The Allee effect describes a scenario in which populations at low numbers are affected by a positive relationship between population growth rate and density, which increases their likelihood of extinction The importance of this dynamic process in ecology has been under-appreciated and recent evidence now suggests that it might have an impact on the population dynamics of many plant and animal species Studies of the causal mechanisms generating Allee effects in small populations could provide a key to understanding their dynamics

Inverse density dependence and the Allee effect

▪ Abstract The pollination of flowering plants by animals represents a critical ecosystem service of great value to humanity, both monetary and otherwise. However, the need for active conservation of pollination interactions is only now being appreciated. Pollination systems are under increasing threat from anthropogenic sources, including fragmentation of habitat, changes in land use, modern agricultural practices, use of chemicals such as pesticides and herbicides, and invasions of non-native plants and animals. Honeybees, which themselves are non-native pollinators on most continents, and which may harm native bees and other pollinators, are nonetheless critically important for crop pollination. Recent declines in honeybee numbers in the United States and Europe bring home the importance of healthy pollination systems, and the need to further develop native bees and other animals as crop pollinators. The “pollination crisis” that is evident in declines of honeybees and native bees, and in damage to web...

https://www.jstor.org/stable/pdfplus/221703.pdf

ENDANGERED MUTUALISMS: The Conservation of Plant-Pollinator Interactions

The ecological significance of spacing among plants in contributing to the maintenance of species richness, particularly in tropical forests, has received considerable attention that has largely focussed on distance- and density-dependent seed and seedling mortality. More recently it has become apparent that plant spacing is also relevant to pollination, which often constrains seed production. While seed and seedling survival is reduced at high conspecific densities, pollination success, by contrast, is positively correlated to local conspecific density. Distance-dependent mechanisms acting on pollination and seed production have now been described for a variety of plants, with relatively isolated plants or fragmented populations generally suffering reduced fecundity due to pollen limitation. Yet there is considerable variability in the vulnerability of plant species to pollination failure, which may be a function of breeding system, life history, the pollination vector, the degree of specialisation among plants and their pollinators, and other indirect effects of habitat change acting on plants or pollinators. As reduced tree densities and population fragmentation are common outcomes of anthropogenically altered landscapes, understanding how pollination processes are affected in such degraded landscapes can inform effective conservation and management of remaining natural areas.

/pdf/pollen-and-seed-dispersal-among-dispersed-plants-5egudw9wrw.pdf

Pollen and seed dispersal among dispersed plants.

Summary


1 We studied reproduction and offspring performance in relation to population size in the declining self-incompatible perennials Primula veris and Gentiana lutea. In both species, reproduction was strongly reduced in small populations, where plants produced fewer seeds per fruit and per plant. Total seed mass per plant was higher in large populations, but individual seeds were smaller, indicating a trade-off between seed number and size. Reproduction was depressed most strongly in populations consisting of less than c. 200 (P. veris) and c. 500 plants (G. lutea), respectively.



2 The inclusion of plant size (an integrated measure of habitat quality) in the statistical models did not change the relationships between fecundity and population size. Pollen limitation or inbreeding depression in small populations are therefore more likely explanations for these patterns than is habitat quality.



3 Germination rate and survival of seedlings in a common environment was not related to population size in either species, although P. veris developed into larger rosettes when seeds were derived from large populations. This suggests that inbreeding depression occurs in small populations of P. veris.



4 In a factorial fertilizer-by-competition experiment with P. veris, offspring from larger populations grew significantly larger and responded more strongly to fertilizer. For this declining species genetic deterioration as a result of habitat fragmentation may therefore aggravate the effects of environmental changes such as habitat eutrophication.



5 Our results suggest that small populations may face an increased short-term risk of extinction because of reduced reproduction, and an increased long-term risk because they are less able to respond to environmental changes.

/pdf/reduced-fecundity-and-offspring-performance-in-small-5cy2a6i1wd.pdf

Reduced fecundity and offspring performance in small populations of the declining grassland plants Primula veris and Gentiana lutea

Contrary to the theory of rational choice, adding an alternative to a set of available options often affects people's judgement of the preexisting options. Here, we show that honeybees (Apis mellifera) and gray jays (Perisoreus canadensis) are also influenced by the addition of an option to a choice set (i.e., by a change in local context). Like humans, our subjects violated basic properties of rational choice. Their relative preference between two original options changed with the introduction of a third, relatively unattractive option. Such context-dependent choice violates the constant-ratio rule. Our subjects increased their relative preference for the more similar of two alternatives, contrary to the similarity hypothesis. The jays also increased their absolute preference for the more similar of two alternatives, in violation of regularity. Thus, the principle of irrelevant alternatives, which assumes that preference between options does not depend on the presence or absence of other options, is violated not only by humans, but also by an invertebrate and a nonhuman vertebrate. These findings contradict the view that nonhuman animals should be immune to such psychological effects and that they should conform with normative accounts, such as rationality or optimal-foraging theory, because their decision-making processes are evolutionarily adaptive. We discuss the potential generality of context-dependent effects and suggest that such effects should be incorporated into decision-making models in behavioral ecology.

Context-dependent violations of rational choice in honeybees (Apis mellifera) and gray jays (Perisoreus canadensis)

We investigated the effects of plant density on reproduction for an insect-pollinated desert mustard ( Lesquerella fendleri (Brassicaceae)) Individual reproductive success, as measured by seeds per fruit, propor- tion of flowers setting fruit, and total seed production, increased with the density of conspecifics within 1 m However, including the density of conspecifics at greater distances (1-3 m) did not significantly increase the amount of variation in reproductive success explained by the regression model This implies that processes oc- curring on a scale of 1 m or less have important effects on reproduction Total seed production also was greater for high-density plants than for otherwise similar plants with a low-density of conspecifics We argue that increased pollinator visitation is the most likely cause of this facilitation and that investigations of the ef- fects of rarity on reproductive success should directly consider density along with more commonly used at- tributes such as population size and fragmentation

Reproductive success increases with local density of conspecifics in a desert mustard (Lesquerella fendleri).

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Reproductive success increases with local density of conspecifics in a desert mustard (Lesquerella fendleri).