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Good Neighbours? Determinants of Aggregation and Segregation among AlpineHerbivoresAuthor(s): Isabel C. Barrio & David S. HikSource: Ecoscience, 20(3):276-282. 2013.Published By: Centre d'études nordiques, Université LavalDOI: http://dx.doi.org/10.2980/20-3-3595URL: http://www.bioone.org/doi/full/10.2980/20-3-3595
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Understanding the structure of biotic communities and how different species coexist is one of the main challenges of community ecology (Agrawal et al., 2007). Mechanisms that allow for coexistence depend on the strength and direction of the interactions among the co-occurring
Manipulative experiments are required to infer mechan-
manipulation is not always feasible for some study organ-isms or broader scale studies, so alternative approaches are
needed. Studies of community structure and species inter-actions often involve identifying and quantifying patterns
Darmon et al. -bution of organisms is a prerequisite for the occurrence of interactions among them. In this context, null model analy-ses can provide a means to study mechanisms where experi-
Such analyses are becoming increasingly popular among population ecologists to investigate interactions among ani-mals (Richard et al.the need to jointly study both spatial structures and biotic interactions (Ritchie et al. et al.obtain a comprehensive understanding of the mechan-isms of coexistence. In this sense, determining the relative role of environmental (habitat) variables and interspecific
Good neighbours? Determinants of aggregation and segregation among alpine herbivores1
Isabel C. BARRIO2, and Instituto Pirenaico de Ecología (CSIC), Avda Nuestra Señora de la Victoria s/n, PO Box 64, Jaca, 22700 Spain, e-mail: [email protected]
David S. HIK,
Abstract: Interspecific interactions often determine the structure and stability of biotic communities. In low-productivity and highly seasonal environments such as the alpine tundra, most interactions occur during a short, snow-free period. The strength and direction of these interactions are likely to be determined by the availability of resources, particularly among species of the same ecological guild. Understanding how species interact in such environments can provide insights into the conditions that facilitate their coexistence. We determined the potential for interspecific interactions among 3 resident medium-sized mammalian herbivores inhabiting the alpine tundra and investigated how they share available space and resources. Overlap in their respective activity areas indicated that these species were aggregated at a landscape scale, but other mechanisms allowed their coexistence at a finer scale. Their distributions were primarily associated with shorter distances to heterospecifics and, secondly, with habitat features related to shelter and escape from predation. Our results suggest that these species can (and do) coexist by partitioning their ecological niches. Competition is likely not a major
herbivores in seasonal, low productivity environments.Keywords: alpine tundra, coexistence, facilitation, interspecific interactions, medium-sized mammals.
Résumé : Les interactions interspécifiques déterminent souvent la structure et la stabilité des communautés biotiques. Dans des environnements très saisonniers et peu productifs, comme la toundra alpine, la plupart des interactions se produisent durant la courte période sans neige. L'intensité et la direction de ces interactions sont probablement déterminées par la disponibilité des ressources, particulièrement entre les espèces d'une même guilde écologique. La compréhension de la façon dont les espèces interagissent dans de tels environnements peut fournir des indices sur les facteurs qui facilitent leur coexistence. Nous avons déterminé le potentiel d'interactions interspécifiques chez 3 mammifères herbivores de taille moyenne résidants de la toundra alpine et avons examiné comment ils se partagent l'espace disponible et les ressources. Le chevauchement de leurs aires respectives d'activité indiquait que ces espèces étaient regroupées à l'échelle du paysage, mais que d'autres mécanismes rendaient leur coexistence possible à plus fine échelle. Leurs distributions étaient associées en premier lieu à des distances plus courtes entre individus hétérospécifiques et, en second lieu, à des caractéristiques de l'habitat reliées à l'abri et à l’évitement de la prédation. Nos résultats suggèrent que ces espèces peuvent coexister (et le font) en se divisant les niches écologiques. La compétition n'est probablement pas un important facteur structurant ces
environnements saisonniers de faible productivité. Mots-clés : coexistence, facilitation, interactions interspécifiques, mammifères de taille moyenne, toundra alpine.
Nomenclature
Introduction
Associate Editor: Daniel Fortin.2Author for correspondence.
ÉCOSCIENCE, VOL. 20 (3), 2013
277
interactions in shaping species’ spatial distribution can pro-vide insights into such mechanisms (Azeria, Ibarzabal &
Interactions among co-occurring organisms may show considerable plasticity depending on the local biotic and
-tive interactions prevail over competition in determining the structure of plant communities in harsh environments (Callaway et al., 2002). However, for herbivores inhabiting less productive ecosystems, competition is thought to be the dominant form of interaction (e.g., Mishra et al.Cheng & Ritchie, 2006), although positive interactions may also play an important role in their coexistence (Gross,
-atures, and low net primary production, tundra ecosystems can support a high biomass of herbivores, among which mammalian herbivores are often abundant (Jefferies, Klein
critical period for herbivores to accrue energetic reserves -
petition for resources might be highly localized in time (Xi,
may be crucial in determining the structure and stability of these herbivore communities and how they respond to environmental changes.
We analyzed the potential for (positive or negative) biotic interactions among 3 alpine mammalian herbivores: hoary marmots (Marmota caligata), arctic ground squir-rels (Urocitellus parryii), and collared pikas (Ochotona collaris). Broadly, these species have similar diets, are ter-ritorial, and can be considered central place foragers, but some differences in their life strategies and behaviour allow them to cope with harsh environmental conditions. Marmots
while pikas remain active in winter below the snow cover. In contrast to marmots, pikas and (to a lesser extent) ground squirrels cache food for over-winter survival and emergence from hibernation, respectively. Following the approach pro-posed by Darmon et al. -bution and co-occurrence of the 3 species. Specifically, we used null models to determine if the overlap between their spatial distribution was random (neutral interaction), larger than expected by chance (positive association), or smaller than expected by chance (avoidance). Then, we analyzed the habitat selection of each species in the presence of the other 2, to determine the extent to which the aggregation or segregation of their distribution patterns could be attributed to environmental factors or to biotic interactions with sym-patric herbivores. Given the low productivity of alpine tun-dra ecosystems, we predicted all 3 species to be aggregated in space as a result of similar diet and habitat requirements. However, we also predicted segregation at a finer scale in order to reduce competition for essential resources, particu-larly access to foraging areas and refuges from predation or perceived predation risk.
MethodsSITE AND STUDY SPECIES
The study was conducted in the Ruby Range N W), southwest Yukon, Canada, in a
valley (about 7.2 km2
to 2000 m, where 3 main herbivore species coexist at rela-tively high densities. Landscapes are composed of a mosaic of alpine meadows and tundra vegetation (Hik, McColl
with boulder fields (i.e., talus patches), which represent
–2
David S. Hik, unpubl. data).Three species of medium-sized herbivores, ranging in
and are the dominant herbivores: hoary marmots, arctic ground squirrels, and collared pikas. All 3 species have been the subjects of extensive study at this site over the past
(caribou, Rangifer tarandus Ovis dalli) or more locally (voles, Clethrionomys spp. and Microtus spp.) or exploit other feeding resources (ptarmigan, Lagopus spp.). Predators are relatively low in abundance (Hik, McColl &
SURVEY
Signs of active presence of the 3 herbivores were
active. One observer (ICB) slowly walked the whole study area inspecting for presence signs everywhere. Signs of
were recorded and geo-referenced using a hand-held GPS receiver. Active burrows were considered to be signs of
the presence of active haypiles (over-winter food stores) were considered signs of presence of pikas. Active burrows of marmots and ground squirrels had fresh pellets or recent
pellets. Activity of haypiles was assessed by the presence of
August, as some pikas may only collect vegetation later in the season (Morrison et al.
Since all 3 species are central place foragers, “high use areas” were defined as those within a certain distance of each species’ respective signs of active presence, accord-ing to the literature available from the study area and
maps of the 3 species yielded a map of herbivore activity
(Table I). To correlate our map to the actual presence of animals we generated a set of random observation points within each intersection category sufficiently represented in
in each category, and the effort was increased in the mar-
Occurrence of the 3 species was assessed by checking points for signs of recent activity (see above) and con-
-ure cannot be considered a true validation of the map,
BARRIO & HIK: SPATIAL DISTRIBUTION OF ALPINE HERBIVORES
because different methods were used, it provides an estimate of the map’s reliability. Overall, the agreement between the activity at observation points and predictions
of direct observations compared to the cumulative activ-ity recorded for the map, we consider this agreement to be very good. To further evaluate potential direct behav-
-
where the mapped distributions of the 3 species overlapped. -
(e.g.marmots and pikas were together. We interpreted these cases as “potential direct interactions”.
STATISTICAL ANALYSES
To test the overlap between the areas of activity of the herbivore species we used a null model (Gotelli &
3 species by generating a number of random points equal to the number of locations recorded in the field (npikas = 206, nmarmots nsquirrels = 746). In the case of pikas, ran-dom points were restricted to talus patches, because the distribution of haypiles is a priori known to be limited to
-domly generated points for pikas lie within a biologically meaningful habitat type. Based on the randomly generated points and the buffer distances considered for each species (see above), we then calculated the randomized high-use areas for each species and their overlap and compared the observed area for each category with the distribution of
P-value for this test.To determine habitat selection of each species we com-
bined our map of species activity with maps of potentially relevant habitat and interspecific variables (distances to locations for the other 2 species). Habitat variables included topographic characteristics (elevation, slope, and aspect at
distances rather than the presence/absence of talus enabled us to account for potential edge effects (Conner, Smith &
FIGURE -
0 500250 1000 m
All 3 herbivores
Marmots and squirrels
Ground squirrel only
Marmot only
No herbivores
N
ÉCOSCIENCE, VOL. 20 (3), 2013
Burger, 2003). Aspect was linearized using a sine trans-
& Hik, 2004). No detailed vegetation map was available at this fine scale, but plant communities are closely deter-mined by local topography in this area (Danby et al.
to low-elevation, southwest-facing gentle slopes.We used variance partitioning to decompose the varia-
tion in the occurrence of each species among 2 blocks of predictors, habitat and interspecific variables, resulting in 3 fractions: the pure effects of habitat alone, those of interspecific variables alone, and the joint effects of both (Figure 2). We used Generalized Additive Models (GAM) with binomial errors and a logit link to calculate the deviance of the general and partial models (Legendre
locations where signs of activity were found and an equal number of random absence points. As the whole area was censused for presence signs, these points can be safely considered true absences. Within each block of predictors
we excluded variables that did not contribute significantly (Pstepwise procedure. We corrected for spatial autocorrelation in all models by including a non-linear spatial term (Bivand,
dimensional smoother of the spatial coordinates. Including the smoother removed spatial trends from the residuals of the model, as assessed by visually inspecting spatial cor-relograms, making the assumption of spatial independence
spatial correlograms.
ResultsThe presence of alpine herbivores was aggregated at a
broader scale, occurring over smaller areas than expected at random (P
the area occupied by herbivores, areas where more than
suggesting some avoidance mechanisms, such as direct avoidance of heterospecifics or segregation among species at a finer scale. Conversely, areas occupied by each species alone were larger than expected at random except for pika-only areas, which did not show a clear trend (PThe main part of the study area was shared by marmots
mainly where the 3 species overlapped (Table I).This spatial pattern was further supported by the habitat
selection of each species. Interspecific variables accounted for the highest proportion of the variance in the occur-rence of the 3 herbivores (Figure 2), while habitat variables
of pure interspecific variables was similar across species
TABLE I. Intersection categories of activity areas of the 3 alpine herbivores. Null model comparison indicates whether the ob-
P-
our map of herbivore activity to the actual presence of herbivores. Absence of values are noted with (.)
Category Area (km2) surface comparison points
arctic ground squirrel
FIGURE 2. Variance partitioning of the factors determining the distribution of 3 mammalian herbivores in the alpine tundra of southwest Yukon. The bars show the contribution of each block to the variance explained for each herbivore, when spatial structures were taken into account.
Hoary marmot
Arctic ground squirrel
Collared pika
Proportion of explained variance
Unexplained variance Habitat Interspecific Shared
Variance partitioning
0 0.20 0.40 0.60 0.80 1.00
BARRIO & HIK: SPATIAL DISTRIBUTION OF ALPINE HERBIVORES
The occurrence of all species was related to shortest dis-tances to heterospecifics and talus patches (Table II). In the case of arctic ground squirrels, within the interspecific
-tion of arctic ground squirrels was also positively linked to southwest-facing slopes.
DiscussionThe 3 alpine mammalian herbivores were aggregated at
a broad scale but segregated at a finer scale, and their dis-tribution was mainly determined by interspecific variables. This scale dependence is not surprising, since processes determining co-occurrence depend on the spatial scale at which species associations are analyzed (Redfern, Ryan & Getz, 2006). Co-occurrence at a coarse habitat scale may result from abiotic constraints on herbivore distribu-tion (Redfern, Ryan & Getz, 2006) and does not neces-sarily imply sympatry at the smaller patch scale, because patches can be exploited differently by each species (Martin
-tion of marmots, pikas, and squirrels may suggest similar ecological requirements, and might be related to environ-mental variables not accounted for in the present study. The distribution of these species overlaps in different parts of their ranges, and similar associations have been described
American pikas (Ochotona princeps), hoary marmots, and Columbian ground squirrels (Urocitellus columbianus) in talus slopes of Glacier National Park (Montana, USA). He hypothesized that, despite their similarities, these species were able to coexist due to differences in their spatial and
although the foraging areas of marmots and squirrels over-lapped, marmots tended to forage closer to boulder fields
-iods (Morrison et al.timing of their energetic requirements suggests differences in the intensity and timing of use of certain areas. Being obligate hibernators, ground squirrels and hoary marmots need to maximize fat gain, and their activity peaks in
increase their foraging behaviour later in the season, as they can rely on dry vegetation to store in their haypiles (Morrison et al.herbivores at a local scale seems then to be a result of eco-logical differences among species, rather than territorial competition for space, as has been described for other taxa
According to niche theory, coexistence requires some form of differentiation or partitioning between species, which might be allowed by neutral relationships, facilita-tion processes, or segregation at finer scales of the species’ ecological niche (e.g.relationships may result from the exploitation of different resources, when species share non-limiting resources or when other mechanisms (e.g., disturbance) prevent com-
-ate the habitat for other co-occurring species (Stachowicz,
3 species has been suggested, it has not been clearly dem-onstrated and no (direct) behavioural interactions have been described (Morrison et al.contrary, it has been hypothesized that these species may
For example, pikas are known to respond to sympatric heterospecific calls from hoary marmots and arctic ground
which may help them to escape from shared predators. Ground squirrels seem to respond as well to such calls, but
Marmota flaviventris). Other mechanisms by which these species may facilitate each other are habitat modification, through sharing burrows (Karels, Koppel & Hik, 2004), and feeding facilitation (Arsenault & Owen-Smith, 2002). For example, pika herbivory dramatically alters plant community composition along the edges of
-ability to the other herbivore species, as has been shown for other grazers (Mysterud et al.strength and direction of the interactions among these spe-cies does not need to be symmetrical, especially among
For example, in our study pikas were positively associated to shorter distances to arctic ground squirrels, but ground squirrels were not affected by the presence of pikas. Further studies identifying the mechanisms by which these species interact will help understand how biotic interactions struc-ture alpine herbivore communities.
Habitat selection by the 3 herbivore species was mainly determined by interspecific variables and to a lesser extent by abiotic habitat variables. In all cases, spatial distribution
TABLE II. Variables explaining the distribution of hoary marmot (a), arctic ground squirrel (b), and collared pika (c) in southwest Yukon, as assessed with Generalized Additive Models. The non-linear terms (s[x,y]) account for the spatial trends in the data.
a) Hoary marmot Estimate (± SE) Z P-value
edf a 2 P-values(x,y b) Arctic ground squirrel Estimate (± SE) Z P-value
2 P-values(x,y c) Collared pika Estimate (± SE) Z P-value
2 P-values(x,y
a edf: estimated degrees of freedom.
ÉCOSCIENCE, VOL. 20 (3), 2013
was associated with closer distances to other herbivores, suggesting a positive effect of the presence of heterospecif-ics. For example, when sharing predators, less preferred prey might select habitats supporting high densities of com-petitors that are a preferred prey because of reduced preda-
mere presence of heterospecifics can also provide valuable information to animals, providing them with cues to habitat
presence of heterospecifics could reflect other habitat vari-ables not accounted for in the present study. In our study, habitat variables mostly reflected areas maximizing shelter from predators and weather extremes, i.e.in the case of ground squirrels, distribution was also related to more productive southwest-facing slopes. Predation risk is a main constraint for the foraging activities of these herbivores (Morrison et al., 2004) even if predator pressure might not be as high as in other habitats (Hik, McColl &
Describing the spatial associations and ecological requirements of coexisting species is a first step to under-standing the potential mechanisms involved (Darmon et al.,
studying interactions when experimental studies are not an option (Richard et al. -gests that these 3 alpine herbivores can (and do) coexist, by partitioning their ecological niches at a finer scale. Their aggregated pattern, together with other lines of evidence, may imply that competition is not playing a major role in
interactions may play an important role in allowing the coexistence of herbivores inhabiting stressful, less product-ive ecosystems (Barrio et al.
year-round) restrict their direct interactions to the summer, when resources are most abundant in the tundra and compe-tition might be less important.
Our work represents a snapshot of a dynamic picture, and changing environmental conditions can reverse the strength and direction of interspecific interactions (Dunson
-able to ongoing climate change (Post et al. -standing these interactions and how they may change will help anticipate the potential responses of alpine herbivore guilds and their cascading effects on ecosystem functioning (Jefferies et al.
AcknowledgementsSpecial thanks are due to F. C. Hik, A. Kolar, and R. Mitten
for their invaluable help in the field, to S. Williamson for assist-ance with GIS, C. Calenge for statistical advice, and C. G. Bueno for helping with the modeling. T. Bao, E. Cameron, S. Nyanumba, K. Peck, A. Shaw, and H. Wheeler provided useful comments on an earlier draft. Funding was provided by the Natural Sciences and Engineering Research Council (Canada). I. C. Barrio was sup-ported by a postdoctoral fellowship provided by the Consejería de Educación, Ciencia y Cultura (JCCM, Spain) and the European Social Fund. We thank Kluane First Nation for permission to con-duct this research on their traditional lands.
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