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Population Ecology

Prey density is an important determinant of carnivore density and influences many aspects of tiger ecology, including territory size, energy expenditures, density of breeding females, number of transients, and many aspects of reproduction as well, including survival rate of cubs and juveniles.

While prey density is important to tiger density, so is prey size. Studies show a strong positive relationship between abundance of large ungulates and tiger densities. The difference surviving and thriving is the availability of large prey in sufficient numbers; tigers cannot survive and reproduce solely on small prey even if small prey is quite abundant.

Because tigers are individually recognizable by their markings, it is possible to estimate their densities using camera-trap surveys in a mark-recapture framework. A survey of 11 sites in India incorporating line-transect sampling of prey abundance and a photographic capture-recapture approach show a strong positive correlation between tiger density and prey density.

The highest tiger densities are found in alluvial grassland-forest mosaics of the Himalayan foothills and moist deciduous forests of peninsular India. The tiger density in Chitwan, an alluvial grassland-forest site, is lower than that of other similar sites, but the loss of several large ungulate species (i.e. water buffalo, barasingha), has undoubtedly affected tiger density in the region. The lowest tiger densities are found in the Russian Far East and the lowland tropical rainforest of Lao PDR, areas with low prey densities.

Though the two sites have vastly different ecological conditions, the land tenure system of tigers in Chitwan is remarkably similar to that in the Russian Far East. In Chitwan, for example, tigresses established and maintained small, exclusive territories in which they hunted and raised their young. Females competed amongst themselves for access to resources vital for rearing young. Side fidelity was strong, with individual females occupying the same territories throughout their reproductive lives. Except during periods of social flux, there was little overlap among the territories of neighboring resident females. The average home range size of radio-tagged tigresses living on the prey-rich floodplain was about 20 km2.

Prey densities in the Russian Far East are five to six times lower than in Chitwan, and territory sizes of females in the Russian Far East are 20 times larger, averaging 400 km2. Nevertheless, tigresses in the region are also territorial, showing little (10%) overlap of neighboring territories, and excluding other females from their territories.

Prey density and female territory size are obviously correlated, but increasing prey density does not always result in more breeding females. The lower limit for female territory size appears to be about 15 – 20 km2, a threshold probably set by a social intolerance, not prey density. There does not appear to be significant reduction in territory size of female tigers as prey biomass density increase from 2,000 to 7,000 kg/km2. However, tiger density does increase with increasing prey biomass, but the additional tigers using these prey-rich areas are probably floaters, transients or dispersers, not breeding individuals. Reproduction in tigers is dependent on acquiring a territory.

The territories of male tigers are large and overlap one of more of the smaller female territories. The much larger territories of males obviously contain more than enough food, water and den sites to satisfy their needs, which emphasizes that females, rather than food, are the most sought-after resources for males. Males compete for access to breeding females and territory sizes expand and contract depending on the number of resident females a make can successfully defend. Given the intense competition among males for access to breeding females, just acquiring a territory and defending it long enough to mate, results in high turnover rates and large differences in reproductive success among territory holders. Intraspecific fighting and incidences of infanticide, especially high-density tiger populations, often accompany changes in territory holders.

The territories of Amur male tigers are 20 times larger than those of tigers in Nepal and India, averaging 1,379 km2, and surprisingly Amur tigers are also territorial. There is little overlap in the territories of neighboring males and resident males exclude other males from their territories.

While high and prolonged levels of human-induced mortality and natural mortality can be devastating to the stability and growth of tiger populations, the results of a 9-year study in Nagarahole, and a 20-year data set from Chitwan, suggest that healthy tiger populations are able to withstand substantial losses and remain demographically viable because of their inherently high reproductive potential. Where prey is adequate tiger populations can grow rapidly. Gestation is short, only 103 days, females breed relatively early (3 years) and recycle rapidly after the loss of a litter. Litter size is commonly three but may be as many as five, and the inter-birth interval can be as short as 20 months.

Historical hunting records from Nepal and India illustrate how tiger population can rebound rapidly from substantial hunting losses as long as the habitat and prey remain intact. Hunting records from the Nepalese Terai—the same region where the Chitwan tiger study was carried out—detail the exact locales where literally hundreds of tigers were killed.

When the same areas were hunted only a few years later, equally high numbers of tigers were killed. Similarly, the tiger population in Russia’s Sikhote-Alin State Biosphere Reserve grew from one individual in 1966 to 24 – 31 individuals over the following 28 years, increasing at an average growth rate of 6% per year. The colonization episode clearly shows how fast tiger populations are capable of increasing when human impact is low and even low-density prey populations are healthy. More precise data from a long-term camera-trapping study in India shows that a Nagarahole tiger population increased by 3% per year and despite annual losses of 23% to emigration and mortality, tiger numbers did not decline.