but may frog species are poisonous and salamanders are almost not

You're correct in mentioning that some frog species are poisonous, or at least contain toxins. However, saying "salamanders are almost not" might need a little more explanation. It's important to consider the following points:

1. Poisonous Frogs: As you mentioned, certain frog species do indeed possess potent toxins in their skin. These include species like the poison dart frogs (generally within the family Dendrobatidae), which can be highly toxic. Others, like some tree frogs, have more mild toxins.

2. Salamanders and Toxins: While it is less common for salamanders compared to some frog species, there are still species of salamanders that can secrete toxins for defense. For example, the rough-skinned newt (Taricha granulosa) from North America has tetrodotoxin in its skin, a very powerful toxin found in pufferfish and one of the most deadly toxins known to science.

3. Toxin Production Mechanisms: Many frogs, especially those in the Dendrobatidae family, produce their own alkaloid toxins. Salamanders that do produce toxins often acquire some from their prey or through other mechanisms; however, it's not as widespread among salamanders as it is in some frogs.

4. Adaptive Defensive Mechanism: The presence of toxins or poisons in these amphibians serves a defensive purpose, protecting them against predators. It is an adaptation that has evolved independently in different lineages of both frogs and salamanders.

In summary, while it's true that we commonly associate toxicity with certain frogs, particularly those from the Dendrobatidae family, and less frequently with salamanders, this does not mean salamanders cannot have toxins or that they are fundamentally non-toxic across the board. Both groups have evolved different strategies to survive in diverse environments and resist various predators.