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Plant Ecology

8 min read

Plant ecology examines the relationships between plants and their living and non-living environment — at scales from individual responses to global vegetation patterns.

Plant community

A characteristic assemblage of plant species growing together in a particular habitat, distinguished by composition, structure, dominance and the environmental conditions to which it is adapted.

Levels of ecological organisation

  1. Individual / autecology — a single species' tolerance and adaptation.
  2. Population — same species in same area.
  3. Community / synecology — multiple species interacting.
  4. Ecosystem — community + abiotic environment.
  5. Biome — large biogeographic region.
  6. Biosphere — globe.

Plant adaptations by environment

Hydrophytes (aquatic plants)

  • Reduced cuticle, large air spaces (aerenchyma), poorly developed root system.
  • Examples: Hydrilla, Nymphaea, Eichhornia (water hyacinth — invasive in Pakistan).

Mesophytes

  • "Average" terrestrial plants; balanced cuticle, stomata, vascular system.
  • Most crop plants.

Xerophytes (drought-adapted)

  • Thick cuticle, sunken stomata, reduced leaves (or spines), water storage tissues, deep or shallow extensive roots.
  • Examples: Calotropis, cacti, Acacia nilotica, Prosopis.

Halophytes (salt-tolerant)

  • Salt glands or bladders, succulence, exclusion mechanisms.
  • Examples: Atriplex, Suaeda, Avicennia marina (mangrove).

Epiphytes

  • Grow on other plants; absorb moisture from air. Common in tropical forests.

Ecological succession

A predictable change in community composition over time:

  • Primary succession — starts on bare substrate (volcanic rock, glacial moraine).
  • Secondary succession — after disturbance on existing soil (fire, abandoned field).

Stages: pioneer community → seral stages → climax community.

  • Hydrosere — succession in water; ends in forest.
  • Xerosere — succession on bare rock; ends in climax forest if climate allows.

Key thinker: F. E. Clements (1916) — monoclimax theory; later modified by Tansley's polyclimax and Whittaker's gradient analysis.

Key Points
  • Primary succession begins on bare substrate; secondary on disturbed soil with prior biota.
  • Climax community is the relatively stable end-stage matched to climate.
  • Niche = role of a species; habitat = address.
  • Ecotone = transition zone between two communities — often shows edge effect with higher diversity.
  • Pakistan's main biomes: Indus alluvial plains, deserts, dry mountains, moist temperate Himalaya, alpine, mangrove.

Population dynamics in plants

  • Density, dispersion (uniform, random, clumped), age structure.
  • Density-dependent factors: competition, herbivory, disease.
  • Density-independent factors: temperature, frost, fire.

Species interactions

  • Competition — for light, water, nutrients (intra- and inter-specific).
  • Mutualism — mycorrhizae (Rhizobium-legume; arbuscular mycorrhizae with most crops).
  • Commensalism — epiphytes on trees.
  • Parasitism — Cuscuta (dodder), Striga, Orobanche (broomrape).
  • Allelopathy — release of chemicals that inhibit competitors (walnut juglone, sorghum sorgoleone).

Biogeography

  • Continental drift explains shared floras of Gondwana fragments.
  • Latitudinal diversity gradient — species richness peaks near the equator.
  • Island biogeography (MacArthur & Wilson, 1967) — species number depends on island area and distance from mainland.

Ecological pyramids and productivity

(Cross-reference: see ecosystem note.)

  • NPP varies from < 100 g C/m²/yr in deserts to > 2,000 g C/m²/yr in tropical rainforests.

Plant biodiversity in Pakistan

  • ~6,000 vascular plant species (Stewart 1972; Flora of Pakistan ongoing).
  • About 400 species are endemic (e.g. Astragalus chitralensis, several Stipa and Astragalus species in mountainous areas).
  • Threatened plants on IUCN Red List include Taxus wallichiana (Himalayan yew, CITES II), Pinus gerardiana (chilghoza, vulnerable), Saussurea costus.

Conservation strategies

  1. In situ — protected areas (national parks, biosphere reserves, sacred groves, community-managed forests).
  2. Ex situ — botanical gardens, seed banks, cryopreservation, in vitro tissue culture.
  3. Legal protection — CITES Appendices I–III for international trade.
  4. Community based — local conservancies, trophy hunting revenue for habitat.

CITES and CBD

  • CITES (1973) — Convention on International Trade in Endangered Species. Pakistan acceded in 1976.
  • CBD (1992, Rio) — Convention on Biological Diversity. Three objectives: conservation, sustainable use, equitable benefit sharing.
  • Nagoya Protocol (2010) — access and benefit sharing.
  • Cartagena Protocol (2000) — biosafety (LMOs).

Examiners reward precise terminology. Distinguish niche vs habitat, ecotone vs ecocline, primary vs secondary succession. Quote Clements 1916 (monoclimax) and MacArthur & Wilson 1967 (island biogeography) as signature references in plant ecology.

Invasive species in Pakistan

  • Prosopis juliflora (mesquite) — widespread in Sindh; outcompetes natives.
  • Parthenium hysterophorus — congress grass; allergenic.
  • Eichhornia crassipes — water hyacinth; clogs waterways.
  • Broussonetia papyrifera — paper mulberry; major aero-allergen in Islamabad.

Control involves mechanical removal, biological control agents, and prevention.

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