How to translate the idiom - "at the bottom of the food chain" into german :)
In German, we often don't express how the food chain is oriented in space. So we don't speak of "top" or "bottom" of the chain, but about the "beginning" ("Anfang") and "end" ("Ende") of the chain.
At this point, it's probably useful to distinguish between the ecological term "food chain" / "Nahrungskette" and the more generally used idiom derived from that.
If you're talking about the ecological term, the "Ende" of the food chain is commonly seen as what would typically be the top of the food chain in English:
Fleischfresser stehen am Ende der Nahrungskette.
Carnivores are at the top of the food chain.
Conversely, the "Anfang" corresponds to the bottom of the food chain in this context:
Plankton steht am Anfang der Nahrungskette.
Plankton is at the bottom of the food chain.
Sometimes, the chain is also seen as having two endings, so it's specified which is the upper end and which is the lower end (here we're talking about up and down again ;) ):
Fleischfresser stehen am oberen Ende der Nahrungskette.
Carnivores are at the upper end of the food chain.
Plankton steht am unteren Ende der Nahrungskette.
Plankton is at the lower end of the food chain.
As an aside, the ecological term "Nahrungskette" or "food chain" is mostly seen as oversimplified, because the actual interdependencies are rarely a linear chain. The more current term would be a "Nahrungsnetz" or "food web".
There's also the related concept of a "Nahrungspyramide" or "food pyramid" ("ecological pyramid", "trophic pyramid" or other terms). While a "Nahrungskette" or "Nahrungsnetz" is focused more on the quantitative aspects ("who eats who"), a "Nahrungspyramide" is focused more on the qualitative aspects ("how much energy goes from one stage to the next").
After all those preliminary remarks, we can get to the answer to the actual question, namely the derived, more general idiom "to be at the bottom of the food chain". This idiom means something like "to occupy in a social hierarchy the position of least importance or power":
In his high school class, he was at the very bottom of the food chain. All the other students bullied him.
If you're talking about this idiom, the German equivalent works exactly the opposite way than what we've seen for the ecological term above. Here, the "bottom" in English generally corresponds to the "Ende" of the chain. So, the example from above could be translated as
In seiner Klasse in der Oberschule war er ganz am Ende der Nahrungskette. Alle anderen Schüler schikanierten ihn.
Some "real life" examples of this usage are
"Die Aktionäre stehen am Ende der Nahrungskette, sie sind die letzten, die Geld aus der Insolvenzmasse bekommen"
Am Ende der Nahrungskette (speaking of an exploited worker)
Kassaei malt schon seit einigen Jahren ein düsteres Bild von seiner Branche, auch dieses Mal: "Werber stehen am Ende der Nahrungskette". Sie hätten ihren Ruf und ihre Relevanz von einst verloren.
Kritk an Corona-Politik: Musikverein sieht sich „am Ende der Nahrungskette“ (typo not corrected ;) )
In German you can say
am Anfang/ Ende der Nahrungskette
(at the beginning/ end of the food chain)
ganz unten/ oben in der Nahrungspyramide
(at the bottom/ top of the food pyramid)
There are three things to consider here.
First: A Nahrungsnetz (food net) may be a more adequate metaphor than a pyramid or a chain to express reality.
Second: Below the question, some commentators argue that the mixing of the two metaphors (bottom of the food chain) was indeed idiomatic in English. In German, however, mixing them would sound odd, i.e., you don't say oben in der Kette or something like that, because oben/ unten are vertical orientations and most (but not all) food chains are oriented horizontally. Therefore, in German, the orientation independet terms am Anfang/ Ende are preferred for chains.
Third: A comment below this answer suggests that there may be a misunderstanding between the terms Ernährungspyramide (a dietary compilation of food products) and Nahrungspyramide (a trophic hierarchy showing the relationships of organisms feeding on each other), probably because both are translated into English as food pyramid. That Nahrungspyramide is the correct German term in the context considered here, I attached an image from the Lexikon der Biologie on spektrum.de: