The number and fitness of offspring

A second key trade-off is that between the number of offspring and their individual fitness. At its simplest, this is a trade-off between the size and number of offspring, within a given total reproductive investment. That is, a reproductive allocation can be divided into fewer, larger offspring or more, smaller offspring. However, the size of an egg or seed is only an index of its likely fitness. It may be more appropriate to look for a trade-off between the number of offspring and, say, their...

Tradeoffs

Any organism's life history must, of necessity, be a compromise allocation of the resources that are available to it. Resources devoted to one trait are unavailable to others. A 'trade-off' is a negative relationship between two life history characteristics in which increases in one are associated with decreases in the other as a result of such compromises. For instance, Douglas fir trees (Pseudotsuga menziesii) benefit both from reproducing and from growing (since, amongst other things, this...

Biotrophic and necrotrophic parasites

The most obvious response of a host to a parasite is for the whole host to die. Indeed, we can draw a distinction between parasites that kill and then continue life on the dead host (necrotrophic parasites) and those for which the host must be alive (biotrophic parasites). Necrotrophic parasites blur the tidy distinctions between parasites, predators and saprotrophs (see Section 11.1). Insofar as host death is often inevitable and sometimes quite rapid, necrotrophic parasites are really...

Complexity and stability in practice populations

Define Terrestrial Food Web

Where c is a constant and 2 is the so-called scaling coefficient. There are grounds for expecting values of 2 to lie between 1 and 2 (Murdoch & Stewart-Oaten, 1989) and most observed values seem to do so (Cottingham et al., 2001). In this range, population variability increases with species richness (Figure 20.8) - a connection between complexity and population instability, as found in May's original model. Overall, therefore, most models indicate that population stability tends to decrease...

Four trophic levels

Brown Trout Food Webs Wiki

In a four-level trophic system, if it is subject to trophic cascade, we might expect that the abundances of the top carnivores and the herbivores are positively correlated, as are those of the primary carnivores and the plants. This is precisely what was found in an experimental study of the food web in Eel River, northern California (Figure 20.4a) (Power, 1990). Large fish (roach, Hesperoleucas symmetricus, and steelhead trout, Oncorhynchus mykiss) reduced the abundance of fish fry and...

Introduction natural selection and adaptation

Bridgeman Figure Drawing

From our definition of ecology in the Preface, and even from a layman's understanding of the term, it is clear that at the heart of ecology lies the relationship between organisms and their environments. In this opening chapter we explain how, fundamentally, this is an evolutionary relationship. The great Russian-American biologist Theodosius Dobzhansky famously said 'Nothing in biology makes sense, except in the light of evolution'. This is as true of ecology as of any other aspect of biology....

When is a population a metapopulation

Two necessary features of a metapopulation have already been established here that individual subpopulations have a realistic chance of experiencing both extinction and recolonization. To this we can add a third, which has been implicit in the discussion thus far. The dynamics of the various subpopulations should be largely independent, i.e. not synchronous. There would, after all, be little hope of stability if when one subpopulation went extinct they all did. Rather, asynchrony guarantees...

Where should we focus conservation effort

Species Richness

Several categories of risk of species extinction have been defined (Mace & Lande, 1991). A species can be described as vulnerable if there is considered to be a 10 probability of extinction within 100 years, endangered if the probability is 20 within 20 years or 10 generations, whichever is longer, and critically endangered if within 5 years or two generations the risk of extinction is at least 50 (Figure 7.17). Based on these criteria, modern and historic extinction rates compared...

Consumption of fallen fruit

Of course, not all plant detritus is so difficult for detritivores to digest. Fallen fruit, for example, is readily exploited by many kinds of opportunist feeders, including insects, birds and mammals. However, like all detritus, decaying fruits have associated with them a microflora, in this case mainly dominated by yeasts. Fruit-flies (Drosophila spp.) specialize at feeding on these yeasts and their by-products and in fruit-laden domestic compost heaps in Australia, five species of fruit-fly...

Diversity indices

Diversity incorporates richness, commonness and rarity An important aspect of community structure is completely ignored, though, when the composition of the community is described simply in terms of the number of species present. It misses the information that some species are rare and others common. Consider a community of 10 species with equal numbers in each, and a second community, again consisting of 10 species, but with more than 50 of the individuals belonging to the most common species...

Net photosynthesis

The rate of photosynthesis is a gross measure of the rate at which a plant captures radiant energy and fixes it in organic carbon compounds. However, it is often more important to consider, and very much easier to measure, the net gain. Net photosynthesis is the increase (or decrease) in dry matter that results from the difference between gross photosynthesis and the losses due to respiration and the death of plant parts (Figure 3.8). Net photosynthesis is negative in darkness, when respiration...

Rankabundance diagrams

Of course, attempts to describe a complex community structure by one single attribute, such as richness, diversity or equitabil-ity, can be criticized because so much valuable information is lost. A more complete picture of the distribution of species abundances in a community makes use of the full array of Pi values by plotting Pi against rank. Thus, the Pi for the most abundant species is plotted first, then the next most common, and so on until the array is completed by the rarest species of...

Simple MSY models of harvesting fixed quotas

Fixed-quota harvesting is extremely risky. The MSY density (Nm) is an equilibrium (gains losses), but when harvesting is based on the removal of a fixed quota, as it is in Figure 15.7, Nm is a very fragile equilibrium. If the density exceeds the MSY density, then hm exceeds the recruitment rate and the population declines towards Nm. This, in itself, is satisfactory. But if, by chance, the density is even slightly less than Nm, then hm will once again exceed the recruitment rate. Density will...

Genetics of small populations significance for species conservation

Theory tells conservation biologists to beware genetic problems in small populations that may arise through loss of genetic variation. Genetic variation is determined primarily by the joint action of natural selection and genetic drift (where the frequency of genes in a population is determined by chance rather than evolutionary advantage). The relative importance of genetic drift is higher in small, isolated populations, which as a consequence are expected possible genetic problems in small...

Natural experiments

We have seen that interspecific competition is commonly studied by an experimenter comparing species alone and in combination. Nature, too, often provides information of this sort the distribution of certain potentially competing species is such that they sometimes occur together (sympatry) and sometimes occur alone (allopatry). These 'natural experiments' can provide additional information about interspecific competition, and especially about evolutionary effects, since the differences between...

Singlegeneration experiments

Given these problems, the alternative 'laboratory' approach, especially with plants (although the methods have occasionally been used with animals), has generally been to follow populations over just a single generation, comparing 'inputs' and 'outputs'. A number of experimental designs have been used. In 'substitutive' experiments, the effect of varying the proportion of each of two species is explored whilst keeping overall density constant (de Wit, 1960). Thus, at an overall density of say...

Mutual antagonism

Figure 8.9c, derived from the Lotka-Volterra model, describes a situation in which interspecific competition is, for both species, a more powerful force than intraspecific competition. This is known as mutual antagonism. An extreme example of such a situation is provided by work on two species of flour beetle Tribolium con-fusum and T. castaneum (Park, 1962). Park's experiments in the 1940s, difficulty proving and, especially, disproving the Principle niche differentiation and interspecific...

Competition for one resource may influence competition for another

Maize Shoot And Roots

Finally, it is worth noting that competition for one resource often affects the ability of an organism to exploit another resource. For example, Buss (1979) showed that in interactions between species of bryozoa (colonial, modular animals), there appears to be an interdependence between competition for space and for food. When a colony of one species contacts a colony of another species, it interferes with the self-generated feeding currents upon which bryozoans rely (competition for space...

Carbon cycle

Opposing forces of photosynthesis and respiration drive the global carbon cycle Photosynthesis and respiration are the two opposing processes that drive the global carbon cycle. It is predominantly a gaseous cycle, with CO2 as the main vehicle of flux between the atmosphere, hydrosphere and biota. Historically, the lithosphere played only a minor role fossil fuels lay as dormant reservoirs of carbon until human intervention in recent centuries (Figure 18.21d). Terrestrial plants use atmospheric...

Sulfur cycle

In the global phosphorus cycle we have seen that the lithospheric phase is predominant (Figure 18.21a), whereas the nitrogen cycle has an atmospheric phase of overwhelming importance (Figure 18.21b). Sulfur, by contrast, has atmospheric and lithospheric phases of similar magnitude (Figure 18.21c). Three natural biogeochemical processes release sulfur to the atmosphere (i) the formation of the volatile compound dimethylsulfide (DMS) (by enzymatic breakdown of an abundant compound in...

Explanation description prediction and control

At all levels of ecological organization we can try to do a number of different things. In the first place we can try to explain or understand. This is a search for knowledge in the pure scientific tradition. In order to do this, however, it is necessary first to describe. This, too, adds to our knowledge of the living world. Obviously, in order to understand something, we must first have a description of whatever it is that we wish to understand. Equally, but less obviously, the most valuable...

Pure and applied ecology

Ecologists are concerned not only with communities, populations and organisms in nature, but also with manmade or human-influenced environments (plantation forests, wheat fields, grain stores, nature reserves and so on), and with the consequences of human influence on nature (pollution, overharvesting, global climate change). In fact, our influence is so pervasive that we would be hard pressed to find an environment that was totally unaffected by human activity. Environmental problems are now...

Compartmentalization

A food web is compartmentalized if it is organized into sub-units within which interactions are strong, but between which interactions are weak. (The most perfectly compartmentalized community possesses only linear food chains.) Do food webs tend to be compartmentalized Not surprisingly, in studies where habitat divisions are major and unequivocal, there is a clear tendency for compartments to map onto habitats. For instance, Figure 20.18 shows the results of a classic study describing the...

Strong interactors and keystone species

Some species are more intimately and tightly woven into the fabric of the food web than others. A species whose removal would produce a significant effect (extinction or a large change in density) in at least one other species may be thought of as a strong interactor. Some strong interactors would lead, through their removal, to significant changes spreading throughout the food web - we refer to these as keystone species. A keystone is the wedge-shaped block at the highest point of an arch that...

Photosynthesis or water conservation Strategic and tactical solutions

Photosynthesis Water Solution

In fact, in terrestrial habitats especially, it is not sensible to consider radiation as a resource independently of water. Intercepted radiation does not result in photosynthesis unless there is CO2 available, and the prime route of entry of CO2 is through open stomata. But if the stomata are open to the air, water will evaporate through them. If water is lost faster than it can be gained, the leaf (and the plant) will sooner or later wilt and eventually die. But in most terrestrial...

The CAM pathway

Plants with a crassulacean acid metabolism (CAM) pathway also use PEP carboxylase with its strong power of concentrating CO2. In contrast to C3 and C4 plants, though, they open their stomata and fix CO2 at night (as malic acid). During the daytime the stomata are closed and the CO2 is released within the leaf and fixed by Rubisco. However, because the CO2 is then at a high concentration within the leaf, photorespiration is prevented, just as it is in plants using the C4 pathway. Plants using...

Nutrients in lakes

Stoichiometry Plants

. shows a pervasive role for nutrients. Like streams, lakes receive nutrients by the weathering of rocks and soils in their catchment areas, in the rainfall and as a result of human activity (fertilizers and sewage input). They vary considerably in nutrient availability. A study of 12 Canadian lakes shows a clear relationship between gross primary productivity (GPP) and phosphorus concentration and demonstrates the importance of nutrients in limiting lake productivity (Figure 17.15). Note that...

Competition between Paramecium species

Caudatum And Bursaria

The fourth example comes from the classic work of the great Russian ecologist G. F. Gause, who studied competition in laboratory experiments using three species of the protozoan Paramecium (Gause, 1934, 1935). All three species grew well alone, reaching stable carrying capacities in tubes of liquid medium. There, Paramecium consumed bacteria or yeast cells, which themselves lived on regularly replenished oatmeal (Figure 8.3a). When Gause grew P. aurelia and P. caudatum together, P. caudatum...

Conclusion

Decomposer communities are, in their composition and activities, as diverse as or more diverse than any of the communities more commonly studied by ecologists. Generalizing about them is unusually difficult because the range of conditions experienced in their lives is so varied. As in all natural communities, the inhabitants not only have specialized requirements for resources and conditions, but their activities change the resources and conditions available for others. Most of this happens...

Factors limiting primary productivity in terrestrial communities

Forest Different Ages

Sunlight, carbon dioxide (CO2), water and soil nutrients are the resources required for primary production on land, while temperature, a condition, has a strong influence on the rate NPP B ratios tend to decrease during successions Table 17.3 Above-ground net primary productivity (ANPP) for forest age sequences in contrasting biomes. (After Cower et al., 1996.) Range of stand ages, in years (no. of stands shown in brackets) Cold temperate Abies baisamea Pinus contorta Pinus densiflora Populus...

Water and temperature as critical factors

Productivity Hump Shape Elevation

Shortage of water may be a critical factor The relationship between the NPP of a wide range of ecosystems on the Tibetan Plateau and both precipitation and temperature is illustrated in Figure 17.9. Water is an essential resource both as a constituent of cells and for photosynthesis. Large quantities of water are lost in transpiration - particularly because the stomata need to be open for much of the time for CO2 to enter. It is not surprising that the rainfall of a region is quite closely...

Net recruitment curves

An alternative general view of intraspecific competition is shown in Figure 5.8a, which deals with numbers rather than rates. The difference there between the two curves (births minus deaths' or 'net recruitment') is the net number of additions expected in the population during the appropriate stage or over one interval of time. Because of the shapes of the birth and death curves, the net number of additions is small at the lowest densities, increases as density rises, declines again as the...

Territoriality

Territoriality is one particularly important and widespread phenomenon that results in asymmetric intraspecific competition. It occurs when there is active interference between individuals, such that a more or less exclusive area, the territory, is defended against intruders by a recognizable pattern of behavior. Individuals of a territorial species that fail to obtain a territory often make no contribution whatsoever to future generations. Territoriality, then, Figure 5.28 Space preemption in...

Geographic variation within species ecotypes

Ecotypes Reciprocal Transplants

The sapphire rockcress, Arabis fecunda, is a rare perennial herb restricted to calcareous soil outcrops in western Montana (USA) - so rare, in fact, that there are just 19 existing populations separated into two groups ('high elevation' and 'low elevation') by a distance of around 100 km. Whether there is local adaptation is of practical importance for conservation four of the low elevation populations are under threat from spreading urban areas and may require reintroduction from elsewhere if...

Aggregation of risk and spatial density dependence

Inverse Density Dependence

How does this stability arise out of aggregation The answer lies in what has been called 'pseudo-interference' (Free et al., 1977). With mutual interference, as predator density increases, predators spend an increasing amount of time interacting with one another, and their attack rate therefore declines. With pseudo-interference, attack rate also declines with parasitoid density, but as a result of an increasing fraction of encounters being wasted on hosts that have already been attacked. The...

What do we mean by a species

Cynics have said, with some truth, that a species is what a competent taxonomist regards as a species. On the other hand, back in the 1930s two American biologists, Mayr and Dobzhansky, proposed an empirical test that could be used to decide whether two populations were part of the same species or of two different species. They recognized organisms as being members of a single species if they could, at least potentially, breed together in nature to produce fertile offspring. They called a...

Restoration of habitats impacted by human activities

Ecology Histogram

The term 'restoration ecology' can be used, rather unhelpfully, to encompass almost every aspect of applied ecology (recovery of overexploited fisheries, removal of invaders, revegetation of habitat corridors to assist endangered species, etc.) (Ormerod, 2003). We restrict our consideration here to restoration of landscapes and waterscapes whose physical nature has been affected by human activities, dealing specifically with mining, intensive agriculture and water abstraction from rivers. Land...

Fecundity schedules and basic reproductive rates

The fecundity schedule in Table 4.1 (the final three columns) begins with a column of raw data, Fx the total number of seeds produced during each period. This is followed in the next column by mx the individual fecundity or birth rate, i.e. the mean number of seeds produced per surviving individual. Although the reproductive season for the Phlox population lasts for 56 days, each individual plant is semelparous. It has a single reproductive phase during which all of its seeds develop...

Introduction

In order to understand the distribution and abundance of a species we need to know its history (Chapter 1), the resources it requires (Chapter 3), the individuals' rates of birth, death and migration (Chapters 4 and 6), their interactions with their own and other species (Chapters 5 and 8-13) and the effects of environmental conditions. This chapter deals with the limits placed on organisms by environmental conditions. A condition is as an abiotic environmental factor that influences the...

The type 2 functional response

Type Predator Functional Response

The most frequently observed functional response is the 'type 2' response, in which consumption rate rises with prey density, but gradually decelerates until a plateau is reached at which consumption rate remains constant irrespective of prey density. (Realistically, even a type 1 response must have a plateau, as in the example above. The distinction is between the deceleration of a type 2 response and the linearity of the type 1 response.) Type 2 responses are shown for a carnivore, a...

Trophic cascades

The indirect effect within a food web that has probably received most attention is the so-called trophic cascade (Paine, 1980 Polis et al., 2000). It occurs when a predator reduces the abundance of its prey, and this cascades down to the trophic level below, such that the prey's own resources (typically plants) increase in abundance. Of course, it need not stop there. In a food chain with four links, a top predator may reduce the abundance of an intermediate predator, which may allow the...

Density or crowding

Betula Pendula What Fertiliser

Of course, the intensity of intraspecific competition experienced by an individual is not really determined by the density of the population as a whole. The effect on an individual is determined, Figure 5.5 (a) The fecundity (seeds per plant) of the annual dune plant Vulpia fasciculata is constant at the lowest densities (density independence, left). However, at higher densities, fecundity declines but in an undercompensating fashion, such that the total number of seeds continues to rise...

Parasites and the population dynamics of hosts

Population Parasite And Host

A key and largely unanswered question in population ecology is what role, if any, do parasites and pathogens play in the dynamics of their hosts There are data (see Section 12.5) showing that parasites may affect host characteristics of demographic importance (birth and death rates), though even these data are relatively uncommon and there are mathematical models showing that parasites have the potential to have a major impact on the dynamics of their hosts. But the point was also made earlier...

Exploitation and interference

Deer Rhum Island Mortality Curve

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. The grasshoppers were one example. Similarly, a competing grass plant is adversely affected by the presence of close neighbors, because the zone from which it extracts resources (light, water, nutrients) has been overlapped by the 'resource depletion zones' of these neighbors, making...

Seasonal and annual trends in primary productivity

Coniferous Forest Seasonal Trends

The large ranges in productivity in Table 17.2 and the wide confidence intervals in Figure 17.1 emphasize the productivity shows considerable temporal variation considerable variation that exists within a given class of ecosystems. It is important to note also that productivity varies from year to year in a single location (Knapp & Smith, 2001). This is illustrated for a temperate cropland, a tropical grassland and a tropical savanna in Figure 17.2. Such annual fluctuations no doubt reflect...

Competition between barnacles

The second study concerns two species of barnacle in Scotland Chthamalus stel-latus and Balanus balanoides (Figure 8.2) (Connell, 1961). These are frequently found together on the same Atlantic rocky shores of northwest a diversity of examples of competition Figure 8.1 (a) Frequency of aggressive encounters initiated by individuals of each fish species during a 72-day experiment in artificial stream channels with two replicates each of 50 Dolly Varden (Salvelinus malma) or 50 white-spotted...

Patterns of distribution dispersion

The movements of organisms affect the spatial pattern of their distribution (their dispersion) and we can recognize three main patterns of dispersion, although they too form part of a continuum (Figure 6.3). Random dispersion occurs when random, regular there is an equal probability of an and aggregated organism occupying any point in space distributions (irrespective of the position of any others). The result is that individuals are unevenly distributed because of chance events. Regular...

A classification of survivorship curves

Survivorship

Life tables provide a great deal of data on specific organisms. But ecologists search for generalities patterns of life and death that we can see repeated in the lives of many species. A useful set of survivorship curves was developed long ago by Pearl (1928) whose three types generalize what we know about the way in which the risks of death are distributed through the lives of different organisms (Figure 4.8). Type I describes the situation in which mortality is concentrated toward the end of...

The type 1 functional response

Species Richness Functioning

The most basic, 'type 1' functional response is that assumed by the Lotka-Volterra equations consumption rate rises linearly with prey density (indicated by the constant, a, in Equation 10.2). An example is illustrated in Figure 10.8. The rate at which Daphnia magna consumed yeast cells rose linearly when the density of cells Figure 10.8 The type 1 functional response of Daphnia magna to different concentrations of the yeast Saccharomyces cerevisiae. (After Rigler, 1961.) Figure 10.8 The type 1...

Foundercontrolled and dominancecontrolled communities

In response to disturbances, we can postulate two fundamentally different kinds of community response according to the type of competitive relationships exhibited by the component species - founder controlled and dominance controlled (Yodzis, 1986). Founder-controlled communities will occur if a large number of species are approximately equivalent in their ability to colonize an opening left by a disturbance, are equally well fitted to the abiotic environment and can hold the location until...

Active and passive dispersal

Like most biological categories, the distinction between active and passive dispersers is blurred at the edges. Passive dispersal in air currents, for example, is not restricted to plants. Young spiders that climb to high places and then release a gossamer thread that carries them on the wind are then passively at the mercy of air currents i.e. 'starting' is active but moving itself is effectively passive. Even the wings of insects are often simply aids to what is effectively passive movement...

Competition between diatoms

Asterionella Synedra Competition

The final example is from a laboratory investigation of two species of freshwater diatom Asterionella formosa and Synedra ulna (Tilman et al., 1981). Both these algal species require silicate in the construction of their cell walls. The investigation was Figure 8.4 (right) Percentage difference in feeding rates (mean SE) at orange-crowned warbler and virginia's warbler nests on plots where the other species had been experimentally removed. Feeding rates (visits per hour to the nest with food)...

Crypsis aposematism and mimicry

An animal may be less obvious to a predator if it matches its background, or possesses a pattern that disrupts its outline, or resembles an inedible feature of its environment. Straightforward examples of such crypsis are the green coloration of many grasshoppers and caterpillars, and the transparency of many planktonic animals that inhabit the surface layers of oceans and lakes. More dramatic cases are the sargassum fish (Histrio pictus), whose body outline mimics the sargassum weed in which...

Vital attributes

Resource Ratio Hypothesis Succession

Noble and Slatyer (1981) were also interested in defining the qualities that determine the place of a species in a succession. They called these properties vital attributes. The two most important relate to (i) the method of recovery after Tilman's resource-ratio hypothesis emphasizes changing competitive abilities beyond just competitive ability Noble and Slatyer's 'vital attributes' Figure 16.14 Tilman's (1988) resource-ratio hypothesis of succession. Five hypothetical plant species are...

Definition and scope of ecology

The word 'ecology' was first used by Ernest Haeckel in 1869. Paraphrasing Haeckel we can describe ecology as the scientific study of the interactions between organisms and their environment. The word is derived from the Greek oikos, meaning 'home'. Ecology might therefore be thought of as the study of the 'home life' of living organisms. A less vague definition was suggested by Krebs (1972) 'Ecology is the scientific study of the interactions that determine the distribution and abundance of...

Reproductive value

Natural selection favors those individuals that make the greatest proportionate contribution to the future of the population to which they belong. All life history components affect this contribution, ultimately through their effects on fecundity and survival. It is necessary, though, to combine these effects into a single currency so that different life histories may be judged and compared. A number of measures of fitness have been used. All the better ones have made use of both fecundity and...

Competition between bedstraws Galium spp

Tansley, one of the greatest of the 'founding fathers' of plant ecology, studied competition between two species of bedstraw (Tansley, 1917). Galium hercynicum is a species which grows naturally in Great Britain at acidic sites, whilst G. pumilum is confined to more calcareous soils. Tansley found in experiments that as long as he grew them alone, both species would thrive on both the acidic soil from a G. hercynicum site and the calcareous soil from a G. pumilum site. Yet, if the species...

Intraspecific competition and densitydependent growth

Intraspecific competition, then, can have a profound effect on the number of individuals in a population but it can have an equally profound effect on the individuals themselves. In populations of unitary organisms, rates of growth and rates of development are commonly influenced by intraspecific competition. This necessarily leads to density-dependent effects on the composition of a population. For instance, Figure 5.12a and b shows two examples in which individuals were typically smaller at...

Nitrogen cycle

The atmospheric phase is predominant in the global nitrogen cycle, in which nitrogen fixation and denitrification by microbial organisms are by far the most important Figure 18.21b . Atmospheric nitrogen is also fixed by lightning discharges during storms and reaches the ground as nitric acid dissolved in rainwater, but only about 3-4 of fixed nitrogen derives from this pathway. Organic forms of nitrogen are also widespread in the atmosphere, some of which results from the reaction of...

Essential resources

Inhibition Essential Resources

Two resources are said to be essential when neither can substitute for the other. Thus, the growth that can be supported on resource 1 is absolutely dependent on the amount available of resource 2 and vice versa. This is denoted in Figure 3.27a by the isoclines running parallel to both axes. They do so because the amount available of one resource defines a maximum possible growth rate, irrespective of the amount of the other resource. This growth rate is achieved unless the amount available of...

The relationship between local and regional species richness

One way to assess the degree to which communities are saturated with species is to plot the relationship between local species richness assessed on a spatial scale where all the species could encounter each other in a community and regional species richness the number of species in the regional pool that could theoretically colonize the community . Local species richness is sometimes referred to as a richness or a diversity and regional species richness as y richness. If communities are...

Symmetric and asymmetric competition

Interspecific competition like intra-specific competition is frequently highly asymmetric - the consequences are often not the same for both species. For instance, with Connell's barnacles, Balanus excluded Chthamalus from their zone of potential overlap, but any effect of Chthamalus on Balanus was negligible Balanus was limited by its own sensitivity to desiccation. An analogous situation is provided by two species of cattail reedmace in ponds in Michigan Typha latifolia occurs mostly in...

Proportion or allowing constant escapement

Two further management strategies are based on the simple idea of availability of a surplus yield. First, a constant proportion of the population can be harvested this is equivalent to fixing a hunting mortality rate and should have the same effect as harvesting at constant effort Milner-Gulland amp Mace, 1998 . Thus, in the Northwest Territories of Canada, 3-5 of the caribou and muskox populations can be killed each year Gunn, 1998 , a strategy that involves the expense of preharvest censuses...

The Lotka Volterra model

The simplest differential equation model is known like the model of interspecific competition by the name of its originators Lotka-Volterra Volterra, 1926 Lotka, 1932 . This will serve as a useful point of departure. The model has two components P, the numbers present in a predator or consumer population, and N, the numbers or biomass present in a prey or plant population. We assume initially that in the absence of consumers the prey population increases exponentially see Section 5.9 increase...

Reproductive allocation and its timing

Reproductive Allocation

If we assume initially that all options sets are convex-outwards, then we can see that relatively low CR habitats should favor a higher reproductive allocation, whilst relatively high CR habitats favor a lower reproductive allocation Figure 4.26a . This pattern can be seen in three populations of the dandelion Taraxacum officinale. The populations were composed of a number of distinct clones that belonged to one or other of four biotypes A-D . The habitats of the populations varied from a...

Hosts as reactive environments resistance recovery and immunity

Any reaction by an organism to the presence of another depends on it recognizing a difference between what is 'self' and what is 'not self'. In invertebrates, populations of phagocytic cells are responsible for much of a host's response to invaders, even to inanimate particles. In insects, hemocytes cells in the hemo-lymph isolate infective material by a variety of routes, especially encapsulation - responses that are accompanied by the production of a number of soluble compounds in the humoral...

Conservation of endangered species

The conservation of species at risk often involves establishing protected areas and sometimes the translocation of individuals to new locations. Both approaches should be based on considerations of the niche requirements of the species concerned. The overwintering habitat in Mexico is absolutely critical for the monarch butterfly Danaus plexippus , which breeds in southern Canada and the eastern United States. The butterflies form dense colonies in oyamel Abies religiosa forests on 11 separate...

Energy flow through contrasting communities

Galapagos Islands Yearly Rainfall

Given accurate values for net primary productivity NPP in an ecosystem, and CE, AE and PE for the various trophic groupings shown in the model in Figure 17.22, it should be possible to predict and understand the relative importance of the different possible energy pathways. Perhaps not surprisingly, no study has incorporated all ecosystem compartments and all transfer efficiencies of the component species. However, some generalizations are possible when the gross features of contrasting systems...

Equilibrium and nonequilibrium views of community organization

It is possible to conceive of a world with just one species of plant or herbivore with supreme performance over an enormous range of tolerance. In this scenario the most competitive species the one that is most efficient at converting limited resources into descendants would be expected to drive all less competitive species to extinction. The species richness we witness in real communities is a clear demonstration of the failure of evolution to produce such supreme species. An extension of this...

Complexity and stability in practice whole communities

Turning to the aggregate, whole community level, evidence is largely consistent in supporting the prediction that increased richness in a community increases stability decreases variability , though a number of studies have failed to detect any consistent relationship Cottingham et al., 2001 Worm amp Duffy, 2003 . First, returning to McNaughton's 1978 studies of US and Serengeti grasslands, the effects of perturbations were quite different when viewed in ecosystem as opposed to population...

Evidence from negatively associated distributions

Difference Richness

A number of studies have used patterns in distribution as evidence for the importance of interspecific competition. Foremost amongst these is Diamond's 1975 survey of the land birds living on the islands of the Bismarck Archipelago off the coast of New Guinea. The most striking evidence comes from distributions that Diamond refers to as 'checkerboard'. In these, two or more ecologically similar species i.e. members of the same guild have mutually exclusive but interdigitating distributions such...

Basic equations

Simple Explanation Density

In Section 4.7 we developed a simple model for species with discrete breeding seasons, in which the population size at time t, Nt, altered in size under the influence of a fundamental net reproductive rate, R. This model can be summarized in two equations The model, however, describes a population in which there is no competition. R is constant, and if R gt 1, the population will continue to increase in size indefinitely 'exponential growth', shown in Figure 5.18 . The first step is therefore...

Unitary and modular organisms

Duckweed Lifecycle

Our 'ecological fact of life', though, implies by default that all individuals are alike, which is patently false on a number of counts. First, almost all species pass through a number of stages in their life cycle insects metamorphose from eggs to larvae, sometimes to pupae, and then to adults plants pass from seeds to seedlings to photosynthesizing adults and so on. The different stages are likely to be influenced by different factors and to have different rates of migration, death and of...

Autochthonous and allochthonous production

North American Photosynthesis

All biotic communities depend on a supply of energy for their activities. In most terrestrial systems this is contributed in situ by the photosynthesis of green plants - this is autochthonous production. Exceptions exist, however, particularly where colonial animals deposit feces derived from food consumed at a distance from the colony e.g. bat colonies in caves, seabirds on coastland - guano is an example of allochthonous organic matter dead organic material formed outside the ecosystem . In...

Effects of consumption on consumers

The beneficial effects that food has on individual predators are not difficult to imagine. Generally speaking, an increase in the amount of food consumed leads to increased rates of growth, development and birth, and decreased rates of mortality. This, after all, is implicit in any discussion of intraspecific competition amongst consumers see Chapter 5 high densities, implying small amounts of food per individual, lead to low growth rates, high death rates, and so on. Similarly, many of the...

The population projection matrix

Life Cycle Graph Matrix

A more general, more powerful, and therefore more useful method of analyzing and interpreting the fecundity and survival schedules of a population with overlapping generations makes use of the population projection matrix see Caswell, 2001, for a full exposition . The word 'projection' in its title is important. Just like the simpler methods above, the idea is not to take the cur rent state of a population and forecast what will happen to the population in the future, but to project forward to...

Consequences for population dynamics of functional responses and the Allee effect

Different types of functional response have different effects on population dynamics. A type 3 response means a low predation rate at low prey densities. In terms of isoclines, this means that type 3 responses stabilize but may be unimportant in practice Figure 10.10 Type 3 sigmoidal functional responses. a The shrews Sorex and Blarina and the deer mouse Peromyscus responding to changing field densities of cocoons of the European pine sawfly, Neodiprion sertifer, in Ontario, Canada. After...

Apparent competition enemyfree space

Competition For Predator Free Space

Another reason for being cautious in our discussion of competition is the existence of what Holt 1977, 1984 has called 'apparent competition', and what others have called 'competition for enemy-free space' Jeffries amp Lawton, 1984, 1985 . Imagine a single species of predator or parasite that attacks two species of prey or host . Both prey species are harmed by the enemy, and the enemy benefits from both species of prey. Hence, the increase in abundance that the enemy achieves by consuming prey...

Models of mutualisms

Bees Mutualism

Several of the previous chapters on interactions have included a section on mathematical models. This is perhaps a good time to remind ourselves why this was - because the models, by separating essence from detail, were able to provide insights that would not be apparent from a catalog of actual examples. For modeling to be a success, then, it is imperative that the 'essence' is correctly identified. What is the essence of a mutualism One might imagine it to be that each partner has a positive...

The demographic approach

For many years, the demographic approach was represented by a technique called key factor analysis. As we shall see, there are shortcomings in the technique and useful modifications have been proposed, but as a means of explaining important general principles, and for historical completeness, we start with key factor analysis. In fact, the technique is poorly named, since it begins, at least, by identifying key phases rather than factors in the life of the organism concerned. For a key factor...

Possible pathways of energy flow through a food web

Dead Organic Matter

Figure 17.22 provides a complete description of the trophic structure of a community. It consists of the grazer system pyramid of productivity, but with two additional elements of realism. Most importantly, it adds a decomposer system - this is invariably coupled to the grazer system in communities. Secondly, it recognizes that there are subcomponents of each trophic level in each most primary productivity does not pass through the grazer system alternative pathways that energy can trace...

Variations in the relationship of productivity to biomass

We can relate the productivity of a community to the standing crop biomass that produces it the interest rate on the capital . Alternatively, we can think of the standing crop as the biomass that is sustained by the productivity the capital resource that is sustained by earnings . Overall, there is a dramatic difference in the total biomass that exists on land 800 Pg compared to the oceans 2 Pg and fresh water lt 0.1 Pg Geider et al., 2001 . On an areal basis, biomass on land ranges from 0.2 to...

Influence of parasitism on community structure

Lisard Transfromation

The incidence of a parasite, like that of other types of exploiter, may determine whether or not a host species occurs in an area. Thus, the extinction of nearly 50 of the endemic bird fauna of the Hawaiian Islands has been attributed in part to the introduction of bird pathogens such as malaria and bird pox van Riper et al., 1986 and changes in the distribution of the North American moose Alces alces have been associated with the parasitic nematode Pneumostrongylus tenuis Anderson, 1981 ....

Aggregated distributions

A more subtle, but more generally applicable path to the coexistence of a superior and an inferior competitor on a patchy and ephemeral resource is based on the idea that the two species may have independent, aggregated i.e. clumped distributions over the available patches. This would mean that the powers of the superior competitor were mostly directed against members of its own species in the high-density clumps , but that this aggregated superior competitor would be absent from many patches -...

Integrated pest management

A variety of management implications of our understanding of pest population dynamics have been presented in previous sections. However, it is important to take a broader perspective and consider how all the different tools at the pest controller's disposal can be deployed most effectively, both to maximize the economic benefit of reducing pest density and to minimize the adverse environmental and health consequences. This is what integrated pest management IPM is intended to achieve. It...

Herbicides weeds and farmland birds

Farmland Birds

Herbicides are used in very large amounts and on a worldwide scale. They are active against pest plants and when used at commercial rates appear to have few significant effects on animals. Herbicide pollution of the environment did not, until relatively recently, arouse the passions associated with insecticides. However, conservationists now worry about the loss of 'weeds' that are the food hosts for larvae of butterflies and other insects and whose seeds form the main diet of many birds. A...

Biological control

Outbreaks of pests occur repeatedly and so does the need to apply pesticides. But biologists can sometimes replace chemicals by another tool that does the same job and often costs a great deal less -biological control the manipulation of the natural enemies of pests . Biological control involves the application of theory about interactions between species and their natural enemies see Chapters 10, 12 and 14 to limit the population density of specific pest species. There are a variety of...

Culture of crops or livestock

At least in terms of geographic extent, some of the most dramatic mutualisms are those of human agriculture. The numbers of individual plants of wheat, barley, oats, corn and rice, and the areas these crops occupy, vastly exceed what would have been present if they had not been brought into cultivation. The increase in human population since the time of hunter-gatherers is some measure of the reciprocal advantage to Homo sapiens. Even without doing the experiment, we can easily imagine the...

The effect of predation on prey populations

Arapaho Prairie The Grasshopper

Returning now to predators in general, it may seem that since the effects of predators are harmful to individual prey, the immediate effect of predation on a population of prey must also be predictably harmful. However, these effects are not always so predictable, for one or both of two important reasons. In the first place, the individuals that are killed or harmed are not always a random sample of the population as a whole, and may be those with the lowest potential to contribute to the...

Mutualistic protectors

Plant Leaves Rich Protein

'Cleaner' fish, of which at least 45 species have been recognized, feed on ectoparasites, bacteria and necrotic tissue from the body surface of 'client' fish. Indeed, the cleaners often hold territories with 'cleaning stations' that their clients visit - and visit more often when they carry many parasites. The cleaners gain a food source and the clients are protected from infection. In fact, it has not always proved easy to establish that the clients benefit, but in experiments off Lizard...

Life cycles

To understand the forces determining the abundance of a population, we need to know the phases of the constituent organisms' lives when these forces act most significantly. For this, we need to understand the sequences of events that occur in those organisms' life cycles. A highly simplified, generalized life history Figure 4.6a comprises birth, followed by a prereproductive period, a period of reproduction, perhaps a postreproductive period, and then death as a result of senescence though of...

Phylogenetic and allometric constraints

The life histories that natural selection favors and we observe are not selected from an unlimited supply but are constrained by the phylogenetic or taxonomic position that organisms occupy. For example, in the entire order Procellariformes albatrosses, petrels, fulmars the clutch size is one, and the birds are 'prepared' for this morphologically by having only a single brood patch with which they can incubate this one egg Ashmole, 1971 . A bird might produce a larger clutch, but this is bound...

The population dynamics of small populations

Small Populations

Much of conservation biology is a crisis discipline. Managers are inevitably confronted with too many problems and too few resources. Should they focus attention on the various forces that bring species to extinction and attempt to persuade governments to act to reduce their prevalence or should they restrict activities to identifying areas of high species richness where reserves can be set up and protected see Section 22.4 or should they identify species at most risk of extinction and work out...

Economic injury level and economic thresholds

Economic injury level defines actual and potential pests Economics and sustainability are intimately tied together. Market forces ensure that uneconomic practices are not sustainable. One might imagine that the aim of pest control is always total eradication of the pest, but this is not the general rule. Rather, the aim is to reduce the pest population to a level at which it does not pay to achieve yet more control the economic injury level or EIL . Our discussion here is informed particularly...

Typical temperatures and distributions

There are very many examples of plant and animal distributions that are strikingly correlated with some aspect of environmental temperature even at gross taxonomic and systematic levels Figure 2.13 . At a finer scale, the distributions of many species closely match maps of some aspect of temperature. For example, the northern limit of the distribution of wild madder plants Rubia peregrina is closely correlated with the position of the January 4.5 C Figure 2.11 a The El Ni o-Southern Oscillation...

Crowding in the Lotka Volterra model

The effects of intraspecific competition, and of a decline in predator consumption rate with predator density, can be investigated by modifying the Lotka-Volterra isoclines. The details of incorporating intraspecific competition into the prey zero isocline are described by Begon et al. 1990 , but the end result Figure 10.7a can be understood without reference to these details. At low prey densities there is no intraspecific competition, and the prey isocline is horizontal as in the...

Multiple equilibria an explanation for outbreaks

Unstable Equilibrium Isocline

When predator and prey populations interact, there can sometimes be sudden changes in the abundance of one or both partners outbreaks or crashes. Of course, this may reflect an equally sudden change in the environment, but ecologists working in a variety of fields have come to realize that there is not necessarily just one equilibrium combination of a predator and prey about which there may or may not be oscillations . There can, instead, be 'multiple equilibria' or 'alternative stable states'....

The nutritional content of plants and animals as food

Aphids And Phloem

As a 'package' of resources, the body of a green plant is quite different from the body of an animal. This has a tremendous effect on the value of these resources as potential food Figure 3.23 . The most important contrast is that plant cells are bounded by walls of cellulose, lignin and or other structural materials. It is these cell walls that give plant material its high fiber content. The presence of cell walls is also largely responsible for the high fixed carbon content of plant tissues...

Niche differentiation and similarity amongst coexisting competitors

It might be imagined that scientific progress is made by providing answers to questions. In fact, progress often consists of replacing one question with another, more pertinent, more challenging question. In this section, we deal with an area where this is the case the questions of how different coexisting competitors are, and how different coexisting species need to be if competition is not to eliminate one of them. Species Species Species 1 2 3 Resource state food particle size, for instance...

Ideal free and related distributions aggregation and interference

Ideal Free Distribution

We can see, then, that consumers tend to aggregate in profitable patches where their expected rate of food consumption is highest. Yet we might also expect that consumers will compete and interfere with one another discussed further in Chapter 10 , thereby reducing their per capita consumption rate. It follows from this that patches that are initially most profitable become immediately less profitable because they attract most consumers. We might therefore expect the consumers to redistribute...