Identification of the selective forces on plant dispersal engenders theoretical argument, empirical study, and speculation. We separate evidence, testable hypotheses, and conjecture surrounding two major questions in dispersal ecology. The first asks what ecological, and ultimately evolutionary, advantages exist in seed dispersal. Astonishingly little is known about the advantages to a parent plant that are actually conferred by investment in dispersal structures. Does dispersal enable seeds and ultimately seedlings to escape mortality near the parent? Is continual recolonization of unstable habitats the primary advantage? Must seeds find rare microhabitats suitable for reestablishment? Such issues are addressed through joint consideration of dispersal and establishment-those stages both mediated by parental provisioning and subject to the highest mortality in the life of a plant. The second broad question asks what general and explicit environmental forces influence the timing and mode of dispersal. Do climates or seasons favor one dispersal mode over another? Do differences in number, size, morphology, or nutritional quality of fruits influence frugivore choice, and consequently differential dispersal of species or individuals within species? Studies of dispersal process and mode should be intimately connected. A general objective of this paper is to explore the degree to which they are integrated and, in so doing, to catalyze their union. We emphasize topics most in need of critical attention: the evolutionary ecology of dispersal process and mode. Excellent recent reviews consider such related topics as dispersal mechanism (131, 184), seed dormancy (1, 30), phytogeography (11, 115, 146), masting and predator satiation (105, 156), and succession (68, 69, 189).

https://www.jstor.org/stable/info/2097067

Ecology of Seed Dispersal

Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by means of mathematical models. These models are very similar,in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some "currency" (Schoener, 1971) -usually energy- and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as "optimal foraging models"; and the theory that embodies them, "optimal foraging theory." The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e., optimal diet); (2) choice of which patch type to feed in (i.e., optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will requre much modification, espicially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated that just energy.

Citation classic - optimal foraging - a selective review of theory and tests

The term phenology is derived from the Greek word phaino meaning to show or to appear. Hence, phenology is defined as the study of the seasonal timing of life cycle events. For plants the seasonal timing of such events can be critical to survival and reproduction. In agriculture the most common failure of introduced crops is the inability to adjust to the seasons imposed by the new, environment (68). In the past few years, interest in the ecology and evolution of timing of life cycle events has grown. Here we review the literature on phenological patterns of germination, flowering, and fruiting (including dispersal).

/pdf/phenological-patterns-of-terrestrial-plants-2339nfk6dp.pdf

Phenological Patterns of Terrestrial Plants

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C 3 species grown in photon irradiances of 200 and 1000 μ mol m - 2 s - 1 . Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high-light-grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light-saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low-light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low-light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2001.00724.x

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

. In the first part of this review, I discuss how we can predict the direct short-term effect of enhanced CO2 on photosynthetic rate in C3 terrestrial plants. To do this, I consider: (1) to what extent enhanced CO2 will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end-product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO2) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO2 concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO2. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long-term response to CO2 may be explained by differences in the sink-source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi-limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short-term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink-source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO2.

Rising Co2 Levels and Their Potential Significance for Carbon Flow in Photosynthetic Cells

Granivores are likely to store food in numerous, widely scattered, small caches if they are unable to defend concentrated large caches against interspecific competitors. This scatterhoarding of seeds makes it impossible for individuals to defend all their scattered caches against intraspecific competitors as well. Optimal spacing of scattered caches should result from a balance between decreasing loss of caches to naive competitors with decreasing density and increasing cost of storage with decreasing density. A mathematical model predicting optimal density is presented. One prediction of the model is that as the (θ) of habitat suitable for seed burial surrounding a seed source is decreased, the average distance (D) a cache is taken from the source by scatterhoarders should increase. Another prediction is that increasing the number of seeds (N) at a source, either by an increase in the size of a single seed crop or by the presence of 2 or more conspecific seed producers in close proximity to one another, should increase D. One trait of trees which increases single crop sizes and average seed dispersal distance (D) is the pattern of withholding energy from reproduction some years to allow unusually large crops during mast years. Three field tests of the model and its predictions were conducted: (1) Juglans nigra seeds were buried at 3 densities and their survival in time from predation by Sciurus niger was found to increase with decreasing density; (2) average distances that J. nigra saplings occurred from 16 parent walnut trees were found to have a statistically significant correlation with the D calculated from the model and the θ of suitable habitat surrounding each tree; (3) Sciurus niger individuals were observed to scatterhoard Juglans nigra seeds in a pattern that maintained a mean cache density that gives low rates of loss to naive competitors as indicated from field test (1). The results strongly suggest maintenance of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed.

A Model for Seed Scatterhoarding: Coevolution of Fox Squirrels and Black Walnuts

Seasonal patterns of insect damage to reproductive tissue of the legume Baptisia australis were studied for three years in native tallgrass priairie. Contrasting seasonal patterns of damage were associated with the major species of insect consumers. The moth Grapholitha tristegana (Olethreutidae) and the weevil Tychius sordidus (Curculionidae), which together infested 80-100% of developing fruits (pods), consistently damaged more seeds on average in early than in late maturing pods. But while late opening flowers were less subject to attack from moths and weevils, they were more subject to attack from chewing insects, particularly blister beetles (Epicauta fabricii, Meloidae), which destroyed >80% of all flowers and developing young pods (including moth and weevil larval inhabitants). The blister beetles arrived late in the flowering season and fed particularly on young reproductive tissue, allowing larger, older pods that had developed from early opening flowers to escape destruction. The relative abundances and impacts of blister beetles, moths, and weevils varied from year to year. Adding to the uncertainty of reproductive success of the host plant were the large and variable amounts of damage to immature buds inflicted by insects (including the blister beetles and weevil adults) and late killing frosts. Thus, timing of flowering is critical to success in seed production for B. australis. The heavy impacts of insects and weather can result in a very narrow window in time (which shifts from year to year) during which B. australis can flower with any success. The opposing pressures exerted by insects and weather on floral reproductive success may act in concert with other features of the plant's biology to foster the maintenance of considerable diversity in flowering times among individuals in local populations of B. australis.

Timing of reproduction in a prairie legume: seasonal impacts of insects consuming flowers and seeds

Gray and fox squirrels show similar preferences for various types of natural foods. Their preferences are based on a combination of two factors; the speed with which they can ingest food energy and the digestibility of the food eaten. The two species of squirrels have essentially the same ability to digest their natural foods. However, five species of nuts differ in their digestibility, ranging in percentage of energy assimilated from about 78% for white oak to 95% in shagbark hickory. The squirrels' efficiency in digesting the kernels of different species of nuts is correlated with the lipid content of the kernels. The niche differences between gray and fox squirrels are not based on food preference or feeding efficiency. Instead, they are probably related to differences in foraging behavior and predator escape behavior which adapt fox squirrels to open forests and forest edges and gray squirrels to dense forests. The annual activity pattern of gray and fox squirrels make the hard—shelled nuts of hickory and walnut the most efficient food in fall and spring and acorns the most efficient food in winter. This change in the relative efficiency of foods results in the squirrels acting as dispersing agents for the seeds of both oaks and hickories in mixed stands of trees.

Food Preferences of Squirrels

We tested our earlier prediction that the food content of buried nuts will influence the density at which fox squirrels (Sciurus niger) scatterhoard each species of nut. We buried black walnuts (Juglans nigra), bur oak acorns (Quercus macrocarpa), and chinquapin oak acorns (Q. muehlenbergii) at varying densities in monospecific and mixed-species grids in three different years and three different seasons within a year, and analyzed their survival from predation over time. We also observed free- ranging squirrels scatterhoard walnuts and bur oak acorns, which we made available in equal numbers. The survival of buried nuts was dependent on the nut species, nut density, and squirrel density. The lowest meaningful density of buried nuts for squirrels was related to the food content of the nut species. Analysis of nut survival at the same burial sites in successive seasons indicated that squirrels did not use memory to return to specific sites where they had previously found nuts. Rather, they foraged preferentially in certain microhabitats. Squirrels were observed burying black walnuts farther away from a source pile than they buried bur oak acorns. The difference is consistent with the difference in food content between the two species and also with the threshhold density below which naive squirrels have no density-dependent effect on buried nuts. Our data on nut survival suggest that squirrel foraging varies with nut density but not with nut spacing.

Density-Dependent Survival of Scatterhoarded Nuts: An Experimental Appoach

Two Scheelea palm populations in Central America were studied: Scheelea rostrata in Costa Rica and Scheelea zonensis in the Panama Canal Zone. Most of the fruits in both populations contained one seed, but small proportions of the fruits on each tree contained two or three seeds. Compared to single-seeded fruits, seedlings from multiseeded fruits are apparently disadvantaged because of less endosperm and competition among seedlings emerging from the same indehiscent fruit. However, since seed predation of Scheelea is heavy, we hypothesized that differential seed predation may counteract this selection pressure against the production of multiseeded fruits and thus maintain multiseeded fruits in Scheelea populations. At least 61% of the fruits in the Costa Rica population and 81% of the fruits in the Canal Zone population were attacked by seed predators. Beetles (Family: Bruchidae) were the predominant preda- tors in the Costa Rica population (84% of attacked fruits), whereas rodents predominated in the Canal Zone population (78% of attacked fruits). Attack by rodents was independent of seed number, but data from Costa Rica suggested that bruchids concentrated predation slightly on one-seeded fruits. Because seed predators were often restricted to one seed, seed survivorship in attacked fruits was directly related to seed number (0%o, 22%, and 41% for attacked 1-, 2-, and 3-seeded fruits, respectively, in the Costa Rica population). As a result of an interaction of these aspects of predation, the proportion of seeds that escaped predation was also directly related to seed number (39wo, 54%, and 60%o for 1-, 2-, and 3-seeded fruits, respectively, in the Costa Rica population). The major difference in predation between the two populations appeared to be the predominance of bruchid predation in the Costa Rica population, which allowed a greater survivorship in attacked multiseeded fruits (29Wo survivorship of bruchid attack versus 12% survivorship of rodent attack). This apparent stronger selection pressure for producing multiseeded fruits coincided with a significantly greater proportion of multiseeded fruits in the Costa Rica population (27%) compared to the Canal Zone population (6%). These findings support the hypothesis that seed predation is a significant factor in maintaining multiseeded fruits in Scheelea populations.

https://www.jstor.org/stable/pdfplus/1939017.pdf

Christopher C. Smith

Papers

A Model for Seed Scatterhoarding: Coevolution of Fox Squirrels and Black Walnuts

Timing of reproduction in a prairie legume: seasonal impacts of insects consuming flowers and seeds

Food Preferences of Squirrels

Density-Dependent Survival of Scatterhoarded Nuts: An Experimental Appoach

Seed Predation and Seed Number in Scheelea Palm Fruits