Dispersal as escape and discovery

Dispersal is the term applied to the process by which individuals escape from the immediate environment of their parents and neighbours, and become less aggre­gated; dispersal may therefore relieve local congestion. But, dispersal can also often involve a large element of discovery. A useful distinction can be made between two types of such 'discovery dispersal'.   First, there is dispersal in which individuals visit and 'explore' a large number of sites before returning and settling in a chosen one. Second, there is dispersal in which individuals visit a succession of locations, but then cease to move (with no element of 'return' to a site previously explored). In fact, this latter category can be split further into cases where the cessation of movement is under the dispersing organism's control, and cases where it is not.

The dispersal of plant seeds is non-exploratory and beyond the control of the seed itself. The discovery aspect of seed dispersal is therefore a matter of chance (although the chances of reaching a suitable site may be increased by the specializations for dispersal that the seeds possess). Animal dispersal, on the other hand, can fall into any of the three categories. Some animals have essentially the same type of dispersal as plant seeds. Many other animals cannot be said to explore, but they certainly control their settlement, and cease movement only when an acceptable site has been found. For example, most aphids, even in their winged form, have powers of flight which are too weak to counteract the forces of prevailing winds. But, they control their take-off from their site of origin, they control when they drop out of the windstream and they make additional, often small-scale flights if their original site of settlement is unsatisfactory. Their dispersal, therefore, involves ‘discovery’, over which they have some, albeit limited, control.

All species disperse, but some are more dispersive than others. Insects living in habitats tat are, by nature, temporary have a more pronounced dispersive phase than insects living in more permanent habitats. In general, dispersal is essential for the persistence of species that exploit temporary stages in a changing community. The descendants of individuals of all successional species are doomed in their local habitats. Yet, even the species of so-called ‘climax’ communities (the relatively stable endpoints of successions) are doomed in the long run unless they colonize new areas.The movements of forests following the advance and retreat of ice-sheets, or of tropical forests following arid periods, are on a different time scale to that usually associated with the dispersal of organisms. However, they make the same point, that in the life of all terrestrial organisms home is sooner or later a dangerous place.

 

 

Dormancy: dispersal in time

 

An organism gains in fitness by dispersing its progeny, as long as the progeny are more likely to leave descendants than if they remained undispersed. Similarly, an organism gains in fitness by delaying its arrival on the scene, so long as the delay increases its chances of leaving descendants. This will often be the case when conditions in the future are likely to be better than those in the present. Thus, a delay in the recruitment of an individual to a population may be regarded as migration in time_' and as an alternative to migration in space, and there are trade-offs between the two types of dispersal. Organisms generally spend their period of delay in a state of d ormancy. This relatively inactive state has the benefit of conserving energy, which can then be used during the period following the delay. In addition, the dormant phase of an organism is often more tolerant of the adverse environmental conditions prevailing during the delay (i_'e_' tolerant of drought, extremes of temperature, lack of light, and so on). Dormancy can be either predictive or consequential. Predictive dormancy is initiated in advance of the adverse conditions, and is most often found in predictable_' seasonal environments. It is referred to as ^,diapause^, in animals_' and in plants as 'innate' or 'primary' dormancy. Consequential (or ‘secondary’) dormancy, on the other hand, is initiated in response to the adverse conditions themselves.

 

 

Intraspecific Competition

 

1. Introduction: The nature and features of intraspecific competition

 

Organisms grow, reproduce, die and migrate. They are affected by the conditions in which they liveand by the resources that they obtain. Yet, no organism lives in isolation. Each, for at least part of its life, is a member of a population composed of individuals of its own species.

Individuals of the same species have very similar requirements for survival, growth and reproduction; but their combined demand for a resource may exceed the immediate supply. The individuals then compete for the resource and, not surprisingly, at least some of them become deprived.

Consider, initially, a simple hypothetical community: a thriving population of grasshoppers (all of one species) feeding on a field of grass (also of one species). In order to provide themselves with energy and material for growth and reproduction,grasshoppers must eat grass; but, in order to find and consume that grass, they must use energy. Any grasshopper might find itself at a spot where there is no grass because some other grasshopper has eaten it. The grasshopper must then move on and expend more energy before it takes in food. The more grasshoppers there are competing for food, the more often this will happen. Yet, an increased energy expenditure and a decreased rate of food intake may all decrease a grasshopper's chances of survival, and also leave less energy available for development and reproduction. Survival and reproduction determine a grasshopper's contribution to the next generation. Hence, the more intraspecific competitors for food a grasshopper has, the less its likely contribution will be.

As far as the grass itself is concerned, an isolated seedling in fertile soil may have a very high chance of surviving to reproductive maturity. It will probably exhibit an extensive amount of modular growth, and will probably therefore eventually produce a large number of seeds. However, a seedling, which is closely surrounded by neighbours (shading it with their leaves and depleting the water and nutrients of its soil with their roots) will be very unlikely to survive, and if it does, will almost
certainly form few modules and set few seeds.                 

We can see immediately that the ultimate effect of competition on an individual is a decreased contribution to the next generation compared with what would have happened had there been no competitors. Intraspecific competition typically leads to decreased rates of resource intake per individual, and thus to decreased rates of individual growth or development, or perhaps to decreases in amounts of stored reserves or to increased risks of predation. These may lead, in turn, to decreases in survivorship and/or decreases infecundity. In many cases, competing individuals do not interact with one another directly. Instead, individuals respond to the level of a resource, which has been depressed by the presence and activity of other individuals. Thus, grasshoppers competing for food are not directly affected by other grasshoppers, but by a reduction in food level and an increased difficulty in finding good food. Similarly, a competing grass plant is adversely affected by the presence of close neighbours because the zone from which it extracts resources (light, water, nutrients) has been overlapped by the 'resource depletion zones' of these neighbours, making it more difficult for the original plant to extract its resources. In all these cases, competition may be described as exploitation, in that each individual is affected by the amount of resource that remains after it has been exploited by others. Exploitation can only occur, therefore, if the resource in question is in limited supply.

In many other cases, however, competition takes a form known as, interference. Here, individuals interact directly with each other, and one individual will actually prevent another from exploiting the resources within a portion of the habitat. For instance, this is seen amongst animals that defend territories and also amongst the sessiIe animals and plants that live on rocky shores and amongst terrestrial plants. For example, the presence of a barnacle on a rock prevents any other barnacle from occupying that same position, even though the supply of food at that position may exceed the requirements of several barnacles. In such cases, space can be seen as a resource in limited supply. Another type of interference competition occurs when, for instance, two red deer stags fight for access to a harem of hinds. In this case, the hinds arc a resource in limited supply, because although either stag, alone, could readily mate with all the hinds, they cannot both do so since matings are limited to the 'owner' of the harem.

Thus, interference competition may occur for a resource of real value (e.g. space on a rocky shore for a barnacle), in which case the interference is accompanied by a degree of exploitation, or for a surrogate resource (a territory, or ownership of a harem), which is only valuable because of the access it provides to a real resource (food within a territory, the right to mate with all females). Interference for a surrogate can replace, in a sense, exploitative competition for a real resource. Hence, whereas with exploitation the intensity of competition is closely linked to the level of resource present and the level required, with interference, intensity may be high even when the level of the real resource is not limiting.

In practice, many examples of competition probably include elements of both exploitation and interference. For instance, our hypothetical grass plant, apart from suffering from the resource depletion of its competitors, would be physically excluded by them from the sites they already occupy. To take a more specific example, adult cave beetles, Neapheanops tellkampfi, in Great Onyx Cave, Kentucky, compete amongst themselves but with no other species and have only one type of food—cricket eggs, which they obtain by digging holes in the sandy floor of the cave. On the one hand, they suffer indirectly from exploitation: beetles reduce the density of their resource (cricket eggs) and then have markedly lower fecundity when food availability is low.But, they also suffer directly from interference: at higher beetle densities they fight more, forage less, dig fewer and shallower holes and eat far fewer eggs than could be accounted for by food depletion alone.

Whether they compete through exploitation or interference, individuals within a species are in essence equivalent, having many fundamental features in common, using similar resources and reacting in much the same way to conditions. However, there are many occasions when intraspecific competition is very one sided: a strong, early seedling will shade a stunted, late one; and an older and larger bryozoan on the shore will grow over a smaller and younger one. The overwinter survival of red deer calves in the resource-limited population on the island of Rhumdeclined sharply as the population became more crowded, but those that were smallest when born were by far the most likely to die.

Intrinsic, heritable differences between individuals may also ensure that competitive interactions are not reciprocal. For instance, tall corn plants will usually shade; and suppress genetically distinct short plants of the same species.

This lack of exact equivalence means that the ultimate effect of competition is far from being the same on different individuals. Weak competitors may make only a small contribution to the next generation, or no contribution at all. Strong competitors may have their contribution only negligibly affected. Indeed, a strong competitor may actually make a larger proportional contribution when there is intense competition than when there is no competition at all (i.e. if they maintain their contribution whilst all around them are losing theirs). In other words, although the ultimate effect of competition is a decrease in reproductive output, this does not always mean a decrease in individual fitness (i.e. 'relative' contribution).

 

Interspecific Competition

1.   Introduction

The essence of interspecific competition is that individuals of one species suffer a reduction in fecundity, survivorship or growth as a result of resource exploitation or interference by individuals of another species. This competition is likely to affect the population dynamics of the competing species, and the dynamics, in their turn, can influence the species' distributions and their evolution. Of course, evolution, in its turn, can influence the species' distributions and dynamics.