From the peaks of the Canadian Rockies to thecity parks of Toronto to the swamps of "Newfoundland, frogs' can be found in moste~9sy:$t~i:nsthat include water. They've beenaround for more than 400 million years, andeven survived 'the disaster that killed off thedinosaurs 65 million years ago. Unfortunately,scientists have noticed recently that frogs andother amphibians are disappearing rapidly.About 30% of North America's frogs and toadsare in trouble. There is evidence to suggest thatfrogs are being affected mainly by the growth ofthe human population.

Why ·Should We Be Concerned?Why is the disapp-earance of frogs considered aproblem? While they are as important as anyother organism in an ecosystem, many scientistsbelieve that frogs are indicators of the health ofthe ecosystems they live in .:. Frogs have two lives (Figure 1). They begin

their lives as eggs and grow to tadpoles in ponds,and then enter the 'second stage of their life asadults in forest and grassland areas. This meansthat a problem in either of these ecosystems willhave an impact on the frogs.

Frogs in Their EcosystemsFrogs are also part of two very different foodchains. A food chain is a step-by-step sequencelinking organisms that feed on each other,starting with a food source such as plants(producers), and continuing with animals andother living things that feed on the plants and oneach other (consumers).

During the tadpole stage of their lives, frogsare herbivores or plant eaters in one type of foodchain (Figure 2), They eat large amounts of bothliving and dead algae (small plant-like organisms).This type of food chain includes decomposerssuch as bacteria and is essential in the recycling ofmatter in ecosystems. Decomposers are organismsthat break down the waste from living and dead

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WillijlThe northern leopard frog, native to Ontario. is one of thethreatened amphibian species. (a) Leopard frogs lay their eggs inponds. (b) Tadpoles develop. (e) Tadpoles grow into immature frogs. ,.~(d)Adult leopard frogs live in fields and around ponds. ~i

.organisms to get nutrients for their own use. As.' -ihey break down the wastes, the nutrients also

secome available for plants and algae.During their adult life, frogs are carnivores,

pr animals that feed on other animals (Figure 3).~Their main diet is insects, but they may also eat.small fish.

.. If frogs were completely wiped out, insectm;; populations would certainly increase. This hasfii;' already happened in Bangladesh, east of India,~f.·where frog populations have been nearly ,~ .eliminated to supply restaurants with delicacies:= The result is a rise in the number of mosquitoes,.' and therefore a rise in the number of cases of. .malaria among humans. Malaria is a disease that

~~,'is transmitted by mosquitoes. The increase int:·malaria can be traced back to the disappearance';", of frogs from the local ecosystems.

Why Are Frogs Disappearing?The worldwide disappearance of frogs is a bit of amystery. In some areas, scientists don't reallyknow what is causing the problem. In other areas,they have identified some probable causes.

Lossof HabitatThe most important factor in frog survival is anappropriate place to live, or a habitat. Frogs needwetlands, ponds, or lakes with clean water thatthey can breed and lay their eggs in. As adultsthey need a place such as a forest or a field wherethey can catch insects. Human activities such asfarming, forestry, road construction, and the-growth of towns and cities take away valuablehabitat from the.frogs.:

Air 'andWater QualityA secoiid cause for the"decline in frogpopulations is pollution. Frogs have lungs, butthey also absorb oxygen through their skin.Since their skin is so thin, it is easy forpollutants to pass through. . '

Acid rain is the most common pollutionproblem. An increase in the acidity of the watercauses reproductive and growth problems fortadpoles. Fewer eggs are fertilized, embryosdevelop more slowly, and some ,tadpoles developdeformed limbs.

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litH'''''The tadpole is a herbivore in this food chain. Organisms in this chainhelp to recycle nutrients in the ecosystem.

grasshopper

Will'.The adult frog is part of a food chain that includes producers (plants)that make the food and consumers (animals and fungi) that feed eitherdirectly or indirectly on the plants.

Ecosystems and Human Activity 9

Ultraviolet RadiationThe thin skin of the frog is also sensitive toultraviolet (UV) radiation from the Sun. Theamount of radiation reaching the Earth's surfaceis increasing because of the damage to theprotective ozone layer (Figure 4). The rate ofhuman skin cancer has also increased because ofthe increase in UV rays. This is another important,reason to study the frog as an indicator of thehealth of ecosystems.

Climate ChangeHuman activities, such as those that use fossilfuels, contribute to global warming, which in turn 'causes climate changes. Frogs are affected byclimate changes in local ecosystems. For example,if the climate becomes drier, frogs will likely losesome of their wetland habitat.

mil"j"A thin layer of ozone blocks harmful UV radiation. The layer isgetting th inner.

BlM 3-1

Hierarchy in Biology

Scientists have observed that living thingshave their place in a distinct hierarchy.This hierarchy starts with the simplestliving things, cells. It goes up to the mostcomplicated systems of organisms, whichinteract with and within their environment.

Examples

biosphere: Earth and the atmospherebiome: tundra, Great Lakesecosystem: forest, wetlandcommunity: inhabitants of soilpopulation: flock of geese, school of fishorganism: one goose, one fishorgan system: digestive system, nervous

systemorgan: stomach, braintissue: muscle, bonecell: neuron, amoeba

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biosphere

biome

ecosystem

+community

population

organism

organ system

+organ

ttissue

tcell

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