Top-down Causation Without Top-down Causes

We argue that intelligible appeals to interlevel causes (top-down and bottom-up) can be understood, without remainder, as appeals to mechanistically mediated effects. Mechanistically mediated effects are hybrids of causal and constitutive relations, where the causal relations are exclusively intralevel. The idea of causation would have to stretch to the breaking point to accommodate interlevel causes. The notion of a mechanistically mediated effect is preferable because it can do all of the required work without appealing to mysterious interlevel causes. When interlevel causes can be translated into mechanistically mediated effects, the posited relationship is intelligible and should raise no special philosophical objections. When they cannot, they are suspect.     

Emergent properties and the context objection to reduction

Reductionism is a central issue in the philosophy of biology. One common objection to reduction is that molecular explanation requires reference to higher-level properties, which I refer to as the context objection. I respond to this objection by arguing that a well-articulated notion of a mechanism and what I term mechanism extension enables one to accommodate the context-dependence of biological processes within a reductive explanation. The existence of emergent features in the context could be raised as an objection to the possibility of reduction via this strategy. I argue that this objection can be overcome by showing that there is no tenable argument for the existence of emergent properties that are not susceptible to a reductive explanation.     

How parasitism affects critical patch-size in a host-parasitoid model: application to the forest tent caterpillar

Habitat structure has broad impacts on many biological systems. In particular, habitat fragmentation can increase the probability of species extinction and on the other hand it can lead to population outbreaks in response to a decline in natural enemies. An extreme consequence of fragmentation is the isolation of small regions of suitable habitat surrounded by a large region of hostile matrix. This scenario can be interpreted as a critical patch-size problem, well studied in a continuous time framework, but relatively new to discrete time models. In this paper we present an integrodifference host-parasitoid model, discrete in time and continuous in space, to study how the critical habitat-size necessary for parasitoid survival changes in response to parasitoid life history traits, such as emergence time. We show that early emerging parasitoids may be able to persist in smaller habitats than late emerging species. The model predicts that these early emerging parasitoids lead to more severe host outbreaks. We hypothesise that promoting efficient late emerging parasitoids may be key in reducing outbreak severity, an approach requiring large continuous regions of suitable habitat. We parameterise the model for the host species of the forest tent caterpillar Malacosoma disstria Hbn., a pest insect for which fragmented landscape increases the severity of outbreaks. This host is known to have several parasitoids, due to paucity of data and as a first step in the modelling we consider a single generic parasitoid. The model findings are related to observations of the forest tent caterpillar offering insight into this host-parasitoid response to habitat structure.     

Influence of synchronization between adult emergence and host plant phenology on the population density of Pseudasphondylia neolitseae (Diptera: Cecidomyiidae) inducing leaf galls on Neolitsea sericea (Lauraceae)

Synchronization between the appearance of herbivorous insects and their host-plant phenology is a critical event, especially for short-lived insects such as gall midges. We studied a natural population of Pseudasphondylia neolitseae (Diptera: Cecidomyiidae) that induces leaf galls on Neolitsea sericea (Lauraceae) to evaluate the effect of synchronization on gall density in the subsequent generation. To do so, we combined quantitative data on host resources with time lag between emergence and host-available seasons. The gamma distribution model was applied to the emergence curve of P. neolitseae and the normal distribution model to the daily changes in the number of host buds suitable for oviposition; the latter model was transformed into an available-resource curve based on the mean number of host buds required for a single female to realize her eggs. By superimposing the emergence curve on the available-resource curve and calculating overlapped area, the degree of synchronization was evaluated more accurately than previous studies, which had treated only the time lag. The number of females that synchronized with host buds affected gall density in the next generation.     

Emergence and resilience of cooperation in the spatial prisoner's dilemma via a reward mechanism

We study the problem of the emergence of cooperation in the spatial Prisoner's Dilemma. The pioneering work by Nowak and May [1992. Evolutionary games and spatial chaos. Nature 415, 424-426] showed that large initial populations of cooperators can survive and sustain cooperation in a square lattice with imitate-the-best evolutionary dynamics. We revisit this problem in a cost-benefit formulation suitable for a number of biological applications. We show that if a fixed-amount reward is established for cooperators to share, a single cooperator can invade a population of defectors and form structures that are resilient to re-invasion even if the reward mechanism is turned off. We discuss analytically the case of the invasion by a single cooperator and present agent-based simulations for small initial fractions of cooperators. Large cooperation levels, in the sustainability range, are found. In the conclusions we discuss possible applications of this model as well as its connections with other mechanisms proposed to promote the emergence of cooperation.     

On the analysis of competition at the level of the individual plant

Summary The extent to which some measure of local crowding can account for the performance of individual plants is examined with reference to populations of two species of annual plant. Only a relatively small proportion of the variation in individual plant yield could be accounted for by measures of local crowding. These included the number of close neighbours, an estimate of the area available to each plant and competitive pressure. A multiple regression that took account of both emergence time and local crowding increased the proportion of variance that could be accounted for up to 50%. Computer simulations of the growth of indivudual plants in monoculture were then caried out in order to determine whether the unexplained variation resulted from fundamental flaws in the models or from unaccounted for sources of variation in the field. The results from the simulations again indicated that only a relatively low proportion of the variation in individual plant yield could be accounted for by emergence time and local density, even though these were known to be the only variables present. These findings are discussed in relation to the relative importance of one-sided and two-sided competition, and the complex cross-correlations that occur between individuals in plant populations. These two factors will make it very difficult for field workers to determine accurately what factors determine individual plant yield and in particular to predict the effects of local crowding on the performance of individual plants.     

Effect of density on magnitude of directional selection on seed mass and emergence time in Plantago wrightiana Dcne. (Plantaginaceae)

Although population density is believed to be an important factor influencing evolutionary processes, surprisingly few studies have documented the existence or nature of density-dependent selection. We quantified the effects of density on directional selection on seed mass (the mass of a sown seed) and emergence time in the greenhouse and field for the annual plant Plantago wrightiana. In the greenhouse, we quantified selection on seed mass and emergence time at each of five planting densities (1 m^–2 to 10,000 m^–2) using the relationship between final plant mass and each trait at each density. We observed no significant selection on either seed mass or emergence time when plants were grown alone. At all higher densities, there was significant selection favoring early emergence and large seed mass, but there were no significant differences among the selection gradients determined at densities greater than individually grown plants. In the field, we detected no relationship between the magnitude of selection for early emergence and density. Our results suggest that selection on seed mass and time of emergence is density dependent, but the relationship between density and the magnitude of directional selection on these traits is not continuously increasing. Over broad ranges of density in the greenhouse and in the field, there was no detectable relationship between density and the magnitude of directional selection.     

香港大蹄蝠出飞时间及其季节变化

于2014年5月至2015年4月,对香港低海拔栖息地的大蹄蝠夜晚出飞活动时间进行了研究。结果显示大蹄蝠出飞时间平均为日落后（14.6±6.1) min,出飞时间与民用曙暮光呈强正相关（r = 0.968, P < 0.0001）,而出飞结束时刻同样与民用曙暮光时间呈现显着相关(r = 0. 977, P < 0. 0001)。。在一年中较冷的12月和1月则没有蝙蝠出飞记录。大蹄蝠虽然在亚洲分布广泛,但是相对于温带地区的蝙蝠物种来说, 关于该物种的基础生态学研究较少。本文对大蹄蝠的季节性出飞行为所开展研究工作,以利有关长远保育的深入研究。     

Diffuse knapweed (Centaurea diffusa Lam.) seedling emergence and establishment in a Colorado grassland

Knapweeds (Centaurea spp.) are damaging invaders of grasslands and other North American rangelands. A field study was conducted to determine conditions that promote diffuse knapweed (C. diffusa) emergence and establishment in a native Colorado grassland (North America). Knapweed was planted in native grassland under treatments with different opening sizes, levels of competition, knapweed seed burial and season of seeding. There was no effect of opening size where competing natives were alive, but knapweed emergence in 5- and 15-cm openings was higher than 0-cm openings where natives were killed. Reducing competition reduced fall diffuse knapweed emergence, but did not affect spring emergence. Seed burial increased knapweed emergence, but the effect varied by season. Although diffuse knapweed emergence reached 35%, only four plants survived from 3,600 seeds. This native grassland did not prevent knapweed emergence or establishment, but both were so low that rapid knapweed invasion is unlikely.