Rain Forest Amphibian Declines

When we peer into amphibian declines, we are looking into the depths of our own ignorance. Ashley Mattoon (1)

In the last 10 – 15 years we have witnessed a surge of interest in the welfare of amphibian species in general and amphibian conservation in particular.

This has been due to the fact that, by the end of the 1990s, researchers were observing the rapid and difficult to explain declines in amphibian populations around the world, specifically in rainforests.

Such declines presented a mystery to many people as they were taking place in undisturbed ecosystems with no apparent human-caused factors such as deforestation. (2)

A number of authorities in this field point out several major causes of amphibian decline.

The most common factors put forward as possible explanations are:

  • Climate Change

This factor has been widely referred to as a likely reason for “simultaneous changes in distant populations”. (3)

B. Waldman and M. Tocher give an excellent explanation of this:

“Even short-term variation in weather patterns can dramatically affect amphibian populations. Amphibians need moist habitats to live and reproduce, so desiccation is an incessant threat. The abundance and composition of the invertebrate prey in their diet is likely to shift with changing environmental conditions. Reproductive behaviors, spatial distributions, and vulnerability to predation and disease are likely to change as conditions become warmer or drier…

Altered weather patterns may be responsible for wide-ranging declines of frog populations in Brazil … and the possible extinction of the golden toad Bufo periglenes in Costa Rica…” (4)

However, the authors go on to mention that changing weather conditions cannot explain many of amphibian declines. (5)

  • Acid Rain

B. Waldman and M. Tocher point out that acid rain was suggested a long time ago as a likely threat for amphibians because of its effects on the larval survivorship and general development. (6)

Yet again, acid rain cannot explain problems with all declining amphibian populations globally.

  • Pesticides, Herbicides and Fertilizers

These chemicals “can retard growth and disrupt the capability of amphibians to survive and reproduce”, especially that they “can spread vast distances through the upper atmosphere in mist or fog to contaminate remote, otherwise pristine, regions”. (7)

  • Habitat Loss / Damage

This factor has been mentioned in a vast number of academic publications and mass media.

We have mentioned in the Tropical Frogs’ article how dependent the frogs are on their specific habitats.

Among other things, the habitat damage can happen through rainforest destruction / deforestation (resulting in habitat loss) or habitat degradation (such as fragmentation of a forest into isolated areas, for example (8)).

B. Waldman and M. Tocher note that local populations of frogs are normally connected through the individuals visiting each other. And this is important for maintaining their genetic diversity. But in a fragmented forest habitat, such interactions between the populations can become very difficult which in turn can lead to the species’ declines and extinction. (9)

  • Pathogens Attacking the Frogs

Researchers discovered that pathogens, such as fungi, can be a serious cause for epidemic die-offs of frog populations. (10)

Chytrid fungus, for example, was known to attack plants and insects before, but never vertebrates. This fungus has now been found to attack frogs and has become a major cause of amphibian declines. (11)

One thesis suggests that a number of negative environmental changes have weakened the frogs’ immune system to the level that they have become susceptible to the threats that they were managing to ward off before. (12)

Many experts rush to point out that it is most likely a combination of many factors that cause the wide-ranging damage to the frogs on a global scale.

B. Waldman and M. Tocher conclude that

“the cumulative effects of environmental stresses on endocrine and immune system function can result in increased mortality, decreased reproductive rates, and ultimately population declines”. (13)

We will not be too wrong to suggest that the factors that we’ve mentioned above are at least some of the reasons why many amphibian species (for example, the golden poison dart frog) have now become endangered animals.