ecology of phasmids - KLUEDO - Universität Kaiserslautern

Life history & potential population growth 31
3 Life cycle, potential population growth, and egg hatching
failure of Metriophasma diocles
3.1 Introduction
Great variation in patterns of population densities clearly exists in herbivorous insects, but generally
most insect species in the tropics occur in low numbers (e.g., Elton 1973; Basset et al. 1992; Basset
1997, 1999; Novotny & Basset 2000). Such low population densities reflect the biotic potential of a
species in the limiting boundaries of habitat capacity, intraspecific competition and regulating
mechanisms within trophic cascades.
A whole wealth of factors acts on individuals in a population thereby affecting net recruitment (i.e.,
births minus deaths). These factors either do not respond to population density or are density-dependent
and can be roughly categorized as: (1) intraspecific density-dependent factors, (2) environmental
density-dependent factors, and (3) density-independent factors.
The upper limit of population size is defined by intraspecific competition. Intraspecific competition as
density-dependent regulation factor increases with population size. Higher competition leads to
increasing death and decreasing birth rates resulting in a population decline (and vice versa). At a
certain density, birth and death rate are equal and there is no net change in population size. This density
is known as carrying capacity (K). The carrying capacity represents the population size the resources of
the environment can just maintain without a tendency to either increase or decrease (Begon et al. 1996).
However, natural populations lack simple carrying capacities. Alongside with intraspecific competition,
populations are regulated by multiple factors reducing the population size below the carrying capacity.
Most factors in the biotic setting of a species respond to population size. Density of prey translates into
predator densities (Volterra 1926; Lotka 1925), and diseases spread faster in higher populated areas
(e.g., Stevenson 1959). Increasing population density also involves decreasing food availability and
higher levels of interspecific competition (reviewed in Schoener 1983).
Other biotic factors like intrinsic food quality or breeding sites on the other hand do not relate to
population density, but their limited nutritious value or availability may act as regulators (Andrewartha
& Birch 1954, 1984). Likewise, abiotic factors such as climate and natural disasters are density
independent and can cause drastic impact on populations (e.g., Willig & Camilo 1991).
Releasing a population from regulating constraints can uncover the impact of the above described
control factors. When there are no limits on its growth, the population of a species will increase
infinitely according to its biotic potential, i.e. the inherent power of an organism to reproduce and
survive (Chapman 1931). The intrinsic rate of natural increase (r) will then be at its maximum. That
means each individual of the population will contribute to population growth with peak reproduction
(Begon et al. 1996). In natural populations, most individuals are not capable of peak productivity