Ecology. Michael Begon

Figure 5.33 Older individuals hold the territories in a black kite population. The age profile of territory holders and non‐territory holding floaters in a black kite, Milvus migrans, population in the Doñana National Park in Spain.

      Source: After Sergio et al. (2009).

      dear enemies and nasty neighbours

The idea that territorial strategies will be favoured that minimise costs to the territory holders also implies that the territory holders should, where possible, tailor their level of effort to the level of threat being posed. This has led to two contrasting hypotheses. The ‘dear enemy’ hypothesis proposes that more effort should be exerted against strangers unfamiliar to the territory holder, lacking territories themselves, than against territory‐holding neighbours, since, once a territorial boundary has been established, it pays both neighbours to minimise their investment in maintaining it (Fisher, 1954). But on the other hand, the ‘nasty neighbour’ hypothesis proposes that more aggression should be displayed against neighbours than strangers, and proposes it especially for group‐living species, where out‐competing your neighbour allows your group to swell in size (Temeles, 1994). There is evidence for both (Figure 5.34), and while the dear enemy effect seen for the rodents in Figure 5.34a appears to be the more common, the nasty neighbour effect does indeed appear to be most often found in group‐living species, such as the ants in Figure 5.34b.

Figure 5.34 ‘Dear enemy’ and ‘nasty neighbour’ effects. (a) Male subterranean rodents, Ctenomys talarum (tuco‐tucos) in Argentina display more aggressive (especially high‐aggressive) behaviour towards unfamiliar than towards familiar opponents in staged contest. Familiarity was gained by previously exposing contestants to the odour of their opponents. In the ‘unfamiliar’ treatment, exposure was to an odour of an animal other than the opponent. Bars are SEs; different letters denote significant differences (P < 0.05). (b) Weaver ant colonies, Oecophylla smaragdina, in Queensland, Australia, behaved more aggressively towards other colonies the less related they were to them in terms of the chemicals in their cuticle (less related = greater ‘spectral distance’).

      Source: (a) After Zenuto (2010). (b) After Newey et al. (2010).

      APPLICATION 5.3 Reintroduction of territorial vultures

      

Having seen that many species compete for territories related to the availability of resources rather than for the resources themselves, it is perhaps not surprising that when we come to manage such species, ensuring the availability of territories is a top priority. A good example comes from a study of bearded vultures, Gypaetus barbatus, which became extinct in the European Alps more than a century ago, and have been the focus of a reintroduction programme since 1986 (Figure 5.35). Captive‐reared individuals were released from four widely dispersed sites from which they spread to new areas, and this spread was monitored in the Valais region of Switzerland (not one of the release sites). During an initial phase, from 1987 to 1994, the sightings were of subadults, and the most important factor explaining the distribution of these sightings was the biomass of ibex, Capra ibex, whose carcasses are an important resource for the vultures. However, during the subsequent phase, from 1995 to 2001, when adults were finally settling in the region, the presence of the vultures was most closely correlated with the distribution of craggy limestone crags, which are the ideal base for their territories, providing nest sites, thermal conditions for soaring, and limestone screes for bone breaking and food storage. Food availability was of only secondary significance. It seems clear, therefore, that future reintroductions in the area should focus precisely on the availability of these viable territories.

Figure 5.35 The importance of good territories for the conservation of bearded vultures. Map of the Valais region of Switzerland, where bearded vultures, Gypaetus barbatus, have spread following their reintroduction. Black squares are 1 km squares where juvenile vultures were sighted during an initial, ‘prospecting’ stage (1987–94). White circles are 1 km squares where adult vultures were sighted during a subsequent, ‘settling’ stage (1995–2001).

      Source: After Hirzel et al. (2004).

5.9 Self‐thinning

      We have seen throughout this chapter that intraspecific competition can influence the number of deaths, the number of births and the amount of growth within a population. We have illustrated this largely by looking at the end results of competition. But in practice the effects are often progressive. As a cohort ages, the individuals grow in size, their requirements increase, and they therefore compete at a greater and greater intensity. This in turn tends to increase their risk of dying. But if some individuals die, then the density is decreased as is the intensity of competition – which affects growth, which affects competition, which affects survival, which affects density, and so on.

In trying to understand these interconnected processes it is important to distinguish three types of study: (i) those in which the ‘final’ performance of competitors is monitored over a range of densities and hence over a range of intensities of competition; (ii) those in which density and performance are monitored together over time as groups of competitors grow and undergo density‐dependent mortality; and (iii) those which seek relationships between density and performance in sets of populations, each observed just once (Weiner & Freckleton, 2010). Each type of study involves density and the performance of either individual competitors or the whole population, but the three tend to be aimed at addressing rather different questions. We examined the first in Section 5.2.2 when we discussed constant final yield. We turn here to the second and third.

      5.9.1 Dynamic thinning lines

Starting with the second, the patterns that emerge in growing, crowded cohorts of individuals were originally the focus of particular attention in plant populations. For example, perennial rye grass (Lolium perenne) was sown at a range of densities, and samples from each density were harvested after 14, 35, 76, 104 and 146 days (Figure 5.36a). Figure 5.36a has the same logarithmic axes – density and mean plant weight – as Figure 5.7: what we referred to previously as a type (i) study. In Figure 5.7, each line represented a separate yield–density relationship