|
AUCO LONG-TERM ECOLOGICAL
RESEARCH site |
|
|
|
|
|
Research Highlights |
|
|
Birds and Predators
|
On the decline and recovery of the bird
assemblage in Auco in response to an extended drought and subsequent ENSO |
|
Auco, and all of the semiarid
region of Chile, is influenced by ENSO (El Niņo Southern Oscillation). Its
absence causes droughts and its presence wet years, which in turn result
in variations in resource levels for avian assemblages. We show that bird
species richness and density follow some of these pulses closely. A total
of 61 bird species, 32 of which were Passeriformes, was sighted during 1993-1997.
Overall, 30 species (49%) were residents and 31 (51%) were migratory. The
most speciose trophic groups were insectivores (34%), carnivores (28%),
and granivores (25%). Bird species richness and density declined from 43
species and 45-50 individuals/ha in spring 1993 to 29 species and 15-20
individuals/ha in autumn 1996. |
|
 |
|
An increase was observed with the onset of El Niņo, reaching totals
of 42 species (a 45% increase from 29) and densities of 55-60 birds/ha in
summer 1997. Similar trends were observed in one of two major food resources
measured: among small mammals but not among terrestrial arthropods. Positive
correlations were found between raptor species richness and density and
small mammal density, and not between insectivorous bird species richness
or density and terrestrial arthropod abundance. Because the climate
was very dry during most of the time of our study, we may have witnessed
the lowest boundary for species richness and bird density. Whether the 1997-98
El Niņo brought the maximum bird species richness and density for the site
is yet to be seen. |
|
 |
|
Back to Top |
|
On the numerical and functional responses of
predators to a long-term decline in mammalian prey in Auco |
|
Occurrence and diet of ten
carnivorous predators (four falconiforms, four owls, and two foxes), as
well as population levels of their mammalian prey, were monitored over 1987-1990
in Auco. Early in this period, small mammals irrupted and then declined
markedly to a density 7% of that at peak. All four falconiforms (Buteo
polyosoma, Falco sparverius, Geranoaetus melanoleucus,
Parabuteo unicinctus) and one owl (Tyto alba) responded
numerically to the decline in mammalian prey by virtually abandoning the
study site. |
|
 |
|
The three other owls (Speotyto cunicularia,
Bubo virginianus, Glaucidium nanum) and the two foxes
(Pseudalopex culpaeus and P. griseus) remained. With few
exceptions, throughout the study predators maintained species-specific preferences
among small mammal species regardless of the absolute and proportional abundance
of these prey. At no time did the two prey species most responsible for
the irruption (the rodents Phyllotis darwini and Akodon olivaceus)
occur in predators' diets out of proportion to their estimated relative
abundance in the field. Predators were clearly unable to prevent the irruption
from occurring. Given the absence of a clear
functional response to the most irruptive species, predators seemed unlikely
to have been responsible for the observed crash.They may, however, be prolonging
the crash and delaying the return of small-mammal populations to typical
densities. |
|
Back to Top |
|
On diet and prey selectivity by the Burrowing
Owl Speotyto cunicularia in Auco |
 On a seasonal basis, we quantified the
diet of Speotyto cunicularia throughout seven years (1987-1993) in
Auco, and detected prey selectivity in comparison to the proportional abundance
of small mammals in the field (the main prey of this owl). The main numerical
component of the diet was insects (79%), specifically families in the order
Coleoptera (Tenebrionidae, Scarabaeidae, Carabidae, and Curculionidae),
and to a lesser extent, arachnids (11%). Vertebrates accounted for only
10% of the total numer of prey, but their energy content largely exceeded
that of insects and arachnids. Among mammals (6% of total prey), the Cricetid
rodents Phyllotis darwini, Akodon olivaceus and Oligoryzomys
longicaudatus, and the Didelphid marsupial Thylamys elegans,
were the most frequent and constant in the diet of the owl. There was a
consistent and significant (P < 0.05) underconsumption of Abrothrix
longipilis, Chinchilla lanigera, Octodon degus, and P.
darwini. However, there was a marginally significant (P < 0.07) overconsumption
of T. elegans. Underconsumed mammals exceeded 45 g on average, whereas
overconsumed ones did not exceed 28 g (with a single exception). Thus, there
seems to be a maximum critical prey size that S. cunicularia es capable
of handling. Amphibians made up only 4% of total prey, whereas reptiles
and birds appeared only as trace elements (<< 1%).
During fall and winter an increase
in the consumption of insects generally occurred, whereas during spring
and summer an increase in the consumption of arachnids and vertebrates was
noted. These results indicate that S. cunicularia is a seasonally
opportunist predator.
|
|
Back to Top |
|
On the long-term dynamics of guild structure
among predatory vertebrates in Auco |
|
During 1987-1990 we monitored
diets and populations of 10 species of predatory vertebrates (two foxes,
four falconiforms, and four owls) in Auco. Pellets (birds) and feces (foxes)
collected monthly contained identifiable remains of small mammals, birds,
reptiles, amphibians, large insects, other arthropods, and (in some fox
feces) fruits.
 For non-mammalian prey, seasonal variation
in availability appeared to be more pronounced than longer-term variation. In contrast, availability of the eight
species of small mammals varied relatively little between predators' breeding
and non-breeding seasons but declined by an order of magnitude through the
first 3 yr of the study. Rather than responding to these shifts in food
availability by diverging in diet during "lean" times and converging
opportunistically during "fat" times, predators in Auco responded
in idiosyncratic ways. Predators of all species tended to maintain species-specific
means in size of prey taken regardless of shifts in the relative availability
of prey size classes. Two owl species maintained strict mammal diets throughout
the decline in mammal abundance, whereas two other owls plus the two fox
species maintained a tight feeding guild based on arthropods as well as
mammals and other food groups.
The four falconiforms failed
to maintain consistent diets from season to season, and consequently ended
up temporarily in unique positions in "diet hyperspace" or else
joined temporarily one of the two consistent guilds. The degree to which
falconiform diets were unique in a given season showed no clear relation
to food abundance. Changes in use of habitat did, however, relate to changes
in food abundance. None of the falconiforms resided continuously in the
study area throughout the period, and their occurrence was particularly
sporadic late in our study even for falconiform species that were not particularly
mammalivorous. |
|
Back to Top |
Home |
|
|
|
