Ecosystem Stability stability inertia resilience

Transcription

1 Ecosystem Stability An ecosystem s stability refers to its apparently unchanging nature over time. Components of ecosystem stability include inertia (the ability to resist disturbance) and resilience (the ability to recover from external disturbance). The diversity of ecosystems at low latitudes (nearer the equator) is generally higher than at higher latitudes (nearer the poles). This photograph shows a forest in Hawaii.

2 Small-scale Medium-scale Time Scales of Change 1 Short-term Medium-term Long-term Very longterm 10 4 Supercell thunderstorms Local communities and populations Communities Drainage basins, soil landscapes Hill slopes, flood plains, glacial moraines and alluvial fans Mountain building Area (km 2 ) Area (m 2 ) Individual organisms and small landforms (soil horizons) Day Month Year Decade Century Millennium Time (years log10 scale) Million years Billion years

3 Large-scale Very large-scale Times Scales of Change 2 Short-term Medium-term Long-term Global circulation Very longterm Phylum Size of Earth 10 8 Class Cyclones and anticyclones Zonobiomes Order Family Tectonic plates form 10 7 Fronts Biomes Genus Species Large tectonic plate movements 10 6 Area (km 2 ) Hurricanes/ cyclones 10 5 Squall lines Communities Major tectonic movements 10 4 Day Month Year Decade Century Millennium Time (years log10 scale) Million years Billion years

4 The Effect of Species Loss Ecological theory suggests that all species in an ecosystem contribute to ecosystem function. Species loss past a certain point is likely to be detrimental to the functioning of the ecosystem and on its ability to resist change (its stability). Ecosystem stability is closely linked with biodiversity but it is not clear what level of biodiversity is required to guard against ecosystem dysfunction. Species play different roles in ecosystems

5 Key Species Species whose influences on ecological communities are greater than would be expected on the basis of their abundance are called key (keystone) species. They are more influential in ecosystem stability than other species because of their pivotal role in some ecosystem function such as nutrient cycling. Elephants are a key species, and can alter the entire structure of the vegetation in those areas into which they migrate. Their pattern of grazing on taller plant species promotes a predominance of lower growing grasses with small leaves. Elephants knock down and consume taller trees and shrubs... allowing lower growing species to predominate

6 Invertebrates as Key Species Termites are key species. They are among the few larger soil organisms able to break down plant cellulose. They shift large quantities of soil and plant matter and have a profound effect on the rates of nutrient processing in tropical environments. Pisaster ochraceous, the ochre star, is also a key species. It feeds on mussels along the coasts of North America. If it is removed, mussels dominate, crowding out most algal species and leading to a decrease in the number of herbivore species. Termite hill, Australia The ochre star, California

7 Diversity and Stability In models of ecosystem function, higher species diversity increases the stability of ecosystem functions such as productivity and nutrient cycling. A low diversity system varies more consistently with environmental variation. A high diversity system is buffered against major fluctuations.

8 Low Diversity Systems Monocultures (single species crops), are low species diversity systems. They are vulnerable to disease, pests, and disturbance. In contrast, natural ecosystems may appear homogeneous, e.g. grasslands, but contain many species which vary in their predominance seasonally. Although natural grasslands may be easily disturbed, e.g. by burning, they are very resilient and usually recover quickly from disturbance. Monoculture Savanna

9 High Diversity Systems The biodiversity of ecosystems at low latitudes is generally higher than that at high latitudes, where climates are harsher, niches are broader, and systems may be dependent on a small number of keystone species. Tropical rainforests are amongst the highest diversity ecosystems on Earth. They are generally quite resistant to disturbance, but once once degraded they have little ability to recover. Deforestation of tropical rainforest

10 Diversity Indices One of the best ways to determine the health of an ecosystem is to measure the variety of organisms living in it. Diversity indices attempt to quantify the degree of diversity and identify indicators for environmental stress or degradation. Natural, unaltered headwater streams are generally high in diversity Artificially managed and channelled rivers are generally low in diversity

11 Photos; Stephen Moore Calculating Diversity Indices Most indices of diversity are easy to use and widely used in ecology. Diversity indices include Simpson s Index for finite populations and the complement of Simpson s Index for an infinite population (below). The index ranges from 0 to almost 1 High diversity stream community DI = 1 pi 2 pi 2 = Ni/N: the proportion of species i in the population Ni = the number of individuals of each species in the sample N = the total number of individuals of all species in the sample Low diversity stream community

12 Ecological Succession Ecological succession is the process by which communities in a particular area change over time. Succession takes place as a result of complex interactions of biotic and abiotic factors. Past communit y Community composition changes with time Present communit y Future communit y Some species in the past community were out-competed or did not tolerate altered abiotic conditions. The present community modifies such abiotic factors as: Light intensity and quality Wind speed and direction Air temperature and humidity Soil composition and water content Changing conditions in the present community will allow new species to become established. These will make up the future community.

13 Early Successional Communities Pioneer community, Hawaii A succession (or sere) proceeds in seral stages, until the formation of a climax community, which is stable until further disturbance. Early successional (or pioneer) communities are characterized by: Simple structure, with a small number of species interactions Broad niches Low species diversity Broad niches

14 Climax Communities In contrast to early successional communities, climax communities typically show: Complex structure, with a large number of species interactions Climax community, Hawaii Narrow niches High species diversity Large number of species interactions

15 Primary Succession Primary succession refers to colonization of a region where there is no pre-existing community. Examples include: Newly emerged coral atolls, volcanic islands Newly formed glacial moraines Islands where the previous community has been extinguished by a volcanic eruption Hawaii: Local plants are able to rapidly recolonize barren areas

16 Primary Succession A classical sequence of colonization begins with lichens, mosses, and liverworts, progresses to ferns, grasses, shrubs, and culminates in a climax community of mature forest. In reality, this scenario is rare. Bare rock and lichens Pioneer species. Mosses and liverworts Ferns, grasses and herbaceous plants Shrubs and fast growing trees Mature, slow growing trees

17 Secondary Succession Secondary succession occurs where an existing community has been cleared by a disturbance that does not involve complete soil loss. Such disturbance events include cyclone damage, forest fires and hillside slips. Cyclone Forest fire

18 Secondary Succession Because there is still soil present, the ecosystem recovery tends to be more rapid than primary succession, although the time scale depends on the species involved and on climatic and edaphic (soil) factors. Mature forest Bare land Pioneer community (annual grasses) Grasses and herbaceous plants Shrubs and small trees Young fast growing trees

19 Deflected Successions Humans may deflect the natural course of succession, e.g. through controlled burning, mowing, or grazing livestock. The resulting climax community will differ from the natural (pre-existing) community. A relatively stable plant community arising from a deflected (or arrested) succession is called a plagioclimax. Grassland and heathland in lowland Britain are plagioclimaxes.

20 Gap Regeneration The reduced sunlight beneath large canopy trees impedes the growth of the saplings below. When a large tree falls, a crucial hole opens in the canopy, allowing sunlight to reach the saplings below. The forest regeneration following the loss of a predominant canopy tree is called gap regeneration. Gap regeneration is an example of secondary succession.

21 Gap Regeneration Cycle Gap regeneration is an important process in established forests in temperate and tropical regions. Gaps are the sites of greatest understorey regeneration and species recruitment. The creation of a gap allows more light to penetrate the canopy and alters other factors that affect regeneration, exposing mineral soils and altering nutrient and moisture regimes.

22 Surtsey Island Surtsey is a volcanic island lying 33 km off the southern coast of Iceland. The island was formed during the eruption of a submarine volcano between 1963 and The island s original area was about 2.7 km 2, but erosion has reduced it to about 1.4 km 2. As an entirely new island, Surtsey has provide researchers with an ideal environment to study primary succession. NOAA Bruce McAdam CC 2.0

23 Colonization of Surtsey The colonization of Surtsey island has been recorded since the island s formation. The first vascular plant discovered on the island was sea rocket in Since then 69 plants species have colonized the island. Colonization of Surtsey can be divided into four stages. 2 nd lag phase : colonization of plants on the northern shores. 1 st lag phase Establishment of gull colony : lag phase - reduced rate of colonization : establishment of gull colony and colonization by associated plants at southern end of island present: second lag phase followed by colonization by bushy plants.

24 Dispersal Routes of Plants The majority of the original colonizing plants arrived on Surtsey via the sea. However birds now account for the vast majority of plant colonization routes. The establishment of bushy vegetation has provided a greater range of nesting sites on the island. Gulls, puffins, graylag geese, and ravens all nest on the island.