The Ecological role of Carnivorous Animals
Carnivores are no doubt the stars in the conservation field. They are often charismatic and playing substantial roles in an ecosystem. Those are the two major reasons for why we love to conserve them. Though we focus on the carnivore conservation globally in the plight of biodiversity depletion, we need to ask ourselves: Does it effective?
The most important assumption underlying the carnivores conservation practice is the belief in the association between carnivores and the all other attributes and components of biodiversity. We believe that conservation of carnivores can benefit the conservation of biodiversity, at least they are positively related. In addition, larger-scale conservation often initiates from conservation of carnivores, which makes it inevitable on the urgency of biodiversity loss.
Nowadays, scientists examine the ecological role of large carnivores and its influence on the ecosystem. On the other hand, conservation practitioners use carnivores as a tool to achieve the conservation goals. To understand the relationship between large carnivores and biodiversity conservation, it is necessary to explore the scientific context and practical application.
The density-dependent process, which indicated the competition, was well-known as the main force structuring nature population, but the following research showed that predation may be another important factor to structure the ecological community and population. Local biodiversity is strongly related to the ability of predators to prevent the monopolization in an ecosystems by a few species. The idea that positive indirect interactions may exist between predators and the biodiversity in the whole system, including the roles, functions and processes originated. As a result, several ecological concepts such as trophic cascades, intermediate disturbance hypothesis and top down forces that form the food web appeared. The concept that predators at top can control the consumers at the bottem became widely accepted at that time. But another saying that the lower trophic level interaction results in the abundance of higher trophic level is argued at the same time. It led to a revoultionary era of considering predators as a driver but not a passengers in the paradigm of community structure.
Since the mid-1960s, several studies showed the ecological control derived by predators. By removing the dominant competitors or consumers, predators are able to control the ontogenetic development such as body size and the population dynamic of other species in the system, which leads to a concept of “Keystone species”. Keystone species is often in low abundance but has disproportionate influence on communities. Apex predator are often the examples, but the reverse may not be true. In contrast, foundation or dominant species are those who are abundant and have a large impact. Most of them are herbivores. However, there is not neceassarily a keystone species in a system. They may be absent is a highly diversed habitat like rain forests. The emphasis is on the ability of carnivores near the top of the trophic level to structure community. Close relationship between prey and their prey could drive the abundance of species in each trophic level.
At the basis of trophic cascades, edible plants are only limited by the available resource. The classic example is the sea otters in Alaska. Sea otters successfully control the sea urchin population and indirectly lead to the kelp recovery. But not all predation can cascade to lower level, we also need to consider the edibility and palatibility of those producers. Biomass such as woody mature plant and megaherbivores can be exceptions. The cascade can also be diluted. The complexity of interaction between carnivores and herbivores could lower the per capita impact especially when the diversity of prey is high. In this case, process of trophic cascade is not obvious.
Four reasons are regarded to influence the power of trophic cascades. First, compensatory community changes. Decline of specific herbivores may restructure the community that maintain the herbivores abundance and prevent the further influence from top predators. Second, poorly defined trophic structure. Complexity of food web make the trophic level obsecured, especially the existence of the omnivory species. Thrid, reduced consumer body size. Body size are seen to be an determinant factor of prey selection. The biogeographic or anthropogenic effects, as a result, reduce power of interaction between predator and herbivores. Fourth, interaction strengths via physiological stress. The primary productivity of the system can regulate the interactions among trophic levels. Predation is dominant when the environmental stress is low, and the competition is dominant when it is high. The relative importance of predation and competition determine the community structure. Therefore, trophic cascades are context dependent.
Except for the variability of trophic cascades, it is more often to detect the predator effects in marine system but not in terrestrial systems. Four possible reasons are as follows:
1) Marine vegetation are relatively more edible than terrestrial vegetation.
2) Body size ratio is more evident between herbivores and plants in marine systems. Less limitation of grazing in marine system since the herbivores can graze vegetation from the top down.
3) Three times more consumption fron herbivores in lentic than territrial food webs.
4) Benthic marine system often has lower environmental stress (higher primary production potential) which drive the predator as a dominant control factor.
After the discussion of the role of carnivores in the torphic level, we are curious about why they are so rare. We all know the inefficient energy pass on to higher trophic level is one of the limitations. But human disturbance is a more evident reason that limiting the abundance of large carnivores today. In addition, extinction rate is higher among big and fierce carnivores. They are often in large size and have more specializations, which make them more vulnerable due to its characteristics. Though large carnivores are once prosperous before human appeared, their numbers and species are soon plummeted from most of the continents (except for Africa). Human proliferation and colonization could be the major event, however, the replacement of the most influential component in trophic levels by human perhaps the most important cause. Ecologists have started to notice that higher trophic levels has higher risk of extinction. The loss of species in higher trophic levels implies the increasing importance of bottom up control. And the truth is human had already shifted the baseline interaction strength from large predators. Except for the influence in terrestrial system, there has been over kill in the sea. Most fish that people like to eat are predatory fish.
Currently, community structure has been changed since human became the ultimate apex predator. The balance in trophic levels is dramatically different from the past. Loss of apex predator weaken the top down control, and the loss of species at lower trophic level further cause the system to be unstable. Restoration of ecological function is difficult since a majority of people have misperceptions about the result of biodiversity loss and don’t understand the changes in communities. It is regard as usual that the ecosystems are mainly composed by meso-predators and herbivores, however, the predator baselie had shifted for a long time. Ecological function such as predation is much more difficult since the abundance of predators have been reduced to a certain number that cause the trophic level dysfunction. In some casees, there might be non-linear realtionship between ecological function and population desities. It is known as “catastrophic shifts in ecosystems”. Therefore, understanding the ecosystem history is necessary and substantial for dealing with the sliding predator baseline, which is an only way towards the restoration of original ecosystem in the past.
Reference:
Ray, J., Redford, K. H., Steneck, R., & Berger, J. (Eds.). (2005). Large carnivores and the conservation of biodiversity. Island Press.
