Fine‐scale spatial variation in fitness is comparable to disturbance‐induced fluctuations in a fire‐adapted species

Abstract

The spatial scale at which demographic performance (e.g., net reproductive output) varies can profoundly influence landscape-level population growth and persistence, and many demographically pertinent processes such as species interactions and resource acquisition vary at fine scales. We compared the magnitude of demographic variation associated with fine-scale heterogeneity (<10 m), with variation due to larger-scale (>1 ha) fluctuations associated with fire disturbance. We used a spatially explicit model within an IPM modeling framework to evaluate the demographic importance of fine-scale variation. We used a measure of expected lifetime fruit production, E F , that is assumed to be proportional to lifetime fitness. Demographic differences and their effects on E F were assessed in a population of the herbaceous perennial Hypericum cumulicola (~2,600 individuals), within a patch of Florida rosemary scrub (400 × 80 m). We compared demographic variation over fine spatial scales to demographic variation between years across 6 yr after a fire. Values of E F changed by orders of magnitude over <10 m. This variation in fitness over fine spatial scales (<10 m) is commensurate to postfire changes in fitness for this fire-adapted perennial. A life table response experiment indicated that fine-scale spatial variation in vital rates, especially survival, explains as much change in E F as demographic changes caused by time-since-fire, a key driver in this system. Our findings show that environmental changes over a few tens of meters can have ecologically meaningful implications for population growth and extinction.

To prosper, live long: understanding the sources of reproductive skew and extreme reproductive success in structured populations

“…In populations where temporal variation in environmental conditions affects demographic rates, we can interpret these results as representing the distribution of outcomes across a cohort of individuals only if each individual in the cohort experiences their own independent sequence of environment states. This may be nearly true if environmental variation is spatiotemporal with a fine spatial grain — patchy fires or local light environments in a forest may be good examples (Coutts et al, 2021; Metcalf et al, 2009).…”