Other plants around a plant can influence its life trajectory. How plants respond to neighbors can affect their numbers and shape the plant community.
Neighbors matter.
Long-term research shows that some rainforest seedlings stay as seedlings for 30 years! During this long stay, they have to deal with neighbors of their own and other plant species. Neighbors may compete for resources, block out light, or harbor pests, parasites and pathogens. In fact, there is now tons of research which shows that what happens in the neighborhood can decide how many species can persist in a plant community. Let’s see how that happens.
Pests, parasites and pathogens home in on their preferred hosts. In many tropical forests, these ‘natural enemies’ that attack plants tend to specialize on a small set of usually closely related species. As a result, plants who have their own species for neighbors will more likely be traced by their pests or attract pathogens. Chances of being found are lower if the plant is in a neighborhood of species different from itself. So, the more numbers there are of your own species, the less well you perform in that neighborhood.
This kind of neighbor effect is strongest during the early life stages of a plant—seeds and seedlings tend to be especially vulnerable to enemies. Nearly half a century ago, two ecologists independently came up with a theory for how enemies maintain plant diversity. Enemies tend to aggregate near their hosts. A seed or seedling will do better when it moves away from larger tree of their own species, usually the parent. These older trees would have built up soil pathogens or attract insects that chomp its leaves. Near these older trees then, many of their own seeds and seedlings will fall to insects or pathogens, freeing up space for seedlings of other species to establish. All thanks to natural enemies.
Scale up this logic, and we see that the more abundant a species becomes, the more likely its progeny will land next to the same species and be attacked by enemies. This kind of self-thinning prevents one of few species from usurping all space. Less abundant species, which are less likely to be found by their enemies, get a chance to produce seedlings. Because most rainforests do not have a seed bank, the seedling bank sets the template for the diversity of the tree community. Without enemies cutting back the population of abundant species, rarer species may get knocked out of the system. In time, this would erode plant diversity. So, natural enemies of plants help keep the plant community diverse.
Neighbors can help too. Many plants form associations with microbes that help them. For example, mycorrhizal fungi are critical partners for many plants. Mycorrhizae stay on or within the roots of host plants. Sometimes mycorrhizae can be quite specific to the plant species. If you benefit from mycorrhiza, it helps to be near your own species or species that cultivate the mycorrhizae you need. Some kinds of mycorrhizae can protect against pathogens too. So, mycorrhizae can shore up the numbers of a species or allow it to remain in the community despite strong competition from other species. At times, mycorrhizae can help a plant deal with stresses such as drought.
Whether neighbors help or hinder, through nutrients or disease, plant species differ in their response to neighbors. Some species are more tolerant of pathogens. They are affected less negatively by crowding of their own species than plant species that more easily succumb to enemies. Another species may harness help from mycorrhizae. Species that can crowd locally can become more abundant. Myriad interactions among neighbors mediate the differences we see among species in their numbers—ecologists call this ‘community structure’.