Ecology Letters

Wright, S. J.; Calderón, Osvaldo

doi: 10.1111/j.1461-0248.2005.00851.xpmid: 16958866

It has recently been reported that humid tropical forests are changing rapidly in response to global anthropogenic change and that these forests experience greater tree mortality and even fire during droughts associated with El Niño events. The former reports are controversial largely because a single method has been used – repeated censuses of tree plots. The latter reports focus on recent extreme El Niño events. Here, we show that flower and seed production both increase during El Niño events in an old‐growth tropical forest in Panama. Flower production, but not seed production, has also increased over the past 18 years. The sustained increase in flower production was greater for 33 liana species than for 48 tree species. These results indicate that moderate El Niño events favour seed production, document long‐term increases in flower production for the first time, and corroborate long‐term increases in the importance of lianas using independent methods. Changes in levels of solar irradiance might contribute to all three patterns.

Johnson, Marc T. J.; Lajeunesse, Marc J.; Agrawal, Anurag A.

doi: 10.1111/j.1461-0248.2005.00833.xpmid: 16958865

Recent research suggests that genetic diversity in plant populations can shape the diversity and abundance of consumer communities. We tested this hypothesis in a field experiment by manipulating patches of Evening Primrose (Oenothera biennis) to contain one, four or eight plant genotypes. We then surveyed 92 species of naturally colonizing arthropods. Genetically diverse plant patches had 18% more arthropod species, and a greater abundance of omnivorous and predacious arthropods, but not herbivores, compared with monocultures. The effects of genotypic diversity on arthropod communities were due to a combination of interactive and additive effects among genotypes within genetically diverse patches. Greater genetic diversity also led to a selective feedback, as mean genotype fitness was 27% higher in diverse patches than in monocultures. A comparison between our results and the literature reveals that genetic diversity and species diversity can have similar qualitative and quantitative effects on arthropod communities. Our findings also illustrate the benefit of preserving genetic variation to conserve species diversity and interactions within multitrophic communities.

Fagan, William F.; Holmes, E. E.

doi: 10.1111/j.1461-0248.2005.00845.xpmid: 16958868

We developed a database of 10 wild vertebrate populations whose declines to extinction were monitored over at least 12 years. We quantitatively characterized the final declines of these well‐monitored populations and tested key theoretical predictions about the process of extinction, obtaining two primary results. First, we found evidence of logarithmic scaling of time‐to‐extinction as a function of population size for each of the 10 populations. Second, two lines of evidence suggested that these extinction‐bound populations collectively exhibited dynamics akin to those theoretically proposed to occur in extinction vortices. Specifically, retrospective analyses suggested that a population size of n individuals within a decade of extinction was somehow less valuable to persistence than the same population size was earlier. Likewise, both year‐to‐year rates of decline and year‐to‐year variability increased as the time‐to‐extinction decreased. Together, these results provide key empirical insights into extinction dynamics, an important topic that has received extensive theoretical attention.

Byrnes, Jarrett; Stachowicz, John J.; Hultgren, Kristin M.; Randall Hughes, A.; Olyarnik, Suzanne V.; Thornber, Carol S.

doi: 10.1111/j.1461-0248.2005.00842.xpmid: 16958869

Although human‐mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat‐providing species.

Helm, Aveliina; Hanski, Ilkka; Pärtel, Meelis

doi: 10.1111/j.1461-0248.2005.00841.xpmid: 16958870

We examined the response of vascular plant species richness to long‐term habitat loss and fragmentation of Estonian calcareous grasslands (alvars). The current number of habitat specialist species in 35 alvars was not explained by their current areas and connectivities but it was explained by their areas and connectivities 70 years ago (R2 = 0.27). We estimated the magnitude of extinction debt in local communities by assuming an equilibrium species richness in 14 alvars that had lost only a small amount of area and by applying this model to the remaining alvars, in which the average area has declined from 3.64 km2 in the 1930s to 0.21 km2 at present. The extinction debt estimated for individual alvars was around 40% of their current species number. Our conclusions are applicable to temperate grasslands in general, which have lost much area because of agricultural intensification and cessation of traditional management.

Bailey, Joseph K.; Wooley, Stuart C.; Lindroth, Richard L.; Whitham, Thomas G.

doi: 10.1111/j.1461-0248.2005.00844.xpmid: 16958871

Recent community genetics studies have shown that specific genotypes of a host plant support distinct arthropod communities. Building upon these findings, we examined the hypothesis that a trophic community consisting of cottonwood trees, a galling herbivore and avian predators could also be related to the genetics of the host tree. We found genetic correlations among phytochemistry of individual tree genotypes, the density of a galling herbivore, and the intensity of avian predation on these herbivores. We detected significant broad‐sense heritability of these interactions that range from H = 0.70 to 0.83. The genetic basis of these interactions tended to increase across trophic levels suggesting that small genetic changes in the cottonwood phenotype could have major consequences at higher trophic levels affecting species interactions and energy flow. These findings show a heritable basis to trophic‐level interactions indicating that there is a significant genetic basis to community composition and energy flow that is predictable by plant genotype. Our data clearly link plant genetics to patterns of avian foraging and show that species interactions are important components of community heritability and ecosystem processes. Overall, these data support the hypothesis that evolution of plant traits can alter trophic‐level interactions and community composition.

Koppel, Johan; Altieri, Andrew H.; Silliman, Brian R.; Bruno, John F.; Bertness, Mark D.

doi: 10.1111/j.1461-0248.2005.00843.xpmid: 16958867

Recent theory suggests that scale‐dependent interaction between facilitation and competition can generate spatial structure in ecological communities. The application of this hypothesis, however, has been limited to systems with little underlying heterogeneity. We evaluated this prediction in a plant community along an intertidal stress gradient on cobble beaches in Rhode Island, USA. Prior studies have shown that Spartina alterniflora facilitates a forb‐dominated community higher in the intertidal by modifying the shoreline environment. We tested the hypothesis that, at a smaller scale, Spartina competitively excludes forb species, explaining their marked absence within the lower Spartina zone. Transplant experiments showed forb species grow significantly better in the Spartina zone when neighbours were removed. Removal of the Spartina canopy led to a massive emergence of annual forbs, showing that competition limits local occupation. These findings indicate that interaction of large‐scale facilitation and small‐scale competition drives plant zonation on cobble beaches. This study is the first to provide empirical evidence of scale‐dependent interactions between facilitation and competition spatially structuring communities in heterogeneous environments.

Stanley Harpole, W.; Tilman, David

doi: 10.1111/j.1461-0248.2005.00836.xpmid: 16958864

Although the distribution of plant species abundance in a Minnesota grassland was consistent with neutral theory, niche but not neutral mechanisms were supported by the ability of species traits to predict species abundances in three experimental grassland communities. In particular, data from 27 species grown in monoculture showed that species differed in a trait, R*, which is the level to which each species reduced the concentration of soil nitrate, the limiting soil nutrient and which is predicted to be inversely associated with competitive ability for nitrogen (N). In these N‐limited habitats, species abundance ranks correlated with their predicted competitive ranks: low R* species, on average dominated. These correlations were significantly different than expected for neutral theory, which assumes the exchangeability of species traits. Additionally, we found that changes in relative abundance after environmental change (N‐addition or disturbance) were not neutral but also were significantly associated with R*.

Hochberg, Michael

doi: 10.1111/j.1461-0248.2005.00856.xpmid: N/A

Halpern, Benjamin S.; Regan, Helen M.; Possingham, Hugh P.; McCarthy, Michael A.

doi: 10.1111/j.1461-0248.2005.00827.xpmid: 16958861

Ecosystems and the species and communities within them are highly complex systems that defy predictions with any degree of certainty. Managing and conserving these systems in the face of uncertainty remains a daunting challenge, particularly with respect to developing networks of marine reserves. Here we review several modelling frameworks that explicitly acknowledge and incorporate uncertainty, and then use these methods to evaluate reserve spacing rules given increasing levels of uncertainty about larval dispersal distances. Our approach finds similar spacing rules as have been proposed elsewhere – roughly 20–200 km – but highlights several advantages provided by uncertainty modelling over more traditional approaches to developing these estimates. In particular, we argue that uncertainty modelling can allow for (1) an evaluation of the risk associated with any decision based on the assumed uncertainty; (2) a method for quantifying the costs and benefits of reducing uncertainty; and (3) a useful tool for communicating to stakeholders the challenges in managing highly uncertain systems. We also argue that incorporating rather than avoiding uncertainty will increase the chances of successfully achieving conservation and management goals.