Impact of an extreme climatic event on community assembly

Abstract

Extreme climatic events are predicted to increase in frequency and magnitude, but their ecological impacts are poorly understood. Such events are large, infrequent, stochastic perturbations that can change the outcome of entrained ecological processes. Here we show how an extreme flood event affected a desert rodent community that has been monitored for 30 years. The flood (i) caused catastrophic, species-specific mortality; (ii) eliminated the incumbency advantage of previously dominant species; (iii) reset long-term population and community trends; (iv) interacted with competitive and metapopulation dynamics; and (v) resulted in rapid, wholesale reorganization of the community. This and a previous extreme rainfall event were punctuational perturbations-they caused large, rapid population- and community-level changes that were superimposed on a background of more gradual trends driven by climate and vegetation change. Captured by chance through long-term monitoring, the impacts of such large, infrequent events provide unique insights into the processes that structure ecological communities.

Fig. 1.

Total number of rodents captured during each monthly trapping session on the control plots (filled circles) and the change in average community composition, as measured by ED, between subsequent 6-month periods (open circles). Three notable events are marked by the vertical dashed lines: 1983 Tropical Storm Octave, 1995 colonization by C. baileyi, and the 1999 flood event. The long-term mean for rodent abundance is indicated by the horizontal dotted line, revealing the period of markedly low rodent abundance preceding the colonization of the site by C. baileyi. A record low number of rodents was captured after the flood in 1999, but total abundance returned to preflood levels within 3 months.

Fig. 2.

Temporal trends in relative abundances (6-month averages) of the four most common rodent species on the control plots. Trends since the beginning of the study (1978–2005) are shown in A, C, E, and G, and trends only since the arrival of C. baileyi in September 1995 are shown in B, D, F, and H. The solid trend lines represent the ordinary least squares regression lines (see Table 1 for relevant statistics) and are extrapolated by using dashed lines. The relative abundances of the two species of pocket mice (Chaetodipus spp.) are significantly higher than expected after the flood and have remained so through the present, whereas those of the historically most abundant species, Merriam's kangaroo rat (D. merriami), and its congener, D. ordii, are significantly lower than expected.

Fig. 3.

Survival of the four most abundant rodent species during the period immediately before and after the flood compared with survival before the flood. Survival is measured as the percentage of marked individuals that survived from one 6-month period to each of three subsequent 6-month periods, as indicated by recapture. Preflood survival is calculated over the course of the entire study (filled circles) and is compared with survival from the 6-month period before the August 1999 flood event to the following periods (open circles). The survivorship of the kangaroo rats (Dipodomys spp.) decreased markedly as a result of the flood, whereas that of the pocket mice (Chaetodipus spp.) remained at or above preflood levels.