https://www.economist.com/technology-quarterly/2021/06/15/compared-with-climate-modelling-of-ecosystems-is-at-an-early-stage

Simulating everything
模拟一切

Compared with climate, modelling of ecosystems is at an early stage

生态系统建模相比气候建模尚处于早期

EVERY FEW weeks from June 1963 until July 1968, Robert Paine, a zoologist, made the journey from Seattle, where he taught at the University of Washington, across Puget Sound to the rocky shores of Mukkaw bay. There, he had found virtually pristine tide pools that teemed with life—limpets, anemones, mussels, seaweeds and purple-and-orange seastars known as Pisaster ochraceus. The unspoiled landscape offered the perfect setting for what was to become a seminal experiment in ecology. On each visit, Dr Paine systematically removed all the seastars he could find from one patch of rock, lobbing them as far as he could into the waves.

从 1963 年 6 月到 1968 年 7 月,动物学家罗伯特・潘恩(Robert Paine)每隔几周就会从他任教的华盛顿大学所在的西雅图出发,穿过普吉特海湾到达穆卡湾的岩石海岸。在那里,他发现了几乎完全处于原始状态的潮汐池,充满了各种各样的生物——帽贝、海葵、贻贝、海藻,还有被称作「赭色海星」(Pisaster ochraceus)的紫色和橙色的海星。未受破坏的景观提供了一个完美的实验环境,而他的实验后来也成为生态学上开创性的实验。每次造访时,潘恩都会有计划地从一片岩石上取下他能找到的所有海星,并全力将它们远远抛入海中。

He did this for five years, all the while carefully documenting how the shoreline communities evolved. Very little changed in the untouched areas. But in his seastar-free zone, everything was altered. Pisaster is a greedy carnivore that feasts on mussels, barnacles, limpets and snails. Released from their predator, these species began to spread out. The acorn barnacles took over first. Later, they were displaced by goose barnacles and mussels. By removing just one species, Dr Paine had triggered a domino effect. Soon, the number of species in the community had dropped from 15 to eight. By 1968, the mussels had taken over completely.

他这样做了五年,与此同时仔细地记录下这些海岸线社区的演变过程。在他没有干预的区域几乎没有变化。但在他的无海星区,一切都变了。赭色海星是一种贪婪的食肉动物,以贻贝、藤壶、帽贝和蜗牛为食。没有了捕食者,这些物种开始扩散。首先接管的是橡子藤壶。后来,它们被鹅藤壶和贻贝取代。通过移除一个物种,潘恩博士引发了多米诺骨牌效应。很快,这个社区中的物种数量从 15 种减少到 8 种。到 1968 年就完全被贻贝占据了。

Dr Paine dubbed Pisaster a“keystone species”;remove it and the ecosystem is transformed. Large herbivores like rhinos are keystone species, spreading seeds of the plants they consume across vast areas, thus maintaining or altering vegetation. In the kelp forests of the Pacific Northwest, sea otters play a keystone role by munching on sea urchins. The urchins graze on kelp and, left unchecked, are capable of wiping out entire kelp forests on which fish and seals depend.

潘恩将赭色海星称为「关键种」——去除它后生态系统就会发生变化。像犀牛这样的大型食草动物是关键种,它们将食用的植物种子散播到广阔的区域,从而维持或改变植被。在太平洋西北部的海藻林中,海獭通过咀嚼海胆而发挥着关键的作用。海胆以海藻为食,如果没有制衡,它们能够消灭鱼类和海豹赖以生存的整个海藻林。

Keystone species illustrate the complex webs of interactions that underpin biodiversity. Understanding, let alone predicting, the impact that removing one species can have on the REST of a non-linear system is devilishly complicated. Even if sensors and ecologists could log the identity and location of every living creature on the planet, such data would be worth little without an understanding of how everything relates to everything else.

关键种反映了生物多样性背后复杂的互动网络。要了解去除一个物种会对非线性系统的其余部分产生什么样的影响是极端复杂的,更不用说预测了。哪怕传感器和生态学家可以记录地球上每个生物的身份和位置,但如果不了解每个事物与其他事物之间的关系,这些数据将毫无价值。

Computer models are ideally suited to providing just that. General circulation models, for example, simulate the planetary climate, linking the physics that govern the formation and disintegration of ice sheets to the huge currents that push water through the ocean, and oceanic temperature gradients to the formation of storm systems over the continents. These models are so complex that they take months to run, even on the world’s most powerful supercomputers. Climate science and policy would be nothing without them.

计算机模型非常适合提供这种关系。例如,大气环流模型用来模拟全球气候,将控制冰盖形成和解体的物理学与海洋中推动水流的巨大洋流联系了起来,也将海洋温度梯度与陆地上风暴系统的形成联系起来。这些模型非常复杂,即使在世界上最强大的超级计算机上也需要几个月才能跑完。没有它们,气候科学和政策将一文不值。

Ecology has few equivalents. One reason is that ecosystems are much harder to simulate.“In a physical system, you have a set of atoms or molecules that behave in a predictable way, even if it is complex,”says Derek Tittensor, a marine-ecosystem modeller at Dalhousie University in Canada. Ecology, by contrast, deals in living things, whose interactions are determined by the unpredictable behaviour of individuals.

几乎没有什么能与生态学相提并论。原因之一是对生态系统的模拟要困难得多。「在物理系统中,你有一组原子或分子以可预测的方式运行,哪怕它很复杂。」加拿大达尔豪斯大学海洋生态系统建模师德里克・蒂滕索(Derek Tittensor)说。相比之下,生态学涉及生命体,决定其相互作用的是个体不可预测的行为。

Added to this is the complexity of the pressures and stresses that modify ecosystems. Carbon dioxide and methane are produced by different processes and behave differently in the atmosphere, but fundamentally they both warm the atmosphere. Burning fossil fuels also produces a mix of particles which cool the climate. These emissions are all very different, but their effects can, to some approximation, be reduced to a single variable known as their“global-warming potential”。Ecosystems, by contrast, are affected by warming temperatures and changing water cycles, but also by chemical pollution, urban encroachment, hunting and overfishing. None of this can be reduced to just one or even a handful of quantitative variables.

此外还有改变生态系统的压力的复杂性。二氧化碳和甲烷由不同的过程产生,在大气中的表现也不同,但总体来说它们都使大气变暖。燃烧化石燃料还会产生使气候降温的颗粒混合物。这些排放都非常不同,但它们的影响在某种程度上可以简化为一个单一的变量,称为「全球变暖潜力」。相比之下,生态系统受到气温升高和水循环变化的影响,但也受到化学污染、城市侵占、狩猎和过度捕捞的影响。所有这些都无法简化为一个甚至少数几个定量的变量。

And so ecosystem modelling remains in its infancy. Statistical models, built on relationships between historical data sets—for example, how the amount of vegetation in a tropical forest tends to grow or shrink as temperatures and rainfall vary—are easier to build, and have progressed furthest. But they cannot capture or predict the dynamic, non-linear ways ecosystems respond to change, including the tipping points at which cumulative damage to an ecosystem suddenly shifts it into a new regime, for example when deforestation tips a region from forest to savannah.

正由于此,生态系统建模仍处于起步阶段。根据历史数据集之间的关系建立的统计模型比较容易建立,进展也最大,一个例子是热带森林中的植被数量如何随着温度和降雨量的变化而趋于增长或收缩。但它们无法捕捉或预测生态系统对变化做出的动态非线性反应,包括累积的损害在达到「临界点」时突然将生态系统转变为新类型,例如森林砍伐使一个地区从森林转变成了稀树草原。

Doing that requires“process-based”or“mechanistic”models, which are harder to build, but can produce non-linearity and emergent behaviour. They are the ecological equivalent of general circulation models, and operate as fully functioning simulations of Earth’s biosphere. They are particularly useful for unpicking what is driving change in an ecosystem. If a fish population is growing, is it because rising temperatures have driven predators away, or because deforestation on land nearby is releasing iron-rich dust which is fertilising the local plankton population?

要捕捉这类动态需要「基于过程的」或「机械论」模型。这些模型更难构建,但能够产生非线性和新兴行为。它们是大气环流模型的生态等价物,运行时模拟全功能的地球生物圈。它们对于找出推动某个生态系统变化的因素特别有用。如果一个鱼类种群在增长,是因为气温升高驱离了捕食者,还是因为附近陆地上的森林砍伐正在释放富含铁的灰尘,为当地的浮游生物种群提供了更多养料?

Marine science has produced a number of process-based models, though they are Less uniform in their design than climate models. Some are built around food chains and the way they move biomass and energy around ecosystems; others focus on how well-suited different species are to particular ecological niches, or group species and their interactions based on body size, which is a reasonable predictor of an organism’s place in the food chain.

海洋科学已经产生了许多基于过程的模型,尽管它们的设计不像气候模型那样统一。有些是围绕食物链及其在生态系统中移动生物质和能源的方式而建立的;还有些则关注不同物种对特定生态位或群体物种的适合程度,或是基于体型大小对物种及其相互作用进行分组(体型是生物体在食物链中所处位置的合理预测指标)。

Over the past decade marine-ecosystem modellers have formed the Fisheries and Marine Ecosystem Model Intercomparison Project. Its goal is to determine how fishing and climate change are likely to alter marine fisheries around the world, which provide 11% of the animal protein humans consume.“Fish-MIP”develops standardised scenarios that can be run across global and regional marine-ecosystem models. As with climate modelling, the idea is to run the same simulations on different models and combine the results into robust projections that can inform policy decisions. Fish-MIP studies suggest that larger fish species, which make up most of what humans consume, are affected most by climate change, as are the tropics, where people tend to be more dependent on catches and more vulnerable to economic instability and poor nutrition.

在过去的十年中,海洋生态系统的建模人员建立了「渔业和海洋生态系统模型比对项目」(Fish-MIP)。其目标是确定捕鱼和气候变化可能会如何改变世界各地的海洋渔业,这些渔业为人类消费提供了 11% 的动物蛋白。Fish-MIP 开发了可以在各种全球和区域海洋生态系统模型上运行的标准化场景。与气候建模一样,其想法是在不同的模型上运行相同的模拟,并将结果组合成可以支撑政策决策的可靠预测。Fish-MIP 的研究表明,较大的鱼类物种(构成人类消费的大部分)受气候变化的影响最大,还有热带地区,那里的人们往往更依赖捕捞,更容易受到经济不稳定和营养不良的影响。

But simulating the effects of fishing operations is more complicated than studying the impact of rising temperatures, as assumptions have to be made about a range of variables, from how the industry will redistribute fishing fleets as fish migrate towards the poles, to how fishing technology will change, and whether changing attitudes towards sustainability will mean more marine protected areas. The climate-modelling community handles such uncertainty by drawing up standardised hypothetical scenarios and producing climate projections for each one. But the scenarios do not yet take into account the ways in which humans effect biodiversity, such as by overfishing.

但是模拟捕捞作业的影响比研究气温上升的影响更为复杂,因为必须对一系列变量进行假设,从随着鱼类向极地迁移,渔业将如何重新分配捕捞船队,到捕捞技术将如何演进,还有改变对可持续性的态度是否意味着出现更多海洋保护区等。气候建模社区处理这种不确定性的方法是制定标准化的假设情景,并为每个情景做出气候预测。但这些情景尚未考虑人类影响生物多样性的方式,例如过度捕捞。

Modelling is far Less advanced for land ecosystems.“Dynamic global vegetation models”can simulate human impacts on plants but do not represent non-human animals. And though there are at least eight global marine-ecosystem models that simulate life in the ocean, there is just one process-based model that includes life on land: the Madingley model, first published in 2014, which represents life both on land and in the ocean.

陆地生态系统的建模远没有那么先进。「动态全球植被模型」可以模拟人类对植物的影响,但里面没有涉及非人类的动物。而虽然至少有八个全球海洋生态系统模型模拟海洋中的生命,包含了陆地生命的基于过程的模型却只有一个——「马丁利模型」(Madingley Model)。它于 2014 年首次发布,同时考虑了陆地和海洋的生命。

Named after the village in Britain where it was devised, it breaks down land and ocean into grid cells that are up to 200 square km (77 square miles). Climatic conditions are set for each cell, which are also populated with organisms, so long as they weigh more than ten micrograms. To simplify the equations involved, the model groups organisms by size, habitat and function. It therefore cannot distinguish between two species of small songbird that live in the same region, but it does simulate interactions between, say, megafauna and their prey.

这个模型以设计出它的英国村庄命名,将陆地和海洋分解成最大 200 平方公里的网格单元。它为每个单元设定气候条件并添上生物,只要这种生物的重量超过 10 微克。为了简化所涉及的方程,该模型按大小、栖息地和功能给生物分组。因此,它无法区分生活在同一地区的两种小型鸣禽,但它确实模拟了比如巨兽与其猎物之间的相互作用。

All this allows for in silico experiments in which all the world’s top predators are wiped out entirely, an extension in space of Dr Paine’s famous seastar experiment but also an extrapolation of current global trends. An assessment in 2014 of 31 of the world’s largest mammalian carnivores found that three-quarters of them were in decline, and 17 occupied Less than half of their historical territory. Using the Madingley model, Selwyn Hoeks at Radboud University in the Netherlands, and his colleagues found that removing all carnivores weighing more than 21 kg triggered a domino effect in food chains with the net result that the total amount of vegetation on Earth decreased. Their results were published in 2020 in the journal Ecography.

所有这一切都让计算机可以推演假如世界上所有顶级捕食者被完全消灭会怎样。这是对潘恩著名的海星实验的扩展,也是对当前全球趋势的推断。2014 年对 31 种世界上最大的哺乳食肉动物的评估发现,其中有四分之三数量正在减少,17 种动物的领地还不到其历史领地的一半。利用马丁利模型,荷兰拉德堡德大学的塞尔温・霍克斯(Selwyn Hoeks)和他的同事们发现,去除所有体重超过 21 公斤的食肉动物会引发食物链中的多米诺骨牌效应,最终结果是地球上的植被总量减少。他们的研究结果发表在 2020 年的《Ecography》期刊上。

Ecologists have long argued that conserving large carnivores has tangible benefits beyond the cuddly feeling of saving tigers. According to the“green Earth hypothesis”,no carnivores means more herbivores and thus fewer plants. Vegetation soaks up carbon dioxide, so Less plant life would amplify global warming. What of the reverse, where all plant life is gradually removed? Changing landscapes, particularly through agriculture, is humanity’s greatest impact on biodiversity, and one that is likely to increase. Expanding agriculture reduces the amount of plant life at the base of food webs. Tim Newbold, of University College London, and colleagues simulated the removal of increasing amounts of vegetation from China, France, Libya and Uganda. They found that once 80% of plant life was gone, entire food chains began to collapse and could not be rebuilt by simply restoring the plants.

生态学家长期以来一直认为,除了拯救老虎让人心头柔软之外,保护大型食肉动物还有切实的好处。根据「绿色地球假说」,没有食肉动物意味着更多的食草动物,从而让植物减少。植被吸收二氧化碳,因此植物的减少会加剧全球变暖。如果反过来,让所有植物都逐渐消失会怎么样呢?地形地貌变化,尤其是因农业造成的变化,是人类对生物多样性最大的影响,而且这种影响很可能还会增加。农业扩张减少了食物网底层的植物量。伦敦大学学院的蒂姆・纽博德(Tim Newbold)和同事模拟了从中国、法国、利比亚和乌干达去除越来越多的植被。他们发现,一旦 80% 的植物消失,整个食物链就会开始崩溃,并且无法通过简单地恢复植物来重建。

As well as predicting outcomes, global ecosystem models make it possible to test policies. What would be the consequence of reintroducing a species from a population bred in captivity? Would the decline of a species be halted or reversed if a percentage of its territorial range were protected, or would it be more efficient to create a corridor between two existing protected areas?

除了预测结果外,全球生态系统模型还可以测试政策。重新引入一个来自人工饲养种群的物种会产生什么后果?如果其领地范围有一部分受到保护,一个物种的衰退能否被阻止或逆转?还是在两个现有保护区之间建立一条走廊会更有效?

Carbon storage, clean water, clean air, abundant crops and fish are all examples of“ecosystem services”that benefit humanity. The principle is undeniable on a grand scale, but the details are harder to map.“We don’t have any frameworks which link biodiversity changes to changes in ecosystem functioning, and on to the services that humans derive from those ecosystems,”says Michael Harfoot of the UN World Conservation Monitoring Centre and co-author of the Ecography paper.

碳储存、清洁水源、清洁空气、丰富的农作物和鱼类都是造福人类的「生态系统服务」的例子。这个原则在宏观上是不可否认的,但细节却更难描绘。「我们没有任何框架能将生物多样性的变化与生态系统功能的变化联系起来,继而再与人类从这些生态系统中获得的服务联系起来。」《Ecography》论文的合著者、联合国世界保护监测中心(UN World Conservation Monitoring Centre)的迈克尔・哈福特(Michael Harfoot)说。

Statistical models try to infer changes in ecosystem services from, for instance, trends in forest cover. But process-based models need further refinement so that changes in temperatures or land use can be linked to changes in biodiversity—and then, in turn, to the functioning of ecosystems and the services they provide.“That is probably the next big frontier for ecosystem modelling,”says Dr Harfoot,“and essentially, also, for conservation.”

统计模型试图从诸如森林覆盖的趋势推断生态系统服务的变化。但基于过程的模型需要进一步完善,以便关联温度或土地使用的变化与生物多样性的变化,进而与生态系统的功能及其提供的服务联系起来。「这可能是生态系统建模的下一个前沿领域,」哈福特说,「也可以说是生态保护的前沿。」

For now, this remains some way off. Today’s ecosystem models are widely compared to where climate models were in their earliest days of development, about 50 years ago.“Given the urgency of the situation, we need ecosystem models to be where climate models will be in ten years’time,”says Dr Newbold.

目前,这还有一段路要走。今天的生态系统模型被普遍拿来与气候模型发展的初期相提并论——那大约是 50 年前了。「鉴于形势的紧迫性,我们需要生态系统模型在十年后达到气候模型届时的水平。」纽博德说

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我们应该学会去理解别人的观点,不仅仅是服从和被告知。

我们应该学会去理解别人的观点,不仅仅是服从和被告知。
我们应该学会去理解别人的观点,不仅仅是服从和被告知。
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