The competitive exclusion principle poses the pressing question of how biodiversity is maintained in nature. Many mechanisms have been proposed to explain diversity and to resolve what has become known as the “paradox of the plankton”. We propose a dichotomy among these mechanisms in order to enable empiricists to begin testing their relative importance. Specifically, the mechanisms can be categorized as being internally generated or as depending on forces external to the competitive community. Here we tested whether the internal competitive dynamics of a phytoplankton assemblage or externally generated resource variability (a disturbance) were more effective at maintaining species diversity over time. We also tested whether the species composition of assemblages was important in determining the persistence of species diversity. We employed controlled microcosm experiments in which we either imposed exogenous variability in nutrient availability via serial dilution or allowed the communities to remain completely undisturbed. We found that species diversity was maintained most effectively in undisturbed microcosms in which only internal dynamics regulated coexistence. We also found that the community composition of the assemblage significantly interacted with the disturbance regime in determining species diversity. This confirmed the importance of internal dynamics and community composition in maintaining species diversity.

Many organisms produce dormant stages, building up large germ banks in the sediments. Timing of emergence from these germ banks is a key feature in the recurrent cycle of dormant and active life stages. We exposed dormant egg bank samples of Daphnia magna from (semi-)permanent ponds to seasonal cues mimicking temperature and photoperiod during early spring, late spring, and summer in the region of sampling and investigated hatching success and response time after incubation. Whereas no differences in hatching success were observed between seasonal treatments, within each seasonal treatment a variable fraction of eggs postponed the hatching response for several days. We link immediate versus short-term delayed hatching to a trade-off between the advantages of quick niche filling and the unpredictability of reproductive success under unstable environmental conditions. A scenario where hatching investment is spread over immediate and delayed hatchers as a bet-hedging strategy is corroborated by environmental data on the risk of abortive hatching in early spring and by modeling results that show that immediate versus short-term delayed hatching is a valid alternative to bet-hedging over years in the investigated habitats to cope with variability in the onset of favourable conditions.

Nomenclature: Daphnia magna Straus, 1820.

We studied the regeneration dynamics of the semi-shade-tolerant invasive tree species Prunus serotina in the understory of 7 pine stands in its introduced range for 4 y, focusing on temporal, spatial, and spatiotemporal patterns. In each 20- × 40-m study plot, we inventoried all trees and shrubs taller than 1 m, counted seedlings in 3 age-height classes (224 subplots), trapped P. serotina seed rain (84 subplots), and identified the P. serotina seed trees. The seed set, seed rain density, and seedling densities of P. serotina all varied between the years and between the study plots, but the temporal stability of the spatial regeneration patterns increased with regeneration stage. There was a clear distinction between (1) seedlings smaller than 20 cm, younger than 6 y, occurring in very high densities and (2) seedlings between 20 cm and 1 m tall, older than 6 y, showing high spatiotemporal stability. Notwithstanding the large year-to-year variation in seed input, P. serotina maintained its regeneration potential in the forest understory by building up a short-lived seedling bank. The seedling bank strategy might represent an efficient way towards site occupancy of shade-tolerant non-native species in forest understories with few shade-tolerant native species.

Nomenclature: Kartesz (1994) for the American species and Lambinon et al. (1998) for the European species.

Disturbances create a variety of legacy types, which can influence subsequent forest dynamics. One method by which legacies may influence succession is by altering herbivore activities. For example, logging slash has been shown to be effective in preventing browsing by large herbivores. Generalizing from studies after logging, we hypothesized that windthrow slash (woody debris) would limit the ability of deer to reach or locate individual plants. Furthermore, another legacy of disturbances is the establishment of recalcitrant vegetation, such as dense fern layers, which subsequently preclude woody plant establishment. To test for potential influences of these 2 types of legacies, we examined the effects of slash and fern abundance on herb and woody community structure and deer herbivory levels within a Pennsylvania windthrow gap. Slash abundance was negatively correlated with woody diversity, richness, black cherry seedling densities, and total seedling densities, but this effect was probably not a consequence of browsing. Fern abundance was negatively correlated with woody richness, red maple densities, total seedling densities, herb diversity, and richness. Fern effects on the woody community appeared to be related to reductions in light availability. Our results contradict the findings of previous studies that show that slash piles serve as refugia from herbivores. We suggest the inconsistencies may be due to a lack of quantitative measures of slash within those studies.

Nomenclature: Gleason & Cronquist, 1991.

Habitat selection theory predicts that consumers should distribute themselves across alternative resources in a fitnessmaximizing manner. One such distribution is the ideal free distribution, where consumer density matches local resource density. Alternatively, if wing wear is an important cost of wing use for bees, there should be relatively more bee consumers in high density patches, resulting in “overmatching”. If the energetic costs of flight influence patch choice, there also should be relatively more consumers in high density patches (where flight costs are lower), resulting in “overmatching”. To examine these hypotheses, I observed the visitation of bumble bees (genus Bombus) across experimental manipulations of plant density in patches of bergamot (Monarda fistulosa) in southwest Alberta, Canada over 2 summers. Bees flew more often and for a greater proportion of time, and hit their wings more frequently, in low-density patches. In 2004, bees “undermatched” resources (relatively few bees in high density patches). In 2005, bees achieved an ideal free distribution. Overall, wing wear and energy costs alone cannot explain bumble bee use of their floral resources (in either year), while the ideal free distribution sometimes can (i.e., in 2005). This study therefore complements the findings of ideal free distributions in other aspects of the foraging ecology of pollinators.

Nomenclature: Moss & Packer, 1983; Natural History Museum, 2009.

Nest survival is an important determinant of overall reproductive success in birds and is often most influenced by factors such as habitat quality and predator communities. We examined the impacts of several variables hypothesized to affect nest survival by monitoring 74 radio-collared female sharp-tailed grouse (Tympanuchus phasianellus) during 2004 and 2005 in northeastern British Columbia, Canada. We tested candidate models using Program MARK and used an information-theoretic approach to assess the effects of habitat characteristics on daily nest survival rates, as well as the influence of female attributes on nest survival. Nest success, defined as hatching at least one egg, was 35% (95% CI = 21, 59) for first nests and 32% (95% CI = 12, 78) for renests. Daily nest survival probabilities increased with the amount of cover provided by woody shrubs at nest sites during first nest attempts and decreased with greater residual cover (i.e., the amount of dead, vegetative material from the previous growing season) at nest sites during renest attempts. As cover provided from shrubs became less important later in the nesting season, and residual vegetation was not a significant predictor of early-season nest survival, our results suggest that habitat features influencing survival of sharp-tailed grouse nests may change seasonally. We further suggest that this trend may be more pronounced at northern latitudes, where growing seasons are shortened and annual temperatures are reduced, which can influence the availability and effectiveness of habitats throughout the duration of the nesting season. We therefore caution against making broad management recommendations when managing habitats to increase nesting success of sharp-tailed grouse, as factors influencing success may vary across the species' range.

Nomenclature: American Ornithologists' Union, 2003.

In the eastern Canadian boreal forest, the term hardwood expansion or encroachment (enfeuillement in French) generally refers to the phenomenon of increasing cover of intolerant hardwoods, notably aspen (Populus tremuloides), following clearcutting. Such expansion is perceived as a local (stand-level) process that is reflected at a larger, regional scale. The objective of this study was to characterize the increase in aspen cover following clearcutting in the clay belt of Abitibi, in northwestern Quebec. Regional data from 3 province-wide forest inventories (1970, 1980, and 1990) conducted by the Quebec Ministry of Natural Resources and Wildlife were analyzed for 2 ecological regions, the Abitibi Plain (western balsam fir-white birch bioclimatic subdomain) and the Matagami Lake Plain (western black spruce-feathermoss bioclimatic subdomain). An assessment was done of forest cover transformations following clearcutting over 3 decades based on time since harvesting, surface deposit/drainage, and ecological region. Results show that mesic clay sites are particularly susceptible to aspen encroachment and that the overall rate of encroachment from the 1970s to the 1990s in the black spruce and balsam fir regions has been about 30%. Moreover, despite the greater abundance of poorly drained and organic sites, less favourable to hardwood establishment, in the more northerly black spruce region, the 2 regions appear to present the same sensitivity (or propensity) to intolerant hardwood expansion. On sensitive sites, management of black spruce forests should include adaptive practices to limit increases of the hardwood component.

Nomenclature: Marie-Victorin, 1995.

Arthropods were inventoried in crop fields and adjacent woody hedgerows of organic and conventional farms situated in south-central Ontario, Canada. The objective was to assess the total abundance, family richness, and composition of beneficial and phytophagous arthropods in the 2 farming systems and to examine the contributions of the vegetation present and landscape features on arthropod abundance and composition. Two hundred and forty sticky traps were installed (24 sites) and 170 sweep net collections were conducted (17 sites) in crop fields and adjacent woody hedgerows of organic and conventional sites. A total of 30 807 individual arthropods belonging to 131 families were recorded during the study. No spatial autocorrelation was detected among the arthropod samples. The study showed that beneficial and phytophagous arthropod abundance differed between organic and conventional sites (only with sweep net) but family richness did not. Beneficial arthropods were more abundant in woody hedgerows, while phytophagous arthropods were more abundant in crop fields. This study also demonstrated a strong relationship between plant and arthropod composition. Habitats (total old field cover, total hedgerow length, and Shannon diversity index, all within 250 m radius) in the surrounding landscape influenced arthropod composition but were not leading factors in explaining richness and abundance. It is therefore of prime importance to consider both local factors (management practices and local vegetation) and regional factors such as landscape features as explanatory variables when attempting to explain biodiversity.

Nomenclature: Gleason, 1963; Hitchcock & Chase, 1971.

The main objective of this study was to analyze between-winter stability of bird communities along latitudinal (950 km) and urban gradients (from small village to towns) in Finland. Birds were surveyed at the same 30-ha study plots using the same methods in 31 villages and town centres in the winters of 1991–1992 and 1999–2000. Species richness did not differ between the 2 study winters, but variation in total abundance of birds increased with increasing urbanization. However, urbanization reduced variation in wintering bird community structure. Species richness, density of wintering birds, and dissimilarity of wintering bird communities did not vary with latitude. According to our results, the level of urbanization was a more important factor than latitude in explaining the structure of the bird community in winter. We assume that the presence of a continuous, rich, and diverse supply of food offered by humans with increasing urbanization may explain variation in species abundances and stability in urbanized ecosystems.

Seed predation can significantly restrict the reproductive output and fitness of individual plants, and its populationlevel consequences may be most severe for plants that are rare or endangered. The Owyhee harvester ant, Pogonomyrmex salinus, actively removes the fruits and seeds of slickspot peppergrass, Lepidium papilliferum, a rare mustard endemic to Idaho. We conducted a field study to investigate the extent to which P. salinus contributes to seed loss in L. papilliferum. On average, individual L. papilliferum exposed to P. salinus experienced a direct loss of > 40% of their mature fruits, whereas plants shielded from ants suffered almost no fruit loss. More than 90% of L. papilliferum seeds placed on the ground beneath plants were scavenged by ants. All L. papilliferum fruits and seeds collected by P. salinus were returned to the ants' nests and transported below ground. A search of 30 middens revealed large quantities of empty L. papilliferum fruit husks, but no intact seeds. Thus, it does not appear that the ants benefit L. papilliferum by dispersing the plant's seeds. No seed predation was detected on plants located > 20 m from a P. salinus colony. We conclude that P. salinus is the main seed predator of L. papilliferum and that in many cases the ants remove and destroy almost all of an individual plant's seeds. Seed removal of this magnitude suggests that P. salinus may significantly limit recruitment of L. papilliferum, which could lead to further decline of this rare species.

Nomenclature: Hitchcock & Cronquist, 1973; Shattuck, 1987; Rollins, 1993.