It’s common that I’m asked, ‘Are ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More regulated top down or bottom up?’ But it’s not that simple. We have a terrible habit of looking for single causal mechanisms for complex systems. This leads to disagreement and makes the science seem confused when it’s simpler. A cursory online search will reveal many papers that argue for one or the other, but this is polemic. In reality, systems are structural [8, 9] and have both components. It’s also true that we’ve mostly exterminated the upper levels though. In order to restore ecosystems we have to start thinking differently. Let’s take a quick look at why rewilding has to focus on higher-level animals, including predators.
Natural philosophy over natural science
Here are two ways we can consider ecology. We can look at what exists now, which is a wildlife-poor version of the past. Or we can imagine something in which humans can thrive. The latter is mostly theoretical because today, we’ve degraded most of the components needed to make it habitable. To construct a modelThe process, either mathematically or in the human brain, of creating an internal version of something that we can refer to, to better understand how it functions and our place within. Scientific modelling is where we take the best knowledge we have and build a version of what will happen, if we assume certain parameters. For example, we might model More of what things need to be in future, requires natural philosophyNatural philosophy concerns itself with the question of 'why' things work [1]. Natural science tries to work out 'how' [1]. Understanding the difference is fundamental to conservation but is largely missing . Natural philosophy has more in kin with ecological wisdom, such as the beliefs, cultures and traditions of first nations peoples all over the world. It embraces an understanding More, not natural science.
So, when we ask the question: what drives ecosystem restoration? The answer is diversity and dynamic stability.
As Professor Stanley Salthe of Columbia University [1] explains, the question is not ‘how?’ that occurs but ‘why?’
‘How?’ means observing intact systems, which is impossible.
John Harte [2] of the University of California says we don’t have enough knowledge to put all these into a model, so instead we tend to choose one or two and ignore others, or we study partial systems, basing models of complex systems on upfront and arbitrary choices about what drives explicit mechanisms
Simon Mustoe, Wildlife in the Balance (2022)
The lack of data means such models raise more questions than they provide answers.
The answer to ‘why?’ is easier. Answer: when balance is restored.
As usual, context is everything. Let’s take a quick look at this question and try to consider it in terms of what it means for humanity.
Missing pieces
I never say ecosystems are top-down controlled but it’s often assumed that’s what I mean. That’s my fault. It’s taken me a while to work out that this seems self-evident. Not because it’s the only mechanism though, but because higher animals are almost always the missing pieces.
The absence of animals from higher trophic levels is leading to collapse in ecosystem stability worldwide.
When a scientist then asks for proof that predators, for example, drive change … that is not the right question. It’s academically interesting but that’s a ‘how’ question, not a ‘why’ question. It’s a distraction from the real issue, which is the lack of predators in the first place.
Bottom-up doesn’t really mean that
Meanwhile, it’s also self-evident that if you remove the bottom of an ecosystem e.g. deforestation, that has a catastrophic impact. But when we’re trying to rewild areas that have existing vegetation and remnant wildlife, we aren’t building from the bottom-up. Those systems might have been in place for 200 years or more. It only takes about 100 years to rebuild functioning wildlife populations.
Removing or tampering with these remaining lower-levels e.g. culling native sea urchins, isn’t really ‘bottom-up’ management at all. Sure, it’s the bottom of what we can see but it’s the top of what remains. Humans are inherently biased only to what our senses commonly perceive. But when we rewild, we start by introducing the herbivores and lower-level predators, so we’re really doing bottom-up management. As you can see, any argument that one or the other applies, is rather academic.
As Hammerschlag et al (2019) say:
Most reviews describing the ecological roles of Aquatic Predators (APs) have focused on food web dynamics involving marine megafaunaThe largest animals that represent the top of the trophic pyramid. These are the final building blocks in ecosystem structures for maximum entropy production. Megafauna can be measured at any spatial scale. The largest animal that ever lived on Earth is the Blue Whale. In a grassland, spiders could be considered megafauna The term is generally reserved for animals larger More. Yet even smaller APs, particularly in freshwater environments, can affect food webs. Growing evidence further suggests that APs are directly and indirectly connected to several other ecosystem functions, including nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More and carbon cycling, habitat modification, disease transmission, and invasion by exotic species. Moreover, APs are linked to socioecological systems (SESs) that encompass relationships between humans and the environment. Indeed, APs provide diverse ecosystem services, including tourism (e.g., whale watching, shark diving;) and food
security, as well as the jobs that depend on them. Less obvious is that APs can help mitigate climate change and lead to bioinspired materials and products that benefit human wellbeing [10].
In other words, it may be ‘top-down’ at any point in the food chain. But even if we are regrowing an entirely new system from nothing, the literature indicates this largely fails without animals integrated from the beginning [3]. To regrow primary producers (trees, seaweed etc) from scratch and rapidly restore function(Of an ecosystem). A subset of ecosystem processes and structures, where the ecosystem does something that provides an ecosystem service of value to people. More, means integrating the full range of wildlife to start with. You have to build the whole vehicle ready to drive.
Three important reasons top-level rewilding is a priority
#1 We’ve killed the top-level animals first
Top-level mechanisms are the first to be removed, as we kill off those parts of the wildlife-driven system faster than anything else. Sharks are removed before bony fish. Wolves are removed before deer. Fish are removed before urchins. Raptors are removed before mice. Urchins are removed before seaweed bacterial infections etc. We have gradually pushed ecosystems into lower levels. To a large degree, it’s always the bigger animals that are exterminated first.
#2 If you remove top predators, ecosystems collapse like a house of cards
We know that top-down effects in ecosystems are hugely important [4-7]. It’s not a question of whether they are more or less important than bottom-up effects. If top-level mechanisms are missing, ecosystems collapse. Top predators in particular, are like the glue that holds food chains together. If you remove them, there are many mechanisms that fail, and you end up with excesses that flow through the food chain.
#3 Humans are big and need other higher-level animals to survive
The ecosystem services humans depend on are nearer the top. Academically, a case can be made for ‘novel type’ structures that could last millions of years based on lower trophic levels e.g. oceans full of algae and jellyfish. But this would be uninhabitable – it’s how the world was before humans could exist.
If we ignore this factor, research to establish how trophic cascades work, become purely speculative. For all practical purposes we need a world humans can live in. This means surrounding ourselves with the right abundance of large animals, to transfer, amplify and concentrate nutrients, so we can sustain our population.
Otherwise we imagine a world that isn’t habitable.
What is missing from our ecosystems?
The key factor is simply to ask what is missing?
In our coastal environment, the larger fish are missing. In our faming environments, birds of prey or songbirds might be missing.
The lower trophic levels are often still present, especially in relatively natural areas.
We must remember that these are dynamic systems that operate as a whole. Again, we can do research on one aspect and draw the wrong conclusions (as often happens in linear ecological studies). Instead, rebuilding lost biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More reforms lost structure and that is why we used to have functioning ecosystems.
References
- Salthe, S (2002) Becoming, Being and Passing: Our Myth from Science (the Second Law and Natural SelectionDarwin’s theory how species are formed, where those that stand in closest competition with those undergoing beneficial modification and improvement, will go extinct faster. Natural selection is by survival of the likeliest, not survival of the fittest. The fittest are only likely to survive because they happen to be most suited to the environment into which they are born. The More) https://www.nbi.dk/~natphil/salthe/
- Harte, J., ‘Maximum entropyThe degree of disorder or chaos in a system, most often used to describe thermodynamic energy but also used the behaviour of information. All else being equal, physics determines that all matter and energy moves towards chaos, therefore biological systems are in a continual state of battling against entropic forces in order to remain stable. The most stable ecosystem is More and ecology. A theory of abundance, distribution, and energetics’. 2011.
- Peter W. Guiden, Nicholas A. Barber, Ryan Blackburn, Anna Farrell, Jessica Fliginger, Sheryl C. Hosler, Richard B. King, Melissa Nelson, Erin G. Rowland, Kirstie Savage, John P. Vanek, Holly P. Jones. Effects of management outweigh effects of plant diversity on restored animal communities in tallgrass prairies. Proceedings of the National Academy of Sciences, 2021; 118 (5): e2015421118 DOI: 10.1073/pnas.2015421118
- Rewilding North America: A Vision for ConservationWhy is animal conservation important? Animal conservation is important, because animals are the only mechanism to create biodiversity, which is the mechanism that creates a habitable planet for humans. Without animals, the energy from today’s plants (algae, trees, flowers etc) will eventually reach the atmosphere and ocean, much of it as carbon. The quantity of this plant-based waste is so More in the 21st Century by Dave Foreman (Island Press 2004).
- Baum, J.K. and Worm, B. (2009), Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology, 78: 699-714. https://doi.org/10.1111/j.1365-2656.2009.01531.x
- Nowicki, RJ, Thomson, JA, Fourqurean, JW, Wirsing, AJ, Heithaus, MR. Loss of predation risk from apex predators can exacerbate marine tropicalization caused by extreme climatic events. J Anim Ecol. 2021; 90: 2041– 2052. https://doi.org/10.1111/1365-2656.13424
- Ashton et al., Predator control of marine communities increases with temperature across 115 degrees of latitude, Science, 376, 6598, (1215-1219), (2022).
- Andersen, K., et al., Assumptions behind size-based ecosystem models are realistic. ICES Journal of Marine Science: Journal du Conseil, 2016. 73: p. fsv211.
- Blanchard, J.L., et al., From Bacteria to Whales: Using Functional Size Spectra to Model Marine Ecosystems. Trends in Ecology & Evolution, 2017. 32(3): p. 174-186.
- Hammerschlag, Neil & Schmitz, Oswald & Flecker, Alexander & Lafferty, Kevin & Sih, Andrew & Atwood, Trisha & Gallagher, Austin & Irschick, Duncan & Skubel, Rachel & Cooke, Steven. (2019). Ecosystem Function and Services of Aquatic Predators in the Anthropocene. Trends in Ecology & Evolution. 34. 369-383. 10.1016/j.tree.2019.01.005.
