Showing papers by "Ragan M. Callaway published in 2005"

A biogeographical approach to plant invasions: the importance of studying exotics in their introduced and native range

Abstract: Summary 1 Most theory and empirical research on exotic invasions is based on the assumption that problematic exotics are much more abundant in the regions where they invade than in the regions where they are native. However, the overwhelming majority of studies on exotic plants have been conducted solely within the introduced range. With few exceptions, ecologists know surprisingly little about the abundance, interaction strengths and ecosystems impacts of even the best-studied exotics in their native range. 2 We argue that taking a biogeographical approach is key to understanding exotic plant invasions. On a descriptive level, unambiguous quantification of distributions and abundances of exotics in native and introduced ranges are crucial. Experiments conducted at a biogeographical scale are also necessary to elucidate the mechanisms that enable highly successful exotics to occur at substantially higher abundance in their introduced vs. native communities. 3 We summarize the leading hypotheses for exotic plant success. We assert that tests of these major hypotheses for invasions (the natural enemies, evolution of invasiveness, empty niche and novel weapons hypotheses) require comparative biogeographical approaches. 4 In addition to focusing on comparative work in the native and introduced range, we also suggest other approaches that could yield important insight into processes that influence exotic success. 5 Increased understanding of invasions has the potential to provide unique insight into fundamental ecological theory, including that on individualistic-holistic structure, the role of trophic interactions in population regulation, and the importance of co-evolution in communities.

The importance of importance

Abstract: Failure to distinguish between 'importance' and 'intensity' of competition has hindered our ability to resolve key questions about the role interactions may play in plant communities. Here we examine how appropriate application of metrics of importance and intensity is integral to investigating key theories in plant community ecology and how ignoring this distinction has lead to confusion and possibly spurious conclusions. We re-explore the relationship between competition intensity and importance for individuals across gradients, and apply our review of concepts to published data to help clarify the debate. We demonstrate that competition importance and intensity need not be correlated and show how explicit application of the intensity and importance of competition may reconcile apparently incompatible paradigms.

Stress tolerance and competitive-response ability determine the outcome of biotic interactions

Abstract: Theoretical models predict that the net outcome of biotic interactions among plants is the sum of co-occurring negative and positive interactions, with facilitation gen- erally increasing in importance with increasing abiotic stress. However, species differ in fundamental niche optima; thus the intensity of stress is relative among species and im- portant only in the context of these relative differences. We tested the hypothesis that the facilitative response of a species is relative to how much abiotic conditions deviate from the optimum conditions for that particular species (stress), and the competitive ''response'' ability of the species (i.e., its ability to tolerate the inhibitory effects of neighbors). In a field experiment, we examined the responses of three co-occurring species with different ecological optima, Bromus erectus, Brachypodium rupestre, and Arrhenatherum elatius ,t o the alleviation of a primary limiting resource (water), and to biotic interactions in a me- soxeric grassland in eastern France. We found that A. elatius had a strong positive response to watering, the response of B. rupestre was moderately positive, and B. erectus did not respond significantly, suggesting that water stress was only important for the first two species. Most importantly, the net outcome of the interaction between each species and its neighbors depended on the degree of water stress it experienced in its natural habitat. For survival, in the control plots we found no significant interactions for B. erectus (not stressed) whereas B. rupestre and A. elatius (stressed species) were facilitated. Enhancing water availability suppressed facilitation of B. rupestre and A. elatius and led to competitive exclusion of B. erectus. In contrast to survival, there was no facilitation for growth in the control plots, and competition intensity increased for all three species with watering. In our experiment the competitive response of a species was inverse to its ability to tolerate stress, indicating a trade-off between these components of the species response. A facili- tative outcome appears to be a function of a species having both a low tolerance to a particular abiotic stress and a strong competitive-response ability.

Natural selection for resistance to the allelopathic effects of invasive plants

Abstract: Summary 1 Exotic plant invasions often cause high mortality in native populations and therefore have the potential to be a powerful selective force. 2 We found that surviving individuals from North American communities that have experienced extensive invasion by Centaurea maculosa have higher tolerances to the Eurasian invader than individuals from communities that did not experience invasion. 3 Some native species grown from the seed of individuals that survived Centaurea invasion were more resistant to the general competitive effects of Centaurea, the root exudates from Centaurea, and to a chemical specific to the root exudates of Centaurea (±)-catechin. 4 Although these results may be confounded by maternal effects, they provide initial evidence that native plant species may evolve to tolerate the effects of an exotic invader, and in particular an invader's novel allelochemistry. 5 Such effects may have long-term implications for plant invasions and the organization of plant communities. Evolved tolerance may ultimately contribute to coexistence among natives and invaders, with our results suggesting that natural plant communities may be more coevolved and less individualistic than currently thought.

Linking patterns and processes in alpine plant communities: a global study

Abstract: 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.

Unpalatable plants protect neighbors from grazing and increase plant community diversity

Abstract: Tasty plants can be protected from herbivores by unpalatable neighbors. We used experimental exclosures, removal of unpalatable species, and transplants of palatable and unpalatable species in subalpine meadows of the Caucasus Mountains of Georgia to study the effects of two unpalatable species on plant communities. We found that Cirsium obalatum and Veratrum lobelianum, two large native perennial herbs that invade after heavy grazing, had strong facilitative effects on communities through their indirect effects on livestock herbivores. These unpalatable invaders had different effects on community composition when livestock were present than when livestock were excluded. Furthermore, removing Cirsium and Veratrum where herbivory was permitted decreased the richness of associated communities, but inside a livestock exclosure removal of these species increased community richness. Transplanted palatable species (Anthoxanthum odoratum and Phleum alpinum) grew larger inside the exclosure, and in the exclosure Cirsium and Veratrum had no effect on their growth. However, outside of the exclosure, Cirsium and Veratrum had strong positive effects on the growth of A. odoratum and P. alpinum. Excluding livestock decreased the growth of Luzula pseudosudetica, another unpalatable species, and Cirsium and Veratrum had no effect on L. pseudosudetica outside the exclosure. In contrast, inside exclosures Cirsium and Veratrum had competitive effects on L. pseudosudetica. Our results indicate that Cirsium and Veratrum, which are in some ways undesirable rangeland weeds, may also play an important role in maintaining species and functional diversity of overgrazed plant communities in the Caucasus.

Insect herbivory stimulates allelopathic exudation by an invasive plant and the suppression of natives

Abstract: Exotic invasive plants are often subjected to attack from imported insects as a method of biological control. A fundamental, but rarely explicitly tested, assumption of biological control is that damaged plants are less fit and compete poorly. In contrast, we find that one of the most destructive invasive plants in North America, Centaurea maculosa, exudes far higher amounts of (±)-catechin, an allelopathic chemical known to have deleterious effects on native plants, when attacked by larvae of two different root boring biocontrol insects and a parasitic fungus. We also demonstrate that C. maculosa plants experimentally attacked by one of these biocontrols exhibit more intense negative effects on natives.

Dual role for an allelochemical: (±)‐catechin from Centaurea maculosa root exudates regulates conspecific seedling establishment

Abstract: Summary 1 Plant allelopathic compounds may have other roles than interspecific interference. We investigated whether ( ± )-catechin, a phytotoxin exuded from the roots of the exotic invader Centaurea maculosa (spotted knapweed), is also one of the factors that regulates C. maculosa recruitment. 2 Adding activated carbon, which adsorbs organic compounds, to soil around C. maculosa adults in the field increased seedling density by 78% 25 days after sowing, and by 34% 32 days after sowing, suggesting that soil-borne compounds inhibited or delayed recruitment. 3 Analysis of field soils near mature C. maculosa revealed that they can contain exceptionally high ( ± )-catechin concentrations (mean = 1.55 mg g − 1 dry soil, with 60% of samples containing ≥ 1.0 mg ( ± )-catechin g − 1 ). 4 In laboratory experiments, treatment with ≥ 1.0 mg ( ± )-catechin mL − 1 reduced seedling root elongation by > 50%, indicating that field concentrations are sufficient to inhibit C. maculosa recruitment. Provided that 10% methanol was used to maintain ( ± )catechin in solution for > 1 day, treatment with ≥ 1.0 mg mL − 1 also reduced C. maculosa germination by > 70%. 5 ( ± )-Catechin maintained in solution with methanol did not significantly reduce C. maculosa seed survival, suggesting that inhibition of germination was due, at least in part, to delayed germination rather than to seed mortality. 6 Depending on the solubility of ( ± )-catechin in soil and on the duration of its effects on recruitment, C. maculosa may avoid intraspecific competition or regulate the timing of seedling establishment by reducing seedling growth or postponing germination in response to its own phytotoxin. 7 Chemical regulation of C. maculosa recruitment, as demonstrated here, suggests a dual role of ( ± )-catechin as an allelochemical and an autoinhibitor. The potential for a single plant root exudate to influence both inter- and intraspecific interactions emphasizes the complex effects that plant secondary metabolites may have on plant population and community structure.

Effects of Acer platanoides invasion on understory plant communities and tree regeneration in the northern Rocky Mountains

Abstract: Quantitative studies are necessary to determine whether invasive plant species displace natives and reduce local biodiversity, or if they increase local biodiversity. Here we describe the effects of invasion by Norway maple Acer platanoides on riparian plant communities and tree regeneration at two different scales (individual tree vs stand scales) in western Montana, USA, using both descriptive and experimental approaches. The three stands differed in community composition with the stand most dominated by A. platanoides invasion being more compositionally homogenous, and less species rich (-67%), species even (-40%), and diverse ( -75%) than the two other stands. This sharp decrease in community richness and diversity of the highly invaded stand, relative to the other stands, corresponded with a 28-fold increase in A. platanoides seedlings and saplings. The dramatic difference between stand 1 vs 2 and 3 suggests that A. platanoides invasion is associated with a dramatic change in community composition and local loss of species diversity; however, other unaccounted for differences between stands may be the cause. These whole-stand correlations were corroborated by community patterns under individual A. platanoides trees in a stand with intermediate levels of patchy invasion. At the scale of individual A. platanoides canopies within a matrix of native trees, diversity and richness of species beneath solitary A. platanoides trees declined as the size of the trees increased. These decreases in native community properties corresponded with an increase in the density of A. platanoides seedlings. The effect of A. platanoides at the stand scale was more dramatic than at the individual canopy scale; however, at this smaller scale we only collected data from the stand with intermediate levels of invasion and not from the stand with high levels of invasion. Transplant experiments with tree seedlings demonstrated that A. platanoides seedlings performed better when grown beneath conspecific canopies than under natives, but Populus and Pinus seedlings performed better when grown beneath Populus canopies, the dominant native. Our results indicate that A. platanoides trees suppress most native species, including the regeneration of the natural canopy dominants, but facilitate conspecifics in their understories.

Effects of Festuca paniculata on the compensatory growth response of Centaurea uniflora in the French Alps

Abstract: The independent effects of herbivores and neighbors on plants are generally negative, and therefore the combined effects of these interactions are generally assumed to have additive or multiplicative negative effects on plant growth. However, because herbivores can stimulate the growth of plants (compensation). and neighbors can facilitate each other, the combined effects of herbivory and plant-plant interactions can be highly variable and poorly predicted by current competition and plant-herbivore theory. In some cases in North America, Festuca species appear to facilitate invasive Centaurea species and enhance their compensatory responses in controlled greenhouse conditions. We explored the interactions between herbivory and neighbor effects in the French Alps by testing the effect of the neighbor, Festuca paniculata L., on the compensatory growth response of defoliated Centaurea uniflora L. over two growing seasons. Seventy percent of aboveground C. uniflora biomass was clipped at each of seven times throughout two growing seasons in the presence or absence of F. paniculata. Centaurea uniflora compensated for severe damage in the first year, but was negatively affected by defoliation in the second year. Defoliating C. uniflora reduced final aboveground biomass by 44% and flower number by 64%, but did not affect survival. Unlike observations for other Centaurea and Festuca species, F. paniculata had significant competitive effects on C. uniflora. Festuca paniculata neither enhanced compensatory responses of C. uniflora nor increased the negative effects of defoliation. Our results show that compensatory responses can weaken over time, but that neighboring plants do not necessarily increase the negative effects of defoliation.