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Large ‘apex’ predators influence ecosystems in profound ways, by limiting the density of their prey and controlling smaller ‘mesopredators’. The loss of apex predators from much of their range has lead to a global outbreak of mesopredators, a process known as ‘mesopredator release’ that increases predation pressure and diminishes biodiversity. While the classifications apex- and meso-predator are fundamental to current ecological thinking, their definition has remained ambiguous. Trophic cascades theory has shown the importance of predation as a limit to population size for a variety of taxa (top–down control). The largest of predators however are unlikely to be limited in this fashion, and their densities are commonly assumed to be determined by the availability of their prey (bottom–up control). However, bottom–up regulation of apex predators is contradicted by many studies, particularly of non-hunted populations. We offer an alternative view that apex predators are distinguishable by a capacity to limit their own population densities (self-regulation). We tested this idea using a set of life-history traits that could contribute to self-regulation in the Carnivora, and found that an upper limit body mass of 34 kg (corresponding with an average mass of 13–16 kg) marks a transition between extrinsically- and self-regulated carnivores. Small carnivores share fast reproductive rates and development and higher densities. Large carnivores share slow reproductive rates and development, extended parental care, sparsely populated territories, and a propensity towards infanticide, reproductive suppression, alloparental care and cooperative hunting. We discuss how the expression of traits that contribute to self-regulation (e.g. reproductive suppression) depends on social stability, and highlight the importance of studying predator–prey dynamics in the absence of predator persecution. Self-regulation in large carnivores may ensure that the largest and the fiercest do not overexploit their resources.
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What is an apex predator?
Arian D. Wallach , Ido Izhaki , Judith D. Toms , William J. Ripple and Uri Shanas
A. D. Wallach (, School of Environment, Charles Darwin Univ., Darwin, Northern Territory, Australia. I. Izhaki,
Dept of Evolutionary and Environmental Biology, Univ. of Haifa, Haifa, Israel. J. D. Toms, Eco-Logic Consulting, Victoria, BC, Canada.
W. J. Ripple, Dept of Forest Ecosystems and Society, Oregon State University, Corvallis, OR, USA. U. Shanas, Dept of Biology and
Environment, Univ. of Haifa - Oranim, Tivon, Israel.
Large ‘ apex predators infl uence ecosystems in profound ways, by limiting the density of their prey and controlling smaller
‘ mesopredators ’ . e loss of apex predators from much of their range has lead to a global outbreak of mesopredators, a
process known as mesopredator release that increases predation pressure and diminishes biodiversity. While the classifi ca-
tions apex- and meso-predator are fundamental to current ecological thinking, their defi nition has remained ambiguous.
Trophic cascades theory has shown the importance of predation as a limit to population size for a variety of taxa (top down
control).  e largest of predators however are unlikely to be limited in this fashion, and their densities are commonly
assumed to be determined by the availability of their prey (bottom up control). However, bottom up regulation of apex
predators is contradicted by many studies, particularly of non-hunted populations. We off er an alternative view that apex
predators are distinguishable by a capacity to limit their own population densities (self-regulation). We tested this idea
using a set of life-history traits that could contribute to self-regulation in the Carnivora, and found that an upper limit
body mass of 34 kg (corresponding with an average mass of 13 16 kg) marks a transition between extrinsically- and self-
regulated carnivores. Small carnivores share fast reproductive rates and development and higher densities. Large carnivores
share slow reproductive rates and development, extended parental care, sparsely populated territories, and a propensity
towards infanticide, reproductive suppression, alloparental care and cooperative hunting . We discuss how the expression
of traits that contribute to self-regulation (e.g. reproductive suppression) depends on social stability, and highlight the
importance of studying predator prey dynamics in the absence of predator persecution. Self-regulation in large carnivores
may ensure that the largest and the fi ercest do not overexploit their resources.
e ecological role of large predators is expressed by their
classifi cation as apex predators ; a term used to denote their
elevated position on the trophic ladder. Apex predators are
primarily known for their role as inhibitors of population
irruptions of prey and smaller predators, an eff ect that
cascades throughout ecological communities and promotes
biodiversity.  e keystone role of apex predators as ecosys-
tem regulators is now fi rmly embedded in ecological theory
(Estes et al. 2011, Ripple et al. 2014). Medium-sized preda-
tors, termed mesopredators , also drive community struc-
ture through a variety of pathways, including predation on
small prey (Roemer et al. 2009). Apex predators limit the
density of mesopredators so that total predation pressure is
contained (Ripple et al. 2014).  e loss of apex predators
removes this inhibiting factor, resulting in mesopredator
release (Crooks and Soul é 1999, Prugh et al. 2009).
Apex predator ’ , ‘ mesopredator ’ and ‘ mesopredator release ’
are terms that have set the tone for our understanding of
a wide range of ecological processes (Estes et al. 2011). How-
ever, the categorization of predators remains ambiguous.
Within each ecosystem the largest extant predators are often
classed as apex predators even if these same species are con-
sidered typical mesopredators elsewhere. For example, cats
and foxes fall easily into the mesopredator group (Crooks
and Soul é 1999), but as introduced species on islands they
are often the largest mammal present and are therefore
classed as apex predators (Rayner et al. 2007, Bergstrom
et al. 2009, Roemer et al. 2009).  e mesopredator release
concept in itself was developed from the study of the coyote
Canis latrans as an apex predator (Crooks and Soul é 1999), a
species frequently placed in the mesopredator group when
in the presence of wolves Canis lupus (Prugh et al . 2009,
Ripple et al. 2013). Some regions contain a rich guild of
large predators making it diffi cult to determine where to
draw the line between apex- and meso-predators (Prugh
et al. 2009). Indeed, some of the world s iconic apex preda-
tors coexist with larger and fi ercer predators (Palomares and
Caro 1999), and many of the world s largest predators are
now extinct. Is the gray wolf therefore a mesopredator in the
presence of larger carnivores?
Predators of all sizes harass, kill and scare predators
smaller than themselves: tigers dominate wolves (Miquelle
et al. 2005), wolves exclude coyotes (Ripple et al. 2013),
coyotes control foxes (Crooks and Soul é 1999), foxes kill
cats (Glen and Dickman 2005), cats suppress rats (Rayner
et al. 2007), and rats displace mice (Wanless et al. 2007).
© 2015  e Authors. Oikos © 2015 Nordic Society Oikos
Subject Editor: James D. Roth. Editor-in-Chief: Dries Bonte. Accepted 12 December 2014
Oikos 000: 001–009, 2015
doi: 10.1111/oik.01977
Predators of all sizes can also induce trophic cascades: pumas
promote tree recruitment by controlling deer (Ripple et al.
2014), sea otters recover kelp forests by eating herbivorous
sea urchins (Estes et al. 2011), cats maintain island produc-
tivity by suppressing rabbits (Bergstrom et al. 2009), plants
benefi t when fi sh reduce dragonfl y predation on pollinating
insects (Knight et al. 2005), and nutrient cycling is infl u-
enced by the stress response of herbivorous grasshoppers to
hunting spiders (Hawlena and Schmitz 2010).
Despite these similarities there appears to be little
functional redundancy between large and small predators.
e loss of the largest of predators has had a dispropor-
tionately disruptive infl uence on ecosystem structure and
function (Ripple et al. 2014); a process coined trophic
downgrading (Estes et al. 2011). Defi ning predator status
comparatively within each system is problematic because
it implies that mesopredators can step into the role of apex
predators as these disappear from the landscape. Studies
suggest the opposite: mesopredators are not eff ective replace-
ments for apex predators (Prugh et al. 2009). Size may in
fact be a reliable predictor of a predator s ecological status,
refl ecting diff erences in evolutionary pressures and adapta-
tion. A mesopredator may therefore remain a mesopreda-
tor even in systems devoid of larger predators, and an apex
predator need not be the single largest.
What then distinguishes apex predators from mesopreda-
tors? One fundamental consequence of size is that large
predators are relatively safe from predation (Promislow and
Harvey 1990). In the absence of an eff ective extrinsic source
of predation, there can be two main mechanisms limit-
ing population growth: 1) the decline in the abundance of
their prey (a bottom up force), and 2) an internal mecha-
nism of self-regulation (a socially mediated force). Although
ecologists have traditionally supported the bottom up view
(Hayward et al. 2007), trophic cascades theory highlights
the role of top down regulation in population dynamics. It
would be surprising if top down forcing infl uences all but
the largest. Indeed, studies frequently fi nd negative rather
than positive correlations between apex predators and their
prey (Estes et al. 2011), hinting that apex predator may not
be bottom up driven. While habitat productivity is not ruled
out as a contributing factor to population density (Carbone
and Gittleman 2002, Jedrzejewski et al. 2007), large preda-
tors may be unique in maintaining their own populations at
sustainable levels.
Body mass may be a good predictor of apex- and
meso-predator status, because it directly infl uences the rate
of extrinsic predation pressure, thus indirectly infl uencing
life-history traits. Across mammals both juvenile and adult
mortality rates increase as body mass declines, and higher
mortality is associated with r-selected life-history vari-
ables (Promislow and Harvey 1990). Among carnivores,
increasing body size is associated with dietary requirements
for larger prey (Carbone et al. 2007) and lower densities
relative to prey biomass (Carbone and Gittleman 2002).
Evolutionary pressures that infl uence body mass may give
rise to similar adaptations in diff erent taxonomic groups,
and the emergence of a self-regulating ecomorph (Flueck
Most large predator populations are subjected to lethal
control (Ripple et al. 2014) and therefore studies of stable
predator populations are rare. Several recent studies have
pointed to the importance of considering the condition of
social stability in large predators when analyzing predator
prey interactions (Wallach et al. 2009, Cariappa et al. 2011,
Ordiz et al. 2013, Cubaynes et al. 2014). Evidence of social
interactions that may enable self-regulation has emerged from
studies of large predators including bears, large cats, large
canids and large otters (Supplementary material Appendix 1
Table A1).  ese studies off er examples where social interac-
tions, rather than resource availability, drive mortality and
fecundity, limit population density and stability, and infl u-
ence the expression of life history traits that slow population
growth rates (Table 1). Where human-caused mortality is low,
large predators may therefore retain relatively constant popu-
lation densities despite diff erences in resource availability.
Here we investigate the hypothesis that predators above
a certain weight threshold are self-regulating, while smaller
predators require extrinsic regulation by a larger predator
(Fig. 1). We conducted an analysis of life-history traits that
may contribute to self-regulation in the Carnivora (hereaf-
ter carnivores). We selected the carnivores because trophic
cascades eff ects have been consistently demonstrated for
several members of this group (Ripple et al. 2014). We found
that carnivores above a threshold mass have life-history traits
conducive to self-regulation.
We conducted a review of life-history traits of terrestrial and
semi-terrestrial species, belonging to twelve carnivore fami-
lies, for which suffi cient information was available (n 121,
Supplementary material Appendix 1 Table A2). We selected
eleven variables representing four major life-history traits,
which we considered likely to contribute to self-regulation,
and analyzed them in relation to upper limit body mass
(ULBM) and average body mass (ABM). Data was sourced
from encyclopedias (e.g. Encyclopedia of Life), online
databases (e.g. Carey and Judge 2002, de Magalhaes and
Costa 2009, IUCN), life-history journals (e.g. Mammalian
Species) and other peer-reviewed sources.
Human hunting can have pronounced eff ects on the
expression of life history traits (Haber 1996, Milner et al.
2007) and few populations have escaped this impact (Ripple
et al. 2014). We therefore chose upper limit values for most
variables (Supplementary material Appendix 1 Table A2)
to account for the potential of individuals in undisturbed
Table 1. Evidence that social interactions enable self-regulation in
large carnivores. For each Family we summarize the number of stud-
ies supporting the propositions that social interactions, rather than
resource availability: drive mortality and fecundity (A), limit density
(B), affect population stability (C), and affect the expression of life
history traits that slow population growth rates (D). The proportion
of studies is shown in brackets, with some studies supporting more
than one proposition. Summarized from studies compiled in the
Supplementary material Appendix 1 Table A1.
Proposition Ursidea Felidea Canidea Mustelidea Total
A 8 (89%) 2 (18%) 4 (27%) 3 (60%) 17 (43%)
B 2 (22%) 8 (73%) 7 (47%) 1 (20%) 18 (45%)
C 4 (44%) 3 (27%) 5 (33%) 1 (20%) 13 (33%)
D 7 (78%) 2 (18%) 2 (13%) 0 11 (28%)
Figure 1. Apex- and meso-predator status are fi xed ecological categories: apex predators are self-regulated and smaller predators are
extrinsically regulated. Antagonistic interactions (dashed arrows) and top down forces (thick arrows) exist within and across both groups,
but the ability to self-regulate (circular arrows) is unique to large predators. Citations for interactions are: 1, 5, 7, 8 Supplementary
material Appendix 1 Table A1; 2 - Murphy et al. 1998; 3, 4, 16, 17 - Palomares and Caro 1999, Gunther and Smith 2004, Jimenez et al.
2008; 6 - Letnic et al. 2011; 9 - Ripple et al. 2013; 10 13 - Crooks and Soul é 1999; 14 - Carlsson et al. 2010; 15 - Glen and Dickman
2005. Artwork by J. Parkhurst.
populations to grow large, mature, form social bonds, hold
territories and provide uninterrupted care for their young.
e relation of life-history traits with ULBM and ABM
showed similar trends (ULBM and ABM values are corre-
lated r 0.97, p 0.0001), and we chose to present ULBM
results because this variable is less likely to be infl uenced by
human activity.
Reproductive strategy (r/K)
We hypothesized that self-regulating carnivores would
employ a K-strategy (i.e. slow life-history) and invest more
energy in fewer off spring compared to extrinsically-regulated
carnivores. Five variables were assessed for this trait: 1) age
at weaning, 2) age at independence (and dispersal), 3) age
at sexual maturity, 4) lifespan and 5) population reproduc-
tive rate (accounting for reproductive suppression of some
e age at weaning and independence provide measures
of parental care. To account for relative parental investment,
both variables were also analyzed in relation to lifespan and
reproductive rate (e.g. age at independence / lifespan / num-
ber of off spring / year). We modifi ed the reproductive rate
variable to account for social carnivores that limit the repro-
duction of some females (off spring / year / average number
of breeding females in a group / average number of sexually
mature females in a group).
Family planning
e limitation of off spring production below the
species maximum reproductive potential is referred to here
as family planning , and we expected this trait to contribute
to self-regulation. We used two binary variables: 1) female
reproductive suppression and 2) infanticide. Female repro-
ductive suppression occurs in social species in which domi-
nant females exclude other sexually mature females from
breeding, or litters of subordinates are killed or abandoned.
Female territoriality
Territoriality is considered an important mechanism for
spacing individuals or groups and limiting population den-
sity (Cariappa et al. 2011). We focused on females because
territorial males may occupy the home range of several
females and reproduce with all of them (e.g. felids and
bears). We included a binary variable female territoriality
and a continuous subset variable female density .
For the subset of female-territorial carnivores, we recorded
the median female territory size and the average number of
females in a social group to calculate an estimate of female
density (group size / territory size). We used the median value
variables were most strongly associated with the fi rst PC
axis (PC1), which is representative of the fast-slow (r-K)
life history continuum.  e piecewise regression identifi ed
a threshold in the relationship between PC1 and ULBM at
33.85 kg (ABM 13 16 kg), with a 95% confi dence inter-
val (CI) between 18.16 63.12 kg (Davies test: p 0.001,
Fig. 2A).  e second PC axis (PC2) was formed by the
socially complex behaviors (e.g. family planning and allo-
parental care) on one side, and high reproductive rates on
the other, with no signifi cant threshold identifi ed. Female
territoriality only appeared as a signifi cant variable in PC3
(Table 2). In no case was a signifi cant threshold detected for
the individual variables included in the PCA.
Across the full carnivore mass range, both PC1 and PC2
were positively correlated with body mass (Pearson s correla-
tion LogULBM: PC1, r 0.73, p 0.001; PC2, r 0.38,
p 0.01). ere was no correlation between PC3 and body
mass (NS). Large carnivores (ULBM 34 kg) had mostly
positive PC1 values (70%) and about half (55%) had positive
PC2 values, refl ecting a K-strategy and a tendency towards
socially complex reproductive behaviors. Small carnivores
(ULBM 34 kg) had predominantly negative PC1 (73%)
and PC2 (69%) values, corresponding with an r-strategy
and more solitary or biparental social groups (Fig. 2B). Both
within and between taxonomic families, large carnivores had
consistently higher PC1 and PC2 values (Fig. 2C). However,
while small carnivore families were clustered together, each
family of large carnivores was distinctly placed along the two
axes (e.g. bears had the highest PC1 values and large canids
had the highest PC2 values).
Female territoriality was ubiquitous and common across
the Carnivora (71% of species), but within the subset of car-
nivores that are female-territorial, female density was nega-
tively correlated with body mass (Spearman s r 0.77,
p 0.001; Fig. 3A) and with PC1 (r 0.41, p 0.01).
Female biomass (female density controlled for standardized
metabolic needs) was also negatively related with body mass
(r 0.48, p 0.001, Fig. 3B), but not with PC1 (NS).
resholds were detected for both density (71.95 kg, 95%
CI 6.67 775.88) and biomass (73.70 kg, 95% CI 4.52
1199.91), but neither threshold was signifi cant (Davies test:
p 0.691 and p 0.913, respectively).
Large carnivores invest more time and a larger portion of
their lifetime in each off spring, relative to small carnivores.
Parental care (age at weaning and independence) and paren-
tal investment (parental care controlled for lifespan and
reproductive rate) were positively correlated with body mass
(parental care: by weaning, r 0.38, p 0.001, by indepen-
dence r 0.52, p 0.001; parental investment: by weaning
r 0.33, p 0.001, by independence r 0.24, p 0.001).
resholds were detected for parental investment (by inde-
pendence: 11.87 kg, 95% CI 3.43 41.02, Davies test:
p 0.1; by weaning 14.0 kg, 95% CI 5.64 34.78, Davies
test: p 0.004), but only the threshold for weaning was
signifi cant (Fig. 4).
Life history traits that may infl uence population regulation
diff er between large and small carnivores, lending support
because territory size may vary widely in relation to habitat
conditions. To account for the diff erences in body mass we
calculated an estimate of carnivore biomass by adjusting the
density to standardized metabolic needs (group size / terri-
tory size ABM 0.75 , Gittleman and Harvey 1982). Data
were obtained for 55 (of 69) female-territorial carnivores.
Cooperative behavior
Cooperative behaviors may be features of self-regulating
species and associated with the ability to secure large terri-
tories and large prey (Creel and Macdonald 1995). We used
cooperative hunting, and two forms of cooperative care: 1)
paternal care and 2) alloparental care, as measures of coop-
erative behaviors, all as binary variables. Carnivores were
included in both categories if their social structure was fl ex-
ible and inclusive of both forms (e.g. red fox Vulpes vulpes ,
Cavallini 1996).
Data analysis
We used a principal components (PC) analysis (SPSS 20),
of the eleven variables for which full datasets were available
(n 73 species), to identify groups of strongly interacting
variables (Jolliff e 2002) across the Carnivora and discretely
for each Family and mass group. We compared the main PCs,
and each of the individual variables, with log-transformed
body mass. We then fi t a piecewise regression to identify a
threshold in the relationships (Toms and Lesperance 2003),
using the segmented package in R (Muggeo 2008). Variables
were log-transformed to meet the assumption of constant
variance in residuals. Binary variables were tested using a
piecewise logistic regression (Toms and Lesperance 2003).
We tested the threshold signifi cance using a Davies test
(Davies 1987, Piepho and Ogutu 2003).  e subset variables
female density and female biomass were correlated separately
with the strongest PCs and with body mass. We compared
the relationship between parental care and parental invest-
ment with body mass separately.
e fi rst three PCs cumulatively accounted for 70% of
the variation in the dataset (Table 2). Reproductive strategy
Table 2. Scores of the eleven life-history variables in the top three
models of the principal component analysis (PCA).
Lifespan 0.854 0.184 0.091
Age at sexual maturity 0.852 0.190 0.048
Age at weaning 0.814 0.174 0.204
Age at independence 0.770 0.359 0.039
Female reproduction
0.388 0.800 0.051
Reproductive potential 0.601 0.424 0.152
Paternal care 0.487 0.580 0.238
Alloparental care 0.328 0.824 0.216
Cooperative hunting 0.222 0.513 0.326
Infanticide 0.044 0.631 0.558
Territoriality (female) 0.266 0.163 0.832
Figure 2. Relationship between the two strongest principal components (PC) and upper limit body mass (ULBM). (A) Relation of PC1 to
log transformed ULBM with the estimated threshold identifi ed at 34 kg (full line), with a 95% confi dence interval of 18 63 kg (dashed
lines). (B) Position of carnivores on the fi rst two PC axes. (C) Position of family and mass groups on the two major axes (average SE),
separated at 34 kg. Blue circles are identifi ed as typical mesocarnivores (ULBM 18 kg), light green circles (in B) denote carnivores that
fall within the lower threshold confi dence zone (ULBM 18 34 kg) and dark green circles are identifi ed as apex carnivores (ULBM 3 4
kg). 1 Canidae, 2 Felidae, 3 Herpestidae, 4 Hyaenidae, 6 Mustelidae, 7 Procyonidae, 8 Ursidae, 9 Ailuridae (red panda,
Ailurus fulgens ), 10 Eupleridae (fossa Cryptoprocta ferox ), 11 Viverridae (common genet, Genetta genetta ).
Figure 3. Density (A) and biomass (B) of territorial female carni-
vores relative to ULBM.  e giant panda, an outlier, is circled in
(B). Blue circles are identifi ed as typical mesocarnivores (ULBM 1 8
kg), light green circles denote carnivores that fall within the lower
threshold confi dence zone (ULBM 18 34 kg) and dark green cir-
cles are identifi ed as apex carnivores (ULBM 34 kg).
Figure 4. Relationship between parental investment (age at weaning
controlled for lifespan and reproductive rate) and body mass with
the estimated threshold identifi ed at an ULBM of 14 kg (full line),
with a 95% confi dence interval of 6 35 kg (dashed lines).
to the proposition that apex- and meso-predator status are
xed. In this analysis of terrestrial and semi-terrestrial car-
nivores, an ULBM of 18 34 kg (ABM 13 16 kg) marked
a transition between extrinsically regulated meso-carnivores
and self-regulating apex carnivores.  is threshold is similar
to the commonly used ABM of 15 kg to distinguish meso-
carnivores from apex-carnivores (Prugh et al. 2009, Ripple
et al. 2014), but is slightly lower than previous studies that
found a dietary threshold at an ABM of 20 kg (Carbone
et al. 2007). Our analysis also helps clarify the ecological
position of carnivore species whose status is ambivalent (e.g.
coyotes are recognized here as apex predators).
Large carnivores probably self-regulate because they
typically invest more in fewer off spring, suppress the repro-
duction of mature females and commit infanticide ( family
planning ), are socially cooperative and hold sparsely popu-
lated territories (Fig. 2 4). Mesocarnivores on the other
hand are unlikely to self-regulate and are instead adapted to
extrinsic-regulation pressure, as suggested by a higher repro-
ductive rate, lower investment in each off spring, scarcity of
family planning and the potential to attain higher densities.
Diff erences between large and small carnivores persist when
controlling for standardized metabolic needs and group size
(Fig. 3), and for lifespan and reproductive rate (Fig. 4).  ese
life-history traits are often shared more closely within mass
groups than within taxonomic groups (Fig. 2C).
Reproductive strategy, the main contributor to the fi rst
PC axis, was the most important trait defi ning carnivore
status. Extended parental care and heavier investment in each
off spring were particularly characteristic of apex carnivores.
Within apex carnivores K-traits and parental investment
increase relative to body mass at a faster rate than in mesocarni-
vores (Fig. 2A, 4).  us apex carnivores increase their K-traits
as body mass increases but this does not consistently occur in
the mesocarnivore group. Predation pressure on mesocarni-
vores of all sizes may be consistently selecting for r-traits.
Family planning , an important contributor to the
second PC axis, was also characteristic of apex carnivores,
particularly in canids. Infanticide is often associated with
reproductive suppression of sexually mature females, and
65% of carnivores that exclude some females from breeding
perform infanticide. In these species the dominant females
kill the young of the subordinate females in their social
group. Where infanticide is used to restrict the reproduc-
tion of females, it most likely acts to limit the size of social
groups and ultimately population density. Overall, 52%
of carnivores that practice infanticide do not suppress the
reproduction of females. In these cases infanticide occurs
when a rival male displaces the resident breeding male, and
the sire ’ s off spring are killed to gain reproductive advantage
(sexually-selected infanticide). Male-driven infanticide has a
substantial infl uence on population density and demography
of bears and large felids (Supplementary material Appendix
1 Table A1). Male-driven infanticide may in some cases
select for larger groups. In banded mongoose, reproductive
suppression is selected against because pup survival increases
when more females in each group reproduce due to male
infanticide (Cant 2000).
Female reproductive suppression is clearly an important
regulation mechanism in social carnivores, occurring in most
large canids (88 100%) and large hyenas (67%) (Supple-
mentary material Appendix 1 Table A2). It may however
also function indirectly in solitary predators. For example,
Ordiz et al. (2008) found that an adult female brown bear
was less likely to produce cubs if her nearest neighbor already
had cubs.  ey argued that this could be considered a form
of reproductive suppression, probably caused by resource
competition among female bears living close to each other.
Cryptoprocta ferox , the largest member of the Eupleridae
and Madagascar s largest carnivore, shares traits with apex
carnivores (Fig. 2C).  e threshold mass is also likely to be
much higher in the pinnipeds whose large body mass is an
adaptation to their marine habitat where they are sub-
jected to predation from even larger predators. Secondly,
the threshold mass identifi ed can also be infl uenced by
the traits investigated and by sample size. Here, parental
investment showed a threshold at a lower position (14 kg)
than the eleven variables combined in PC1 (ULBM 34
kg), possibly due to the larger sample size of the former,
and the absence of many medium-sized carnivores in the
latter. Lastly, it remains unclear whether habitat size infl u-
ences predator status and self-regulation. Several islands
are too small to support large carnivores but do contain
medium-sized carnivores, which our analysis suggests are
mesopredators. Whether plasticity in the expression and
evolution of life-history traits can enable mesopredators
to self-regulate and function as apex predators on small
islands remains unknown.
e ecological roles of large carnivores vary greatly, and
only some function ecologically as apex predators . Large
carnivores that are primarily vegetarian will have ecological
eff ects that diff er from those that are carnivorous. Despite
this, self-regulation within large predators may provide a
distinct ecological function. For example, apex carnivores
are less likely to become invasive . A notable case is the
contrasting ecologies of the red fox and the dingo Canis dingo ,
two canids that migrated to Australia.  e fox, a mesopreda-
tor, correlates positively with resource availability, and in the
absence of regulation by dingoes reaches high densities and
can drive the extinction of their prey. By contrast the dingo,
an apex predator, forms stable population densities across a
wide productivity gradient when socially stable, and contrib-
utes signifi cantly to the preservation of Australia s biodiver-
sity (Wallach et al . 2009, Letnic et al. 2011).
Identifying whether predators are primarily self- or
extrinsically- regulated requires long-term studies of socially
stable populations. Human persecution of predators is
a major infl uence not only on their numbers, but also on
their social structure (Haber 1996, Wallach et al . 2009,
Ordiz et al. 2013). In turn, social stability determines preda-
tor prey dynamics, and the relative importance of bottom
up and top down forces driving population size.
Infl uence of social stability on life-history
e expression of self-regulation in apex carnivores stems
from social interactions, and is therefore subject to the
condition of social stability. Reproductive strategy (r/k)
variables are responsive to conditions of population density,
demographics and stability. In apex carnivore populations
subjected to human hunting, age at sexual maturity (and
primipatry) declines, reproductive rate increases, parental
care shortens and demography skews towards juveniles. In
non-exploited populations of large canids, off spring often
remain within their natal group for several years, delaying
primipatry and reducing litter production (Haber 1996).
Social stability generally acts to promote the expression
of K-traits by slowing down population turnover rates
(Supplementary material Appendix 1 Table A1).
In apex carnivores, female territoriality contributes
to self-regulation by maintaining low densities (Fig. 3,
Supplementary material Appendix 1 Table A1), but on its
own territoriality is not a signifi cant predictor of predator
status (Table 2). Territoriality is unlikely to contribute to
self-regulation if territories shrink in response to increased
densities, as has been observed in several small carnivore
populations (Cavallini 1996, Benson et al. 2006). Flexibility
in territorial behavior is probably advantageous for smaller
carnivores that have to adjust their space use in relation to
the threat of larger carnivores (Cavallini 1996), while terri-
torial stability is important for large predatory carnivores to
buff er patchy or variable resources and for protection from
dangerous conspecifi cs (Supplementary material Appendix 1
Table A1).
Cooperative behaviors were more pronounced in large
carnivores (Table 2).  e predisposition for cooperative
hunting is in line with a tendency towards hypercarnivory
in large carnivores (Carbone et al. 2007). While cooperative
hunting of large prey is not unique to large carnivores (some
ant species hunt prey thousands of times their size, Dejean
et al. 2010), nor is it obligate (not all large carnivores are
carnivorous), the most complex forms of cooperative hunting
have been observed primarily in large carnivores (MacNulty
et al. 2009, Bailey et al. 2013).
Alloparental care was more common in the large
carnivore group, and was associated with family planning in
the second PC (Fig. 2B). In large social carnivores therefore,
the association of alloparental care with female reproductive
suppression and infanticide provides a high carer:off spring
ratio. Paternal rearing, on the other hand, was affi liated
with an r-strategy and with small carnivores in the fi rst PC
(Fig. 2B). In both solitary and biparental carnivores, female
breeding is unrestricted. An r-strategy without family plan-
ning is a condition conducive to high reproductive output
and is more common in mesocarnivores: 48% of small carni-
vores versus 7% of large carnivores have negative PC1 values
and are solitary or biparental.
e results of our study were robust despite the high
level of noise in the dataset.  e quality of life history
knowledge varies between species and traits: research eff ort
is biased towards a small number of carnivores (Ripple et al.
2014); much data are derived from captive animals; and data
sourced from wild populations may be equally biased due
to anthropogenic eff ects (Milner et al. 2007). Additionally,
life-history traits vary with habitat conditions (Carbone and
Gittleman 2002, Jedrzejewski et al. 2007). While these biases
are unlikely to be confounding in this study, we do expect
that advances in life history studies of wild populations with
minimal anthropogenic eff ects (particularly predator control)
will help clarify the mechanisms regulating population size.
Our analysis identifi ed a threshold at approximately 34
kg (ULBM), but there are several reasons not to consider
this weight overly prescriptive. Firstly, the threshold mass
that diff erentiates apex- from meso-predators is likely to vary
between taxonomic groups. In this study the threshold mass
was strongly infl uenced by three families that contributed the
highest number of species: the canids, felids and mustelids.
Defi ning a threshold mass at the Order level was necessary
in order to obtain a suffi cient sample size, but it may obscure
diff erences between families. For example, the 12 kg fossa
health (Wallach et al. 2009, Ordiz et al . 2013). Carnivores
subjected to hunting undergo markedly diff erent popula-
tion dynamics.  ere are few studies that have investigated
how the loss of individual animals infl uences populations,
and fewer still that have determined the drivers of popula-
tion density in protected populations. We are only recently
beginning to appreciate the profound importance of large
carnivores for the health of ecosystems. Apex predators may
keep the proverbial balance of nature not only by limit-
ing the populations of those they hunt, but also by limiting
themselves. Whether humanity can achieve a similar feat is
an important question to consider.
Are we apex primates?
Our earliest ancestors were prey species most likely top down
regulated by large carnivores (Rose and Marshall 1996), but
we have evolved into the fi ercest predator on the planet,
free of extrinsic top down regulation and are arguably apex
predators in our own right. In the words of Louis C. K. we
got out of the food chain (Oh My God, HBO, 2013). And
yet, after surpassing a population size of 7 billion in 2012,
triggering a sixth mass extinction and severely depleting
non-renewable resources, one would hesitate to argue that
humans are self-regulating. Current human society appears
to be a classic case of mesopredator release, destined to end
in a Malthusian collapse (Ehrlich and Ehrlich 2013). How-
ever, when we consider that self-regulation in apex carnivores
is dependent upon a state of social stability, we can refl ect
upon our own condition as that of a socially disrupted apex
primate. And social instability can be redressed.
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Supplementary material (available online as Appendix
oik.01977 at ).
Appendix 1.
... Study organisms were assigned to one of four trophic levels: primary producers, herbivores, meso-predators, and top-predators, based on how they perform and function in their natural ecosystem. We note that whereas the classification of top (i.e., apex) and meso-predators are fundamental to current ecological thinking, their distinction remains ambiguous (e.g., Wallach et al., 2015). Here we classified 'top-predators' based on their ecological role and their elevated trophic position in their food web. ...
... However, it should be noted that when splitting trophic levels among developmental stages (Fig. S5A), we still find the same pattern reported for OA when developmental stages are pooled, signifying a robust result that is independent of species ontogeny. In addition, although top-predators could be pooled with meso-predators due to the ambiguity associated with these trophic distinctions (Wallach et al., 2015), doing so would provide little change to the meta-analysis resultwith consumers still showing less tolerance to OA than predators. Even so, we clearly need more studies on climate change effects on top-predators to allow for stronger inferences on the effects of OA and OW on this important trophic position. ...
Full-text available
Marine ecosystems are currently facing a variety of anthropogenic perturbations, including climate change. Trophic differences in response to climate change may disrupt ecological interactions and thereby threaten marine ecosystem function. Yet, we still do not have a comprehensive understanding of how different trophic levels respond to climate change stressors in marine ecosystems. By including 1278 experiments, comprising 236 different marine species from 18 different phyla in a meta-analysis of studies measuring the direct effect of ocean acidification and ocean warming on marine organisms, we found that higher trophic level species display greater tolerance to ocean acidification but greater sensitivity to warming. In contrast, marine herbivores were the most vulnerable trophic level to both acidification and warming. Such imbalances in the community and a general reduction of biodiversity and biomass in lower trophic levels can significantly disrupt the system and could drive negative bottom-up effects. In conclusion, with ocean acidification and elevated temperatures, there is an alarming risk that trophic disparity may disrupt species interactions, and thereby drive community destabilization under ocean climate change.
... The upper limit on the number of territories that can be supported effectively caps breeding by a cooperatively breeding social carnivore, limiting the influence of increased natality on population growth (Fuller et al., 2003). This limitation may be evidence of how territoriality can be a self-regulating mechanism for a population (Wallach et al., 2015). ...
Full-text available
As climate change accelerates in northern latitudes, there is an increasing need to understand the role of climate in influencing predator-prey systems. We investigated wolf population dynamics and numerical response in Denali National Park and Preserve in Alaska, United States from 1986 to 2016 under a long-term range of varying climatic conditions and in the context of prey vulnerability, abundance, and population structure using an integrated population modeling approach. We found that wolf natality, or the number of wolves added to packs, increased with higher caribou population size, calf:cow ratio, and hare numbers, responding to a 1-year lag. Apparent survival increased in years with higher calf:cow ratios and cumulative snowfall in the prior winter, indicators of a vulnerable prey base. Thus, indices of prey abundance and vulnerability led to responses in wolf demographics, but we did not find that the wolf population responded numerically. During recent caribou and moose population increases wolf natality increased yet wolf population size declined. The decline in wolf population size is attributed to fewer packs in recent years with a few very large packs as opposed to several packs of comparable size. Our results suggest that territoriality can play a vital role in our study area on regulating population growth. These results provide a baseline comparison of wolf responses to climatic and prey variability in an area with relatively low levels of human disturbance, a rare feature in wolf habitat worldwide.
... Entscheidend ist dabei nicht allein die finale Größe einer Art, sondern vielmehr ihre Wechselbeziehungen mit Beutetieren und konkurrierenden Kleinräubern. Demnach verfügt nur ein Gipfelräuber (Spitzenprädator, Topräuber) über weitgehende Freiheit von natürlichen Feinden und somit die Selbstregulierung seiner Populationsdichte (Wallach et al. 2015). Natürlich sind spezifische Faktoren wie etwa im zwischenartlichen Verhalten ebenbürtiger Fleischfresser denkbar (Lehman et al. 2016), aber paläontologisch kaum auflösbar. ...
... focuses on ecological values, as indicated by the presence of "Environmental Science Ecology", "Zoology" and "Biodiversity Conservation" in the top three list of the WOS research areas (Fig. 3). Several studies link conservation of biodiversity and mesopredator release (Courchamp et al., 1999;Nimmo et al., 2015;Ritchie and Johnson, 2009), focusing on the ecological roles of apex predators in the stabilization of ecosystems (Wysong et al., 2020), food webs (Ripple et al., 2014;Scoleri et al., 2020), and prey species (Wallach et al., 2015;Wysong et al., 2020). ...
Human activities severely impact the distribution and behaviour of apex predators in numerous terrestrial and aquatic ecosystems, with cascading effects on several species. Mesopredator outbreaks attributable to the removal of an apex predator have often been recorded and described in the literature as “mesopredator release”. During recent decades several examples of the phenomenon have been observed and studied in many different parts of the world. In this paper, we quantitatively reviewed the existing literature on mesopredator release using two software packages (VOSviewer and CiteSpace) to investigate patterns and trends in author keywords through occurrences and temporal analyses, and creating relative network maps. The results showed that even though the general scientific interest in mesopredator release has increased in recent decades, the vast majority of studies focus on canid species, leaving many other species or entire taxa (e.g., reptiles) understudied and under-described. The connection between invasive species and mesopredator release has only recently been more extensively explored and also the effects of apex predators declining in aquatic ecosystems are still only partially investigated. Due to the increasing effect of biological invasions, overfishing, and either the decline or the rise of apex predators in different parts of the world, we expect an even higher increase in interest and number of published documents on the subject. We also encourage widening the research focus beyond canids to include other important taxa.
... They find that the cyclic dynamics described by the Lotka-Volterra model emerge. Further, Wallach et al. (2015) propose that apex predators self-regulate their own population to make sure that there is no over-exploitation, i.e. keep their own population at a certain level, which limits the pressure on the prey population. ...
... Asterisks denote significance (P < 0.05). (Wallach et al. 2015b); interspecific cooperative hunting between carnivores (coyotes and badgers -Taxidea taxus) (Minta et al. 1992); and that human persecution alters the development of personalities in juvenile hyenas (C. crocuta) (Greenberg and Holekamp 2017). ...
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Apex predators structure ecosystems by hunting mesopredators and herbivores. These trophic cascades are driven not only by the number of animals they kill, but also by how prey alter their behaviors to reduce risk. The different levels of risk navigated by prey has been likened to a “landscape of fear.” In Australia, dingoes are known to suppress red fox populations, driving a trophic cascade. However, most of what we know of this relationship comes from circumstances where predators are persecuted, which can affect their social and trophic interactions. Utilizing camera traps, we monitored fox behavior when accessing key resource points used by territorial dingoes, in a region where both predators are protected. We predicted that foxes would avoid and be more cautious in areas of high dingo activity. Indeed, foxes avoided directly encountering dingoes. However, contrary to our expectations, foxes were not more cautious or vigilant where dingo activity was high. In fact, fox activity and scent-marking rates increased where dingo scent-marking was concentrated. Further, foxes were increasingly confident with increasing levels of conspecific activity. Our results suggest that responses to the threat of predation are more complex than fear alone. In socially stable conditions, it is possible that prey may develop knowledge of their predators, facilitating avoidance, and reducing fear.
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The decline and extirpation of large carnivore populations can lead to cascading effects in natural ecosystems. An understanding of large carnivore population densities, distribution and dynamics is therefore critical for developing effective conservation strategies across landscapes. This is particularly important in island environments where species face increased extinction risk due to genetic isolation coupled with local losses of finite habitat. The Sri Lankan leopard Panthera pardus kotiya is one of two remaining island-living leopards on Earth and the only apex predator in Sri Lanka. Despite its iconic status in Sri Lanka, robust research on the species has been limited to only a handful of scientific studies, limiting meaningful scientific recommendations for the species’ conservation and management. In this study, we conducted a single season camera trap survey in Sri Lanka’s largest protected area, Wilpattu National Park (1,317 km²), located in the country’s northwest. Our objective was to estimate key ecological state variables of interest (density, abundance, sex-specific movement and spatial distribution) of this leopard subspecies. Our results indicate that Wilpattu National Park supports a density of 18 individuals/100 km² (posterior SD=1.5; 95% HPD interval=16–21) with a mean abundance of 144 (posterior SD=15) individual leopards and a healthy sex ratio (f:m=2.03:1). The estimated activity range for male leopards >2 years old was 49.53 km² (Posterior SD=3.43; HPD interval=43.09–56.41) and for female leopards >2 years old was 22.04 km² (Posterior SD=1.82; HPD interval=18.34–25.65). This density falls at the higher end of published estimates for the species anywhere in its global range, based on similar methods. Given Sri Lanka’s limited size, this national park system should be considered as a critical stronghold that maintains a source population of leopards, contributing to the long-term population viability of leopards in the larger landscape.
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The global expansion of road networks threatens apex predator conservation and ecosystem functioning. This occurs through wildlife-vehicle collisions, habitat loss and fragmentation, reduced genetic connectivity and increased poaching. We reviewed road impacts on 36 apex predator species and assessed their risk from current roads based on road exposure and species vulnerability. Our findings reveal all apex predators are exposed to road impacts. Eight of the ten species with the highest risk occur in Asia, although other high-risk species are present in the Americas, Africa and Europe. The sloth bear suffers the highest risk of all apex predators, followed by the tiger and dhole. Based on species risk from roads, we propose a widely applicable method to assess the potential impact of future roads on apex predators. We applied this method to proposed road developments in three areas: the Brazilian Amazon, Africa, and Nepal, to locate high-impact road segments. Roughly 500 protected areas will be intersected by these roads, threatening core apex predator habitats. We advocate the need for rigorous road development planning to apply effective mitigation measures as an urgent priority and to avoid construction in wilderness areas and predator strongholds.
Per-and poly-fluorinated alkyl substances (PFAS) are a group of chemicals used in a wide variety of commercial products and industrial applications. These chemicals are persistent, can accumulate in humans' and animals' tissues and in the environment, representing an increasing concern due to their moderate to highly toxicity. Their global distribution, persistence and toxicity led to an urgent need to investigate bioaccumulation also in marine species. In 2013 PFAS contamination was detected in a vast area in Veneto region, mainly in Adige and Brenta rivers. In order to investigate any relevant presence of these substances in marine vertebrates constantly living in the area, PFAS were measured in hepatic tissue samples of 20 bottlenose dolphins (Tursiops truncatus) stranded along the northern Adriatic Sea coastline between 2008 and 2020. Using high performance liquid chromatography-mass spectrometry, 17 target PFAS (PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnA, PFDoA, PFTrDA, PFTeDA, PFBS, PFHxS, PFOS, PFDS, PFHpS, PFPeS), were quantified in the samples. PFAS profiles were generally composed of the same five dominant PFAS (PFOS > PFUnA > PFDA ≈ PFDoA ≈ PFTrDA). The greatest PFOS concentration found was 629,73 ng/g wet weight, and PFOS accounted until 71% in the PFAS profiles. No significant differences between sexes were found, while calves showing higher mean values than adults, possibly indicating an increasing ability in the elimination of PFAS with age. Finally, a temporal analysis was carried out considering three different periods of time, but no temporal differences in concentrations were found. The results suggest that long-chain PFAS are widespread in bottlenose dolphins along the North Adriatic Sea. Furthermore, they represent a baseline to investigate the impact of PFAS on marine mammals' conservation and health. Filling an important gap in the knowledge of PFAS accumulation in bottlenose dolphins, this study highlights the relevant role of Environmental and Tissue Banks for retrospective analyses on emergent contaminants.
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Global change is severely affecting ecosystem functioning and biodiversity globally. Remotely sensed ecosystem functional attributes (EFAs) are integrative descriptors of the environmental change —being closely related to the processes directly affecting food chains via trophic cascades. Here we tested if EFAs can explain the species fitness at upper trophic levels. We took advantage of a long-term time series database of the reproductive success of the Golden Eagle (Aquila chrysaetos) – an apex predator at the upper trophic level – over a 17-year period across a bioclimatic gradient (NW Spain; c. 29,575 km²). We computed a comprehensive database of EFAs from three MODIS satellite-products related to the carbon cycle, heat dynamics and radiative balance. We also assessed possible time-lag in the response of the Golden Eagle to fire, a critical disruptor of the surface energy budget in our region. We explored the role of EFAs on the fitness of the Golden Eagle with logistic-exposure nest survival models. Our models showed that the reproductive performance of the Golden Eagle is influenced by spatiotemporal variations in land surface temperature, albedo and vegetation productivity (AUC values from 0.71 to 0.8; ΣWi EFAs from 0.66 to 1). Fire disturbance also affected ecological fitness of this apex predator—with a limited effect at 3 years after fire (a time-lagged response to surface energy budget disruptions; ΣWi Fire= 0.62). Our study provides evidence for the influence of the matter and energy fluxes between land surface and atmosphere on the reproductive success of species at upper trophic levels.
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Four cases where large predators caused Grey Wolf (Canis lupus) mortality are recorded. We describe two incidents of Cougars (Puma concolar) killing Wolves in Montana and one incident of a Cougar killing a Wolf in Alberta. We report the first recorded incident of a Grizzly Bear (Ursus arctos) killing a Wolf in the western United States
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Human-caused harassment and mortality (e.g. hunting) affects many aspects of wildlife population dynamics and social structure. Little is known, however, about the social and physiological effects of hunting, which might provide valuable insights into the mechanisms by which wildlife respond to human-caused mortality.To investigate physiological consequences of hunting, we measured stress and reproductive hormones in hair, which reflect endocrine activity during hair growth. Applying this novel approach, we compared steroid hormone levels in hair of wolves (Canis lupus) living in Canada's tundra–taiga (n = 103) that experience heavy rates of hunting with those in the northern boreal forest (n = 45) where hunting pressure is substantially lower.The hair samples revealed that progesterone was higher in tundra–taiga wolves, possibly reflecting increased reproductive effort and social disruption in response to human-related mortality. Tundra–taiga wolves also had higher testosterone and cortisol levels, which may reflect social instability.To control for habitat differences, we also measured cortisol in an out-group of boreal forest wolves (n = 30) that were killed as part of a control programme. Cortisol was higher in the boreal out-group than in our study population from the northern boreal forest.Overall, our findings support the social and physiological consequences of human-caused mortality. Long-term implications of altered physiological responses should be considered in management and conservations strategies.
Black bears (Ursus americanus) or grizzly bears (Ursus arctos) visited 8 of 55 cougar-killed (Felis concolor) ungulates in Glacier National Park (GNP), Montana, from 1992 to 1995, and 19 of 58 cougar kills in Yellowstone National Park (YNP), Wyoming, from 1990 to 1995. Bears displaced cougars from 4 of 8 carcasses they visited in GNP and 7 of 19 in YNP. Cougar predation provided an average of 1.9 kg/day (range = 0-6.8 kg/day) of biomass to bears that fed on cougar-killed ungulates. This biomass was an important percent (up to 113%) of the daily energy needs of bears when compared to their caloric requirements reported in the literature. We suggest that ungulate carrion resulting from cougar predation is important nutritionally to bears in some regions and seasons. Cougars that were displaced from their kills by bears lost an average of 0.64 kg/day of ungulate biomass, or 17-26% of their daily energy requirements. Biologists modelling or measuring cougar predation rates should be aware that losses to scavengers may be significant.
Principal component analysis has often been dealt with in textbooks as a special case of factor analysis, and this tendency has been continued by many computer packages which treat PCA as one option in a program for factor analysis—see Appendix A2. This view is misguided since PCA and factor analysis, as usually defined, are really quite distinct techniques. The confusion may have arisen, in part, because of Hotelling’s (1933) original paper, in which principal components were introduced in the context of providing a small number of ‘more fundamental’ variables which determine the values of the p original variables. This is very much in the spirit of the factor model introduced in Section 7.1, although Girschick (1936) indicates that there were soon criticisms of Hotelling’s method of PCs, as being inappropriate for factor analysis. Further confusion results from the fact that practitioners of ‘factor analysis’ do not always have the same definition of the technique (see Jackson, 1981). The definition adopted in this chapter is, however, fairly standard.
We demonstrate the use of piecewise regression as a statistical technique to model ecological thresholds. Recommended procedures for analysis are illustrated with a case study examining the width of edge effects in two understory plant communities. Piecewise regression models are ‘‘broken-stick’’ models, where two or more lines are joined at unknown points, called ‘‘breakpoints.’’ Breakpoints can be used as estimates of thresholds and are used here to determine the width of edge effects. We compare a sharp-transition model with three models incorporating smooth transitions: the hyperbolic-tangent, benthyperbola, and bent-cable models. We also calculate three types of confidence intervals for the breakpoint estimate: an interval based on the computed standard error of the estimate from the fitting procedure, an empirical bootstrap confidence interval, and a confidence interval derived from an inverted F test.We recommend use of the inverted F test confidence interval when sample sizes are large, and cautious use of bootstrapped confidence intervals when sample sizes are smaller. Our analysis demonstrates the need for a careful study of the likelihood surface when fitting and interpreting the results from piecewise-regression models.
Conference Paper
Background/Question/Methods The process of energy and materials transfer governs ecosystem production, food chain length and species diversity. Herbivores are pivotal in this process because their low plant assimilation efficiencies constrain transfer rates and ecosystem carbon: nitrogen balance. Theory suggests that physiological stress due to predation risk could be an important determinant of trophic transfer efficiency and ecosystem nutrient budget. We tested this assertion in a series of field and laboratory experiments. We reared grasshopper herbivore nymphs with or without risk of spider predation and measured their metabolic rate, nutritional requirements, and excretion and body elemental composition. We also calculated how the risk of spider predation affects elemental composition of uneaten plant material entering the soil organic matter pool. Results/Conclusions We show that shifting nutrient demand of herbivores facing predation risk causes carbohydrate carbon to become an important nutrient to fuel stress-heightened respiration. This had cascading effects on C:N balance of herbivore body tissue, fecal material and uneaten plant material. Fear from predation risk thus represents an endogenous mechanism regulating materials transfer via chronic exacerbation of herbivore physiological stress and shift in what becomes a limiting nutrient.
Suppose that the distribution of a random variable representing the outcome of an experiment depends on two parameters xi and theta and that one wishes to test the hypothesis xi = 0 against the alternative xi > 0. If the distribution does not depend on theta when xi = 0, standard asymptotic methods such as likelihood ratio testing or C(α) testing are not directly applicable. However, these methods may, under appropriate conditions, be used to reduce the problem to one involving inference from a Gaussian process. This simplified problem is examined and a test which may be derived as a likelihood ratio test or from the union-intersection principle is introduced. Approximate expressions for the significance level and power are obtained.
1. Understanding the population dynamics of top predators is essential to assess their impact on ecosystems and to guide their management. Key to this understanding is identifying the mechanisms regulating vital rates.2. Determining the influence of density on survival is necessary to understand the extent to which human-caused mortality is compensatory or additive. In wolves (Canis lupus), empirical evidence for density-dependent survival is lacking. Dispersal is considered the principal way in which wolves adjust their numbers to prey supply or compensate for human exploitation. However, studies to date have primarily focused on exploited wolf populations, in which density-dependent mechanisms are likely weak due to artificially low wolf densities.3. Using 13 years of data on 280 collared wolves in Yellowstone National Park, we assessed the effect of wolf density, prey abundance and population structure, as well as winter severity, on age-specific survival in two areas (prey-rich vs. prey-poor) of the national park. We further analysed cause-specific mortality and explored the factors driving intraspecific aggression in the prey-rich northern area of the park.4. Overall, survival rates decreased during the study. In northern Yellowstone, density-dependence regulated adult survival through an increase in intraspecific aggression, independent of prey availability. In the interior of the park, adult survival was less variable and density-independent, despite reduced prey availability. There was no effect of prey population structure in northern Yellowstone, nor of winter severity in either area. Survival was similar among yearlings and adults, but lower for adults older than 6 years.5. Our results indicate that density-dependent intraspecific aggression is a major driver of adult wolf survival in northern Yellowstone, suggesting intrinsic density-dependent mechanisms have the potential to regulate wolf populations at high ungulate densities. When low prey availability or high removal rates maintain wolves at lower densities, limited inter-pack interactions may prevent density-dependent survival, consistent with our findings in the interior of the park.This article is protected by copyright. All rights reserved.