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Selective harvesting of large mammals: how often does it result in directional selection?

2011; Wiley; Volume: 48; Issue: 4 Linguagem: Inglês

10.1111/j.1365-2664.2011.02006.x

1365-2664

Atle Mysterud,

Ecology and biodiversity studies

Journal of Applied EcologyVolume 48, Issue 4 p. 827-834 FORUMFree Access Selective harvesting of large mammals: how often does it result in directional selection? Atle Mysterud, Corresponding Author Atle Mysterud Correspondence author. E-mail: atle.mysterud@bio.uio.noSearch for more papers by this author Atle Mysterud, Corresponding Author Atle Mysterud Correspondence author. E-mail: atle.mysterud@bio.uio.noSearch for more papers by this author First published: 13 May 2011 https://doi.org/10.1111/j.1365-2664.2011.02006.xCitations: 96AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Summary 1. Harvesting of large mammals is usually not random, and directional selection has been identified as the main cause of rapid evolution. However, selective harvesting in meat and recreational hunting cultures does not automatically imply directional selection for trait size. 2. Harvesting selectivity is more than a matter of hunter preference. Selection is influenced by management regulations, hunting methods, animal trait variance, behaviour and abundance. Most studies of hunter selection only report age- or sex-specific selection, or differences in trait size selection among hunting methods or groups of hunters, rather than trait size relative to the age-specific means required for directional selection. 3. Synthesis and applications. Managers aiming to avoid rapid evolution should not only consider directional selection and trophy hunting but also mitigate other important evolutionary forces such as harvesting intensity per se, and sexual selection processes that are affected by skewed sex ratios and age structures. Introduction There is increasing concern about the possible long-term evolutionary consequences of heavy human harvesting (Harris, Wall & Allendorf 2002; Festa-Bianchet 2003; Allendorf et al. 2008; Allendorf & Hard 2009; Darimot et al. 2009). Such effects have been linked to strong directional selection for specific phenotypic traits, such as against large fish because of mesh sizes of closing nets (Jørgensen et al. 2007) or against large trophy males because of hunter preference (Coltman et al. 2003; Garel et al. 2007). Directional selection effects of trophy hunting on size are well documented for bighorn sheep Ovis canadensis Shaw in Canada (Coltman et al. 2003). Trophy hunting is widespread (Courchamp et al. 2006; Johnson et al. 2010), so these results should be taken seriously. However, most harvesting of large mammals is not a result of trophy hunting. Moreover, management regulations often restrict large mammal hunters from following their preferences. When comparing red deer Cervus elaphus L. harvest statistics across 11 European countries (Milner et al. 2006), the proportion of calves in the harvest varied from 10% to 40%, while males typically accounted for 40–60% of the remainder, i.e. male and female harvests were of similar magnitude. Trophy bulls usually make up a very small proportion of the harvest. Directional selection are sometimes reduced by counter selection pressure on small, young males (Mysterud & Bischof 2010), and trophy males are often shot at the age of trophy culmination (Apollonio, Andersen & Putman 2010). The degree of size selection may strongly differ between the age classes that are targeted in both males (Mysterud & Bischof 2010) and females (Proaktor, Coulson & Milner-Gulland 2007). Different selection pressures arise from harvesting aimed at meat provisioning, subsistence, recreation or population control. It therefore cannot be taken for granted that harvesting always induces strong directional selection as a result of hunter preferences for large sized individuals. Identifying the level and pattern of selection is crucial for predicting expected rates of evolutionary responses to large mammal hunting. Here, it is argued that: (i) there are currently few studies documenting directional selection for body or trophy size despite claims on the contrary (Tenhumberg et al. 2004; Allendorf & Hard 2009) and (ii) that in many cultures, large mammal harvesting is not expected to induce strong directional selection in trait size. Harvest selectivity in mammals is complex because of highly variable environments, management culture and regulations. We also need to broaden our focus beyond trophy harvesting when considering evolutionary effects. The mechanisms of harvest selectivity The factors affecting patterns of harvest selectivity in terrestrial ecosystems can be broadly organized into: (i) hunter preferences and (ii) opportunities to be selective via (a) management regulations (quotas; economic costs etc.), (b) hunting methods (stalking vs. drives etc.), (c) animal trait variation (strength of hunter cues, appearance), (d) animal behaviour, (e) animal abundance, (f) population structure (sex ratio and age structure) and g) habitat openness. Table 1 gives an overview of common traits targeted by hunters in terrestrial ecosystems and the cues the hunters use to separate individuals at the within-species level most relevant for directional selection. The hunters' preferences are likely to differ depending on hunter motivation (i.e. meat, recreation or trophy), level of knowledge and skill (use of guides), cultural background, religion (taboos), individual ethics and animal trait variation. For example, trophy hunters using guides shot larger moose Alces alces L. in Alaska, because guides took client hunters to areas with lower population densities and therefore larger moose (Schmidt, Ver Hoef & Bowyer 2007). More importantly, strong directional selection for size is often unlikely because of limited (or redirected) opportunities for hunter selection because of both direct and intentional factors such as quotas or economic costs of high pricing and also time limitations, cost of lost opportunity, and indirect and non-intentional factors through animal behaviour and abundance (Table 2). Table 1. Traits used for direct hunter preference or selection Trait targeted Hunter cue Examples References Age: juvenile vs. yearling/subad. Juvenile traits (short jaw), small body size, overall appearance (fur colour etc.) Cervids Age: yearling/subad. vs. adult Body size, antler or horn size Cervids Sex: male vs. female Sexual traits (presence of penis) Cervids Secondary sexual traits (presence, size or form of horns, antlers, tusks; colour of mane) Elephants, lions Panthera leo L., cervids Kurt, Hartl & Tiedemann 1995; Whitman et al. 2004 Sexual body size dimorphism Chamois Females: reproductive status Offspring vs. no offspring at heel Moose Ericsson 2001 Brown bear Bischof et al. 2009 Chamois Rughetti & Festa-Bianchet 2011 Female size Body size Cervids Male size Trophy or body size Cervids, bovids Coltman et al. 2003 Special trophies –'oddities' Parück vs. normal Roe deer Capreolus capreolus L. Colour morphs Black morphs vs. normal Springbok Antidorcas marsupialis Sundevall, roe deer White morphs vs. normal Springbok Silver morphs vs. normal Fox Vulpes v. fulvus Desmarest Haldane 1942 Table 2. Mechanisms affecting the level of hunter selection beyond hunter preferences Factor Effect on selectivity Mechanism Type of selection Management Quota: size More selective if small quota Law enforcement and time limitation Direct Quota: specificity Less selective the more specific the quota Law enforcement Direct Quota: scale Less selective if quota for a region or team rather than for individual Competition among hunters Direct Duration of hunting season More selective the longer the hunting season Time limitation, but depletion may reduce selectivity Opportunity for direct Size of hunting estate Less selective on smaller estates Fewer to choose from Opportunity for direct Price Less selection the more costly to shoot the larger one Not all hunters have endless amount of money Direct Hunting implementation Hunting method Stalking more selective than drive hunt More time to assess differences when animals are calm Indirect, and opportunity for direct Use of guides Use of guides increase selectivity Guides assess size better; know where largest animals are living Direct, opportunity for direct Use of dogs Use of dogs may lower selectivity Preference of dogs might differ from preference of hunter Indirect, and opportunity for direct Trapping Use of traps may change selectivity Traps differ in specificity due to variation in catchability Indirect Animal Trait variation (Table 1) More opportunities to select if animals differ in traits Hunters ability to select differ, and differences can affect chances of being observed Indirect, and opportunity for direct Animal population density (and skewed sex ratio) More selective the more to choose from Time limitation to find animals at low density Opportunity for direct Grouping behaviour More opportunities to select if animals in herds Easier to assess differences in size when individuals are close Indirect, and opportunity for direct Mother-offspring bond More selective if strong bond If offspring do not follow mothers closely, more difficult to separate mothers from non-mothers Indirect, and opportunity for direct Sexual segregation More (or less) selection if sex groups spatially segregate Spatial search of hunter increase likelihood of shooting a given sex (but may decrease selection in some cases) Indirect, and opportunity for direct Home range size More selection for animals with large home ranges More exposed if large home range size Indirect Activity levels More selection for animals that are more active More active more exposed Indirect Habitat use More selective harvest in open habitat (or farmland) More vulnerable if using open areas (or farmland) Indirect, and opportunity for direct Individual personality More selection for animals that expose themselves more Animals may differ in their propensity to take risk Indirect Landscape factors Habitat More open habitat increase selection Easier to see what is available Opportunity for direct If there is little opportunity for choice, for instance because of a low population density (Tenhumberg et al. 2004), a skewed sex ratio leading to low density of one sex (Nilsen & Solberg 2006), a high quota relative to population size (Solberg et al. 2000), a short duration of hunting season, or small estate size, selectivity will be reduced. For example, moose hunters did not select for male age (older being larger), in a situation where a female-biased sex ratio and a young male age structure limited the opportunities to select (Nilsen & Solberg 2006). High competition among hunters is likely to produce the same effect. For example, for large carnivores in Scandinavia, quotas are given for large regions rather than to an individual hunter. Low selectivity was found for brown bears Ursus arctos L. under such management regulations (Bischof et al. 2009). A given hunter might prefer to shoot a very large male, but might not risk passing up a small bear because the quota for the area might be filled before the hunter encountered a large bear, and furthermore, the hunter has nothing to lose by shooting the small bear as there is no individual quota. Similar effects result as a consequence of team hunting on the same estate. Clearly, a lack of appropriate cues may in many cases limits the hunters' ability to select. Selection may decrease when there is low sexual body size dimorphism or lack of visual secondary sexual characters. By contrast, habitat openness promotes gregariousness, which can increase the likelihood of selection. Climate affects movement such as the timing of migration and can also affect opportunities for selection. We currently have rather limited knowledge of how much animal behaviour affects harvest-related selection in mammals, but it is likely to be an important factor. For example, young birds were more prone to being shot than adult birds because of difference in behaviour (Bunnefeld et al. 2009). Furthermore, selection on bold personality with fast growth has been found in fisheries (Biro & Post 2008). In Fennoscandia, hunting of cervids is often carried out with the aid of dogs (either on a leash or barking), which is known to increase moose harvesting success by up to 56% (Ruusila & Pesonen 2004). Drive hunting in Europe is carried out both with and without the aid of dogs (Apollonio, Andersen & Putman 2010). In Nicaragua, dogs sometimes selected non-target prey species (Koster 2008), and it is possible dogs can be selective of scent from, e.g. rutting males, affecting selectivity. The spatial hunting behaviour of humans may also influence the selective pressures exerted (Schmidt, Ver Hoef & Bowyer 2007). Limited empirical evidence of directional selection In a recent review of terrestrial ecosystems, Allendorf & Hard (2009) pointed out that selection is important for trait evolution. This conclusion was based on theoretical modelling which indicated that size-selective harvesting can cause shifts in trait values. However, the few empirical cases that were listed consisted of different kinds of hunters shooting different kinds of animals without any evidence that the total harvest differed in terms of trait mean from what was available in the population. There was thus no clear link from theory (directional selection→evolution) to data (non-random harvest). A broader review undertaken here (Table 3) reveals that there are no clear-cut examples of directional selection apart from the case study of bighorn sheep arising from trophy hunting. The most common documentation of selective harvesting comes from comparing different groups of hunters or using different hunting strategies or methods, or comparisons of age or sex classes rather than size directly (Table 3). That hunters select adults over calves is not evidence of directional selection acting on size, this would require comparison with age-specific mean size within a population (the unit for evolution). The population mean or availability in the population is known only rarely, and these studies therefore cannot say with confidence that selection is directional. Table 3. Studies of harvesting selection of mammals in terrestrial ecosystems. Dir. Sel. = evidence of direct selection on trait size (or character) Species Trait Assumed mechanism Selectivity comparison Population average or availability known Dir. sel. Reference Ungulates Bighorn sheep Male trophy size Hunter preference Rams of different sizes Yes Yes Coltman et al. 2003 Chamois Female reproductive status Management Populations with different management No Yes? Rughetti & Festa-Bianchet 2011 Female horn size Hunter preference No selection found No No Rughetti & Festa-Bianchet 2011 Red deer Age and sex Management Mortality of marked individuals Yes No Langvatn & Loison 1999 Male body mass Hunter method Monteria vs. trophy-stalking vs. management catch vs. bycatch No No Martínez et al. 2005 Male body and trophy size Hunter method Commercial vs. selective monteria No No Torres-Porras, Carranza & Pérez-González 2009 Roe deer Male body mass Management Local vs. client hunters; early vs. late season; habitat openness No No Mysterud, Tryjanowski & Panek 2006 Age and sex Hunter vs. lynx Lynx lynx L. No No Andersen et al. 2007 Moose Female reproductive status Management Survival with or without reproduction (marginally sign.) Yes Yes? Ericsson 2001 Age and sex Hunter preference (females, none found for males) Age groups; within season decline No No Nilsen & Solberg 2006 Age and sex Management Sex-specific age groups; years with low and high quota relative to population size No No Solberg et al. 2000 Male trophy size Hunter preference; implementation With or without aid of guides No No Schmidt, Ver Hoef & Bowyer 2007 Calf size None found Male vs. female calves No No Moe et al. 2009 Elk Cervus elaphus L. Age and sex Hunter preference Hunters vs. wolves Canis lupus L. No No Wright et al. 2006 White-tailed deer Odocoileus virginianus Zimmermann Age and sex Trapability Trapability of marked individuals Yes No Hiller et al. 2010 Age and sex Hunter method Archery vs. firearm No No Mattson & Moritz 2008 Male age Management Mortality of marked individuals Yes No Webb, Hewitt & Hellickson 2007 Age Hunter preference(?) Mortality of marked individuals Yes No Pac & White 2007 Disease prevalence (CWD=Chronic Wasting Disease) None found (animal behaviour hypothesized) Periods of different harvesting methods No No Grear et al. 2006 Mule deer Odocoileus hemionus Rafinesque Age and condition Assumed none Hunter vs. mountain lions Puma concolor L. No No Krumm et al. 2010 Wild boar Age and sex Hunter method Espera vs. Monteria hunt No No Braga et al. 2010 Age Hunter preference(?) Hunters vs. wolves vs. estimated population Yes No Nores, Llaneza & Alvarez 2008 Age and sex Hunter preference(?) Mortality of marked individuals Yes No Toïgo et al. 2008 Age and sex None found Mortality of marked individuals Yes No Keuling et al. 2010 Large carnivores Brown bear Body mass Hunter method Moose vs. bear specialist hunters No No Bischof et al. 2008 Age and sex Animal behaviour Mortality of marked individuals Yes No Bischof et al. 2009 Mountain lion Age and sex Hunter preference(?) Mortality of marked individuals Yes No Cooley et al. 2009 There is little doubt trophy hunting is directionally selective, but the level of directional selection for hunters targeting meat, subsistence, recreation or population control rather than trophies is not well documented. We do know that foreign trophy stalkers select differently than local hunters (Martínez et al. 2005; Mysterud, Tryjanowski & Panek 2006) and in some cases, selection is based on size. A lower level of selection will strongly affect the expected rate of evolutionary response. Harvesting selection will always work against forces of natural selection (Ratner & Lande 2001). It is not known when selection pressure from harvesting is strong enough to alter the fitness landscape. The level of trait heritability is clearly also critical, but not the focus here. In the bighorn sheep case (Coltman et al. 2003), the smaller males became the more successful breeders, which is a quite extreme example of harvest-driven directional selection. However, for elephants Loxodonta africana L. in Tarangire National Park, Tanzania, the larger males retained a higher mating success even under poaching pressure (Ishengoma et al. 2008), suggesting that the fitness landscape did not change qualitatively. In Alpine chamois Rupicapra rupicapra L., horn length appears to have a limited role in male reproductive success, and hunter selection was regarded as unlikely to yield an evolutionary response in males (Rughetti & Festa-Bianchet 2010) or females (Rughetti & Festa-Bianchet 2011). Harvesting effects are expected to be stronger in small populations (Hard, Mills & Peek 2007; Steenkamp, Ferriera & Bester 2007), and effective population size might become an issue (Sæther, Engen & Solberg 2009). For large populations, there is less likely to be uniform harvest pressure, and regions with limited harvesting might buffer selective effects of harvesting through migration (Tenhumberg et al. 2004). Harvesting intensity is itself important A mild preference for large quarry size, for example in cultures where animals are hunted for meat, does not imply an absence of evolutionary effects but we need to consider other mechanisms. Even non-selective harvesting may theoretically affect trait evolution (Bischof, Mysterud & Swenson 2008). The intensity of harvesting per se and the timing of the harvest relative to the age of first reproduction may be important in this context because life expectancy is a crucial fitness component in large mammals. The same harvest pressure is thus more important for males than females, because of lower life expectancy of males (Toïgo & Gaillard 2003). Under heavy harvest pressure, individuals that begin reproduction at a young age and at a light weight have a greater chance of reproducing at least once compared with those that begin reproduction at heavier weights, later in life (Proaktor, Coulson & Milner-Gulland 2007). However, we do not know how strong harvest pressures need to be to outweigh the (high) cost of early reproduction. Population differences in harvesting pressure have been shown to correlate with the proportion of juveniles reproducing in wild boar Sus scrofa L. (Servanty et al. 2009) but no trend towards earlier maturation was found for red deer in populations where a high proportion of non-breeding juveniles are harvested (Mysterud, Yoccoz & Langvatn 2009). Hunter preference for non-reproducing females is common (Table 1). Furthermore, skewed sex ratios and age structure in harvested populations may lead to a relaxation of sexual selection processes. Limited intra-male competition for mates in harvested populations with skewed population structure is suggested by observations of younger males rutting more in synchrony with older males (Mysterud et al. 2008) and a reversal towards female-biased dispersal (Pérez-González & Carranza 2009) in red deer. Lower levels of sexual selection might favour development of lower male body- and trophy sizes, as suggested by growth patterns of moose (Mysterud, Solberg & Yoccoz 2005; Tiilikainen et al. 2010). Conclusion Understanding the mechanisms by which harvesting might affect trait evolution is crucial for management to select efficient mitigative efforts. It is emphasized here that harvesting, although selective, is not always expected to be strongly directional as a result of hunter preferences for large-sized individuals. 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