.
Data analysis design and potential bias in radio-tracking
studies of animal habitat use
F. Palomares and M. Delibes
Estaciôn Bioldgica Doñana,
CSIC, Avda. Maria
Luiss s/n,
41013
Sevilla, Spain
Habitat preference, type of behaviour and pattern of daily activity are closely
related: an animal selects a particular habitat to perform a particular activity at a
particular time. These should be analysed jointly, otherwise, results can be inconsistent (HARRIS et at., 1990). In the last thirty years radio-telemetry has enabled great
progress in the study of the ecology and behaviour of free-living auimal.s, including
habitat preferences (AMLANER & MACDONALD, 1980; Mnoi, 1983). However, results
of radio-tracking studies of habitat use on animals can be misleading. For instance,
SERVHEEN (1983) did riot detect the use by grizzly bears Ursus arctos of the habitat
providing more food. SmvwroN and MAJOR (1982) found that martens Manes
americana used the habitat richer in raspberries I?ubus idaeus, one of their main
foods, far less than expected, and ARTHUR eta!. (1989) found that fisher, Manes
pennanti, used a lesser variety of habitat types than expected during summer, since
a greater diversity of food types (likely .to be found in most forest. types) was
a’saiiable to them in this stason. These authors, like others (e. g. LIT’S MTIS el at.,
1986; B1osci & HARE5TAD, 1990; CRAWSHAW & QUIGLEY, 1991), estimated habitat
use from the proportion of radio-fixes in each habitat, irrespective of the time of
day and the activity being performed. Their results strongly suggest that most
fixes were on inactive animals, and so foraging habitat was underestimated, with
potentially important management and/or ecological consequences.
In order to reduce the bias associated with the time of tracking, some authors
have considered day and night localizations separately, assuming that one period
roughly corresponds to activity and the other to rest (e.g. ANDELT, 1981;
HAROLD5ON & FRITZELL, 1984; ART0I5 et at., 1990). However, in our knowledge
results from these attempts have not been contrasted to direct measures of habitat
use.
In this paper we review data analysis designs used in some radio-tracking
studies of the recent literature on habitat use, and proved with Egyptian mongooses,
Herpesles ichneumon, radio-tracking data, how analysis design can affect the obtai-
ned results.
L
-
REVIEW OF RECENT LITERATURE
Issues from January 1989 to October 1991 of the ten following scientific
journals were searched for papers using radio-tracking data for habitat use analysis
in animals: Acia Oecologica, The Canadian Field-Naturalist, Canadian Journal of
Zoology, Holartlc Ecology, Journal of Animal Ecology, Journal of Applied Ecology,
Journal of Wildljfe Management, Journal of Zoology, Journal of Mammalogy, and
Wild4fe Monographs. Eight of them provided one or more suitable references, and
a total of 40 papers (24 on mammals and 16 on birds) were included in the review.
For the review, only papers performing a full analysis on animal habitat use and
relating habitat types to number of locations in each one, were included. For each
paper, information on data analysis design was collected.
Most papers (62.5%) analyzed habitat use irrespectively of activity being
performed by the animals and the time of day. This was more frequent in mammals
(70.8%) than in birds (50%). Another frequent design consisted in analyzing habitat
use at different time periods of the day: daytime and nightime (15%), or usually 4
periods (10%). This design was more frequent (37.5%) in birds than in mammals
(16.6%). Finally, only in S cases (12.5%, 3 in mammals and 2 in birds), habitat use
was estimated distinguishing between active and inactive bearings.
Review proved that most papers may supply information about an “average
habitat” that actually animals never select (see below). Besides, few of them
discussed the possible problems with their sampling or data analysis design (see
ARTHUR et al. (1989? as a good exception).
IMPORTANCE OF DATA ANALYSIS DESIGN ON HABITAT USE RESULTS
We will use radio-tracking data on Egyptian mongoose habitat use to illustrate
the importance that data analysis design has on the obtained results.
We were able to continually track on foot and at short distances (30-bOrn)
30 Egyptian mongooses in different seasons and zones of Doflana National Park,
SW Spain (ca. 37°N, 6°30’W) from November 1985 to September 1989 (PALOMARE5,
1986, 1990). Egyptian mongooses were equipped with radio-collars containing
motion sensor. Radio-tracking was carried out without disturbing their behaviours:
in more than 40% of 82 direct observations, radio-tracked mongooses were unaware
of our presence and in only 20% they fled at full speed. Moreover, no appreciable
differences in activity time were detected in days when we performed a short
distance tracking and days when we used triangulation (PALOMARE5, 1990). This
allowud us to know the actual time spent by the mongooses in each habitat, at
each hourly time and while they were active or resting. Hence, we could estimate
habitat preference in three ways: (a) for the whole 24 hours-period, (b) separately
for day and night, and (c) separately for the active and resting time. The comparison
of these three approximations will answer the following question as well: when all
24-hours localizations are either pooled, or considered separately for day and night,
what type of habitat selected by mongooses is being favoured: resting habitat,
foraging (and traveling) habitat or a misleading mixture of both?
For the purposes of our analysis we selected three adult females living in quite
similar zones and radio-tracked intensively for 28 independent 24 hours periods
from November 1985 to April 1986, the rainy period of the year. This minimized
potential variations due to age, sex, zone and season.
Ada tEcologica
We distinguished eight habitat types in the study area. They are listed in
decreasing order of vegetal cover: brambles (BR), hygrophitic scrubland (HS),
xerophitic scrubland (XS), dune hollows (DH), marshes (MA), rushes (RU), bare
sand dunes (BD) and grassland-bracken (GB). Since BD and GB (lacking in scrub)
were never used by the radio-tracked mongooses, so the analysis was limited to
the other six habitats.
Habitat use was determined from the amount of time the animals spent in
each habitat type. Six estimates of habitat use were made:
1. Overall habitat use: percent of the total study time spent in each habitat.
This is equivaLent to an analysis of habitat use by pooling all radio-tracking fixes.
2. Night time habitat use: percent of the time between sunset and sunrise
spent in each habitat type. This is equivalent to an analysis of habitat use employing
only nocturnal fixes.
3. Day time habitat use: like 2, but between sunrise and sunset. This is equivalent to an analysis of habitat use employing only diurnal fixes..
4. Resting habitat use: percent of actual resting time spent in each habitat,
from the daily end of activity to the beginning of further activity.
5. Active habitat use: percent of the active time spent on each habitat.
6. “Siesta” habitat use: percent of the resting time, intercalated between
activity, spent on each habitat.
The results of the six analyses were compared by the Similarity Index (SI) of
COUWELL and FUTUyMA (1971) and the Tukey test (ZAR, 1984). Habitat use diversity
(B) was estimated by the index of LuviNs (1968).
A detailed description of study area, characteristics of habitat types, habitat
use of these animals, and method of radio-tracking can be found, in PALOMARE5
and DeLmus (1990).
I. — Percentage of time spent by mongooses in six djfferent habitats, and estimated by six
approximations: pooled data, separating nocturnal and diurnal periods, and separating night resting
period, “siesta” period and activity period.
TABLE
Habitat
serubland
Hyg.
scrubland
Brambles
Dune
hollows
Marshes
Rushes
Night
Day
9.9
3.4
16.3
10.5
65.2
10.8
75.4
6.3
4.0
4.1
6.9
3.3
0.0
Overall
Resting
Siesta
Activity
3.4.
8.8
26.8
10.2
55.1
10.8
75.4
11.7
65.9
13.0
35.6
5.6
6.9
4.6
8.1
3.4
0.0
7.8
0.0
5.7
3.6
6.1
14.7
.
‘
Mongooses selected different habitats for resting, siestas and activity (table I).
Diversity of used habitat was low for resting (B = 0.69), intermediate for siestas
(B= hO) and higher for activity (B=2.13).
Overall habitat use was very similar to that for siestas (SI = 0.955) and resting
(SI 0.900), but much less similar to habitat used for activity (SI 0.505). Since
=
=
mongooses devoted 75% of each 24hours period to resting (PALOMARES & DamEs,
1992), this result was predictable.
By comparing the overall habitat use result with that for the active period,
some important foraging habitats like XS, in which mongooses search for rabbit
warrens, and RU, where they search for carrion (PALOMARES & DELmEs, 1990)
appear underestimated (q=4.80, p.<0.0O1 for XS and q=4.19, p<O.Ol for RU).
On the other hand, BR, which is mostly used for resting, appears overestimated
(q6.28, p <0.001).
These results explain why in some studies where all localizations were pooied,
foraging habitats appear systematically less used than expected. This could be
because the animals, like the mongooses in our study, devoted a short time to
forage, and that many of the radio-fixes corresponded to resting individuals.
Another possibility is that the animals were mainly nocturnal, while radio-tracking
was made predominantly by day. In both cases, the importance of the habitat
selected for activity would be underestimated. In animals with diurnal activity,
habitat selected for foraging would be overestimated if radio-tracking is mainly
carried out during the day.
The indepcndent analysis of night and day habitat use improved the results to
some extent (table I). The time spent in each habitat from sunset to sunrise
described quite accurately night resting habitat use (SI =0.985) and even habitat
used for siestas (51=0.870), but not activity habitat use (S10.570). On the other
hand, day time spent in each habitat described better siesta habitat (51=0.855),
than foraging (or activity) habitat (SI = 0.785). However, the use of several habitat
types was again underestimated, specially XS (q=2.88, p<O.O5), while the use of
BR w.s overestimated (q4.l0,p<O.Ol). This bias is due to the fact that mongooses remained inactive during part of the day: on average mongooses rested for 16
consecutive hours (including the whole night, 2 hdurs after sunrise, and an hour
before sunset) and 2 hours during siestas (PALOMARES & DELIBE5, 1992).
The extent of the bias in the separate analysis of daytime and nightime data
obviously depends on the çircadian pattern of activity of the studied animal. As
periods of activity usually fail to coincide precisely with sunrise and sunset (e. g.
KAvANAU & RAM05, 1975), a greater subdivisiOn of the 24 hours cycle, would make
the results more precise.
CONCLUSIONS AND RECOMMENDATIONS
In summaring, resting and foraging habitats of animals arc likely to be different
because cues for habitat selection should be different for resting (i. e. safety) and
for hunting (i. e. food availability). The results presented here illustrate the problems
associated with the study of habitat use by radio-tracked animals:
I. As the patterns of habitat use for resting and foraging usually dìfTer, the
pooling of all the radio-tracking localizations provided a biased information about
both patterns.
2. The separate analysis of daytime and nighttime data provides a greater
accuracy, but information may be biased towards foraging or resting habitat
depending on the activity time budget of the animal.
These results strongly suggest that:
I. Sampling oi data analysis design should be conducted to allow habitat use
analysis during different activities (at least for active and inactive individuals), in
order to be able to distinguish habitat selection in each case.
2. When the discrimination between active and inactive animals is not technically possible, the limitations that it entails should be taken into account and
results discussed accordingly (e. g. see ARTHUR eta!., 1989).
ACKNOWLEDGEMENTS
The research was supported by Consejo Superio do Investigaciones Cientificas and Dirección
General de Investigacibn cientifica y Tbcnica (project PB87-0405). One of the authors (FP) has a
postdoctoral grant of Consejo Superior de Investigaciones Cientificas. We thank Dr. V.0. NAM5 for
reviewing the manuscript and N. BUSTAMANTE for reviewing the English. An anonymous referee provided
helpful comments on the original manuscript.
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