Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving … directly follows.
—Darwin, Charles. 1861. “Conclusion.” In On the Origin of Species, 425. New York: D. Appleton and Company.
This is true in more ways than Darwin knew. He was talking about evolution (of the “higher animals”), but not only is death a hammer by which natural selection beats species into “endless forms”, death from disease is the reason so many of these endless forms can coëxist. The outcome of increased mortality from disease is reduced competition between plant species. It is not an obvious outcome of one organism harming another, but it emerges when we consider the spatial dynamics of a population.
First, let's consider an adult tree in fruit. The seeds of the tree land all around, in decreasing amounts as we get further from the trunk. The highest density is right by the base of the tree. Seeds aren't the only thing the tree produces though. Both through the air and under the ground, spores of fungi and oömycetes are also spreading. Adult gall midges and seed weevils fly out from its canopy. Larval mites are blown on the breeze. These, too, land all around the tree, at highest density nearest the trunk, right among most of the seeds! The seeds are less likely to survive the closer they are to their parent tree. The denser the stand of trees of this species gets, the fewer survive to adulthood.
Now look at the figure below. The black line shows the density of seeds as we get further from the tree (moving right). The yellow line shows the density of species-specific pathogens. In this case, the seeds can spread further than the pathogens. As pathogens are more abundant closer to the tree, the death rate of seedlings (purple) is higher. This leads to a dip in seedling density (green) immediately around the tree.
This large dip in seedling density appears in the rather extreme case where seeds can spread further than pathogens, but it is ilustrative of a more general effect. We can directly observe this in tropical tree seedlings, which have a higher death rate at higher densities unless fungal pathogens are excluded with fungicide1. The death rate immediately around the tree is higher, leading to overdispersion: adult trees are further apart from each other than we would expect based on the distribution of seeds. This is the Janzen-Connell Effect, originally described to explain the mysterious diversity of tropical trees2, most of which more-or-less do the same thing. Ecologists were confused why one species did not simply outcompete all of the others, since they all occupy a similar niche.
The Janzen-Connell Effect is part of a wider body of work known as Coëxistence Theory. This seeks to explain why, and under what circumstances, different species can coëxist. In many ways it is a field in its infancy: ecosystems are incredibly complex, and being able to predict their dynamics is not something we can do yet. What coëxistence theory has provided is a set of smaller explanations and predictions, and many of these are quite profound. Considering the Janzen-Connell effect, we see the importance of pathogens in maintaining the diversity of an ecosystem. This leads us to the prediction that in their absence, plant diversity would decrease. This is an easily testable hypothesis — we simply take a plant community, remove the pathogens, and see what happens. This is generally done using biocides, particularly fungicides. The result is exactly as we expected: when pathogens are removed, the diversity of the community decreases3. In the same experiment, it was shown that fungi are more important than oömycetes in this effect — when oömyceticide is sprayed, the decrease in diversity is less than when fungicide is sprayed.