The evolutionary assembly of forest communities along environmental gradients: recent diversification or sorting of pre-adapted clades?
Alexander G. LinanJonathan A. MyersChristine E. EdwardsAmy E. ZanneStephen A. SmithGabriel ArellanoLeslie CayolaWilliam Farfan‐RíosA C.Karina García‐CabreraSebastián Gonzales-CaroM. Isabel LozaManuel J. MacíaYadvinder MalhiBeatriz Nieto‐ArizaNorma SalinasMiles R. SilmanJ. Sebastián Tello
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Summary Biogeographic events occurring in the deep past can contribute to the structure of modern ecological communities. However, little is known about how the emergence of environmental gradients shape the evolution of species that underlie community assembly. In this study, we address how the creation of novel environments lead to community assembly via two non-mutually exclusive processes: 1) the immigration and ecological sorting of pre-adapted clades (ISPC), and 2) recent adaptive diversification (RAD). We study these processes in the context of the elevational gradient created by the uplift of the Central Andes. We develop a novel approach and method based on the decomposition of species turnover into within- and among-clade components, where clades correspond to lineages that originated before mountain uplift. Effects of ISPC and RAD can be inferred from how components of turnover change with elevation. We test our approach using data from over 500 Andean forest plots. We found that species turnover between communities at different elevations is dominated by the replacement of clades that originated before the uplift of the Central Andes. Our results suggest that immigration and sorting of clades pre-adapted to montane habitats is the primary mechanism shaping communities across elevations.Keywords:
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Abstract Dispersal evolves under the action of opposing forces. Some factors, like the cost of dispersal, select against dispersal. Several other factors, however, select for dispersal. In this chapter we focus on three main factors: (1) the temporal variability of the environment will often select for dispersal; (2) dispersal may also be adaptive if it reduces competition between relatives; and (3) dispersal may represent a way to escape the cost of inbreeding.
Inbreeding avoidance
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Many organisms live in ephemeral habitats, making dispersal a vital element of life history. Here, we investigate how dispersal rate evolves in response to habitat persistence, mean habitat availability and landscape pattern. We show that dispersal rate is generally lowered by reduced habitat availability and by longer habitat persistence. However, for habitats that persist for an average of ten times the length of a generation, we show a clear non–monotonic relationship between habitat availability and dispersal rate. Some patterns of available habitat result in populations with dispersal polymorphisms. We explain these observations as a metapopulation effect, with the rate of evolution a function of both within–population and between–population selection pressures. Individuals in corridors evolve much lower dispersal rates than those in the mainland populations, especially within long, narrow corridors. We consider the implications of the results for conservation.
Metapopulation
Persistence (discontinuity)
Ballooning
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Metapopulations consist of a number of sub-populations connected
through the dispersal of migrants. In many theoretical approaches
often rather simple dispersal patterns are investigated, while
typically complex dispersal patterns are observed in nature. This
especially is the case for larvae dispersal in marine environments
driven by ocean currents.
In a recent work Vuilleumier and Possingham investigate the role of
symmetry in dispersal patterns and draw the conclusion, that
asymmetric dispersal has a distinct negative impact on population
viability. Our results based on artificial dispersal patterns,
however, suggest that symmetry actually only could have a vanishing
impact. The simulation results are in good agreement with tests on
realistic dispersal patterns of mussel larvae in the Baltic Sea.
We demonstrate the importance of artificially generated dispersal
patterns for the disentanglement of the complexity intrinsic to
natural systems. Although many aspects of larvae dispersal might not
yet have been understood we can draw the conclusion, that asymmetry
might not be as severe as expected by Vuilleumier and Possingham. That
is of course good news for many systems of biological relevance.
Metapopulation
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Theoretically, dispersers should target the habitat where prospects for fitness will be highest. Aiming for a habitat similar to the natal area (natal habitat-biased dispersal) has been hypothesized as a probable rule of thumb for dispersers, but has received very little empirical support to date. We investigated similarities between natal and post-dispersal settlement sites with radio-collared Siberian flying squirrels ( Pteromys volans L.). Juveniles born in small patches and raised in nests close to patch edge settled in small patches and used nests close to edges after dispersal. In addition, post-dispersal use of dreys (versus cavities) was similar to that observed in natal sites. However, the quality of settlement habitat was unrelated to the quality of the natal site, which suggests that natal experience on average-quality habitats may not lead to ecological traps for flying squirrels. This study provides evidence that habitat selection at the landscape scale is influenced by habitat of natal area.
Ecological trap
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Abstract Animal dispersal and subsequent settlement is a key process in the life history of many organisms, when individuals use demographic and environmental cues to target post‐dispersal habitats where fitness will be highest. To investigate the hypothesis that environmental disturbance (habitat fragmentation) may alter these cues, we compared dispersal patterns of 60 red squirrels ( Sciurus vulgaris ) in three study sites that differ in habitat composition and fragmentation. We determined dispersal distances, pre‐ and post‐dispersal habitat types and survival using a combination of capture–mark–recapture, radio‐tracking and genetic parentage assignment. Most (75%) squirrels emigrated from the natal home range with mean dispersal distance of 1,014 ± 925 m (range 51–4,118 m). There were no sex‐related differences in dispersal patterns and no differences in average dispersal distance, and the proportion of dispersers did not differ between sites. In one of the sites, dispersers settled in patches where density was lower than in the natal patch. In the least fragmented site, 90% of animals settled in the natal habitat type (habitat cuing) against 44–54% in the more strongly fragmented sites. Overall, more squirrels settled in the natal habitat type than expected based on habitat availability, but this was mainly due to individuals remaining within the natal wood. In the highly fragmented landscape, habitat cuing among emigrants did not occur more frequently than expected. We concluded that increased habitat fragmentation seemed to reduce reliable cues for habitat choice, but that dispersing squirrels settled in patches with lower densities of same‐sex animals than at the natal home range or patch, independent of degree of fragmentation.
Sciurus
Habitat Fragmentation
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Dispersal ability (i.e., investment in dispersing structures) can vary across plant species or populations, such as between core compared to leading populations of invasive plants. However, in heterocarpic plants, which produce propagules with varying dispersal abilities, dispersal potential can also vary via investment in the proportion of dispersing morphs (termed dispersal rate). Nevertheless, very little is known about the interplay between investment in dispersal ability vs. dispersal rate or how each is affected by varying environmental pressures.This study examined the interplay between dispersal ability and dispersal rate across the invasion route of the heterocarpic plant Heterotheca subaxillaris. Capitula of H. subaxillaris were collected from eight populations along its invasion route in the Eastern Mediterranean coastal plain. The dispersal ability of the dispersing pappus-bearing achenes was measured as the ratio between pappus width and biomass. Dispersal rate was calculated as the ratio between the number of dispersing achenes and total achenes per capitulum.Dispersal ability and dispersal rate were found to be negatively correlated across populations of H. subaxillaris, with a greater investment in pappus width in populations at the leading edge of the invasion compared to a greater proportion of dispersing achenes in core populations.Our results suggest a trade-off might exist between dispersal ability and dispersal rate, which could change along the invasion route of heterocarpic plants such as H. subaxillaris and contribute to their invasive success. This study highlights the importance of examining both dispersal traits when studying the dispersal potential of heterocarpic species.
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Abstract Sex-biased dispersal is common in group-living animals. Due to differences in local demographic and environmental factors, sex-biased dispersal presents many irregular patterns. In this study, a habituated, individually identified Yunnan snub-nosed monkey Rhinopithecus bieti group was observed over 9 years; 192 dispersal events, including 97 male dispersal events (25 natal dispersal and 72 secondary dispersal) and 95 female dispersal events (34 natal dispersal and 61 secondary dispersal) were observed. Males and females showed different dispersal paths, dispersal ages, and dispersal patterns. Females had 2 dispersal paths, whereas males had 4 paths. In terms of age of dispersal, the male age of natal dispersal was younger than for females. Males prefer single dispersal, whereas females prefer parallel dispersal. Our study indicates that the dispersal pattern of R. bieti should be classified as a bisexual dispersal pattern. The differences in dispersal path, average age at dispersal, and dispersal path pattern indicate that Yunnan snub-nosed monkeys may still retain a loose matrilineal social system.
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It has been argued that nonstandard mechanisms of dispersal are often responsible for long-distance dispersal in plants. For example, plant seeds that appear to be adapted for wind dispersal may occasionally be dispersed long distances by birds, or vice versa. In this paper, we explore whether existing data on dispersal distances, colonization rates, and migration rates support the idea that dispersal processes suggested by the morphology of the dispersal unit are responsible for long distance dispersal. We conclude that the relationship between morphologically defined dispersal syndrome and long-distance dispersal is poor. This relationship is poor because the relationship between the morphology of dispersal units and the multiple processes that move seeds are often complex. We argue that understanding gleaned from the often anecdotal literature on nonstandard and standard means of long distance dispersal is the foundation for both statistical and mechanistic models of long-distance dispersal. Such models hold exciting promise for the development of a quantitative ecology of long-distance dispersal.
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Abstract Because plants are mostly sessile, dispersal of pollen and seeds during reproduction determines changes in gene frequencies within populations and plant distributions on a broader scale. Selection favours a mixture of local and distant dispersal and for nearly all plants this is achieved with a single type of dispersal structure because dispersal is relatively ineffective on average whatever the dispersal mode; this ineffectiveness probably accounts for the rarity of dimorphic dispersal structures. Empirical studies show that most dispersal is local, but far‐dispersal events, because of their potential to increase fitness though colonisation of unoccupied habitat patches, are probably what largely determines the selection of dispersal structures. The effectiveness of dispersal can be described in terms of distance, and the ability to reach favourable habitats. Effectiveness in achieving distance, a result of the interaction of the dispersal mode and agent with the environment, results in the typical leptokurtotic dispersal curve. There is no evidence that the shape of the dispersal curve itself is selected.
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