Factors Affecting Mammal Populations Neotropics

Factors affecting mammal populations in the neotropics and the challenges to managing these populations.


Different species of mammals have evolved to live in nearly all-terrestrial and aquatic habitats on the planet. Mammals inhabit every terrestrial biome, from deserts to tropical rainforests to polar icecaps. Mammals first appeared 180 million years ago. Today, there are more than 4,600-5,000 living species, placed in 26 orders. Placental mammals represent about 90% of mammals. The most diverse groups of Amazon mammals are bats and rodents, both of which are largely nocturnal and relatively small. Carnivores are represented by several cats (including the jaguar), otters (including the giant otter, the world's largest), weasels and relatives, and rare and little-known wild dogs. Aquatic mammals include two species of dolphin, and the rare Amazon manatee.

In general, mammalian population can be affected by Biotic (food) and abiotic (space availability) factors. These factors may severely limit population size. Many ecologists believe that species ranges are shaped in large part by biotic interactions. Predation and competition may be important on the small scale to limit populations.

Populations normally have a maximum density near the center of their geographic range (total area occupied by the species).

There are several factors that affect mammalian populations in the neotropics:

  • Climate change
  • Loss of habitat- due to deforestation
  • Pollution- due to modern agricultural techniques, mining activities
  • Over hunting- particularly hunting of herbivores as food for man
  • Removal of predator and indiscriminate killing.

1. Climate change

Climate change may seem to have little impacts on homeothermic animals. However, this is a misconception, dealing only with the adult mammal. The damage that heat can potentially cause on developing embryos must not be ignored, as this have a direct impact on the mammalian population. Embryo damage and death occur at environmental temperatures that pose little danger to adults. Embryo mortality can reach 100% in unacclimatized ewes, which are not dangerously stressed (Thwaites 1985). Environmental heat that drives up the female's core temperature kills embryos in large numbers. A rapid increase in environmental temperature increases heat damage and death to embryos. . In a worst-case greenhouse, massive embryo death could produce a positive feedback between embryo and adult numbers that could trigger reduction in population numbers, and collapse in vulnerable regions.

High environmental temperatures can reduce male fertility by damaging, or killing, sperm; however, the dominant effects of heat upon lowered fertility occur among females. By stressing the female, environmental heat triggers blood flow changes that reduce the flow of blood to the uterine tract, damaging or killing developing embryos. It is well documented that conception rates and fertility decrease in the summer in temperate zones, and in the subtropical and tropical climates.

“Human disruption of the carbon cycle threatens to produce a super-interglacial warming beyond the evolutionary experience of modern mammal.” Researches found that there is a correlation between climate change and extinction of mammals, at the end of the last ice age. It therefore becomes easy to predict the future of mammalian viability in face of an accelerated climate change. Mammals would not be able to adapt to the abrupt and radical temperature increase. Large mammals would in particular be affected by changes in the climate.

Climate influences animal distributions both directly and indirectly through effects of vegetation.

2. Loss of habitat.

“Pressures for land use have been a major cause of tropical rain forest loss and fragmentation throughout the world (Donald 2004), and a major cause of increases in rates of species extinction in recent decades (Laurance et al. 2002).” The same is applicable to the neotropics.

“In Amazon basin countries (N = 9) human population approaches 300 million people and is growing at a rate of 2.6% per year. Average population density for these countries is estimated at 20.2 + 15.0 people/km2.” The population is expected to double in the next 25-35 years. World population is rising at about 77 people million per year.

With such drastic increase in population size (as well as population density), there is an increasing demand for land space, for human settlements. The rising population also correlates to the increasing demand for food, which in turn results in clearing of vast stretches of forested area for agriculture.

Mammals, especially large mammals e.g. Jaguar, require a large range of territory. Wildlife Conservation Society reveals that the animal (Jaguar) has lost 37% of its historic range, with its status unknown in an additional 18%. Jaguars are killed when they are found to destroy livestock. They also hunt cattles, but the clearing of grazing area poses a problem for these cats. With the clearing of land for human survival, force these mammals to migrate to other area. But the problem only gets worst. The mammals would have a smaller territory, and in large population of mammals, there will be fierce competition. Survival of the fittest will ensue. Another important danger that Neotropical terrestrial mammals encounter, due to land clearing, is the reduction of food supply. Many mammals may die of starvation.

“average country landmass protected in Mesoamerica in the Amazon basin from Neotropics is only about 10-12% (see Earthtrends :www.earthtrends.wri.org)”

“Deforestation in Mesoamerica for the period 1990-2000 is estimated at 440,000 ha/yr, and similar trends in loss of forest cover are evident in the case of Amazon basin countries, where deforestation for the same period is estimated at 3.5 million ha/year (FAO statistics http://faostat.fao.org; earthtrends.wri.org).”

The mammals particularly primates, are sometimes considered as pest to agricultural lands near the Amazon forest. However only 5% crop raiding occurs in the neotropics by primates compared to 59 % in Asia. Aquatic mammals are also affected due to habitat lost e.g. the giant otter of Brazil, whose population is affected by hydroelectric dams, which cause fragmentation of populations.

Flood control structures are very harmful to manatees. Many manatees get trapped and crushed in canal locks and flood gates (Marmontel et al. 1997)

Overall, over 10% of the approximately four million km2 of Amazonian forest has been cleared and deforestation has continued at a rate of more than 10,000km2 per year (Fearnside 1990; Brown and Brown 1992). In coastal Ecuador, 96% of the forests have been destroyed and only 1% of the coastal dry forest remains.

3. Pollution- due to modern agricultural techniques, mining activities

The rise in human population demands more food production. This demand is constantly changing, for better quality of food products. For better quality of food products, crop plants are sprayed with insecticides, and other chemicals the increase the yield and quality of food products. The chemicals used are normally toxic to animals. They (chemicals) manage to get into streams and other waterways. The pollutants can then move to other locations, covering a large range. In mining activities, many toxic chemicals are also used, e.g. Mercury, used for the extraction of gold. These also make its way into the rivers. The chemicals then are absorbed by phytoplankton, a food source for many fishes.

Pollution of such, affects both terrestrial and aquatic mammals.

Terrestrial mammals, drinks water from these rivers. The water may get into their system, and can ultimately lead to death. Large populations of a single species can suffer the same fate. Herbivores are especially vulnerable, since they can consume plants that are in close proximity to the polluted water source.

Bioaccumulation and Biomagnifications occur in the fish that consumes toxic chemicals. Carnivorous aquatic mammals (seals) normally feed on fishes.

Contamination of food resources with organochlorine pesticides and PCBs is attributed to decline of Eurasian otters (Lutra lutra). Organochlorine pesticides have limited use in developed countries. It is mainly used by developing countries. These chemicals can be dispersed over a large range of area.

Human activities, such as pollution and littering, harm manatee habitats as well. Pesticides, detergents, and other industrial chemicals pollute the water in which manatees live. Litter in the water, such as fishing line and plastic bags, can get tangled around manatees and kill them.

4. Overhunting

Trapping and hunting of the Giant otter (Pteronura brasiliensis), have led to a decrease in their population. The otters are used for their pelt. The decline of about 15000 jaguar was attributed to overhunting, for their skin. Another aquatic mammal that is hunted, is dolphins.

Herbivores such as deers, when over hunted, cause these populations to decline. The decline of the deer, would translate into a shortage to food for its predators, and therefore the predator population will also be similarly affected.

Overharvsting of animals, is responsible for extinction of 23% of animals, since 1600.

In the Amazon, larger species are hunted for food, and some are also hunted for their skins, claws, teeth and other parts which are used in the curio trade or for medicinal purposes.

5. Removal of predator and indiscriminate killing.

This may occur mostly to the members of the Carnivora family. Human settlements are sometimes situated close to the cat’s territory. These cats are seem by the settlers as a threat, and are killed. When these top predaors are non-existant in that area, it allows the quick growth of the prey’s population (mostly herbivores). The large population will devastate large areas of vegetation- too quick for it’s regrowth to feed the ever-growing population, which is not checked by predators. Soon, the population of the herbivores would plummet, due to food shortage (secondary effect). The death of one species or population can cause the decline or elimination of others, a process known as secondary extinction.

Trans-migration, particularly in Brazil, has compounded the effects of shifting cultivation. Introduction of cattle to large areas of Central and South America has provided vampire bats with a ready food source. As a result, populations have increased bringing with them a range of problems.

In brazil, the Giant otter ( treatened with extinction) is killed by fish farmers, who complain that the otters kill the fishes at their farms. The otters are also sometimes killed be fish nets set by fishermen.

Challenges to managing these mammalian populations

Overexploitation, habitat destruction and fragmentation, the introduction of exotic species, and other anthropogenic pressures threaten mammals worldwide. In the past 5 centuries, at least 82 species have gone extinct. IUCN have named 1,000 (or 25%) of mammals as being at some risk of extinction.

In order to have a successful and sustainable wildlife, there must be planning and implementation of such plans, involving all stakeholders.

Mammals that are used as food, e.g. Labba (capybara), Agouti, can be farmed (husbandry) in order to keep the wild population in check.

Some Mammals occupy a larger territory (especially members of the order Carnivora), and live in very remote areas sometimes. This may pose difficulty in monitoring their population, and population decreases could go on unnoticed. In the neotropics, majority of the countries are developing countries. Theses countries are thus unable to finance research on population changes, and problems affecting mammals. Much funds provided are dependent on developed countries, and this may be limited.

Many smaller species will do quite well in the future, others, including tapirs, jaguars, and larger monkeys will require concerted conservation efforts, including public education and changes in the attitudes of local people and tourists alike. Rainforest ranching of some species may help to protect wild population.

Although husbandry is recommended for some wildlife animals, it would be very costly.

In terms of pollutants, other effective pesticide should be used as alternatives, with less harmful effect.

It is very difficult to avoid having endangered mammals killed by people. To avoid this, there will be a need for persons to monitor activities of people. This is almost impossible to achieve, due to the large range that is required to monitor.

Protections of these animals are sometimes hampered by political motives. Below are some relevant points on managing mammalian population in the neotropics: -In 2004, 87,871 ha of the Pantanal in Mato Grosso State (brazil) has been designated a Ramsar site because in contains the rare Giant otter. (Tapir conservation) The Neotropical otter is considered near threatened on the Brazilian list of threatened species. (Alarcon 2004) -The size of the wild manatee population is determined by an aerial survey, and the last count, in March of 1999, found 2,353 manatees, so the population is endangered (SMC). Manatees are protected by the U.S. Endangered Species Act of 1973, the U.S. Marine Mammal Protection Act of 1972, the Florida Endangered and Threatened Species Act of 1977, and the Florida Manatee Sanctuary Act of 1978 (Marmontel et al. 1997). Even with this protection, manatees continue to die of natural and human-related causes.

-According to the “Save the Manatee Club”, 69% of manatee deaths occur form natural of undetermined causes.

-On 7 January, 2008 United States Fish and Wildlife Service Director H. Dale Hall approved an unprecedented decision by the George W. Bush Administration to abandon jaguar recovery as a federal goal under the Endangered Species Act. The decision is the first of its kind in the 34-year history of the Endangered Species Act. . Some critics of the decision said that the jaguar is being sacrificed for the government's new border fence, which is to be built along many of the cat's typical crossings between the United States and Mexico. Currently, in the majority of Latin American countries, no specific legal structure exists for the protection of the jaguar. “Sport hunting of depredatory jaguars as a generator of funds for their conservation has been proposed by some researchers (Swank and Teer, 1992), and rejected by others. Among them, Rabinowitz (1995) asserts that it creates more problems than it solves, as it is simply a system to kill jaguars for profit. It does not protect the jaguar, not does it resolve the problem of cattle depredation. It is highly unlikely that such a system could be appropriately regulated when the majority of Latin American countries have demonstrated neither the inclination, nor the organization to make people comply with instituted hunting laws.” An increase in number of protected areas in the neotropics could to some extent help to properly monitor and manage mammalian population.


- Saving the Jaguar:A small Caribbean nation struggles to save a rare beast


-Managing Manatees - Final


-Neotropical Otters (Lontra longicaudis) http://jrscience.wcp.muohio.edu/fieldcourses07/PapersCostaRicaArticles/NeotropicalOttersLontralo.html

-Conclusions and Priorities for Otter Conservation http://www.iucn.org/dbtw-wpd/html/Otter/14.html





By Rafael Hoogesteijn (Translated by Beth Kuperman, Wildlife Conservation Society)


-Animal diversity web.


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