Green plants contain a pigment is chlorophyll which traps the energy from the Sun and converts it into chemical energy to be utilized by the members of the next trophic level. So, the first trophic level in a food chain is always a green plant. Green plants use sunlight and carbon dioxide from the atmosphere to produce carbohydrates by the process of photosynthesis. Since plants produce food for themselves, they are known as producers. So, they are the first link in any food chain. Producers always occupy the first trophic level of every food chain because only they have the ability to trap solar energy with the help of chlorophyll and synthesize their own food. The maximum number of trophic levels is hardly 3 to 5 in terrestrial ecosystems and 4 to 6 in aquatic ecosystems. The lesser number of trophic levels is due to consumption of lot of energy through respiration.

green plants, through the process of photosynthesis, are capable of converting sunlight energy into chemical energy in the form of glucose. They are autotrophs, meaning they can synthesize their own organic molecules using inorganic substances, such as carbon dioxide and water, in the presence of sunlight. Green plants play a crucial role in ecosystems by capturing solar energy and converting it into a usable form of energy. This energy is then transferred to higher trophic levels as organisms consume plants or other organisms that have fed on plants. As such, green plants serve as the primary source of energy for all other trophic levels in the food chain.

The maximum number of trophic levels in a food chain can vary depending on the specific ecosystem and its complexity. In general, food chains tend to have fewer trophic levels compared to food webs, which represent the interconnectedness of multiple food chains in an ecosystem.

Dr ahmad El Rayes thank you for your contribution to the discussion

The maximum number of trophic levels is hardly 3 to 5 in terrestrial ecosystems and 4 to 6 in aquatic ecosystems. The lesser number of trophic levels is due to consumption of lot of energy through respiration. A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5. The first trophic level contains the greatest number of organisms and is comprised mainly of plants. The organisms in this layer are called primary producers because they get their energy from an abiotic source. The first trophic level is made up of a large number of individuals i.e. producers who are capable of making their own food by trapping the energy of the sun. Green plants use sunlight and carbon dioxide from the atmosphere to produce carbohydrates by the process of photosynthesis. Since plants produce food for themselves, they are known as producers. So, they are the first link in any food chain.A food chain always starts with a producer, which is an organism that makes food. This is usually a green plant, because plants can make their own food by photosynthesis. The first trophic levels are producers in a food chain which includes green plants. It is the lowest trophic level. The reason behind producers placed on the first trophic level is that they have the capability to transfer solar energy into food.