Species diversity
What determines: 1) Species distributions? 2) Species diversity? Patterns and processes At least 120 different (overlapping) hypotheses explaining species richness... We are going to think about general principles and the most obvious hypotheses
Hierarchy... and some terms Species species richness, species diversity Populations Communities community assembly Ecosystems and biomes Environmental variation... Habitat
What explains the distribution of plant species (and thus the local species richness) in Skåne? Why are there no lions in Skåne?
Floran i Skåne Tyler et al. (2007)
Drivers of diversity What controls species distributions and therefore levels of species diversity??
Species diversity Ecology and/or chance?
Scale The same general processes operate on all spatial and temporal scales
Adaptation?
Habitat and adaptation Adaptation Process of evolution by natural selection Basic physiological and morphological characteristics of a species determine whether a species can grow in a particular place Large scale patterns often determined by physiological constraints. e.g. climate Fine scale patterns Determined by e.g. local variation in soil properties
Adaptation Natural selection Operates on individuals Better relative survival and reproductive fitness... depends on available genetic variation... large scale adaptation within species... speciation
Large scale patterns often determined by physiological constraints (e.g. climate) Large scale distribution (European beech) Modern natural distribution of Fagus sylvatica according to Atlas Florae Europaeae (Jalas & Suominen, 1972 99), bold black line; and areas where F. sylvatica would be the dominant tree under natural conditions (Bohn et al., 2000), shaded areas.
Fine scale patterns; often determined by e.g. local variation in soil properties Fine scale distribution (mine tailings with very high levels of lead and zink)
Why may there be problems with predicting future distributions from present distributions and climatic limits?
Why may there be problems with predicting future distributions from present distributions and climatic limits? Other factors are involved Purely spatial processes ( false correlations ) Historical processes (present distribution may not be in equilibrium with present-day climate) The distributions and species diversity that we observe at the present day are a complex reflection of processes that operate/have operated on different spatial and temporal scales
Adaptation and ecology Non-adaptive processes: history, chance [The impacts of these two types of driver will be a central thread in my lectures and in the exercise at the end of the biodiversity theme]
History Immigration patterns Barriers Chance [we have already discussed how ancient chance events may still be evident in patterns of present day genetic variation within species...]
(Dispersal) Horse chestnut Cameraria ohridella
Big seeds!
Dispersal
Species pool Regional species pool Species within a region (scale!) The species in the pool are a subset of the species that could potentially exist under the particular climatic and environmental conditions in that particular region Filtered by historical processes: migration, dispersal, previous regional processes
Habitat heterogeneity Adaptation Different environmental conditions within a particular area (applies at different scales) Land-cover heterogeneity
Habitat heterogeneity Scale!
Agricultural intensification Fig. 1. The multivariate and interacting nature of farming practices and some of the routes by which farming practice impacts on farmland birds. Arrows indicate known routes by which farming practices indirectly or directly affect farmland bird demography and therefore local population dynamics and finally total population size.the goal of manipulating farming practice is to impact on population size. Rather than identifying key routes through this web to change in a piece-wise fashion (e.g. insecticide usage), we suggest that management designed to increase habitat heterogeneity is likely to benefit the organisms in such a way as to meet the management goals Benton, Vickery & Wilson (2003) Trends Ecol Evol
Competition
Competition Complete competitors cannot coexist" Competition for light (shading): above ground Competition for nutrients: below ground Competition can lead to exclusion of most species OR to the coexistence of many species
Competitive exclusion Fertile (= high levels of nutrients) and moist conditions mean that vegetation grows vigorously and there is high biomass (productivity is high) There is competition for light and no room for non-vigorous species complete competitors cannot coexist
Niche Species ecological situation Abiotic variables (e.g. soil moisture or nutrients) Biotic interactions (e.g. competition or interactions with predators, mycorrhizal fungi etc. etc.) [gaps for establishment... regeneration niche ]
Niche separation Complete competitors cannot coexist" Low-nutrient and dry conditions are stressful Competetion for nutrients leads to... Co-existence of functionally different species through niche separation: different ways of acquiring the scarce nutrients (functional diversity) Highly diverse systems are often nutrient-poor [In the short term, sorting of existing species and, in the long term character displacement and speciation] [Fundamental and realized niches]
Functional diversity What kind of things do organisms do within a community or ecosystem? Traits (functional or otherwise) = particular characteristics of an organism (influence a species fundamental niche) Rooting depth, ways of acquiring nutrients, leaf area, plant height, leaf thickness, how seeds are dispersed etc. etc. adaptation to different environments competition dispersal Communities can be characterized in terms of the diversity of functional traits ( functional diversity )
Functional differentiation Functional diversity
Intermediate disturbance hypothesis Highest diversity at intermediate levels of disturbance Level can refer to the intensity, frequency or size of the disturbance... or the time since disturbance Mixture of different processes (dispersal and competition) e.g. Gentle ( intermediate ) disturbance keeps fine scale habitat heterogeneity (microhabitat diversity) e.g. Open ground. Initial colonization by a few species more species arrive and the first ones hang on the environment changes as succession proceeds (soil changes that may be influenced by the plants themselves). the vegetation becomes closed and there are less gaps for establishment e.g. Productivity: (disturbed) low productivity intermediate high productivity (competitive exclusion) Connell 1978: Science
Intuitively sensible hypothesis, but can be explained, or viewed in a range of different ways Has been a lot of recent controversy about this hypothesis Different ways of looking at ecology: empirical (based on data and observations) and logical (based on theories about how nature should work) Interplay between productivity and disturbance Huston 2014 Ecology
Species pool If we have a set of species within a region or local area, what determines whether they will occur in a particular patch of apparently suitable habitat? Dispersal gives access to the habitat History of loss plus fragmentation coupled with a poor dispersal ability that stops recolonization... Real habitat specialists... Habitat may not actually be suitable Local extinction... lose species from the regional or local pool
Regional Species Pool Zobel 1997 Trends Ecol. Evol.
Landscape and habitat fragmentation Landscape modification and habitat fragmentation are key drivers of global species loss (Fischer & Lindenmayer 2007: Global Ecol. Biogeogr.) Connectivity, patch size, edge effects, habitat heterogeneity, local extinction
Fragmentation Patches of habitat are large and close to each other: they support large populations Patches become smaller and more sharply separated: they support smaller populations The distance between fragments increases: the smallest patches can no longer support populations: dispersal and gene flow are limited
Edge effects Edge effects: fertilizer and nutrients spread in from adjacent habitats Unexpected effects of fragmentation on species diversity?
Edge effects Edge effects: fertilizer and nutrients spread in from adjacent habitats Unexpected effects of fragmentation... High diversity in small fragments as a result of edge effects What kind of diversity are we interested in? Habitat specialists
Spatial structure in a fragmented landscape Fragment size Habitat heterogeneity Probability of colonization Reduced edge effects (habitat quality) Connectivity Dispersal Realized connectivity Surrounding habitat matrix Local species pool
Fragmentation and stochastic processes As the area of habitat become smaller and fragments of habitat become more and isolated Edge effects may reduce the suitability of the habitat Species will be lost (increasingly randomly) from small habitat patches
Extinction Local extinction (loss of a species from a habitat fragment) Then regional extinction Gradual process: not just all individuals that die suddenly Fragmentated habitats: what influences the probability of local extinction? dispersal, connectivity, landscape context Loss of genetic variation contributes to the problem of small populations. Genetic diversity is important!
Extinction debt Habitat fragmentation... fragments were once part of a larger, more connected area of habitat Inertia.. the properties of the communities in the fragments still reflect earlier times What will happen to species richness if there is no further habitat fragmentation in the future?
Grasslands Truncated succession Grazing Low levels of nutrients Intermediate disturbance Fine-scale habitat heterogeneity... gaps ( regeneration niche ) Fine scale niche-differentiation (competition for nutrients) [habitat fragmentation and history] Grassland specialists and habitat generalists
Dark diversity Regional species pool contains many species that are ecologically suited to a particular habitat... But samples from plant communities typically innclude only a proportion of the species that could be there: the missing species = the Dark diversity Functional traits that explain the dark diversity?
Neutral model Everything depends on neutral (not influenced by ecological processes) processes such as dispersal and species physiological tolerances. Competition unimportant The model depends on the assumption that dispersal is neutral and that dispersal traits are non-adaptive (i.e. not subject to natural selection)
Dispersal
But neutral (i.e. non-adaptive and random) processes are contributing to patterns of distribution and diversity on all spatial and temporal scales. Examples Large scale: immigration history and chance Very fine scale: the processes involved in fine-scale establishment in grassland. Small gaps and the probability of a seed from a particular species finding a gap.
5-year study, grasslands on Öland The carousel model Species mobility on the scale of the small [0.001 m 2 and 0.25 m 2 ]quadrats used implies both appearance and disappearance [of species] from these quadrats. "We postulate that in homogeneous, grazed, nutrient- and water-deficient environments many species can reach virtually all microsites within the plot, which we express through the idea of the carousel model. We also question the usefulness of the niche concept and re-interpret it by stating that all species of this plant community have the same habitat niche, while most of them are short-lived and have the same regeneration niche. The essential variation amongst the species is their individual ability to establish or re-establish by making use of favourable conditions appearing in microsites in an unknown, complex spatio-temporal pattern. Van der Maarel, & Sykes 1993. Small-scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept. Journal of Vegetation Science 4: 179-188.
Characterizing species diversity Alpha and Beta diversity Within and between samples or habitats Alpha diversity = Species diversity Beta diversity = Species turnover
[ Quadrats = samples]
Range expansion; immigration history (a lot of chance events (non-adaptive processes)) Basic ecological amplitude: can the species grow in a region? Abiotic environmental factors (adaptive variation) Get the species into the regional species pool (chance + general ecology and availability of suitable habitats) Don t lose species as a result of habitat fragmentation (stochastic processes) Habitat heterogeneity Biotic interactions (competition) All this goes on on different spatiotemporal scales!
Agricultural intensification & Habitat heterogeneity Farmland biodiversity: is habitat heterogeneity the key? Benton, Vickery & Wilson 2003 TRENDS in Ecology and Evolution: 18 Dark Diversity Dark diversity: shedding light on absent species Pärtel, Szava-Kovats & Zobel 2011 TRENDS in Ecology and Evolution: 26 General text book Land Mosaics R.T.T. Forman (in the library)