Christopher J. Lortie

Positive interactions among alpine plants increase with stress

Plants can have positive effects on each other. For example, the accumulation of nutrients, provision of shade, amelioration of disturbance, or protection from herbivores by some species can enhance the performance of neighbouring species. Thus the notion that the distributions and abundances of plant species are independent of other species may be inadequate as a theoretical underpinning for understanding species coexistence and diversity. But there have been no large-scale experiments designed to examine the generality of positive interactions in plant communities and their importance relative to competition. Here we show that the biomass, growth and reproduction of alpine plant species are higher when other plants are nearby. In an experiment conducted in subalpine and alpine plant communities with 115 species in 11 different mountain ranges, we find that competition generally, but not exclusively, dominates interactions at lower elevations where conditions are less physically stressful. In contrast, at high elevations where abiotic stress is high the interactions among plants are predominantly positive. Furthermore, across all high and low sites positive interactions are more important at sites with low temperatures in the early summer, but competition prevails at warmer sites.

LINKING PATTERNS AND PROCESSES IN ALPINE PLANT COMMUNITIES: A GLOBAL STUDY

Predictable relationships among patterns, processes, and properties of plant communities are crucial for developing meaningful conceptual models in community ecology. We studied such relationships in 18 plant communities spread throughout nine Northern Hemisphere high-mountain subalpine and alpine meadow systems and found linear and curvilinear correlative links among temperature, precipitation, productivity, plant interactions, spatial pattern, and richness. We found that sites with comparatively mild climates have greater plant biomass, and at these sites strong competition corresponds with overdispersed distribution of plants, reducing intraspecific patchiness and in turn increasing local richness. Sites with cold climates have little biomass, and at these sites a high proportion of species benefit from strong facilitative effects of neighbors, leading to an aggregated distribution of plants. Sites with intermediate, or relatively moderate climates are intermediate in biomass, and at these sites interactions are weak (or competition may be counterbalanced by facilitation), corresponding with a nearly random distribution of plants. At these sites species richness is lower than average. We propose that the relationship between interspecific spatial pattern and community richness reflects niche differentiation and/or construction, which allows for the coexistence of more species than would be possible with random, unstructured spatial distributions. Discovering the mechanisms that drive the relationships described here would further link functional and structural components of plant communities and enhance the predictive capability of community ecology.

THE SPECIALIZATION HYPOTHESIS FOR PHENOTYPIC PLASTICITY IN PLANTS

Adaptive plasticity in plants is commonly interpreted for fitness estimates like size and fecundity. The specialization hypothesis, however, predicts that plasticity in such characters is not a product of selection but, rather, a product of specialized (i.e., ecotypic) adaptation to particular environmental conditions. In response to a recent test of this hypothesis (Emery et al. 1994), we refine its predictions to recognize that the evolution of specialized ecotypes may be accompanied by an increase, decrease, or no change in the plasticity of size or fecundity. These predictions depend on whether specialization is associated with the less favorable or more favorable end of an environmental gradient and on whether specialization to one end of the gradient comes at a cost of reduced performance at the other end. We argue that, for size or fecundity characters, a plastic response to environmental deterioration is adaptive only if the alternative is dormancy or death and is generally less adaptive than phenotypic stability. Based on analysis of reaction norms for reciprocal transplants, we illustrate how it is possible to reject the specialization hypothesis and how to recognize results that are consistent with this hypothesis.

Tree invasions: a comparative test of the dominant hypotheses and functional traits

Trees act as ecosystem engineers and invasions by exotic tree species profoundly impact recipient communities. Recently, research on invasive trees has dramatically increased, enabling the assessment of general trends in tree invasion. Analysing 90 studies dealing with 45 invasive tree species, we conducted a quantitative review and a meta-analysis to estimate the relevance of eight leading hypotheses for explaining tree invasions. We also tested whether species functional traits (growth rate, density/cover, germination, biomass and survival) equally promote tree invasiveness. Overall, our results suggest that several hypotheses, linked to invasibility or invasiveness, are pertinent to explain tree invasions. Furthermore, more than one hypothesis has been supported for a given species, which indicates that multiple factors lead to the success of invasive tree species. In addition, growth rate appears to be the most efficient predictor of invasiveness for invasive trees and could thus be used as a means to identify potential alien tree invasions. We conclude that further investigations are needed to test the consistency of some hypotheses across a broader pool of invasive tree species, whilst experimental studies with the same tree species across a larger range of sites would help to reveal the full suite of factors that affect tree invasions.

Alpine cushion plants inhibit the loss of phylogenetic diversity in severe environments

Biotic interactions can shape phylogenetic community structure (PCS). However, we do not know how the asymmetric effects of foundation species on communities extend to effects on PCS. We assessed PCS of alpine plant communities around the world, both within cushion plant foundation species and adjacent open ground, and compared the effects of foundation species and climate on alpha (within-microsite), beta (between open and cushion) and gamma (open and cushion combined) PCS. In the open, alpha PCS shifted from highly related to distantly related with increasing potential productivity. However, we found no relationship between gamma PCS and climate, due to divergence in phylogenetic composition between cushion and open sub-communities in severe environments, as demonstrated by increasing phylo-beta diversity. Thus, foundation species functioned as micro-refugia by facilitating less stress-tolerant lineages in severe environments, erasing a global productivity - phylogenetic diversity relationship that would go undetected without accounting for this important biotic interaction.

Partitioning net interactions among plants along altitudinal gradients to study community responses to climate change

Summary Altitudinal gradients provide a useful space-for-time substitution to examine the capacity for plant competition and facilitation to mediate responses to climate change. Decomposing net interactions into their facilitative and competitive components, and quantifying the performance of plants with and without neighbours along altitudinal gradients, may prove particularly informative in understanding the mechanisms behind plant responses to environmental change. To decouple the inherent responses of species to climate from the responses of plant–plant interactions to climate, we conducted a meta-analysis. Using data from 16 alpine experiments, we tested if changes in net interactions along altitudinal gradients were due to a change in the performance of target species without neighbours (i.e. environmental severity effects only) or with neighbours (neighbour trait mediated effects). There was a global shift from competition to facilitation with increasing altitude driven by both environmental severity and neighbour trait effects. However, this global pattern was strongly influenced by the high number of studies in mesic climates and driven by competition at low altitude in temperate climates (neighbour trait effect), and facilitation at high altitude in arctic and temperate climates (environmental severity effect). In Mediterranean systems, there was no significant effect of competition, and facilitation increased with decreasing altitude. Changes in facilitation with altitude could not unambiguously be attributed to either neighbour trait effects or environmental severity effects, probably because of the opposing stress gradients of cold and aridity in dry environments. Partitioning net interactions along altitudinal gradients led to the prediction that climate change should decrease the importance of facilitation in mesic alpine communities, which might in turn exacerbate the negative effects of climate change in these regions. In xeric climates, the importance of facilitation by drought-tolerant species should increase at low altitudes which should mitigate the negative effect of climate change. However, the importance of facilitation by cold-tolerant species at high altitudes may decrease and exacerbate the effects of climate change.

Plant invasions, generalist herbivores, and novel defense weapons

One commonly accepted mechanism for biological invasions is that species, after introduction to a new region, leave behind their natural enemies and therefore increase in distribution and abundance. However, which enemies are escaped remains unclear. Escape from specialist invertebrate herbivores has been examined in detail, but despite the profound effects of generalist herbivores in natural communities their potential to control invasive species is poorly understood. We carried out parallel laboratory feeding bioassays with generalist invertebrate herbivores from the native (Europe) and from the introduced (North America) range using native and nonnative tetraploid populations of the invasive spotted knapweed, Centaurea stoebe. We found that the growth of North American generalist herbivores was far lower when feeding on C. stoebe than the growth of European generalists. In contrast, North American and European generalists grew equally well on European and North American tetraploid C. stoebe plants, lending no support for an evolutionary change in resistance of North American tetraploid C. stoebe populations against generalist herbivores. These results suggest that biogeographical differences in the response of generalist herbivores to novel plant species have the potential to affect plant invasions.

Don't diss integration: a comment on Ricklefs's disintegrating communities.

Ricklefs’s recent call to investigate ecological processes at large scales helps focus ecologists’ attention on an undoubtedly important topic. However, we believe that some of his accompanying arguments for the primacy of such work and, in particular, for the need to “disintegrate” the local community concept are flawed. We revisit Ricklefs’s main tenets and demonstrate that research on local communities is a vital part of understanding processes and diversity across a range of spatial and temporal scales. The integration of research across spatial scales expands our horizons and understanding of ecology and evolution, and this should not be unnecessarily constrained to one extreme or the other.

Bang for Your Buck: Rejection Rates and Impact Factors in Ecological Journals

When choosing where to submit their research for publication, most ecologists are concerned with journal impact factor, but they are also concerned with the likelihood that their manuscripts will be accepted. Based on a survey of ecologists, we found different degrees of relative concern for these journal attributes depending on author experience and gender. However, the ability of authors to choose among journals based on these journal attributes is limited: while journal impact factors are published regularly, journal rejection rates are not. We obtained, by permission, rejection rate data for a sample of 60 ecology journals for the year 2004. As expected, journals with higher impact factors also have higher rejection rates, but the ratio of (rejection rate) / (impact factor) increases sharply with decreasing impact factor below 1.76. Journals with impact factors below this value therefore provide relatively low payback in terms of impact against cost as estimated by rejection rate. We discuss alternative possible interpretations of this relationship and alternative criteria that might affect an authors decision about journal choice. Most importantly, our analysis indicates that the ability to make informed choices requires that journals publish their rejection rates annually. PREAMBLE How does an author decide where to submit a paper for publication? This important decision is made routinely throughout the career of a typical researching scientist/academic. The choices made can profoundly affect the trajectory, rate of progress, and status of ones research career. Necessarily, the subject category of the journals under consideration must be concordant with the research topic of the paper. However after this, most authors in the field of ecology at least, are usually still presented with more than one choice of a topically suitable subset of candidate journals. And, when the paper is rejected (an experience that few if any manage to avoid completely), the author is commonly inclined to iteratively select alternative journals.

The importance of urban backgardens on plant and invertebrate recruitment: a field microcosm experiment

Private backgardens in cities have the capacity to enhance ecological function and connectivity. The importance of increased habitat availability was tested experimentally using microcosms with soil or with soil and vegetation added into Toronto backgardens. Local attributes of the backgardens were recorded, and within the microcosms, recruitment of seeds, plants, and winged and wingless invertebrates were recorded. Natural recruitment by all organisms was significant into the microcosms in the 20 backgardens tested. Invertebrate abundance and diversity, incidental seed recruitment, and aboveground vegetation growth was enhanced regardless of whether only bare soil was added or soil with vegetation. The number of woody plants (non-herbaceous plants with hard lignified tissues or woody parts e.g., stems and are adapted to survive from one year to the next i.e., through winter) in the backgardens predicted seed recruitment and number of plant species predicted winged invertebrate abundance. Hence, simple augmentation of private backgardens—even at an ecologically small-scale—can enhance the capacity of urban systems to provide appropriate habitats for organisms within a difficult matrix for many species.