Prompted by Dmitrii's question, I poked around in my database for some more concrete examples of German naturalists in the New World who don't entirely fit the stereotype dominated by Humboldt.
Georg Wilhelm Freyreiss (born 1789 in Frankfurt; son of a shoemaker; originally trained as a salesman) was a wandering naturalist who first traveled to St Petersburg, where he made the acquaintance of Georg Heinrich von Langsdorff (b. 1774), a Hessian physician and naturalist who resided for many years in Russia and was an early scientific explorer of Siberia. When Langsdorff became Russian consul to Brasil in 1812, he invited Freyreiss to accompany him. Freyreiss mostly sent specimens from Bahia back to European universities, and in 1818 he briefly became a professor of natural history in Rio, eventually returning to found the colony Leopoldina in Bahia. The aristocratic explorer Max zu Wied-Neuwied was also a patron and collaborator for a time. Freyreiss's specimens ended up in Moscow, Stockholm, Frankfurt, Leiden, and Berlin. He was a founding member of the Senckenberg Science Society in Frankfurt (1818).
Carl Friedrich Philipp von Martius (b. 1794 in Erlangen) was the son of a court pharmacist; he started out studying medicine, but came into the orbit of the naturalist Johann Baptist Spix. The two men joined forces for a research trip to Brazil (1817-1820) sponsored by King Maximilian of Bavaria. Along with animal specimens von Martius brought some 2700 insects and 6500 botanical specimens back to Europe. He also studied the language and culture of the indigenous Brazilians, frequently relying on racist physiognomy. He was knighted for his efforts upon returning, and he eventually became a professor and head of the botanical garden in Munich. He attended numerous German Versammlungen and became a member of the Moscow Society of Naturalists.
Christian Sartorius (b. 1796 in Darmstadt) was trained as a theologian and worked briefly as a high school teacher. He had been active in student fraternities and was later arrested for his liberal politics. He emigrated to Mexico in 1824, where he ran a farm and then a silver mine, and began writing pieces for European readers to supplement his income. He returned to Germany in 1849-1852 (perhaps hopeful about new political options?), but then returned to Mexico for the remainder of his life. He was a member of the Darmstadt Association for Geology and Applied Sciences and attended one German Versammlung in 1852.
Hermann Burmeister (b. 1807 in Stralsund the son of a toll official for the Swedes), originally slated for a career as a salesman, studied natural history and medicine in Greifswald and then Halle (where he became a fraternity member). He earned a PhD for work in entomology and moved back to Stralsund before taking a commission as a military surgeon but then teaching natural history at a high school in Berlin. With some further publications he then succeeded his old teacher at Halle in 1837, also setting up and running the university’s natural history museum. He traveled to Brazil in 1850-52, and also undertook trips to Italy and South Africa, bringing back hundreds of animal specimens and 8,000 insects. Dissatisfied with his position in Halle (he had been somewhat disadvantaged by his leftist politics in 1848), he undertook a lengthy trip to South America, returning with hundreds of animal specimens and thousands of birds. He eventually accepted a position in Buenos Aires in 1861. There he was involved in all manner of new scientific institutions. He died in 1892. He attended one German Versammlung (Bremen) and was a member of the Halle Science Society, the German Geological Society, the Stettin Entomological Society, and the Russian Entomological Society.
Karl Wilhelm (Carlos) Moesta (b. 1825 in Hessen in modest circumstances) was an astronomer who was the founding director of the national observatory in Chile, 1852-1867. He attended at least one German Versammlung as well as a meeting of the German Astronomical Society and the first German congress of meteorologists.