Johannes Järemo

Plant compensatory growth : herbivory or competition?

The presence of an overcompensatory response to damage in some plant species has recently created a debate concerning whether this trait is an adaptation to herbivory, or simply a physiological consequence of adaptations to competition for light. According to the latter hypothesis, competition for light favors fast vertical growth and strong apical dominance. The removal of apical dominance by damaging the primary shoot allows the growth of secondary shoots and hence increases productivity. We compare predictions of these two hypotheses in a model-system where plants are exposed to both a risk of damage and a risk of competition. Compensatory seed production is assumed to depend on the number of dormant buds that can be activated by damage, and on the seed production of surviving shoots. In accordance with earlier theoretical analyses, we expect that intensive herbivory can favor overcompensatory seed production. In contrast, competition for light should at best lead to exact compensation when the competitive environment remains unchanged. Competition acts against overcompensation for two reasons. First, competitive plants should have poor resource reserves to support compensatory growth. Second, competition for light is assumed to favor unbranched architecture and thus. activation of many secondary shoots should not increase the seed yield. However, we cannot exclude the possibility that plants adapted to competition may overcompensate when grown singly. In spite of this caveat, it is likely that overcompensation requires damage related adaptations that may evolve only under intensive and relatively predictable risk of damage.

Plant Adaptations to Herbivory: Mutualistic versus Antagonistic Coevolution

The discovery of overcompensatory responses to damage in some plant species has inspired attempts to classify some plant-herbivore interactions as mutualism. Since the debates over plant-herbivore mutualism and overcompensation have been intense, we attempt to outline three conceptual models of plant-animal interactions and evaluate the status of interactions with help of three different fitness criteria: relative fitness, absolute fitness, and mean absolute fitness. Our three plant-animal interactions are assumed to represent plant-pollinator mutualism, plant-herbivore antagonism and the evolution of overcompensation, respectively. Each case describes how absolute fitness, and consequently also the other two fitness criteria, is assumed to change with animal encounters for plants with special adaptations to cope with those encounters and for plants with no such adaptations. As a result, all these types of interactions may be considered as mutualism when taken relative fitness only into account. Obviously, this criterion is too weak because any trait, evolving under natural selection, should improve fitness relative to other, alternative traits. Absolute fitness increases with animal encounters for the adapted phenotype in the first and in the last case and thus, in the context of absolute fitness, overcompensation in plants would indicate plant-herbivore mutualism. However, absolute fitness as such may not be sufficient when discussing the evolutionary history of plant-animal mutualism. In the light of mean fitness, overcompensation as presented in earlier studies does not represent mutualism between plants and herbivores. Mean absolute fitness in plant populations decrease with the risk of herbivore attack in our model of overcompensation, while the reverse trend characterises our plant-pollinator model. We, therefore, suggest that mean absolute fitness may well provide an appropriate criterion for distinguishing mutualistic and antagonistic plant-animal interactions in coevolutionary contexts. In order to evaluate our model-system, we compare the predicted patterns with empirical data on the grassland biennial Gentianella campestris.

Endemism Predicts Intrinsic Vulnerability to Nonindigenous Species on Islands

While numerous efforts have been made to identify and quantify factors controlling invasibility of biological communities, less attention has been given to analyzing the expressions of vulnerability to nonindigenous species (NIS). Using the International Union for the Conservation of Nature and Natural Resources Red List database for birds, mammals, and amphibians and the Invasive Species Specialist Group global invasive species database as sources of information, we developed a new indicator for the relative intrinsic vulnerability of islands to NIS. It was calculated from the residuals to the global relationship between the impact of NIS and their exposure to the islands. The impact of NIS was expressed as the proportion of indigenous species threatened by NIS, and the exposure was the number of invasive NIS per number of native species. The residuals corresponded to the variability in impact, about 60%, that was not explained by exposure. The proportion of endemic species on the islands was positively correlated with the relative intrinsic vulnerability and explained about 60% of its variability. The robust relationship between endemism and intrinsic vulnerability reinforces the role of long‐term isolation for the fate of island indigenous species to biological invasions and is useful in identifying vulnerable environments without having a specific invader in mind.

Reproductive effort tactics : balancing pre- and postbreeding costs of reproduction

Costs of reproduction are most frequently evaluated in terms of postbreeding survival and fecundity costs. Such demographic costs are expected to follow as the female drains her somatic resources into reproduction. However, some reproductive tactics may lead to costs of reproduction that are expressed in terms of prebreeding survival. We propose an optimality model where total absolute effort may originate from two different components. The first component measures the amount of resources that the female accumulates for reproduction during the prebreeding period. The second component represents the amount of resources drained from somatic demands relative to non-reproductive individuals. While our model allows both components to imply pre-and postbreeding survival costs, we mainly focus on the case where accumulation effort implies costs on prebreeding survival and somatic effort implies costs on postbreeding survival. Effective fecundity is assumed to be a function of both components. We consider accumulation and somatic effort as alternative options of an organisms reproductive effort tactic, and solve for optimal tactics by maximizing fitness over a single breeding season, assuming a constant total investment in reproduction. The present analysis suggests that the evolution of accumulation effort requires that marginal prebreeding costs due to accumulated resources remain low relative to marginal postbreeding costs implied by somatic effort. When the total investment in reproduction increases, optimal somatic effort increases relative to optimal accumulation effort. Our analysis demonstrates that natural selection may well favour different effort tactics satisfying the energy demands of reproduction, some of which may involve optimization of the balance between pre- and postbreeding costs of reproduction. For organisms relying on tactics implying prebreeding costs, empirical studies monitoring postbreeding survival only may not reveal the major costs of reproduction.

Plant compensatory growth: a conquering strategy in plant-herbivore interactions ?

We present a theoretical analysis that considers the phenotypic trait of compensatory growth ability in a context of population dynamics. Our model depicts a system of three interactors: herbivores and two different plant types referred to as ordinary and compensating. The compensating plant type has the ability to increase its intrinsic rate of biomass increase as a response to damage. This compensatory growth ability is maintained at the expense of a reduced growth rate in the absence of damage, where the ordinary plant type has the higher growth rate. Analysis of this system suggests that, even though a compensatory capacity of this kind will not imply an increase in equilibrium plant density, it will give a competitive advantage in relation to other plants, in the presence of a sufficiently efficient herbivore. Invasion of compensating plants into a population of non-compensating plants is facilitated by a high compensatory growth ability and a high intrinsic rate of plant biomass increase. Conversely, an ordinary plant can invade and outcompete a compensating plant when the herbivore is characterised by a relatively low attack rate, and/or when plant intrinsic growth rate is decreased.

On the consequences of pre- and postbreeding costs in the evolution of reproductive effort tactics

Abstract:Models of reproductive effort have generally concentrated on the evolution of total reproductive effort, without considering the option for specific components of effort to evolve. Further, most analyses have assumed postbreeding costs of reproduction. We present an optimization model that evaluates the option for components of effort with different temporal distributions of survival costs, affecting either adult survival before offspring independence (prebreeding cost) or survival after offspring independence (postbreeding costs). We assume that total absolute effort is fixed, and that the two components therefore, are constrained in their variation. The results show that, in most cases, optimal effort tactics that imply prebreeding costs must have lower marginal effects on survival than tactics that imply postbreeding costs. Our analysis also shows that the marginal benefits on effective fecundity of effort components may influence the optimal effort tactics. An age-structured version of the mo...

Associations of invasive alien species and other threats to IUCN Red List species (Chordata: vertebrates)

Apart from acting synergistically or additively, threats to species may be associated or disassociated. Here we link global data on threatened Chordata species, mainly birds, mammals, and amphibians, with a probabilistic methodology to test whether the impact from invasive alien species co-occurs purely randomly, associated, or disassociated with impact from nine other major threats to biodiversity listed in the International Union for Conservation of Nature Red List database. Impacts from several of the other threats, in particular from natural disasters, are associated with the impact from invasive alien species. Three of the threats of anthropogenic origin, namely habitat loss, harvesting, and human disturbance, co-occur randomly with impact from invaders, and we suggest several explanations to this unexpected relationship, such as ambiguous evidence for associations between them and human-induced disturbances. Impact from invasive alien predators has a strong association with impact from native predators, indicating that similarity in autecology affects co-occurrences between threats. The threat from invasive predators is disassociated from intrinsic factors on islands, probably because species suffering from for instance inbreeding problems have low densities and rarely encounter invasive alien predators. The analysis of co-occurrence of impact from invasive alien species and other threats is a first step to understand and mitigate vulnerability of a community to the simultaneous exposure to invasive alien species and other threats. Association or disassociation between threats may depend on correlations between exposures and sensitivity to the threats or on the presence of one threat increasing or decreasing the sensitivity to another.