Janzen's early work focused on the careful and meticulous documentation of species in Costa Rica, and in particular on ecological processes and the dynamics and evolution of animal-plant interactions. amongst them a "kind of mass flowering", which Alwyn Howard Gentry in his classification of flowering named Type 4 or "big bang" strategy. Janzen proposed many hypotheses that inspired decades of work by tropical and temperate ecologists (see below).
Miguel Altieri in his textbook
Agroecology: The Science of Sustainable Agriculture says: "Janzen's 1973 article on tropical agroecosystems was the first widely read evaluation of why tropical agricultural systems might function differently from those of the temperate zones". In 1985, realizing that the area in which they worked was threatened, Janzen and Hallwachs expanded the focus of their work to include tropical forest restoration, expansion (through land purchases) and conservation. As of 2017, some 10,000 new species in the
Area de Conservacion Guanacaste have been identified thanks to the efforts of parataxonomists. Janzen and Hallwachs have supported species barcoding initiatives at both national and international levels through the
Instituto Nacional de Biodiversidad (INBio), CBOL (
Consortium for the Barcode of Life) and iBOL (
International Barcode of Life).
Influential hypotheses Janzen is known for proposing "characteristically imaginative and unorthodox" hypotheses. These hypotheses have received varying degrees of support, but are notable for having inspired a large and sustained body of research, as evidenced by the extremely high citation rates of many of his papers for decades after they are published. is now known as the
Janzen-Connell hypothesis, as Janzen and
Joseph Connell independently proposed the idea in 1970-1971. They both suggested that the high diversity of tropical trees was due, in part, to specialist enemies attacking seeds or seedlings that were particularly close to the parent tree or particularly densely clustered, thus preventing any one species from becoming dominant. Another influential idea comes from Janzen's 1967 paper 'Why mountain passes are higher in the tropics'. It proposes that tropical mountains are more of a barrier to species dispersal than temperate mountains because tropical species are less able to tolerate changes in temperature with elevation, having evolved and lived in relatively stable climates. In a 1977 paper 'Why fruits rot, seeds mould, and meat spoils', Janzen proposed that microbes render food inedible (or at least distasteful) to vertebrates not just as a by product of microbe-microbe competition or accidental waste products, but as an evolutionary strategy to repel vertebrates consumers, who would otherwise eat the food resource and the microbes themselves. Evidence is mixed, and it is hard to test whether compounds evolved to deter other microbes or vertebrates, but the idea has been widely incorporated into studies of vertebrate feeding from humans to dinosaurs.
Coevolution of plants and animals •
Coevolution of a mutualistic system in New World tropics between species of
Acacia (
Mimosoideae;
Leguminosae), v. gr.,
Acacia cornigera, and the ant
Pseudomyrmex ferruginea (
Formicidae).
Acacia spp in the Neotropics are protected by ants against defoliation; for this, the ants are rewarded by means of special organs and physiology that
Acacia has evolved.
Tropical habitat restoration Tropical dry forests are the world's most threatened forest ecosystems. In middle America there were 550 000 km2 of dry forests at the beginning of the 16th century; today, less than 0.08% (440 km2) remains. They have been cleared, burnt and replaced by pastures for cattle raising, at an ever-faster rate during the last 500 years. It is one of the oldest, largest and most successful
habitat restoration projects in the world. As of 2019, it consists of . One is also faced with the difficulties of changing a culture that coevolved with, profited from, and can become miserable with such a system. For this reason ACG was conceived as a cultural restoration project, which, to paraphrase its natural counterpart, ought to be grown as well. ACG integrates complementary processes of experimentation,
habitat restoration and cultural development. The techniques used include: • Active restoration, artificial dispersal of propagules from plant species native to the Guanacaste habitats ==Personal life==