JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.The contemporary literature accepts that disturbance strongly influences patterns of species diversity, and that the relationship is peaked, with a maximum at intermediate levels of disturbance. We tested this hypothesis using a compilation of published species diversity-disturbance relationships that were gleaned from a literature search of papers published from 1985 through 1996 and from references therein. We identified 116 species richness-, 53 diversity-, and 28 evenness-disturbance relationships in the literature, which we grouped according to shape of relationship (nonsignificant, peaked, negative monotonic, positive monotonic, or U-shaped). We tested the relationships between the strength and shapes of these relationships and attributes of the community, disturbance, and sampling and study design. Nonsignificant relationships were the most common, comprising 35% of richness, 28% of diversity, and 50% of evenness studies. Peaked responses were reported in only 16% of richness, 19% of diversity, and 11% of evenness cases. Explained variation in the three measures of diversity was variable among studies but averaged -50%. It was higher when few samples and few disturbance levels were examined and when organisms within the samples were not exhaustively censused, suggesting that procedural artifact contributes to these relationships. Explained variation was also higher in studies in which disturbance was measured as a gradient of time passed since the last disturbance (mean r2 = 61%), vs. studies of spatial variation in richness (mean r2 = 42%). Peaked richness relationships had the greatest odds of being observed when sampled area and actual evapotranspiration were small, when disturbances were natural rather than anthropogenic in origin, and when few disturbance levels were examined. Thus, on average, diversity-disturbance relationships do not have consistently high r2 and are not as consistently peaked as the contemporary consensus would suggest. It has been hypothesized that in communities where post-disturbance succession is not driven by competitive hierarchies, disturbance intensity and frequency may have little effect on species richness (Reice 1985, Chesson and Huntly 1997). Huston (1979, 1994) suggests productivity may influence the shape of the diversity-disturbance relationship. Unpredictable, severe, episodic disturbances may have greater effects on richness than do predictable, moderate events to which communities may adapt (Reice et al. 1990). Wootton's (1998) models predict that, in multitrophic systems, IDH should hold for species competing ...

The contemporary literature accepts that disturbance strongly influences patterns of species diversity, and that the relationship is peaked, with a maximum at intermediate levels of disturbance. We tested this hypothesis using a compilation of published species diversity–disturbance relationships that were gleaned from a literature search of papers published from 1985 through 1996 and from references therein. We identified 116 species richness–, 53 diversity–, and 28 evenness–disturbance relationships in the literature, which we grouped according to shape of relationship (nonsignificant, peaked, negative monotonic, positive monotonic, or U‐shaped). We tested the relationships between the strength and shapes of these relationships and attributes of the community, disturbance, and sampling and study design. Nonsignificant relationships were the most common, comprising 35% of richness, 28% of diversity, and 50% of evenness studies. Peaked responses were reported in only 16% of richness, 19% of diversity, and 11% of evenness cases. Explained variation in the three measures of diversity was variable among studies but averaged ∼50%. It was higher when few samples and few disturbance levels were examined and when organisms within the samples were not exhaustively censused, suggesting that procedural artifact contributes to these relationships. Explained variation was also higher in studies in which disturbance was measured as a gradient of time passed since the last disturbance (meanr2 = 61%), vs. studies of spatial variation in richness (meanr2 = 42%). Peaked richness relationships had the greatest odds of being observed when sampled area and actual evapotranspiration were small, when disturbances were natural rather than anthropogenic in origin, and when few disturbance levels were examined. Thus, on average, diversity–disturbance relationships do not have consistently highr2 and are not as consistently peaked as the contemporary consensus would suggest.

Aim  Anthropogenic habitat loss is usually cited as the most important cause of recent species’ extinctions. We ask whether species losses are in fact more closely related to habitat loss than to any other aspect of human activity such as use of agricultural pesticides, or human population density (which reflects urbanization).Location  Canada.Methods  We statistically compared areas in Canada where imperiled species currently occur, versus areas where they have been lost. Using multiple regressions, we relate the numbers of species that had suffered range reductions in an ecoregion to variables that represent present habitat loss, pesticide use and human population density.Results  We find high losses of imperiled species in regions with high proportions of agricultural land cover. However, losses of imperiled species are significantly more strongly related to the proportion of the region treated with agricultural pesticides. The relationship between species losses and area treated with pesticides remains significant after controlling for area in agriculture.Main conclusions  Our results are consistent with the hypothesis that agricultural pesticide use, or something strongly collinear with it (perhaps intensive agriculture more generally), has contributed significantly to the decline of imperiled species in Canada. Habitat conversion per se may be a less important cause of species declines than how that converted habitat is used.

Disturbance is often cited as one of the main factors determining patterns of species diversity. Several models have predicted qualitatively that species richness should be highest at intermediate intensities and/or frequencies of disturbances, but none indicate whether this effect should be strong (statistically accounting for much variability in diversity) or only subtle. Empirical evidence on the point is very mixed. This study examines Markov models of the dynamics of six real communities. We derive the predicted changes in species richness and evenness when these communities are subjected to quantified disturbance frequency and intensity gradients. We also use several different sampling intensities (i.e. numbers of individuals counted) to determine how this affects richness‐disturbance relationships. Our models predict that peaked responses of diversity to disturbance should be less common than monotonic ones. Species richness should vary, on average, by only 3% over gradients of no disturbance to complete disturbance. In the most extreme case, richness varied two‐fold over this gradient. Moreover, richness may increase monotonically, decrease monotonically, or be a peaked function of disturbance, interacting in a non‐intuitive fashion with both the sampling intensity and the community in question. These results are broadly consistent with a review of published richness‐disturbance relationships. Evenness varies somewhat more strongly along disturbance gradients, but the effect is still small. We conclude that extant models provide little reason to believe that disturbance should play more than a subtle role in determining patterns of diversity in nature, contrary to most contemporary literature.

To determine why some bats concentrate their activity over calm water rather than turbulent water, we studied the effects of surface clutter and running-water noise on the foraging activity of Myotis lucifugus (little brown bats), which commonly fly within 0.5 m of the water surface, and Eptesicus fuscus (big brown bats), which forage at greater heights, in southwestern Alberta, Canada. In paired experiments over calm water, artificial clutter reduced the activity of M. lucifugus, but not that of E. fuscus, compared with natural conditions. Playbacks of the sound of turbulent water reduced the activity of both M. lucifugus and E. fuscus in paired experiments over calm water. Clutter is an obstacle to flight and produces extraneous background echoes that must be discriminated from prey echoes. Water noise may also interfere with prey detection. The result may be reduced foraging efficiency by bats and a preference for calm bodies of water over turbulent ones.