From rainforest to reef: how loss of apex predators is deconstructing the earth’s biomes

By Evangeline Wilby

‘Flagship’ species are species that are used to gain public attention for conservation efforts because they are likeable organisms that act as ambassadors for their ecosystem.1 These flagship species are often large, notable species that are apex predators and therefore losing them is a much greater issue than loosing ecosystem aesthetic. Losing apex predators has severe knock-on effects for the whole ecosystem and will affect ecosystem processes that numerous important species are involved in. The decline in apex predators is often due to human impact and anthropogenic pressure that can reduce functional diversity, leading to unstable environments and leaves species functionally extinct.2 This deconstructs ecosystems and is being seen in biomes across the world. From the rainforest to the reef, the tropics to the tundra, this is a widespread issue with too many examples that needs immediate management action.

A historic example of biome transformation caused by human impact is the conversion of Steppe to tundra in the artic (Siberia).3 Steppe is an area of flat, unforested grassland where megaherbivores such as reindeer, horses, Bisons and historically the Woolley mammoth inhabit. Modelling and simulations have suggested that human hunting is one of the  main variables that determines a conversion from steppe to tundra. Hunting targets large organisms, so megaherbivores have historically been at great risk.4 Lower rates of these megaherbivores cause less grazing,  less soil disturbance and nutrient cycling, leading to low quality, low nutrient soil and an unproductive ecosystem.3 Important ecosystem processes that these organisms played included the trampling of shrublands, and fertilising grasses with their faeces. In areas known to be excluded from human hunting, the woolly mammoth survived a lot later, suggesting that human hunting did play a large part in their extinction.3 The consequences of the loss of these organisms has caused the conversion of steppe to tundra and therefore the biome has lost a lot of ecosystem process, productivity, and biodiversity.

Another example of human hunting affecting a biome can be seen in tropical rainforests, where large birds/mammals are hunted for food and sport.5 The impact can be examined by comparing the community composition of tropical forest that has been exposed to hunting to an area that hasn’t. A study that did this found that forest exposed to hunting had few demographically stable species with 72% of them decreasing.5 Loss of large birds has knock on effects for seed dispersal and these birds cannot carry out their ecological dispersal function, fundamentally altering the forest composition. Instead, smaller birds/mammals are relied on to substitute the dispersal roles, likely to have negative feedback for most clades within the biome.

In the temperate subtidal, human impact is causing a trophic-level dysfunction in the Kelp forest, notably in the western north Atlantic.6 These kelp forests were once inhabited by top apex predators such as Atlantic cod, haddock and wolfish, but due to fisheries reducing numbers, the density of the top apex predators is not high enough to limit the prey species at the next trophic level. These phase changes have caused a takeover of sea urchins and now invertebrate predators such as crabs and these phases are happening more and more rapidly, with ecologically important species repeatedly being lost from the higher trophic levels. It is the disruption of these species interactions that is causing the deconstruction of the kelp forest ecosystem, which will only worsen with increased human fishing efforts.7 The kept forest is an important ecosystem because it provides organic carbon particulates which are relied upon for consumption by many other organisms.8

One study suggests that overfishing is the leading cause of ecological extinction in coastal ecosystems too.9 Overfishing drives certain apex species so close to extinction that they become functionally or ecologically extinct, meaning they can no longer significantly interact with other species and fulfil their niche. For example, the historic decrease of sea turtles as an apex predator in sea grass meadows has led to a reduction of sea grass nutrient cycling in the ecosystem. Without turtles to consume seagrass, it grows longer creating shading in the ecosystem preventing necessary access to sunlight and provides conditions for slime moulds that can cause disease.9 Seagrasses provide a sink for carbon and can absorb vast amounts to help combat global warming, so protecting them is critical, but without the presence of turtles to maintain the meadows, this ecosystem may be lost.10

Overfishing also causes removal of these predatory fish in coral reef ecosystems too.11 With removal of top predators, there is loss of interaction with the sea urchin prey group species, which then thrives, dominates, and reduces the crustose coralline algae that normally help to recruit coral species to the reef. This can be seen in studies that compare the crustose coralline algae species interactions between areas with fishing polices to areas without, where it was seen that areas without fishing polices had half of these algal species compared to those with fishing policies.12 Maintaining the coral reef is of high importance because it supports such an array of biodiversity, and the fundamental dynamics of the habitat can therefore not be lost.13  

Hunting and overfishing together in the open ocean has had catastrophic effects on a particular food web. Whaling after world war II caused a huge decline of great whales and therefore their main predator, killer whales, had a dietary shift to smaller marine organisms such as pinnipeds (sea otters/lions).14 Overfishing does have some explanatory power to initial declines of species such as pinnipeds, but more recent declines seem to fall in conjunction with removal of the great whale from further up the food chain. The energy requirements for killer whales would then mean they had to consume vast numbers of pinnipeds in the absence of great whales.14 In turn, the loss of whales and pinnipeds has caused an increase in krill and some species such as the Adeline penguin, have shifted their diet to take advantage of this. As overfishing increases, it is likely that krill itself will be depleted, which will leave the Adeline penguins without a viable food source, showing again the complex array of species interactions that are affected when an apex predator is removed by humans.15

Since many of these ecosystem deconstructions are caused by humans hunting and overfishing, conservation strategy and management action should focus on this. Further research into areas where marine protected areas with fishing quotas or gear restrictions will be key as well as hunting policies that limit the number of apex predators that can be removed. Not only are these ‘top’ species some of the most appreciated in the public eye, but they are also most necessary for the functioning of healthy stable biomes, and  by extension, a healthy, stable planet.

Word count: 1115

References:

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2Estes JA, Terborgh J, Brashares JS, Power ME, Scheffer M. Trophic Downgrading of Planet Earth. Science (American Association for the Advancement of Science); Science. 2011; 333 (6040): 301-306. 10.1126/science.1205106.

3Zimov SA, Chuprynin VI, Oreshko AP, Chapin FS, Reynolds JF, Chapin MC. Steppe-Tundra Transition: A Herbivore-Driven Biome Shift at the End of the Pleistocene. The American Naturalist. 1995; 146 (5): 765-794. 10.1086/285824.

4Child KR, Darimont CT. Hunting for Trophies: Online Hunting Photographs Reveal Achievement Satisfaction with Large and Dangerous Prey. Human dimensions of wildlife. 2015; 20 (6): 531-541. 10.1080/10871209.2015.1046533.

5 Terborgh J, Nuñez-Iturri G, Pitman NCA, Valverde FHC, Alvarez P, Swamy V, et al. Tree Recruitment in an Empty Forest. Ecology (Durham); Ecology. 2008; 89 (6): 1757-1768. 10.1890/07-0479.1.

6 Steneck RS, Vavrinec J, Leland AV. Accelerating Trophic-Level Dysfunction in Kelp Forest Ecosystems of the Western North Atlantic. Ecosystems (New York). 2004; 7 (4): 323-332. 10.1007/s10021-004-0240-6.

7 Steneck RS, Graham MH, Bourque BJ, Corbett D, Erlandson JM, Estes JA, et al. Kelp forest ecosystems: biodiversity, stability, resilience, and future. Environmental Conservation; Envir.Conserv. 2002; 29 (4): 436-459. 10.1017/S0376892902000322.

8 Filbee-Dexter K. Ocean Forests Hold Unique Solutions to Our Current Environmental Crisis. One Earth. 2020; 2 (5): 398-401. doi.org/10.1016/j.oneear.2020

9 Jackson JB, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, et al. Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science (American Association for the Advancement of Science); Science. 2001; 293 (5530): 629-638. 10.1126/science.1059199.

10 Duarte CM, Marbà N, Gacia E, Fourqurean JW, Beggins J, Barrón C, et al. Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows: SEAGRASS COMMUNITY METABOLISM. Global Biogeochemical Cycles. 2010; 24 (4): n/a. 10.1029/2010GB003793.

11 O’Leary J,K., McClanahan TR. Trophic cascades result in large-scale coralline algae loss through differential grazer effects. Ecology (Durham); Ecology. 2010; 91 (12): 3584-3597. 10.1890/09-2059.1.

12 O’Leary J,K., Potts DC, Braga JC, McClanahan TR. Indirect consequences of fishing: reduction of coralline algae suppresses juvenile coral abundance. Coral Reefs. 2012; 31 (2): 547-559. 10.1007/s00338-012-0872-5.

13 Bellwood DR, Wainwright PC, Fulton CJ, Hoey AS. Functional versatility supports coral reef biodiversity. Proceedings of the Royal Society.B, Biological sciences; PROC R SOC B. 2006; 273 (1582): 101-107. 10.1098/rspb.2005.3276

14 Springer AM, Estes JA, van Vliet GB, Williams TM, Doak DF, Danner EM, et al. Sequential Megafaunal Collapse in the North Pacific Ocean: An Ongoing Legacy of Industrial Whaling? Proceedings of the National Academy of Sciences – PNAS; Proc Natl Acad Sci U S A. 2003; 100 (21): 12223-12228. 10.1073/pnas.1635156100.

15 Emslie SD, Patterson WP. Abrupt recent shift in 13C and 15N values in Adelie penguin eggshell in Antarctica. Proceedings of the National Academy of Sciences – PNAS. 2007; 104 (28): 11666-11669. 10.1073/pnas.0608477104.

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