Title: Land and deep-sea mining: the challenges of comparing biodiversity impacts

Authors: Katona et al.

Journal & Year: Biodiversity and Conservation, 2023

BLUF: Comparing biodiversity impacts between terrestrial and deep-sea mining is fraught with scientific, ecological, and philosophical challenges. This 2023 review highlights why direct, quantitative comparisons between these domains remain unreliable — and argues for a more nuanced, multidimensional approach to assessing environmental tradeoffs. As societies confront increasing demand for energy transition metals, the insights presented here are essential for informed decision-making. For the deep-sea mining sector, the paper supports the need for tailored biodiversity frameworks and underscores the value of environmental rigor in securing long-term social and regulatory legitimacy.

In the global pursuit of a low-carbon future, pressure is mounting to secure vast quantities of critical minerals. This has revived interest in polymetallic nodules found on the abyssal plains of the Clarion-Clipperton Zone (CCZ), a resource with the potential to offset supply constraints currently borne by terrestrial mining.

However, concerns over biodiversity loss — both on land and in the deep sea — are increasingly shaping public and policy discourse. This study investigates why biodiversity tradeoffs between these two realms are so difficult to assess with scientific consistency.

While ‘biodiversity’ is central to environmental impact assessments, it lacks a standardized, universally applicable definition. Depending on context, it may refer to genetic variability, species richness, functional diversity, or ecosystem integrity. These dimensions do not transfer neatly across ecosystems, especially between terrestrial habitats and the deep-sea floor.

Differences in sampling techniques, data availability, and biological baselines all contribute to a lack of comparability. The paper identifies three major phases in biodiversity assessment — measurement, computation, and interpretation — and shows how each is subject to limitations.

In the measurement phase, inconsistency in collection tools (e.g., trawls, remotely operated vehicles, eDNA) and taxonomic focus introduces sampling bias. Many deep-sea organisms are microscopic, rare, or undescribed, and often evade detection using standard morphological surveys. Even molecular approaches like eDNA struggle to resolve functional or ecological roles without extensive reference libraries.

Computation presents its own set of challenges. Widely used indices such as Simpson’s or Shannon’s require robust and harmonized input data, which is rarely achievable in the deep sea. Combining metrics across taxa or size classes often leads to skewed results, while the majority of deep-sea biodiversity remains undocumented.

Interpretation is equally complex. Quantitative scores do not account for the ecological or cultural value of species — a problem made more acute by the absence of iconic flagship species in the abyss. By contrast, terrestrial species such as elephants, tigers, or gorillas benefit from strong societal recognition, complicating efforts to apply equal weighting to deep-sea biodiversity.

The study helpfully categorizes seven key barriers to meaningful biodiversity comparison:

  1. Ambiguous Definitions: Biodiversity lacks a single, agreed-upon meaning, complicating cross-system analysis.
  2. Measurement Inconsistencies: Sampling methods and survey designs differ widely between land and sea, distorting data.
  3. Ecological Incommensurability: The structural and functional contrasts between ecosystems make direct comparison unreliable.
  4. Knowledge Gaps: Deep-sea biodiversity is vastly under-described; most eukaryotic marine species are still unknown.
  5. Value Attribution: Biodiversity scores can conceal differences in ecological function or public salience.
  6. Functional Uncertainty: High species richness does not always equate to greater ecosystem resilience or service delivery.
  7. Index Limitations: Composite biodiversity metrics may ignore species roles, interdependencies, or recovery timelines.

A particularly important consideration is ecological recovery. While some terrestrial environments may regain partial biodiversity within decades of disturbance, deep-sea ecosystems operate on geological timescales.

Processes such as polymetallic nodule formation occur over millions of years, and recovery of ecological structure following disturbance is likely to be extremely slow or incomplete. These realities raise difficult questions about the long-term ecological cost of deep-sea mining, but also compel a balanced comparison with terrestrial alternatives, which often impact forests, watersheds, and indigenous communities in ways that are socially and environmentally profound.

The review calls for the development of integrated, multidimensional assessment frameworks that move beyond simplistic metrics. Life cycle analysis (LCA) is proposed as one such tool for evaluating broader environmental tradeoffs, including greenhouse gas emissions, habitat alteration, and waste generation.

However, LCA alone is insufficient to capture the full spectrum of biodiversity impacts. The authors argue that ecological, cultural, and functional values must be layered into any comparative framework — particularly when operating at the frontier of scientific knowledge, as deep-sea mining does.

Though framed as a critique of current biodiversity science, the paper ultimately offers a constructive path forward. It encourages investment in improved taxonomic resolution, standardized sampling protocols, and public data sharing.

Importantly, it highlights the value of humility in ecological comparisons.