Discuss the importance of top carnivores in an ecosystem, and explain the role that wildlife management plays?

Question

Looking for educated answers.

Answer 1

Top carnivores, including eagles, tigers and great white sharks are predestined by their perch at the apex of food webs to be big in size and sparse in numbers. They live on such a small portion of life’s available energy as always to skirt the edge of extinction, and they are the first to suffer when the ecosystem around them starts to erode (Wilson 1993:36).

The removal of a top carnivore from an ecosystem can have an impact on the relative abundance of herbivore species within a guild. In the absence of predation, usually one or two species come to dominate the community. The consequence of this is often a direct alteration of the herbaceous vegetation fed on by the predatory guild or assemblage. Top carnivores have an important role to play in the structuring of communities and ultimately of ecosystems. Thus, the preservation of carnivores becomes an important consideration in the discipline of conservation biology (Eisenberg 1989:7).


These quotes from two eminent ecologists contrast the “bottom-up” and “top-down” control processes that both regulate species abundance and structure the numbers and kinds of species that co-exist in an area (Power 1992). Most conservation biologists agree that top-down and bottom-up processes occur concurrently in many ecosystems and that change and stress in Asian forest ecosystems may produce markedly different outcomes when mediated through bottom- and/or top-down processes.

Obtaining details on the role of top carnivores in structuring ecosystems has been an elusive goal because of the daunting task posed by the large spatial scales required to investigate these processes and the very few places where it is possible to do so (Seidensticker and McDougal 1993). Recognizing and understanding the details imbedded in these patterns and processes is paramount in our efforts to prevent extinction and the loss of biodiversity and to restore natural abiotic and biotic processes that sustain biodiversity.

In this essay I examine the food and spatial requirements of the top carnivore in Asian forests, the tiger (Panthera tigris); what the tiger needs to survive; and how top predators shape community structure.

Big carnivores need big prey to survive

There is an overall correlation between body mass and the mass of their most common prey in the Carnivora. In general, carnivores weighing 21.5 kg or less feed mostly on prey that is less than 45% of their own mass and carnivores above this size feed mostly on prey that is more than 45% of their own mass. This dichotomy is the consequence of mass-related energetic requirements, i. e. the hunting times required to obtain the prey (energy) to balance the net energy expenditure. As carnivore mass increases, the total energy expenditure increases, and so too must hunting time to obtain prey in order to balance the energy budget (Carbone et al. 1999).

The Felidae can be divided into three clusters according to their mass. The cats in each cluster have diets and strategies particular to and appropriate for the size group to which they belong. Twenty-nine species of cats are less than 20 kg in mass and they eat small prey weighing less than1 kg in size, the medium sized cats such a leopards (40-60 kg) feed on larger prey - 2 kg up to their own mass and larger.

The two very large cats – tigers and lions (140-160 kg) - are the specialized predators of large hoofed mammals (ungulates). In Asian forests tigers need wild swine (Sus sp.) and large deer (Axis sp., Cervus sp.), prey that are from one half to greater in mass than tigers are themselves (Emmons 1992). Where wild cattle such as gaur (Bos gaurus) and bantang (B. javanicas) occur in the tiger’s range, tigers select these very large ungulates (~1000 kg) over smaller ungulates (Seidensticker and McDougal 1993, Karanth and Sunquist 1995).

Because of mass-specific energy requirements, tigers can not survive where they do not have access to ungulate prey that is at least about half their own body mass or larger. For example if only 20-kg muntjac (Muntiacus muntjac) are available, a tigress on a maintenance diet would need to kill one every 2-3 days. A tigress feeding two large cubs would need to kill one muntjac every 1-2 days or 183-365 muntjac a year (Sunquist et al. 1999).

This scenario may approximate the situation in many tropical rain forests that support a very low density of ungulate prey (Eisenberg and Seidensticker 1976) and thus will only support tiger densities of only 0.5 to 2 tiger/100 km2 (Carbone et al., ms.). The importance of large ungulate prey in the lives of tigers is so imperative that tiger distribution can be accurately predicted by mapping the preferred habitat of key prey species (Miquelle et al. 1999).

Thus, the top carnivores in Asian forests are at risk when their key prey are depleted. Ungulate management is an integral component of tiger conservation and depleted prey density due to intensive legal and illegal harvests by people is a critical tiger conservation issue. When large ungulates decline through over-hunting by humans, carrying capacity for breeding female tigers is reduced, cub survival rate decreases and the tiger population size declines rapidly (Karanth and Stith 1999). This is the norm in much of the tiger’s potential and actual range today (Wikramanayake et al. 1999). This is an important axis in the ways in which ecosystems “erode,” as Wilson described in the epigraph, and how this erosion of available energy has a bottom-up impact on ecosystem structure.

A principal result of the first scientific effort to understand tiger ecology (Schaller 1967:326) was that, “All evidence indicates that the tiger was the main factor limiting the growth of the populations of these species (chital Axis axis, gaur, sambar Cervus unicolor, barasingha Cervus duvaucelii).” And “…there is a density level based on intraspecific intolerance which both maintains the tiger population at or near optimum levels both in relation to the food resources and independent of them.” (p.330).

This is a top-down effect. Tigers are such splendid and powerful predators that our vision of this top-down process drives our notions and emotions about them and has obscured and delayed our recognizing the tiger’s vulnerability to eroding ecosystems through the depletion of key prey populations. Even the strong correlation between the tiger’s large mass and its dependence on large ungulate prey has not been incorporated into the thinking of many conservationists.

The critical issue key prey species depletion was not paramount on the tiger conservation agenda until 1997 (Karanth and Stith 1999, Miquelle et al. 1999), when it was also recognized, that over most of the remaining forest tracts through south and southeast Asia, key prey populations, populations necessary to sustain tiger populations, are missing (Rabinowitz 1999, Karanth et al. 1999, Sunquist et al. 1999).

Top carnivores face multiple risks through forest fragmentation
The survival of tigers is further compromised in the source-sink conditions created at the hard- and even soft-edges of isolated reserves. Because of their high energy requirement, the territories of wide-ranging, top carnivores are large and they are therefore exposed to threats at reserve boundaries.

Indeed, conflict with people on the borders of protected areas throughout the world is the major cause in mortality of wide-ranging, top carnivores, through hunting and poisoning, collisions with vewhicles, and diseases from domestic animals. Unless the killing of carnivores that venture outside of park boundaries can be prevented through expanded “no-kill zones,” hunter education, and significantly enlarged protected areas (in most cases), these parks and reserves will not sustain wildlife assemblages with top carnivores intact in the long-term (Woodroffe and Ginsberg 1998).

Unlike leopards which can move with shadows from moon light through relatively open and barren landscapes, tigers are comparatively weak in their dispersal capabilities (Smith 1993, Sunquist et al 1999). They simply don’t like to cross open areas. Forest fragmentation creates barriers to dispersal that fragment tigers into many distinct population segments. The present estimate of about 160 distinct population segments (Wikramanayaka et al. 1999) is a minimum estimate because the forest-cover data-layer used in these analyses did not of course capture the continual march of forest fragmentation.

Further, the proliferation of roads through remaining forest tracts across Asia further fragments habitats and places remaining tigers at risk. Roads provide poachers access. And tigers do not help themselves in this environment because they seem to prefer to travel along the edges of roads where they are flanked by forest. As sight-orientated predators, open roadsides increase the area tigers can scan for prey as compared to closed forest habitat. And open roadsides provide an environment where the tiger’s key prey come to feed on the early successional vegetation that grows there (Seidensticker 1986, Kerley et al. ms.).

Where roads have been improved to enable the speed of vehicles to increase, tiger-vehicular collisions are becoming more frequent (Bittu Sahgal, per. Comm. 1999.). Improving the quality and decreasing transport times on national transportation networks is high development priority through most of Asia and attracts major international donor backing. The barriers and sinks that roads create when they cut through reserves or cut through critical landscape-connecting corridors can in some circumstances be mitigated through features incorporated into roadway design, such as underpasses and fencing. Such features have been usefully employed in the transportation architecture in southern Florida (USA) to reduce Florida panther (Puma concolor coryi) collision-mortality and re-establish landscape connectivity (Maehr 1997:80). Strategies to offer incentives that result in closing or limiting access to logging roads into remote areas is a priority for top carnivore conservation efforts.

Top carnivores control meso-carnivores
The last track of a Javan tiger (Panthera tigris sondaica) was found in the rain forest of Ujung Kulon National Park, a 600 km2 peninsula on the west end of Java, Indonesia, in 1965. Today, Ujung Kulon supports a leopard (Panthera pardus) population. When A. Hoogerwerf (1970) documented the lives of large mammals in Ujung Kulon in the 1930s, leopards were absent, but tigers occurred at low density and they killed adult bantang. In the tropical moist evergreen forest of Khoa Yai National Park, Thailand, tigers occur at low density and there are no leopards (W. Brockelman, person. com.1986). Tigers catch and kill leopards where they can.

In case after case, the larger mammalian predator has been found to reduce or exclude populations of a smaller one (Palomares and Caro 1999). In the absence of large, dominant carnivores, smaller omnivores and carnivores can undergo population explosions, a phenomenon known as “meso-predator release.” This in turn, results in local extinctions of vulnerable prey populations (Soule et al. 1988).

Resource limitations appear to drive inter-specific competition (Weins 1993). If tigers eliminate leopards in habitats where appropriate prey are scarce, what happens where appropriate sized prey are both diverse and abundant? In the tropical moist deciduous/dry forest mosaic in the Nagarahole National Park, India, the principle prey of tigers, leopards and dholes (Cuon alpinus) reach densities of 66 animals/km2 comprised of gaur, sambar (Cervus unicolor), wild swine (Sus scrofa), chital (Axis axis), muntjac (Muntiacus muntjac) and hanuman langur (Presbytis entelleus). With this abundant and diverse prey base, leopards, tigers and dhole (Cuon alpinus) coexist.

The two solitary, stalking predators and the group-hunting coursing predator selectively killed different prey types in terms of species, size, and age-sex classes. This selective predation, which facilitated coexistence of the three predators, was mediated primarily by the adequate availability of prey in different size classes. This study by Karanth and Sunquist (1995, 2000) suggests that ecological factors such as adequate availability of appropriate-sized prey, dense cover, and high tree density are primary factors in structuring predator communities in these tropical forest types. In contrast, behavioral factors, such as differential habitat selection and inter-specific social dominance drive the structure of the large predator community in savanna habitats. In Nagarhole, however, both tigers and dholes were socially dominate over leopards, but there was no spatial exclusion of leopards as there is in rainforests with low key prey abundance.

In a survey of leopard food habits from areas in Asia and Africa, a pattern in the adaptability of leopard feeding emerged (Seidensticker 1991). Leopards easily switch prey. If there is one or more abundant ungulate species in the 20-50 kg size range present in a habitat, the leopard focuses its hunting on those animals. Leopards do not seem to spend much time hunting primates in those habitats even though primates may be quite abundant, and leopards do not spend much time pursuing smaller prey animals. When ungulate prey in the 20-50 kg size class is present but not at high density, leopard do prey on primates and primates may make up as much as 30% of their diet. In habitats where 20-50 kg size ungulate prey are essentially absent, leopards switch and eat a high proportion of primates.

Where prey of any kind occurs at very low density, leopards take just about any size animal they can capture: small ungulates up to 50 kg, porcupines, foxes, jackals, and primates as encountered. In the absence of a larger predator such as the tiger, the very large ungulates (>50 kg) escape predation by leopards, although their young do not. From this review, it is apparent that the consequences of losing the top carnivore, followed by a meso-predator release, will be mediated by the structure of the available prey assemblage. The structure of the available prey assemblage will be mediated through vegetation structure, productivity of the habitat, and to what extent humans have altered the habitat and impacted the prey assemblage.

Top carnivores play a pivotal role in the maintenance of biodiversity in Asian forests
Recent reviews (Soule and Terborgh 1999) and research conducted in large natural systems (Clark et al. 1999) point to the pivotal role of predation in structuring and preserving biodiversity in terrestrial communities. The top-down force that top carnivores exert on selected prey populations has been long appreciated. An understanding of the importance and the means by which top carnivores maintain the overall biodiversity in ecological systems is only recently emerging. This pivotal role has yet to be incorporated into the doctrine of conservation planning. Similarly, the ecological, behavioral, and security needs of top carnivores have been slow to be recognized, and even slower to be included our preservation and recovery thinking.

In our battle against extinction, we must come to fully understand how essential ecological processes such as predation work and how humans alter ecosystems in terms of the degree of change, the degree of sustained control, the spatial extent of change, and its abruptness. (Angermeier 2000). The eroding of connectivity and habitat quality has been of concern to conservationists seeking to preserve biodiversity. Top carnivores are the first to go in this erosion and degradation process. We have not fully understood the impact of poacher’s bullets and snares as they selectively removed top carnivores from relatively intact Asian forests. We now appreciate how important predation is in structuring and maintaining biodiversity.

Maintaining a reserve network large enough to sustain viable populations of the top carnivores has emerged as an overarching conservation objective (Soule and Terborgh 1999). We must shift our conservation thinking from viewing top carnivores as an isolated part of ecosystem management to viewing the maintenance of viable populations of top carnivores as essential components of an integrated system of sustainable ecosystem management.
The critically endangered top carnivores are indicators of ecosystems in decline. As we set our conservation sights on recovery, top carnivores can be the stars in our ongoing efforts that restore and maintain biodiversity. But the star-power of top carnivores, their flagship and umbrella role, is more than symbolic. Without top carnivores our efforts to stem the loss of biodiversity will not prevail.