The way that a group of individuals
of the same species interacts over time in a certain defined space can be
called population dynamics. Population dynamics of a certain species can be
driven by both interactions with individuals of the same species (intraspecific interactions) and with
individuals from other species (interspecific interactions). Populations are also
dependent on resources like space, light, water, or food, and have a carrying capacity, the number of
individuals that a habitat can support due to limited resources. Some
populations can also grow and interact with resources and other species
differently when they are below a certain threshold size or density. A good
understanding of species population dynamics can help us understand species’
distributions – why a species is found where it is. When habitats become
fragmented by natural disturbances like fire and earthquakes, or manmade
disturbances like agriculture and construction, the effects on population
dynamics can be complicated. Understanding these effects is critical in
conservation efforts.
Births, deaths, immigration, and emigration, which add up to
population size, are all influenced by competition among individuals and
between species for resources like space, light, water, or food. <http://bio1152.nicerweb.com/Locked/media/ch53/53_03PopulationDynamics-L.jpg>.
This study manipulated different
aspects of fragmented orchard meadow habitats in the Leine-Weser region in
southern Lower-Saxony, Germany, from 1998 to 2003, and observed the response of
populations of Osmia rufa, a solitary
bee, and its natural enemies, in order to tease apart the effects of habitat
fragmentation and to determine which are the most influential on bee population
dynamics. They hypothesized that there are three main factors: nesting
resources, pollen availability, and natural enemies. O. rufa is a solitary mason bee that nests in pre-existing holes in
twigs or logs, or in hollow plant stems. They are fairly general feeders. Their
enemies include a variety of parasites, most of which attack larvae in the
nest. In order to quantify and compare these three factors, the researchers
placed artificial nests in the center of each meadow, and then each winter
(when the larvae were dormant) the artificial nests were retrieved to determine
birth, mortality, and parasitism rates. Population growth rate was compared
with habitat connectivity and area as well as parasitism and mortality rates.
This study provided strong evidence
that nest availability has a bottom-up controlling effect on O. rufa populations: after 5 years with
artificial nests, populations grew to be, on average, 35 times larger. Food
availability had no significant impact on populations, even when they were
hugely inflated by the presence of artificial nest sites. However, the authors
speculate that more specialized feeders could be limited by food availability.
They found that small populations were more vulnerable to parasitic enemies and
hypothesized that this is because in denser populations, it is less likely that
nests will be left totally unguarded when parents leave to forage. This is
important because it suggests that dwindling populations already limited by
lack of nesting sites are increasingly threatened by predators as their numbers
shrink. Regarding the effects of spatial aspects of habitat, the scientists
found that although the bees are capable of migrating much farther than their
typical foraging distance of less than one kilometer, about 80% of females
built their nests at the same site where they were born.
Cross-section of an O.
rufa nest. These bees construct partitions between each larvae and fill
each chamber with pollen. <http://upload.wikimedia.org/wikipedia/commons/8/87/Osmia_rufa_nest.jpg>
Although
similar studies have been conducted about larger herbivores, this piece sheds
light on the population dynamics of pollinators. Red mason bees are
particularly economically important, as they are widely managed to pollinate
orchards. While this study finds that
providing these bees with nesting material increases their population, it
cannot speak to other, more specialized bees that are more dependent on native
plants. For these bees, spatial arrangement and food availability may be more crucial
to population dynamics, so strategic placement of food or nesting resources
along corridors or in way stations could be a management option to preserve
other more species that are more threatened by the effects of habitat
fragmentation.
References:
Steffan-Dewenter, Ingolf and Susan Schiele. 2008. Do
Resources or Natural Enemies Drive Bee Population Dynamics in Fragmented
Habitats? Ecology 89(5): 1375-1387. http://www.esajournals.org/doi/pdf/10.1890/06-1323.1
Links:
Osmia rufus factsheet:
http://hymettus.org.uk/downloads/Info_sheets_2010/Osmia_rufa_infosheet.pdf
More information on solitary bees and bee conservation: http://www.xerces.org/pollinator-conservation/native-bees/#
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