Community ecology

Abstract Community ecology is the study of interactions among species and the resulting diversity, composition, and structure of communities of organisms. Diversity is comprised of the number, types and relative abundances of species in the community. Most communities have a few common species and many rare ones. Within-community diversity is called alpha diversity and is compared among samples using rarefaction; between-community diversity is beta diversity, and differences among communities are visualized through ordination. The collection of interactions among pathogens and plants can be described as a bipartite network, with structural features like modularity and connectedness that influence pathogen spread. Maintenance of plant diversity requires mechanisms that counteract competition, which would otherwise lead to the decline of plant species over time. These include equalizing mechanisms such as the growth-resistance tradeoff, and stabilizing mechanisms, such as niche partitioning, the storage effect, and conspecific negative density dependence. Pathogens are commonly responsive to the density of their hosts, so diseases can be important stabilizing mechanisms promoting plant diversity (Janzen–Connell Hypothesis). Pathogen spillover from one species that is a competent host (supporting pathogen reproduction) to a neighboring species that is susceptible can lead to apparent competition. Plant-soil feedbacks occur when a plant changes its associated soil microbial community in a way that either benefits (enhancing mutualists) or harms (enhancing pathogens) subsequent generations of that species or of other plants. The spread and impact of pathogens can be modulated by a plant community’s composition and structure. Increasing plant diversity sometimes reduces disease pressure, called the dilution effect.