A. Population dynamics significantly influence both community diversity and functional structure in the northern Chihuahuan Desert grassland/shrubland mosaic.
Changes in population sizes over time can lead to increases or decreases in the diversity of species present. Population booms of a particular species may allow it to dominate and outcompete other species, decreasing overall diversity. Population crashes of a keystone species can open up niche space and allow other species to establish, increasing diversity. The relative abundances of species also shift with population fluctuations, altering community structure.
Functionally, population dynamics shape the mix of ecological roles within the community. Increases or decreases in pollinator populations alter pollination services. Changes in grazer populations affect nutrient cycling through their impacts on primary production. Fluctuations in predator populations shift the regulation of herbivores and small mammals. So population dynamics rearrange the functional structure of the community by altering the balance of ecological niches filled.
B. Successional patterns differ substantially between plants and microbes in this desert ecosystem during both primary and secondary succession.
Primary Succession:
- Plants - Perennial grasses and shrubs slowly establish first on newly exposed parent material. Larger shrubs and trees eventually establish, with mesquite, juniper, and creosote bush becoming dominant over long timescales.
- Microbes - Cyanobacteria and other microorganisms rapidly colonize new substrates within days/weeks. This facilitates establishment of plants. Microbial community composition and diversity changes over months/years as conditions become less extreme.
Secondary Succession:
- Plants - Early colonizing annuals and perennials establish first after disturbance removes existing vegetation. Mid-successional grasses give way to shrubs like mesquite and creosote over decades. Climax community depends on disturbance severity.
- Microbes - Generalist bacteria and fungi spike rapidly after disturbance utilizing available nutrients. Over months/years, community rediversifies and shifts back toward undisturbed composition.
Late in succession, competitive exclusion structures both plant and microbial communities. Superior competitors exclude other species, decreasing diversity. Disturbance resets this by removing dominants. Dispersal allows recolonization from refugia, increasing diversity again.