Colleen K. Kelly

On the Adaptive Value of Physiological Integraton in Colonal Plants

In clonal plants, genetically identical ramets arise from a common stolon or rhizome. Anatomical connection often allows physiological integration, the translocation of resources from a larger mother ramet to a developing daughter ramet. Translocation of a limiting resource can reduce the mothers growth while increasing the daughters growth. Our models predict patterns in resource translocation; the models assume that fitness increases with the expected biomass a genet attains over a season of vegetative growth in a stochastic environment. In each model a ramets growth depends nonlinearly on its level of a limiting resource. If resource availability varies both spatially and temporally, and a ramets growth does not depend on its size, an analytical approximation for total genet growth leads to several new predictions. If a ramets growth increases as a concave function (i.e., a function with decreasing positive slope) of resources level, physiological integration should increase when spatial variance increases and spatial convariance is negative. If a ramets growth increases as a convex function (increasing positive slope) of resource level (the less likely case), spatial variance—convariance in resource availability has the opposite effect on translocation. Independently of the concavity or convexity of the growth function, increasing temporal variance in the mother ramets resource availability reduces translocation, and increasing temporal variance in the daughter ramets resource availability increases translocation. When a ramets growth increases with both its resource level and its size, translocation and growth in one time interval influence the value of future physiological integration. For this case a stochastic dynamic programming model demonstrates how translocation can depend on time and the sizes of the mother and daughter ramets, as well as on spatiotemporal resource variability. The predictions qualitatively match those deduced from the model of size—independent growth, although translocation often declines late in the season of vegetative growth. The dynamic model also indicates that a large mother ramet should always share resource with a daughter ramet. But a smaller mother ramet should often abandon a daughter and allocate all its available resource to its own growth.

Plant Foraging: A Marginal Value Model and Coiling Response in Cuscuta Subinclusa

Stems of the parasitic plant Cuscuta subinclusa (Convolvulaceae) discrim- inate among host species and invest in resource acquisition (coil around the stem of a host plant) relative to host quality in a way predicted by a marginal value model of resource use. C. subinclusa can be said to forage in that: (1) stem coiling, the necessary antecedent and determinant of resource uptake, precedes exploitation of host materials; and (2) mean coiling on a host species is proportional to mean reward per unit of investment (measured as biomass accumulation per unit length of stem in coil). Preliminary evidence indicates that coiling in C. subinclusa is induced by host bark flavonoids. Larger parasites are more likely to overwinter and set seed a second season than smaller ones, and parasites that start from overwintered tissue are significantly larger at flowering than are those that have started from seed. Thus, seed set is correlated with parasite size at the end of the season, linking the foraging response and fitness of the plant.

PHYLOGENY, NICHES, AND RELATIVE ABUNDANCE IN NATURAL COMMUNITIES

Community structure refers to the number of species in a community and the pattern of distribution of individuals among those species. We use a novel way of representing community structure to show that abundance within closely related pairs of co-occurring tree species in a highly diverse Mexican forest is more equitable than is abundance within more distantly related pairs. This observation is at odds with the fundamental assumption of neutral models of community structure, i.e., that species are interchangeable. The observed patterns suggest niche apportionment, in which interaction is focused pairwise between congeners but falls away from the phylogenetic structure above the genus level. Thus niche processes may significantly affect community structure through regulating relative abundance in a substantial proportion of species, which in turn potentially enhances community stability. One such mechanism of stable coexistence has already been shown to be active in this forest.

Plant competition, temporal niches and implications for productivity and adaptability to climate change in water‐limited environments

Summary We review plant competition in water-limited environments with focus on temporal niche dynamics and examine implications for diversity–productivity relationships and the response of ecosystem productivity to changes in water availability. The main theses under examination are that (i) plant functional types (PFTs) have distinct resource pulse use and coexist through mechanism of temporal resource use complementarity; and (ii) species of same PFT (functionally redundant species) coexist through distinct recruitment niches. In water-limited systems, opportunities for plant resource uptake and growth fluctuate through time, dependent on precipitation patterns. Species differ in the sensitivities of germination, seedling mortality and adult productivity to pulses of water availability, and this generates opportunity for temporal niche diversification. We illustrate this in two case studies. Case study I. Savannas: This is an example of niche separation between two distinct plant functional types (PFTs), trees and grasses. Several models suggest that the two PFTs have complementary resource pulse use, which regulates their abundances, but other models suggest that tree abundance is regulated by the narrow recruitment niche of trees. Overly restrictive recruitment niches can cause a mismatch between resource availability, PFT composition and ecosystem productivity. Case study II. The tropical dry forest: Here, we examine niche separation between closely related species of same PFT. These species commonly have distinct temporal recruitment niches based on differences in seed and seedling traits. A diversification of recruitment niches may be necessary for sympatric speciation and has the effect of broadening of the recruitment ‘portfolio’ of a phylogenetic lineage and PFT. Synthesis: Functional diversity, characterized by differences in adult resource use, optimizes ecosystem function in a pulsed resource environment only if PFT abundances are regulated by adult resource use. Regulation through recruitment niches tends to uncouple plant productivity from resource availability. However, we hypothesize that a diversification of recruitment niches within PFTs may help alleviate recruitment limitations and help communities attain a PFT composition that optimizes resource use and permits adaptation to climate change.

An analytical model assessing the potential threat to natural habitats from insect resistance transgenes

We examine the role of ecological interactions on effective gene flow from genetically manipulated plants to their wild relatives. We do so by constructing and applying to oilseed rape (OSR) an analytical model for interaction between plants with and without an insect resistance (IR) allele in natural communities, incorporating documented levels of herbivore variability. We find that with reasonable values of advantage to the IR allele, little concomitant disadvantage (physiological costs of the allele) restricts it to low proportions of the natural population for large numbers of generations. We conclude that OSR IR transgenes are unlikely to pose an immediate threat to natural communities. Our model identifies those factors best able to regulate particular transgenes at the population level, the most effective being impaired viability of seeds in the period between production and the following growing season, although other possibilities exist. Because solutions rely on ratios, limiting values of regulating factors are testable under controlled conditions, minimizing risk of release into the environment and offering significant advancement on existing testing programmes. Our model addresses folivory but is easily modified for herbivory damaging the seed or directly affecting seed production by infested plants, or for pathogens altering seed survival in the seedbank.

Have clonal plant biologists got it wrong? The case for changing the emphasis to disintegration

The paucity of knowledge addressing the role of fragmentation in clonal plant ecology emphasizes the need for plant biologists to focus attention onto the ecological and evolutionary contexts of fragmentation as a trait in its own right, not merely the absence of integration.

On the economics of plant growth: Stolon length and ramet initiation in the parasitic clonal plantCuscuta europaea

SummaryPrevious studies have suggested (1) that prior expenditure determines continued stolon extension, resulting in the hypothesis that inter-ramet distance (the direct consequence of stolon extension rule) should be correlated with stolon volume or (2) that inter-ramet distance will be shorter under resource-rich conditions, in order to better exploit a rich resource patch. Contradicting both of these hypotheses and consistent with the prediction of an animal foraging model of prey choice, stolon diameter of the parasitic clonal plantCuscuta europaea is negatively correlated with ramet initiation; neither stolon length nor volume significantly predict ramet initiation. InC. europaea, stolon diameter is an indicator of resources available to the outgrowing stolon. The greater probability of ramet initiation for resource-poor stolons ofC. europaea means that ramets will have shorter stolons (inter-ramet distances) in poorer patches. In contrast, the only formal models previously proposed have assumed that resource availability is negatively associated with ramet length generally, even in the face of contradictory evidence. Inspection of the literature shows marked interspecific variation in stolon response patterns to resource enrichment and I suggest that understanding (as opposed to intraspecific prediction) of the response pattern inC. europaea requires understanding of the causes underlying the observed interspecific variation. I postulate that likely causes are variation among species in sun/shade adaptation, commonly encountered resource patch size and/or pressures of herbivory on ramet aggregation.

Temporal niche dynamics, relative abundance and phylogenetic signal of coexisting species

The coexistence of similar species accounts for some 30% of diversity within communities, yet the coexistence and relative abundance of similar species is a continuing ecological conundrum. Using close phylogenetic relatedness as a measure of similarity, we previously demonstrated that neither classic niche theory nor neutral theory can explain the relative abundances of co-occurring pairs of similar tree species in a diverse tropical forest. Here, we show that the stable, focused competition of a temporal niche dynamic fits the distribution of observed fractional abundances (pairwise relative abundances). Previously published, independent evidence of temporal dynamics in this community supports our results; our model identifies additional criteria for field tests of differential sensitivity (DS) temporal dynamics. The success of temporal dynamics at explaining the observed distribution—and the failure of alternative hypotheses to do so—indicates that current diagnostics of community structure and assembly needs general re-examination.

Comparison of population structures and ecology of a congeneric pair of common and rare neotropical tree species

We compared the general ecology and population profiles of Jatropha standleyi and J. chamelensis, two ecologically similar, dioecious forest trees in order to illuminate why they differ in range size, an important measure of relative commonness and rarity. Jatropha standleyi is widespread throughout the tropical deciduous forest in Mexico and the endangered J. chamelensis is restricted to a small region on the Pacific coast of Mexico. Using data collected over the same time period from a site central to the distributions of both species, we found significant differences in population size distributions and seedling mortality, but no discernible differences in seedling or adult growth rates, local abundance or local distribution. The observed difference in relative rarity could be explained by a number of hypotheses depending on whether the populations are at a stable equilibrium or not. The high ratio of pre-reproductive individuals may indicate a growing, rather than stable, population for J. chamelensis. Jatropha standleyi is more widespread and has a significantly lower ratio of pre-reproductive to mature individuals. Alternatively, both populations may be at equilibrium with J. chamelensis restricted to its limited range by large-scale environmental constraints or niche differences not elucidated in this study. In either case however, the greater and less variable seed output of the J. standleyi population will also mean a greater number of rare long-distance dispersal events and a better chance of establishing a persistent remote population for J. standleyi than for J. chamelensis. The dioecy to be found in both these species would exacerbate the limiting effects of low seed production on the ability of a species to ‘travel’ successfully. A more extensive field investigation of mortality and factors contributing to mortality in sub-canopy and pre-reproductive individuals would allow more light to be shed on relative population processes in these species.

Temporal dynamics and ecological process

1. Introduction Colleen K. Kelly, Michael G. Bowler and Gordon A. Fox Part I. Observing Temporal Processes in Nature: 2. The storage effect: definition and tests in two plant communities Peter Chesson, Nancy J. Huntly, Stephen H. Roxburgh, Marissa Pantastico-Caldas and Jose M. Facelli 3. What temporal processes in trees tell us about competition, community structure and speciation Colleen K. Kelly, Michael G. Bowler, Gordon A. Fox, J. Arturo Solis-Magallanes, J. Marcela Ramos-Tapia, Pilar Lopera Blair, Susan Schwinning, John N. Williams and Jeffrey B. Joy 4. Testing the storage effect with long-term, observational data Peter B. Adler 5. Seedling herbivory and the temporal niche Mick E. Hanley and Rebecca J. Sykes 6. Temporal variation in density dependence in an herbaceous community Norma L. Fowler and Craig M. Pease 7. Population and community dynamics in variable environments: the desert annual system D. Lawrence Venable and Sarah Kimball 8. Temporal niches, ecosystem function, and climate change Susanne Schwinning, Gordon A. Fox and Colleen K. Kelly Part II. Application to Specific Questions: 9. Evolution of synchronized and intermittent reproduction (masting) of trees: key role of regeneration dynamics Yoh Iwasa, Akiko Satake and Yuuya Tachiki 10. Spatiotemporal variation can promote coexistence more strongly than temporal variation Robin E. Snyder 11. Roles of pollinator attraction and environmental fluctuation in inducing flowering synchrony Akiko Satake, Yuuya Tachiki and Yoh Iwasa 12. Temporal dynamics and the spread of insect resistance transgenes Michael G. Bowler, Felix Breden and Colleen K. Kelly 13. Concluding remarks Gordon A. Fox, Michael G. Bowler and Colleen K. Kelly Index.