Loss of Apex Predators in Dual-Apex Systems
By Tania Esteban, Samuel Ross, Jessica Rushall, & Louise Shuttleworth
Apex predators are in global decline. The description of possible complex interactions between apices in dual-apex systems calls for further research.
Apex predators occupy the highest trophic level of an ecosystem, thus do not have natural predators themselves. They are capable of affecting ecosystem functioning through consumer-control and strong trophic interactions. There are currently unknown interactions between apex predators and mega-herbivores in systems where both are present. Because of this, the loss of apex predators from a ‘dual-apex system’ could affect communities in a highly complex manner. The decline of apex predators should be considered in systems with both mega-herbivores and apex predators. Tambling et al., (2013) explored this concept in an African ecosystem where lions (apex predators) and elephants (mega-herbivores) co-exist. In this article we discuss the potential effects of apex predator loss in this ecosystem.
Mega-herbivores perform important functional roles in ecosystems. For example, elephants alter plant community architecture through trampling and overgrazing1. Direct aggressive interactions between elephants and other animal species also occur in these systems2, highlighting the key role of mega-herbivores in influencing species dynamics. Elephants also have indirect impacts on other herbivores through exploitation competition over resources, and depending on the system, are sometimes able to outcompete smaller herbivores1.
Figure 1. Weighted trophic interactions between species in the presence of apex predators: (a) with; and (b) without mega-herbivores. Interaction strengths are depicted by line thickness.
Lions are important apex predators in African ecosystems. They exert consumer-control, through predation on small and medium/large sized ungulate species, such as duiker and kudu respectively1. Like elephants, lions are classed as a flagship species because they are globally renowned, captivating, and of conservation concern. Lions are classed as vulnerable3, and are in decline because of hunting and persecution; diseases including CDV; and habitat loss due to agriculture and urbanisation4. If lion numbers continue to fall, large detrimental impacts on these ecosystems might be seen.
In multiple apex systems, interactions between apices are likely. Despite a lack of literature on the topic, there are potentially undescribed interactions between species occupying these apex roles.
Apex predators are likely to interact indirectly with other ‘apex consumers’ including mega-herbivores5. One of these indirect interactions can be facilitation by one apex on another; for example facilitation of predatory success of apex predators by mega-herbivores through environmental modification1. Another of these indirect interactions between apices would be the loss of one apex from the system. It is widely recognised that the loss of consumer-control has widespread effects, with the impacts of this loss propagating through the ecosystem. On a larger scale, trophic downgrading is a global threat, as systems are driven towards simplicity when consumer-control by apex predators is lost5.
In an African thicket ecosystem, Tambling et al., (2013) studied the interaction between lions (apex predators) and elephants (mega-herbivores). In this system elephants facilitate predatory success of lions through overgrazing and trampling of dense thicket vegetation. This allows access into the dense vegetation, which lions utilize because they are sit-and-wait predators. Lions will preferentially select for foraging habitats that maximise cover, over abundance or value of prey6. Therefore, modification by elephants facilitates an increase in encounter rates between lions and their smaller ungulate prey, as these species predominantly inhabit this thicket vegetation1. In the absence of mega-herbivore facilitation, lions predominantly feed on larger prey species, as they do not have access into the dense thicket vegetation in which the smaller species reside [see figure 1].
As briefly discussed by Tambling et al., (2013), loss of apex predators in these systems could lead to multi-directional trophic cascades. Compared to unidirectional trophic cascades, impacts of predator loss can radiate through the system in a nonlinear manner. For example, apex loss could propagate down through trophic levels and ‘rebound’ back up towards the second apex (mega-herbivore) through changes in populations of smaller herbivores.
In a classic trophic cascade, apex predator loss results in increases in herbivore populations7. The extent of population responses to predatory release depends on ecosystem structure. In the African thicket ecosystem where mega-herbivores facilitate lion predatory success on small ungulates, if lions were lost from this system, the resulting population changes of small herbivores would be greater than in the absence of facilitation by elephants. Where elephants are not present, lions mostly cannot access small prey species that frequent dense thicket vegetation so they predominantly prey on larger ungulate species1, resulting in greater proportional population increases in larger ungulates if predatory release were to occur.
Following the exploitation ecosystem hypothesis, if consumer-control is lost, systems are limited purely by primary productivity so the extent of primary production determines trophic complexity8. If lions were lost from a dual-apex system, the ‘second apex’ would likely be affected, as mega-herbivores would face increased competition due to predatory release of other herbivores [See Fig. 2a]. Systems that support mega-herbivores face increased herbivory initially, so when apex predators in these ecosystems are lost, mega-herbivore populations are at greatest risk of collapse due to competitive exclusion of these species with low rates of secondary production9.
Although Tambling et al. (2013) studied facilitation in dual-apex systems, as far as we are aware there is no current research into the effects of apex predator loss in these ecosystems. The African thicket ecosystem should be used as a model for future studies into dual-apex interactions, as exploration of connections in this novel system was valuable. As briefly outlined, complex multi-directional trophic cascades have not been widely recognised and described. If we are to gain valuable insight into the impacts of apex predator loss, we must research this area further, in different dual-apex systems, as currently little is known about the consequences of apex predator declines. Equally, the role of consumer control in structuring ecosystems is not widely recognised5. This further highlights the need to consider the effects of apex predator loss in ecosystems globally, and the importance of preserving all types of apex consumers in an increasingly downgraded world.
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