Climate Change: Alters temperature and precipitation patterns, affecting forest dynamics.
Summary
A forest ecosystem is an intricate web of interactions among various biotic and abiotic components. Herbivory by leaf-eating insects is a key process that influences plant health, nutrient cycling, and energy flow, playing a crucial role in maintaining the balance and functioning of the ecosystem.
New research sheds light on relationships between plants and insects in forest ecosystems
U.S. Forest Service researchers and partners published new findings on how leaf-eating insects affect forest ecosystems worldwide.
The findings of this study provide an improved understanding of the intricate relationship between herbivorous insects and forest ecosystems.
Bernice C. Hwang, lead author
Department of Physical Geography and Ecosystem Science
Lund University, Lund, Sweden.
Hwang and fellow researchers are aware of how large herbivores cycle nutrients in forests.
They know much less, however, about how leaf-eating insects impact forest carbon and nutrient cycling.
And they were determined to find out.
Their first step was establishing a global network of 74 plots within 40 mature, undisturbed broadleaved forests.
They analyzed leaf litter and green leaves for carbon, nitrogen, phosphorus and silica concentrations.
Leaf production, how much insects feed on leaves, and fluxes in nutrient levels from other sources also informed their research.
Chistian Giardina and Nels Johnson of the Pacific Southwest Research Station and other researchers found that insects play a significant role in releasing and cycling vital nutrients in forest ecosystems.
This was particularly true for warmer climates like those in tropical forests.
Their findings suggest that a warming climate can affect how plants and herbivores interact.
Changes in those relationships have important consequences for carbon and nutrient cycling in broadleaved forests on a global scale.
The researchers hope this new knowledge can contribute to a better understanding of forest ecosystems and inform efforts to conserve them.
I believe this analysis will be a benchmark to compare against for its field.
.Nels G Johnson, co-author
Pacific Southwest Research Station
USDA Forest Service
Hilo, Hawai’i, USA.
Abstract
Herbivorous insects alter biogeochemical cycling within forests, but the magnitude of these impacts, their global variation, and drivers of this variation remain poorly understood. To address this knowledge gap and help improve biogeochemical models, we established a global network of 74 plots within 40 mature, undisturbed broadleaved forests. We analyzed freshly senesced and green leaves for carbon, nitrogen, phosphorus and silica concentrations, foliar production and herbivory, and stand-level nutrient fluxes. We show more nutrient release by insect herbivores at non-outbreak levels in tropical forests than temperate and boreal forests, that these fluxes increase strongly with mean annual temperature, and that they exceed atmospheric deposition inputs in some localities. Thus, background levels of insect herbivory are sufficiently large to both alter ecosystem element cycling and influence terrestrial carbon cycling. Further, climate can affect interactions between natural populations of plants and herbivores with important consequences for global biogeochemical cycles across broadleaved forests.
Introduction
Herbivory is an important mediator of ecosystem nutrient cycling and primary production across biome types1,2. A wide diversity of herbivores shape the form, function, and biochemistry of plants, exhibiting deep and taxonomically diverse co-evolutionary linkages to plants3. The impacts of mammalian herbivores and those of a small group of insects that cause extensive but rare mass defoliation events have received significant attention4,5. However, a more cryptic, diverse, and extensive community of insects is responsible for near-continuous and ubiquitous background levels of herbivory. The seemingly minor contributions of background insect herbivory to ecosystem processes under non-outbreak conditions may be substantial over the long term and over large spatial scales, with ecosystem consequences that likely differ from the more charismatic yet sporadic outbreak events6. The magnitude of these impacts, the variation of these impacts across the world’s forests, and importantly for terrestrial ecosystem modeling, the drivers of this variation all remain poorly quantified6,7.
An expanded focus on insect herbivores is also warranted because they create important feedbacks between plants and soils mediated by a wide variety of mechanisms8. One key direct, immediate feedback occurs via transfer of labile nutrients from green leaves to the soil in the form of excreta, cadavers, leachate, unconsumed leaf fragments, and prematurely abscised leaves (Fig. 1)8,9. Relative to leaf litter, herbivory-related insect deposits are typically enriched with labile forms of nutrients, and in many forests, insect-mediated nutrient fluxes are comparable to or even exceed fluxes from other inputs of relatively labile, mineral forms of nutrients9,10. In contrast, most of the nutrients in leaf litter and are resorbed and retained within plant biomass, or they are released in relatively recalcitrant forms11. When folivores alter the fluxes of limiting nutrients such as nitrogen (N) and phosphorus (P), they also have the potential to influence plant growth and ecosystem carbon (C) cycling11,12. Silicon (Si) is increasingly investigated in plant science research because silica enhances plant structural integrity, reduces the impact of stressors such as herbivory and drought, and correlates with C sequestration13,14. However, the biogeochemical dimensions of herbivory impacts on Si remain understudied15. Though global analyses of herbivory exist e.g.16, the flux of nutrients associated with insect herbivory and insect deposits remains poorly understood8, as is the potential impact of climate on these fluxes.
To test fundamental hypotheses about the magnitude of nutrient fluxes mediated by background levels of insect herbivory, the variation of these fluxes across the Earth’s angiosperm forests, and the global-scale drivers of this variation, we established 74 plots within 40 mature, undisturbed broadleaved forests representing nearly the full range of broadleaved forests on Earth17. We used standardized methods to regularly collect and analyze green and freshly senesced leaves throughout the growing season for one or two years at each plot for: (i) foliar biomass production, (ii) foliar herbivory, and (iii) foliar C, N, P and Si concentrations. We then used these measures to calculate annual stand-level element fluxes generated during leaf consumption by entire, natural communities of insect herbivores under non-outbreak conditions10. We later compared these fluxes to other sources of labile nutrients (atmospheric N, atmospheric P, and bedrock weathered P). Furthermore, we investigated potential abiotic and biotic drivers of these insect-mediated C and nutrient fluxes. High nutrient availability, warm temperatures, and low water stress can positively influence foliar nutrient concentrations, foliar biomass production, and insect abundance with positive synergistic overall effects on insect-mediated element fluxes18,19,20. Consequently, warm mean annual temperatures (MAT) and high soil nutrient concentrations could increase insect herbivory and thus insect-mediated element fluxes in nutrient-limited but not water-limited (i.e., low dryness) systems. Therefore, we hypothesized that:Fig. 1: Hypothesized effects of insect herbivory on ecosystem element cycling in a broadleaved forest.
Although herbivores exert a wide variety of other direct and indirect effects, our study focused on one major direct effect of herbivores—removal of foliar matter. Briefly, insect foliar herbivory (H) constitutes an important pathway for labile carbon and nutrients to move from green foliage to the soil—via excreta, cadavers, unconsumed leaf fragments, early abscised leaves, and leachate. Once these products of H enter soil, contained nutrients alter a range of processes that support soil microbial communities and plant growth. Foliar production (FP), while not tested here, would be negatively affected by H at the individual target plant level, but neutrally or positively affected by H at the stand level as adjacent non-target trees, composed of herbivore resistant genotypes or species, benefit from access to additional fluxes of growth limiting nutrients. Similarly, greenleaf nutrients (FE) would decline at the target plant level as a result of H, but for similar reasons as FP, would remain unchanged or even increase at the stand level. REE represents the difference in element content between green and freshly senescent leaves, the quantity of which would be absorbed by the tree prior to senescence. Hc (FP x FE x H) represents the gross amounts of elements consumed by insect folivores, and Hi (LEH + Hc – LE) refers to the additional (net) element inputs from insect folivores due to release of nutrient rich green leaf material prior to resorption10. In all cases, the subscript E refers to elements. Arrow sizes denote the relative size of the flux. Herbivory-related calculations are fully described in Supplementary Table 1. Tree silhouette adapted from NikhomTreeVector/Shutterstock.com.
H1—The flux of N and P from plant to soil mediated by insect herbivores would meet or exceed other major labile fluxes of these elements, such as atmospheric deposition (for N and P) and bedrock weathering (for P).
H2—Insect-mediated element fluxes would increase with increasing mean annual temperature, decrease with increasing dryness (potential evapotranspiration/mean annual precipitation), and increase with increasing soil nutrient concentration.
H3—Observed responses of insect mediated element fluxes to MAT, dryness and soil nutrient concentrations would be driven in equal measure by similar responses from foliar herbivory, biomass production and foliar element concentrations.
Here, we show that background levels of insect herbivory can have profound impacts on biogeochemical cycling in broadleaved forests. For some localities, these fluxes exceed atmospheric deposition inputs, showing that background levels of insect herbivory are large enough to both alter ecosystem element cycling and, because primary productivity in most forests is nutrient-limited, influence terrestrial C cycling. Insect-mediated fluxes of N and P are especially high in tropical forests compared to temperate and boreal forests. Further, we show that all insect-mediated element fluxes increase strongly with MAT. These results reveal how climate can affect interactions between natural populations of plants and herbivores with important consequences for C and nutrient cycling across global broadleaved forest biomes.
Hwang, B.C., Giardina, C.P., Adu-Bredu, S. et al. The impact of insect herbivory on biogeochemical cycling in broadleaved forests varies with temperature. Nat Commun 15, 6011 (2024). https://doi.org/10.1038/s41467-024-50245-9
Copyright: © 2024 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Because they have a deep-seated need to credit their putative designer with everything, they will claim, with no evidence at all, that it must have arranged ecosystems to suit the species that live in it, and because this alleged designer is perfect, the ecosystems it designed must be perfect.
But this is childish nonsense, of course, because an ecosystem is the species that live and interact with one another and their environment so the system is dynamic and changing - something that would not be possible for a perfect system. As this piece of research shows, an ecosystem like a forest is also a delicate balance, like any dynamic system, and a small change in one component can have a large effect elsewhere in the system. For example, changes in the population of herbivorous insects, which is itself in a dynamic relationship with the population of predators, can impact soil fertility and the carbon cycle in the system. Similarly, small changes in ambient temperature will affect the growth of herbivorous insects and the rate of soil reactions, as will fluctuations in rainfall.
There is nothing perfect in a system which can change in response to unpredictable external influences and to which the species living within it respond to those changes to become better at surviving within it by a process that selects against those less able to adapt. The whole basis of dynamic ecosystems is not the perfection of the environment for the species within it, but its tendency to hostility.
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