Population

Series of events that must occur before competition between two individuals will influence populations or communities. ronments - recall that one of the conditions for competition was the existence of limited resources. Like so many concepts in ecology, the issue is generally one of semantics - in this case it is about what we define as 'resource-rich' and 'resource-poor'. There is no good way to quantify the division between these two. Hence, most studies find evidence for both...

Agamospermy

Agamospermy is the production of seed without fertilization (i.e. the fusion of gametes - sperm and ovum). There are three main types of agamospermy (diplospory, apospory and adventitious embryony) but there are numerous and often complex variations (Fig. 5.2). Normally, meiosis occurs and the gamete (ovum) contains one copy of all chromosomes, i.e. the ovum is 'haploid'. After being fertilized by sperm, the seed will have the normal number of copies of chromosomes (i.e. it will be...

Literature Cited

Aarssen, L.W. (1989) Competitive ability and species coexistence a plant's-eye view. Oikos 56, 386-401. Aarssen, L.W. (1992) Causes and consequences of variation in competition ability in plant communities. Journal of Vegetation Science 3, 165-174. Agnonin, C., Caussanel, J.-P. and Meynard, J.-M. (1996) Competition between winter wheat and Veronica hederifolia influence of weed density and the amount and timing of nitrogen application. Weed Research 36, 175-187. Alphalo, P.J., Ballare, C.L. and...

R FR Wavelength nm

Spectral photon distribution in the 400-800 nm wavelength region on an overcast day in (a) full daylight, and (b) filtered through a canopy of ash (Fagus) (redrawn and adapted from Pons, 2000). Seedling emergence to floral primordium To initiation of seed set S To completion of seed set Fig. 7.6. The effect of photoperiod on the duration from emergence, to floral primordia, to first flowering, to seed set initiation and to completion of seed set in wild mustard (from data in Huang et...

Describing the Distribution and Abundance of Populations

A population is a group of individuals of the same species found in the same place at the same time. Populations are characterized in terms of their distribution and abundance. The distribution of a species can be mapped using historical data, field observations and remote sensing. Individuals within a population will not be evenly distributed throughout their range. Abundance can be measured as frequency, density, cover or biomass. Abundance and distribution do not necessarily reflect a...

Seed Germination

Seeds that are not dormant may not germinate if they have not encountered favourable environmental conditions these are termed 'quiescent' (Foley, 2001). They are 'seeds in waiting'. Quiescent seeds are able to germinate immediately once they encounter favourable environmental conditions, but may revert to secondary dormancy (Fig. 6.6). The critical factor for seeds is to be able to germinate at an appropriate time, a daunting task since environmental conditions vary on small spatial scales and...

Senescence Programmed Death

It is easy to understand why plants die when they are eaten, trampled or run out of water. It is not as easy to understand why plants die naturally, unthreatened by external forces at the end of their life cycle. 'Senescence' is the programmed process of deterioration that leads to the natural death of a plant (or plant part). It is an internally controlled process that determines the life span of a whole organism or its parts. Knowledge of when and why a weed senesces has implications for weed...

Days after emergence

Change in shoot height of wild mustard (Sinapis arvensis) over time at five temperature regimes (redrawn from Huang et al., 2001). Fig. 7.3. Effect of light intensity on (a) shoot and root dry weight, and (b) inflorescence number of Canada thistle (adapted and redrawn from Zimdahl et al., 1991). Fig. 7.3. Effect of light intensity on (a) shoot and root dry weight, and (b) inflorescence number of Canada thistle (adapted and redrawn from Zimdahl et al., 1991). thresholds) is 24.5 C...

Density of species

Four types of competition experiments (a) partial additive, (b) replacement series, (c) additive series, and (d) complete additive (redrawn from Silvertown and Lovett Doust, 1993). 1993) (Table 10.4). Obviously having more density combinations will give a better understanding of competitive effects, but with it comes an increased requirement of time and money. Numerous reviews have been written examining various aspects of competition experiments (Cousens, 1991 Goldberg and Barton,...

Sheep grazing treatments

Number of mouse-eared chickweed (Cerastium fontanum) seedlings emerging from plots with various combinations of herbivory. Sheep grazing in the summer was managed so that pastures were maintained at either 3- or 9-cm heights. In the winter pastures were either grazed (+) or not grazed (-). In addition, subplots were maintained either with or without slug herbivores (redrawn from Clear Hill and Silvertown, 1997). the information you are looking for when you increase their density....

What Determines the Outcome of Competition

One of the reasons weeds are so successful is because they adapt rapidly to new environmental conditions, including the 'competitive neighbourhood' of other weeds, crops and plants in general. Weeds do not 'know' how competitive others are - if others are much better competitors, the weed simply dies without reproducing. If a weed is at a competitive disadvantage but still produces offspring, there should be selection for the offspring to develop better competitive abilities (as long as the...

Basic Community Concepts and Diversity

Tropical Rainforest Sketch

An ecological community consists of groups of species found together at the same time and space. Communities exist at any temporal or spatial scale. Species within a community may or may not be interdependent. When researching a community, the researcher must first decide where community boundaries are and what species to include. These are both fairly arbitrary decisions. Species diversity is a measure of the number of species present (richness) and their relative abundances (evenness)....

Special adaptations

Rameta Klon

Dentaria bulbifera (adventitious and axillary buds, aomunl apices, tarions, plant fragments, budding plants) Fig. 5.4. Classification of clonal growth types based on Central European vegetation (Klimes et al., 1997 with permission of Backhuys Publishers). Table 5.3. Definitions of structures common in clonal species with weed examples. A horizontal, underground structure connecting ramets. It may bear roots and leaves and it may be cordlike or fleshy An above-ground, horizontal branch...

A

Schematic diagram of three community types (a) natural community with no human disturbance, (b) natural community with human disturbance and (c) agricultural system. Fig. 1.1. Schematic diagram of three community types (a) natural community with no human disturbance, (b) natural community with human disturbance and (c) agricultural system. individual weedy species without providing a broad background. Our goal is to provide you with a link between the fields of weed science, plant...

Frequency Mean densityMean cover

no. quadrats with ( no. present per quadrat) ( estimated cover per Quadrat ei o species present no. quadrats quadrat area quadrat) no. quadrats Q size no. quadrats x 100 _ (4x0.1) + (38x0.01) + (15x0.005) 144 x 100 0.6 Fig. 10.5. The effect of quadrat size on estimates of a population's density, cover and frequency. There is no frequency value for a 'true' population density of common species. Various measures of abundance will be affected by quadrat size in different ways (Krebs, 1999)....

Age years

Survivorship curve of Drummond phlox (Phlox drummondii) shown on arithmetic and logorithmic scales (data from Leverich and Levin, 1979). Fig. 3.10. Idealized survivorship curves shown on (a) log scale, and (b) arithmetic scale. Type I shows low early mortality and high late mortality. Type II shows constant mortality rate over time. Type III shows high early mortality and low mortality late in life. Fig. 3.10. Idealized survivorship curves shown on (a) log scale, and (b) arithmetic...

Sexual Reproduction

Diagram For Dicliny

Benefits of sexual reproduction relate to increased fitness - specifically, this includes the production of genetically variable offspring (often expressed as hybrid vigour) that allows at least some offspring to survive in a heterogeneous environment. Costs of sexual reproduction include disrupting well-adapted genotypes and the cost of producing reproductive structures. Plants can have complex combinations of gender expression for example, they can be exclusively male or female, or both male...

Sexual Reproduction in Asexually Reproducing Species

Clonal reproduction rarely occurs to the total exclusion of sexual reproduction, although there are examples of this, such as Japanese knotweed. Clonal growth may occur at the expense of seed production (creeping buttercup, Ranunculus repens, and Canada goldenrod). A trade-off occurs between these two methods of reproduction because only a finite amount of resources is available to allocate to reproduction (Abrahamson, 1980). The allocation of resources to sexual vs. asexual reproduction will...

Exploitation and Interference Competition

There are two general mechanisms of competition. In 'exploitation competition', weeds (as with all plants) compete with one another by being better at exploiting the resources, i.e. obtaining more of them than others (Fig. 8.2). In 'interference competition', there can be a direct removal of a resource (taking it away from another who already captured it) or the occupation of a resource that may not be needed immediately, but denying it to others raises relative fitness (Murphy, 1999). In...

Getting Away from Your Parents I Seed Dispersal Dispersal in Space

Seeds are dispersed away from parents for several reasons. First, it avoids seedlings competing with their parents or siblings. Competition among siblings or between parent and offspring is more intense because they tend to have similar resource requirements and strategies to acquire these resources. Being dispersed away from the maternal plant also decreases the likelihood of mating with a sibling, which could lead to inbreeding depression. Second, dispersal increases colonization...

Summary

Designing an experiment is the 'art of science' because it involves imagination and creativity. There is no one correct way to answer an ecological question. However, designing experiments also requires pragmatism. When an ecologist is interested in a specific ecological phenomenon, there is much work required to plan and design the experimental methodology. Lack of planning results in unusable data. There is a trade-off between the various types of controlled greenhouse-type experiments and...

Effect of species 2 on species

The continuum of species interactions redrawn from Bronstein, 1994a . If we use the 'cat and mouse' example, the cat species 1 has a negative effect on the mouse species 2 , and the mouse has a positive effect on the cat. Herbivores include mammals such as grazing deer and zebra, sap-sucking insects, seedling-eating molluscs, root-feeding larvae, leaf-eating grasshoppers, and seed-eating mice and beetles. Plant herbivores range from specialists typically invertebrates , that eat only...