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|-Chapter 2: Ecology|
The conservation or protection of the actual biodiversity of the planet requires
knowlegde of the way nature is working, how biodiversity comes into existence and how it
maintains. Nature is not a static but a dynamic phenomenon. This chapter is focussed on
the ecological aspects of biodiversity and the meaning of ecology for the conservation of
As local populations within a metapopulation fluctuate in size, they become vulnerable to extinction during periods when their numbers are low. Extinction of local populations is common in some species, and the regional persistence of such species is dependent on the existence of a metapopulation. Hence, elimination of much of the metapopulation structure of some species can increase the chance of regional, continental or worldwide extinction of species.
The structure of metapopulations varies among species. In some species one population may be particularly stable over time and act as the source of recruits into other, less stable populations. In other species, metapopulations may have a shifting source. Any one local population may temporarily be the stable source population that provides recruits to the more unstable surrounding populations. As conditions change, the source population may become unstable, as when disease increases locally or the physical environment deteriorates. Meanwhile, conditions in another population that had previously been unstable might improve, allowing this population to provide recruits.
Overall, the population ecology and dynamics of all species is a complex result of their genetic structure, the life histories of the individuals, fluctuations in the carrying capacity of the environment, the relative influences of all the different kinds of density-dependent and density-independent factors that limit population growth, the spatial distribution of individuals, and the pattern of movement between populations. It is therefore not surprising that there are often great fluctuations in the numbers of individuals in local populations and that the long-term persistence of species may often require the conservation of many, rather than a few, populations.
The landscape in The Netherlands is very fragmentated. Forests that use to be widespread became more and more reduced in size ever since the Middle Ages. Heaths and moors became very fragmentated and small-sized since the last century. The effects on the flora and fauna of these reductions and fragmentations are not well documentated, but its seems likely that the disappearance of the Black Stork (Ciconia nigra), the Lesser Spotted Eagle (Aquila pomarina), the Middle Spotted Woodpecker (Dendrocopus medius) and the Great Grey Shrike (Lanius excubitor) is totally or at least partly the result of the shrinking and fragmentation of their original habitats (foliage woodlands on wet soils, heathers and moors).
Research shows that the fragmentation is the main cause for the (temporal) disappearance of a great number of species (ants, amphibians, mammals and plants) in suitable habitats.
Fragementation not only effects the number and spreading of species, it also has influence on the genetic composition of populations. Under normal conditions populations have sufficient genetic variation. Individuals within a population are genetically different. The rate of homozygoty is generally low. The latter may cause deficienties such as high mortality and a low rate of reproduction. Normally these deficiencies are the result of inbreeding (inbreeding depression). The disappearance of the Middle Spotted Woodpecker (Dendrocopus medius) is probably caused by inbreeding depression as a result of fragmentation.
Fragmentation causes local extinction of species within local
populations. Therefore measurements to be taken in order to maintain or regain species in
a fragmentated landscape or region should deminish the chances for extinction or increase
the chances for recolonization. Such measurement are the improvement and enlargement of
the habitats of local populations and the making of connections between local populations.
Two or more divergent phenotypes in an environment may be favoured
simultaneously by diversifying selection. No natural environment is homogeneous; rather,
the environment of any plant or animal population is a mosaic consisting of more or less
dissimilar subenvironments. This is called environmental heterogeneity. There is
environmental heterogeneity with respect to climate, food resources, and living space:
spatial heterogeneity. Also, the heterogeneity may be temporal (temporal heterogeneity),
with change occurring over time, as well as spatial, with dissimilarity found in different
areas. Species cope with environmental heterogeneity in diverse ways. One strategy is the
selection of a generalist genotype that is well adapted to all of the subenvironments
encountered by the species. Another strategy is genetic polymorphism, the selection of a
diversified gene pool that yields different genotypes, each adapted to a specific
A life history is the sequence and timing of events that occur between birth and death. Populations from different parts of the geographic range that a species inhabits may exhibit marked variations in their life histories. The patterns of variation seen within and among populations are referred to as the structure of populations. These variations include breeding frequency, age at which reproduction begins, the number of times an individual reproduces during its lifetime, the number of offspring produced at each reproductive episode (clutch size), the ratio of male to female offspring produced, and whether reproduction is sexual or asexual. These differences in life history characteristics can have profound effects on the dynamics, ecology, and evolution of populations.
Populations often can be divided into one of two extreme types, based on their life-history strategy. Some populations, called r-selected, are considered opportunistic because their reproductive behaviour involves a high intrinsic rate of growth (r)--individuals give birth once at an early age to many offspring. Populations that exhibit this strategy often have been shaped by an extremely variable and uncertain environment. Because mortality occurs randomly in this setting, quantity of progeny rather than quality of care serves the species better. In another strategy, called K-selected, populations tend to remain near the carrying capacity (K), the maximum number of individuals that the environment can sustain. Individuals in a K-selected population give birth at a later age to fewer offspring. This equilibrial life history is exhibited in more stable environments where reproductive success depends more on the fitness of the offspring rather than on their numbers.
Differences in life-history strategies greatly affect population dynamics. Populations in which individuals reproduce at an early age have the potential to grow much faster than populations in which individuals reproduce later. The effect of the age of first reproduction on population growth can be seen in the life tables for a particular species. Life tables were originally developed by insurance companies to provide a means of determining how long a person of a particular age could be expected to live. They are used not only by demographers of human populations but also by plant, animal, and microbial ecologists to make projections about the life expectancies of nonhuman populations. The number of individuals in a closed population (a population in which neither immigration nor emigration occurs) is governed by the rates of birth (natality), growth, reproduction, and death (mortality). Life tables are designed to evaluate how these rates influence the overall growth rate of a population
In an ideal environment, one that has no limiting factors, populations grow at a geometric rate or exponential rate. Human populations, in which individuals live and reproduce for many years and in which reproduction is distributed throughout the year, grow exponentially. Exponential population growth can be determined by dividing the change in population size (N) by the time interval (t) for a certain population size (N). The growth curve of these populations is smooth and becomes increasingly steeper over time. The steepness of the curve depends on the intrinsic rate of natural increase for the population. Insects and plants that live for a single year and reproduce once before dying are examples of organisms whose growth is geometric. In these species a population grows as a series of increasingly steep steps rather than as a smooth curve.
Populations rarely grow smoothly up to the carrying capacity and then remain there. Instead, fluctuations in population numbers are the norm. In a few species, such as, lemmings, lynx, and Arctic foxes, populations show regular cycles of increase and decrease spanning a number of years. The causes of these fluctuations are still under debate by population ecologists, and no single cause may provide an explanation for every species. Most major hypotheses link regular fluctuations in population size to factors that are dependent on the density of the population, such as the availability of food.
Population ecologists commonly divide the factors that regulate the size of populations into density-dependent and density-independent factors. Density-independent factors, such as weather and climate, affect the same proportion of individuals in a population regardless of population density. In contrast, the effects of density-dependent factors intensify as the population increases in size. For example, some diseases spread faster in dense populations than in sparse populations. Similarly, competition for food and other resources rises with density and affects an increasing proportion of the population. The dynamics of most populations are influenced by both density-dependent and density-independent factors, and the relative effects of the factors vary among populations.
The size of other populations varies within tighter limits. Some fluctuate close
to their carrying capacity, others fluctuate below this level, held in check by
various ecological factors, including predators and parasites. The tremendous expansion of
many populations of weeds and pests that have been released into new environments in which
their enemies are absent suggests that predators, grazers, and parasites all contribute to
maintaining the small sizes of many populations. To control the explosive proliferation of
these species, biological control programs have been instituted. With varying degrees of
success, parasites or pathogens inimical to the foreign species have been introduced into
the environment. The European rabbit (Oryctolagus cuniculus) was introduced into Australia
in the 1800s, and its population grew unchecked, wreaking havoc on agricultural and
pasture lands. The myxoma virus subsequently was released among the rabbit populations and
greatly reduced them. Populations of the prickly pear cactus (Opuntia) in Australia and
Africa grew unbounded until the moth borer (Cactoblastis cactorum) was introduced. Many
other similar attempts at biological control have failed, illustrating the difficulty in
pinpointing the factors involved in the vulnerability of species, population and
As species adapt to one another and to their communities, they form niches and
guilds. The development of more complex structures allows a greater number of species to
coexist with one another. The increase in species richness and complexity acts to buffer
the community from environmental stresses and disasters, rendering it more stable.
Diverse communities usually are healthy communities. Long-term ecological
studies have shown that species-rich communities are able to recover faster from
disturbances than species-poor communities. Species-rich grasslands in the Midwestern
United States maintain higher primary productivity than species-poor grasslands. Each
additional species lost from these grasslands has a progressively greater effect on the
drought-resistance of the community. Similarly, more diverse plant communities in
Yellowstone National Park show greater stability in species composition during severe
drought than less diverse communities. And, in the Serengeti grassland of Africa, the more
diverse communities show greater stability of biomass through the seasons and greater
ability to recover after grazing.
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